Characterisation of precipitates in a Mg-7Gd-5Y-1Nd-0.5Zr alloy aged to peak-ageing plateau

The precipitates in a Mg-7Gd-5Y-1Nd-0.5Zr alloy aged to peak-ageing plateau (aged at 210 °C from 18 h to 180 h) were studied using transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Precipitation at the peak-ageing plateau involves the formation of the β′ phase and a new type of plate-shaped phase precipitated along the (0001)α habit plane. The ellipsoid morphology of β′ was illustrated according to [0001]α and [101¯0]α zone axes observations. The orientation relationship between the β′ precipitates and matrix satisfies [001]β′//[0001]α and (100)β′//(112¯0)α, respectively. The TEM and HRTEM results indicate that the β′ precipitates have a base-centred orthorhombic structure with lattice parameters a = 0.64 nm, b = 2.22 nm and c = 0.52 nm. Further, atomic-scaled HAADF-STEM observations suggest a Mg7RE-type and not a Mg15RE-type structure for the β′ phase. The positive thermal stability of the alloy at 210 °C ageing is attributed to the coherent relationship with the matrix and the formation of an interlaced network of the β′ precipitates. Aside from the β′ phase, a large amount of precipitate particles were observed to form on the (0001)α habit plane in the peak-aged samples. The new type of precipitates have a plate shape, of approximately 5 nm wide along the [101¯0]α direction of the matrix and approximately 5 nm long along the [2¯110]α direction. A completely coherent relationship with the matrix can also be observed.