Influence of the TiO ₂ Oxide Layer Phase Composition on the Fatigue Behavior of Heat‐Resistant Titanium Alloys

ABSTRACT This study investigates the effect of long‐term exposure (625°C, 1000 h) on the fatigue resistance of VT18U, VT8, and VT25U titanium alloy sheet stock. X‐ray diffraction analysis revealed that rutile dominated the TiO 2 scale on all alloys, whereas VT25U contained a significantly higher anatase fraction (≈32%) than the oxide on VT18U (≈17%) and VT8 (≈16%). Mechanical testing, XRD, fractography, and microstructural analysis indicate that the oxide phase composition along with the oxide thickness alone controls the time to fatigue crack initiation. The results suggest that rutile‐rich oxides, which have a higher elastic modulus than mixed anatase–rutile oxides, reach their critical strain earlier and initiate a crack at the stress concentrator. These findings show that the control of anatase/rutile ratio can mitigate oxidation‐induced fatigue degradation and expand the safe temperature–time exposure limits for high‐temperature service of titanium alloys.