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Titanium alloys can suffer from halide-associated stress corrosion crackingat elevated temperatures e.g., in jet engines, where chlorides and Ti-oxidepromote the cracking of water vapour in the gas stream, depositing embrittlingspecies at the crack tip. Here we report, using isotopically-labelledexperiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy arestrongly enriched (>5 at.%) in oxygen from the water vapour, far greater thanthe amounts (0.25 at.%) required to embrittle the material. Surprisingly,relatively little hydrogen (deuterium) is measured, despite careful preparationand analysis. Therefore, we suggest that a combined effect of O and H leads tocracking, with O playing a vital role, since it is well-known to causeembrittlement of the alloy. In contrast it appears that in alpha+beta Tialloys, it may be that H may drain away into the bulk owing to its highsolubility in beta-Ti, rather than being retained in the stress field of thecrack tip. Therefore, whilst hydrides may form on the fracture surface,hydrogen ingress might not be the only plausible mechanism of embrittlement ofthe underlying matrix. This possibility challenges decades of understanding ofstress-corrosion cracking as being related solely to the hydrogen enhancedlocalised plasticity (HELP) mechanism, which explains why H-doped Ti alloys areembrittled. This would change the perspective on stress corrosion embrittlementaway from a focus purely on hydrogen to also consider the ingress of Ooriginating from the water vapour, insights critical for designing corrosionresistant materials.

A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys