Abstract: As a part of the systematical investigation of the phase relationships of the Zn-Al base alloys,decomposi- tion of the supersaturated phase β_S in a eutectoid Zn-Al-Cu-Si alloy was studied by hardness testing,opti- cal,electron microscopy and X-ray diffraction.Com- pared to the monotectoid Zn-Al-Cu-Si alloy,a similar sequence of decomposition was found to consist of three stages of phase transformation,which were in accordance to various equilibrium reactions at the temperatures above the ageing temperatures.

Key words: Zn-Al base alloys, phase transformation, X-ray diffraction