Solidification microstructure selection and characteristics in the zinc-based Zn system

Abstract

A solidification microstructure selection diagram has been determined for ZnMg alloys containing up to 4.5 wt% Mg over the growth velocity range 0.001–8 mm/s at temperature grandient 15 K/mm by means of the Bridgman technique. Results are compared with the cmicrostructures of chill cast wedges made from the same alloys. The observed transition from equilibrium eutectic E αβ ( αZn- β Mg 2Zn 11) to nonequilibrium E αγ ( αZn- γ MgZn 2) to E αβ and back to E αγ with increasing V is explained on the basis of competitive growth. Secondary dendrite arm spacing λ 2 of αZn decreases with increasing V such that λ 2V 1 3 is constant at fixed alloy concentration in agreement with predictions of arm coarsening theory. Eutectic interphase spacing λ decreases with increasing V such that λV 1 2 is constant at 7.3 ± 1.3 μ m 3 2 s −1 2 for rod-like E αβ compared with 4.6±1.6 μ m 3 2 s −1 2 for lamellar/spiral E αβ , both values being in reasonable agreement with predictions of the Jackson-Hunt model. Higher hardness levels for E αβ than E αβ at the same growth velocity are attributed to the larger volume fraction of hard intermetallic phase at the same Mg-content in E αβ .