Morphology and Crystallography of α Precipitates in β Ti–Mo Binary Alloys

The morphology and crystallography of α(hcp) precipitates formed in the β(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, α precipitates form on β grain boundaries and grow into the interior of β grains. An α precipitate formed intragranularly is lath-shaped, and has the Burgers orientation relationship, ((1\bar10)β//(0001)α, [111]β//[11\bar20]α) with respect to its β matrix. The habit plane of α lath is near {111}β. In the Ti-40 mass%Mo alloy, a fine α phase distribution, probably due to the prior β phase separation, was observed below 823 K although non-uniform α precipitation on β grain boundaries and dislocations occurs above 873 K. An α precipitate has the near-Potter orientation relationship ((0\bar11)β//(1\bar101)α, [111]β//[11\bar20]α), and is plate-shaped with the {013}β habit plane. The validity of the Ti–Mo phase diagram currently proposed is discussed in terms of the coherent phase separation of β phase and the equilibrium volume fraction of α phase. It is concluded that the region where β phase separation occurs should be present in the much lower temperature range than the currently proposed Ti–Mo binary phase diagram.