Macroscopic kinetics of discontinuous precipitation in a Mg-8.5 wt% Al alloy

The macroscopic kinetics of discontinuous precipitation in two Mg-8.5 wt% Al alloys with different initial average grain sizes has been studied at 220°C by quantitative optical metallography, hardness measurements and integrated intensity measurements of the precipitate peaks during in situ X-ray experiments. The kinetics observed can be described by Johnson-Mehl-Avrami's law, which indicates that a reaction front stops when it meets an immobile grain boundary. Moreover, the precipitation nodule nucleation probably takes place via the mechanism observed and described by Fournelle and Clark, as can be infered from the grain size dependence of the nucleation rate.