Characterisation and modelling of precipitate evolution in an Al–Zn–Mg alloy during non-isothermal heat treatments

Abstract This paper describes the response of a precipitate microstructure to various types of non-isothermal temperature changes, namely reversion, ramp heating and thermal cycles, met in heat-affected zones (HAZs) of arc-welds, in an Al–Zn–Mg alloy in the T6 and T7 states. During these thermal histories, the precipitate size and volume fraction are quantitatively measured by small-angle X-ray scattering (SAXS). Reversion experiments are characterised by a fast dissolution stage at almost constant average precipitate size, followed by a coarsening stage that affects the surviving precipitates. Ramp heating experiments show a dissolution behaviour, which is temporarily interrupted for low heating rates, either due to phase transition when the initial precipitates are of a metastable nature (the case of T6 initial state), or due to a dynamic competition between the average and critical radii during the temperature increase (the case of the T7 material). The HAZ of arc-welds is characterised by a gradual increase of the dissolved precipitate fraction as one gets closer to the weld line. In the zone immediately before complete dissolution, precipitate coarsening is observed. A simple model, based on the growth/dissolution of precipitates distributed in size classes, has been adapted to the present ternary alloy. This model, calibrated using the reversion experiments, has proven its predictive nature in all the other thermal cycles investigated (ramp heating and welding). The predictions of the model are used in order to improve the physical insight into the range of material behaviour observed in the experiments.