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Superior creep strength of a heat resistant AX52 magnesium alloy is ascribed to the grain boundary eutectic Al 2 Ca phase covering the primary -Mg grains. The eutectic phase is stable in morphology at temperatures below 473 K, while it collapses during long term exposure at temperatures higher than 473 K. The microstructural change of the alloy during high temperature exposure is characterized by the decrease in the grain boundary coverage by the eutectic phase. The creep strength of the alloy decreases with the decrease in the grain boundary coverage, and the correlation between the creep strength and the grain boundary coverage is discussed.

The eutectic phase in a 1.72 mass pct calcium added AM50 die-cast alloy homogenized at 673 K has been investigated by X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The XRD and EDS experiments indicate that the eutectic phase consists of a Al 2 Ca phase with the C15 structure and contains 10.76 atomic pct magnesium in the equilibrium state. The solubility lobe of the Al 2 Ca phase lies parallel to the equi-66.7 at%Al composition line in the Mg-Al-Ca ternary grid, indicating that magnesium preferentially substitutes the calcium site of the Al 2 Ca phase.

The internal stress during high temperature creep was investigated for the die-cast AM50 magnesium alloy at 473 K through the straintransient dip test technique. The microstructure of the alloy is characterized by fine -Mg grains together with the -Mg 17 Al 12 particles located on the grain boundaries. The internal stress is proportional to the applied stress, when the applied stress ranges below the yield stress. The ratio of the internal stress to the applied stress is 0.36 for the alloy, which is about half of those for conventional pure metals such as Cu, -Fe and Ni. The small internal stress is a characteristic feature in the creep for the alloy.

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