Phase Transformation And Superplasticity Deformation Mechanism In Ti₂AlNb-based Alloys

At the basis of an ordered orthorhombic structure, Ti₂AlNb-based alloyshave been of interest for high-temperature structural applications for aviation andaerospace, primarily because of their excellent specific strength and modulus aswell as their creep and thermal fatigue resisitance. Recently, reseachers all overthe world paid lots of attetion to its thermo-mechanical processing andapplications. However, the thermal stability and high-temperature superplastic（HTSP） deformation mechanism of Ti₂AlNb-based alloys remain unclear. In thisstudy, vacuum arc remelting （VAR） and severe plastic deformation （SPD） wereused to obtain intermetallic Ti₂AlNb-based alloy sheets. Phase transformationand microstructure microstructural evolution at the superplasticity deformationwere determined with OM, SEM, XRD and TEM methods to studysuperplasticity deformation mechanism.With temperature increasing, a single O-phase region appeared at thetemperature less than600℃, followed by O+β/B2+α2-Ti3Al phases at800℃to900℃, then β/B2+α2-Ti3Al two-phase zone at950℃to1050℃, and single BCC-phase higher then1100℃.As the heat-treated temperature increased from600℃to900℃, micro-hardness decreased from509HV to342HV, then at900℃to1100℃, the hardness showed an increasing trend.Hot-rolled intermetallic Ti₂AlNb alloy has good superplasticity in thetemperature range875-960℃and at an initial strain rate2×10-4s-1-1.67×10-3s-1. Amaximum elongation of319%at950℃under an initial strain rate of4×10-4s-1was obtained, and a steady state flow stage appeared in the true stress-straincurves. A fractured specimen has two parts: static and dynamic tensile. Both ofthem undergone phase transformation and grain growth of O, α2-Ti3Al and β/B2phase during deformation. In addition, in the superplastic deformation part someother microstructural characteristics accompanied, like α2grains elongating alongthe direction of external force, B2grains equiaxing and cavities nucleating andgrowing at the interface of α2and B2grains.The strain rate sensitivity （m） was0.36-0.5, the activation energy （Q） was243-270kJ/mol, and the grain size factor （p）3at the superplasitcity temperaturerange. For Ti₂AlNb-based alloys, grain boundary slide should be the dominantdeformation mechanism, while lattice dislocations and atom diffusion were cooperated with the main mechanisms. The constitutive equation at the steadysate deformation of Ti2AlNb-based alloys at elevated temperature was given as:...