• My interests are interdisciplinary in nature and are intended to identify microstructural phenomena and mechanisms that govern the physical behavior at large at the macroscopic scale of metals, metal alloys: nanomaterials, thin films, intermetallic alloys.
• The current research topics are: (1) the engineering of multiscale microstructures coupled with the identification of the underlying mechanisms of deformation and fracture, (2) micromechanical modeling and the coupling properties (physical and mechanical) induced by microstructure refining (grain size, second phase), (3) the link microstructure - physical and mechanical properties of plant fibers used as reinforcements (composites).
Keywords: Physical Metallurgy and Structural, Mechanical behavior; Microstructure electron microscopy, physical properties of materials, crystal plasticity.
Abstract: Bulk nanostructured cobalt was processed using a bottom-up strategy. Nanostructured particle agglomerates of about 50 and 240 xa0;nm in diameter were synthesised using a polyol route and subsequently consolidated by Spark Plasma Sintering (SPS). The microstructure of the starting powders and of the processed bulk samples was studied and characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns of the as-prepared powders showed predominantly a face centred cubic (fcc) crystalline phase, whereas both fcc and hexagonal close packed (hcp) phases were found within the consolidated samples. A sample with the highest relative mass density (94.5%) was obtained from the small powder particles. TEM observations revealed a lamellar substructure with a high density of nanotwins and stacking faults in every grain in the sample with the highest density. Brillouin light scattering (BLS) and quasistatic compression tests were used to investigate the mechanical properties of the consolidated samples. The two techniques yielded Young modulus values of 168 xa0;GPa and 130 xa0;GPa, respectively, in the sample with the highest density. This sample also exhibited a yield stress higher than 1 xa0;GPa after the compression test, which is mainly attributed to the lamellar-like structure occurring in almost every grain of the polycrystalline aggregate.
Abstract: Commercial-purity (99 wt pct), bulk, ultrafine-grained aluminum samples were produced by a two-step process that combines powder consolidation by hot isostatic pressing and dynamic plastic deformation. The compaction step yielded crystallographic texture-free specimens with an average grain size of approximately 2 mu m. Then, some of the consolidated specimens were deformed dynamically at room temperature at an initial strain rate of 370 seconds(-1) and up to an axial strain of epsilon = 1.25. After dynamic plastic deformation, the grain size and the dislocation density were approximately 500 nm and 10(14) m(-2), respectively. The yield strength was approximately 77 MPa for the as-consolidated sample, which increased up to approximately 103 MPa and 120 MPa for the impacted samples along the axial and radial directions, respectively. The compression stress as a function of strain showed saturation behavior for the axially deformed samples, whereas the specimens deformed along the radial direction exhibited significant strain softening. The latter behavior is explained mainly by the weakening of the crystallographic texture that occurred because of the strain-path change along the radial direction.
Abstract: Physical properties of compressed earth blocks reinforced with plastic wastes are compared to those of non- reinforced ones. These bricks are made with two clayey soils from two deposits of Congo located in Brazza- ville and Yengola. Mineralogical and geotechnical analysis revealed that the soil of Brazzaville is mainly composed of kaolinite whereas that of Yengola is a mixture of kaolinite and illite. The amounts of clay (46 and 48%, respectively) are higher than those usually recommended for bricks’ production without stabilizers. Despite this difference of mineralogical compositions, the physical properties of these soils are quite similar. The compressive strength of the resulted bricks compacted with an energy of 2.8 MPa is about 1.5 MPa, which is the lower limit value allowed for adobes. Reinforcing with polyethylene waste nets increased the strength by about 20% to 30% and slightly enhanced resistance to water, Young’s modulus and strain to fail- ure. However, the reinforcement had no significant effect either on bricks’ curing length or on their shrinkage.
Abstract: The Raffia textilis fiber has interesting specific mechanical properties among other vegetables fibers. But its production remains entirely based on empirical knowledge. The fibers are dried in the open air and in the shade for about 48 hours. This study explores the effect of the drying temperature, from 30 to 70 degrees C, on its drying kinetics. It was found that the drying duration passes from 55 min at 30 degrees C to 20 min at 70 degrees C. Among the three models used to simulate the drying kinetics, the Page model yields the best results. The values of the parameters of this model agree with the hypothesis that the water diffusion is one-dimensional. The activation energy of water in the fiber varies from 49 to 71 KJ/mol, depending on the model used. The effective diffusion coefficient is about 3x10(-14) m(2). s(-1) at 30 degrees C. This low value justifies the traditional use of the raffia leaves for house roofs.
Abstract: Polycrystalline Zn with an average grain size of about 300 gm was deformed by direct impact Hopkinson pressure bar at a velocity of 29 m/s. An inhomogeneous grain structure was found consisting of a center region having large average grain size of 20 gm surrounded by a fine-grained rim with an average grain size of 6 gm. Transmission electron microscopy investigations showed a significant dislocation density in the large-grained area while in the fine-grained rim the dislocation density was negligible. Most probably, the higher strain yielded recrystallization in the outer ring while in the center only recovery occurred. The hardening effect of dislocations overwhelms the smaller grain size strengthening in the center part resulting in higher nanohardness in this region than in the outer ring. (C) 2011 Elsevier Inc. All rights reserved.
Abstract: Aluminum/alumina nanocomposites were processed by hot isostatic pressing at 450 degrees C and 550 degrees C. In the bulk material sintered at 550 degrees C, the composite microstructure was formed by in situ phase transformation of the native amorphous layer on the Al particle surfaces into nanocrystalline alumina dispersoids. The microstructure consisted of an aluminum matrix containing both ultrafine and coarse grains as well as embedded gamma-Al(2)O(3) nanocrystals. The large grains in the matrix stopped the crack propagation during deformation thereby increasing the toughness of the composite. When fracture occurred during deformation at 200 degrees C in air, the heat released due to oxidation smelts aluminum resulting in filament formation between the fracture surfaces. The samples sintered at 450 degrees C and 550 degrees C had similar crystallite size and dislocation density in the matrix while in the former specimen crystallization of the amorphous phase did not occur. Additional annealing of this sample in a calorimeter resulted in the formation of nanocrystalline Al(2)O(3) accompanied by an endothermic peak at about 527 degrees C and mass-reduction of about 3%, probably as result of gaseous products release. The stresses induced by the volume change during crystallization of alumina yielded an increase of the dislocation density in the Al matrix. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Co powders having nominal average particle size of 50 and 240 nm were synthesized using a polyol method and then consolidated by hot isostatic pressing (HIP) or the emerging spark plasma sintering (SPS) compaction processes. Bulk polycrystalline aggregates were obtained, having average grain sizes of about 200 and 300 nm, respectively. It is found that both nanoparticles and consolidated samples exhibit a soft ferromagnetic behavior. The magnetization reversal likely occurs by nucleation/propagation process. However, a curling process can be involved in the magnetization reversal for the smaller particles. The dynamic measurements provide for the consolidated samples magnetic parameters corresponding to bulk cobalt with vanishing anisotropy. The contribution of the intergranular region is found to be negligible. We can infer that the used consolidation routes insure a good magnetic interfacial contact between the particles. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: Near fully dense polycrystalline nickel with a random crystallographic texture was consolidated by hot isostatic pressing of blends of nanocrystalline and conventional microcrystalline powders with different volume fractions. The transformation process resulted in a composite-like microstructure constituted by clusters of soft coarse grains regularly distributed in a hard ultrafine-grained matrix. It was found that the ultrafine-grained matrix hinders the coalescence of the coarse grains component during sintering resulting in a smaller grain size than in the fully coarse-grained counterpart in a sort of "barrier effect". This effect was found to depend on the volume fraction of the ultrafine-grained matrix. Conversely, during the hot isostatic pressing, the plastic deformation of the coarse-grained fraction is preferred to that of the ultrafine-grained fraction, because of greater dislocation activity in the former type of grains, resulting in lower defect densities in the ultrafine-grained matrix ("shielding effect"). It is shown that as a result of the interplay between the coarse-grained and ultrafine-grained components during sintering, the mechanical behavior of the composite materials cannot be obtained by a linear interpolation between the characteristics of fully ultrafine-grained and coarse-grained counterparts. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The present work focuses on the transformation of high-purity Ni powder blends of controlled volume fractions (40 and 60 %) of nanometre-sized (100 nm) and micrometre-sized (544 nm) particles into bulk samples as part of a strategy for producing ultrafine-grained materials usefully exhibiting both strength and ductility. The process involved cold isostatic pressing at 1.5 GPa and sintering. The resulting bulk samples had relative densities near 95 %, were texture-free, and exhibited two different grain size distributions with an average value of 600 +/- 30 nm. The mechanical properties were investigated by compression and microhardness tests, both at room temperature, and compared to the behaviour of a sample processed from micrometre-sized powder only. Samples prepared from the blends exhibited high yield stresses of 440 and 550 MPa after compression, and they did sustain work hardening. Tests conducted before and after compression up to 50 % deformation showed the same relative amount of hardness increase around 20 %, which was three times lower than that of the monolithic sample for which a decrease of the average grain size close to 26 % was measured.
Abstract: Bulk Co samples having a mean grain size of similar to 300 nm were processed by hot isostatic pressing of a high purity Co nanopowder synthesized by chimie douce. The grain interior exhibited a highly faulted nanoscale lamellar microstructure comprising an intricate mixture of face-centered cubic, hexagonal close-packed phases and nanotwins. Room temperature compression tests carried out at a strain rate of similar to 2 x 10(-4) s(-1) revealed a yield stress of similar to 1 GPa, a strain to rupture of similar to 5%. During straining it was found that the hexagonal close-packed phase content increased from 55% to 65% suggesting a deformation mechanism based on stress-assisted face-centered cubic to hexagonal close-packed phase transformation. In addition, an apparent activation volume of similar to 3b(3) was computed which indicates that the deformation mechanism was controlled by dislocation nucleation from the numerous boundaries. Nonetheless, in such an intricate the overall mechanical properties are discussed in term of a complex interplay between microstructure, lattice dislocation plasticity, transformation-induced plasticity and possibly twin-induced plasticity. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Bulk nanocrystalline and ultrafine-grained metals are materials having grain size in the submicron range and have motivate considerable attention due to their interesting physical and mechanical properties. An important issue in the field of submicron grain-sized materials is how to achieve both high strength and high ductility? It has been suggested that, one strategy for enhancing the ductility of high-strength nanocrystalline materials is to develop a bimodal grain-size distribution, in which the fine grains provide strength, and the coarser grains enable strain hardening. In this paper, we report on the micromechanical behaviour of bulk nickel samples having bimodal microstructures. The samples were processed by hot isostatic pressing of blends of nano and micrometer-sized powder particles. The resulting microstructure is a bimodal randomly distributed grains considered here as a mixture of two unimodal log-normal distributions. An efficient modelling approach (i.e. a generalized self-consistent approach) previously developed by Jiang and Weng (2004a,b) is then applied to such experimental data to investigate, among others, local plastic strains and internal stresses fields as well as local magnitudes deviations. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: High purity electrolytic nickel (99.99%) samples deformed dynamically in compression using a direct impact Hopkison pressure bar test at the velocities of 10.9, 28.2 and 70.6 m s(-1) were investigated. The dislocation density increased with increasing the impact velocity up to 28.2 m s(-1) resulting in an increase of nanohardness and quasi-static compressive flow stress. At the same time, a decrease of the fraction of Sigma 3 coincident site lattice boundaries was observed for the benefit of Sigma 1 low angle grain boundaries having misorientations lower than 15 degrees. Increasing the velocity to 70.6 m s(-1) led to a decrease of the dislocation density, in parallel with the regeneration of Sigma 3 boundaries. As a consequence, the nanohardness decreased to a similar value as in the initial state. These observations suggest possible dynamic recovery/recrystallization that might have occurred at the highest impact velocity. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Fine-grained aluminum (700-1000 nm) was processed by dynamic severe plastic deformation of coarse-grained (3mm) pure aluminum (99.999wt.%). The resulting microstructure was characterized by transmission electron microscopy (TEM) and X-ray profile analyses. It is observed that the grain size determined by TEM departs from measurements made by X-ray profile analysis. In the latter case, the average crystallite size determined over the global crystallographic or on the deformation-induced texture components, namely {123} < 751 >, {100} < 011 >, and {223} < 154 >, yields similar values (similar to 225 nm). By contrast, the dislocation density determined on these texture components is about two times higher than the one measured on the global texture. The difference might be related to the specificities of the induced crystallographic texture. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: Bulk ultrafine-grained nickel specimens having grain sizes in the range of 0.25-5 mu m were processed by a spark plasma sintering method. The resulting microstructures were characterized by electron backscattering diffraction, transmission electron microscopy and X-ray diffraction analysis. Compression tests were carried out at room temperature and at a strain rate of 1.6 x 10(-4) s(-1). It was found that the fine-grained microstructure and the presence of NiO phase were the main strengthening factors in the as-processed bulk materials. The contribution of the oxide phase to strengthening was even more pronounced for lower grain sizes. This contribution was calculated as the difference between the measured strength and the value obtained from a Hall-Petch plot of oxide-free samples, and this yielded a flow stress increment of about 635 MPa for the lowest grain size studied here. In addition, a transition from work-hardening to -softening occurred for materials having a mean grain size smaller than about 300 nm and having boundaries that could have been weakened by the presence of a high amount of NiO phase. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Ultrafine-grained aluminum materials were processed by hot isostatic pressing of aluminum nanopowders (99.7 wt.% purity). Quasi-static compression tests were carried out at a strain rate of 2 x 10(-4) s(-1) 200 degrees C. Scanning electron microscopy investigations of fracture surfaces or cavities that were formed during straining reveal the presence of filaments. The number and dimensions of the filaments depend on the shielding effect of the native amorphous alumina film that forms on the surface of the nanoparticles in the starting powder. After crystallization of the amorphous, extensive filament formation is observed. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We report on the first measurements of the physico-mechanical properties of the raffia textilis fiber. This fiber is the epidermis of the leaflet and is used to fabricate many ethnographical items. Scanning electron microscopy reveals a layered structure: a top layer with a tile-like structure, and a bottom layer with a honeycomb-like structure. X-ray diffraction and FTIR-ATR show the presence of cellulose I(beta) with a crystallinity index of 64%. Tensile tests give a Young's modulus of 30 GPa, a tensile strength of 500 +/- 97 MPa, and a total elongation between 2% and 4%. The fiber density is 0.75 +/- 0.07, conferring to it the highest known specific mechanical properties among all studied raw vegetable fibers. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: Ultrafine-grained samples were produced front a Ni nanopowder by hot isostatic pressing (HIP) and spark plasma sintering (SPS). The microstructure and mechanical behavior of the two specimens were compared. The grain coarsen in observed during the SPS procedure was moderated due to a reduced temperature and time of consolidation compared with HIP processing. The smaller grain-size and higher nickel-oxide content in the SPS-processed sample resulted in a higher yield strength. Compression experiments showed that the specimen produced by SPS reached a maximal flow stress at a small strain, which was followed by a long steady-state softening while the HIP-processed sample hardened until failure. It was revealed that the softening of the SPS-processed sample resulted front microcracking along the grain boundaries.
Abstract: This paper is devoted to the analysis of the specific constraints imposed on the mean free path, the nucleation and the multiplication rate of dislocations in strained polycrystals whose grain size is smaller than about 100 nanometres. It is demonstrated that the extent of the microdeformation stage, at the end of which the yield stress should be measured, is definitely larger than the generally accepted value of 0.2% proof strain for average grain sizes smaller than 125 nanometres. This analysis is confirmed experimentally in two instances. Systematic experimental analysis of the elastic-plastic transition in these materials is a necessary requirement in order to obtain relevant values of the flow stress in these materials.
Abstract: The present work focuses on understanding the mechanical behavior of bulk ultrafine-grained nickel specimens processed by spark plasma sintering of high purity nickel nanopowder and subsequently deformed under large amplitude monotonic simple shear tests and strain-controlled cyclic simple shear tests at room temperature. During cyclic tests, the samples were deformed up to an accumulated von Mises strain of about epsilon(VM) = 0.75 (the flow stress was in the 650-700 MPa range), which is extremely high in comparison with the low tensile/compression ductility of this class of materials at quasi-static conditions. The underlying physical mechanisms were investigated by electron microscopy and X-ray diffraction profile analysis. Lattice dislocation-based plasticity leading to cell formation and dislocation interactions with twin boundaries contributed to the work-hardening of these materials. The large amount of plastic strain that has been reached during the shear tests highlights intrinsic mechanical characteristics of the ultrafine-grained nickel studied here. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: A generalized self-consistent approach, recently proposed by Jiang and Weng (2004) [B. Jiang, G.J. Weng, A generalized self-consistent polycrystal model for the yield strength of Nanocrystalline materials, journal of the Mechanics and Physics of Solids 52 (2004a) 1125-1149; B. Jiang, G.J. Weng, A theory of compressive yield strength of nano-grained ceramics, International journal of Plasticity 20 (2004b) 2007-2056.] for investigating the so-called "breakdown" of the Hall-Petch law in the case of nanocrystalline (NC) materials, is revisited and reformulated following an incremental small strain scheme. The NC material is modelled as a composite material that takes each oriented grain and its immediate grain boundary to form a pair, which in turn is embedded in the infinite effective medium with a property representing the average orientation of all these pairs. The plastic deformation of the inclusion phase takes into account the dislocation glide mechanism whereas boundary phase is modelled as an amorphous material. As an application, the model's parameters are identified under an optimization code with respect to data stated from pure copper submitted to tensile load. The aggregate is composed of spherical randomly distributed grains with a grain-size distribution following a log-normal statistical function. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: The present work attempts to pinpoint scaling constraints associated with plastic deformation of nanograined polycrsytals in relation to the analysis of the available data on their mechanical properties. In doing so, it is shown in particular that the systematic transfer to nanograined polycrystal metals of concepts applied with success to coarse-grained polycrystals counterpart has no solid basis. To uncover the physics of plastic flow in bulk nanograined polycrystal metals, simple arguments and recommendations are presented and discussed.
Abstract: A novel experimental methodology to produce ultrafine-grained metallic microstructures, which is applied on aluminum is proposed in this work. In fact, the ultrafine-grained aluminum polycrystal is made from commercial purity powder by a combination of hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 mu m. The material is then subsequently impacted, using a falling weight at a maximum impact velocity of 9.2 m/sec. The resulting material shows a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture perpendicular to the impact direction. The mechanical properties of the impacted material are then characterized under compression tests at room temperature under a strain rate of 10(-4) S(-1). The effect of the change of the deformation path on the mechanical response parallel and perpendicular to the impact direction is also investigated. These results are discussed in relation with microstructure. Further, a new extension of a micromechanical approach developed by Abdul-Latif et al., [2] is proposed to predict the grain size effect on the enhancement of the mechanical strength of polycrystals. Within the framework of small strain hypothesis, the elastic anisotropy of the grain and grain rotation are neglected for the sake of simplicity. The local inelastic deformation heterogeneity is determined through the slip theory. It is assumed that the yield strength increases linearly with decreasing grain size as in Hall-Petch relationship. It is obviously recognized that the model with its new extension describes fairly well the effect of the grain size on the strain-stress behavior of the sub-micrometer aluminum. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: Ultrafine-grained samples were consolidated from Ni nanopowders with the nominal particle size of 50 and 100 nm by Hot Isostatic Pressing (HIP) and Spark Plasma Sintering (SPS). The higher nickel-oxide content and the smaller grain size of SPS-processed samples result in a higher yield strength at room temperature compared with HIP-processed specimen. It is found that during compression the dislocation density increases while the twins decay in both samples, indicating that the deformation is mediated mainly by dislocations. As a consequence of the higher oxide content, the flow stress of the SPS-processed samples saturates at small strain values while the HIP-processed specimen shows strain hardening even at the strain value of 0.35. After saturation of the flow stress for SPS-processed samples the deformation is most probably mediated rather by grain rotation or grain boundary-related mechanisms such as sliding and/or decoliesion instead of dislocation motion.
Abstract: Ultrafine-grained aluminum microstructures were processed from commercial purity powder by combining hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 mu m. The material was then subsequently impacted, using a falling weight at a strain rate of 300s(-1). The resulting material showed a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture parallel to the impact direction. The mechanical properties of the impacted material were subsequently characterized under compressive tests at room temperature at a strain rate of 10(-4)s(-1). The effect of the change of the deformation path on the mechanical response parallel (DN) and perpendicular (DT) to the impact direction was also investigated. These results are here discussed in relation with microstructure and texture evolution.
Abstract: Ultrafine-grained (uf-g) and microcrystalline-grained (mc-g) irons have been fabricated by hot isostatic pressing of nanopowders. The mechanical properties have been characterized by compressive tests at room temperature and the resulting microstructures and textures have been determined by combining electron back scatter diffraction and transmission electron microscopy. A transition of the deformation mode, from work hardening to work softening occurs for grain sizes below similar to 1 mu m, reflecting a transition of the deformation mode from homogeneous to localized deformation into shear bands (SBs). The homogeneous deformation is found to be lattice dislocation-based while the deformation within SBs involves lattice dislocations as well as boundary-related mechanisms, possibly grain boundary sliding accommodated by boundary opening. (C) 2006 Elsevier B.V. All rights reserved.
Abstract: The correlation between the microstructure and the yield strength of a specimen produced by hot isostatic pressing (HIP) of commercial purity aluminum nano-powder was studied. It was found that the bulk sample can be regarded as a composite containing microcrystalline grains embedded in an ultrafine grained matrix. The composite-like microstructure results in a bimodal hardness distribution as shown by nanoindentation. The yield strength values for both the ufg matrix and the mc grains were calculated from the characteristic parameters of the microstructure. The yield strength of the composite estimated by using a simple rule of mixture was in good agreement with the value determined by compression test. It was revealed that the majority of the strengthening can be attributed to the dislocations in the ufg matrix and the alumina dispersoids formed during HIP process. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: By extracting the variation of the plastic strain rate from measurements of the stress-strain curves of thin films of varying thickness, the large extent of the microdeformation stage was determined for tensile deformation of free-standing thin films, as well as for films on substrates deformed by cyclic heating. The stress varies dramatically with strain during this stage. It is demonstrated that this behaviour is common to most fine-grained polycrystals, and that the extent of the microdeformation stage is much larger than the conventional 0.2% proof strain, and depends both on the material as well as on the measurement technique. Therefore a careful analysis of this stage is essential in measuring the mechanical behaviour of these materials.
Abstract: A commercial purity aluminum nano-powder has been consolidated by the hot isostatic pressing technique. The bulk material, in addition to the unavoidable native Al2O3 phase, has a microstructure comprising a fraction of microcrystalline grains (>= 500 nm) that are embedded in an ultrafine-grained matrix (150 nm). True stress-true strain curves acquired from compressive tests at room temperature showed a rapid and brief hardening at the early stage of the deformation followed by a short stress plateau and linear work softening. The material yielded at 390 MPa - 10 times the flow stress of the coarse-grained counterpart material. The per cent reduction to rupture was about 20% mostly due to the presence of microcrystalline grains acting against crack propagation. In addition, it is shown that microcrystalline grains deform via dislocation-based mechanisms. while present evidence suggests that the matrix deforms by a cooperative grain boundary sliding. ((c)) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: Brillouin light scattering has been used to investigate elastic properties of a monocrystalline and of <111> textured polycrystalline 3C polytype silicon carbide films that have been deposited on silicon substrate by chemical vapor deposition. Taking advantage from the detection of different acoustic modes, a complete elastic characterization of the films has been achieved. The three unknown elastic constants of the monocrystalline 3C-SiC, namely, c(11)=395 GPa, (c(11)-c(12))/2=136 GPa, and c(44)=236 GPa have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes traveling parallelly to the film surface. These determinations are in agreement with the frequency of the observed Rayleigh surface mode, of the pseudosurface mode, and of the bulk waves propagating at different angles from the normal of the single crystal film plane and consistent with existing theoretical calculations of beta-SiC elastic constants. Finally, the calculated Voigt average values of the effective elastic constants for the (111) textured 3C-SiC polycrystalline film using the single crystal constants provides a good agreement with our experimental results (C-11=500 GPa, C-33=535 GPa, C-44=165 GPa, C-66=210 GPa, and C-13=50 GPa) and compare fairly well with the alpha-SiC published one. These results confirm that the elastic constants of silicon carbide are slightly influenced by the polytypism. (C) 2004 American Institute of Physics.
Abstract: Direct elastic properties measurements of beta-SiC films have been made using the interferometric strain gage displacement (ISDG) technique and compared with data acquired by the Brillouin light scattering (BLS) technique. BLS permits to selectively determine the three independent elastic constants (c(11) = 395 GPa; (c(11) - c(12))/2 = 136 GPa and c(44) = 236 GPa) of a beta-SiC single crystal epitaxial film from the analysis of a number of different surface acoustic modes. The calculated Voigt average values of the elastic constants for the <111> textured polycrystalline films (C-11 = 500 GPa, C-33 = 534 GPa, C-44 = 166 GPa, C-66 = 201 GPa, C-13 = 62 GPa) using the single crystal constants provides good agreement with experimental results on Young's modulus measured by the ISDG technique. Nevertheless, BLS gave more accurate values of Poisson's ratios. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Commercial purity Al has been processed by hot isostatic pressing consolidation of nanopowders. The resulting bulk material contained a small fraction of microcrystalline grains (>300 nm) embedded in a matrix of ultrafine grains (150 nm). The mechanical properties under compressive tests at various temperatures and at a strain rate of 2x10(-4) s(-1) have been investigated. At room temperature a brief hardening was observed followed by a stagnation of the flow stress up to failure. From 150 to 200 degreesC softening occurred after a short hardening stage. A change in the behavior showed up at 300 degreesC: a quasi perfect plasticity was observed subsequently to a yield point. The major deformation mechanisms include dynamic recovery, grain fragmentation and sliding. Besides, when tested up to rupture at elevated temperature filament formation where observed at the fracture surface. The formation mechanism of the filaments is possibly linked to the presence of a viscous-like phase at grain boundaries.
Abstract: Room and elevated temperature tensile, creep and high-cycle fatigue properties of electrodeposited LIGA Ni microsamples have been measured and are being used to predict the reliability of LIGA Ni MEMS structures. Tensile specimens with dimensions of hundreds of microns have been LIGA fabricated and characterized in terms of their underlying microstructure, elevated temperature tensile and creep, strength and their high-cycle fatigue performance. The stiffness of these LIGA Ni structures was found to be reduced by the introduction of porosity during the plating process. The strength of these structures was observed to decrease dramatically at temperatures above 200 degreesC. At stresses significantly below the yield strength, substantial creep deformation was observed at moderately elevated temperatures. The fatigue life of the LIGA Ni microsamples increased with decreasing stress amplitude in a manner comparable to what has been reported for wrought Ni. An apparent fatigue limit was observed for the LIGA Ni microsamples, but the importance of underlying microstructure and component geometry on the fatigue life was also highlighted. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: Silicon carbide is a very attractive material for a variety of applications. Originally considered for use in high power and high temperature electronics because of its large bandgap, designers of MEMS are now considering use of silicon carbide because of its stability at high temperatures, resistance to corrosives, high stiffness, and radiation resistance. However, as with any new structural material, its mechanical properties must be measured for design information. This research measures the elastic modulus, strength, and Poisson's ratio of two different silicon carbides using microtensile testing. One material is a 0.5-1 mum thick film from Case Western Reserve University. Preliminary results give an average of 420 GPa for elastic modulus, a strength of 1.2 GPa, and a Poisson's ratio of 0.19. The second material is from Massachusetts Institute of Technology with an average thickness of 30 microns. Preliminary results show an elastic modulus of 430 GPa, a strength of 0.49 GPa, and a Poisson's ratio of 0.24. In addition to the most recent results, techniques used to obtain these results, microstructure investigations, and a comparison of the materials are detailed.
Abstract: Role of the residual stresses on the mechanical properties of metal-matrix composites is studied. It is shown that the stress relaxation can be responsible for the morphologies and spatial distribution of precipitates. Direct measurements of the residual stress is also emphasized and the influence of dislocations in the accommodation process and during interface crossing is exemplified. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: TEM study of deformed samples complemented by a new approach combining image analysis and simulation of the dislocation motion have been carried out to study the precipitate/dislocation interaction in Al-Mg-Si alloys (AA-6XXX). The analysis of HRTEM images allows a direct measurement of the strain field around precipitates and is further introduced in the simulation of dislocation propagation. In the case studied here, the simulation indicates that for an applied stress close to the yield stress, dislocations' motion in the matrix occurs by both the by-pass of precipitates through the activation of the Orowan mechanism and the shearing of precipitates. This is in agreement with TEM observations on deformed samples showing numerous dislocations loops along with laths shearing.
Abstract: Double-faulted ribbons of stacking faults on the same (111) plane have been found in a L1(2) pseudobinary Ni3Ge-Fe3Ge compound. They stem from the interaction of coplanar (110) superdislocations giving rise to (112) dislocations, which subsequently dissociate into three identical :(112) Shockley partials. It is shown that, provided that a larger core relaxation of the 1/3 (112) partial dislocations is assumed, the threefold dissociated configuration has a lower energy than the antiphase-boundary dissociated configuration.
Abstract: Compressive tests at a constant strain rate conducted on a pseudo-binary L1(2) Ni55Fe20Ge25 intermetallic in a wide range of temperatures (from room temperature to 823 K) show the occurrence of a positive flow stress anomaly behaviour with a peak of flow stress occurring around 600 K. The induced dislocation substructures (morphology and core) were investigated by means of transmission electron microscopy (TEM) in weak beam conditions. In the domain of the increase of the flow stress, the dislocation substructure consists of screw dislocations locked in a Kear-Wilsdorf (KW) configurations as commonly observed in L1(2) alloys. With increasing temperature, gliding superdislocations are found to interact strongly with dislocations, in complete KW configurations. This interaction leads to a non-negligible quantity of dislocation dipoles. In the domain of the decrease of the flow stress. the most striking feature was the presence of a relatively high density of superlattice stacking faults. A close observation shows that the faulted defects exhibit in fact two linked faulted ribbons of unequal widths bounded by three superpartials having the same 1/3<112> Burgers vector and lying in the same plane. The observed mechanical behaviour is discussed in relation with the TEM investigations. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Strain localization was observed in mild steel submitted to sequential tests. The mechanisms of deformation within the bands of localization were studied at macroscopic. mesoscopic and microscopic scales, via dislocation substructures observations, local strain and rotation fields measurements within the grains. The conclusion is that microstructural effects govern the localization rather than textural effects. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Annealed (O temper) and heavily cold-rolled (H temper) sheets of commercial AA5182 aluminium alloy have been submitted to in-plane shear tests, monotonically and with strain reversals. Mechanical behaviour and dislocation patterning are analysed and compared for two directions of testing: in the rolling direction. and 60degrees from it. No influence of strain reversal is exhibited in 0 temper in both cases, whereas softening occurs in H temper after reverse shearing in the rolling direction. These results are discussed with respect to microstructural and textural anisotropy. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Use of digital processing of HRTEM images to determine stress fields around precipitates is exemplified in the case of aluminum alloys. Behavior of dislocations in these stress fields is simulated and compared to experimental observations. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Cyclic shear tests of various amplitudes have been carried out on aluminium-3004 alloys. The macroscopic behaviour shows the occurrence of either cyclic softening or cyclic hardening, depending on the initial state of the alloy (recrystallised, recovered or extra-hardened). This macroscopic behaviour is discussed with the help of both isotropic and kinematic concepts and in connection with the texture, the development of dislocation substructures and its evolution upon straining. (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: Cyclic shear tests with various constant strain amplitudes have been carried out to characterize the microstructural development under cyclic straining of aluminium-3004 and aluminium-5182 alloys. Substructural instabilities that occur inside the microstructure, their morphologies and crystallographies are reported. The hypothetical influence of these inhomogeneities on the observed macroscopic behaviour is discussed. (C) 1997 Elsevier Science S.A.
Abstract: The cluster variation method has been used to calculate antiphase boundaries (APBs) on {112} planes in a stoichiometric AB compound with the B2 ordered structure. Atoms interact through a nearest-neighbour pair ordering term and the basic irregular tetrahedron is chosen as the maximum cluster for the entropy evaluation. The free energy of the APB shows a monotonic decrease with increasing temperature and vanishes at T(c) with zero slope as expected since the B2<->disorder transition is second order. The excess internal energy exhibits a maximum at around 0.75T(c) before vanishing at T(c). The APB profile (extension of the local disorder) widens as the temperature increases, becoming flat and infinitely wide at T(c). An important amount of disorder already exists at 0.2T(c). Using Brown's analysis, the APB drag phenomenon is evaluated as a function of temperature for a dissociated superdislocation which would glide on {112}. A small twinning-antitwinning asymmetry is detailed. The stress takes a high value already at 0.2T(c) because of the disorder existing locally at the APB. Similarly to {110} planes, the stress exhibits an extended maximum around 0.6T(c). These calculations are compared with the case of beta-CuZn. Calculated and measured APB energies are consistent provided that the atomic position relaxation is taken into account. It is not thought that the disorder existing at the {112} APBs in thermal equilibrium, which results in a high APB drag stress at low temperatures, plays a part in the glide plane change from {112} to {110} occurring in beta-brass between 77 K and room temperature. Contrary to higher temperatures, the diffusion is too slow for this APB to rearrange during superdislocation movement.
Abstract: In the general framework of studies of the anomalous increase in plastic strength with temperature which is common to a number of ordered alloys, the effect of test temperature on the deformation microstructure has been investigated in beta-CuZn (B2 structure). Single crystals have been deformed in compression at different temperatures between room temperature and 200-degrees-C along axes near [001] and [111BAR]. The deformation microstructure has been observed under weak-beam conditions. The superdislocations with Burgers vector a<111> dissociated into two superpartials bounding an antiphase boundary (APB) have been found to dominate the microstructure over the whole temperature range although, in the near [111BAR] orientation of the applied stress, deformation involves glide in the a<001> direction at the peak temperature. The relative density of climb-dissociated dislocations with mixed character has been observed to increase with temperature. In addition, the energies of the APBs on {110} and {112} planes have been measured at room temperature.
Abstract: Antiphase boundaries (APBs) on {110} planes in a stoichiometric AB compound with the B2 ordered structure have been investigated using the cluster variation method. The basic irregular tetrahedron of the b.c.c. lattice is chosen as the maximum cluster for the entropy approximation, and the ordering energy of nearest-neighbour pairs is employed to evaluate the internal energy. The excess free energy due to the APB shows a monotonic decrease with increasing temperature and vanishes at T(c) as it should, considering that the B2<-->disorder transition is second order. The excess internal energy exhibits a maximum at around 0.7T(c) before vanishing at T(c). Up to about 0.3T(c), the APB structure keeps the sharp profile of the pure shear structure at 0K. Then it locally disorders and widens, the profile becoming flat and infinitely wide at T(c). At each temperature, there exists a number of equilibrium APB configurations with almost the same energies corresponding to the same APB but located at different positions along the <110> APB normal. The calculated APB structures are used to evaluate, as a function of temperature, the stress necessary to move a <111> superdislocation dissociated into two 1/2 <111> superpartials bounding an APB. On the leading partial the stress is almost proportional to the long-range order parameter of the homogeneous system, which decreases monotonically from OK to T(c). On the trailing partial, the stress is proportional to minus the degree of order existing locally at the boundary. This quantity shows a sharp increase between 0.3T(c) and 0.6T(c) so that the resulting stress on the superpartial due to the diffuse APB reaches a maximum at about 0.65T(c). Comparison with the critical resolved shear stress peak in beta-brass shows excellent agreement in terms of peak temperatures, the calculated stress at the peak being about three times the experimental value. This difference is discussed considering the competition between the dynamics of APB profile widening by diffusion and that of dislocation motion.
Abstract: This analysis of our present understanding of the mechanical properties of nanograined metallic polycrystals focuses at first on the necessity of improving the knowledge and control of the various features that constitute the microstructure of as-processed nanograined and ultrafine-grained metallic material, as well as their evolution in the course of plastic flow. In a second step it is shown, in particular, that the geometrical and kinematical constraints imposed by the small grain size challenge the meaning generally accepted for fundamental quantities such as yield stress, strain-hardening rate and elastic-plastic transition. Finally, a thorough discussion of the possible deformation mechanisms is given, with special emphasis on the importance of dislocation nucleation at grain boundaries.
