Fatigue and Corrosion Fatigue of Thermally Oxidized Ti6Al4V Alloy

The current study examines the impact of thermal oxidation on the fatigue strength of Ti6Al4V, tested in both air and at 0.9% NaCl aqueous environment. Thermal oxidation at 600 °C for 5 h produced the rutile oxide layer with approximate thickness of 0.68 μm on the Ti6Al4V surface. Surface mean roughness increased from 0.054 µm in untreated samples to 0.069 µm in oxidized samples. The fatigue limit of the Ti6Al4V alloy tested in air was 750 MPa, while thermal oxidation reduced the fatigue limit by 34% to 500 MPa. In the NaCl solution, the oxidized alloy exhibited the fatigue limit of 550 MPa. The similar fatigue behavior of the oxidized alloy in both air and NaCl solution suggests that the increased stress intensity from cracks in the oxide layer had a more significant effect than corrosion fatigue under the test conditions applied in the present study. The brittleness of the oxide layer enhanced the fatigue nucleation, making it the more critical factor in reducing the alloy's fatigue strength than corrosion fatigue. Additionally, the brittle oxide layer and increased surface roughness were key contributors to the decreased fatigue performance of the oxidized Ti6Al4V alloy.