Comparison of flow-stress measurements on high-purity tungsten single crystals with the kink-pair theory

Abstract

The flow stress of high-purity tungsten single crystals, oriented for single slip, was measured in dependence on temperature and strain rate in the temperature regime 60 to 800 K by successive tensile deformations at constant strain rate and in correlated stress-relaxation tests. Three regimes in the dependence of the flow stress and its strain-rate sensitivity on the temperature were detected. They constitute a challenging feature of the mechanical properties of high-purity b.c.c. metals. The interpretation of the results is based on the motion of a0/2〈111〉 screw dislocations which is rate-controlled by thermally activated formation of kink pairs on screw dislocations. The kink-pair theory of the flow stress developed by A. Seeger provides excellent tools to interpret the temperature and strain-rate dependence of the flow stress of b.c.c. metals. Comparison between theory and experiment enables quantitative tests of the theory and allows the determination of kink properties. It is found that for low stresses (regime I, T > 600 K) the fundamental process of kink-pair formation takes place on {211} plane with a formation enthalpy of two isolated kinks of a kink pair 2Hk ≅ 2.1 eV. For intermediate stresses (regime II, 220 K ≤ T ≤ 600 K) the fundamental kink-pair formation process takes place on {211} with 2Hk ≅ 1.75 eV. For high stresses (regime III, T ≤ 220 K) the fundamental process of kink-pair formation takes place on {110} glide planes with 2Hk ≅ 1.3 eV.