Temperature and Strain-rate Dependence of the Flow Stress of BCC Transition Metal Crystals in Cyclic Deformation

Abstract

After uniaxial cyclic deformation of ultra-pure Ta single crystals well into stress saturation at a sufficiently high temperature, the temperature dependence of the flow stress was measured between 80 K and 450 K at different plastic shear-strain rates in the range 2×10-5 s-1 to 6×10-3 s-1. A recent theory by Seeger describing the thermally activated contribution to the flow stress, σ*, in terms of formation and migration of kink pairs in screw dislocations is critically tested and found to account for the data very well. The enthalpy of kink formation is deduced as Hk = (0.49 ± 0.01) eV in good agreement with recent internal-friction data, and the kink height a as 1.74 ± 0.05 times the minimum lattice period on {110} planes. The latter result is independent of the crystallographic orientation of the crystals and of doping with nitrogen up to atomic concentrations of about 10-3. It is interpreted to mean that, at least between 250 K to 450 K, the kinks in <111> screw dislocations connect dislocation segments in adjacent positions of minimum energy on {211} planes and not on {110} planes.