PhasetransformationandmicrostructurecontrolofTi2AlNb-basedalloys:Areview

In recent years,the Ti2 AlNb-based alloys are selected as potential alloys for elevated tempe rature 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 mecha nisms of O phase,phase transformation and microstructure control of Ti2 AlNb-based alloys are reviewed.Ti2 AlNb-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 Widmannstatten 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 Ti2 AlNb-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/βgrains 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 Ti2 AlNbbased 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.