Microplasticity in b.c.c. metals

Abstract

Stress relaxation was studied in polycrystalline Nb, Mo and W of 99.95% purity at 77 – 298 K. The relaxation following pre-straining to stresses not exceeding 70 – 80% of the macro-yield level was logarithmic, with the slope s = dσ/d ln ( t + t o) proportional to the tensile stress σ. At higher stresses the relation between s and σ was again linear, but d s/d σ assumed a new, constant, value. These observations, certains anomalies in the relaxation behaviour apparent particularly with high stresses and pre-staining rates, and the observed temperature dependence of s, are consistent with an interpretation in which mainly edge dislocations, impeded in their movement by structural defects and by impurities, control the relaxation in the low-stress region; at higher stresses dynamic recovery involving migration of jogged screw dislocations appears to be rate determining.