Maximum yield strength under quasi-static and high-rate plastic deformation of metals

The dependence of the yield strength of metals on the grain size and initial dislocation density in a wide range of strain rates has been analyzed within a unified approach. It has been shown that the barrier stress and characteristic time of plastic relaxation completely determine the shear strength of metals for all strain rates. The existence of alternative (to dislocation glide) mechanisms of plastic deformation in the material, limits the increase in the yield strength with increasing strain rate and leads to the appearance of a maximum in the dependence of the yield strength on the grain size. It has been found that, at extremely high strain rates, the maximum yield strength corresponds to grain sizes of the order of several hundred nanometers. This has been explained by the dislocation starvation effect of the material.