Microstructure and damping behaviour of consolidated magnesium chips

Chips produced by turning a commercial grade pure magnesium billet were consolidated by solid state recycling technique of cold compaction followed by hot extrusion. The cold compacted billets were extruded at four different temperatures: 250 degrees C, 300 degrees C, 350 degrees C and 400 degrees C. For the purpose of comparison, cast magnesium (pure) billets were extruded under similar conditions. Extruded products were characterized for damping properties. Damping capacity and dynamic modulus was measured as a function of time and temperature at a fixed frequency of 5 Hz 10 to 14% increase in damping capacity was observed in chip consolidated products compared to reference material. Microstructural changes after the temperature sweep tests were examined. Chip boundaries present in consolidated products were observed to suppress grain coarsening which otherwise was significant in reference material. The present work is significant from the viewpoint of recycling of machined chips and development of sustainable manufacturing processes. (C) 2012 Elsevier B.V. All rights reserved.

Accumulative roll bonding of aluminum alloys 2219/5086 laminates: Microstructural evolution and tensile properties

The present study describes the course of microstructure evolution during accumulative roll bonding (ARB) of dissimilar aluminum alloys AA2219 and AA5086. The two alloys were sandwiched as alternate layers and rolled at 300 degrees C up to 8 passes with 50% height reduction per pass. A strong bonding between successive layers accompanied by substantial grain refinement (similar to 200-300 nm) is achieved after 8 passes of ARB. The processing schedule has successfully maintained the iso-strain condition up to 6 cycles between the two alloys. Afterwards, the fracture and fragmentation of AA5086 layers dominate the microstructure evolution. Mechanical properties of the 8 pass ARB processed material were evaluated in comparison to the two starting alloy sheets via room temperature tensile tests along the rolling direction. The strength of the 8 pass ARB processed material lies between that of the two starting alloys while the ductility decreases after ARB than that of the two constituent starting alloys. These differences in mechanical behavior have been attributed to the microstructural aspects of the individual layer and the fragmentation process. (C) 2011 Elsevier Ltd. All rights reserved.

Microstructure and compression behavior of chip consolidated magnesium

Chips produced by turning a commercial purity magnesium billet were cold compacted and then hot extruded at four different temperatures: 250, 300, 350, and 400 degrees C. Cast billets, of identical composition, were also extruded as reference material. Chip boundaries, visible even after 49: 1 extrusion at 400 degrees C, were observed to suppress grain coarsening. Although 250 degrees C extruded chip-consolidated product showed early onset of yielding and lower ductility, fully dense material (extruded at 400 degrees C) had nearly 40% reduction in grain size with 22% higher yield strength and comparable ductility as that of the reference. The study highlights the role of densification and grain refinement on the compression behavior of chip consolidated specimens.

Microstructure and texture evolution during accumulative roll bonding of aluminium alloys AA2219/AA5086 composite laminates

Accumulative roll bonding of two aluminium alloys, AA2219 and AA5086 was carried out up to 8 passes. During the course of ARB, the deformation inhomogeneity between the two alloy layers results in interfacial instability after the 4th pass, necking of the AA5086 layers after the 6th pass and fracture along the necked regions after the 7th and 8th pass. The EBSD analysis shows deformation bands along the interfaces after 8 passes of ARB. The ARB-processed materials predominantly show characteristic deformation texture components. The weak texture after the 2nd pass results from the combination of a weakly-textured starting AA2219 layer and a strongly-textured starting AA5086 layer. A strong deformation texture forms due to the high imposed strain after a higher number of ARB passes. Subgrain formation and related shear banding induces copper/S components in the case of the small elongated grains, while planar slip leads to the formation of brass component in the large elongated grains.

Asymmetric and symmetric rolling of magnesium: Evolution of microstructure, texture and mechanical properties

In the present study, asymmetric rolling was carried out for incorporating a shear component during the rolling at different temperatures, and was compared with conventional (symmetric) rolling. The microstructures were investigated using electron back-scatter diffraction (EBSD). The strain incorporated was compared with the help of grain orientation spread (GOS). GOS was eventually used as a criterion to partition the microstructure for separating the deformed and the dynamically recrystallized (DRX) grains. The texture of the partitioned DRX grains was shifted by similar to 30 degrees along the c-axis from the deformed grains. The mechanism of dynamic recrystallization (DRX) has been identified as continuous dynamic recovery and recrystallization (CDRR). The partitioned deformed grains for the higher temperature rolled specimens exhibited a texture similar to the room temperature rolled specimen. The asymmetric rolling introduces a shear component which shifts the texture fibre by similar to 5-10 degrees from the conventional rolling texture. This led to an increase in ductility with little compromise on strength. (c) 2012 Elsevier B.V. All rights reserved.

Effect of selenium addition on the GeTe phase change memory alloys

Compositional dependent investigations of the bulk GeTe chalcogenides alloys added with different selenium concentrations are carried out by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron probe micro-analyzer (EPMA) and differential scanning calorimetry (DSC). The measurements reveal that GeTe crystals are predominant in alloys up to 0.20 at.% of Se content indicating interstitial occupancy of Se in the Ge vacancies. Raman modes in the GeTe alloys changes to GeSe modes with the addition of Se. Amorphousness in the alloy increases with increase of Se and 0.50 at.% Se alloy forms a homogeneous amorphous phase with a mixture of Ge-Se and Te-Se bonds. Structural changes are explained with the help of bond theory of solids. Crystallization temperature is found to be increasing with increase of Se, which will enable the amorphous stability. For the optimum 0.50 at.% Se alloy, the melting temperature has reduced which will reduce the RESET current requirement for the phase change memory applications. (C) 2012 Elsevier B.V. All rights reserved.

Optical Properties Study of S40Se60-xSbx Bulk Alloys

Bulk samples of S40Se60-xSbx (with x=10, 20, 30, 40 at. %) were prepared from high purity S, Se and Sb by melt quenching method. XRD studies revealed that all the samples have poly-crystalline phase. The variation in optical properties with composition has been investigated by XPS and Raman spectroscopy. The intensity of XPS core level spectra changes with addition of Sb clearly interprets the optical properties change due to compositional variation. The Raman shift and new peak formation in these samples clearly shows the structural modifications due to Sb addition.

Phase relationships and magnetic properties of TbxDy1-xFe1.95 alloys

The effect of Tb/Dy ratio on the structural and magnetic properties of (Tb,Dy)Fe-2 class of alloys has been investigated using nine alloys of TbxDy1-xFe1.95 (x = 0-1) covering the entire range. Our results indicate that the three phases viz. (Tb,Dy)Fe-2 (major phase), (Tb,Dy)Fe-3 and(Tb,Dy)-solid solution (minor phases) coexist in all the alloys. The volume fraction of pro-peritectic (Tb,Dy)Fe-3 phase however, has a minimum at x = 0.4 and a maximum at x = 0.6 compositions. The volume fraction of this phase decreases upon heat treatment at 850 degrees C and 1000 degrees C. A Widmanstatten type precipitate of (Tb,Dy)Fe-3 was observed for Dy-rich compositions (0 <= x <= 0.5). The microstructural investigations indicate that the ternary phase equilibria of Tb-Dy-Fe are sensitive to Tb/Dy ratio including the expansion of (Tb,Dy)Fe-2 phase field which is in contrast to the pseudo-binary assumption that is followed in available literature to date. The lattice parameter, Curie temperature and coercivity are found to increase with Tb addition. Split of (440) peak of (Tb,Dy)Fe-2 observed in x >= 0.3 alloys indicate, a spin reorientation transition from 100] to 111] occurs with Tb addition. (C) 2012 Elsevier B. V. All rights reserved.

Microstructural Evolution and Some Unusual Effects During Thermo-Mechanical Cycling of Sn-Ag-Cu Alloys

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time, and thereby influences the long-term reliability of microelectronic packages. Accurate prediction of this aging behavior is therefore critical for joint reliability predictions. Here, we study the precipitate coarsening behavior in two Sn-Ag-Cu (SAC) alloys, namely Sn-3.0Ag-0.5Cu and Sn-1.0Cu-0.5Cu, under different thermo-mechanical excursions, including isothermal aging at 150 degrees C for various lengths of time and thermo-mechanical cycling between -25 degrees C and 125 degrees C, with an imposed shear strain of similar to 19.6% per cycle, for different number of cycles. During isothermal aging and the thermo-mechanical cycling up to 200 cycles, Ag3Sn precipitates undergo rapid, monotonous coarsening. However, high number of thermo-mechanical cycling, usually between 200 and 600 cycles, causes dissolution and re-precipitation of precipitates, resulting in a fine and even distribution. Also, recrystallization of Sn-grains near precipitate clusters was observed during severe isothermal aging. Such responses are quite unusual for SAC solder alloys. In the regime of usual precipitate coarsening in these SAC alloys, an explicit parameter, which captures the thermo-mechanical history dependence of Ag3Sn particle size, was defined. Brief mechanistic description for the recrystallization of Sn grains during isothermal aging and reprecipitation of the Ag3Sn due to high number of thermo-mechanical cycles are also presented.

Microstructure and tensile properties of consolidated magnesium chips

In this study tensile properties of consolidated magnesium chips obtained from solid state re-cycling (SSR) has been examined and correlated with the microstructure. Chips machined from as-cast billet of pure magnesium were consolidated through SSR technique, comprising of compaction at ambient conditions followed by hot extrusion at four different temperatures viz., 250, 300, 350 and 400 degrees C. The extruded rods were characterized for microstructure and their room temperature tensile properties. Both ultimate tensile strength and 0.2% proof stress of these consolidated materials are higher by 15-35% compared to reference material (as cast and extruded). Further these materials obey Hall-Petch relation with respect to strength dependence of grain size. Strain hardening behavior, measured in terms of hardening exponent, hardening capacity and hardening rate was found to be distinctly different in chip consolidated material compared to reference material. Strength asymmetry, measured as a ratio of compressive proof stress to tensile proof stress was higher in chip consolidated material. (C) 2012 Elsevier B.V. All rights reserved.

Isothermal transformation of beta-phase in Cu-rich Cu-Al-Sn alloys

Microstructural changes resulting from isothermal decomposition of the beta-phase have been studied in Cu-rich binary Cu-Al and ternary Cu-Al-Sn alloys containing up to 3 at.% Sn at temperatures from 873 to 673 K. Results are presented as TTT diagrams. The decomposition occurs in several stages, each of which involves the establishment of metastable equilibrium between beta and one or more of the product phases alpha, beta(1) and gamma(2). Addition of Sn has been shown to increase the stability of the ordered beta(1)-phase in relation to beta. In alloys containing more than 2 at.% Sn, the beta(1) emerges as a stable phase. At low Sn concentrations beta(1) is metastable. An important new finding is the existence of three-phase equilibrium microstructure containing alpha, beta(1) and gamma(2). Increasing addition of Sn alters the morphology of beta(1) from rosettes to dendrites and finally to Widmanstatten needles.

Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys

Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation.

Effect of temperature and strain rate on the deformation behavior and microstructure of a homogenized AZ31 magnesium alloy

The plastic deformation behavior and dynamic recrystallization (DRX) in homogenized AZ31 Mg alloy was investigated in uniaxial compression in the temperature range between 150 and 400 degrees C with strain rates ranging from 10(-3) to 10(2) s(-1). Twinning was found to contribute significantly during the early stages of deformation. The onset of twinning was examined in detail by recourse to the examination of the appearance of first local maxima before peak strain in the stress-strain responses and the second derivative of stress with strain. High strain hardening rate was observed immediately after the onset of twinning and was found to increase with the Zener-Hollomon parameter. DRX was observed at temperatures above 250 degrees C whereas deformation at lower temperatures (< 250 degrees C) leads to extensive twinning at all the strain rates. At intermediate temperatures of 250-300 degrees C, plastic strains tend to localize near grain/twin boundaries, confining DRX only to these regions. Increase in the temperature promotes non-basal slip, which, in turn, leads to uniform deformation; DRX too becomes uniform. Deformation behavior in three different regimes of temperature is discussed. The dependence of critical stress for the onset of DRX and peak flow stress on temperature and strain rate are also described. (C) 2013 Elsevier B.V. All rights reserved.

Ultranarrow and Widely Tunable Mn2+-Induced Photoluminescence from Single Mn-Doped Nanocrystals of ZnS-CdS Alloys

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width <= 150 meV in the orange-red region and a surprisingly large spectral width (>= 180 meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (similar to 370 meV) covering the deep green-deep red region and (ii) exhibit widths substantially lower (similar to 60-75 meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

Temperature dependence of indentation recovery ratios in austenitic and martensitic shape memory alloys

Instrumented microindentation (IM) on two Ni-Ti shape memory alloys (SMAs), where one is austenitic and the other is martensitic at room temperature, were conducted from 40 to 150 degrees C. Results show that the depth and work recovery ratios, eta(d) and eta(w) respectively, are complementary to each other. While eta(d) decreases gradually with temperature for austenite, it drops markedly for the martensite in the martensite-to-austenite transformation regime. These results affirm the utility of IM for characterizing SMAs.