Precipitation hardening of Mg-Ca(-Zn) alloys

Binary Mg-1Ca alloy exhibits a moderate precipitation-hardening response during isothermal ageing at 200°C. The ternary addition of a small concentration of Zn (1wt%) to the binary alloy leads to a substantial increase in peak hardness and an accelerated rate of ageing. The maximum hardness achievable (63 HV) is comparable with that of ZC63, Mg-8Al-1RE and Mg-1.3RE alloys. This significant increase in maximum hardness appears to be associated with a refined distribution of precipitates. The primary intermetallic phase in as-cast Mg-1Ca-1Zn alloy is a ternary phase with an atomic composition of 69.4 ± 1.4 at.%Mg, 27.0 ± 0.3 at.%Ca, and 3.6 ± 1.0 at.%Zn. This phase has a hexagonal crystal structure (point group 6/mmm), with lattice parameters of approximately a = 0.61 ± 0.01 nm, c = 1.02 ± 0.01 nm, which appears isomorphous with Mg2Ca (space group P63/mmc). Two solid state precipitate phases have been observed in as-cast microstructure of Mg-1Ca-1Zn alloy, a hcp phase with a = 0.623 nm, c = 1.012 nm, and a hep phase with a = 0.556 nm, c = 1.042 nm. These two precipitate phases have a thin plate shape, and have identical orientation relationships with respect to the matrix phase: (0001)P // (0001)m, [2110]P // [1010]m.