Grain size dependent shear instabilities in body-centered and face-centered cubic materials

The grain size dependence of the mechanical response has been a topic of immense interest in nanostructured (NS) materials. Most NS metals produced by grain refinement processes show characteristically different work hardening and rate sensitivity from their coarse-grained counterparts. These differences, coupled with novel deformation mechanisms in NS materials, have direct implications on the stability of the plastic deformation process. Recently, we proposed a model for grain rotation based geometric softening within a visco-plastic constitutive setting to investigate grain size dependent shear instabilities in NS metals. Using this model, we investigate the effects of internal material hardening on the shear instabilities in NS body-centered cubic and face-centered cubic metals at quasi-static loading rates. At small grain sizes, the model predicts occurrence of the shear bands for both b.c.c. and f.c.c. materials as a consequence of the dominance of geometric softening over material hardening. However, in NS face-centered cubic materials the occurrence of instability is delayed due to elevated rate sensitivity compared NS body-centered cubic materials.