GH4169合金高温疲劳裂纹扩展的微观损伤机制

Microscopic damage mechanisms during fatigue crack propagation at high temperature in GH4169 superalloy

空气环境对高温合金在高温下的损伤行为有显著影响.为了研究标准热处理态GH4169合金在高温疲劳裂纹扩展过程中的微观损伤机制,在空气环境中进行650℃、初始应力强度因子幅ΔK=30 MPa·m^（1/2）和应力比R=0.05的低周疲劳裂纹扩展试验.使用扫描电镜（SEM）及能谱（EDS）对试样的断口、外表面和剖面进行观察和分析.实验结果表明：疲劳主裂纹以沿晶方式萌生并扩展,随后沿晶二次裂纹出现,并且其数量和长度沿主裂纹方向逐渐增加,进入快速扩展阶段后,断口呈现韧窝组织形貌;在裂纹扩展过程中,δ相与基体的界面发生氧化,使得沿晶二次裂纹沿界面扩展并产生偏折,从而起到阻碍二次裂纹扩展的作用;试样外表面的主裂纹周围出现晶界氧化损伤区,其尺寸和晶界开裂程度沿主裂纹扩展方向逐渐增大.

The air environment strongly influences the damage behaviors of superalloys at high temperatures. To investigate the microscopic damage mechanisms during high-temperature fatigue crack growth in standard heat-treated GH4169 superalloys,low-cycle-fatigue crack growth tests were conducted at 650 ℃ with initial stress intensity factor ΔK = 30 MPa·m^（1/2） and stress ratio R = 0. 05 under the air environment. The fracture surface,outside surface,and central sectioned surface of the specimen were observed and analyzed using scanning electron microscopy（ SEM） and energy-dispersive spectrometry（ EDS）. The results show that the main fatigue crack initiates and propagates intergranularly,followed by the appearance of intergranular secondary cracks,whose quantity and length increase along the propagating direction of the main crack. In the rupture stage,a dimpled morphology appears on the fracture surface. Oxidation occurs at the interfaces between δ phases and the matrix during the fatigue crack propagation process,which leads to secondary cracks propagating along the interfaces. This leads to their inflection,which in turn retards their propagation. A grain-boundary oxidation damage zone exists at the outside surface of the specimen near the main crack. The size and degree of grain-boundary cracking increase along the propagating direction of the main crack.