中文摘要：

研究了热暴露（700℃,10 000 h）对高强度全片层γ-Ti Al合金Ti-44Al-4Nb-4Hf-0.2Si-1B表面缺陷损伤容限的影响,采用扫描电子显微镜研究了热暴露导致γ-Ti Al合金的显微组织变化,并将之与在交变载荷下的表面短裂纹行为和长裂纹扩展行为联系起来.研究发现,在热暴露后,该合金的疲劳强度提高,且长疲劳裂纹启裂门槛值改善,但热暴露导致该合金短裂纹效应的尺寸范围明显增大.采用Kitagawa-Takahashi线图的形式总结和分析了实验结果,分析了热暴露引起的疲劳强化、疲劳失效的非安全短裂纹的尺寸变化以及长裂纹的启裂门槛值的变化,定量确定了热暴露对表面缺陷的损伤容限.长期热暴露所导致的材料内部应力释放、偏聚缓解、缺陷钝化显著影响裂纹尖端的应力状态,更有利于增大长裂纹的启裂抗力并减缓长裂纹的扩展速率.

英文摘要：

The effect of thermal exposure（ 700 ℃ for 10 000 h） on the surface defect tolerance of a high strength fully lamellar g-Ti Al alloy,Ti- 44Al- 4Nb- 4Hf- 0. 2Si- 1B,has been studied. The microstructural evolution due to thermal exposure was detected by using scanning electron microscope（ SEM）,and correlated with the short crack behaviors and the long crack propagation behaviors. It has been found that long-term thermal exposure can result in the increase in the fatigue strength and the threshold of fatigue crack growth resistance,and a significant improvement in the short crack effect size range. The results are interpreted in the framework of the Kitagawa-Takahashi diagram. The exposure-induced fatigue strengthening,the unsafe size range of short cracks,and the fatigue threshold of long cracks were analyzed. The surface defect tolerance was determined quantitatively. The release in the internal stress,the ease in the segeragation,and the passivation in the defects due to long-term thermal exposure can exert a crucial role in the stress state of the crack tip,therefore favoring the enhanced the resistance of crack and thus slowing down the long crack growth rate.