Abstract
In order to clarify the fatigue behavior of Ni-base superalloy Udimet 720 Li, four point bending fatigue tests of plain specimens were carried out at room temperature. Initiation and propagation behavior of major cracks was observed by plastic replica technique. At the extremely early stage of cycling, cracking of TiN particles was generated. The cracked TiN particle always became the starting site of a micro crack. Though some particles embedded in the surface were cracked due to the stress cycling, the generation of cracking TiN particles was stopped before 30% of fatigue life. On the other hand, the growth rate of a small crack can be determined by a term σanl, not by ΔK. The n is a material constant and was 3.6. The relation between the fluctuation of dl/dN due to the microstructure and the small crack growth law was investigated. It was concluded that the dl/dN of microcracks can be evaluated based on the small crack growth law and dl/dN calculated from a smoothed crack growth curve.
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