Precipitation in a Ag-Containing Mg-Y-Zn Alloy

The precipitation in a Mg-6Y-2Ag-1Zn-0.6Zr (wt pct) alloy during isothermal aging at and above 473 K (200 °C) has been examined systematically using conventional transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and three-dimensional atom probe. The precipitation involves the formation of G.P. zones, metastable γ″ and γ′ phases, and equilibrium 14H and δ phases. The G.P. zones are a single atomic layer on (0001) α . The γ″ is the key strengthening phase and it has an ordered hexagonal structure (P6/mmm, a = 0.556 nm, c = 0.424 nm). The γ″ forms as (0001) α plates with a thickness of a single unit cell height and tends to develop into stacks with irregular spacing when the alloy is exposed to longer aging time at 473 K (200 °C). The γ″ is gradually replaced by γ′ and 14H during prolonged aging at 473 K (200 °C) and/or temperatures above 473 K (200 °C). The equilibrium δ phase is also found in grain boundaries. The results are compared with findings in other but similar alloy systems, and approaches for enhancing γ″ nucleation rate and hence further improvement in alloy age-hardening response are discussed. The effects of Ag on precipitation are also discussed.