Grain-scale in-situ study of discontinuous precipitation in Mg-Al

Competitive continuous and discontinuous precipitation (CP and DP) have been studied in-situ for the first time in magnesium alloys, using the Mg-Al system as an exemplar. CP forms first, and strongly affects the subsequent migration of the high angle grain boundary (reaction front, RF) behind which DP occurs. It has been demonstrated that in contrast with expectations from classical DP theory, the RF does not migrate with a steady-state velocity, but instead proceeds in an irregular stop-start fashion. Furthermore, the RF velocity is not constant but varies from grain to grain, by a factor of 4 for the conditions investigated. This growth behaviour can be explained by the interaction of the RF with CP. Whilst a mean-field model has been demonstrated to correctly predict the overall CP and DP kinetics, it is shown that the irregular motion of the RF is due to local effects. A simple model has been developed that demonstrates how the locally depleted solute field around a CP leads to arrest of a segment of the RF until sufficient diffusion occurs along it to reactivate RF motion. Zener pinning and boundary curvature also play an important, but secondary role. The results have implications for controlling DP, which is usually considered undesirable.