AbstractThe mechanical behaviour of an Al–Li–Mg–Cu–Zr 8090 alloy at a temperature of 515°C and strain rates in the range 10−4−10−2s−1was measured by tensile testing. The greatest strain rate sensitivity was measured in the middle of that strain rate range, and did not change significantly with strain. Large abrupt changes in strain rate during testing showed that the strain rate history had a significant effect, especially slow prestraining which gave a relative increase in flow stress and a reduction in rate sensitivity compared with testing at a constant rate to the same strain. The evolution of grain size was measured, and there was evidence that this aspect of the material microstructure could be used to explain the observed behaviour. This view was reinforced by the ability of a transition model of superplasticity, together with a simple model of the evolution of grain size distribution, to reproduce the essential features observed in testing with large changes in strain rate.MST/3351