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 500°C. 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 500°C 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