Abstract:The morphology and crystal structure of the precipitates in Mg-7Gd-3Y-1Nd-1Zn-0.5Zr (wt.%) alloy with fine plate-like 14H-LPSO structures aged at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-？eld scanning transmission electron microscopy (HAADF-STEM). Fine plate-like 14H-LPSO structures precipitate after heat treatment at 500 °C for 2 h, and β-type phases precipitate after the alloy is aged at 240 °C. The long-period atomic stacking sequence of 14H-LPSO structures along the [0001]_α direction is ABABCACACACBABA. After being aged at 240 °C for 2 h, the β-type phases are the ordered solution clusters, zig-zag GP zones, and a small number of β′ phases. The peak hardness is obtained at 240 °C for 18 h with a Brinell hardness of 112, the β-type phases are β'''' phases and local RE-rich structures. After being aged at 240 °C for 100 h, the β-type phases are β'''', β₁ and β''''_F phases. β′ phases nucleate from the zig-zag GP zones directly without β″ phases, and then transform into β₁ phase by β''''→β''''_F→β₁ transformations. The Zn not only can form LPSO structure, but also is the constituent element of β₁ phases. LPSO structures have a certain hindrance to the coarsening of β'''' and β₁ along á0001？_α.

Key words: magnesium alloy; ageing; precipitates; long-period stacking ordered (LPSO) structures; HAADF-STEM