The flow stress of metals at low temperatures

Abstract

The collapse of narrow V-shaped notches formed in glide dislocations at points of close approach to dislocations threading the glide plane is shown to facilitate intersection without thermal activation. The force effecting intersection, which is a component of the line tension of the notches, subsequently assists the glide dislocation to overcome the drag of jogs formed in the process. A consequence of this dual function of the force is the Cottrell-Stokes law, i.e. the strain invariance, at any given temperature and strain rate, of the ratio τT/τG, where τT and τG are the temperature dependent and independent parts of the flow stress respectively. Linear and parabolic work-hardening laws are deduced from the model. On equating the slip distance to grain size in the parabolic relation the flow stress in found to be linear in (grain sk)^−1/2, which is known to apply for example to zinc. The activation energy of the jog migration associated with τT, estimated from low temperature flow stress data, is equal to about ½ ev in copper.