Plastic flow of tungsten-based composite under hot compression

Liquid-phase sintered tungsten composite specimens with a 92.5W-5.25Ni-2.25Fe composition were tested for temperature and strain-rate effects during hot deformation. The flow stress was measured for samples tested at constant strain rates of 0.01, 0.1 and 1 s^-1 and at temperatures ranging from 25 to 1100°C for reductions in height of 30, 50 and 70%. The results show that the true stress/true strain response of the tungsten composite depends on both the test temperature and, to a lesser extent, the applied strain rate, with the rate of strain hardening decreasing with increasing temperature and strain rate. Optical microscopic observations showed a dramatic increase in grain deformation and micro-crack density as the strain rate, temperature and reduction in height are increased. Initial cracking occurred preferentially at tungsten-tungsten boundaries or at the tungsten grain/matrix interface, then cracking propagated along a minimum fracture energy path. Brittle failure of tungsten grains is mainly found at 1100°C. The results are modelled mathematically using a strain-, strain rate- and temperature-dependent equation.