Anand Krishna Kanjarla

The ductility of Magnesium alloys is often limited due to the strong basal texture. Initial attempts were made to reduce the intensity of the basal textures by adding rare earth(RE) elements. However, owing to the cost and scarcity of RE elements, alloys with calcium and tin were recently introduced. Among them, Mg-2Sn-2Ca alloys are the most promising, with high strength reported in the extruded condition. In this work, we report, for the first time, the mechanical properties of the alloy in the sheet form. In plane mechanical anisotropy in compressive behaviour is studied along with the detailed characterisation of texture, microstructure and the deformation twins in the material. Unlike strongly basal textured Mg alloys, deformation twinning was also observed in samples loaded along the normal direction. Crystal plasticity simulations were performed to confirm the slip and twin activity. Furthermore, a detailed analysis of deformation twins revealed that the so called Schmid twins accommodate strain by both twin growth and slip, whereas the non-Schmid twins accommodate the strain exclusively by crystallographic slip.

An investigation was conducted to understand the role of microstructure on the asymmetry of mechanical properties in quasi-static tension and compression in a two-phase titanium alloy Ti-5Al-3Mo-1.5V. Different microstructures were obtained (lamellar colony, basketweave and martensitic)on adopting different cooling rates after solution treatment in single phase β region. The microstructure with martensite exhibited higher strain to failure and lower yield strength than basketweave. Detailed TEM microstructural analysis helped reveal the existence of compression twins and strain-induced phase transformation in compression and tension tests respectively, both of which are relatively rare to be observed in two-phase titanium alloys.

As the need for more fuel-efficient vehicles having lesser carbon emissions is growing, the applicability of light weight metal such as magnesium having low density and high ratio of strength to weight have gained momentum to replace aluminum and steel in aerospace and automobile industry [1]. However, poor formability and low ductility of magnesium alloys due to inherent hexagonal close packed (HCP) structure and low hardness, strength and wear resistance posses' practical problems in its applicability at room temperature. Since, limited slip systems are available at room temperature; the deformation of magnesium alloy is complicated as compared to FCC metal alloys and highly texture dependent. There is considerable difference in the critical resolved shear stress of basal slip system and other slip systems (prismatic and pyramidal) at room temperature and hence imparts high anisotropy in the mechanical properties of Magnesium. Basal slip system {0 0 0 2} 〈1 1 2 0〉 can be easily activated in magnesium alloys but it alone can't accommodate for the general plastic deformation and other non basal slip systems are needed to improve room temperature formability [2]. Microstructure and texture modification can be done for activating non-basal slip systems and hence suitable thermomechanical processing needs to be formulated to accomplish this goal. Copyright © 2013 MS&T'13®.

In this paper, an infinitesimal-strain based FFT formulation is extended to account for deformation twinning in hexagonal close-packed (HCP) materials. A model called the Complex Voxel (CV) model is developed that includes twinning as pseudo-slip and accounts for interaction between parent grains and corresponding twin variants by assuming a Taylor-type approximation at the voxel level. The macroscopic deformation behavior of Magnesium alloy AZ31, a representative HCP material, is simulated in three different loading directions. Detailed analysis of twinning dominated deformation reveals that: (a) Global Schmid factors indicate twin variant selection and the rate of twin growth; (b) Basal slip is the dominant active slip system during the twin nucleation and twin growth stages; (c) A comparison of numerical results of the present study with the published experimental and numerical studies indicate that the evolution of local stress states in parent grains and corresponding twin variants depend on their orientation with respect to the loading direction and/or the neighbouring grains. Further, the need of immediate experimental evidence to assist assumptions made during modelling deformation twinning is discussed.

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.

The structure and morphology of the reinforcing material play an important role in the vibration damping characteristics of polymer composites. In this work, multiwalled carbon nanotubes (MWCNTs) with different structures and morphologies are incorporated into a polymer matrix. The vibration damping characteristics of the nanocomposites, in Oberst beam configuration, are studied using a free vibration test in cantilever mode. Inner tube oscillation is established as the vibration damping mechanism by correlating the extent of the loss factor obtained from the two nanocomposites with the dissimilarities in the structure and morphology of the two varieties of MWCNTs. Inner tube oscillation is simulated using molecular dynamics (MD). Since the open-ended double walled CNT (DWCNT) models used in earlier studies over predict the damping, we propose a capped DWCNT model. This can simulate the atomic interactions at the end caps of the tube. This study indicates that the contributions to the observed damping have their origins in the interaction between atoms that constitute the inner and outer tubes rather than the inter-tube frictional energy loss.

The study of microstructure and its relation to properties and performance is the defining concept in the field of materials science and engineering. Despite the paramount importance of microstructure to the field, a rigorous systematic framework for the quantitative comparison of microstructures from different material classes has yet to be adopted. In this paper, the authors develop and present a novel microstructure quantification framework that facilitates the visualization of complex microstructure relationships, both within a material class and across multiple material classes. This framework, based on the stochastic process representation of microstructure, serves as a natural environment for developing relational statistical analyses, for establishing quantitative microstructure descriptors. In addition, it will be shown that this new framework can be used to link microstructure visualizations with properties to develop reduced-order microstructure-property linkages and performance models.

We present a microstructure and work hardening sensitive reduced-order model using random forest regression for predicting damage initiation in dual-phase (DP) steels. The ductile damage behavior of banded and non-banded DP steels is evaluated for various degrees of ferrite hardening. The banded microstructures show a superior damage resistance which is further improved by ferrite hardening. A general framework to rank the severity of damage initiation in various classes using a statistical fitting procedure is introduced. The regression-based and statistical fitting-based models can successfully quantify the damage initiation and group the various classes into three major clusters. The proposed framework is a step toward developing more effective and invertible reduced-order structure–property correlations

Twinning is an important deformation mechanism in hexagonal close-packed (hcp) metals such as Mg, Zr, Ti, and Be. Twinning in hcp materials is a multiscale process that depends on microstructural and mechanical response details at the mesoscale, microscale, and atomic scales. Twinning can generally be understood as a two-step process, a nucleation step followed by propagation. The nucleation of twins is governed by both material details such as the defect configurations at potential nucleation sites within grain boundaries, as well as the highly local mechanical field near grain boundaries. These two factors, the material and mechanical, must align for a successful nucleation event. In this article, we present a stochastic constitutive law for nucleation of twins and describe its implementation into a homogenized crystal plasticity simulation. Twin nucleation relies on the dissociation of grain boundary defects under stress into the required twinning partials. This dissociation is considered to follow a Poisson process where the parameters of the Poisson distribution are related to the properties of the grain boundaries. The rate of the process is a direct function of the local stress concentration at the grain boundary. These stress concentrations are randomly sampled from a distribution calibrated to full-field crystal plasticity simulations. © 2013 TMS (outside the USA).

Gas turbine components are often shot-peened to improve surface integrity and fatigue capability. Compressive Residual Stress (CRS) and increased surface hardness are the key outcomes of shot peening. But the CRS is observed to relax during the service life, and its stability depends on the amount of cold work during the shot peening process and the applied load during service life. This study develops a numerical framework capable of accurately estimating the amount of cold work using a dislocation density-based crystal plasticity model. The proposed framework captures the evolution of microstructure during the shot peening process, incorporating the effects of grain size, orientation, and strain rate. The simulation methodology is used to investigate the influence of loading conditions on work hardening and relaxation. The numerical predictions are shown to agree with available experimental observations. The proposed method demonstrates the capability to optimize microstructure and shot peening parameters to reduce the relaxation of CRS during service life.