An observation on microstructure of a casting Zn-40 Wt pct al alloy

The microstructure of a casting Zn-40 wt pct Al alloy was studied by transmission electron microscopy (TEM). The results show that primary α’ dendrites decompose into η-Zn phases and an aluminum matrix during cooling and at ambient aging. There are three typical morphologies corresponding to the different microcompositions or cooling conditions: coherent platelike η-Zn phases, spherical η-Zn phases coexisting with the platelike η-Zn phases, and the full equilibrium structure of spherical η-Zn phases and an aluminum matrix. The formation mechanism of these three kinds of microstructures is discussed. On the surface of the α’ dendrite, there is a cell with fcc structure, which should be β-Zn phase by the peritectic reaction. In the interdendritic regions, there are proeutectic β-Zn phases with equiaxed grains and lamellar colonies which are ternary eutectic β-Zn, η-Zn, and ε-CuZn4 phases, with the orientation relationships of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$(\overline {12} 1)_{\beta - Zn} //(0\bar 110)_{\eta - Zn} //(0\bar 110)_{\varepsilon - CuZn_4 } and [125]_{\beta - Zn} //[4\overline {22} 3]_{\eta - Zn} //[4\overline {22} 3]_{\varepsilon - CuZn_4 } $$ \end{document} The T’ or T phase (Al4ZnCu3), which has an ordered bcc structure (a=0.296 nm), was found.