Crossover from ω-phase to α-phase precipitation in bcc Ti-Mo

In Ti-Mo the low-temeprature instability of the bcc lattice leads to precipitation of metastable \ensuremath{\omega} particles. Their coarsening has been found to be controlled by Mo diffusion, because of the low solubility of Mo in the \ensuremath{\omega} phase. Irrespective of the annealing temeperature, the coarsening of \ensuremath{\omega} particles was found to stop when their average diameter reached \ensuremath{\simeq}160 \AA{}. Such a stagnation of the microstructure can be understood as an effect of elastic inhomogeneity of the two-phase system. The shape of the \ensuremath{\omega} particles was determined as a prolate ellipsoid of revolution. The aspect ratio of these coherent particles was constant at 2:1 not only during their coarsening but also in the state of stable particle size. Upon further annealing, the stablization of the two-phase microstructure was destroyed by the nucleation of platelike \ensuremath{\alpha}-Ti particles that grew at the expense of \ensuremath{\omega} phase. The rapid growth of \ensuremath{\alpha} particles during the dissolution of \ensuremath{\omega} particles is explained by a structural transition \ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\alpha}. After the dissolution of the \ensuremath{\omega} phase, the following growth of the semicoherent \ensuremath{\alpha}-Ti particles mainly took place on the incoherent rim of the plates. This led, in contrast to the self-similar coarsening of coherent \ensuremath{\omega} precipitates, to continuously increasing aspect ratios of \ensuremath{\alpha}-Ti particles. Their shape evolution was found consistent with predictions from literature, based on the elastic anisotropy of the crystal lattice.