Characterisation of precipitate phases in magnesium alloys using electron microdiffraction

Recent results of the characterisation of the structure, morphology and orientation of fine-scale, strengthening precipitate phases in selected magnesium alloys using transmission electron microscopy and microdiffraction are reviewed. The strengthening precipitate phases in Mg-Y-Nd alloys, aged to maximum 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 structure (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)(beta1)//(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 phase 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 orientation relationships have been observed between beta and the matrix phase. Most precipitates have an irrational orientation relationship that approximates 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 morphology with habit plane parallel to (0001)(alpha). A minor fraction of precipitates posses an orientation relationship that is of the form (1 (1) over bar0)(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 [0001](alpha), and their faceted surfaces are parallel to {1 (1) over bar 00}(alpha). A few precipitates are observed to have an orientation relationship 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.