Fatigue strength of ion-nitrided Ti-6Al-4V alloy in high cycle region

摘要

Though various kinds of surface treatment methods were applied to improve the wear resistant properties of Ti-6Al-4V alloy, it has been reported that the brittle surface layer of this alloy deteriorated their fatigue properties. Furthermore, when this alloy is nitrided by gas and salt bath, the fatigue strength of nitrided one decreases as compared with that of non-nitrided one. On the other hand, as the ion-nitriding method is treated in clean environment, the surface of the material could become free from hydrogen and oxygen. There are few reports about the fatigue strength of ion-nitrided titanium alloys. Fatigue tests have been accomplished by rotating bending fatigue testing machine using a partial shallow notch specimen after different temperatures of heat treatment. In addition, the specimen surface has been successively observed by replica method during tests and it seems clear that the fatigue strength is improved by removing the chemical compound surface layer, because this chemical compound layer deteriorates the fatigue strength of nitrided specimen. From the above results, the surface layer would be requested to have the equivalent toughness of the matrix to improve its fatigue strength according to the view point of practical use.