CROSSOVER FROM OMEGA-PHASE TO ALPHA-PHASE PRECIPITATION IN BCC TI-MO

In Ti-Mo the low-temperature instability of the bcc lattice leads to precipitation of metastable omega particles. Their coarsening has been found to be controlled by Mo diffusion, because of the law solubility of Mo in the omega phase. Irrespective of the annealing temperature, the coarsening of omega particles was found to stop when their average diameter reached congruent-to 160 angstrom. Such a stagnation of the microstructure can be understood as an effect of elastic inhomogeneity of the two-phase system. The shape of the 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 stabilization of the two-phase microstructure was destroyed by the nucleation of platelike alpha-Ti particles that grew at the expense of omega phase. The rapid growth of alpha particles during the dissolution of omega particles is explained by a structural transition omega --> alpha. After the dissolution of the omega phase, the following growth of the semicoherent alpha-Ti particles mainly took place on the incoherent rim of the plates. This led, in contrast to the self-similar coarsening of coherent omega, precipitates, to continuously increasing aspect ratios of alpha-Ti particles. Their shape evolution was found consistent with predictions from literature, based on the elastic anisotropy of the crystal lattice.