Phase transformation and microstructure control of Ti₂AlNb-based alloys: A review

In recent years, the Ti₂AlNb-based alloys are selected as potential alloys for elevated temperature applications to replace conventional Ni-based superalloys owing to their good creep resistance and oxidation resistance which are related to the O precipitates. In this paper, the precipitation mechanisms of O phase, phase transformation and microstructure control of Ti₂AlNb-based alloys are reviewed. Ti₂AlNb-based alloys generally consist of B2/β, α₂, and O phase with different morphologies which are derived from the various heat treatment processes, including equiaxed α₂/O particles, bimodal microstructure, and Widmannstätten B2/β + O structures etc. As a newly developed strengthening phase, O precipitates can be precipitated from the B2/β matrix or α₂ phase directly as well as generated by means of peritectoid reaction of α₂ phase and bcc matrix. Microstructural control of the Ti₂AlNb-based alloys can be implemented by refining the original B2/β grain size and regulating the O precipitates. Multidirectional isothermal forging (MIF) and powder metallurgy technique are two effective methods to refine the original B2/β grains and the morphology and size of O precipitates can be regulated by adding alloying components and pre-deformation process. Moreover, the phase diagram as well as coarsening behavior of Ti₂AlNb-based alloys in ageing process is also reviewed. For the further application of these alloys, more emphasis should be paid on the deep interpolation of microstructure-property relationship and the adoption of advanced manufacturing technology.