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

Strengthening precipitate phases in a Mg-Y-Nd based alloy (WE54), aged at 250 degrees C, have been characterised using transmission electron microscopy. Precipitation at 250 degrees C involves formation of three separate metastable phases, {11 (2) over bar 0}(alpha) platelets of an as yet unidentified phase, and the phases designated beta' and pl, preceding formation of the equilibrium phase beta. All three phases beta', beta(1) and beta are present in significant fractions in peak-aged samples. The beta' phase has a base-centred orthorhombic structure, with a potential point group of mmm. The beta(1) phase has an f.c.c. structure (space group Fm (3) over bar 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 beta has an f.c.c. structure (space group, F (4) over bar 3m, a = 2.2+/-0.1 nm), which makes it isomorphous with Mg5Gd. The formation of beta(1) phase is shown to generate significant shear strain energy, and a mechanism of shear strain energy accommodation is proposed, involving nucleation in association with beta' phase. With prolonged ageing at 250 degrees C, the beta(1) phase transforms in situ to the equilibrium beta phase. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AN rights reserved.