Kinetics of flow stress in ultra-pure tantalum single crystals in stress/temperature regime III

Abstract

The temperature dependence of the critical resolved shear stress (CRSS), τ, of ultra-pure tantalum single crystals (RRR ≥ 14000) observed below 250 K for a range of shear-strain rates $\dot{\gamma } = 2\times 10^{ - 5} - 6\times 10^{ - 3} \,{\text{s}}^{ - 1}$ was analyzed within the framework of a kink-pair nucleation model of flow stress. The CRSS/strain-rate data follow the model formulation $\tau^{ 1/ 2} = C + D\,{ \ln }\dot{\gamma }$, where C and D are positive constants, for each deformation temperature T in the range 78–250 K. Evaluation of the various slip-parameters of flow stress points to (211)$[\bar{1}11]$ slip system responsible for the yielding of ultra-pure tantalum single crystals in the so-called stress/temperature regime III (T < 250 K). The value of the pre-exponential factor $\dot{\gamma }_{\text{o}}$ in the Arrhenius-type equation for the shear-strain rate $\dot{\gamma }$ is found to be of the order of 10⁵ s⁻¹, which is substantially lower than that $\left( {\dot{\gamma }_{\text{o}} \sim 10^{7} \,{\text{s}}^{ - 1} } \right)$ determined in the stress/temperature regime II (250–400 K) and contradicts the assumption invariably made in most of the theoretical models of flow stress that $\dot{\gamma }_{\text{o}}$ is a constant over a wide temperature range.