Analysis on the critical strain associated with the onset of portevin-Lecatelier effect of substitutional F.C.C. alloys

Flow instability of substitutional f.c.c. alloys, which is a common feature of the Portevin-LeChatelier effect, usually develops after a critical strain is reached. With ∈. and T as the tensile strain rate and deformation temperature, respectively, the Arrhenius equation that ∈. ∝ ∈_c^α exp (−Q/kT) is obeyed in numerous systems. Rationalization of the critical strain exponent, α, based on the conventional Cottrell model is not in agreement with empirical data. A proper modification regards ∈_c as related to some critical mobile dislocation density in which the local vacancy enrichment near dislocation core region is considered. Dislocation interception should also play a role in affecting the average dislocation velocity. The rationalization based on the above concepts reveals a correct grain size dependence.