MORPHOLOGY AND CRYSTALLOGRAPHY OF ALPHA-PRECIPITATE IN BETA-TI-MO BINARY-ALLOYS

The morphology and crystallography of alpha(hcp) precipitates formed i n the beta(bcc) matrix were examined in the Ti-Mo binary alloys with v arious Mo contents below the currently proposed monotectoid temperatur e by means of transmission electron microscopy. In the Ti-10, 20 and 3 0 mass%Mo alloys, alpha precipitates form on beta grain boundaries and grow into the interior of beta grains. An alpha precipitate formed in tragranularly is lath-shaped, and has the Burgers orientation relation ship, ((1 (1) over bar 0)(beta)//(0001)(alpha), [111]beta//[11 (2) ove r bar 0](alpha)) with respect to its beta matrix. The habit plane of a lpha lath is near {111}(beta). In the Ti-40 mass%Mo alloy, a fine alph a 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 pr ecipitate has the near-Potter orientation relationship ((0 (1) over ba r 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 equilibr ium volume fraction of alpha phase. It is concluded that the region wh ere beta phase separation occurs should be present in the much lower t emperature range than the currently proposed Ti-Mo binary phase diagra m.