The effect of gas nitriding on fatigue behaviour in titanium alloys

Abstract

The fatigue behaviour of gas-nitrided Ti6Al4V alloy and Ti15Mo3Al alloy has been studied under rotating bending, and the results obtained were compared with those for annealed or untreated materials on the basis of detailed observations of crack initiation, growth and fracture surfaces. Nitriding was performed using smooth specimens for 4 h and 15 h at 850°C in Ti6Al4V alloy and for 20 h and 60 h at 750°C in Ti15Mo5Zr3Al alloy in pure nitrogen gas. The depths of the nitrided layers obtained were approximately 25 μm and 65μm for the former, and 130 μm and 200 μm 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 was increased. However, fatigue strength was reduced by nitriding for 15 h. Similar results were obtained in Ti15Mo5Zr3Al: fatigue strength was decreased by nitriding for 20 h, and the fatigue lives of the material nitrided for 60 h were shorter than those of the annealed material, but the fatigue limit was slightly increased. The reduction in fatigue strength of the nitrided materials in both alloys was primarily attributed to premature crack initiation in the nitrided layer. The role of the nitrided layer in crack initiation is also discussed in terms of results for pure titanium in a previous report.