Stress relaxation and dynamic recovery in cobalt at low temperatures

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Abstract
Stress relaxation in polycrystalline cobalt of 99•999% purity was studied at constant strains of up to 8% at 77—384°K. After the application of a small plastic strain increment the applied tensile stress [sgrave] diminished logarithmically with time; from the variation of the slopes of the isothermal relaxation curves (d[sgrave]/d log10 t) with temperature and pre-strain the rate controlling energy barrier was determined to be 1°61 ± 0•1 ev. This appears to be related to the formation of vacancies by intersection jogs dragged by basal dislocations. The relaxation contributed to dynamic recovery, manifest in the linear decrease of the coefficient of ‘parabolic’ work-hardening d[sgrave]2/dσ from 18•3 × 107 at 77°K to 5•8 × 107 Kg2/cm4 at 384°K.