AbstractPlane strain compression tests have been carried out at strain rates between 0·5 and 10 s−1and temperatures in the range 275–510°C, both under nominally isothermal conditions and with temperature decreasing. Also, temperature or strain rate have been changed in the interval between two deformations. In all cases, the stress–strain curves obeyed a mechanical equation of state, described by constitutive relationships in terms of strain and instantaneous value of Zener–Hollomon parameter Z. When the value of Z varies slowly during deformation, flow stress is uniquely related to subgrain size and to dislocation density within subgrains, but these relationships break down in transition structures developed after a change of Z between two deformations. The existence of an equation of state for mechanical behaviour, but not for microstructure, is considered to result from important contributions of both dislocation velocity and density to hot strength.MST/1066