The examination of the athermal flow stress component of vanadium and copper polycrystals by means of the decremental unloading technique

Abstract

On vanadium and copper polycrystals the applicability of the decremental unloading technique for the measurement of the athermal flow stress component is examined especially in the range of low temperatures. It is shown that generally one has to distinguish between the measured critical stress τ c and the true internal stress τ i . Both stresses are equal only under special conditions. For copper τ c can be identified at high as well as at low testing temperatures. In constrast with copper τ c for vanadium is identical with τ i only at higher temperatures. The temperature dependence of τ c for b.c.c. metals can be explained by a model of Guiu. In this case τ c can be divided into two components. One depends only on temperature (and is equal to zero for f.c.c. metals) whereas the second, which corresponds to τ i , depends only on the strain. The strain dependence of τ c is the same for both materials. Further examination shows that work hardening is essentially caused by the athermal stress component.