Effect of Aging on Crack Initiation and Propagation of Ni-Base Super Alloy 718
Norio KAWAGOISHI, Koji KARIYA, Syogo YOSHIMI, Yuzo NAKAMURA and Kazuhiro MORINO
Abstract:In order to investigate the effect of aging condition on initiation and propagation properties of a fatigue crack, rotating bending fatigue tests were carried out using partially notched specimens of Ni-base supper alloy, Alloy 718, at 500C. Aging conditions selected were the under-aged and the over-aged conditions with nearly the same hardness. Fatigue strength of the over-aged alloy was higher than that of the under-aged one in wide life region till 108 cycles. Moreover, fatigue strength at 500C was larger than that at room temperature in both alloys. Most of fatigue life was occupied by the growth life of a small crack in both alloys. The main reason for higher fatigue strength of the over-aged alloy was an increase in crack growth resistance. Many slip bands were observed on the specimen surface of the over-aged alloy in comparison with results of the under-aged one. This related to the difference in crack growth resistances in both alloys. Key Words:Fatigue, Alloy 718, Aging condition, Elevated temperature, Rotating bending, Crack initiation, Crack propagation