On the Precipitation in an Ag-Containing Mg-Gd-Zr Alloy

The evolution of precipitates in a high-strength Mg-2.4Gd-0.4Ag-0.1Zr (at. pct) alloy was investigated using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitation of Mg-2.4Gd-0.4Ag-0.1Zr includes β- and γ-type precipitates, the latter involving a hitherto unreported precipitation sequence that is the focus of the present study. The β-type precipitation sequence is described as follows: supersaturated solid solution (S.S.S.S.) → ordered solute clusters → zigzag GP zones → β′ → \( \beta_{F}^{'} \) → β ₁ → β. Compared with the precipitation sequence of the Mg-Gd system, the proposed β-type precipitation sequence includes ordered solute clusters, zigzag GP zones, and \( \beta_{F}^{'} \), but excludes β″. The strain field around the coarsened β′ phase is supposed to stimulate the formation of the \( \beta^{\prime}_{F} \) phase. Furthermore, the \( \beta_{F}^{'} \) phase provides preferential nucleation site for the β ₁ phase. The γ-type precipitation sequence is proposed as follows: S.S.S.S. → basal GP zones → γ′′′ → γ″ → γ. The crystal structures, morphologies, and orientations of the basal GP zone, γ′′′, γ″, γ phases were comprehensively examined and established herein. The results are described in the context of other, but similar, alloy systems. A holistic description of the precipitate evolution in Ag-containing Mg-Gd alloys is discussed and rationalized.