The mechanism of the ω→α phase transformation following isothermal aging has been studied on a single crystal of a Ti-14 mass%Mo alloy. For isothermal aging at 623 K, the specimen showed a maximum hardness at 1×10⁶ s and subsequent softening. At 3×10⁵ s to 1×10⁶ s the ω particle abruptly grew so far as about 100 nm by combining each other. During further aging, however, the change in ω size was not observed. A part of the large scale ω was transformed to the α phase after aging 1×10⁶ s. The orientation relation between the ω and α phases was revealed to be (11\bar20)_ω\varparallel(0001)_α and [0001]_ω\varparallel[11\bar20]_α by the image of HR-TEM and model simulation. The α phases formed according to the orientation relation obeyed the well-known Burgers relation. However, the growth direction of precipitate was not [0001]_α.
For isothermal aging at 823 K for 1×10⁵ s, needle-like precipitates of Type 1α and Type 2α were observed. It was elucidated using micro-beam diffraction technique for the individual precipitates that both types of α phases obeyed the Burgers relation, and that the difference between Type 1α and Type 2α was merely in the morphology of growth direction of the precipitates. It was suggested that Type 1α was directly formed from the β matrix through the transformation mechanism dominated by thermal diffusion, while Type 2α was formed via ω from β.