Nano microstructure development and solidification of Zn-6 wt% Al hypereutectic alloy

Abstract

Zn-6 wt% Al hypereutectic alloy was melt in a mild steel crucible under a purge gas of nitrogen, and solidified with five different cooling rates of 0.04, 0.10, 0.22, 0.40, 10.00 °C/s. The influence of cooling rates on the solidification process of the alloy was investigated via thermal analyses based on temperature measurements recorded on cooling curves. The corresponding first and second derivatives of the cooling curves were derived to reveal the details of phase changes and the development sequences of microstructure features occurring during solidification. The microstructure analyses by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) indicate that four different phases, the primary γ-ZnAl phase, the eutectic β-Zn phase, and the eutectoid α-Al and eutectoid β-Zn phases decomposed from the eutectic γ-ZnAl phase appear in all the samples solidified under the difference cooling rates. But, the quantitative analyses reveal that the decomposition rates of the primary γ-ZnAl and the average diameter of the eutectoid phases are reduced with increasing the cooling rates. The highest cooling rate of 10.00 °C/s suppresses the growth of the eutectoid phases and led to the formation of the nano-sized eutectoid α-Al and eutectoid β-Zn phases, and consequently refined the microstructure significantly.