采用急冷和深过冷快速凝固技术,研究了冷却速率与过冷度对Ti_{75 - x} Al _x Nb₂₅ (x = 22、45,原子分数,%)合金相组成、凝固组织演变、B2相形成机制及显微力学性能的作用。在自由落体条件下,随着液滴直径减小,Ti₅₃Al₂₂Nb₂₅合金凝固液滴中B2相由粗大枝晶向等轴晶转变,Ti₃₀Al₄₅Nb₂₅合金凝固液滴的B2相形核位置由γ-TiAl相晶粒内部向晶界处转移,B2相的体积分数逐渐减小。在电弧熔炼和真空吸铸条件下,随着冷却速率的增加,Ti₅₃Al₂₂Nb₂₅合金B2相枝晶尺寸显著减小,Ti₃₀Al₄₅Nb₂₅合金凝固组织发生了由非规则(γ + B2)相层片→规则(γ + B2)相层片→针状(γ + B2)的转变。自由落体条件下Ti_{75 - x} Al _x Nb₂₅合金显微硬度随液滴直径的减小逐渐增加,显微硬度的最大值分别为11.57和7.7 GPa,分别较吸铸样品增加了64%和22%,表明深过冷耦合大冷速能有效提高合金的显微力学性能。

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 Ti_{75 - x} Al _x Nb₂₅ (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 Ti_{75 - x} Al _x Nb₂₅ 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.