Microstructure evolution of Mg–9Gd–2Er–0.4Zr alloy during solid solution treatment

Abstract

Microstructure evolution of the cast Mg-9Gd-2Er-0.4Zr alloy during solid solution treatment at temperature of 460–520 °C for 3–12 h was investigated by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that the grain size and the shape of second phase were obviously changed with time and/or temperature going on. At 460 °C for 3 h, the morphology of the Mg5(GdEr) phase was changed into fragmentized island morphology and the volume faction of the phase decreased. After solution solid treatment at 460 °C for 6 h, the Mg5(GdEr) phase was already completely dissolved, but some cuboid-shaped RE-rich phase precipitated. As the temperature increased, the morphology of the Mg5(GdEr) phase was transformed into the same morphology as that at 460 °C for 6 h. It was suggested that the microstructure evolution of the alloy during the solid solution treatment was concluded as follows: Mg5(GdEr) eutectic phase→Gd/Er atom diffusing into matrix→spheroidic Mg5(GdEr) phase→cuboid-shaped RE-rich phase→grain boundary immigration.