Natural Aging in Mg-Zn(-Cu) Alloys

Abstract

Natural aging in Mg-Zn-Cu alloys results in a significant hardening due to the formation of a high number density of shearable precipitates. For alloys containing Zn and Cu in the compositional ranges of both cast and wrought alloys (Zn = 2 to 2.4 at. pct, and Cu = 0.15 to 1.2 at. pct), hardening produced by natural aging (T4) for 4 to 8 weeks (91 to 104 VHN) almost equals that produced by artificial aging (93 to 109 VHN). The density of the precipitates in the T4 condition of Mg-Zn-Cu alloys was found to be of the order of precipitate density in a typical aged aluminum alloy (1024 precipitates/m3). An addition of even a trace amount of Cu to Mg-Zn alloy enhances the nucleation of the precipitates and accelerates the kinetics of precipitation, in particular during natural aging. This is believed to be a result of the enhanced favorable interaction between Cu, Zn, and vacancies. The strengthening is produced mainly by the formation of a very high density of the coherent disc-shaped Guinier–Preston 1 (GP1) zones and the prismatic precipitates perpendicular to the magnesium basal plane.