The effects of microalloying on the precipitate microstructure at grain boundary regions in an Mg-Zn-based alloy

Publication date: 5 April 2017
Source:Materials & Design, Volume 119
Author(s): B. Langelier, G. Sha, A. Korinek, P. Donnadieu, S.P. Ringer, S. Esmaeili
Microalloying additions to Mg-Zn base alloys can refine precipitation and improve hardening, but their effect on the microstructure at the grain boundary regions are seldom analyzed. Here the grain boundary microstructure is examined in an Mg-4Zn (wt.%) alloy, which has been microalloyed with Ce-Ca. This combination of elements has previously been shown to successfully enhance ductility, texture, and precipitation hardening, compared to binary Mg-Zn. Coarse grain boundary precipitates are found with or without microalloying, but precipitate-free zones (PFZs) for β′1 that surround the boundaries are far narrower with Ce-Ca microalloying additions. Furthermore, fine basal precipitates containing Ca are found uniformly distributed up to the boundary, making those zones devoid of β′1 not truly precipitate-free. Electron microscopy and atom probe analysis of early-stage ageing conditions reveals that Ca readily forms clusters with Zn, and forms fine ordered GP zones, while Zn also segregates to the grain boundaries. The tendency of Ca to homogeneously form clusters and precipitates reduces Ca migration to the grain boundaries, which has a beneficial effect on producing the refined precipitate distributions at the grain boundary regions.

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