Structural changes of precipitates in an Mg-5at%Gd alloy studied by transmission electron microscopy

Structural changes of precipitates in an Mg-5 at%Gd (Mg₉₅Gd ₅) alloy by aging at 200°C and 250°C have been studied by ordinary high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). In the early stage of precipitation by aging at 200°C up to 5 h, a short-range ordered structure exists in Gd-enriched regions with an about 2 nm size, and nuclei of an Mg₇Gd structure (β′ phase) occur in the short-range ordered structure by aging at 200°C for 5 h. The β′ phase with the Mg₇Gd structure is formed as definite precipitates by aging at 200°C for 10h. The β′ precipitates are joined along the b-axis and form a plate-shape with a thickness of about 20 nm along the a-axis, and lengths of about 200 nm along the b-axis and 200 nm along the c-axis, and the connection of planar precipitates along the three directions, which are parallel to the [210]_m-typed directions of the Mg-matrix, forms a two-dimensional cell structure, by top-aging at 200°C for 100h.