Abstract 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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