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

The morphology and crystallography of α(hcp) precipitates formed in the α(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, a 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, ((110)_β//(0001)_α, [111]_β//[1120]_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 ((011)_β//(1101)_α, [111]_β//[1120]_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.