Abstract: Recent findings have linked low hysteresis in shape memory alloys with phase compatibility between austenite and martensite. To investigate the evolution of microstructure as phase compatibility increases and hysteresis is reduced, transmission electron microscopy was used to study the alloy system Ti50Ni50-xPdx, where the composition is systemically tuned to approach perfect compatibility. Changes in morphology, twinning density and twinning modes are reported, along with special microstructures occurring when compatibility is achieved. In addition, the interface between austenite and a single variant of martensite was studied by high-resolution and conventional electron microscopy. The low energy configuration of the interface detailed in this article suggests that it plays an important role in the lowering of hysteresis compared to classical habit plane interfaces.
Abstract: Although it is well known that Fe-Mn-C TWIP steels exhibit high work-hardening rates, the elementary twinning mechanisms controlling the plastic deformation of these steels have still not been characterized. The aim of the present study is to analyse the extended defects related to the twinning occurrence using transmission electron microscopy. Based on these observations, the very early stage of twin nucleation can be attributed to the pole mechanism with deviation proposed by Cohen and Weertman or to the model of Miura, Takamura and Narita, while the twin growth is controlled by the pole mechanism proposed by Venables. High densities of sessile Frank dislocations are observed within the twins at the early stage of deformation, which can affect the growth and the stability of the twins, but also the strength of these twins and their interactions with the gliding dislocations present in the matrix. This experimental evidence is discussed and compared to recent results in order to relate the defects analysis to the macroscopic behaviour of this category of material. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: The size, morphology and configuration of Ni4Ti3 precipitates in a single-crystal Ni–Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction.
Abstract: The effects of high dose ion irradiation in beta Cu-Zn-Al were investigated between room temperature and 150 degrees C. Single crystal samples with surface normal close to [001](beta) were irradiated with 300 keV Cu+ ions. Microstructural changes were characterized using transmission electron microscopy. Irradiation induced cavities located on the surface exposed to the irradiation were observed. The morphology, size and density distribution of these cavities were analyzed as a function of different irradiation conditions. The shape and location of the cavities with respect to the irradiation surface were not affected by irradiation temperature or irradiation dose. Instead, the cavity size distribution showed a bi-modal shape for a dose of 15 dpa, regardless of irradiation temperature. For a dose of 30 dpa the bi-modal distribution was only observed after room temperature irradiation. The diffusion effects of vacancies produced by irradiation are analyzed in shape memory Cu-Zn-Al alloys, which main characteristic is the diffusionless martensitic transformation. Particularly, the cavity size distributions were analyzed in terms of nucleation, growth and coalescence. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: The purpose of this study is to develop methodologies for the characterization of Ni-silicides by analytical TEM techniques. The measurements are performed in STEM mode to allow a good spatial resolution. Initial examination of the Ni-silicide phases is done by energy-dispersive X-ray spectroscopy (EDS). It is shown that quantification results with high accuracy and precision are obtained when a Ni-silicide reference layer is used as a standard, and an absorption correction is applied to the data. The EDS quantification results are then used as a reference for the quantitative electron energy-loss spectroscopy (EELS) analysis. It is shown that EELS analysis yields quantification results with high accuracy and precision, when the EELS data are treated with a model based approach. The EELS analysis is combined with a study of the Ni-L/sub 2,3/ near-edge structure (ELNES). The ELNES of the phases can be distinguished either visually or by calculating the branching ratio from the spectra with the model based approach. This paper shows that the different Ni-silicide phases can be distinguished with these EELS based methods. The accuracy and precision of the EDS and EELS results are compared.
Abstract: Ti6Al4V sheet material is subjected to simple shear deformation with strain ratio's of 10%, 30% and 50%. Optical microscopy, transmission electron microscopy and electron backscatter diffraction techniques are applied to study the presence and morphology of deformation twins. Only the {1 0 (1) over bar 2} type of twins seems to be present with a volume fraction below 1%. These {1 0 (1) over bar 2} twins show a high density of basal stacking faults of the ... ABABACAC ... type identified using atomic resolution transmission electron microscopy. A resolved shear stress analysis shows that twins most often occur on those planes with the highest resolved shear stresses, but that the starting texture is not beneficial for the occurrence of twins. It is further suggested that a transitory strain hardening regime observed around 530 MPa might be related with the onset of twinning. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution
of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni–Ti wires subjected to a non-conventional
heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes
taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond
pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an
optimal nanograined microstructure (20–50 nm) that is partially recovered through polygonization and partially recrystallized and that
has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures
annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds
200 nm.
Abstract: Ni-Ti is one of the most popular shape-memory alloys, a phenomenon resulting from a martensitic transformation. Commercial Ni-Ti-based alloys are often thermally treated to contain Ni4Ti3 precipitates. The presence of these precipitates can introduce an extra transformation step related to the so-called R-phase. It is believed that the strain field surrounding the precipitates, caused by the matrix-precipitate lattice mismatch, lies at the origin of this intermediate transformation step. Atomic-resolution transmission electron microscopy in combination with geometrical phase analysis is used to measure the elastic strain field surrounding these precipitates. By combining measurements from two different crystallographic directions, the three-dimensional strain matrix is determined from two-dimensional measurements. Comparison of the measured strain matrix to the eigenstrain of the R-phase shows that both are very similar and that the introduction of the R-phase might indeed compensate the elastic strain introduced by the precipitate.
Abstract: The 3D morphology and distribution of lenticular Ni4Ti3 precipitates in the austenitic B2 matrix of a binary Ni51Ti49 alloy has been investigated by a slice-and-view procedure in a dual-beam focused ion beam/scanning electron microscope system. Due to the weak contrast of the precipitates, proper imaging conditions need to be selected first to allow for semi-automated image treatment. Knowledgeable imaging is further needed to ensure that all variants of the precipitates are observed with equal probability, regardless of sample orientation. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.27Ti49.73 for the matrix, leading to an increase of 125 degrees for the martensitic start temperature. Also, the expected relative orientation of the different variants of the precipitates could be confirmed.
Abstract: A sharp drop in hysteresis is observed for shape memory alloys satisfying the compatibility condition between austenite and martensite, i.e. lambda(2) = 1, where lambda(2) is the middle eigenvalue of the transformation strain matrix. The present work investigates the evolution of microstructure by transmission electron microscopy as the composition of the Ti50Ni50-xPdx system is systemically tuned to achieve the condition lambda(2) = 1. Changes in morphology, twinning density and twinning modes are reported along with twinless martensite and exact austenite-martensite interfaces. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: By changing the testing temperature, an austenitic Fe-Mn-Al-Si alloy presents either epsiv -martensite transformation or mechanical twinning during straining. In order to understand the nucleation and growth mechanisms involved in both phenomena, defects and particularly stacking faults, were characterized by transmission electron microscopy. It is observed that the character of the stacking faults also changes (from extrinsic to intrinsic) together with the temperature and the activated mode of plasticity. [All rights reserved Elsevier].
Abstract: Recording the electron energy loss spectroscopy data cube with a series of energy filtered images is a dose inefficient process because the energy slit blocks most of the electrons. When recording the data cube by scanning an electron probe over the sample, perfect dose efficiency is attained; but due to the low current in nanoprobes, this often is slower, with a smaller field of view. In W. Van den Broek et al. [Ultramicroscopy, 106 (2006) 269], we proposed a new method to record the data cube, which is more dose efficient than an energy filtered series. It produces a set of projections of the data cube and then tomographically reconstructs it. In this article, we demonstrate these projections in practice, we present a simple geometrical model that allows for quantification of the projection angles and we present the first successful experimental reconstruction, all on a standard post-column instrument. [All rights reserved Elsevier].
Abstract: High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni-Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-ECmethod). As drawn Ni-Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni-Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni-Ti wire are comparable to those observed in straight annealed wires.
Abstract: A functionally graded multilayer Ni-Ti thin film was deposited on a SiO2/Si substrate by d.c.sputtering using a ramped heated Ni-Ti alloy target. The stand-alone films were crystallized at 500 degrees C in vacuum better than 10(-7) Torr. Transmission electron microscopy micrographs taken along the film cross section show two distinct regions, thin and thick, with weak R and B2 phases, respectively. The film compositions along the thickness were measured and quantified using the standard-less EELSMODEL method. The film deposited during the initial thermal ramp (thin regions) displays an average of 54 at.% Ni while the film deposited at a more elevated target temperature (thick regions) shows about 51 at.% Ni.
Abstract: Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject to severe degradation while others from the same period survived the ravages of time. A series of dark blue, green-blue and purple enamel glass paints from the same region (Northwestern Europe) and from the same period (16-early 20th centuries) has been studied by means of a combination of microscopic X-ray fluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim of better understanding the causes of the degradation. The chemical composition of the enamels diverges from the average chemical composition of window glass. Some of the compositions appear to be unstable, for example those with a high concentration of K2O and a low content of CaO and PbO. In other cases. the deterioration of the paint layers was caused by the less than optimal vitrification of the enamel during the firing process. Recipes and chemical compositions indicate that glassmakers of the 16-17th century had full control over the color of the enamel glass paints they made. They mainly used three types of coloring agents, based on Co (dark blue), Mn (purple) and Cu (light-blue or green-blue) as coloring elements. Blue-purple enamel paints were obtained by mixing two different coloring agents. The coloring agent for red-purple enamel, introduced during the 19th century, was colloidal gold embedded in grains of lead glass. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The microstructure of a 55 mu m diameter, cold-worked Ni-Ti microwire is investigated by different transmission electron microscopy techniques. The surface consists of a few hundred nanonleter thick oxide layer composed of TiO and TiO2 with a small fraction of inhomogeneously distributed Ni. The interior of the wire has a core-shell structure with primarily B2 grains in the 1 mu m thick shell, and heavily twinned B19' martensite in the core. This core-shell structure can be explained by a concentration gradient of the alloying elements resulting in a structure separation due to the strong temperature dependence of the martensitic start temperature. Moreover, in between the B2 part of the metallic core-shell and the oxide layer, a Ni3Ti interfacial layer is detected.
Abstract: Lanthanum carbonate is a new phosphate binder that is poorly absorbed from the gastrointestinal tract and eliminated largely by the liver. After oral treatment, we and others had noticed 2-3 fold higher lanthanum levels in the livers of rats with chronic renal failure compared to rats with normal renal function. Here we studied the kinetics and tissue distribution, absorption, and subcellular localization of lanthanum in the liver using transmission electron microscopy, electron energy loss spectrometry, and X-ray fluoresence. We found that in the liver lanthanum was located in lysosomes and in the biliary canal but not in any other cellular organelles. This suggests that lanthanum is transported and eliminated by the liver via a transcellular, endosomal-lysosomal-biliary canicular transport route. Feeding rats with chronic renal failure orally with lanthanum resulted in a doubling of the liver levels compared to rats with normal renal function, but the serum levels were similar in both animal groups. These levels plateaued after 6 weeks at a concentration below 3 mu g/g in both groups. When lanthanum was administered intravenously, thereby bypassing the gastrointestinal tract-portal vein pathway, no difference in liver levels was found between rats with and without renal failure. This suggests that there is an increased gastrointestinal permeability or absorption of oral lanthanum in uremia. Lanthanum levels in the brain and heart fluctuated near its detection limit with long-term treatment (20 weeks) having no effect on organ weight, liver enzyme activities, or liver histology. We suggest that the kinetics of lanthanum in the liver are consistent with a transcellular transport pathway, with higher levels in the liver of uremic rats due to higher intestinal absorption.
Abstract: The patterns of Ni release from Nitinol vary depending on the type of material (Ni-Ti alloys with low or no processing versus commercial wires or sheets). A thick TiO2 layer generated on the wire surface during processing is often considered as a reliable barrier against Ni release. The present study of Nitinol wires with surface oxides resulting from production was conducted to identify the sources of Ni release and its distribution in the surface sublayers. The chemistry and topography of the surfaces of Nitinol wires drawn using different techniques were studied with XPS and SEM. The distribution of Ni into surface depth and the surface oxide thickness were evaluated using Auger spectroscopy, TEM with FIB and ELNES. Ni release was estimated using either ICPA or AAS. Potentiodynamic potential polarization of selected wires was performed in as-received state with no strain and in treated strained samples. Wire samples in the as-received state showed low breakdown potentials (200 mV): the improved Corrosion resistance of these wires after treatment was not affected by strain. It is shown how processing techniques affect surface topography, chemistry and also Ni release. Nitinol wires with the thickest surface oxide TiO2 (LIP to 720 nM) showed the highest Ni release, attributed to the presence of particles of essentially pure Ni whose number and size increased while approaching the interface between the surface and the bulk. The biological implications of high and lasting Ni release are also discussed. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: By changing the testing temperature, an austenitic Fe-Mn-Al-Si alloy presents either epsilon-martensite transformation or mechanical twinning during straining. In order to understand the nucleation and growth mechanisms involved in both phenomena, defects and particularly stacking faults, were characterized by transmission electron microscopy. It is observed that the character of the stacking faults also changes (from extrinsic to intrinsic) together with the temperature and the activated mode of plasticity. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: A single crystal of Cu/sub 74.08/Al/sub 23.13/Be/sub 2.79/ undergoes a martensitic phase transition at 246 and 232 K under heating and cooling, respectively. The phase fronts between the austenite and martensite regions of the sample are weakly mobile with a power-law resonance under external stress fields. Surprisingly, the martensite phase is elastically much harder than the austenite phase showing that interfaces between various crystallographic variants are strongly pinned and cannot be moved by external stress while the phase boundary between the austenite and martensite regions in the sample remains mobile. This unusual behavior was studied by dynamical mechanical analysis (DMA) and resonant ultrasound spectroscopy. The remnant strain, storage modulus, and internal friction were recorded simultaneously for different applied forces in DMA. With increasing forces, the remnant strain increases monotonously while the internal friction peak height shows a minimum at 300 mN. Transmission electron microscopy shows that the pinning is generated by dislocations which are inherited from the austenite phase.
Abstract: Carbonaceous spherules of millimeter size diameter and found in the upper soils throughout Europe are investigated by TEM, including SAED, HRTEM and EELS, and Raman spectroscopy. The spherules consist primarily of carbon and have an open cell-like internal structure. Most of the carbon appears in an amorphous state, but different morphologies of nano- and microdiamond particles have also been discovered including flake shapes. The latter observation, together with the original findings of some of these spherules in crater-like structures in the landscape and including severely deformed rocks with some spherules being embedded in the fused crust of excavated rocks, points towards unique conditions of origin for these spherules and particles, possibly of exogenic origin. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Electron energy loss spectroscopy (EELS) investigations were carried out on NiTi shape memory alloys. The composition of lens-shaped precipitates is determined to be Ni4Ti3 by model-based EELS quantification, and the Ni-depleted zone in the B2 matrix surrounding the Ni4Ti3 precipitates was quantified. The Young's modulus Y-m of the B2 matrix with 51 at.% Ni and the Ni4Ti3 precipitates was evaluated to be about 124 and 175 GPa, respectively. The intensity of the Ni L-3 edge for the precipitate is slightly higher than that for the B2 phase. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Experimental studies of the martensitic phase transition of Ni63.5Al36.5 by means of differential scanning calorimetry and x-ray diffraction show a coexistence interval of 30 K for the austenite and martensitic phase. Within a smaller interval of 10 K near the upper limit of the stability of the martensitic phase no strain broadening of x-ray diffraction lines was observed for either phase. With decreasing temperature, diffraction profiles broaden, with a maximum reached when the sample is fully transformed into the martensitic phase. This observation is analyzed in terms of heterogeneous strain broadening due to imperfect microstructural self-accommodation of the martensitic grains. No strain broadening occurs when the martensitic phase is not fully interconnected but forms isolated islands inside an austenite matrix. It is concluded that large stress fields result from the coalescence of martensitic regions in the sample. The calibration of the excess entropy from the measurements of the specific heat confirms the Landau potential of Salje et al ( 2007 Appl. Phys. Lett. 90 221903). Imperfect self-accommodation does not contribute to a measurable increase of the entropy of the sample.
Abstract: Electron energy-loss spectroscopy (EELS) investigations and first-principles calculations were carried out on different aspects of a NiTi shape memory alloy. The composition of lens-shaped precipitates is determined to be Ni4Ti3 by model-based EELS quantification and the Ni-depleted zone in the B2 matrix surrounding those precipitates is quantified. EELS spectra show that the intensity of the normalized Ni L-3 peak of both the Ni4Ti3 precipitates and Ni-depleted zones is higher than that of matrix regions with the nominal composition, which is confirmed by first-principles simulations. The total amounts of 3d electrons, however, are the same for both elements in the different Ni-Ti structures and regions. The Young's and bulk moduli of the B2 matrix with 51 at.% Ni and the Ni4Ti3 precipitates were evaluated and the precipitates are found to be harder than the matrix, while first-principles calculations indicate that the bulk modulus for the Ni4Ti3 precipitate is higher by 5% than that for the equiatomic NiTi B2 phase. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Electron energy-loss spectroscopy (EELS) investigations and first-principles calculations were carried out on different aspects of a Ni-Ti shape memory alloy. The composition of lens-shaped precipitates is determined to be Ni/sub 4/Ti/sub 3/ by model-based EELS quantification and the Ni-depleted zone in the B2 matrix surrounding those precipitates is quantified. EELS spectra show that the intensity of the normalized Ni L/sub 3/ peak of both the Ni/sub 4/Ti/sub 3/ precipitates and Ni-depleted zones is higher than that of matrix regions with the nominal composition, which is confirmed by first-principles simulations. The total amounts of 3d electrons, however, are the same for both elements in the different Ni-Ti structures and regions. The Young psila s and bulk moduli of the B2 matrix with 51at.% Ni and the Ni/sub 4/Ti/sub 3/ precipitates were evaluated and the precipitates are found to be harder than the matrix, while first-principles calculations indicate that the bulk modulus for the Ni/sub 4/Ti/sub 3/ precipitate is higher by 5% than that for the equiatomic NiTi B2 phase. [All rights reserved Elsevier].
Abstract: At the proper composition and after receiving the appropriate thermomechanical treatment, NiTi shape memory alloys show shape memory and superelastic behaviour. These effects are due to the martensitic transformation, and a proper understanding of the microstructural aspects of this transformation is essential to develop accurate models describing the macroscopical behaviour. In this work, transmission electron microscopy combined with in situ straining is used to investigate microstructural features of the stress-induced transformation. Two kinds of samples are investigated: a superelastic polycrystalline NiTi sheet and a single crystal of NiTi, both overstoichiometric in Ni. Regarding the superelastic material, the martensite plates induced by straining are reversible upon relaxing and do not seem to consist of more than one variant. In the single-crystal sample, the induced martensite plates are not reversible and contrasts possibly due to twinning are observed. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The martensitic phase in the PtTi near-equiatomic alloy was extensively studied by electron microscopy techniques. A new 3 (3) under bar long-period stacking based on the expected B 19 structure was observed as well as twinning on (111) planes. Based on electron diffraction and high-resolution imaging and the B2 to B19 transformation mechanism a new model was proposed and the agreement with experiments was checked by simulation of electron diffraction patterns and high-resolution images. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The microstructure of a Co38Ni33Al29 ferromagnetic shape memory alloy was determined by conventional transmission electron microscopy (TEM), electron diffraction studies together with advanced microscopy techniques and in situ Lorentz microscopy. Rod-like precipitates, 10-60 nm long, of hexagonal close-packed epsilon-Co were confirmed to be present by high-resolution TEM. The orientation relationship between the precipitates and B2 matrix is described by the Burgers orientation relationship. The crystal structure of the martensite obtained after cooling is tetragonal L1(0) with a (1-11) twinning plane. The magnetic domain structure was determined during an in situ cooling experiment using the Fresnel mode of Lorentz microscopy. While transformation proceeds from B2 austenite to L1(0) martensite, new domains are nucleated, leading to a decrease in domain width. with the magnetization lying predominantly along a single direction. It was possible to completely describe the relationship between magnetic domains and crystallographic directions in the austenite phase though complications existed for the martensite phase. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Experimental studies of the martensitic phase transition of Ni/sub 63.5/Al/sub 36.5/ by means of differential scanning calorimetry and X-ray diffraction show a coexistence interval of 30 K for the austenite and martensitic phase. Within a smaller interval of 10 K near the upper limit of the stability of the martensitic phase no strain broadening of X-ray diffraction lines was observed for either phase. With decreasing temperature, diffraction profiles broaden, with a maximum reached when the sample is fully transformed into the martensitic phase. This observation is analyzed in terms of heterogeneous strain broadening due to imperfect microstructural self-accommodation of the martensitic grains. No strain broadening occurs when the martensitic phase is not fully interconnected but forms isolated islands inside an austenite matrix. It is concluded that large stress fields result from the coalescence of martensitic regions in the sample. The calibration of the excess entropy from the measurements of the specific heat confirms the Landau potential of Salje et al (2007 Appl. Phys. Lett.90 221903). Imperfect self-accommodation does not contribute to a measurable increase of the entropy of the sample.
Abstract: In this paper, we present a study on the effect of annealing a cold-rolled near-equi-atomic Ni-Ti alloy strip by transmission electron microscopy. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain B2 nanograins of around 20 nm diameter, which are preferentially oriented along the [1, -1,-1] direction, normal to the rolling direction. As-received samples were annealed for 30 min at different temperatures up to 800 degrees C. With the increase in annealing temperature, crystallization starts in the amorphous bands at around 350 degrees C and finally ends up with the coarsening of the grains in the entire sample. The annealing of the samples at 450 degrees C entirely transforms the amorphous bands to crystalline bands and as off 600 degrees C Ni3Ti2 precipitates are formed. At 800 degrees C the grain size increases to 30-50 mu m. Diffraction patterns from such grains reveal the presence of diffuse intensity around reciprocal lattice positions 1/3 < 110 >* indicating the forination of the R-phase. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: Ni/sub 4/Ti/sub 3/ precipitates play an important role in the shape memory and superelastic behaviour of thermo-mechanically treated Ni-Ti material. The 3D morphology and distribution of such precipitates with lenticular shape and rhombohedral atomic structure in the austenitic B2 matrix of a binary Ni-rich Ni-Ti alloy has been elucidated via a slice & view procedure in a dual-beam FIB/SEM system. With the sequence of cross-section SE images obtained from the SEM, a 3D reconstruction has been achieved after proper alignment and image processing, from which both qualitative and quantitative analysis can be performed. Careful imaging is needed to ensure that all variants of the precipitates are observed with equal probability, regardless sample orientation. Moreover, due to the weak contrast of the precipitates, proper imaging conditions need to be selected to allow for semi-automated image treatment. Finally, a volume ratio of 10.2% for the Ni/sub 4/Ti/sub 3/ precipitates could be calculated, summed over all variants, which yields a net composition of Ni/sub 50.36/Ti/sub 49.64/ for the matrix, leading to an increase of 113 degrees for the martensitic start temperature Ms. Also, the expected relative orientation of the different variants of the precipitates could be confirmed. In the near future, other quantitative measures on the distribution of the precipitates can be expected.
Abstract: Alloys with a microstructure analogous to the gamma / gamma ' system of Ni-based superalloys are found in the Pt-Al system. A two-phase microstructure consisting of Pt/sub 3/Al precipitates in an fcc (Pt) solid solution was developed to mimic the basic microstructure of Ni-based superalloys. The Pt/sub 3/Al precipitates were found to consist of plates that are twin related and have a specific orientation relationship with respect to the matrix. Each plate contains a high density of thin platelets lying perpendicular to the /i c/-direction. The precipitates were expected to crystallise in the D0'/i c/ unit cell. However, the experimental results showed that a different ordered structure, based on two modifications of the D0'/i c/ unit cell, occurred instead. The observed matrix/precipitate orientation relationship could be justified by misfit considerations. [All rights reserved Elsevier]
Abstract: The application of Nitinol in a wide variety of medical implants is progressively increasing because of its unique mechanical properties, durability and biocompatibility. However, as Nitinol consists of about 50 at.% of toxic Ni, certain applications are still hindered by the concern of free Ni release in the surrounding tissue. The latter is controlled by the structure of near-surface layers and can be strongly affected by various surface treatments. A proper application of advanced cross-section sample preparation techniques allows us to characterize the Nitinol near-surface structure down to the nanoscale by means of transmission electron microscopy (TEM). Elemental maps of the Ti, O and Ni distribution, concentration profiles, quantification of composition as well as atomic resolution images at the surface of a Nitinol tubing are presented and the results obtained with different sample preparation and analytical characterization techniques are compared. In addition to a strong decrease of Ni towards the surface of the oxide layer and a Ti depleted layer underneath the oxide, also a possible transformation from TiO to TiO2 is documented.
Abstract: Alloys with a microstructure analogous to the gamma/gamma' system of Ni-based superalloys are found in the Pt-Al system. A two-phase microstructure consisting of Pt3Al precipitates in an fee (Pt) solid solution was developed to mimic the basic microstructure of Ni-based superalloys. The Pt3Al precipitates were found to consist of plates that are twin related and have a specific orientation relationship with respect to the matrix. Each plate contains a high density of thin platelets lying perpendicular to the c-direction. The precipitates were expected to crystallise in the D0'c unit cell. However, the experimental results showed that a different ordered structure, based on two modifications of the D0'c unit cell, occurred instead. The observed matrix/precipitate orientation relationship could be justified by misfit considerations. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Conventional transmission electron microscopy (CTEM) and electron diffraction studies together with advanced microscopy techniques (high-resolution transmission electron microscopy (HRTEM) and energy-filtered transmission electron microscopy (EFTEM)) were carried out in order to characterize the microstructure of an as-east Co38Ni33Al29 shape memory alloy. Nanosized precipitates of face-centered cubic alpha-Co were observed in the specimen by means of CTEM, selected area diffraction and HRTEM. EFTEM measurements confirm that these precipitates are Co-rich. The orientation relationship between the largest precipitates and the B2 matrix is confirmed to be close to the Kurdjumow-Sachs orientation. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Focused ion beam specimen preparation has been used for NiTi samples and SrTiO3/SrRuO3 multilayers with prevention of surface amorphization and Ga implantation by a 2-kV cleaning procedure. Transmission electron microscopy techniques show that the samples are of high quality with a controlled thickness over large scales. Furthermore, preferential thinning effects in multicompounds are avoided, which is important when analytical transmission electron microscopy measurements need to be interpreted in a quantitative manner. The results are compared to similar measurements acquired for samples obtained using conventional preparation techniques such as electropolishing for alloys and ion milling for oxides.
Abstract: The influence of the charge transfer between metal and oxygen atoms on the EELS integrated cross sections has been studied experimentally and theoretically in titanium and manganese oxides of different valence. It is demonstrated that the behavior of the metal L /sub 2,3/ integrated cross section can be connected with the formal valence of the compound. The charge depletion is most noticeable in the energy-loss region 10-20 eV above the threshold while, at greater energy windows, the increasing contribution of continuum states masks the effect of the charge transfer. This contribution of continuum states is independent of the local chemical environment and can be calculated in a free atom model. Complementary, the integrated cross section near the threshold can be reliably calculated by the LAPW method provided that the simulation of EELS accounts for the matrix elements and angular dependence of the scattering. The LAPW method is successful near the threshold, however, at higher energy-losses, the standard LAPW basis set is insufficient with respect to the continuum states. Extending the basis set with extra localized orbitals allows one to account partially for the contribution of the continuum states and therefore to enlarge the applicability range of the LAPW calculations. The created core hole might noticeably affect the intensity scale of integrated cross sections but this can be modeled by introducing a core-hole in calculations also. Finally it is demonstrated that the effect of charge transfer on EELS cross sections is adequately reproduced by the LAPW calculations
Abstract: The martensitic phase in an equiatomic PtTi alloy was studied by electron microscopy, neutron and X-ray diffraction, and differential scanning calorimetry. The expected orthorhombic B19 structure with Type I twinning on {111} planes was found plus a new 3 (3) under bar long-period microtwin stacking based on this B19 structure, also including plate twinning. The measured microtwin volume fraction for the B19 is found to be larger than the value calculated from crystallographic theory, which can be explained by the fact that no martensite lattice parameters at the transformation temperature are known. An atomic model is suggested for the new long period structure in agreement with the electron diffraction and high resolution electron microscopy data. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: ITEM specimens of a LaAlO3/SrTiO3 multilayer are prepared by FIB with internal lift out. Using a Ga+1 beam of 5 kV, a final cleaning step yielding top, top-angle, side, and bottom-angle cleaning is performed. Different cleaning procedures, which can be easily implemented in a dual beam FIB system, are described and compared; all cleaning types produce thin lamellae, useful for HRTEM and HAADF-STEM work up to atomic resolution. However, the top cleaned lamellae are strongly affected by the curtain effect. Top-angle cleaned specimens show an amorphous layer of around 5 nm at the specimen surfaces, due to damage and redeposition. Furthermore, it is observed that the LaAlO3 layers are preferentially destroyed and transformed into amorphous material, during the thinning process.
Abstract: The martensitic phase transformation in Ni-Al (63.5 at. % Ni) is described by a Landau-type Gibbs free energy G(Q)=1/2A Theta(s)(coth(Theta(s)/T)-coth(Theta(s)/T-C))Q(2)+1/4BQ(4)+1/6CQ(6), with A=5.6 J K-1 mol(-1), B=-3493 J mol(-1), C=4901 J mol(-1), T-C=86 K, and Theta(s)=257 K. The dispersion (Ginzburg) energy was estimated to be g=60 J mol(-1) nm(2). This potential was determined from high resolution diffraction data to determine the spontaneous strain as proxy for the order parameter Q and differential scanning calorimetry measurements for the calibration of the excess entropy of the martensitic phase transformation. The Gibbs free energy is compared with equivalent functions of Ni-Ti, quartz, KMnF3, BaTiO3, and SrTiO3. (C) 2007 American Institute of Physics.
Abstract: The martensitic phase transformation in Ni-Al (63.5 at. % Ni) is described by a Landau-type Gibbs free energy G(Q)=1/2A Theta /sub s/(coth( Theta /sub s//T)-coth( Theta /sub s//T/sub C/))Q/sup 2/+1/4BQ/sup 4/+1/6CQ/sup 6/, with A=5.6 J K/sup -1/ mol/sup -1/, B=-3493 J mol/sup -1/, C=4901 J mol/sup -1/, T/sub C/=86 K, and Theta /sub s/=257 K. The dispersion (Ginzburg) energy was estimated to be g=60 J mol/sup -1/ nm/sup 2/. This potential was determined from high resolution diffraction data to determine the spontaneous strain as proxy for the order parameter Q and differential scanning calorimetry measurements for the calibration of the excess entropy of the martensitic phase transformation. The Gibbs free energy is compared with equivalent functions of Ni-Ti, quartz, KMnF/sub 3/, BaTiO/sub 3/, and SrTiO/sub 3/.
Abstract: In this paper we present a transmission electron microscopic study on the effect of annealing on the microstructure of a cold-rolled Ni50.2Ti49.8 ribbon. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain amorphous nanopockets and B2 nanograins, the latter at around 20 urn diameter and preferentially oriented with their normal along the (111) direction and perpendicular to the strip surface. As-received samples were annealed for 30 min at different temperatures up to 800 degrees C. Crystallization starts in the amorphous bands at around 350 degrees C and finally ends up with the coarsening of the grains in the entire sample. Annealing of the samples at 450 degrees C entirely transforms the amorphous bands into crystalline bands. At 800 degrees C the grain size increases to 30-50 pm with a formation of a tweed kind of morphology inside the grains when observed at room temperature. Diffraction pattems from such grains reveal the presence of diffuse intensity around 1/3(110)* indicating the formation of the R-phase. NiTi2 precipitates form at 450 degrees C while annealing at 600 degrees C and higher yields Ni3Ti2 precipitates. For samples annealed at 500 degrees C for a longer time, Ni4Ti3 precipitates have been observed along with the austenite to martensite transformation in the grains. (c) 2007 Elsevier Ltd. All rights reserved.
Abstract: Energy filtered TEM, EFTEM, provides three-dimensional data, two spatial and one spectral dimension. We propose to acquire these data by measuring a series of images with a defocused energy filter. It will be shown that each image is a projection of the data on the detector and that reconstruction of the data out of a sufficient number of such projections using a tomographic reconstruction algorithm is possible. This technique uses only a fraction of the electron dose an energy filtered series (EFS) needs for the same spectral and spatial resolution and the same mean signal-to-noise ratio. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The appearance of lanthanum in liver cells as a result of the injection of lanthanum chloride into rats is investigated by advanced transmission electron microscopy techniques, including electron energy loss spectroscopy and high-resolution transmission electron microscopy. It is demonstrated that the lysosomes contain large amounts of lanthanum appearing in a granular form with particle dimensions between 5 and 25 nm, whereas no lanthanum could be detected in other surrounding cellular components.
Abstract: Concentration gradients surrounding Ni4Ti3 Precipitates grown by appropriate annealing in a Ni(51)Ti(49)B2 austenite matrix are investigated by electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM). Concentration gradients of similar to 1.0-2.0 at.% in Ni within the surrounding B2 matrix can be detected by both EELS and EFTEM, revealing a Ni depleted zone in the matrix. Besides the concentration gradients, the EELS integrated cross-section of the Ni L-2,L-3 edges for the Ni-depleted region increased slightly, when compared with a matrix region away from the precipitate and not depleted in Ni. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The atomic coordinates of the crystal structure of nanoscale Ni4Ti3 precipitates in Ni-rich NiTi is refined by means of a least-squares method based on intensity measures of electron-diffraction patterns. The optimization is performed in combination with density functional theory calculations and has yielded an R3 symmetry with slightly different atomic positions when compared with the existing structure. The new unit cell offers a better understanding of the lattice deformation from the B2 matrix.
Abstract: Composition gradients surrounding Ni4Ti3 precipitates in a Ni51Ti49 B2 austenite matrix are investigated by a combination of analytical TEM techniques, including electron energy loss spectroscopy (EELS), energy filtered TEM (EFTEM) and energy dispersive X-ray spectroscopy (EDX). A Ni depleted zone in the surrounding matrix was detected. The size and concentration change of the depletion zone compensate very well for the excess Ni in the precipitate. Since different precipitates are often formed in nanoscale proximity, depletion zones from different precipitates can enforce one another in certain regions. A new quantification technique of EELS confirmed the 4:3 atomic ratio between Ni and Ti for the precipitate. No Ni-rich zone in the vicinity of grain boundaries was detected in the solution-treated samples.
Abstract: The structure of Ni4Ti3, which occurs as a precipitate in Ni-rich NiTi, has already been determined of having the R-3 space group. This was done using basic electron diffraction by Tadaki et al. also providing an approximation of the atomic coordinates. In the present work a least squares refinement method based on quantitative electron diffraction is used to optimise the structure of the Ni4Ti3 unit cell. This method, in combination with Density Functional Theory (DFT) calculations, results in a structure for which the composition and R-3 space group of the Tadaki model is maintained but the atom positions are clearly shuffled. The R-factor (similar to the one used in X-ray diffraction refinement) drops from 12.5% for the original Tadaki structure to 8.2% for the newly refined one, and moreover the change of atomic coordinates explains in a better manner the shrinking of the Ni4Ti3 unit cell in reference to the NiTi matrix. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Strain fields and concentration gradients around Ni4Ti3 precipitates in Ni-rich Ni-Ti have been quantified using advanced detection and analyzing tools of transmission electron microscopy. Strains up to 2% have been detected at distances up to 50 nm away from the precipitate-matrix interface while Ni-depletion down to 49.5 at.% Ni has been found in regions of the same dimensions. In all cases the values of the measured strain fields as well as concentration gradients perfectly compensate the related deformation and concentration changes estimated from the lattice parameter and composition of the precipitates. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The properties of the shape memory behaviour of Ni-rich binary NiTi are strongly dependant on the thermal history of the material. In this respect the changing of transformation temperatures of the underlying martensitic transformation and the occurrence of multiple step transformations are the most important phenomena. Part of the explanation is found in the presence of Ni/sub 4/Ti/sub 3/ precipitates in the B2 matrix after particular heat treatments, The formation of these precipitates changes the Ni concentration of the matrix and induces a strain field, with both of these aspects expected to be of importance. In this work atomic resolution and analytical TEM (transmission electron microscopy) techniques are used to obtain quantitative information concerning these two main features. Furthermore, the known structure of Ni/sub 4/Ti/sub 3/ is refined by a least squares optimization of quantitative electron diffraction data. The high-resolution TEM results show that there are strains up to 2% in the matrix surrounding the precipitates and they gradually increase until a maximum is reached when moving away from the interface. Analytical results reveal a global decrease of Ni content; in the matrix when sufficient precipitates are present and a gradient in their close vicinity. The refinement of the structure shows atomic displacements, thereby increasing our understanding of the shrinking of the precipitate lattice with respect to the matrix
Abstract: High-resolution transmission electron microscopy and image-processing techniques are used to measure the strain-fields surrounding coherent Ni4Ti3 precipitates in an austenitic Ni51Ti49 matrix. Images are recorded in the [1 1 (1) over bar](B2) and the [1 0 (1) over bar](B2) zones, and the {1 1 0}(B2) interplanar spacings are used to determine the strain induced by both small (50 nm diameter) and large (300 nm diameter) precipitates. From these observations, the maximum strain in the surrounding matrix is mapped and identified as compressive or tensile. Interactions between strain fields of different precipitates are also investigated. A simple model for the observed strain is proposed and compared to the classical Eshelby solution for an ellipsoidal inclusion. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Lattice deformations and concentration gradients surrounding Ni4Ti3 precipitates grown by appropriate annealing in a Ni51Ti49 B2 austenite matrix are determined by a combination of TEM techniques. Quantitative Fourier analysis of HRTEM images reveals a deformed nanoscale region with lattice deformations up to 2% while EELS and EDX indicate a Ni depleted zone up to 150 nm away from the matrix-precipitate interface.
Abstract: It is increasingly apparent that the spatial distribution of Ag and Cu nanoparticles in lustre glazes is an important factor in determining the colour displayed by the decoration. Using STEM-EDX of cross sections, this distribution can easily be imaged, as is demonstrated in two modern reconstructions of lustre decorations from Granada, Spain. On the nanoscale, the difference in colour can be related to a different spatial distribution of the Ag and Cu particles, with the brownish gold variant consisting of a top layer of Ag nanoparticles with an underlying layer of Cu nanoparticles while in the yellowish golden lustre Cu specks are unevenly dispersed.
Abstract: Concentration gradients surrounding Ni4Ti3 Precipitates grown by appropriate annealing in a Ni51Ti49 B2 austenite matrix are investigated by a combination of analytical TEM techniques. A Ni depleted zone in the surrounding matrix up to 150 nm away from the matrix-precipitate interface was detected. The size and concentration change of the depletion zone perfectly compensate for the excess Ni in the precipitate. Moreover, since different precipitates are often found in nanoscale proximity, depletion zones from different precipitates can reinforce one another in certain regions. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: The properties of the shape memory behaviour of Ni-rich binary NiTi are strongly dependant on the thermal history of the material. In this respect the changing of transformation temperatures of the underlying martensitic transformation and the occurrence of multiple step transformations are the most important phenomena. Part of the explanation is found in the presence of Ni4Ti3 precipitates in the B2 matrix after particular heat treatments. The formation of these precipitates changes the Ni concentration of the matrix and induces a strain field, with both of these aspects expected to be of importance. In this work atomic resolution and analytical TEM (transmission electron microscopy) techniques are used to obtain quantitative information concerning these two main features. Furthermore, the known structure of Ni4Ti3 is refined by a least squares optimization of quantitative electron diffraction data. The high-resolution TEM results show that there are strains up to 2% in the matrix surrounding the precipitates and they gradually increase until a maximum is reached when moving away from the interface. Analytical results reveal a global decrease of Ni content in the matrix when sufficient precipitates are present and a gradient in their close vicinity. The refinement of the structure shows atomic displacements, thereby increasing our understanding of the shrinking of the precipitate lattice with respect to the matrix.
Abstract: Microstructural observations of crystalline particles created by controlled thermal treatments in an initially amorphous Ti50Ni25Cu25 melt-spun ribbon have been carried out by means of transmission electron microscopy (TEM). After annealing at temperatures close to the crystallization temperature during short times (10 min at 693 K), a low dispersion of micrometric particles embedded in the amorphous matrix is obtained. Their lattice structure is cubic (austenite) and many of them exhibit a single crystallographic orientation, although particles with several grains have also been observed. In the latter case, each grain grows radial and simultaneously from the center of the particle, producing a spherical crystalline area divided in several crystallographic grains. Most of them show rather planar interfaces between adjacent grains in TEM images and twinning relationships in selected area electron diffraction patterns (SAEDP). However, the directions of the twinning planes in the SAEDP do not match with the traces of the interfaces between the corresponding grains observed in the TEM images. The experimental results indicate that the growth of the particular grains does not occur independently, and suggest that additional topological conditions yield the interfaces different from the twinning planes. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Binary NiTi with a composition of 51 at.% Ni was heat treated to form lens-shaped Ni4Ti3 precipitates that are coherent or semi-coherent with the B2 matrix. High resolution transmission electron microscopy (HRTEM) was used to study the internal structure of the precipitates, precipitate-precipitate and matrix-precipitate interfaces and the deformation of the B2 matrix near a precipitate. Observations were made in the (110)(B2) and (111)(B2) zones and compared with computer simulated high resolution images. The (111) B2 observations made it possible to study the [001](H) zone orientation of Ni4Ti3 (direction defined according to the hexagonal unit cell of Ni4Ti3) which corresponds to the normal of the central plane of the discs. In these images the superperiodicity of the 4:3 ordering is clearly visible confirming the known atomic structure. Close to the precipitate the B2 matrix is deformed, as determined by measuring the interplanar spacing from the HRTEM images. The observed deformations are compared with theoretical models for the stress field. (C) 2004 Elsevier B.V All rights reserved.
Abstract: In the present contribution some recent results of advanced transmission electron microscopy applications on Ni-Ti based shape memory materials will be reviewed. These will include the precipitate growth in binary Ni51Ti49 in which lens-shaped Ni4Ti3 precipitates are grown and which induce particular strain fields in the surrounding matrix influencing any ensuing martensitic transformation. Special image treatment methods are used to measure the actual lattice deformations close to these precipitates. Also the crystallisation process occurring in Ti50Ni25Cu25 melt-spun ribbons is investigated in which the focus is on the domain, interface and surface structure of micron-sized spherical B2 particles growing inside the amorphous material. When particles grow larger, martensite plates as well as small planer defects are observed. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: A partially crystallized amorphous Ti50Ni25Cu25 melt-spun ribbon showing spherical particles in martensite has been investigated. Microstructural observations support the hindering of the martensitic transformation as well as the production of additional autoaccommodated structures nearby the interface compared with the ones used inwards. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: The influence of the charge transfer between metal and oxygen atoms on the EELS integrated cross sections has been studied experimentally and theoretically in titanium and manganese oxides of different valence. It is demonstrated that the behavior of the metal L-2,L-3 integrated cross section can be connected with the formal valence of the compound. The charge depletion is most noticeable in the energy-loss region 10-20 eV above the threshold while, at greater energy windows, the increasing contribution of continuum states masks the effect of the charge transfer. This contribution of continuum states is independent of the local chemical environment and can be calculated in a free atom model. Complementary, the integrated cross section near the threshold can be reliably calculated by the LAPW method provided that the simulation of EELS accounts for the matrix elements and angular dependence of the scattering. The LAPW method is successful near the threshold, however, at higher energy-losses, the standard LAPW basis set is insufficient with respect to the continuum states. Extending the basis set with extra localized orbitals allows one to account partially for the contribution of the continuum states and therefore to enlarge the applicability range of the LAPW calculations. The created core hole might noticeably affect the intensity scale of integrated cross sections but this can be modeled by introducing a core-hole in calculations also. Finally it is demonstrated that the effect of charge transfer on EELS cross sections is adequately reproduced by the LAPW calculations.
Abstract: Measurements of absolute positions of electron energy loss spectroscopy (EELS) core-loss edges in a transmission electron microscopy (TEM) are hampered by noticeable errors caused by instabilities of the primary energy of the incident electrons. These instabilities originate from a continuous drift and random ripple of the high tension and are unavoidable in the present generation of TEM and scanning TEM microscopes. However, more precise measurements are desired, for instance, to study the shift of the edge onset between atoms of different valency or chemical environment, the so-called chemical shift. A solution to this problem is presented by collecting a series of short low-loss acquisitions immediately followed by core-loss ones. To ensure a minimal time lapse between core-loss and low-loss acquisitions, all operations must be computer controlled. Accumulation of a number of acquisitions and their summation corrected for energy drift allows to cancel the energy instabilities and to relate the core-loss EELS spectra to the absolute energy scale. A practical algorithm is presented as well as the necessary calibrations for such a procedure. Also, examples of spectra collected using this principle and the resulting measured chemical shifts in several metal-oxides are presented. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The electron-energy-loss spectra of NiO have been calculated by the ab initio full-potential linearized augmented plane-wave method within the LDA+U approximation and compared with experimental spectra obtained in transmission electron microscopy. The results of standard calculation schemes generally agree with the experimental O K edge of NiO, though the calculated spectrum looks a little contracted along the energy axis in comparison with the experimental one. However, reducing the magnetic moment at Ni atoms by a temperaturelike parameter allows us to obtain a solution which fits very closely to experiment. Possible reasons for the disagreement of standard calculation with experiment are discussed.
Abstract: Transmission electron microscopy (TEM) has been used to investigate the nanoscale morphology of some contemporary lustre glazes. High-resolution TEM, electron energy-loss spectroscopy and energy-dispersive X-ray analysis data imply that two kinds of nanoparticles are present in the lustre layer, namely metallic Ag and metallic Cu particles. Moreover, these particles appear separated in the material. The dense top layer consists of Ag particles and the particles occurring below this upper layer are metallic Cu. A depth profile of the sizes of the nanoparticles with respect to their penetration depths has been drawn. The particle sizes are mainly situated in the range of 5 nm to 15 nm, though smaller and larger particles occur frequently.
Abstract: A set of Roman glass fragments, excavated at Bliesbruck-Reinheim (French-German border), were analysed by electron probe microanalysis (EPMA) and micro-synchrotron radiation-induced x-ray fluorescence (mu-SRXRF) in order to determine the major, minor and trace chemical composition. Based on this analysis, five classes of mono-coloured glass could be discerned. However, one piece of this set was not mono-coloured, but consisted of a mixture of brown and white opaque glass. This fragment was investigated using EPMA and transmission electron microscopy (TEM) in order to gain a better insight into its microstructure. Both colours proved to contain small crystalline inhomogeneities. Structural information reveals the form in which the detected elements are present and helps to explain the appearance (e.g. colour) of the material. Copyright (C) 2004 John Wiley Sons, Ltd.
Abstract: Electron energy loss spectroscopy was used to investigate the bonding of plasma deposited carbon films. The experimental conditions include the use of a specific collection angle for which the shape of the spectra is free of the orientation dependency usually encountered in graphite due to its anisotropic structure. The first quantification process of the energy loss near-edge structure was performed by a standard fit of the collected spectrum, corrected for background and multiple scattering, with three Gaussian functions followed by a comparison with the graphite spectrum obtained under equivalent experimental conditions. In a second approach a fitting model directly incorporating the background subtraction and multiple scattering removal was applied. The final numerical results are interpreted in view of the deposition conditions of the films and the actual fitting procedure with the related choice of parameters.
Abstract: Some micrometric spherical particles containing several b.c.c. grains are obtained from an initially amorphous Ti50Ni25Cu25 meltspun ribbon after a controlled thermal treatment. The microstructure and crystallography of these crystalline particles have been investigated by TEM, showing that each of the grains grows to the same distance from the center of the particle, keeping the isotropic growth of the entire system. The number of grains in each particle is variable, but self-accommodating arrangements have always been found. Most of the grains show {112}(B2) (sometimes {114}(B2)) planes unchanged when moving to an adjacent grain, as typical twin crystals. However, the interfaces between adjacent grains usually do not match with these invariant planes. The morphology and self-accommodating arrangements found in these multi-grain particles are unusual for b.c.c. structures, but resemble in some way the case of multiple twinned particles present in other materials, as the five-fold twinned structures in f.c.c. systems. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: The crystal structure of orthorhombic martensite in ternary TiNi-based compounds has been several times examined, although, in all cases, the structure was not completely solved as the positions of atoms in the unit cell remained unknown. In the present work, the X-ray patterns of TiNi-Cu and TiNi-Pd films were refined by the Rietveld method revealing important regularities in the location of Ti and Ni atoms. In particular, the (010) atomic planes in the orthorhombic martensite structure are always distorted and loose the planar centre of symmetry. This observation was confirmed by ab-initio calculations, where the atomic positions were varied in order to minimise the energy of electron bands.
Abstract: The atom positions in the structure of the R-phase in a TiNi(Fe) intermetallic compound have been refined using the multi-slice least-squares method based on electron diffraction data obtained from 50 nm sized regions in a CM30 FEG TEM instrument. The refinement accounts for dynamic scattering including the specimen thickness and crystal misorientation as refining parameters. Compared with the original positions in the (111) planes of the parent B2 phase, each third Ti and Ni plane in the R-phase separates into three layers with different z-coordinates. The final data reveal a centre of symmetry in the R-phase structure leading to the P (3) over bar space group in contrast to earlier suggestions of a P3 or P (3) over bar lm group. Finally, it is suggested that the fine scale antiphase-like domains observed in the R-phase appear due to mirror planes over which the above mentioned atomic shifts in the structure are reversed. This phenomenon is expected to cause an artificial symmetry increase in the space group determination when using CBED with a probe size close to the size of the domains.
Abstract: A low dispersion of micrometric particles embedded in the amorphous matrix is obtained by controlled thermal treatments in an initially amorphous Ti50Ni25Cu25 melt-spun ribbon. The microstructure and crystallography of the crystalline particles have been investigated by means of TEM. Many of these particles are nearly spherical and exhibit a single crystallographic grain of austenite, the crystalline-amorphous interfaces revealing a stepped surface with preferential crystallographic planes. In addition to this, some particles with several grains in austenite have also been observed, most of them showing rather planar interfaces between adjacent grains in TEM images and twinning relationships in SAEDP. The experimental results indicate that the growth of the particular grains does not occur independently, and suggest that additional topological conditions, different from the twinning planes, yield the interfaces.
Abstract: The near-surface area of the commercial TiNi wire has been thinned by ion milling and investigated by transmission electron microscopy (TEM). The local chemical composition has been characterized by the combination of Energy-Dispersive-X-ray-Spectroscopy (EDX) and Electron-Energy-Loss-Spectroscopy (EELS) enabling us to measure simultaneously the content of heavy Ti, Ni, Cu and light O, C elements. It has been found that the wire mantle consists of the following consequent layers: 1. Austenitic TiNi with a small amount of fine martensitic plates. Lamellar shaped voids are observed near the edge of the TiNi phase. 2. Few distinct titanium oxide layers with the total thickness of about 5 mu m and the thickness of the individual layers of 0.2-0.5 mu m. The stoichiometry of titanium oxides is close to 50:50 but varies noticeably from layer to layer. EELS elemental mapping allows to visualize clearly the Ti and O distribution along the layers. 3. Outer TiO2 layer of 1 mu m in thickness. The developed technique of ion milling and the combination of TEM, EDX and EELS has been shown to be suitable for characterization of commercial TiNi products and predicting the in-vivo release of Ni-ions from their surface
Abstract: Using transmission electron microscopy the development of photothermographic materials has been investigated from a microstructural point of view. The early stages of development were examined by quenching the partially processed materials. Within a few seconds after the beginning of thermal processing, small silver filaments nucleate at the corners of AgBr grains and grow towards the matrix. Each filament is a single crystal of fcc silver, however there is no clear orientation relationship with the AgBr mother lattice. In contradiction to the widely adopted point of view, the interfaces between silver bromide and silver behenate crystals do not play a direct role in the development process, since the nucleation and growth of silver filaments occurs even in the absence of such interfaces. The filament growth was also observed by heating inside the microscope column under the electron beam, although in this case the growth mechanism and the source of Ag ions were different from those for the development in real conditions. At the second stage, silver initiates the reduction of silver ions in the matrix around AgBr grains. The reduced silver is deposited as a coagulate of small metallic particles forming large and dense broccoli-type silver conglomerates.
Abstract: The atomic configurations at macrotwin interfaces between microtwinned martensite plates in Ni65Al35 material are investigated using transmission electron microscopy. The observed structures are interpreted in view of possible formation mechanisms for these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular {110} austenite planes, enclose a final angle larger or smaller than 90degrees. Two different configurations, a crossing and a step type are described. Depending on the actual case, tapering, bending and tip splitting of the smaller microtwinvariants are observed. The most reproducible deformations occur in a region of approximately 5-10 nm width around the interface while a variety of structural defects are observed further away from the interface. These structures and deformations are interpreted in terms of the coalescence of two separately nucleated microtwinned martensite plates and the need to accommodate remaining stresses. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Abstract: We present a theoretical study of macrotwins arising in cubic to tetragonal martensitic transformations. The results help to explain some features of such macrotwins observed in Ni65Al35.
Abstract: It is known that the neutral image tone of a developed photographic film becomes brownish when the thickness of the original silver halide tabular crystals is reduced. We investigate by electron microscopy to what extent the silver filament structure has changed and how it induces the shift in image tone. Therefore, two samples of AgBr {111} tabular crystals with average thicknesses of 160 nm and 90 nm respectively, are compared. It is shown that the dimensions and defect structure of the filaments are comparable, but that the 90 nm crystals result in a more widely spaced structure, which explains the shift in image tone on a qualitative level. The influence of the addition of an image toner, i.e., phenylmercaptotetrazole, on the filament structure is also investigated. An even more open filament structure of longer, but smaller filaments was observed.
Abstract: Plasma cleaning is a useful technique to clean transmission electron microscopy (TEM) sample surfaces from carbon hydroxide. Plasma cleaning could then be applied to carbonaceous samples such as graphite, diamond, amorphous carbon films as well as to any sort of powder deposited in holey carbon films. We evaluate the time necessary to remove a carbon film deposited on former on copper grids such as those used in TEM. The film still proves completely stable under the electron beam even after 6 minutes of plasma cleaning with the shield and no re-contamination is observed. During sample preparation of diamond by ion milling an amorphous surface layer of considerable thickness (5 to 10 nm) can be created. This shows complete disappearance of that peak after a plasma cleaning time of 6 minutes with the shield, while the diamond ELNES remains unchanged. Plasma cleaning refreshes the surface by removing amorphous layers, it also seems to differentially react with the surface on an atomic scale
Abstract: High-resolution transmission electron microscopy was performed on compacted Ni3Al nanostructured material prepared by the inert gas condensation technique. From electron diffraction data an incomplete L1(2) ordering of the crystallites is observed in the free particles as well as in the room temperature compacted samples. However, a completely ordered L1(2) structure with much bigger and well-defined crystallites exhibiting several defects is observed in material compacted and annealed at 773 K. Sharp crystallite boundaries as well as amorphous material and voids are observed in between crystallites in all samples, the former being dominant in the annealed material, the latter in the as-prepared one.
Abstract: We measured the electrical resistivity R(T) and specific heat C-p(T) between room temperature (RT) and 4.2 K as well as the microstructure by transmission electron microscopy (TEM) of a Ni-52 Ti-48 SMA quenched from 1000degreesC (B2-Phase range) to RT and then annealed for 1h at T=380degreesC, 550degreesC and 650degreesC. In the "as quenched" and the "650degreesC annealed" state no martensitic transformations (MT's) occur. The diffraction patterns show faint reflections originating from coherent Ni4Ti3 precipitates in an early state of formation. Additional reflections of the type 1/2 <110>, 1/2 <111> and 1/3 <110> result from various lattice displacement waves, which are precursors of the MT's to the B19' and R-phase, respectively. Indeed, high resolution TEM micrographs of the [001] zone of the "as quenched" sample reveal transverse 1/2 <110> <110> lattice displacement waves, precursors of the B19' martensite. The coherent Ni4Ti3 precipitates, homogeneously distributed on a small length scale, binder the MT's in the "as quenched" and the "650degreesC annealed" state, and thus only the precursors appear. When annealed at T=380degreesC, however, coherent Ni4Ti3 precipitates with a length of 10nm are clearly visible in TEM. These precipitates trigger the NIT from the B2 to the R-phase on cooling, as evidenced also by anomalies in R(T) and C-p(T). Annealing at T-550degreesC leads to the well known two step MT's from the B2 to the R-phase and then into the B19'-phase. These martensitic transitions are clearly seen as additional peaks in the specific heat and anomalies in the resistance, while the "as quenched" and 650degreesC annealed samples show weak features in R(T) and C-p(T).
Abstract: EELS L-32 ionization edges in several Ni-based intermetallic compounds have been studied and interpreted in terms of the distribution of electrons in the valence d -bands. It is demonstrated that the integral EELS cross-sections change only slightly upon the formation of intermetallic compounds and therefore the charge transfer between atoms is negligible. On the other hand, the changes in the fine energy-loss near-edge structure (ELNES) of the Ni L-3 edge can be readily detected indicating an important redistribution of d -electrons at the Ni site with alloying. These features are well reproduced by ab-initio calculations with a FLAPW method in its WIEN97 implementation. In contrast to the drastic effect of chemical environment, structural transformations in the investigated intermetallics result in smaller ELNES changes, which can be detected by only exceptional instruments with a higher energy resolution.
Abstract: Glass treated on its surface with silver compounds and an aluminosilicate. such as ochre or clay, at higher temperatures (between 550 and 650 degreesC) accepts a wide variety of a yellow colour. It is the aim or this study to investigate the parameters of the manufacturing process affecting the final colour of silver stained glass and to correlate them with the final colour and colour intensity. Therefore, defined mixtures of ochre and a silver compound (AgCl, AgNO3, Ag2SO4, Ag3PO4, Ag2O) were prepared and applied on soda-lime glass. The firing process was modified within the range from 563 to 630 degreesC and glass samples were analysed after treatment with energy dispersive X-ray fluorescence analysis (EDXRF), scanning electron microscopy (SEM/EDX). transmission electron microscopy (TEM), as well as ion beam analysis (IBA) with an external beam. Within the scope of IBA simultaneous measurements using particle-induced X-ray emission (PIXE), particle-induced gamma-ray emission (PIGE). and Rutherford backscattering spectrometry (RBS) were carried out in order to obtain the thickness of the Ag-rich surface layer and the depth distribution of Ag. By means of TEM the microstructure of the silver particles was visualised. XRF results show that the lowest amount of Ag Could be detected on glass samples treated with silver stain mixtures containing AgCl and Ag2O. A low kiln temperature (e.g. 563 degreesC) results in a higher silver concentration at the Surface and lower penetration depths. Furthermore. the results obtained with SEM/EDX at cross-sections of the glass samples could be confirmed by PIXE, PIGE, RBS, and TEM.
Abstract: The atom positions in the structure of the R-phase in a TiNi(Fe) intermetallic compound have been refined using the multi-slice least-squares method based on electron diffraction data obtained from 50 nm sized regions in a CM30 FEG TEM instrument. The refinement accounted for dynamic scattering including the specimen thickness and crystal misorientation as refining parameters. Compared with the original positions in the (I I I) planes of the parent B2 phase, each third Ti and Ni plane in the R-phase separates into three layers with different z-coordinates. This agrees with previous experiments in which it was concluded that the R-structure belongs to the space group P3 rather than P3 I in as determined earlier by convergent beam electron diffraction (CBED). However, our data reveal a centre of symmetry in the R-phase structure leading to the P3 space group (R-factor of 5.5%). Finally, it is suggested that the fine scale antiphase-like domains observed in the R-phase appear due to mirror planes over which the above mentioned atomic shifts in the structure are reversed. This phenomenon is expected to cause an artificial symmetry increase in the space group determination when using CBED with a probe size close to the size of the domains.
Abstract: Different microstructural configurations of Ni-Al martensite are described on the meso- and atomic-scale, based on transmission electron microscopy images at conventional and atomic resolution. The observed structural features are described in close correlation with modern linear and non-linear continuum models, explaining the existence of particular orientations and stress accommodating features such as lattice reorientation, twin tapering and bending. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: A fragment of 17th century ruby red Kunckel glass has been investigated to elucidate its internal structure. It is known that gold nanoparticles can cause a red colour It will be demonstrated that the gold particles found in this piece display a cubo-octahedral morphology and have the right sizes to provoke the proper colour Furthermore, iron containing particles were abundantly found in the glass matrix. The larger ones proved to be alpha-Fe2O3 particles and it is suggested that they too have an influence on the colour. The composition and structure of the smaller particles was investigated using HRTEM and EELS and most of them are also alpha-Fe2O3, although some are characterised as Fe3O4. These Fe3O4 particles may have caused an internal reducing environment that facilitated the reduction of gold ions into gold particles.
Abstract: The structure of GP-zones in an industrial, 7xxx-series Al-Zn-Mg alloy has been investigated by transmission electron microscope methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)zones are formed over a wide temperature range, from room temperature to 140-150 degreesC, independently of quenching temperature, The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450 degreesC, by aging at temperatures above 70 degreesC. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated < 110 > domains, The structural relation to the eta'-precipitate is discussed. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Abstract: The bending and tapering of microtwin needles at the interface between two martensite laminates in Ni65Al35 are addressed. A theory is presented, based on the assumption that the microstructure is essentially stress-free; it predicts a linear relationship between the bending and tapering of the needles. The predictions of this theory are compared to experimental data obtained by high-resolution transmission electron microscopy. The agreement between theory and experiment is good, thus bridging the gap between continuum theory and discrete atomic structure, particularly in the region where the bending and tapering are largest.
Abstract: A positron lifetime experiment is performed on samples produced by the compaction of nanocrystalline Ni3Al powder synthesized by the inert-gas condensation technique. In the lifetime spectrum we observe two components corresponding to defects. Computer (virtual) samples of n-Ni3Al are obtained using molecular dynamics combined with the Metropolis Monte Carlo technique. Positron lifetime calculations are then performed on selected regions of simulated samples. For this purpose, a new computational technique based on a generalization of the atomic superposition method for non-periodic systems was developed. Lifetimes calculated in this way are compared to experiment.
Abstract: Ni-Al material consisting 62...65%Ni was rapidly quenched to room temperature by the melt-spinning technique and studied using X-ray diffraction, different transmission electron microscopy (TEM) modes and calorimetry measurements. Similar to bulk material, the initial 132 structure undergoes a martensitic transformation to the Llo or 14M structure. However, the transformation proceeds very inhomogeneously and results in a mixed microstructure consisting of transformed and untransformed regions. The structure of the transformed regions varies from faulted Llo to faulted 14M and shows a variety of morphological features. EDX and EELS nanoprobes reveal that the special structural state of the melt-spun material is explained mainly by solute segregation appearing during the crystallisation process. Thus, contrary to most of other melt quenched materials, in Ni-Al, solute segregation can not be suppressed by the rapid quenching procedure. Ageing at 1200degreesC restores the compositional homogeneity and results in more homogenious martensitic transformation resembling that in bulk Ni-Al material.
Abstract: On one hand, the lattice measurements of the austenite and the martensite cells for one CuZnAl and one CuAlBe alloys permit to determine the nature of the phase transformation i.e. an "exact" interface between the parent phase and an untwinned martensite variant. Hence, for single crystals, the yield surface looks like a polygon with several sectors. Each sector is defined by its peculiar activated variant. On the other hand, a "micro-macro" integration allows to predict the yield curve for biaxial proportional mechanical tests on polycrystals. This homogenisation process is in agreement with the prediction of a phenomenological approach at the macroscopic scale and fits well the yield experimental points.
Abstract: Phase transformations and precipitation mechanisms in Ti50Ni25Cu25 material, starting from an amorphous planar-cast ribbon, were studied by in-situ transmission electron microscopy (TEM). On heating, the cubic B2 austenite crystallisation occurs around 470 degreesC after which the orthorhombic B19 martensite is formed on cooling. Annealings up to 700 degreesC yield the identification of Cu3Ti2 and Ti2Ni precipitates. These precipitations drastically alter the matrix composition, resulting in the formation upon cooling of the monoclinic B19 ' martensite with different types of long-period stacking sequences. Also the B19 ' martensite is still recognised in the latter samples. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: EELS L-3.2 ionization edges in several Ni-based intermetallic compounds have been studied and interpreted in terms of the distribution of electrons in the valence d bands. It is demonstrated that the integral EELS cross sections change only slightly upon the formation of intermetallic compounds and, therefore, the charge transfer between atoms is negligible. On the other hand, changes in the energy loss near edge fine structure (ELNES) of the Ni L-3 edge can be readily detected, indicating an important redistribution of d electrons at the Ni site. Full-potential linearized augmented-plane-wave calculations of the density of states (DOS) and simulations of the Ni L-3 edge EELS profiles show that these changes correspond to a hybridization between the Ni d band and d bands of the alloying elements. In contrast, structural transformations in the investigated intermetallic compounds do not significantly affect the ELNES as the typical energy resolution of EELS is not sufficient to track the slight difference in the DOS between various structural modifications of intermetallic compounds.
Abstract: The structure of melt-spun Ti50Ni25Cu25 ribbons after various heat treatments has been studied by X-ray and TEM analyses. As-received amorphous Ti50Ni25Cu25 ribbons, when annealed at 500degreesC for a few minutes, crystallise into the B2 structure, which fully transforms into the orthorhombic martensite upon cooling. The X-ray scans of Ti50Ni25Cu25 ribbons annealed for 5 min. at 500degreesC show the well developed orthorhombic pattern, which was used for refinement of the orthorhombic martensitic structure of TiNi-Cu alloys. The refined structure of the orthorhombic martensite in TiNi-Cu alloys differs from the standard B19 type suggested previously. X-ray investigation reveals a shift of atoms in the (010)(ORT) layers from the centro-symmetric positions towards the [001](ORT) direction. The longer annealing of Ti50Ni25Cu25 ribbons at 500degreesC causes TiCu precipitation and results in appearance of untransformed B2 grains at room temperature. Annealing at 700degreesC causes Ti3Cu4 precipitation with significant Cu depletion of the matrix resulting in the appearance of the mixed orthorhombic and monoclinic martensite.
Abstract: Microstructural investigations with transmission electron microscopy were carried out on quenched Ni-Ti alloys with 52 and 54.5 at% Ni. For the Ni52Ti48 specimen long time exposed diffraction patterns of a single grain show besides the expected reflections of the B2-phase, two sets of extra reflections in different zones. The first type of spots is explained by lattice displacement waves, which are regarded as precursors of the martensitic Ni-Ti phases, B 19' and R-phase, respectively. The second set of reflection with more diffuse intensity than the other reflections is related to Ni4Ti3 precipitates in an early state of formation. For the Ni-richer Ni54.5Ti45.5 alloy only Ni4Ti3 precipitates in an early state of formation are found but no precursors of the B 19'- and R-phase.
Abstract: The atomic configurations at macrotwin interfaces between microtwinned martensite plates in Ni65Al35 material are investigated using high resolution transmission electron microscopy (HRTEM). The observed structures are interpreted in view of possible formation mechanisms of these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular {110} austenite planes, enclose a final angle larger or smaller than 90degrees, measured over the boundary. Two different configurations, one with crossing microtwins and the other with ending microtwins producing a step configuration are described. The latter is related with the existence of microtwin sequences with changing variant widths. Although both features appear irrespective of the material's preparation technique, rapid solidification seems to prefer the step configuration. Depending on the actual case, tapering, bending and tip splitting of the small microtwin variants is observed, Severe lattice deformations and reorientations occur in a region of 5 - 10 nm around the interface while sequences of single plane ledges gradually bending the microtwins are found up to 50 nm away from the interface, These structures and deformations are interpreted in view of the need to accommodate any remaining stresses.
Abstract: We investigated NixTi180-x alloys with 51 less than or equal to x less than or equal to 54 at% Ni, quenched from 1000degreesC (B2 phase region). SANS measurements reveal the presence of precipitates on a length scale from 0.9nm (x = 51.5) to 1.32nm (x = 54), increasing in volume fraction from 0.3 % to 6 %. Diffuse diffraction spots in TEM identify the precipitates as Ni4Ti3, HREM shows their coherence. Additional reflections for x = 52 can be related to precursors of B19' martensite or R-phase, respectively. We assume that the strain fields of the Ni4Ti3 precipitates are the regions of R-phase formation, i.e. the precipitates trigger the R-martensite, responsible for the SME. The high value of the absolute resistivity (rho similar to 100muOmegacm) and the increase of rho(T)up to 40% between R.T. and 4.2K very likely originate from strain fields due to Ni4Ti3, precipitates in the early stage of formation.
Abstract: The properties of elemental and bimetallic free, supported and assembled nanoclusters are modeled at the atomic scale and the models are discussed on the basis of experimental observations. This way, the memory of some free cluster properties in nanostructured materials may be evaluated. The combination of molecular statics with High Resolution Transmission Electron Microscopy (HRTEM) allows one to predict fine detail of the lattice relaxation of a truncated octahedral gold cluster deposited on MgO. Metropolis Monte Carlo (MC) predicts that a lattice mismatch may contribute to disordering in deposited Cu/sub 3/Au nanoclusters. In both Cu-Au and Ni-Al free clusters, offset of equilibrium stoichiometry may result in segregation of Au or Al at the cluster surface. An ordered stoichiometric core is surrounded by a disordered mantle where the excess species resides. Different modeling methods predict different nanometer scale textures. Therefore, cluster assembled Ni/sub 3/Al alloys formed by condensation and pressing are modeled in two different ways. Both make use of a combination of Molecular Dynamics and MC. Whatever the model nanostructure, the segregation properties of free clusters remain in the nanostructured material. This segregation is one possible cause that can inhibit the formation of a metastable martensitic phase as observed in bulk Ni-Al alloys. The occurrence of vacancy clusters and voids is hardly identified by HRTEM. On the other hand, their distribution and sizes are sensitive to the nanostructure modeling. Therefore, a new characterization method is developed, which combines positron lifetime spectroscopy with the calculation of positron lifetimes from selected areas of the model samples
Abstract: An overview is given of non-tabular crystals with {111} surfaces occurring in emulsions of silver bromide and silver chloride tabular grains. The crystal defects are characterized by transmission electron microscopy (TEM) and their influence on the growth process is discussed. It is proven that in all cases of non-tabular growth twins on two non-parallel {111} type planes are present. Different morphologies have been observed in AgBr and AgCl emulsions. The most frequently occurring morphologies are needle and tetrahedron shaped crystals in AgBr and thick tabular crystals with an incomplete microtwin as well as crystals consisting of a tetrahedral shaped part and a flat triangular to trapezoidal part in AgCl. A comparison will be made between the silver bromide and the silver chloride crystals.
Abstract: Positron lifetime spectroscopy is an effective tool to study various types of defects in materials including nanostructured ones. The size of free volumes associated with defects can be estimated using the lifetime components found in the measured spectra. Positron lifetime experiments are performed on nanocrystalline Ni-Al samples synthesized by the inert-gas condensation technique. The samples are further characterized by means of X-ray diffraction, electron diffraction and microscopy techniques, as well as density measurements. In the lifetime spectra we observe three lifetime components corresponding to different annihilation sites in the samples. These lifetimes are compared with the results of simulations of positron lifetimes in modeled Ni-Al samples obtained using molecular dynamics and Monte Carlo calculations. Finally, we present positron lifetime results for nanocrystalline Ni/sub 3/Al samples which were produced or annealed at different temperatures
Abstract: Different structural configurations of nanoparticles have been investigated by advanced transmission electron microscopy (TEM) techniques. The cluster-surface interaction of Au clusters, produced in a laser vaporization source and deposited with low energy on MgO cubes, is investigated by high-resolution transmission electron microscopy (HRTEM). A dilation of the Au lattice that perfectly accommodates the misfit with the MgO lattice is measured and modeled with Molecular Dynamics (MD). To study the chemical ordering in bimetallic clusters, Au-Cu alloy clusters with different stoichiometries are produced and deposited in the same way as the Au clusters. Electron diffraction (ED) ring patterns obtained from these alloy clusters lying on amorphous carbon, are indexed as face-centered cubic, which is confirmed by HRTEM. This indicates that clusters of Au-Cu alloys are solid solutions, i.e. no ordering takes place in these clusters. Assembled nanoclusters of Ni /sub 3/Al, produced by the inert gas condensation (IGC) technique and pressed under high pressure (2 GPa), are investigated by HRTEM. These studies indicate that nanocrystalline Ni/sub 3/Al consists of small crystallites of random crystallographic orientations separated by grain boundaries, with the presence of several nanoscaled voids. From the selected area electron diffraction (SAED) ring pattern, an incomplete Ll/sub 2/ ordering is concluded
Abstract: Defects in AgCl tabular grains with {111} surfaces are characterized by transmission electron microscopy (TEM) and their influence on the growth process is discussed. In the tabular crystals, twins parallel to the tabular face as well as dislocations along different directions are observed. The twins induce the tabular growth, while the dislocations do not influence the morphology. In 10 to 30% of the crystals that have been characterized, thickness growth is observed and it is shown that in all cases twins on other planes than the tabular ones are present. Two configurations occur more frequently and are analyzed in detail. For the first group, twins parallel to the tabular face as well as a microtwin along a non-parallel {111} plane that ends inside the crystal are present. In the crystals of the second group, only one extra non-parallel twin occurs giving rise to a bicrystal built up by a tetrahedral shaped part and a flat triangular or trapezoidal part. More complex twin configurations give rise to various, less characteristic morphologies. From these observations, it appears that the thickness growth is related to the presence of the twins on planes other than on the tabular ones.
Abstract: NixAl100 - x material with x = 62.5 or 65 was rapidly quenched to room temperature by the melt-spinning technique and studied using X-ray diffraction, different transmission electron microscopy (TEM) modes and calorimetry measurements. Similar to bulk material, the initial B2 structure undergoes a martensitic transformation to the L1(0) or 14M structure. However, the transformation proceeds very inhomogeneously and results in a mixed microstructure consisting of transformed and untransformed regions. The structure of the transformed regions varies from faulted L1(0) to faulted 14M and shows a variety of morphologies and features like wave-like interfaces and curvature of twin planes. The potential factors responsible for such an inhomogeneous behaviour, i.e. internal stresses, lattice defects, incomplete atomic ordering and compositional variations, are investigated and discussed. Finally, we conclude that the special structural state of the melt-spun material is explained mainly by solute segregation appearing during the crystallisation process. Thus, contrary to most other melt-quenched materials, in Ni-Al, solute segregation cannot be suppressed by the rapid quenching procedure. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.
Abstract: The occupancies of the different atomic species in the L2(1) austenitic phase of the Cu66.9Zn23.7Al9.4 alloy have been determined by electron diffraction using the multi-slice least-squares method taking dynamical scattering into account. The 4a site (0, 0, 0) is occupied by aluminium and zinc, while the 4b site (1/2, 1/2, 1/2) is occupied by zinc and copper, in both cases in a random fashion. (C) 2000 Published by Elsevier Science Ltd.
Abstract: Multishell tabular grains have a higher speed than pure AgBr tabular grains. Usually the shells differ in size and iodide content, but also the precipitation method for the iodide containing shells has an influence on the iodide incorporation. A TEM investigation was performed to determine the defect structure of multishell AgBr (111) tabular crystals containing a shell with a low iodide concentration and one with a high iodide concentration. The twins that induce tabular growth and stacking fault contrast in the region of the iodide shells have been observed, similar to previously studied AgBr/Ag(Br,I) coreshell crystals. Moreover in some of the crystals dislocations have been observed, sometimes even an entire network. The number of dislocations formed varies for the different methods of iodide addition. Also variations in average thickness between the different iodide addition methods have been observed. A higher number of dislocations and thicker crystals point towards a higher local concentration of iodide. These observations allow deciding which iodide incorporation method is most useful for a preferred dislocation pattern.
Abstract: Defects in AgG tabular crystals with {111} surfaces are characterised by transmission electron microscopy (TEM) and their influence on the growth process is discussed. In the tabular crystals, twins parallel to the tabular face as well as dislocations along different directions are observed. The twins induce the tabular growth, while the dislocations do not influence the morphology. In 10 to 30% of the crystals that have been characterised, thickness growth is observed and it is shown that in all cases twins on other planes than the tabular ones are present. Two configurations occur more frequently and are analysed in detail. For the first group, twins parallel to the tabular face as well as a microtwin along a non-parallel {111} plane and ending inside the crystal are present. In the crystals of the second group only one extra non-parallel twin occurs giving rise to a bicrystal built up by a tetrahedral shaped part and a flat triangular or trapezoidal part. More complex twin configurations give rise to various, less characteristic morphologies.
Abstract: Epitaxial nanoscale [001] films of NixAl100-x (x = 52.5) have been prepared by physical vapour deposition on to a thin him of Ag [001] on NaCl (001) faces with occasional hillocks. The Ag film contains numerous dislocations and stacking faults and has a rms surface roughness of 2 nm. The Ni-Al film is ordered in the B2 structure and reveals many dislocations as well as antiphase boundaries between ordered domains. The formation of subgrains in the Ni-Al film results in severe height variations up to 30 nm across the surface. A cross-sectional model for the growth of both films is presented.
Abstract: Phase transitions are often accompanied by pre-transition effects. These effects have been studied on an atomic scale by high resolution electron microscopy. For the diffusion controled phase transitions we discuss short range ordering effects in Ni-Mo and Cu-Pd. For the displacive transitions we discuss the martensitic nucleation in Ni-Al and the nanoscale domain formation in the Ni-Ti R phase.
Abstract: Ni39.6Mn47.5Ti12.9 material annealed for one hour at 900 degrees C was investigated by different electron microscopy techniques. Coherent dodecahedron shaped precipitates with sizes between 20 to 100 nm and faceted by lozenge shapes of {110} type planes are observed. Selected area and microdiffraction patterns reveal an overall unit cell with a size of 3x3x3 units of the bcc lattice of the matrix and a body centred symmetry without screw axes. Finally a ternary gamma-brass type atomic structure of space group I4 (3) over bar m is suggested for these precipitates in accordance with the obtained symmetry constraints, the EDX measurements and HRTEM images. This is the first time this type of structure was found in an alloy completely consisting of transition metal elements. A second one-dimensional long period antiphase boundary structure with antiphase boundaries parallel with the {100} planes of the B2 austenite and correlation lengths of a few nm is occasionally observed.
Abstract: Structural characteristics and mechanical properties of TiN and (Ti,AI)N monolayers, and multilayered (superlattice) (Ti,AI)N/TiN coatings deposited on ASP 23 high-speed steel by different PVD techniques are compared. Coating characteristics such as morphology, composition, cross-sectional structure, hardness and elastic modulus were determined. The effect of TIN interlayers on the hardness and the elasticity of multilayered (Ti,Al)N/TiN coatings was investigated. The friction and wear of these coatings were investigated in unlubricated fretting tests (mode I) performed in air of different relative humidities. The relationship between wear volume and dissipated energy was derived for (Ti,Al)N-based coatings, and the dependence upon relative humidity was checked and compared to the one found earlier for TiN coatings. (Ti,Al)N-based coatings exhibit a lower wear rate (mu m(3) J(-1)) at increasing relative humidity, whereas TiN has a constant wear rate independently of the relative humidity. Multilayered (Ti,Al)N/TiN coatings show an improved fretting wear resistance in comparison to monolayer (Ti,AI)N coatings. (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: "Sialons" have several interesting mechanical, thermal and chemical properties which make them candidates for high temperature applications. Solid solutions in the Si-Al-O-N system were synthesised using nitrogen and oxygen implantation into Si33Al67, Si45Al55 and Si67Al33 thin films deposited by DC magnetron sputtering on glassy carbon substrates. Nitrogen has been implanted firstly at 50 and then at 20 keV, oxygen was post-implanted at 50 keV. The different implantation doses ranged from 1 to 10x10(17) ions/cm(2), High depth resolution profiles were obtained using RES and resonant nuclear reaction, the chemical bonds were investigated using LEEIXS and these results are correlated to the film structure measured by cross section TEM, The TEM micrographs show a columnar structure perpendicular to the substrate surface in unimplanted coatings. Nevertheless, when nitrogen is implanted grain formation is observed and after oxygen post-implantation gas bubbles appear at the film depth where the maximum oxygen concentration is observed. The correlation of these results with RES and LEEIXS measurements indicates that nitrogen should be enclosed in these bubbles. Nanohardness was also measured. The highest values are observed in samples post-implanted with 1x10(17) O/cm(2) where nanohardness increases from 3 to 10 Gpa. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Abstract: The martensitic transformation in splat-cooled Ni-37.5at.%Al and Ni-35at.%Al material is investigated by different transmission electron microscopy (TEM) techniques. It was found that the extreme conditions of the rapid quench produce much smaller grains with a higher dislocation density than in bulk material. The martensite start temperature M-s is also increased slightly, probably due to internal stresses, leading to small martensite plates often grouped into self-accommodating configurations inside the Ni-37.5at.%Al B2 grains. The 3R martensite microstructure of completely transformed grains in Ni-35at.%Al reveals several peculiar features again pointing towards internal stress configurations. Among these are stepped martensite-martensite interfaces within a former B2 grain, alternating sequences of martensite microtwin stackings on both sides of a grain boundary between former B2 grains and small arrowhead configurations with a 14M-like stacking observed at such grain boundaries. (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: The present contribution contains examples of case studies on martensitic and related phase transformations investigated using different advanced transmission electron microscopy techniques. Although the emphasis is on particular results of atomic resolution electron microscopy, examples of recent applications using other electron microscopy modes are also referred to. Within the chosen examples the emphasis is on peculiar features directly related to the phase transformation under study. These include inherited lattice defects in Cu-Zr martensite, electron radiation induced transformation in Ni-Mn-Ti, interface lattice deformations in Ni-Al and the nanostructure of the Ni-Ti R-phase.
Abstract: Homogenized Ni39.6Mn47.5T12.9 material was investigated by different electron microscopy techniques. Apart from the martensite precursor distortions typical for B2 phase alloys undergoing a thermoelastic martensitic transformation upon cooling, coherent dodecahedron-shaped precipitates with sizes between 20 and 100 nm and faceted by lozenge shapes of {110}-type planes are observed. Selected-area and microdiffraction patterns reveal an overall unit cell with a size of 3 x 3 x 3 units of the bcc lattice of the matrix and a body-centred symmetry without screw axes. Finally a ternary gamma-brass-type atomic structure of space group 14(3) over bar m is suggested for these precipitates in accordance with the obtained symmetry constraints, the energy-dispersive X-ray measurements and high-resolution transmission electron microscopy images. This is the first time this type of structure is found in an alloy completely consisting of transition-metal elements.
Abstract: The decomposition of a free-standing monophase B2 Ni62.5Al37.5 thin film with nanoscale dimensions, for thickness as well as lateral grain size, is investigated by in situ and subsequent high resolution transmission electron microscopy. The atomic structure, microstructure and texture are unchanged until a temperature of 450 degrees C is reached above which a two-phase mixture of B2 + L1(2) is formed. The predominant texture of the B2 film is inherited in the L1(2) grains and even improved during the passage of the transformation front. As a result many small angle tilt boundaries exist in between neighbouring [001] L1(2) grains. Around 1000 degrees C some of the L1(2) grains grow in thickness forming microcrystals and using material from nearby grains thus deteriorating the integrity of the film. Occasionally also fcc Ni microcrystals with up to 15 at.% disordered Al are observed. (C) 1998 Elsevier Science S.A. All rights reserved.
Abstract: Recrystallization and grain growth were studied in hot extruded B2 Fe-Al by X-ray texture measurements and Orientation Imaging Microscopy (OIM). As-extruded material contained ultra-fine grains of 02-04 mu m. Such grains contained high dislocation densities, but mostly possessed high angle boundaries with the immediate neighbors. Depending on the annealing conditions, a combination of fine equiaxed and/or large columnar grains was observed. The fine equiaxed grains were at least 10 times larger than the as-extruded grain size and contained lower dislocation densities-which points towards primary recrystallization. Large columnar grains were observed in samples with fast heating and/or prolonged annealing at higher temperatures and were possibly formed by grain growth. Samples with predominantly equiaxed grain structure contained a similar texture as in the as-extruded material-a strong [110] fiber. Formation of the large columnar grains reduced the [110], but increased the [111] fiber. Possible selection of [111] grains for grain growth is not due to grain size advantage, as grain size distributions of all three fibers were similar, but can be understood from the consideration of grain boundary character distribution (GBCD) or mesotexture. (C) 1998 Elsevier Science Limited. All rights reserved.
Abstract: The martensitic transformation in splat-cooled Ni62.5Al37.5 and Ni65Al35 material is investigated by different transmission electron microscopy techniques. It was found that the extreme conditions of the rapid quench produce much smaller grains with a higher dislocation density than in bulk material. The martensite start temperature M-s is also slightly increased, probably due to internal stresses, leading to small martensite plates inside the Ni62.5Al37.5 B2 grains. The microstructure of completely transformed grains in Ni65Al35 reveals several peculiar features again pointing towards internal stress configurations. (C) 1998 Elsevier Science Limited. All rights reserved.
Abstract: The microstructure of the martensite structures in CuZr is investigated by conventional and high resolution electron microscopy. In Part I of these two sequential papers the details of the micro- and macrotwin arrangements are presented and discussed. The correlation between the microtwinning and the superstructure is confirmed and atomic models for twin interfaces, interface steps and twin tapering are derived. A comparison with earlier findings in the isostructural B19' NiTi martensite is made. The microtwins are (001) compound twins while the macrotwins are (021) Type I twins. A spear arrangement with a ((3) over bar 32) midrib is also observed. (C) 1998 Acta Metallurgica Inc.
Abstract: The present paper covers the results of different transmission electron microscopy studies on silver halide microcrystals. Pure AgBr as well as core-shell AgBr-AgBrI crystals are investigated. In the former parallel and non-parallel twinning modes yielding tabular and needle- or tetrahedral-shaped microcrystals, respectively, are discussed. Also the short-range-order of Ag+ interstitials around the twin planes as determined from diffuse intensity in reciprocal space is described. The latter yields a technique to determine the variant in which dislocations are located in certain core-shell microcrystals. The introduction of iodine also results in the presence of inclined stacking faults in the shell and of long-range iodine ordering on the crystal surface. (C) 1998 Wiley-Liss, Inc.
Abstract: The atomic and microstructure of Ni-rich B2 NixAl100-x thin films with nanoscale dimensions,for thickness as well as lateral grain size, and produced by evaporation onto NaCl and KCl (001) surfaces are investigated by high resolution electron microscopy. Different textures and orientation relationships with the substrate are found exhibiting grains with [001], [011] and [111] zones parallel with the film normal. Local distortions including precursor-like domains and surface shearing as well as dislocations are observed in the grains. No micromodulations as in bulk martensite precursors are visible. No martensitic transformation was observed when cooling the free standing films to -170 degrees C. (C) 1998 Acta Metallurgica Inc.
Abstract: AgCl(I) crystals with (1 0 0) surfaces are studied with conventional transmission electron microscopy for their microstructure in relation with their growth habits. The crystals are prepared according to two different growth methods which give slightly different results. The presence of dislocations of mixed character that end at only two surfaces are in both cases responsible for the tabular growth, but the formation of these dislocations is different. In the first case, a stacking fault is formed which is eliminated by dislocations. In the second case, the incorporation of iodide impurities is at the origin of the formation of dislocations. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: In Part II of these two sequential papers the details of two different planar defects are presented and discussed in view of their relation with the martensitic transformation. It is shown that the lattice displacements and habit planes of the defects can be explained by assuming non-conservative anti-phase boundaries existing in the parent B2 phase, that is, prior to martensitic transformation. The main difference between both defects is the sequence of shuffled (001) planes which remains unchanged in the first type but skips one plane in the second type. The microtwin sequence is also clearly affected by the second type. (C) 1998 Acta Metallurgica Inc.
Abstract: A metastable body centred cubic (bcc) phase with lattice parameter a = 0.280 +/- 0005 nm is obtained upon rapidly solidifying Ni-9at.% Zr and Ni-8at.% Zr-1 at.% X (= Si, Ti, Mo) alloys. This bcc (A2) phase and its decomposition behaviour are studied by X-ray diffraction, differential scanning calorimetry (DSC), conventional transmission electron microscopy, transmission electron diffraction and high resolution electron microscopy. Both continuous heating experiments in DSC and ex-situ isothermal annealing studies suggest that the bcc phase is relatively more stable in the ternary alloys. Upon aging, the bcc phase decomposes into equilibrium phases (alpha Ni) and Ni5Zr. While (alpha Ni) forms directly from the bcc phase, formation of layered structures precedes the formation of the other equilibrium phase Ni5Zr. Using hexagonal representation, the lattice parameters of the layered structure in the ordered state are a approximate to 0.471 nm and c approximate to 1.612 nm and those in the disordered state are a approximate to 0.471 nm and c approximate to 0.403 nm. The morphology of the decomposition products at a very early stage is found to be dominated by strain energy. The microstructural evolution as a function of temperature is also reported and discussed.
Abstract: Tetrahedral AgBr crystals present in emulsions precipitated at high pAg values are investigated by conventional electron microscopy and electron diffraction. The defect structure of these crystals is determined to consist of two non-parallel (111)-type twin planes enclosing an angle of 70.5 degrees. A growth model is presented explaining the morphology of both needle crystals and tetrahedral crystals in terms of the influence of twin planes on the growth behaviour of the crystals.
Abstract: A new method is presented to determine from top views whether a tabular silver halide crystal contains an even or an odd number of twin planes. The model is based on the occurrence of diffuse intensity contours in diffraction patterns of the crystals. This implies that this information can be correlated with simultaneous observations of the shape of the tabular crystals (hexagonal or triangular). Moreover, it will be shown that this method allows one to determine from top views in which variant of an even twinned crystal dislocations are present.
Abstract: An overview will be given of some recent results of microstructural electron microscopy studies on martensitic systems. The precursor structure observed in a ternary Ni-Mn-Ti alloy and its correlation with the ensuing stacking sequence is presented. New types of materials of a well known alloy, in casu splat-cooled and nanoscale thin films of Ni Al are investigated revealing particular morphologies of the microtwin stackings in the martensite. The complex microstructure with two martensite structures, multiple macro-and microtwinning and different out-of-phase type defects in CuZr is also discussed.
Abstract: Selected results of electron microscopic investigations on the atomic and microstructure of Ni-Al austenite and martensite in new types of materials are presented. It is shown that splat-cooled material behaves similar as normal homogenised bulk material whereas in evaporated thin films the austenite is strongly stabilised.
Abstract: A large number of materials have a highly degenerate ground state and therefore a complex microstructure. Because of this degenerate state, phase transitions between the different phases play an important role. High resolution techniques in electron microscopy and nano-scale chemical analysis allow to study not only the microstructure but also the interfaces down to an atomic scale. We focus particularly on the ambiguity of alloys oil approaching the phase transition. The short range order (SRO) in ''1 1/20'' type alloys and the microstructure of ''tweed'' and needle formation in martensite like alloys with composition Ni5Al3 are considered in more detail.
Abstract: Defect structures in CuZr martensite are studied by conventional and high-resolution transmission electron microscopy. Two different types of planar defects are observed. It is concluded that both defect types can be inherited from anti-phase boundaries in the austenite phase. With respect to the martensitic transformation, both defect types can be explained by the different shuffles of atoms after or during the distortion to the monoclinic angle.
Abstract: A full understanding of the chemical sensitization of silver halide microcrystals with nanoscale silver/gold-sulfide clusters requires knowledge of both their nature and distribution on the microcrystal surface. Because direct electron microscopy studies of sensitized silver halide microcrystals are almost impossible, due to the electron-induced release of photolytic silver, one must resort to reliable preparation techniques such as carbon replication or gelatin encapsulation. In the present study different replication techniques are investigated and compared. For cubic and octahedral silver bromide microcrystals, the carbon replica technique in combination with the complexing agent 1,2,4-triazolium-thiolate is favored, because the traditional complexing agent, sodium thiosulfate, itself creates silver sulfide clusters as artifacts, which hampers the investigation of genuine silver-sulfur sensitization centers. Gelatin encapsulation, an alternative to carbon replication, shows severe reduction problems created during the hardening process of the gelatin. Tabular crystals, on the other hand, can be replicated by the latter process without the need for hardening.
Abstract: The results of a temperature study of AgBr-AgBrI(10%I) tabular core-shell microcrystals are presented. Stacking faults, which are present in the shell of the microcrystal, are found to disappear upon heating the microcrystals to 200 degrees C and higher. A model for the disappearance of these stacking faults, involving the movement of partial dislocations in the stacking fault plane, is presented. Stacking faults are observed to be removed most easily from parts with only one reentrant angle at the twin planes, i.e., in hexagonal microcrystals or at the long segments of triangular microcrystals.
Abstract: Homogenised and quenched Ni65Al35 samples were heated and studied in situ in a CM20 electron microscope up to 900 degrees C. The Ni5Al3 phase first forming around 550 degrees C in the quenched L1(0) microtwinned martensite starts to decompose around 800 degrees C yielding B2 precipitates in a twinned L1(2) matrix. The latter twinning is a remainder of the microtwinning in the original room temperature martensite. Also the crystallographic relations between precipitates and matrix can be traced back to the original formation of twinned martensite plates within the austenite. Some aspects of the dynamics of the process are discussed on the basis of snap shots and video recordings.
Abstract: The present paper gives a review of results of recent studies investigating the fundamentals of the martensitic and related phase transformations in Ni-Al. For the former case, the emphasis will be on the microstructure of martensite plates. The latter include the metastable Ni2Al omega-like and stable Ni5Al3 bainitic phases. These phases will be discussed in view of their atomic structure, nucleation, growth and effect on the martensitic transformation. A separate chapter will deal with precursor effects.
Abstract: Needle shaped AgBr crystals are investigated using electron diffraction and conventional transmission electron microscopy. It will be shown that the particular morphology and growth of the needles result from the defect structure of the crystals, which consists of three non-parallel twin planes, i.e. two (111) type and one (221) type twin planes. Unilateral growth is shown to occur in a [110] type direction, being the intersection of the two non-parallel (111) twin planes. On both ends of the needle, three self-perpetuating reentrant grooves are present, responsible for the rapid growth of the needle.
Abstract: The occurrence of long period fringes in mixed AgBr-AgBrI core-shell-type tabular crystals is explained in terms of a periodically varying composition in iodine content in the surface layer. Changes in fringe spacing are observed for different precipitation conditions and are explained in terms of a different iodine content. The difference in fringe morphology and direction in the core and shell region is attributed to the differences in interaction between the matrix and the surface layer.
Abstract: The formation of the Ni5Al3 phase in multiply twinned Ni65Al35 martensite was investigated by in situ transmission electron microscopy heating and high resolution electron microscopy of ex situ annealed samples. It is found that reordering in the thin foils starts around 300 degrees C, the domain size increasing with temperature. At each temperature the final stage is reached within a few minutes and without any change in the morphology of the martensite plates. As a result of the slight increase of about 1% in the c/a ratio, stacking faults are introduced in completely transformed samples in the small L1(0) twin variants.
Abstract: The nucleation and growth mechanisms of Ni5Al3 precipitates and microtwinned plates in B2 austenite and 2M (3R) martensite phases are described on the basis of conventional and high resolution electron microscopy. In the Ni62.5Al37.5 B2 austenite matrix short annealings at 550 degrees C introduce three-pointed star shaped precipitates consisting of twin related parts of different variants of the Ni5Al3 structure. Longer annealings result in plates growing separately from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix. Annealing of Ni65Al35 2M martensite plates induces simple reordering into the Ni5Al3 phase, increasing the fct c/a ratio by about 1%. As a result stracking faults are introduced in the smallest twin variants.
Abstract: The formation and growth of Ni5Al3 precipitates found inside an annealed B2 matrix in Ni62.5Al37.5 samples is described on the basis of conventional and high resolution electron microscopy images and electron diffraction patterns. Short anneals introduce three-pointed star shaped precipitates consisting of twin related segments with different variants of the Ni5Al3 structure. Longer anneals result in separate plates growing from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix. Two different growth mechanisms were observed and are discussed. The lattice distortions are found to be much higher in the small precipitates than in the large microtwinned plates.
Abstract: The martensitic phases in stoichiometric CuZr have been investigated by SAED and HREM. Detailed diffraction tilt series reveal a superstructure of the B19' (NiTi type) unit cell to be the major phase which also exhibits multiple microtwinning within one martensite plate. In some plates evidence for the basic B19' structure is found. From HREM a confirmation of the unit cell symmetry and dimensions for the superstructure was obtained and a suggestion for the type of shuffles yielding the superstructure could be given.
Abstract: The formation of the Ni5Al3 phase in L1(0) martensite of Ni65Al35 material can be delayed by using slow quenching rates after the homogenisation of the material. This is of potential interest for the use of Ni-Al as a high temperature shape memory alloy. Based on the present results indicating a strong influence of the quenching rate on the ordering sequence in this material, some difficulties in previous interpretations can be clarified.
Abstract: In this contribution the results of some electron microscopy studies on microstructures related with phase transitions in a variety of materials will be presented. The materials include binary and ternary alloys, high T-c superconductors as well as C-60 and C-70 fullerenes, while the transitions can be diffusional, displacive or both.
Abstract: Different twinning arrangements in Ni5Al3 plates grown inside the B2 phase in Ni62.5Al37.5 samples are described on the basis of conventional and high resolution electron microscopy images and electron diffraction. The plate morphology is comparable with that of pure 2M martensite plates. The choice of ordered variants within a plate is dictated by the ordering coherency of the twin plane. No dislocations were observed at the primary plate-matrix interfaces. An explanation using the streaking due to aperiodic twinning is given for unexpected reflections.
Abstract: The occurrence and origin of diffuse intensity contours in electron micrographs of AgBr crystals are investigated. The observations are interpreted in terms of a model, which attributes diffuse scattering to the presence of predominant atom or vacancy clusters of a particular polyhedral type. It is shown that irrespective of the crystal morphology, interstitial Ag ions order in AgBr material in clusters of finite size along 001 type planes. A different geometry of the diffuse intensity locus observed for triangular and hexagonal tabular grains is explained in terms of the different twin plane morphology of these grains.
Abstract: Microtwin sequences in thermoelastic NixAl100-x martensite plates of different size and composition (62.5 less-than-or-equal-to x less-than-or-equal-to 66.0) were investigated by conventional and high-resolution electron microscopy. The average twin width increases with increasing twin length and with increasing Ni content. No systematic behaviour of the volume fraction of microtwin variants could be detected, partially due to the numerous stacking irregularities. Such irregularities are attributed to changing boundary conditions during the transformation, leading to multiple needle-shaped microtwins.
Abstract: The Ni-rich Ni-Al beta2 phase (CsCl structure) exhibits the precipitation of a metastable Ni2Al phase, which is closely related to an underlying lattice instability of the [111] phonon branch in the beta2 phase. The Ni2Al precipitates undergo both the Bain distortion and twinning together with the martensitic transformation of the beta2 matrix. The resulting structure of the Ni2Al precipitates in the martensite is presented and discussed.
Abstract: An electron-irradiation-induced transformation from cubic beta2 austenite to monoclinic 7M martensite was observed in situ under high resolution electron microscopy conditions in a thin foil of Ni63Al37 beta2 phase (B2, CsCl structure), with the start temperature of the martensitic transformation below but very close to room temperature. The structure of the martensite is consistent with thermoelastic or stress-induced martensite at this composition, including the existence of multiple stacking faults. The transformation is described by a gradual increase in the shear and shuffle amplitudes already existing in the precursor domains. Possible origins of the transformation are discussed.
Abstract: The structure of the metastable Ni2Al phase, which has long been a matter of controversy, has been carefully re-examined by means of high-resolution transmission electron microscopy (HREM) and electron microdiffraction. First, it is concluded that theas-quenched NixAl100-x(60 less-than-or-equal-to x less-than-or-equal-to 65) material already exhibits a partial omega-type collapse in a one-dimensional fashion which and is consistent with the anomalous dip in the phonon dispersion curve. Ni2Al precipitates are formed on annealing by thermal decomposition of the high-temperature NixAl100-xB2 phase and still retain the small omega-type shuffle. The amount of displacement in the well developed Ni2Al phase was estimated to be between 20 and 50% of the ideal omega collapse; this was determined by means of a combined technique of HREM and microdiffraction together with dynamical calculations of HREM images and diffraction intensities.
Abstract: Cubic silver chloride microcrystals (size between 10 and 50 nm) were investigated with high-resolution electron microscopy. The print-out effect that occurs due to reaction with the electron beam was used to investigate the orientational relationship between the AgCl crystal and the Ag particles that are-formed on the AgCl crystal. The particles are found to retain the same cubic directions as the AgCl crystal with a macroscopic form of a truncated octahedron.
Abstract: Elastic and inelastic neutron-scattering and high-resolution electron-microscopy examinations of the beta /sub 2/ (B2) phase of Ni/sub x/Al/sub (100-x)/ alloys show a direct relationship between the anomalous changes in lattice dynamical behavior and the evolution of static premartensitic structural configurations as the beta /sub 2/ cools toward its martensitic transformation temperature, /b M//sub S/. The resulting microstructure is a fine-scale mosaic assembly of nonuniformly distorted and modulated domains, in which {110}<11 macr 0> shear-plus-shuffle displacements give rise to the {110}/sub B2/ micromodulations of ~1.3 nm wavelength. These displacements are derived from the unusually low energy of the Sigma /sub 4/< zeta zeta 0>-TA/sub 2/ phonon mode and its anomalous temperature-dependent incomplete softening at zeta =0 (viz., the elastic constant /b C/') and at zeta =0.16. These inhomogeneously strained domains (ISDs) are believed to be centered on low strain-amplitude defects. They are viewed as strain embryos of the product 7/b M/(5,2 macr ) martensite but are generally too weak to act as potent nucleation centers. Similar ISD configurations develop at defects with higher strain amplitudes (e.g. dislocations, grain boundaries) and these are the most likely sites for heterogeneous nucleation to occur
Abstract: Inelastic-neutron-scattering and electron-microscopy studies were performed on single crystals of the cubic phase of Ni(x)Al100-x alloys for x = 50, 58, 62.5, and 63.9 at. %. The [110]-TA2 phonon branch, corresponding to atomic displacements along the [110] direction, is shown to exhibit anomalous behavior in that the entire branch is very sensitive to composition and exhibits a "kink" whose position in q space is also sensitive to x. A peak in the elastic diffuse scattering is also present at the same q as the kink. Amplitude-contrast electron-microscope images display a "tweed" pattern characteristic of many martensitically transforming materials. The high-resolution (phase-contrast) electron image reveals microscopic deviations of the cubic structure and can be viewed as embryos of the low-temperature (5,2BAR) martensitic phase. The displacement fields associated with these distortions are shown to be the origin of the elastic central peak observed in the experiments. Temperature-dependence studies show a substantial softening of the phonon branch in the vicinity of the kink and the elastic diffuse scattering increases as T approaches T(M), the martensitic-transformation onset temperature. The mode never becomes completely soft. Furthermore, the softening is narrowly restricted to the [110BAR] direction, which is perpendicular to [110]. Studies of the phonon dispersion curve under uniaxial stress show a softening of the phonon mode near the q value of the kink, which can be interpreted in terms of Clapp's proposed localized-soft-mode theory of nucleation of materials undergoing martensitic transformation.
Abstract: Microtwin sequences in the Ni-Al 3R martensite phase are investigated at different compositions using conventional and high resolution electron microscopy and electron diffraction. The results show, upon increasing the Ni content, an increase of the c/a ratio, an increase of the c and a decrease of the a parameter of the fct lattice and a decrease of the twin density. The various compositions were obtained by annealing samples with the same bulk composition at different decomposition temperatures.
Abstract: Tabular silver bromide grains with iodide uniformly mixed in the shell were investigated with conventional transmission electron microscopy. The shell region was found to contain a large number of stacking faults parallel with the {111BAR} edges and in some cases edge dislocations with a Burgers vector of 1/2[11BAR0] type. An atomic model for the formation of the stacking faults during the growth is presented.
Abstract: The results of a HREM study of the different ordered phases that are stable at different temperatures in the Pt/sub 3/V alloy are presented. The existence of these phases is discussed briefly in terms of the calculation of the ordering energy using the Ising model and interaction parameters calculated by Bieber and Gautier (1986). The experimental results are discussed in decreasing annealing temperature
