Effect of Cd on matrix structure ordering and aging precipitation evolution in a Mg-Gd-Cd solid-solution alloy

Abstract

The formation and evolution of Gd-rich precipitates companying with the matrix structure ordering in a Mg97Gd2Cd1 (at.%) solid-solution alloy aged at 200 °C have been systemically investigated using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The results show that Gd-rich precipitation dynamics during the aging treatment are noticeably affected by a continuous ordering transformation in the matrix. The ordering transformation process involving mainly re-distribution of Cd atoms was revealed to occur in the following way: random super-saturated solid solution (S.S.S.S.) → B19-type ordered domains → D019-type ordered domains. Four Gd(Cd)-rich precipitates, G.P. I zone, G.P. II zone, β′ and β1 phases, have been observed to be formed in sequence to coexist with the various ordered domains. Based on the HAADF-STEM characterization on the aging microstructures at different aging stages, it can be concluded that the Cd-addition and related matrix structure ordering can play significant roles in modifying the early-stage G.P. zone structure, altering the morphology of β′ precipitates and promoting the forming ability of the β1 precipitate.