Nilesh P. Gurao

Crystallographic texture in Materials

Many naturally occurring as well as man made materials comprise of large number of crystallites with a preferred orientation. The preferred orientation, popularly known as texture, governs various structural and mechanical properties of these materials. Texture may develop during variety of processes like solidification, plastic deformation, annealing and phase transformation. It is, therefore, possible to tailor texture in materials to enhance a particular property. Traditionally, X-ray and neutron diffraction had been used to study texture in materials. It has been very recently that other techniques based on synchrotron X-rays and SEM based Electron Backscattered Diffraction have been developed for complete characterization of texture in materials. In the present review, the authors discuss the pole figures and more comprehensive orientation distribution function methods for texture analysis. In addition, a brief account of texture evolution in various technologically important materials, ranging from common metals and alloys to intermetallics, ceramics and polymers along with some naturally occurring materials like rocks, ice, bones etc. has been given. The present review is particularly aimed at readers newly initiated in this field rather than the experts.

Deformation mechanisms during large strain deformation of nanocrystalline nickel

In this letter, a conclusive evidence of the operation of planar slip along with grain boundary mediated mechanisms has been reported during large strain deformation of nanocrystalline nickel. Dislocation annihilation mechanism such as mechanical recovery has been found to play an important role during the course of deformation. The evidences rely on x-ray based techniques, such as dislocation density determination and crystallographic texture measurement as well as microstructural observation by electron microscopy. The characteristic texture evolution in this case is an indication of normal slip mediated plasticity in nanocrystalline nickel.

Evolution of Crystallographic Texture and Microstructure During Cold Rolling of Twinning-Induced Plasticity (TWIP) Steel: Experiments and Simulations

A systematic investigation of the evolution of deformation microstructure and texture of twinning-induced plasticity (TWIP) steel during cold rolling has been carried out using electron backscatter diffraction and X-ray diffraction, as well as viscoplastic self-consistent simulations. It is found that extensive twinning leads to the formation of the strong Brass {110}〈112〉 and Goss {110}〈001〉 components in TWIP steel even at low strains. At higher reduction, heterogeneous deformation contributes to further strengthening of Brass (Bs) component. The origin and stability of Bs component as well as the impact of the evolution of texture and microstructure on mechanical anisotropy is further explored using viscoplastic self-consistent simulations.

Evolution of texture and grain boundary microstructure in two-phase (α + β) brass during recrystallization

The evolution of texture and microstructure during recrystallization is studied for two-phase copper alloy (Cu–40Zn) with a variation of the initial texture and microstructure (hot rolled and solution treated) as well as the mode of rolling (deformation path: uni-directional rolling and cross rolling). The results of bulk texture have been supported by micro-texture and microstructure studies carried out using electron back scatter diffraction (EBSD). The initial microstructural condition as well as the mode of rolling has been found to alter the recrystallization texture and microstructure. The uni-directionally rolled samples showed a strong Goss and BR {236}⟨385⟩ component while a weaker texture similar to that of rolling evolved for the cross-rolled samples in the α phase on recrystallization. The recrystallization texture of the β phase was similar to that of the rolling texture with discontinuous ⟨101⟩ α and {111} γ fiber with high intensity at {111}⟨101⟩. For a given microstructure, the cross-rolled samples showed a higher fraction of coincident site lattice Σ3 twin boundaries in the α phase. The higher fraction of Σ3 boundaries is explained on the basis of the higher propensity of growth accidents during annealing of the cross-rolled samples. The present investigation demonstrates that change in strain path, as introduced during cross-rolling, could be a viable tool for grain boundary engineering of low SFE fcc materials.

Generalized scaling of misorientation angle distributions at meso-scale in deformed materials.

Scaling behaviour has been observed at mesoscopic level irrespective of crystal structure, type of boundary and operative micro-mechanisms like slip and twinning. The presence of scaling at the meso-scale accompanied with that at the nano-scale clearly demonstrates the intrinsic spanning for different deformation processes and a true universal nature of scaling. The origin of a ½ power law in deformation of crystalline materials in terms of misorientation proportional to square root of strain is attributed to importance of interfaces in deformation processes. It is proposed that materials existing in three dimensional Euclidean spaces accommodate plastic deformation by one dimensional dislocations and their interaction with two dimensional interfaces at different length scales. This gives rise to a ½ power law scaling in materials. This intrinsic relationship can be incorporated in crystal plasticity models that aim to span different length and time scales to predict the deformation response of crystalline materials accurately.

Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel

The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N2) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.

Simulation of Deformation Texture Evolution During Multi Axial Forging of Interstitial Free Steel

Bulk texture measurement of multi-axial forged body center cubic interstitial free steel performed in this study using x-ray and neutron diffraction indicated the presence of a strong {101}〈111〉 single texture component. Viscoplastic self-consistent simulations could successfully predict the formation of this texture component by incorporating the complicated strain path followed during this process and assuming the activity of {101}〈111〉 slip system. In addition, a first-order estimate of mechanical properties in terms of highly anisotropic yield locus and Lankford parameter was also obtained from the simulations.

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.

Deformation behaviour of Titanium in torsion

The evolution of microstructure and texture in Hexagonal Close Pack commercially pure titanium has been studied in torsion in a strain rate regime of 0.001 to 1 s-1. Free end torsion tests carried out on titanium rods indicated higher stress levels at higher strain rate but negligible change in the strain-hardening behaviour. There was a decrease in the intra-granular misorientation while a negligible change in the amount of contraction and extension twins was observed with increase in strain rate. The deformed samples showed a C1 fibre (c-axis is first rotated 90° in shear direction and then +30° in shear plane direction) at all the strain rates. With the increase in strain rate, there was an increase in the intensity of the C1 fibre and it became more heterogeneous with a strong {11 6} component. In the absence of extensive twinning, pyramidal slip system is attributed for the observed deformation texture. The present investigation, therefore, substantiates the theoretical prediction of increase in strength of texture with strain rate in torsion.

Texture evolution during annealing of large-strain deformed nanocrystalline nickel

The recrystallization behaviour of cold-rolled nanocrystalline (nc) nickel has been studied at temperatures between 573 and 1273 K using bulk texture measurements and electron back-scattered diffraction. The texture in nc nickel is different from that of its microcrystalline counterpart, consisting of a strong Goss (G) and rotated Goss (RG) components at 773 K instead of the typical cube component. The texture evolution in nc Ni has been attributed to the prior deformation textures and nucleation advantage of G and RG grains.