Precipitate evolution in Mg-7Gd-3Y-1Nd-1Zn-0.5Zr alloy during isothermal ageing at 240 °C
(1. Guobiao (Beijing) Testing & Certification Co., Ltd., Beijing 100088, China;
2. National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials, GRINM Group Co., Ltd., Beijing 100088, China;
3. China United Test & Certification Co., Ltd., Beijing 100088, China;
4. State Key Laboratory of Non-ferrous Metals & Processes, GRIMAT Engineering Institute Co., Ltd., Beijing 101407, China)
2. National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials, GRINM Group Co., Ltd., Beijing 100088, China;
3. China United Test & Certification Co., Ltd., Beijing 100088, China;
4. State Key Laboratory of Non-ferrous Metals & Processes, GRIMAT Engineering Institute Co., Ltd., Beijing 101407, China)
Abstract: The morphology and crystal structure of the precipitates in Mg-7Gd-3Y-1Nd-1Zn-0.5Zr alloy during isothermal ageing at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-？eld scanning transmission electron microscopy (HAADF-STEM). After under-ageing for 2 h, the precipitates in the alloy are ordered solute clusters with rare earth atomic columns exhibiting hexagonal ring structure, zigzag GP zones and β'''' in its early formation. After peak-ageing for 18 h, the precipitates are mainly β'''' and new rod-like β''''p accompanied with β''''. After over-ageing for 100 h, the precipitates are β'''', β1, long-period stacking ordered (LPSO) building block known as γ′ and 14H-LPSO. β'''' has the three-dimensional shape of convex lens with smaller length-to-width ratio viewed along á0001？α than that in the EW75 alloy. The excellent thermal stability of this alloy can be attributed to the γ'''' and 14H-LPSO retarding the growth of β'''' and β1, low diffusion rate of rare earth atoms and physical character of β'''' and β1.
Key words: magnesium alloy; ageing; precipitate; long-period stacking ordered (LPSO) phase