镍基单晶高温合金850 ℃高周疲劳断裂行为

Fracture Behavior of Nickel-Based Single Crystal Superalloy During High-Cycle Fatigue at 850 °C

摘要:

对镍基单晶高温合金在850 ℃的高周疲劳断口特征及损伤机制进行研究。结果表明：在850 ℃时单晶高温合金高周疲劳裂纹一般萌生于试样亚表面或内部的缺陷位置。在应力比R=0.05的条件下，随疲劳载荷降低，高周疲劳寿命逐渐增加。高周疲劳断口主要以八面体滑移机制为主，在大应力低寿命时断口呈单个或多个滑移面特征，部分断口沿垂直小平面起源后再沿{111}滑移面扩展；在低应力高寿命时，断口沿亚表面或内部起源后易沿多个滑移面交替扩展，断口呈多个滑移面特征。通过电子背散射衍射分析和透射电镜分析可知，高周疲劳断口表面存在氧化行为，断口截面由外往内依次为氧化层、畸变层和基体层。其中，氧化层主要成分为Ni、Co氧化物；畸变层内主要为长条状或短棒状的Al、Ta、W氧化物形貌分布；基体层为单晶层。本研究对大量高周疲劳断口进行系统分析得到其裂纹萌生及扩展规律，应力比为0.05更接近涡轮叶片实际服役过程中的振动模态，为涡轮叶片的失效断裂机制奠定了基础。

Abstract:

The high-cycle fatigue fracture characteristics and damage mechanism of nickel-based single crystal superalloys at 850 °C was investigated. The results indicate that high-cycle fatigue cracks in single crystal superalloys generally originate from defect locations on the subsurface or interior of the specimen at 850 °C. Under the condition of stress ratio R=0.05, as the fatigue load decreases, the high-cycle fatigue life gradually increases. The high-cycle fatigue fracture is mainly characterized by octahedral slip mechanism. At high stress and low lifespan, the fracture exhibits single or multiple slip surface features. Some fractures originate along a vertical small plane and then propagate along the {111} slip surface. At low stress and high lifespan, the fracture surface tend to alternate and expand along multiple slip planes after originating from subsurface or internal sources, exhibiting characteristics of multiple slip planes. Through electron backscatter diffraction and transmission electron microscope analysis, there is obvious oxidation behavior on the surface of the high-cycle fatigue fracture, and the fracture section is composed of oxidation layer, distortion layer, and matrix layer from the outside to the inside. Among them, the main components of the oxidation layer are oxides of Ni and Co. The distortion layer is mainly distributed in the form of elongated or short rod-shaped oxides of Al, Ta, and W. The matrix layer is a single crystal layer. Crack initiation and propagation mechanism were obtained by systematical analysis of a large number of high-cycle fatigue fractures. In addition, the stress ratio of 0.05 is closer to the vibration mode of turbine blades during actual service, providing effective guidance for the study of failure and fracture mechanisms of turbine blades.