Analysis of flow-stress measurements of high-purity tungsten single crystals

Abstract

High-purity W single crystals oriented for single slip were deformed in tension at a constant plastic strain rate between 26 and 800 K. Specimens were isothermally deformed up to their ultimate tensile strain at several temperatures (type-A tests). Another part was deformed successively in small strain intervals at different- decreasing- temperatures (type-B tests) after a first deformation at high temperatures (≥600 K). Tests were combined with stress–relaxation experiments to determine the strain-rate sensitivity of a relevant shear stress. Type-A tests exhibit a parabolic shape of the work-hardening curves with a rather high work-hardening rate at the beginning, which decreases continuously thereafter. In contrast to this behaviour the short stress-strain curves within the short strain intervals of type-B tests show no work hardening but a plateau after yielding. According to this behaviour a critical resolved shear stress (CRSS) and its strain-rate sensitivity can be determined unambiguously. For type-B tests, it turns out that below 775 K, the temperature dependence of the CRSS is non-uniform and exhibits three regimes of different shape. The activation volume and activation enthalpy are determined on the basis that an Arrhenius equation holds for the shear–strain rate.