Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy

Abstract Strengthening precipitate phases in a Mg–Y–Nd based alloy (WE54), aged at 250°C, have been characterised using transmission electron microscopy. Precipitation at 250°C involves formation of three separate metastable phases, {11 2 0} α platelets of an as yet unidentified phase, and the phases designated β′ and β1, preceding formation of the equilibrium phase β. All three phases β′, β1 and β are present in significant fractions in peak-aged samples. The β′ phase has a base-centred orthorhombic structure, with a potential point group of mmm. The β1 phase has an f.c.c. structure (space group Fm 3 m , a=0.74±0.01 nm), which renders it isomorphous with a family of intermetallic compounds of the general form Mg3X, where X represents Nd, Ce, La, Pr, Dy and Sm. The equilibrium phase β has an f.c.c. structure (space group, F 4 3 m , a=2.2±0.1 nm), which makes it isomorphous with Mg5Gd. The formation of β1 phase is shown to generate significant shear strain energy, and a mechanism of shear strain energy accommodation is proposed, involving nucleation in association with β′ phase. With prolonged ageing at 250°C, the β1 phase transforms in situ to the equilibrium β phase.