| Investigations of fatigue failure mechanism have been carried out to study the contrary fatigue properties of nitrided steel and titanium. In the case of nitrided steel, the starting point of fatigue failure is the surface of specimen. The fatigue limit received nitriding treatment is improved by the increase of surface slip resistance. On the other hand, in the case of nitrided titanium, the hardened compound layer formed on surface by nitriding treatment is broken in brittle manner by the concentrated stress located at the boundary between compound layer and matrix grain. Also, the intensity of this concentrated stress is depended on the slip length of matrix grain and the slip length is decided by the grain size. Consequently, in nitrided titanium Hall-Petch relationship between matrix grain size and fatigue limit remarkably appears. Furthermore, as the hardness of this compound layer is very high(Hv>2000), i.e. low toughness, the brittle fracture of it happens in low stress level and directly results in the overall fracture of specimen. By this reason, the fatigue limit is decreased. |
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