Initiation and propagation behaviour of microcracks in Ni-base superalloy Udimet 720 Li

In order to clarify the fatigue behaviour of polycrystalline Ni-base superalloy Udimet 720 Li and its statistical characteristics, four point bending fatigue test of plain specimens were carried out at room temperature. Initiation and propagation behaviour of all the cracks in each specimen was observed using the plastic replication technique. At extremely early stages of cycling, cracking of some TiN particles occurred. Fatigue cracks were initiated from these cracked particles. The growth rate of a small crack can be determined by a term sigma(a)(l), not by Delta K, where n is the material constant and is 3.6 for this alloy. Moreover, the relationship between the small crack growth law and fluctuation of growth rate caused by the inhomogeneity of the microstructure was studied. With regard to statistical characteristics of fatigue behaviour, the investigation of initiation lifetime of cracks and crack density was performed. The distribution characteristics of crack length, microcrack propagation life and growth rate of microcracks was examined by assuming a Weibull distribution and log-normal distribution. (C) 1998 Elsevier Science Ltd. All rights reserved.