PRECIPITATION OF METASTABLE EPSILON-PHASE IN A HYPEREUTECTIC ZINC ALUMINUM-ALLOY CONTAINING COPPER

Copper additions are known to increase the strength of zinc-aluminium based alloys, but the mechanism responsible has not been establish. The structure of pressure-diecast alloy ZA8 containing copper has been examined by transmission electron microscopy and direct evidence for the formation of a transitional copper-rich phase has been obtained. During both the eutectic solidification and the solid-state transformation of the high-temperature beta-phase, copper had become concentrated in the zinc-rich product (eta), and on cooling was rejected from solid solution as a dense precipitation of particles 80 nm in diameter and 2-3 nm in thickness. The precipitate was identified by electron diffraction as the metastable cph epsilon-phase (CuZn4), and its orientation relationship with the matrix and habit plane were determined. The asymmetrical orientation relationship adopted was shown to reduce elastic strain between the two cph structures of different c/a ratios. The dense precipitation of epsilon-phase was unaffected by ageing for five years at ambient temperatures, and is thought to be responsible for the high strength of zinc-based alloys containing copper.