On the different grain-size dependences of flow stress and spall strength of nanocrystalline Cu under shock loading

Abstract

Understanding the material response and material strength under dynamic loading is crucial for optimized design of advanced material serving in extreme conditions. Flow stress and spall strength are typical measured material strengths in shock loading. However, the correlation of the two strengths is not well understood. Here we use large-scale molecular dynamics simulations to demonstrate that flow stress and spall strength of nanocrystalline Cu have obviously different variation tendencies upon grain refinement at nanoscale. The flow stress reveals a transition from Hall-Petch (HP) to inverse Hall-Petch (IHP) behaviors as grain size decreases. The HP - IHP transition of flow stress is mainly attributed to the competition of grain boundaries strengthening effect by blocking and absorbing dislocations and the grain boundaries weakening effects including GB sliding and grain rotations. However, the grain size dependence of spall strength mainly shows an inverse Hall-Petch relationship, i.e., spall strength generally decreases as grain size decreases. This is mainly due to the role of grain boundaries as preferred void nucleation sites. For finer grain size, the larger volume fraction of grain boundaries and junctions facilitates damage nucleation and results in larger amount of voids and lower tensile strength.