Cellular growth during the transient directional solidification of Zn-rich Zn–Cu monophasic and peritectic alloys

Zn–Cu alloys located in the monophasic and hypoperitectic ranges of compositions of Zn-rich Zn–Cu alloys were directionally solidified under unsteady-state heat flow conditions. The experimental cooling curves allow solidification thermal parameters: tip cooling rate (Ṫ), tip growth rate (VL) and temperature gradient (GL) to be experimentally determined. The observed microstructural evolution of both alloys has shown that a regular cellular morphology prevails along the whole castings lengths. Only the regions very close to the cooled casting surface showed the presence of plate-like cells due to very high cooling rates (higher than 25 K/s). The cell spacing (λc) was measured along the castings lengths, and experimental correlations between λc and experimental solidification thermal parameters have been established. Power laws with −0.55 and −1.1 exponents expressing λc as a function of Ṫ and VL, respectively, were found to better represent the growth of cells under transient heat flow conditions for both alloys experimentally examined. The predictions furnished by the Hunt–Lu model underestimate the experimental cell spacings found for both alloys examined. It was shown that the proposed equations relating λc as a function of Ṫ are able to represent both the steady-state and unsteady-state cellular growth of monophasic and peritectic Zn-rich Zn–Cu alloys.

► The microstructure of monophasic and hypoperitectic Zn-rich Zn–Cu alloys is cellular.
► The presence of plate-like cells occur only for cooling rates higher than 25 K/s.
► The Hunt–Lu model do not match the cell spacings of monophasic and hypoperitectic alloys.
► Equations encompassing steady-state and transient cellular growth are proposed.