Evolution of texture and microstructure during hot torsion of a magnesium alloy

A systematic study of the evolution of the microstructure and crystallographic texture during free end torsion of a single phase magnesium alloy Mg-3Al-0.3Mn (AM30) was carried out. The torsion tests were done at a temperature of 250 degrees C to different strain levels in order to examine the progressive evolution of the microstructure and texture. A detailed microstructural analysis was performed using the electron back-scattered diffraction technique. The observed microstructural features indicated the occurrence of continuous dynamic recovery and recrystallization, starting with the formation of subgrains and ending with recrystallized grains with high angle boundaries. Texture and microstructure evolution were analysed by decoupling the effects of imposed shear and of dynamic recrystallization. Microstructure was partitioned to separate the deformed grains from the recovered/recrystallized grains. The texture of the deformed part could be reproduced by viscoplastic self-consistent polycrystal simulations. Recovered/recrystallized grains were formed as a result of rotation of these grains so as to reach a low plastic energy state. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Evolution of microstructure and texture in Ni49.4Ti38.6Hf12 shape memory alloy during hot rolling

This paper deals with the evolution of microstructure and texture during hot rolling of hafnium containing NiTi based shape memory alloy Ni49.4Ti38.6Hf12. The formation of the R-phase has been associated with the precipitation of (Ti,Hf)(2)Ni phase. The crystallographic texture of the parent phase B2 as well as the product phases R and B19' have been determined. It has been found that the variant selection during the B2 -> R phase transformation is quite strong compared to the case of the B2 -> B19' transformation. During deformation, the texture of the austenite phase evolves with strong Goss and Bs components. After transformation to martensitic structure, it gives rise to a 011]parallel to RD fiber. Microstructure and texture studies reveal the occurrence of partial dynamic recrystallization during hot rolling. Large strain heterogeneities that occur surrounding (Ti,Hf)(2)Ni precipitates are relieved through extended dynamic recovery instead of particle stimulated nucleation.

Role of convection in microstructure evolution during solidification

In this study, we present a new computational approach for studying the effect of melt convection on solidification at the micro-scale level. Models for dendritic and eutectic growth are developed on the basis of the enthalpy technique and incorporate the presence of flow in the domain. Simulation results show the growth and motion of dendrites and evolution of eutectic lamellae and their interaction with melt flow. The present study provides the foundation for development of an efficient generalized micro-scale solidification model, which can potentially be coupled with system-scale models based on the same framework.

Effect of equal channel angular pressing on grain refinement and texture evolution in a biomedical alloy Ti-13Nb-13Zr

Evolution of texture and concomitant grain refinement during Equal Channel Angular Pressing (ECAP) of Ti - 13Nb - 13Zr alloy has been presented. Sub-micron sized equiaxed grains with narrow grain size distribution could be achieved after eight pass at 873 K. A characteristic ECAP texture evolved in alpha phase till four passes while the evolution of characteristic ECAP texture in the beta phase could be observed only beyond the fourth pass. On increasing the deformation up to eight passes, the texture in alpha phase weakens while the beta phase shows an ideal ECAP texture. A weaker texture, low dislocation density and high crystallite size values in alpha phase suggest the occurrence of dynamic recrystallization. The absence of texture evolution in beta phase till four passes can be attributed to local lattice rotations. The characteristic ECAP texture in the eight pass deformed sample is attributed to delayed dynamic recrystallization in the beta phase. (C) 2013 Elsevier Inc. All rights reserved.

Evolution and stability of phases in a high temperature shape memory alloy Ni49.4Ti38.6Hf12

Ni49.4Ti38.6Hf12 shape memory alloy has been characterized for structure, microstructure and transformation temperatures. The microstructure of the as-cast sample consists of B19' and R-phases, and (Ti,Hf)(2)Ni precipitate phase along the grain boundaries in the form of dendrites. The microstructure of the solution treated sample contains only B19' martensite phase, whereas a second heat treatment after solutionizing results in reappearance of the R-phase and the (Ti,Hf)(2)Ni grain boundary precipitate phase in the microstructure. A detailed microstructural examination shows the presence of precipitates having both coherent and incoherent interface with the matrix, the type of interface being dictated by the crystallographic orientation of the matrix phase. The present study shows that the (Ti,Hf)(2)Ni precipitates having coherent interface with the matrix, drive the formation of the R-phase in the microstructure. (C) 2013 Elsevier Ltd. All rights reserved.

Chromium isotope variations (delta Cr-53/52) in mantle-derived sources and their weathering products: Implications for environmental studies and the evolution of delta Cr-53/52 in the Earth's mantle over geologic time

Here we report chromium isotope compositions, expressed as delta Cr-53/ 52 in per mil (&) relative to NIST 979, measured in selected Cr-rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes. The main objectives of this study were: (i) to further constrain the isotope composition of the Earth's mantle Cr inventory and its possible variation during geological history, based on the analysis of globally distributed and stratigraphically constrained mantle-derived chromites; and (ii) to investigate the magnitude and systematics of Cr isotope fractionation during oxidative weathering and secondary alteration (i. e., hydration, serpentinization) of the magmatic Cr sources. Specifically, we analyzed delta Cr-53/ 52 in a set of globally distributed mantle-derived chromites (FeMgCr2O4, n = 30) collected from various locations in Europe, Asia, Africa and South America, and our results confirm that a chromite-hosted Earth's mantle Cr inventory is uniform at - 0.079 +/- 0.129& (2SD), which we named here as a ` canonical' mantle d 53/ 52 Cr signature. Furthermore our dataset of stratigraphically constrained chromites, whose crystallization ages cover most of the Earth's geological history, indicate that the bulk Cr isotope composition of the chromite-hosted mantle inventory has remained uniform, within about +/- 0.100&, since at least the Early Archean times (similar to 3500 million years ago, Ma). To investigate the systematics of Cr isotope fractionation associated with alteration processes we analyzed a number of secondary Cr-rich minerals and variably altered ultramafic rocks (i. e., serpentinized harzburgites, lherzolites) that revealed large positive delta Cr-53/ 52 anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization. The degree of aqueous alteration and serpentinization was quantified by the abundances of fluid-mobile (Rb, K) elements, and by the Loss On Ignition (LOI) parameter, which determines the amount of structurally bound water (OH/ H2O) present in secondary hydrated minerals like serpentine. Overall, we observed that altered ultramafic rocks that yielded the highest LOI values, and the lowest amounts of fluid mobile elements, also yielded the heaviest delta Cr-53/ 52 signatures. Therefore, we conclude that secondary alteration (i.e., hydration, serpentinization) of ultramafic rocks in near-surface oxidative environments tend to shift the bulk Cr isotope composition of the weathered products to isotopically heavier values, pointing to a dynamic redox cycling of Cr in the Earth's crustal and near-surface environments. Hence, if validated by future

Evolution of ferromagnetism from a frustrated state in LixNi(2-x)O2 (0.67 < x < 0.98)

Two-dimensional triangular-lattice antiferromagnetic systems continue to be an interesting area in condensed matter physics and LiNiO2 is one such among them. Here we present a detailed experimental magnetic study of the quasi-stoichiometric LixNi2-xO2 system (0.67

Environments of extended radio sources in the Australia Telescope Low-Brightness Survey

We present a study of the environments of extended radio sources in the Australia Telescope Low-Brightness Survey (ATLBS). The radio sources were selected from the ATLBS Extended Source Sample, which is a well defined sample containing the most extended of radio sources in the ATLBS sky survey regions. The environments were analysed using 4-m Cerro-Tololo Inter-American Observatory Blanco telescope observations carried out for ATLBS fields in the Sloan Digital Sky Survey r(') band. We have estimated the properties of the environments using smoothed density maps derived from galaxy catalogues constructed using these optical imaging data. The angular distribution of galaxy density relative to the axes of the radio sources has been quantified by defining anisotropy parameters that are estimated using a new method presented here. Examining the anisotropy parameters for a subsample of extended double radio sources that includes all sources with pronounced asymmetry in lobe extents, we find good evidence for environmental anisotropy being the dominant cause for lobe asymmetry in that higher galaxy density occurs almost always on the side of the shorter lobe, and this validates the usefulness of the method proposed and adopted here. The environmental anisotropy parameters have been used to examine and compare the environments of Fanaroff-Riley Class I (FRI) and Fanaroff-Riley Class II (FRII) radio sources in two redshift regimes (z < 0.5 and z > 0.5). Wide-angle tail sources and head-tail sources lie in the most overdense environments. The head-tail source environments (for the HT sources in our sample) display dipolar anisotropy in that higher galaxy density appears to lie in the direction of the tails. Excluding the head-tail and wide-angle tail sources, subsamples of FRI and FRII sources from the ATLBS appear to lie in similar moderately overdense environments, with no evidence for redshift evolution in the regimes studied herein.

Transverse single spin asymmetry in e+p↑→e+J/ψ+X and transverse momentum dependent evolution of the Sivers function

We extend our analysis of transverse single spin asymmetry in electroproduction of J/ψ to include the effect of the scale evolution of the transverse momentum dependent (TMD) parton distribution functions and gluon Sivers function. We estimate single spin asymmetry for JLab, HERMES, COMPASS, and eRHIC energies using the color evaporation model of charmonium production, using an analytically obtained approximate solution of TMD evolution equations discussed in the literature. We find that there is a reduction in the asymmetry compared with our predictions for the earlier case considered by us, wherein the Q2 dependence came only from DGLAP evolution of the unpolarized gluon densities and a different parametrization of the TMD Sivers function was used.

Spatial variations of effective elastic thickness over the Ninetyeast Ridge and implications for its structure and tectonic evolution

We present new data on the strength of oceanic lithosphere along the Ninetyeast Ridge (NER) from two independent methods: spectral analysis (Bouguer coherence) using the fan wavelet transform technique, and spatial analysis (flexure inversion) with the convolution method. The two methods provide effective elastic thickness (T-e) patterns that broadly complement each other, and correlate well with known surface structures and regional-scale features. Furthermore, our study presents a new high resolution database on the Moho configuration, which obeys flexural isostasy, and exhibit regional correlations with the T-e variations. A continuous ridge structure with a much lower T-e value than that of normal oceanic lithosphere provides strong support for the hotspot theory. The derived T-e values vary over the northern (higher T-e similar to 10-20 km), central (anomalously low T-e similar to 0-5 km), and southern (low T-e similar to 5 km) segments of the NER. The lack of correlation of the T-e value with the progressive aging of the lithosphere implies differences in thermo-mechanical setting of the crust and underlying mantle in different parts of the NER, again indicating diversity in their evolution. The anomalously low T-e and deeper Moho (similar to 22 km) estimates of the central NER (between 0.5 degrees N and 17 degrees S) are attributed to the interaction of a hotspot with the Wharton spreading ridge that caused significant thermal rejuvenation and hence weakening of the lithosphere. The higher mechanical strength values in the northern NER (north of 0.5 degrees N) may support the idea of off-ridge emplacement and a relatively large plate motion at the time of volcanism. The low T-e and deeper Moho (similar to 22 km) estimates in the southern part (south of 17 degrees S) suggest that the lithosphere was weak and therefore younger at the time of volcanism, and this supports the idea that the southern NER was emplaced on the edge of the Indian plate. (C) 2013 Elsevier B.V. All rights reserved.

Crystallographic texture and microstructure evolution during hot compression of Ti-6Al-4V-0.1B alloy in the (alpha plus beta)-regime

Microstructure and texture are known to undergo drastic modifications due to trace hypoeutectic boron addition (similar to 0.1wt.%) for various titanium alloys e.g. Ti-6Al-4V. The deformation behaviour of such an alloy Ti-6Al-4V-0.1B is investigated in the (+) phase field and compared against that of the base alloy Ti-6Al-4V studied under selfsame conditions. The deformation microstructures for the two alloys display bending and kinking of lamellae in near and softening via globularization of lamella in near phase regimes, respectively. The transition temperature at which pure slip based deformation changes to softening is lower for the boron added alloy. The presence of TiB particles is largely held attributable for the early softening of Ti-6Al-4V-0.1B alloy. The compression texture of both the alloys carry signature of pure phase defamation at lower temperature and phase transformation near the transus temperature. Texture is influenced by a complex interplay of the deformation and transformation processes in the intermediate temperature range. The contribution from phase transformation is prominent for Ti-6Al-4V-0.1B alloy at comparatively lower temperature.

Co-evolution of specific amino acid in sigma 1.2 region and nucleotide base in the discriminator to act as sensors of small molecule effectors of transcription initiation in mycobacteria

The transcription from rrn and a number of other promoters is regulated by initiating ribonucleotides (iNTPs) and guanosine tetra/penta phosphate (p)ppGpp], either by strengthening or by weakening of the RNA polymerase (RNAP)-promoter interactions during initiation. Studies in Escherichia coli revealed the importance of a sequence termed discriminator, located between -10 and the transcription start site of the responsive promoters in this mode of regulation. Instability of the open complex at these promoters is attributed to the lack of stabilizing interactions between the suboptimal discriminator and the 1.2 region of sigma 70 (Sig70) in RNAP holoenzyme. We demonstrate a different pattern of interaction between the promoters and sigma A (SigA) of Mycobacterium tuberculosis to execute similar regulation. Instead of cytosine and methionine, thymine at three nucleotides downstream to -10 element and leucine 232 in SigA are found to be essential for iNTPs and pppGpp mediated response at the rrn and gyr promoters of the organism. The specificity of the interaction is substantiated by mutational replacements, either in the discriminator or in SigA, which abolish the nucleotide mediated regulation in vitro or in vivo. Specific yet distinct bases and the amino acids appear to have co-evolved' to retain the discriminator-sigma 1.2 region regulatory switch operated by iNTPs/pppGpp during the transcription initiation in different bacteria.

Differential evolution based 3-D guidance law for a realistic interceptor model

This paper presents a novel, soft computing based solution to a complex optimal control or dynamic optimization problem that requires the solution to be available in real-time. The complexities in this problem of optimal guidance of interceptors launched with high initial heading errors include the more involved physics of a three dimensional missile-target engagement, and those posed by the assumption of a realistic dynamic model such as time-varying missile speed, thrust, drag and mass, besides gravity, and upper bound on the lateral acceleration. The classic, pure proportional navigation law is augmented with a polynomial function of the heading error, and the values of the coefficients of the polynomial are determined using differential evolution (DE). The performance of the proposed DE enhanced guidance law is compared against the existing conventional laws in the literature, on the criteria of time and energy optimality, peak lateral acceleration demanded, terminal speed and robustness to unanticipated target maneuvers, to illustrate the superiority of the proposed law. (C) 2013 Elsevier B. V. All rights reserved.

Structural Evolution and Phase Stability of Hume-Rothery Phase in a Mechanically Driven Nanostructured Ag-15 at. pct Sn Alloy

The paper reports phase evolution in mechanically driven Ag-15 at. pct Sn alloy powder starting with elemental powders in order to establish the feasibility of designing nanocomposites of a Ag-Sn solid solution. This alloy lies in the phase field of the hexagonal zeta-phase which is a well-known Hume-Rothery electron compound with an electron-to-atom ratio of about 1.45 and hexagonal crystal structure (a = 0.2966 nm, c = 0.4782 nm). Through a systematic use of X-ray diffraction and transmission electron microscopy, the results establish the formation of the zeta-phase which co-exists with the Ag solid solution during the initial phase of milling. Mechanical milling for long duration (55 hours) destabilizes the zeta-phase. A complete solid solution of Ag with a grain size of similar to 8 nm could be achieved after 60 hours of milling. Additional milling can induce decomposition of the solid solution that results in a reappearance of zeta-phase. We present a detailed thermodynamic calculation which indicates that complete Ag solid solution of the present alloy composition would be possible if the crystallites size can be reduced below a certain critical size. In particular, we show that both Ag and zeta-phase grain sizes need to be taken into account for determining the metastable equilibrium and the phase change that has been experimentally observed. Finally, we argue that recrystallization processes set a limit to the achievable size of the nanoparticles with metastable Ag solid solution.