ラジカル窒化したAlloy718の引張圧縮荷重下における中高温疲労特性

抄録

Effect of radical nitriding on fatigue strength of Alloy 718 was investigated at room temperature and 500°C under push-pull loading. Nitrided alloy used was with compound layer of 4μm. Fatigue strength was increased at room temperature by nitriding, though there was no or little difference in fatigue strengths between aged alloy and nitrided one at 500°C. All of fracture occurred from specimen surface in both alloys at room temperature. On the other hand, fracture origins were specimen surface at high stress levels and subsurface at low stress levels in both alloys at 500°C. Surface crack in the aged alloy initiated in ductile manner and the one in nitrided alloy was in brittle manner. The increase in fatigue strength at room temperature was mainly caused by the suppression of crack initiation by compound layer. The reasons for the change in fracture origin from specimen surface to subsurface at 500°C were the suppressions of crack initiation by compound layer in the nitrided alloy and the one of propagation of small crack by oxide induced crack closure effect in the aged alloy. In both of aged and nitrided alloys, fatigue strength at 500°C was larger than that at room temperature in high cycle region where subsurface fracture occurred.