THE EFFECT OF GAS NITRIDING ON FATIGUE BEHAVIOR IN TITANIUM-ALLOYS

The fatigue behaviour of gas-nitrided Ti-6Al-4V alloy and Ti-15Mo-5Zr- 3Al alloy has been studied under rotating bending, and the results obt ained were compared with those for annealed or untreated materials on the basis of detailed observations of crack initiation, growth and fra cture surfaces. Nitriding was performed using smooth specimens for 4 h and 15 h at 850-degrees-C in Ti-6Al-4V alloy and for 20 h and 60 h at 750-degrees-C in Ti-15Mo-5Zr-3Al alloy in pure nitrogen gas. The dept hs of the nitrided layers obtained were approximately 25 mum and 65 mu m for the former, and 130 mum and 200 mum for the latter. In Ti-6Al-4V , the fatigue lives of the material nitrided for 4 h were shorter than those of the corresponding annealed material, but the fatigue limit w as increased. However, fatigue strength was reduced by nitriding for 1 5 h. Similar results were obtained in Ti-15Mo-5Zr-3Al: fatigue strengt h was decreased by nitriding for 20 h, and the fatigue lives of the ma terial nitrided for 60 h were shorter than those of the annealed mater ial, but the fatigue limit was slightly increased. The reduction in fa tigue strength of the nitrided materials in both alloys was primarily attributed to premature crack initiation in the nitrided layer. The ro le of the nitrided layer in crack initiation is also discussed in term s of results for pure titanium in a previous report.