Oxidation-induced embrittlement of Ti6242S alloy under fatigue loading at room temperature and 550°C

Abstract

The effect of pre-oxidation at 600 ◦C for 3000 h on the fatigue behavior of a forged bimodal Ti6242S alloy was investigated through fatigue lifetime analysis. An embrittlement threshold stress was identified, beyond which fatigue life is significantly reduced at both room temperature (RT) and 550 ◦C, with threshold stresses of 645MPa and 450MPa, respectively. Below this threshold at RT, pre-oxidation does not influence fatigue life, whereas at high temperatures, lifetimes are systematically reduced by at least an order of magnitude compared to non-oxidized references. The formation of a 100 μm oxygen-rich layer (ORL) modifies crack front geometry from elliptic to crescent-shaped and alters crack initiation mechanisms, leading to faceted crack initiation around the specimen diameter and within the ORL. Fractographic analysis shows that propagation follows a two-stage process, with short faceted crack growth occurring within the ORL before transitioning to long-crack propagation with striations. Despite these changes, pre-oxidation does not affect the long crack growth rate or the stress intensity factor at fracture. The embrittlement threshold is therefore discussed in relation to the accelerated propagation of short faceted cracks in the ORL. TEM observations further suggest that the reduction in fatigue life may be linked to a shift in crack initiation mechanisms and enhanced short-crack growth rate within the ORL.

Keywords: Titanium alloys, Ti6242S, Fatigue, Oxygen-embrittlement, crack initiation, Crack propagation