Strain rate sensitivity of thermally activated dislocation motion across fields of obstacles of different kind

The thermally activated motion of dislocations across fields of randomly distributed obstacles of two types is studied. The two types have either the same strength and different dependence of the activation energy on the applied force, or different strength but same activation behavior. The objective is to determine how the two sub-populations of obstacles contribute to defining the strain rate sensitivity and the flow stress. Above a threshold stress, dislocation motion undergoes a transition from smooth ("unzipping") to jerky, i.e. obstacles are bypassed in a correlated manner at high stresses. In the jerky regime, the strain rate sensitivity parameter depends exclusively on the ratio of the applied stress to the mechanical threshold stress of the respective array, the dynamics exhibiting near-critical behavior. This regime appears to be essential for the deformation of real crystals. When obstacles are bypassed in the unzipping mode, the strain rate sensitivity is controlled by the strong obstacles. These results have implications for the finite temperature superposition of contributions of the two types of obstacles to the overall flow stress. (C) 2008 Elsevier B.V. All rights reserved.