The high temperature BCC phase (beta) of titanium undergoes a martensitic transformation to HCP phase (alpha) upon cooling, but can be stabilized at room temperature by alloying with BCC transition metals such as Mo. There exists a metastable composition range within which the alloyed beta phase separates into alpha+beta upon equilibrium cooling but not when rapidly quenched. Compositional partitioning of the stabilizing element in asquenched beta microstructure creates nanoscale precipitates of a new simple hexagonal £s phase, which considerably reduces ductility. These phase transformation reactions have been extensively studied experimentally, yet several significant questions remain: (i) The mechanism by which the alloying element stabilizes the beta phase, thwarts its transformation to £s and how these processes vary as a function of the concentration of the stabilizing element is unclear. (ii) What is the atomistic mechanism responsible for the non-Arrhenius, anomalous diffusion widely observed in experiments, and how does it extend to low temperatures? How does the concentration of the stabilizing elements alter this behavior? There are many other £s forming alloys that such exhibit anomalous diffusion behavior. (iii) A lack of clarity remains on whether £s can transform to alpha -phase in the crystal bulk or if it occurs only at high-energy regions such as grain boundaries. Furthermore, what is the nature of the alpha phase embryo? (iv) Although previous computational results discovered a new o → alpha transformation mechanism in pure Ti with activation energy lower than the classical Silcock pathway, it is at odds with the alpha /beta / o orientation relationship seen in experiments.
展开▼
