IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

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®.

World University root node

Department of Metallurgical and Materials Engineering

Materials Science and Technology Conference and Exhibition 2013, MS and T 2013

Texture and microstructure developments during friction stir processing of magnesium alloy AZ31

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.

Acta Materialia

Spatially resolved in situ strain measurements from an interior twinned grain in bulk polycrystalline AZ31 alloy

In this work, the evolution of internal lattice strains in face-centered cubic stainless steel under uniaxial tension is studied using a recently developed full-field elasto-viscoplastic formulation based on fast Fourier transforms. The shape of the diffraction peaks is simulated, and the predicted lattice strains (peak shift and broadening) are compared with the experimental measurements obtained by in situ tensile neutron diffraction. Detailed analysis of the lattice strain distributions reveal that 1 0 0 and 1 1 0 transverse families exhibit a bimodal nature, and that transverse lattice strains are more sensitive to local grain interactions compared with longitudinal lattice strains. A comparison with the results of a mean-field formulation indicates that type III (intragranular) stresses play a much larger role than type II (intergranular) stresses in diffraction peak broadening. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Study of internal lattice strain distributions in stainless steel using a full-field elasto-viscoplastic formulation based on fast Fourier transforms

We present the infinitesimal-strain version of a formulation based on fast Fourier transforms (FFT) for the prediction of micromechanical fields in polycrystals deforming in the elasto-viscoplastic (EVP) regime. This EVP extension of the model originally proposed by Moulinec and Suquet to compute the local and effective mechanical behavior of a heterogeneous material directly from an image of its microstructure is based on an implicit time discretization and an augmented Lagrangian iterative procedure. The proposed model is first benchmarked, assessing the corresponding elastic and viscoplastic limits, the correct treatment of hardening, rate-sensitivity and boundary conditions, and the rate of convergence of the numerical method. In terms of applications, the EVP-FFT model is next used to examine how single crystal elastic and plastic directional properties determine the distribution of local fields at different stages of deformation. © 2011 Elsevier Ltd. All rights reserved.

International Journal of Plasticity

An elasto-viscoplastic formulation based on fast Fourier transforms for the prediction of micromechanical fields in polycrystalline materials

Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and growth of deformation twins and twin variant selection. Statistically based EBSD analyses of deformed microstructures in HCP metals indicate that the nucleation of deformation twins depends on, among other factors, the local stress fields arising from neighboring grain interactions at grain boundaries [1]. Inspired by these findings a probability model for twin nucleation was developed [2,3], based on the activation of defect sources statistically occurring in grain boundaries. This nucleation model was implemented in a Visco-Plastic Self-Consistent (VPSC) code. Because the latter is based on an Effective Medium assumption and the inclusion formalism, it only provides average stress values in the grains, and the nature of local stress fields at grain boundaries had to be considered in a heuristic manner. In order to have better insight on the effect of local textures on twin nucleation, in this work we employ a viscoplastic full field Fast Fourier Transform (FFT) method as a numerical tool for conducting virtual experiments to study the role of crystal orientation and local neighbor grain interactions on stress localization close to the interfaces and, consequently, on twin nucleation in hexagonal materials, such as Zr and Mg. © (2012) Trans Tech Publications, Switzerland.

Materials Science Forum

On the role of local grain interactions on twin nucleation and texture evolution in hexagonal materials

Fully recrystallized commercial-purity aluminum sheets were deformed by limiting dome height tests, the following strain modes: uniaxial tension (US), near plane strain tension (PS), and equibiaxial tension (BS) were identified using standard procedure. The deformation texture developments differed significantly depending on the strain mode. Although the full constraints Taylor (FCT) model captured the texture developments in US, it failed to reproduce deformation textures in PS and especially in BS. The Advanced LAMEL (ALAMEL) model and the crystal plasticity finite element method (CPFEM) were, however, successful with respect to all three strain modes. Microtexture data brought out interesting observations of orientation gradients. First, the orientation gradients increased from US to PS to BS. Second, such gradients were mostly around initial (or prior deformation) grain boundary regions. A simple algorithm, and an associated computer program, was developed to demarcate such near boundary gradient zones (NBGZs). The area fraction and severity of NBGZ seemed to affect the texture development; FCT was reasonably successful at low NBGZ, whereas high NBGZ required the ALAMEL and the CPFEM models that are capable of addressing strain heterogeneity and grain interactions. © 2010 The Minerals, Metals & Materials Society and ASM International.

Department of Mathematics

Fulltext

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Strain mode dependence of deformation texture developments: Microstructural origin

In plastically deforming polycrystals, grains interact with their surroundings resulting in heterogeneous deformation fields both at inter- and intragrain level. When the crystal plasticity finite element method (CPFEM) is employed to predict such heterogeneity, splitting of grains is occasionally observed. In the present study, detailed local strain analysis is performed in such subdivided grain and its neighbors. The role of local interactions at the grain boundaries and of the initial orientation of the grain is investigated. It is shown that a hard grain surrounded by relatively softer grains is prone to subdivide when subjected to opposing shears.

Symposium: Structural Transitions and Local Deformation Processes at and near Grain Boundaries

Finite Element Study of Intragrain Plastic Heterogeneity near a Triple Junction

Micromechanical models aimed at simulating deformation textures and resulting plastic anisotropy need to incorporate local plastic strain heterogeneities arising from grain interactions for better predictions. The ALAMEL model [Van Houtte, P., Li, S., Seefeldt, M., Delannay, L. 2005. Deformation texture prediction: from the Taylor model to the advanced Lamel model. Int. J. Plasticity 21, 589-624], is one of the models in which the heterogeneous nature of plastic deformation in metals is introduced by accounting for the influence of a grain boundary on the cooperative deformation of adjacent grains. This is achieved by assuming that neighbouring grains undergo heterogeneous shear rates parallel to the grain boundary. The present article focuses on understanding the plastic deformation fields near the grain boundaries and the influence of grain interaction on intra-grain deformations. Crystal Plasticity Finite Element Method (CPFEM) is employed on a periodic unit cell consisting of four grains discretised into a large number of elements. A refined study of the local variation of strain rates, both along and perpendicular to the grain boundaries permits an assessment of the assumptions made in the ALAMEL model. It is shown that the ALAMEL model imbibes the nature of plastic deformation at the grain boundaries very well. However, near triple junctions, the influence of a third grain induces severe oscillations of the stress tensor, reflecting a singularity. According to CPFEM, such singularity can lead to grain subdivision by the formation of new boundaries originating at the triple junction. © 2010 Elsevier Ltd. All rights reserved.

Assessment of plastic heterogeneity in grain interaction models using crystal plasticity finite element method

A CPFE model was used for an assessment of the assumptions used by the ALAMEL model concerning grain interactions. A finite element mesh was constructed for a multicrystal consisting of four grains. There were 17496 integration points per grain. The main goal was to capture the complex nature of the plastic fields in the vicinity of the grain boundaries. The distribution of strain rates, both along and perpendicular to the grain boundaries, confirms that the basic assumptions of the ALAMEL model are qualitatively correct, except at triple junctions. Splitting of one of the grains was occasionally observed, which has also been observed experimentally. © (2010) Trans Tech Publications.

Modelling study of deformation texture and plastic heterogeneity at grain boundaries and triple junctions

The development of texture was studied during electrophoretic deposition in alumina suspensions containing plate shaped alumina particles. The mechanism of platelet orientation during EPD was examined with respect to the influence of the electric field, gravity and hydrodynamic forces. This was realized by using two different deposition cells, with vertically or horizontally positioned deposition electrode. The texture of the green deposit was further enhanced during sintering by templated grain growth in which the platelet shaped alumina particles were growing at the expense of the fine grained matrix. The sharp 'fiber texture' obtained after templated grain growth during sintering of the deposit was characterized by means of x-ray diffraction and Electron Backscatter Diffraction (EBSD).

Key Engineering Materials

Textured α-alumina through electrophoretic deposition and templated grain growth

Plastic deformation in metals is heterogeneous in nature. This heterogeneity is induced by variety of factors at different length scales. The present study involves understanding the contribution of grain boundaries towards such a heterogeneous deformation field at mesoscopic scale. Finite element method coupled with crystal plasticity theory is used to deform a simple set-up of four grains. Each grain is discretized into large number of elements to effectively capture the complex nature of the plastic fields in the vicinity of the grain boundaries. Distribution of strain rates, both along and perpendicular to the grain boundaries is analyzed. The observations from above simulations are then used to enrich the microstructural description and grain interaction effects in a Taylor-type micromechanical model. A new model, aimed at predicting deformation textures in face centered cubic materials, is presented. It takes into account the near neighbor interaction as well as the influence of global texture on the deformation behavior of an individual grain. This is achieved by an iterative scheme for solving the field equations for a system consisting of a grain boundary segment (and the associated grain boundary zones) embedded in a homogenous media. Deformation textures predicted by the new model are presented along with a quantitative comparison with both experimental and simulated textures from other micromechanical models.

Ceramic Transactions

Plastic heterogeneity due to grain boundaries and its influence on global deformation textures

Crystal plasticity based finite element modeling (CPFEM) is applied to finely discretized, digital microstructures, statistically representative of textured polycrystalline aggregates. The first problem treated is the prediction of the planar anisotropy of a textured zinc sheet, where CPFEM, VPSC and a simplified "multisite" modeling of neighbor interactions are found to perform better than the conventional Taylor model. Then, CPFEM is used in order to assess the effect of grain shape and local lattice orientation on the onset of shear banding in plane strain compression of fcc and hcp polycrystals.

CPFEM investigation of the effect of grain shape on the planar anisotropy and the shear banding of textured metal sheets

A hierarchical multilevel method is presented for the plastic deformation of polycrystalline materials with texture-induced anisotropy. It is intended as a constitutive material model for finite element codes for the simulation of metal forming processes or for the prediction of forming limits. It consists of macroscopic models of which the parameters are to be identified using the results of two-level (meso/macro) or three-level (micro/meso/macro) models. A few such two-level models are presented, ranging from the full-constraints Taylor model to the crystal-plasticity finite element models, including the grain interaction models GIA, LAMEL and ALAMEL. Validation efforts based on experimental cold rolling textures obtained for steel and aluminium alloys are shortly discussed. An assessment is also given of the assumptions of the LAMEL and ALAMEL models concerning stress and strain rate heterogeneity at grain boundaries, based on the results of a crystal plasticity finite element study. Finally a recent three-level model which also looks at the microscopic level (dislocation substructure) is discussed. © 2006 Elsevier SAS. All rights reserved.