A detailed HAADF-STEM study of precipitate evolution in Mg–Gd alloy

The magnesium-gadolinium (Mg–Gd) system provides an ideal model alloy for precipitation hardening. Evolution of precipitates in a Mg–16.2Gd–0.4Zr (wt.%) alloy during isothermal ageing from 150 °C to 300 °C was systemically studied using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The widely accepted four-stage precipitation sequence of the Mg–Gd(–Zr) alloy system has been accordingly updated to: super-saturated solid solution (S.S.S.S.) → ordered solute clusters → GP zones → β′ → β_F ^’ + tail-like hybrid structures → β₁ → β. In addition to carefully revealing an updated precipitation sequence, new insights into the relationships amongst different precipitate phases were obtained and rationalised. (1) Solute clusters and GP zones form during the early stages of ageing and share the same structural unit; (2) Formation of β′ phase is accomplished via ordered stacking of zigzag GP zones along 101¯0_α; (3) Coarsening of β′ phase promotes the formation of β_F ^’ phase and tail-like hybrid structures; (4) β_F ^’ phase facilitates the nucleation of β₁ phase; and (5) β₁ and β phases maintain a low-energy Σ9 interface and β₁ phase finally transforms to equilibrium β phase in situ. The precipitate evolution at the grain boundaries during ageing were also investigated and grain boundary precipitate free zones were observed in Mg–16.2Gd–0.4Zr alloy aged at 250 °C or above.