The role of crystallography and thermodynamics on phase selection in binary magnesium–rare earth (Ce or Nd) alloys

In-depth understanding of secondary-phase selection during solidification and ageing is a crucial factor underpinning microstructure design and alloy development. Experimental observations show significantly different behavior of secondary phase selection between the Mg–Nd and Mg–Ce systems. Mg–Nd alloys show a significant degree of metastability of secondary phase selection during solidification, with NdMg12 forming upon slow cooling and NdMg3 upon fast cooling. However, at high heat treatment temperatures and long enough times, Nd5Mg41 forms. In contrast, Mg–Ce alloys form CeMg12 under a wide variety of casting conditions, and this phase remains stable even after long annealing times at high temperatures. However, both Mg–Ce and Mg–Nd alloys undergo a similar precipitation sequence during ageing. Based on recently developed thermodynamic data and crystallographic matching calculations, the phase selection in these two alloy systems under different processing regimes is well explained by the competition between the driving force (reduction in bulk Gibbs energy) and the nucleation energy barrier (increment of surface energy). The approach used here can be also applied to similar phase selection problems in other alloy systems.