Precipitate evolution in Mg-7Gd-3Y-1Nd-1Zn-0.5Zr alloy during isothermal ageing at 240°C

The morphology and crystal structure of the precipitates in Mg-7Gd-3Y-1Nd-1Zn-0.5Zr alloy during isothermal ageing at 240 degrees C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field 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 beta' in its early formation. After peak-ageing for 18 h, the precipitates are mainly beta' and new rod-like beta(p)' accompanied with beta'. After over-ageing for 100 h, the precipitates are beta', beta(1), long-period stacking ordered (LPSO) building block known as gamma' and 14H-LPSO. beta' has the three-dimensional shape of convex lens with smaller length-to-width ratio viewed along < 0001 >(alpha) than that in the EW75 alloy. The excellent thermal stability of this alloy can be attributed to the gamma' and 14H-LPSO retarding the growth of beta' and beta 1, low diffusion rate of rare earth atoms and physical character of beta' and beta(1).