Fatigue behaviour has been studied on gas-nitrided smooth specimens of commercial pure titanium, alpha/beta Ti-6Al-4V alloy and beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behaviour of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behaviour depended on the strength of the materials. Fatigue stregnth was increased by nitriding in pure titanium, while it was decreased in Ti-6Al-4V alloy and Ti-15Mo-5Zr-3Al alloy. Based on detailed observations of fatigue crack initiation, growth and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.
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