Effect of changing strain rate on stress-strain behaviour during high temperature deformation

Abstract

Plane strain compression tests have been carried out on a ferritic stainless steel at a nominally constant temperature of 917°C and at strain rates of 0.15–5 s −1, and on commercially pure aluminium and an Al-1% Mg alloy at 450°C and strain rates in the range 0.4–12 s −1. Tests have been conducted both at constant strain rate and with strain rate increasing or decreasing from one constant rate to another in a controlled manner after steady state conditions had been established. A wide range of rates of change of strain rate have been studied, with initial and final strain rates differing by up to one and a half orders of magnitude.The ferritic stainless steel and the Al-1% Mg alloy follow a mechanical equation of state under all test conditions of changing strain rate in the sense that the flow stress is dependent only on the instantaneous strain rate and not on the way this strain rate is reached. On the other hand the commercial purity aluminium shows deviations from a mechanical equation of state, which increase systematically with the rate of change of strain rate but are the same magnitude for increasing and decreasing strain rate.