Abstract:
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 alpha' dendrites and tiny rounded alpha' particles, followed by the peritectic reaction whereby a zinc-rich beta 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 beta and eta phases, the beta joining the peritectic beta and the eta remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the beta phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of alpha and eta, and some lamellar colonies spread into the low-aluminium alpha'-phase cores of the dendrites to form coarse lamellar products. The bulk of the alpha', 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 alpha constituent of the decomposed peritectic and eutectic beta phases were small particles of a phase which was identified as the transitional alpha'(m) phase containing 30.2% Al or 14.8% Al, with an fee crystal structure and lattice parameter (at 14.8% AI) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding alpha 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 mu m in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich eta 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 c p h epsilon-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566.
