Work softening yield drops

Abstract

Nabarro's expression for the activation energy required for a glide dislocation to cut through a localised obstacle, such as a forest dislocation, is used to estimate the low-temperature relaxation processes which occur in the densely dislocated walls of a work hardened material. Because such activations involve some time delay the mechanical state of the wall material responds slowly to an externally imposed stress transient. As a result, a rapidly applied stress overshoots, leading to a high upper yield point. And when the ensuing general plastic deformation brings this rise to a halt, the same relaxation processes in the walls then lead to a yield drop down to a lower yield stress.