铸造镍基高温合金K445的热疲劳行为

铸造镍基高温合金K445的热疲劳行为THERMAL FATIGUE BEHAVIOR OF NICKEL BASE SUPERALLOY K445

利用开有V形缺口的平板试样，研究了一种新型铸造高温合金K445在最高温度分别为800、850、900℃，最低温度为室温的热循环下的热疲劳行为. 通过光学显微镜和扫描电镜观察合金的组织和热疲劳裂纹形貌，研究热疲劳损伤机制. 结果表明，热疲劳主裂纹主要从V形缺口处萌生，沿晶界扩展，而二次裂纹则穿晶扩展. 当最高循环温度为800℃时，碳化物的组成和分布起主要作用，(Ti, Ta)C的开裂处以及碳化物与基体分离的界面处是裂纹优先形成区域. 当最高循环温度为900℃时，高温氧化的作用占主导地位. 应力辅助作用下的晶界氧脆是此时的主要损伤机制.

A study of thermal fatigue (TF) behavior of cast nickel-base superalloy K445 was conducted between the temperatures ranging from 800℃ to 900℃ and RT, using plate specimens notched to induce crack initiation. Conventional optical microscopy (OM) and scanning electron microscopy (SEM) examinations were conducted to investigate the damage mechanisms of thermal fatigue. Almost all the primary fatigue cracks at elevated temperatures propagate intergranularly and the secondary crack is transgranular. At 800℃, coarse MC carbides either at grain boundaries or within the grain interior, as well as decohesion of MC/matrix interface, are the preferential sites of crack initiation and propagation. At 900℃, crack propagation is an oxidation-dominant and carbide-assisted process. Stress assisted grain boundary oxygen (SAGBO) embrittlement is the principle mechanism of the environmental degradation to the alloy at the crack-tip region.

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