Dislocation kinetics at high strain rates

Abstract

Dislocation theory is applied to the superposition of thermal-activation kinetics and phonondrag control. The effect of the various parameters is analyzed in detail; in particular, reasonable dependencies of the mobile dislocation density on stress and structure are accounted for. It is concluded that the transition is generally quite sharp and commences at stresses not much less than 0.9 of the mechanical threshold (the flow stress in the absence of thermal activation, i.e. at zero temperature). At very high strain rates, the glide kinetics should be entirely controlled by drag under the applied stress, without any significant contribution from the obstacle strength. This theory should describe the actual transition between the two glide resistance mechanisms as a function of stress or strain-rate at constant structure. It is demonstrated that the observed transition to a linear relation between the strain-rate and the flow stress at constant strain, even if this were a comparison at constant structure, could not be due to viscous drag control; at most it could be an empirical description of the transition regime near its inflection point.