Characterization of β' phase precipitates in an Mg-5 at% Gd alloy aged in a peak hardness condition, studied by high-angle annular detector dark-field scanning transmission electron microscopy

The beta' phase precipitated in a 95 at%Mg-5 at%Gd (Mg95Gd5) alloy aged in a peak hardness condition (at 200 degrees C for 100hrs) is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scaled HAADF-STEM observations of the beta' phase can propose a new structure model with an orthorhombic unit cell of a = 0.64 nm, b = 2.28 nm and c = 0.52 nm and a composition Of Mg7Gd. The beta' precipitates have a plate-shape with an about 40 nm width along the [001](m) of the Mg-matrix and an about 100 nm length along [210](m)-typed directions, and joining of the plate-shaped precipitates establishes a two-dimensional cell structure parallel to the (001)(m) plane of the Mg-matrix.