A revised criterion for the Portevin – Le Chatelier effect based on the Strain Rate Sensitivity of the Work Hardening Rate

13 Oct 2011

P. van Liempt; Tata Steel RD&T IJmuiden, The Netherlands
J. Sietsma; Dept. Materials Science and Engineering, Delft, The Netherlands

An improved analysis is presented of the stability of plastic deformation under conditions where dynamic strain ageing (DSA) occurs, which leads to instabilities known as the Portevin-Le Châtelier (PLC) effect. It is shown that PLC instabilities can occur for conditions that are not covered by the currently prevailing criterion [Y. Estrin and L. P. Kubin., Mat. Sci. Eng., A 137 (1991) 125-134], which focuses on a negative strain rate sensitivity of the flow stress, caused by interactions of solutes with thermally activated glide of mobile dislocations. The present analysis recognizes that the strain rate sensitivity of the flow stress consists of two contributions, one associated with glide of mobile dislocations, the second with work hardening, related to storage of immobile dislocations. In the present paper, an instability criterion is proposed that takes into account the possibility of a negative strain rate sensitivity of the work hardening rate, which is caused by diffusion of solutes to immobile dislocations. The latter contribution leads to an extended instability criterion. This criterion also provides an explanation for the existence of a critical strain above which instabilities occur. In this paper previously published tensile test data are used to show that a negative strain-rate sensitivity of the work hardening rate, which significantly influences the occurrence of the PLC-effect, can indeed occur under DSA conditions.

Paper published in Metallurgical and Materials Transactions A, 13 October 2011, Volume 6 / 1975 - Volume 42 / 2011