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 (10 24 precipitates/m 3 ). 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.

Precipitation and Hardening in Magnesium Alloys

“…Although G.P. zones, together with Zn clusters and G.P.1 zones, were reported in recent studies [57,58] to form in a Mg-2.8 at. pct Zn alloy aged at temperatures 295 K, 343 K, 371 K, and 433 K (22°C, 70°C, 98°C, and 160°C), no compelling experimental evidence was provided to support the existence of such Zn clusters and G.P.…”

Characterisation of precipitates in an aged Mg–Zn–Ti alloy

“…This indicates that Ti is very effective in promoting the nucleation of the precipitates. In comparison to the binary alloy and other Mg-Zn based alloys [2,10,[13][14][15], Mg-Zn-Ti alloy exhibited a noticeably higher fraction of the blocky precipitates, believed to correspond to the ␤ 1 phase (Fig. 3b).…”

“…strength, can be produced by ageing at much lower temperatures, in particular at ambient temperature [1] and most likely also at intermediate temperatures (up to about 100 • C) [2]. The precipitation in the Mg-Zn based alloys generally takes place through the formation of a number of intermediate phases [2][3][4][5][6][7][8][9][10] The decomposition of the supersaturated solid solution (SSSS) initially occurs through clustering of solute atoms [2,10]. Depending on the ageing temperature and alloy composition, three types of coherent Guinier-Preston (GP) zones may form: GP1 zones as plates on {1 12 0} Mg [11], GP2 zones as oblate spheroids on {0 0 0 1} Mg [11], and GP zones as discs on {0 0 0 1} Mg [12].…”

“…The formation of the latter precipitates generally leads to a loss of mechanical properties due to their very sparse distribution. Alloys aged at ambient temperature are strengthened primarily by the combination of the GP1 zones, solute (co)clusters and prismatic precipitates of an unknown phase [1,2,10,13]. At intermediate temperatures, a large number of precipitates typical for both artificial and natural ageing may form [2], many of which are yet to be fully characterised.…”

Effects of Cu addition on microstructure and mechanical properties of as-cast Mg-6Zn magnesium alloy

“…This can be explained by the following reasons. On one hand, the addition of Cu can alter the solidus of the alloys, which shortens the solidification time and thus refines the microstructure [17] . On the other hand, the CuMgZn intermetallic compound possesses high melting point and good thermal stability [18] .…”