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℃. 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℃ 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.
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