AN OBSERVATION ON MICROSTRUCTURE OF A CASTING ZN-40 WT NOT AL-ALLOY

The microstructure of a casting Zn-40 wt pet Al alloy was studied by t ransmission electron microscopy (TEM). The results show that primary a lpha' dendrites decompose into eta-Zn phases and an aluminum matrix du ring cooling and at ambient aging. There are three typical morphologie s corresponding to the different microcompositions or cooling conditio ns: coherent platelike eta-Zn phases, spherical eta-Zn phases coexisti ng with the platelike eta-Zn phases, and the full equilibrium structur e of spherical eta-Zn phases and an aluminum matrix. The formation mec hanism of these three kinds of microstructures is discussed. On the su rface of the alpha' dendrite, there is a cell with fee structure, whic h should be beta-Zn phase by the peritectic reaction. In the interdend ritic regions, there are proeutectic beta-Zn phases with equiaxed grai ns and lamellar colonies which are ternary eutectic beta-Zn, eta-zn, a nd epsilon-CuZn4 phases, with the orientation relationships of (<(12)o ver bar>1)(beta-Zn)//(0 (1) over bar 10)(eta-Zn)//(0 (1) over bar 10)( epsilon-CuZn4) and [125](beta-Zn)//[4<(22)over bar>3](eta-Zn)//[4<(22) over bar>3](epsilon-CuZn4) The T' or T phase (Al4ZnCu3), which has an ordered bcc structure (a = 0.296 nm), was found.