High pressure-induced elimination of grain size softening in nanocrystalline metals: Grain boundary strengthening overwhelming reduction of intragranular dislocation storage ability

Abstract

Recent experimental studies indicate that the yield strength of nanocrystalline (NC) metals under high hydrostatic pressure exhibits monotonous strengthening as the grain size decreases. In other words, the softening with decreasing grain size known as the inverse Hall–Petch (HP) behavior is eliminated by high hydrostatic pressure. To reveal the underlying mechanism of inverse HP elimination, here we propose pressure-dependent crystal plasticity constitutive models for both grain boundaries (GBs) and grain interiors (GIs) considering size-dependent intragranular dislocation storage. It is found that the elimination of inverse HP behaviors is facilitated by the hydrostatic pressure-induced GB strengthening and is hindered by GI softening attributed to the reduction of intragranular dislocation storage ability. The tension–compression asymmetry of the yield strength of NC metals becomes remarkable with decreasing grain size and eventually saturates as the grain size decreases down to several nanometers. Moreover, high hydrostatic pressure-induced suppression of GB plastic deformation relieves the shear localization and restricts the coalescence of microcracks and microvoids in adjacent grains with possible crack extension prohibited. Consequently, the ductility and fracture strength of NC metals are enhanced. Our results provide novel and fundamental insights for understanding high-pressure strengthening of NC metals and pave the way for the rational design and fabrication of NC metals of ultra-strength and ultra-ductility via GB engineering.