Thermodynamic Analysis of As-Cast and Heat-Treated Microstructures of Mg-Ce-Nd Alloys

Abstract

The phase relationships in the Mg-rich corner of the Mg-Ce-Nd system have been investigated through the evaluation of selected compositions in the as-cast and heat-treated condition. Consistent thermodynamic CALPHAD-type assessments have also been generated for the Mg-Ce-Nd system [1].It is shown that this system reveals a significant degree of meta-stability under technologically relevant solidification conditions (i.e., permanent-mould or high pressure die casting). This is simulated in thermodynamic calculations by suppression of the RE5Mg41 phase and reasonable agreement is found with the as-cast micro structures. After heat treatment these micro structures transform, depending on alloy composition, into phase assemblies consistent with the calculated stable equilibrium phase diagram. It is the elucidation of such meta-stable phase formation and the subsequent transformation from the as-cast to the heat treated state that is a particular strength of the thermodynamic approach and which makes it a powerful tool for alloy development.Mg-Ce-Nd Liquidus surface.The solid dots in Figure 1 indicate alloys exhibiting experimentally evidenced primary precipitation of the CeMg12 phase in the as-cast condition, whereas the stable equilibrium phase diagram suggests RE5Mg41 to be the primary phase. The thermodynamic calculation, however, reveals that a relatively small undercooling will result in the (metastable) primary CeMg12 phase precipitation in all these alloys, as observed after casting. More results and the phase transformations towards equilibrium after heat treatment are given in detail by Gröbner et al. [1].KeywordsRare earthCALPHADCastingIsothermal heat treatments