Origin mechanism of heterostructure nanograins with gradient grain size suppressing strain localization

Abstract

Strain localization is a common deformation induced instability in many nanograined metals. Gradient nanograined metals are known to suppress the strain localization by inner coarse grains (CGs) coordinating plastic deformation. However, in this study, we found that using micropillar compression, heterostructure nanograins (NGs) with gradient grain size could suppress the strain localization and thus provide extra strain hardening without the CGs coordinating the plastic deformation. The gradient distribution of the grain size in Ti–6Al–4V alloy with heterostructure NGs plays a significant role in suppressing the strain localization. First, the gradient grain size distribution in NGs causes shear band delocalization by activating the stress-driven grain boundary migration and dislocation slip, which largely suppresses shear band localization at the low strain level. Second, when the gradient grain size distribution disappears, gradient nanograins (GNGs) change to homogeneous ultrafine grains (UFGs) stored with a large number of dislocations at the high strain level. This can refine the UFGs to NGs by dislocation-induced dynamic recrystallization and coordinate the plastic deformation in the shear bands, thereby suppressing the shear band localization at the high strain level. Hence, heterostructure NGs with gradient grain size contribute to the extra strain hardening by suppressing the strain localization. This study not only provides new insights into how GNGs suppress the strain localization, but also offers a new strategy for designing heterostructure NGs using grain boundary engineering (gradient distribution of the grain size) to improve the ductility of nanograined metals.