MECHANISM OF OMEGA-]ALPHA TRANSFORMATION IN BETA-TI ALLOY

The mechanism of the omega-->alpha 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 x 10(6) s and subsequent softening. At 3 x 10(5) s to 1 X 10(6) s the omega particle abruptly grew so far as about 100 nm by combining each other. During further aging, however, the change in omega size was not observed. A part of the large scale omega was transformed to the alpha phase after aging 1 X 10(6) s. The orientation relation between the omega and alpha phases was revealed to be (1120BAR)omega//(0001)alpha and [0001]omega//[1120BAR]alpha by the image of HR-TEM and model simulation. The alpha phases formed according to the orientation relation obeyed the well-known Burgers relation. However, the growth direction of precipitate was not [0001]alpha. For isothermal aging at 823 K for 1 x 10(5) s, needle-like precipitates of Type 1alpha and Type 2alpha were observed. It was elucidated using micro-beam diffraction technique for the individual precipitates that both types of alpha phases obeyed the Burgers relation, and that the difference between Type 1alpha and Type 2alpha was merely in the morphology of growth direction of the precipitates. It was suggested that Type 1alpha was directly formed from the beta matrix through the transformation mechanism dominated by thermal diffusion, while Type 2alpha was formed via omega from beta.