Solidification paths and eutectic intermetallic phases in Mg–Al–Ca ternary alloys

The solidification of Mg–Al–Ca ternary alloys in the α-Mg solidification region was investigated by microstructural and thermal analysis, and the liquidus projection was determined. The liquidus temperature decreases with additions of Al and Ca. Solidification is terminated with the formation of C14 (Mg 2Ca), C36 ((Mg,Al) 2Ca) or A12 (β-Mg 17Al 12) eutectic compounds, in the order of increasing Al content. The L → α + C36 eutectic has a saddle point at 807 K, and with decreasing temperature, two ternary invariant transformations, L → α + C14 + C36 (787 K) and L + C36 → α + A12 (725 K) are observed. The stability of the intermetallic C36 phase during annealing at 573 K was examined in die-cast Mg–5Al–3Ca based alloys. The C36 phase transformed to the C15 (Al 2Ca) phase by a shear-assisted mechanism.