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

Strengthening precipitate phases in a Mg-Y-Nd based alloy (WE54), aged at 2 50 degrees C, have been characterised using transmission electron microscop y. Precipitation at 250 degrees C involves formation of three separate meta stable phases, {11 (2) over bar 0}(alpha) platelets of an as yet unidentifi ed phase, and the phases designated beta' and pl, preceding formation of th e equilibrium phase beta. All three phases beta', beta(1) and beta are pres ent in significant fractions in peak-aged samples. The beta' phase has a ba se-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 intermetalli c 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 stra in energy, and a mechanism of shear strain energy accommodation is proposed , involving nucleation in association with beta' phase. With prolonged agei ng at 250 degrees C, the beta(1) phase transforms in situ to the equilibriu m beta phase. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AN rights reserved.