The rate dependence of the portevin-Le chatelier effect

Abstract

Van den Beukel's approach to the Portevin-Le Chatelier effect which connects its occurrence with dynamic strain aging and the associated negative microscopic strain rate sensitivity of the flow stress is re-examined. Thermally activated dislocation motion affected by segregating solutes is modelled taking into account that between two stress drops or strain bursts, the effective stress can vary with time. A general criterion for the occurrence of the Portevin-Le Chatelier effect, which incorporates the influence of the applied stress rate or strain rate, is obtained by examining the conditions for the vanishing of the microscopic strain rate sensitivity. Several consequences are investigated and assessed numerically, with particular reference to Al-Mg alloys: the existence of two limiting strain rates between which the macroscopic, apparent strain rate sensitivity is negative and discontinuous deformation is observed; a decrease in the amplitude of the effect with increasing applied rates; and the rate dependence of the critical stress for the onset of instability. It is concluded that with the modifications proposed, the model establishes a realistic connection between dynamic strain aging and the rate-dependent macroscopic manifestations of the Portevin-Le Chatelier effect.