Microstructural characterisation of as cast and homogenised Mg–Gd–Nd–Zr alloys

Abstract

Microstructure of as cast and homogenised Mg–xGd–2Nd–0˙5Zr (x=6, 8, 11, wt-%) alloys has been investigated by using of optical microscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. It was observed that all the as cast alloys contained primary α-Mg solid solution, eutectic structures, cuboid shaped phases, rodlike precipitates and Zr rich clusters. The eutectic phase in Mg–11Gd–2Nd–0˙5Zr was characterised to be of Mg₅Gd prototype with stoichiometry near Mg₅(Nd _x Gd_1–x , x≈0˙43). Microstructural evolution of the alloys during homogenisation can be divided into three coinstantaneous processes as follows: dissolving of eutectic phases, growth of cuboid shaped phases and coarsening of grains. After appropriate homogenisation, the alloys exhibit microstructure with little eutectic phases, small amount of cuboid shaped phases and relatively fine grains. The cuboid shaped phase with a face centred cubic crystal structure (a≈0˙56 nm) was identified to be a GdH₂ type compound.