Identification of the metastable phase α′_m in a Zn/Al alloy containing Cu and Mg

Abstract

Samples of the commercial pressure-diecast alloy ZA8 containing 8.1% Al, 1.1% Cu and 0.024% Mg have been examined by transmission electron microscopy (TEM). During decomposition of the high temperature f c c β phase, on cooling after casting, a transitional phase was formed within the α phase in both dendrites and eutectic. This phase was identified as the metastable phase α′ _m containing 11% Al or 23% Al, with a f c c crystal structure and lattice parameter (at 11%) of about 0.394 nm. It had adopted a symmetrical cube/cube orientation relationship with the surrounding f c c α phase. The stability of this metastable phase at ambient temperatures was greatly increased by the presence of copper.