A Transmission Electron Microscopy Study of Decomposition in the Zn-Al Eutectoid Alloy

There has been considerable interest in the Zn-Al eutectoid alloy in recent years mainly because it exhibits superplastic deformation under certain conditions. A feature of particular importance is that the microstructural requirements for superplasticity can be achieved.in this alloy by heat treatment alone. However in spite of this, the mechanisms of phase transformation have not been widely studied; some aspects of these are described in this paper. The eutectoid reaction takes place at a composition of 78 wt% Zn and a temperature of 276°C by the decomposition of the high temperature γ phase (F.C.C.) to the Zn-rich β phase (C.P.H.) and the Al-rich α phase (F.C.C.) terminal solid solutions. Experiments have mainly involved quenching directly from the γ - phase field and isothermally transforming at various temperatures below 276°C.