Phase and microstructure pattern selection of Zn-rich Zn–Cu peritectic alloys during laser surface remelting

Peritectic solidification has attracted increasing attention as a lot of important binary alloys, such as Fe–Ni, Zn–Cu, Fe–C and Ti–Al, exhibit peritectic reaction during solidification. In order to investigate the solidification behavior of Zn-rich Zn–Cu peritectic alloy containing nominally up to 7.8 wt.% Cu, a series of laser surface remelting experiments were performed. With the increase in growth velocity, Zn–Cu alloys with Cu content below 3.0wt.% showed an evolutional sequence from low-velocity η planar interface → lamellar structures → η shallow cells and finally to high-velocity η planar interface. The Zn-4.0 wt.%Cu alloy showed a similar transitional sequence except that irregular η cells appeared when low-velocity planar interface became unstable. In contrast, ε cell/dendrite was the typical microstructure of the Zn-7.8 wt.% Cu alloy within the whole scanning velocity range. Based on the maximum interface temperature criterion, a eutectic growth model under rapid solidification conditions (TMK model) and a self-consistent numerical model for the cellular and dendrite growth were applied to establish a phase and microstructure pattern selection map, which drew a clear whole picture of the relationship between phase/microstructure and solidification conditions of this series of alloys. Regarding the microstructure feature, our investigation revealed the range of the solidification velocity and chemical composition of lamellar structures as dominant microstructure and their lamellar spacing displayed a considerable range of the average value as a function of growth velocity. The relationship between the lamellar spacing and the growth velocity was further analyzed by using the TMK eutectic model, and the results showed the same overall trend as the experimental results.