Abstract: A single-phase stable icosahedral quasicrystalline sample of high quality with the composition Al62.5Cu25Fe12.5 was produced by the spray forming technique. The material was further investigated by mechanical milling under an argon atmosphere to avoid oxidation during milling. At the initial stages of milling (within 5âh) a significant broadening of the diffraction peaks was observed, indicating a reduction of crystallite size and the introduction of lattice strain, which can be linked to phason strain of the quasilattice. Line broadening was noticed to increase with increasing milling time and in the material milled for longer time only a few broad diffraction peaks, which can be identified as a nanoscale bcc phase (i.e. disordered B2 phase, aââ 2.9âÃ), were visible. At this stage the diffraction signals belonging to the quasicrystals were no longer observable, indicating a complete transformation of the quasicrystals into the bcc phase. Finally, the bcc phase formed during milling transformed back to the quasicrystalline phase during subsequent annealing treatment. The microhardness measured on the milled powders was found to decrease with increasing milling time, most likely as a consequence of the increased volume fraction of the ductile bcc phase. Attempts are made to rationalize the structural transformation.
Abstract: In the present investigation, tribological characteristics of spray formed and hot pressed Alâ6.5Si, Alâ12.5Si and Alâ12.5Siâ25Sn alloys have been studied. The alloys were spray formed in an environmental chamber using nitrogen gas at 1.0 MPa gas pressure and a deposition distance of 350 mm. The preforms were vacuum hot pressed, at appropriate temperatures, to achieve its densification, The spray formed alloys showed considerable microstructural refinement with uniform distribution of Si particles in AlâSi alloys and that of Sn and Si particles in the matrix of primary α-Al phase of AlâSiâSn alloys. Dry sliding wear behaviour of these materials was studied as a function of sliding distance, applied load and sliding velocity. The wear response parameters such as wear rate, seizure pressure and frictional force were measured. A pressure-sliding velocity (PâV) limit diagram was established from the data obtained. The PâV diagram indicated that the allowable pressure range for wear seizure increased with increasing Si content. A considerable increase in the wear resistance of the alloy and the seizure pressure is observed with Sn addition. The improved wear properties of the alloy containing Sn are discussed in light of the microstructural features of spray formed alloy and the nature of the debris particles.
Abstract: A 12 mm thick Al85Y8Ni5Co2 plate was spray deposited on a 30 mm thick copper substrate pre-heated to 383 K. The deposit microstructure consists of an amorphous phase, 50â150 nm fcc-Al grains, 0.2â0.7 μm Al2Y and Al3Y intermetallic phases and some unidentified phases. The hardness of different microstructural features and the compressive strength of the deposit, after extrusion at 723 K, were evaluated. The small size overspray particles as well as the deposit show a glass transition phenomenon. The total crystallization energy of the deposit is 121 J/g compared to 83.9 J/g for the small sized overspray powder, indicating its high metastability. A compressive strength of 925 MPa and a deformation strain of 9% were achieved for the deposit after extrusion. The novel microstructural features in the deposit are attributed to the âchilling effectâ on highly undercooled or partially solidified large-size droplets during deposition onto the pre-heated substrate, and the rapid heat extraction thereof due to a close contact at the deposit/substrate interface.
Abstract: Highly dense bulk samples were produced by spark plasma sintering (SPS) through
combined devitrification and consolidation of partially amorphous Al85Y8Ni5Co2 gas atomized
powders. The microstructure of the consolidated samples shows a mixed structure containing
crystalline, ultrafine-grained and amorphous/nanocrystalline particles. The sintered sample
exhibits a remarkable high strength of about 1050 MPa combined with 3.7 % fracture strain.
Abstract: In the present investigation, a multicomponent glass-forming Al83Y5La5Ni5Co2 (at. pct) alloy was spray deposited on a copper substrate to produce an 8-mm-thick plate. The substrate was 30-mm thick and heated to a temperature of 160 °C prior to spray deposition. The temperature of the substrate and the deposit was measured during and after deposition. The deposits as well as oversprayed powders were characterized in terms of the microstructural features by optical microscopy and scanning electron microscopy (SEM). The phase constitution and transformation were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The oversprayed powder revealed different microstructural characteristics showing crystalline, partially crystalline, and fully featureless particles. The spray deposit showed large fraction of featureless regions with embedded dendrites of 1- to 10-μm size intermetallic phases. These regions were observed to have a nanocrystalline structure with an average grain size of approximately 100 nm. The XRD analysis also revealed the nanocrystallinity in terms of a halo and peak broadening. These microstructural features have been attributed to the deposition of undercooled liquid on a highly conductive copper substrate and rapid heat extraction from the droplets due to proper metallic contact between the deposit and the substrate. These results have been discussed in light of processing conditions and the microstructural evolution of droplets in flight and during deposition.
Abstract: In the present investigation, different Al based alloys such as AlâSiâPb, AlâSi, AlâSiâFe and 2014Al + SiC composites have been produced by spray forming process. The microstructural features of monolithic alloys and composite materials have been examined and their wear characteristics have been evaluated at different loads and sliding velocities. The microstructural features invariably showed a significant refinement of the primary phases and also modification of secondary phases in Al-alloys. The Pb particles in AlâSiâPb alloy were observed to be uniformly distributed in the matrix phase besides decorating the grain boundaries. The spray formed composites showed uniform distribution of SiC particles in the matrix. It was observed that wear resistance of AlâSi alloy increases with increase in Pb content; however, there is not much improvement after addition of Pb more than 20%. The coefficient of friction reduced to 0.2 for the alloy containing 20%Pb. A sliding velocity of 1 msâ1 was observed to be optimum for high wear resistance of these materials. Alloying elements such as Fe and Cu in AlâSi alloy lead to improved wear resistance compared to that of the base alloy. The addition of SiC in 2014Al alloy gave rise to considerable improvement in wear resistance but primarily in the low pressure regime. The wear rate seemed to decrease with increase in sliding velocity. The wear response of the materials has been discussed in light of their microstructural features and topographical observation of worn surfaces.
Abstract: Role of liquid phase stability on the glass forming ability (GFA) has been reviewed and the alloy systemshave been analyzed by introducing a contribution factor (wg) to the characteristic temperature Tg, in the gamma parameter. The kinetics of glass formation for various alloy systems has been found to vary with liquid phase stability in metastable state. The GFA of a fragile liquid is found to be more responsive to the contribution of metastable stability compared to strong liquid.
Abstract: In the present investigation, discontinuous SiC particle reinforced 2014 Al alloy based metal matrix composites have been produced by spray forming process. The composites contained average particle sizes of 17, 30 and 58 μm in the range of 5â11 vol.%. The composites were tested for their compressive flow behaviour, in unlubricated condition, at strain rates of 0.01, 0.1 and 1.0 sâ1 and at temperatures of 150, 300 and 450 °C. The flow stress for 30 μm size particle reinforced composite increased with increasing particle content from 0 to 8.5 vol.%, but decreased at 11 vol.%. The flow stress invariably decreased at larger strain values during deformation. The increase in particle size from 17 to 30 μm led to increase in flow stress at 300 °C, whereas, it decreased at 450 °C. The strain rate sensitivity (m) for 30 μm size particle reinforced composite was close to 0.16 up to 8.5 vol.%, whereas, for the composite with 17 μm size particles it decreased to 0.13 with increasing volume fraction up to 8.6 vol.%. The m values increased from 0.13 to 0.15 with increase in particle size from 17 to 58 μm. The variation in flow behaviour has been attributed mainly to particle fracture and debonding at particle/matrix interface, confirmed by microstructural features of the deformed samples. The major particle fracture events were recorded at low temperature and low strain rate of deformation. The composite with 30 μm size particles showed enhanced restoration process based on the low value of calculated apparent activation energy for diffusion (80â100 kJ molâ1). This deformation behaviour of the composites has been discussed in light of microstructural observations and the void formation during deformation.
Abstract: The present study shows feasibility of synthesising steel-aluminium based metal intermetallic laminate (MIL) composites by an industrially viable and repetitive roll bonding process and subsequent annealing. Roll bonding process has been utilised to achieve solid state bonding of interstitial free (IF) steel and aluminium with simultaneous reduction in sheet thickness. Further annealing of roll bonded laminate sheets gives rise to IF steel-aluminides/aluminium multilayer composites. Formation of Al5Fe2 phase at the interface is confirmed by X-ray diffraction (XRD) analysis.
Abstract: In the present study, an icosahedral single phase bulk quasicrystalline material based on Al62Cu25.5Fe12.5 has been synthesised by a spray forming route. Microstructural characterization showed an average grain size of 10 μm. The oversprayed fine powder showed the presence of β- and λ-phases, whereas, the deposit consited of the fully single phase bulk quasicrystalline material with compositional homogeneity. The hardness and fracture toughness measurements were carried out at different indentation loads of 50â500 g. The hardness values varied in the range 10.4â8.1 GPa and fracture toughness was seen to decrease with increasing load. The varionan of hardness with load, which is known as indentation size effect (ISE), has been established clearly. Fracture toughness value was constant in the load range from 200 to 500 g at 1.2 MPa m1/2. The cracking pattern after indentation at higher load has been observed to be intergranular as well as transgranular. The evolution of the single phase bulk quasicrystalline material has been discussed in light of the unique combination of atomization and deposition process elements in spray forming technique.
Abstract: Aluminium alloy (AA2014) based metal matrix composites were produced by spray atomization and codeposition
process. Various sizes of SiCp (6 to 58 $m) were used as reinforcement with different volume
fractions. Microstructures of deposits as well as overspray powder particles were examined. Porosity and SiCp
content were measured at different positions in the sprayed billet. An increase in the incorporated SiCp volume
fraction leads to an increase in the porosity. The SiCp volume fraction showed its nearly uniform distribution
along the billet radius. The microstructure of composites showed a uniform distribution of particles. The grain
size decreases with increase in the volume fraction of reinforcement particulates and is sensitive to the
particulate size. Microstructural examination of overspray powder particles shows that the basic mechanism for
the incorporation of smaller particles is the collision between reinforcement particles and the liquid droplets
during flight. Numerical calculations, based on interfacial energy balance, showed that the large size particle
can be easily incorporated but smaller ones require a high relative velocity to penetrate into the droplet.
