Structure and thermal stability of primary intermetallic particles in an Mg–Zn casting alloy

The structure and thermal stability of primary intermetallic particles formed in an Mg–8 wt.%Zn casting alloy have been characterized using transmission electron microscopy. Most primary intermetallic particles in the as-cast microstructure are of the Mg7Zn3 phase, and they decompose into α-Mg and Mg4Zn7 phases during post-casting heat treatments. The resultant Mg4Zn7 phase is metastable and, during prolonged heat treatments, it is gradually replaced by the equilibrium phase MgZn that has a hitherto unreported monoclinic structure.