Rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys

Rapid solidification experiments, including laser remelting, melt-spinning and wedge casting, were carried out to investigate the rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys containing up to 9.0 at% Ag. For comparison, Bridgman solidification experiments of the same alloys were also carried out for growth velocities ranging from 0.02 to 4.82 mm/s, which were lower than that of 12–54.5 mm/s for laser remelting and that in the order of 102 mm/s for melt-spun samples. Optical images and transmission electron microscopy (TEM) showed that instead of the typical structure consisting of primary dendrites of ϵ surrounded by peritectic η, a two-phase plate-like η+ϵ with (or without) primary dendrites of ϵ was observed in Zn–3.1, 4.4, 6.3 and 9.0 at% Ag alloys when the growth velocity was higher than a critical value. It was found that the higher was the alloy concentration, the higher was the critical growth velocity for the formation of fully two-phase plate-like η+ϵ. From the TEM micrographs, the volume fraction of ϵ in the fully two-phase plate-like η+ϵ increased from 0.09 to 0.50 with increase in alloy concentration from 3.1 to 6.3 at% Ag. A plausible analysis was proposed to interpret the dependence of microstructural transitions on the growth velocity in Zn–3.1 to 9.0 at% Ag alloys, that is, primary dendrites of ϵ in a matrix of peritectic η→two-phase plate-like η+ϵ with primary dendrites of ϵ→ fully two-phase plate-like η+ϵ.