Phase Relationships in Zn-Al Based Alloys

Abstract

The present investigation started with the establishment of equilibrium phase diagrams of Zn-Al-Si and Zn-Al-Cu-Si systems. Subsequently, hardness testing on alloys of the whole composition range in both systems was carried out at various temperatures to get a general survey of phase transformation in the quenched-ageing processes. Five alloys with pronounced age-hardening responses, i.e. Al-rich, monotectoid, and eutectoid alloys in the Zn-Al-Cu-Si system and Al-rich and monotectoid alloys in Zn-Al-Si systems, were studied in detail using X-ray diffraction examination. Eleven sets of X-ray diffractograms and seven fraction transformation curves revealed the distinct mechanisms of phase transformations in Zn-Al, Zn-Al-Si, Zn-Al-Cu, and Zn-Al-Cu-Si systems. Two of these five alloys were investigated in the transmission electron microscope to explain the two typical structural changes occurring as a result of quench-ageing processes in Zn-Al-Si and Zn-Al-Cu-Si systems. Using the previously established equilibrium phase diagrams and the mechanisms of phase transformation of the four alloy systems, the phase relationships between equilibrium and non-equilibrium states in multi-component Zn-Al based alloys were obtained. A general rule of phase transformation of supersaturated alloys was derived based on thermodynamic and kinetic considerations, which may be applicable to other alloy systems. The effect of the phase transformations on mechanical and physical properties was studied using tensile strength, impact, dimensional stability and wear property tests. This allowed the selection of particular alloys for commercial purposes, especially for use as plain bearings or structures which require good tribological properties.