Morphology and crystallography of α precipitates in β Ti-Mo binary alloys

The morphology and crystallography of alpha(hcp) precipitates formed in the beta(bcc) matrix were examined in the Ti-Mo binary alloys with various Mo contents below the currently proposed monotectoid temperature by means of transmission electron microscopy. In the Ti-10, 20 and 30 mass%Mo alloys, alpha precipitates form on beta grain boundaries and grow into the interior of beta grains. An alpha precipitate formed intragranularly is lath-shaped, and has the Burgers orientation relationship, ((1 (1) over bar 0)(beta)//(0001)(alpha), [111]beta//[11 (2) over bar 0](alpha)) with respect to its beta matrix. The habit plane of alpha lath is near {111}(beta). In the Ti-40 mass%Mo alloy, a fine alpha phase distribution, probably due to the prior beta phase separation, was observed below 823 K although non-uniform alpha precipitation on beta grain boundaries and dislocations occurs above 873 K. An alpha precipitate has the near-Potter orientation relationship ((0 (1) over bar 1)(beta)//(1 (1) over bar 01)(alpha), [111](beta)//[11 (2) over bar 0](alpha)), and is plate-shaped with the {013}(beta) habit plane. The validity of the Ti-Mo phase diagram currently proposed is discussed in terms of the coherent phase separation of beta phase and the equilibrium volume fraction of alpha phase. It is concluded that the region where beta phase separation occurs should be present in the much lower temperature range than the currently proposed Ti-Mo binary phase diagram.