Yield-stress and stress relaxation measurements in cobalt

Abstract

Yield stress of polycrystalline cobalt (5N) was studied in the temperature range 15–300 K and interpreted by a theoretical model which provided an agreement with experimental results. Logarithmic stress relaxation was also studied in polycrystalline cobalt at natural strains of up to 8% at 15, 25, 40, and 55 K. Activation energies, i.e., 1.27±0.35 eV, deduced from a single barrier model of logarithmic stress relaxation, i.e., u0=k(T0+AT2)×[m+2.3 (dσ/ds)c] where T0 and A are constants), which allows for ‘‘quantum’’ effects below T≤70 K, favors vacancy formation and dynamic recovery as the rate determining process. The occurrence of structural effects such as ‘‘strain enhancement’’ (semiempirical fact) at low temperatures, suggests that a single barrier model is not sufficient to explain the ‘‘anomalies’’ appertaining to the temperature dependence of the yield stress and of the accompanying modes of plastic deformation. However, both quantum and structural effects seem to play an important role in the determination of low temperature deformation peculiarities.