Metallurgical Synthesis Methods for Mg-Al-Ca Scientific Model Materials

Further improving the mechanical and corrosion properties of Mg-based alloys requires the investigation of underlying causalities between synthesis, processing, and microstructure, ideally down to the near atomic scale. However, such investigations with high resolution characterisation techniques require model materials of exceptionally controlled chemical composition and microstructure. In this context, the Mg-Al-Ca system, being attractive for offering significant improvement of the inherent brittleness of Mg alloys, is exceptionally challenging from a metallurgical perspective due to the inclusion of components with high reactivity and vapor pressures. Here, we demonstrate the applicability of various synthesis methods from arc-melting over solution growth to diffusion couples, extending to parameters for thermomechanical processing. Suitable pathways to overcome the specific challenges of the Mg-Al-Ca system are demonstrated, as well as the persistent limitations of the current state-of-the-art metallurgical laboratory technology. Solid solution sheet or wrought materials are ideally produced by induction melting in steel crucibles and casting under elevated pressure, with intermittent annealing steps between hot rolling passes to ensure homogenous chemical composition without excessive grain growth. Composite microstructures strongly depend on the solidification rate, and special care needs to be taken with regards to crucible reactions with increasing Al concentrations. Binary intermetallic compounds can be successfully produced in bulk by a variety of techniques, but the interplay between reactivity and evaporation of the melt when both Ca and Al are present required more complex approaches such as the Bridgman method for the synthesis of ternary compounds.