A mechanism for the negative strain-rate sensitivity of dilute solid solutions

A new mechanism is proposed for dynamic strain ageing and the negative strain-rate sensitivity (SRS) exhibited by dilute solid solutions containing mobile solute atoms. The mechanism is based on the strength variation of dislocation junctions due to the presence of solute clusters on forest dislocations. The strength of a Lomer-Cottrell lock in which the mobile dislocation is free of solute, while the forest dislocation is clustered, is studied by using an orientation-dependent line tension model. It is shown that the junction strength increases with the size of the cluster on the forest dislocation (binding energy of the forest dislocation to its cluster). The cluster forms by lattice diffusion and its size depends on the time lapsed from the formation of the respective dislocation segment. Therefore, the average size of clusters on new forest dislocations is smaller the larger the imposed strain rate. Consequently, the average strength of junctions decreases (after a transient) upon an increase of the strain rate, which leads to negative SRS. A model including the results of the mesoscopic analysis is developed to capture this mechanism. The model reproduces qualitatively a number of key features observed experimentally at the macroscopic scale. The new mechanism does not require solute diffusion to take place sufficiently fast for clustering of mobile dislocations to happen during their arrest time at obstacles, as assumed in previous models of the phenomenon. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.