Plastic deformation-Microstructure correlation

An important aspect of plastic deformation of polycrystalline materials is the development of preferred orientation of crystals, which is referred to as 'crystallographic texture‘. Since most of the mechanical and physical properties of crystals are anisotropic, the properties of a polycrystalline aggregate will depend on how the individual grains are oriented. It is, therefore, fundamental to understand how preferred orientations are formed when a polycrystalline material is subjected to plastic deformation. In my thesis work, I was focusing on how microstructural parameters such as stacking fault energy (SFE) and grain size would effect the deformation mechanisms and, eventually, the texture development in Nickel-Cobalt alloys.

Some of the interesting observations are:

1. Medium SFE materials, such as Ni-40Co alloy, undergo sharp transition from a predominant {112}<111> texture to a texture comprising largely {110}<112> grains, at very large strain. This dramatic grain reorientation is stimulated by microscopic copper-type shear banding selectively within {112}<111> grains. Though shear banding caused by strain hardening is common in medium SFE materials, its definitive role in texture transition, like the one observed in Ni-40Co alloy, is not reported before. Read More.

(Figure: Shear banding within a {112}<111> grain after 95% thickness reduction in Ni-40Co alloy, captured by SEM-EBSD technique. Magnified image shows the local reorientation within the shear band towards {111}<112>)

2. The Wassermann's theory of brass-type texture in low SFE materials through twinning, and the texture transition from copper-type to brass-type in the intermediate deformation stage is conditional. Wassermann's proposed chain of events depends on many factors, one such factor is the initial texture. For instance, in low SFE Ni-60Co alloy, by weakening the starting texture, the texture transition event is entirely skipped. The texture is completely brass-type throughout the deformation process. However, the final texture is rather close to Goss than towards ideal brass. The texture intensity is due to the synergistic effects of deformation twinning and shear banding. From this work, we proposed that Wassermann's postulation is a necessary condition but not a sufficient condition. Read More.

(Figure: Propensity of twinning within a single {112}<111> grain at small thickness reductions in Ni-60Co alloy, captured by employing 'pseudo-in-situ method' in SEM-EBSD. Parent grain and twinned volumes are partitioned out to highlight the change in volume fraction with reduction levels.)

3. In nanocrystalline regime, the textures are qualitatively similar for the respective alloy composition, though the deformation mechanisms are quite different with respect to their microcrystalline counterparts. Due to small grain sizes, dislocation activity is suppressed, which subsequently retards the texture evolution. Stress-induced grain growth revives dislocation activity at large strains, which triggers texture development. In the absence of twinning and shear banding, the brass-type texture in nanocrystalline Ni-60Co alloy is proposed to be caused by partial dislocation activity. This claim is validated by crystal plasticity simulations. Read More. (Figure: Schematic showing the proposed mechanism for the activity of partial dislocations within a grain in the nanocrystalline low SFE alloy. Two partial dislocations having the same Burgers vectors but opposite signs are emitted from a grain boundary with a time lag. The stacking fault left behind by the leading partial (b) is cleared by the trailing partial (-b) of opposite sign. Both the partials are absorbed in the opposite grain boundary.)