Abstract: 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 wherebya 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 inter dendritic 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 a n aluminum matrix. The Cu-rich ε phase particles form in all η phases.
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.
Fig.5 Changes of lamellar spacing of α+η decomposed from β phase through branching of existing lamellae (C) and nucleation of fresh lamellae at advancing interface (D)
Fig.6 Microstructures of products decomposed from α′ phase (a) —SEM image of lamellar products decomposed from α′ phase; (b) —TEM image of impingement of two lamellar cells in primary α′ phase; (c) —General TEM view of lamellar structures decomposed from α′ phase through discontinuous decomposition
