Anand Krishna Kanjarla

In this paper, we report for the first time, to our knowledge, spatially resolved measurements of strain gradients across a grain containing twins, located in the bulk of a polycrystalline Mg AZ31 sample. We also report orientation mapping on three parallel sections from the bulk of the sample. We use for such purpose the technique of differential-aperture X-ray microscopy (DAXM) based on synchrotron X-rays. The DAXM technique allows us to map crystallographic strains with sub-micron-sized spatial resolution. The results of this experiment confirm indirect evidence from previous experiments having less spatial resolution that important stress gradients exist in the vicinity of twin boundaries. Such a result is relevant to understanding twin growth and de-twinning, since both mechanisms are affected by stress-driven twin dislocations at the interface. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Inclination angle (IA), which is that between the c-axis of a twin and the global loading direction, satisfactorily captures trends observed in area fractions of both Schmid and non-Schmid twins. The detailed analyses also show that the non-Schmid twins form preferentially in smaller grains compared to that of Schmid ones.

Hot deformation studies of a newly designed γ + α2 based TiAl alloy of composition Ti–42Al–6Nb–3Cr–0.1B at.% (nominal) realized through ingot metallurgy route using double vacuum arc remelting were carried out. Hot isothermal compression testing was performed in Gleeble™ 3500 at different temperatures ranging from 1123 to 1373 K at 50 K intervals and strain rates of 0.001–1 s−1. Processing maps were developed using an approach of dynamic material modeling of the flow curves to establish the safe hot working regime. Strain rate sensitivity and Zener–Holloman parameters were calculated and constitutive equation was derived. Microstructural investigation revealed dynamic recrystallization and activation of multiple twin systems as the main softening mechanisms operating at optimum hot working conditions. Safe hot working temperature and strain rate regime for the alloy was found to be in the temperature range of 1323–1373 K and strain rate range of 0.001–0.01 s−1.

A β-solidified nearly lamellar γ-TiAl (α2+γ) based alloy was investigated to understand the evolution of microstructure during hot isothermal compression test at 1050 °C (below eutectoid). Cylindrical samples were compressed to different true strains up to 60%. The deformed samples were then characterized using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) to understand the active deformation mechanisms. Dynamic flow softening behavior, i.e., dynamic recrystallization (DRX) type flow was observed. While no new strain induced phases were observed, there were minor changes in the phase fractions as a function of strain. Kinking and bending of lamellae at certain areas in the microstructure of compressed samples were predominant. Preliminary analysis by TEM indicated the presence of twinning in both γ and β phases.

The effect of deformation temperature (77, 298 and 573 K) on the evolution of slip and twinning modes in Zircaloy-4 was studied. Samples of Zircaloy-4 were uniaxially compressed to different strains at a nominal strain rate of 10−3 s−1. The compression direction was chosen so as to activate twinning. Microstructural examination was carried out using electron backscatter diffraction to relate the evolution of microstructure and texture to the micro-mechanics and work-hardening behaviour. In conjunction with experiments a visco-plastic self-consistent crystal plasticity model was used to determine the evolution of deformation modes. Extensive twinning was observed at all the test temperatures. Twinning activity at 298 and 573 K was comparable, but was lower than that at 77 K. A strong basal texture parallel to the compressive direction was found at 298 and 573 K. The VPSC model predicted well the texture evolution at 298 and 573 K. The basal texture developed at 77 K was found to have partially reoriented back to the initial texture due to secondary twinning. Prism slip was the dominant deformation mode with secondary slip becoming more active at higher temperatures.

Twinning in hexagonal close-packed (hcp) metals is a multi-scale process that depends on the microstructural and mechanical response details at the polycrystalline aggregate, grain, micro, and atomic scales. Twinning can generally be regarded as a two-step process, a nucleation event followed by propagation and growth. This articles presents a stochastic model for the nucleation of deformation twins in hcp polycrystals. Twin nucleation is modeled through its dependence on lower length scale material details, such as the defect configurations at potential nucleation sites within grain boundaries, and mechanical details such as highly localized stress concentrations at the microscale in a probabilistic manner. These two aspects, the material and mechanical, must align for a successful nucleation event. The nucleation process is cast as a survival model parameterized by the local stress at the grain boundary. The model gives an explicit form for the probability distribution for the critical stress values required for twin nucleation. The model is implemented into a viscoplastic self-consistent (VPSC) crystal plasticity framework in order to test its predictive capability against previously reported statistical characterization in deformed zirconium at multiple temperatures. For implementation in VPSC, the stress concentrations are sampled from a distribution calibrated to full-field crystal plasticity simulations and a three-dimensional model of grain neighbors and distribution of grain boundary areas are implemented. © 2013 Elsevier Ltd. All rights reserved.