An electron metallographic study of pressure die-cast commercial zinc–aluminium-based alloy ZA27

The microstructure of ZA27 pressure die-castings was examined by scanning and transmission electron microscopy after ageing for 5 years at ambient temperatures. Solidification began with the formation of compact aluminium-rich α′ dendrites and tiny rounded α′ particles, followed by the peritectic reaction whereby a zinc-rich β phase formed around the edges of the primary phases. The extremely high cooling rate during solidification reduced the extent of the peritectic reaction so that the liquid became highly enriched with zinc and solidification was completed by eutectic formation of β and η phases, the β joining the peritectic β and the η remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the β phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of α and η, and some lamellar colonies spread into the low-aluminium α′-phase cores of the dendrites to form coarse lamellar products. The bulk of the α′, however, decomposed into a very fine mixture of zinc-rich phases in an aluminium matrix. These structures are consistent with solidification under conditions of high undercooling. Enclosed within the α constituent of the decomposed peritectic and eutectic β phases were small particles of a phase which was identified as the transitional α′m phase containing 30.2%Al or 14.8%Al, with an fcc crystal structure and lattice parameter (at 14.8%Al) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding α phase. This metastable phase was probably stabilized by copper. Copper became concentrated in the eutectic liquid during the first stages of solidification, and was rejected from the liquid in the form of discrete irregular particles, 1–2 μm in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich η phase in the form of a dense precipitation of small particles of 70–120 nm diameter and 2–3 nm thick. Both of these particles were identified as the metastable cph ε-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566.