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′ → βF′ → β₁ → 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 βF′, but excludes β″. The strain field around the coarsened β′ phase is supposed to stimulate the formation of the βF′ phase. Furthermore, the βF′ phase provides preferential nucleation site for the β₁ phase. The γ-type precipitation sequence is proposed as follows: S.S.S.S. → basal GP zones →  → γ″ → γ. The crystal structures, morphologies, and orientations of the basal GP zone, , 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.