An electron metallographic study of the commercial zinc-based pressure-die-cast alloy 3

Abstract

The solidification structure of pressure-die-cast commercial alloy 3 was examined using scanning electron microscopy and thin foil transmission electron microscopy techniques. It was found that the rapidly cooled alloy commenced solidification by the formation of small rounded primary zinc (η) particles followed by pseudo-primary β particles and then a fine eutectic of β + η. The high temperature β phase subsequently decomposed into α-aluminum and η, the phases stable at ambient temperature, but an intermediate transitional phase was found in alloys examined shortly after casting. Chemical analysis by energy-dispersive spectroscopy showed that this phase contained zinc with 11.8 wt.% Al, suggesting that it was a transitional phase since none of the stable phases in the AlZn system has that composition. This transitional phase had almost entirely disappeared after aging for a period of 5 years. Precipitates had formed during or immediately after casting within the central regions of the zinc-rich primary dendrites. These were identified as the aluminium-rich α solid solution with f.c.c. structure. The orientation relationship between the phases was determined as [0001] ƞ[111] α (11 20) ƞ(1 10)α Each grain contained two families of precipitates with a common (0001) η habit plane, each adopting one of the two non-equivalent variants of the orientation relationship. Since this work had shown that aluminium precipitation from η was completed rapidly, the long-term dimensional changes found in zinc alloy castings on aging are considered to be due to the gradual replacement of the zinc-rich metastable phase by equilibrium α and η.