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Grain-scale creep straining processes in die-cast and permanent-mold-cast Mg–Al–Ca alloys have been studied. High-temperature strain mapping with electron backscattered diffraction analysis reveals favorably oriented grains deforming preferentially during creep. Displacements of microstructural-scale markers applied along grain boundaries and within grain interiors demonstrate no significant contribution of grain boundary sliding to creep. Creep rates at 175 °C display an inverse dependence on grain size, with fine-grained die-cast material possessing the highest creep resistance. Implications for the design of future alloys are discussed.

The partitioning of elements during solidification of ternary and quaternary Mg-Al-Ca alloys has been measured by an electron microprobe segregation mapping technique. Differences in the degree of partitioning of Al and Ca to the α-Mg phase in as-solidified MRI230D, AX44, AXJ530, and MRI153M alloys was observed. MRI230D has a higher level of Al and Ca in the primary α-Mg phase, while only a higher level of Al is observed in MRI153M, compared with AX44 and AXJ530. While the increased levels of Al in the MRI alloys may be due in part to higher bulk Al concentrations, the higher level of Ca in the primary phase of MRI230D is attributable to the presence of Sn in this alloy. The differences in the partitioning of Al and Ca to the primary α-Mg phase during solidification influence both solid solution and precipitation strengthening, which can explain observed differences in creep behavior.

Dislocation substructures of three crept Mg–Al–Ca-based alloys have been characterized using transmission electron microscopy. At low temperature, most dislocations are basal 〈a〉 type, while at higher temperatures, basal and non-basal 〈a〉 dislocations are observed with isolated 〈c〉 or 〈c + a〉 dislocations. Although creep resistance varies substantially, no major differences in crept dislocation substructures are observed. Differences in creep behavior are not attributable to different creep mechanisms and may be due to strengthening of primary alpha phase instead.

Precipitation of the Al2Ca phase in the α-Mg phase of the Mg-Al-Ca–based AXJ530 die-cast alloy and the effect of precipitation hardening on the creep resistance were studied. Precipitation of the Al2Ca phase is suppressed in the die-casting process due to its high cooling rate, but precipitation was observed during subsequent isothermal heat treatments above 448 K. The precipitate has a disk-shaped morphology parallel to the basal plane of the α-Mg matrix phase and increases hardness by aging in the temperature range of 448 to 573 K. The creep resistance of the die-cast material at 448 K was improved by a factor of 1.5 ∼ 2, by a peak-age heat treatment at 523 K for 1 hour. Improvement in strength is limited by the low volume fraction of the precipitates; however, the precipitates are effective obstacles against dislocations gliding on nonbasal planes during creep deformation.

The effect of Sr additions on the microstructure and strength of a Mg-5Al-3Ca alloy was investigated by replacing Ca with Sr. In the as-cast alloys, the major intermetallic compound observed at the α-Mg dendrite cell boundary region changes from the Al-rich (Mg, Al)2Ca phase to the Mg-rich Mg17Sr2 phase with increasing Sr content. This results in an increased Al solute content in the α-Mg phase in the high-Sr containing alloys. More than 1 wt pct of Sr enhances the solid-solution strength of the α-Mg phase, resulting in the increased compression strength of the alloys.

A thermodynamic description of the Mg–Al – Sr ternary in the Mg– Al-rich region was developed based on the microstructures, the crystal structures and compositions of the phases in three samples in the as-cast and 673 K annealed states. The gross compositions of the three alloys were identified for investigation based on a calculated isotherm using the description obtained from those of the constituent binaries via extrapolation. The calculated liquidus projection using the developed description was found to be consistent with the primary phases of solidification obtained from two additional alloys in the as-cast state. In addition, the model-calculated isotherm and liquidus projection are able to rationalize results reported in a recent publication for this ternary. This paper demonstrates the power of using a preliminary calculated phase diagram for selecting key alloys for experimentation, on basis of which a reliable thermodynamic description can be developed.

The crystal structure of a eutectic (Mg,Al)2Ca Laves phase in as-cast AXJ530 (Mg–5Al–3Ca–0.15Sr) was identified as C36 by electron diffraction analysis. The C36 phase transformed to C15(Al2Ca) during annealing at 573 K. The orientation relationship for the C15 and C36 phases is reported.