快速凝固 Ti-Al-Nb 合金 B2 相形成机制与显微力学性能

Ti-Al-Nb alloys are widely used in the aerospace industry and are promising candidate materials for turbine engines owing to their relatively low density, high specific strength, and good oxidation resistance. Here, the effects of the cooling rate and undercooling on phase constitution, microstructure evolution, B2 phase formation, and micromechanical properties of the rapidly solidified Ti₇₅ - _xAl_xNb₂₅ (x = 22, 45, atomic fraction, %) alloy were investigated. With a decrease in the droplet diameter, the primary B2 phase of Ti₅₃Al₂₂Nb₂₅ alloy transforms from coarse dendrite to equiaxed grain under free fall. For the rapidly solidified Ti₃₀Al₄₅Nb₂₅ alloy droplet, the nucleation and growth of the B2 phase transforms from the center of the γ dendrite to γ-grain boundaries, and the volume fraction of the B2 phase decreases with the droplet diameter. Under the condition of arc melting and vacuum suction casting (VSC), with an increase in the cooling rate, the average diameter of the B2 dendrite of the Ti₅₃Al₂₂Nb₂₅ alloy decreases from 515 to 370 μm. For the Ti₃₀Al₄₅Nb₂₅ alloy, the solidified microstructure changes from irregular (γ + B2) lamellae to regular (γ + B2) lamellar, to acicular (γ + B2) microstructure, and Al segregation is inhibited. The microhardness of Ti₇₅ - _xAl_xNb₂₅ alloy increases with a decrease in the droplet diameter, and the maximum microhardness of each alloy is 11.57 GPa and 7.7 GPa, respectively, which are 64% and 22% higher than that of VSC, respectively, thereby indicating that the coupled effect of a large cooling rate and high undercooling can effectively enhance the microhardness of the Ti-Al-Nb alloy.