采用冷坩埚定向凝固技术制备了定向柱状晶的Ti-47Al-2Cr-2Nb合金铸锭, 在改变抽拉速率的情况下, 所获得的铸锭具有不同片层间距的全片层组织. 通过拉伸性能实验, 测得其室温极限抗拉强度最高达到652 MPa, 伸长率最大达到1.5%, 而高温极限抗拉强度最高达到490 MPa, 伸长率最大达到5.0%. 通过高周疲劳性能实验, 绘制了抽拉速率分别为1.0和1.2 mm/min时定向凝固合金的应力-循环次数(S-N)曲线以及应力比R为0.1时的疲劳极限值. 对比断口形貌分析表明, 室温时试样拉伸断裂方式为脆性断裂, 而经高周疲劳实验后断裂方式为脆性解理断裂; 高温时试样拉伸断裂方式则为大部分脆性断裂与少部分延性断裂并存. 对高周疲劳试样断口的分析表明, 疲劳裂纹在相界面和B₂相附近萌生, 据此基于塑性钝化理论分析了其裂纹扩展机制, 并绘制了高周疲劳裂纹扩展模式图.

TiAl-based alloys have recently received considerable attention as one of the promising candidates for application in aero engine blades by replacing the Ni-based superalloys because of their unique properties, such as high specific strength, high specific stiffness and good oxidation resistance. However, there are some shortcomings limiting the application of TiAl-based alloys, namely, their brittleness and poor processing properties. Nevertheless, aero engine blades usually suffer a variety of cyclic loadings during the period of services, which finally results in fatigue failure. According to statistics, fatigue failure, mainly high cycle fatigue (HCF), occupies almost 80% failure modes of gas turbine blades in aero engines. Consequently, more and more researches about fatigue behavior of blade materials have been done in the last tens of years. However, there are less relevant results about TiAl-based alloys, especially HCF properties. Recently, the advancement of directional solidification (DS) of TiAl-based alloys using cold crucible has revealed that the ductility can be enhanced at room and elevated temperature. For purpose to verify the influence of DS structures on the tensile and HCF properties, TiAl-based alloy in composition with Ti-47Al-2Cr-2Nb (atomic fraction, %) was prepared and evaluated in this work. Directionally solidified Ti-47Al-2Cr-2Nb alloy ingots with different withdrawal rates (1.0, 1.2 and 1.4 mm/min) were prepared by cold crucible method under alter electromagnetic field in a vacuum furnace. Based on these ingots, macro and microstructures have been characterized by methods of digital camera, OM, SEM and XRD. Furthermore, the tensile properties at room and high temperature (800 ℃) as well as HCF properties at room temperature have been measured respectively. So, the relationship between microstructures and mechanical properties of TiAl-based alloy, especially HCF properties, was demonstrated reasonably and mechanism in which HCF cracks propagated was discussed. The results show that the comprehensive mechanical properties of Ti-47Al-2Cr-2Nb alloy can be significantly improved after directionally solidified using cold crucible. The tensile strength reaches 652 MPa at room temperature with the maximum elongation of 1.5%. Meanwhile, the tensile strength at 800 ℃ attains 490 MPa with the elongation of 5.0%. Based on the data of HCF test at room temperature with the stress ratio of 0.1, the equations of stress amplitude-number of cycles to failure (S-N) curve at different withdrawal rates are calculated. The fatigue limits are 300 and 247 MPa with the withdrawal rates of 1.0 and 1.2 mm/min, respectively, namely, with the increase of withdrawal rate, the fatigue fracture resistance decreases. The mode of HCF fracture of directionally solidified Ti-47Al-2Cr-2Nb alloy behaves in brittle cleavage fracture. And micro-cracks which can propagate along and perpendicular to the lamellae at the same time are observed between α₂/g lamellae and around B₂ phases.