Effects of cell morphology and macrosegregation of directionally solidified Zn-rich Zn-Cu alloys on the resulting microhardness

Zn-1.0 wt.%Cu (monophasic) and Zn-2.2 wt.%Cu (hypoperitectic) alloys were investigated by transient directional solidification, and the results include the cell spacing (lambda(c)) and correlation with the experimental tip cooling rate ((T) over dot), macrosegregation, microhardness (HV) and the qualitative variation on eta and epsilon phases by XRD spectra. Only regions very close to the cooled casting surface of the Zn-2.2 wt.%Cu alloy casting, showed plate-like cells due to very high cooling rates (> 16 K/s) associated with a significant Cu enrichment. The Zn-2.2 wt.%Cu alloy exhibited HV values 2-3 times higher than those found for the Zn-1.0 wt.%Cu alloy. It is shown that the microhardness of the Zn-2.2 wt.%Cu alloy is directly influenced by both the inverse Cu segregation and by the morphologies of eta and epsilon phases. Modified Hall-Petch equations are proposed relating HV to lambda(c) for both alloys. (C) 2012 Elsevier B.V. All rights reserved.