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