Probing the ultimate limits of metal plasticity

TL;DR

This paper uses atomistic simulations to identify the maximum strain rate in tantalum beyond which dislocation motion ceases and twinning dominates, revealing a new plastic flow regime where tantalum behaves like a viscous fluid.

Contribution

It introduces the first fully dynamic atomistic simulations of tantalum plasticity, predicting the ultimate strain rate limit and the transition to twinning as the dominant deformation mechanism.

Findings

Dislocations cannot relieve load above a certain strain rate.

Twinning becomes dominant beyond the ultimate strain rate.

Tantalum exhibits a viscous fluid-like flow in the new plastic state.

Abstract

Along with high strength, plasticity is what makes metals so widely usable in our material world. Both strength and plasticity properties of a metal are defined by the motion of dislocations - line defects in the crystal lattice that divide areas of atomic planes displaced relative to each other by an interatomic distance. Here we present first fully dynamic atomistic simulations of single crystal plasticity in metal tantalum predicting that above certain maximum rate of straining - the ultimate limit - the dislocations can no longer relieve mechanical loads and another mechanism, twinning, comes into play and takes over as the dominant mode of dynamic response. At straining rates below the ultimate limit, the metal attains a path-independent stationary state of plastic flow in which both flow stress and dislocation density remain constant indefinitely for as long as the straining conditions remain unchanged. In this distinct state tantalum flows like a viscous fluid while still remaining a strong and stiff metal.