Plate Preparation, Microstructure And Mechanical Properties Of Low-Density Ti₂ AlNb-Based Alloys

Intermetallic alloys based on the ordered orthorhombic phase (O phase) Ti₂AlNb are continually developing as attractive materials for aerospace applications due to their desirable properties like a unique combination of strength and ductility, moderate oxidation and good creep resistance. They were developed to replace nickel based alloys for high temperature application, i.e. jet engine parts or valves. However, the poor workability due to the coarse grain size and higher density compared with TiAl and Ti₃Al because of the high Nb content serevely limits the engineering application of Ti₂AlNb-based alloys.A novel Ti-22Al-23(Nb, Mo, V, Si) intermetallic alloy with high specific strength was produced by induction skull melting (ISM), and subsequent hot isostatic pressing (HIP). Rather low density of 5.01 g/cm³ in Ti₂AlNb-based intermetallic alloy was achieved by partial substituting of niobium with such strongβ-stabilizing elements, as molybdenum and vanadium. Special thermomechanical processing, which included multi-step isothermal forging, hot rolling, and intermediate annealing, was developed in order to refine initial coase-grained microstructure and improve both room temperature strength and ductility of intermetallic. Microstructure evolution during thermomechanical processing and corresponding mechanical properties of material were analyzed.The as-cast alloy with an average grain size of about 500μm was characterized with O, B2 andα₂. Thick O-laths distributed in the B2 matrix andα₂ mainly exited in the original B2 boundaries. Theσ_b is 966MPa and 484MPa respectively at room temperature and 650℃, but the ductility is poor. Initial coase-grained microstructure was siginificantly refined by appropriate thermomechanical processing, and theα₂ phase in a small volume percentage, which became much round or short rod-like granular, disperses in the matrix consists of very fine O-laths and B2 phase. Theσ_b is 1524MPa and 766MPa respectively at room temperature and 650℃. Especially, the elongation reaches to 19% at 700℃,which indicates excellent high-temperature plastic deformation capacity and provides good materials for hot rolling. By using hot pack rolling, the average grain size was refined to 1.5μm, and the Ti₂AlNb-based alloy sheets with a size of 464×180×1.3mm³ were prepared. At room temperature,σ_0.2,σ_b, andδis 1062MPa, 1213MPa, and 5.51% respectively. At 650℃,σ_0.2,σ_b, andδis 767MPa, 883MPa, and 12% respectively. The Ti₂AlNb-based alloy sheets show a considerable comprehensive mechanical performance.The hot pack rolled Ti₂AlNb-based alloy sheets exhibited excellent superplastic deformability at tensile conditions of 800-1000℃and 6.25×10^-3s^-1-2×10^-4s^-1. An elongation of 668% is obtained at the optimum deformation condition of 950℃, 4×10^-4s^-1. The superplastic deformation mechanism for the present Ti₂AlNb-based alloy was grain boundary sliding (GBS). In the primary period, the movement of dislocation already exists in the original materials and recrystallization is the main cooperation mechanism of GBS. After the original dislocation is exhausted and deformed microstructure has recrystallized completely, the microstructure consists of equiaxed grains with stable size, and at this time, the movement of dislocation newly created during deformation and diffusion creep is the main cooperation mechanism of GBS.