The
microstructure of
permanent mold casting ZA27 alloy was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) after
natural aging for 18 months. The solidification begins with the formation of aluminum rich α′ dendrites, followed by the peritectic reaction whereby a zinc rich β phase forms around the edges of the primary phase. Finally, the solidification is completed by formation of
eutectic β and η
phases, the β phase joining the peritectic β phase and the η phase remaining as thin film in the interdendritic regions. In some regions, the eutectics form in the form of fibrous β within η phase. The morphologies of the eutectics are determined by the size of the eutectic pools. On cooling after
Casting and on aging, both of the β phases decompose into well formed eutectoid α+η lamellae or irregularly complex structures through cellular reaction. These cell colonies nucleate on the eutectic η phase and grow into the low aluminum α′ phase cores of the dendrites to make the α′ phases form lamella structures. But the growth of these lamellae is always retarded by the continuous decomposition of the α′ phase in the dendritic cores and they decompose into mixture of fine zinc rich η phase in an aluminum matrix. The Cu rich ε phase particles form in all η phases.