Microstructure and Phase Evolution in Mg-Gd and Mg-Gd-Nd Alloys With Additions of Zn, Y and Zr

Microstructure and phase evolution in Mg-Gd and Mg-Gd-Nd based alloys with additions of Zn, Zr and Y were analyzed in the as-cast, solution treated and aged conditions. Alloys has been investigated after solution treatment at 540°C for 24hr followed by isothermal aging at 175°C up to 32 days by using of Vickers hardness, optical microscopy, scanning electron microscopy equipped with EDS, X-ray diffraction and transmission electron microscopy. It was found that the as-cast alloys contained primary α-Mg matrix, eutecticlike structures, cuboid-like phases and Zr-rich clusters. The homogenized and quenched alloys contained primary α-Mg solid solution, smaller amount of divorced eutectic compounds, enlarged cuboid-like particles and Zr-rich clusters. The eutectic phase was Mg₅Gd prototype with the composition Mg₅(Gd_xNd_1-x, x≈0.2). The compositions of the cuboid shaped particles are characterized by enlarged amount of Gd and can be written as Mg₂(Gd _x Y_1-x) with x≈0.85 in the Mg-5Gd based alloy, and Gd₄(Y_xNd_1-x) with x≈0.5 in the Mg-6Gd-3.7Nd based alloy. The cuboid shaped particles grew during aging and reached ~3µm average size. Precipitation of ß″ and ß′ phases during aging was observed. Mg-6Gd-3.7Nd based alloy reached a maximum value of microhardness after 16 days of aging; in Mg-Gd based alloy, microhardness increased more slowly and reached a maximum value after 32 days of aging.