Characterisation of precipitate phases in magnesium alloys using electron microdiffraction

Recent results of the characterisation of the structure, morphology and ori entation of fine-scale, strengthening precipitate phases in selected magnes ium alloys using transmission electron microscopy and microdiffraction are reviewed. The strengthening precipitate phases in Mg-Y-Nd alloys, aged to m aximum hardness at 250 degreesC, have been found to include two metastable precipitate phases beta' and beta (1), and the equilibrium precipitate beta . The beta' phase has a globular form, a base-centred orthorhombic structur e (Potential point group of mmm), and an orientation relationship such that (100)(beta')//(1 (2) over bar 10)(alpha), [001](beta')//[0001](alpha). The beta (1) phase has an f.c.c. structure (space group Fm (3) over barm), and an orientation relationship that may be described by ((1) over bar 12)(bet a1)//(1 (1) over bar 00)(alpha), (100)beta (1)//[0001](alpha), and forms as plates parallel to {1 (1) over bar 00}(alpha). The beta phase has an f.c.c . structure (space group F (4) over bar 3m) and also forms as plates on {1 (1) over bar 00}(alpha), with an orientation relationship with the matrix p hase that is identical to that observed for beta (1) phase. Precipitates in Mg-Al alloys, aged isothermally at 200 degreesC, invariably have the b.c.c . structure of the equilibrium precipitate phase beta (Mg17Al12). Three ori entation relationships have been observed between beta and the matrix phase . Most precipitates have an irrational orientation relationship that approx imates to the Burger's relationship, (001)(beta)//(0001)alpha, [1 (1) over bar1]beta//[2 (1) over bar(1) over bar0](alpha), and a faceted lath morphol ogy with habit plane parallel to (0001)(alpha). A minor fraction of precipi tates posses an orientation relationship that is of the form (1 (1) over ba r0)(beta)//(1 (1) over bar 00)(alpha), [111](beta)//[0001](alpha), and have a prismatic rod morphology. The long axes of these rods are parallel to [0 001](alpha), and their faceted surfaces are parallel to {1 (1) over bar 00} (alpha). A few precipitates are observed to have an orientation relationshi p such that (1 (1) over bar0)(beta) similar to //(1 (1) over bar 00)(alpha) , [11 (5) over bar](beta) similar to //[0001](alpha) and a rod shape, with their long axes apparently inclined with respect to [0001](alpha). (C) 2001 Elsevier Science Ltd. All rights reserved.