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

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 stress threshold was identified, beyond which fatigue life is significantly reduced at both room temperature (RT) and 550 °C, with threshold stresses of 645 MPa and 450 MPa, 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 stress 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 increased short crack growth rate within the ORL. • Exposure of Ti6242s bimodal specimens to 600 °C air for 3000 h led to oxidation and formation of a 100 µm oxygen-rich layer. • A stress threshold above which strong reduction of fatigue life occurs is identified for pre-oxidized specimens. • The life debt induced by pre-oxidation is mainly attributed to a sharp reduction in short crack life above the load threshold. • Fast faceted short crack growth occurred in the oxygen-rich layer, inducing a crescent-shaped crack propagation front on the fracture surface of pre-oxidized samples. • TEM observations suggest that the embrittlement load threshold corresponds to the transition from a plasticity-driven to an elastic crack initiation mechanism.