Crack tip mechanics effects on environment-assisted cracking of beta-titanium alloys in aqueous NaCl

The objective of this research is to understand the effects of crack tip mechanics on environment-assisted cracking (EAC) of α-precipitation hardened β-Ti alloys. Precracked Ti–8V–6Cr–4Mo–4Zr–3Al and Ti–15Mo–3Nb–3Al are prone to severe EAC in aqueous NaCl when stressed under fixed or rising displacement. The latter is more damaging and establishes a lower bound threshold of the stress intensity ( K ) well below K _IC . EAC is intergranular and occurs at fast growth rates (d a /d t , up to 150 μm/s) for five orders of magnitude of loading rate, d K /d t . Cracking, due to hydrogen-environment embrittlement, is rate limited by lattice diffusion of hydrogen. EAC at fast d a /d t or high d K /d t requires process zone embrittlement sites very near to the crack tip or enhanced H transport. Subcritical d a /d t versus K depends on loading format through crack tip strain rate (ε˙_CT ) differences governed by d K /d t , d a /d t and creep. A continuum model approximates the contributions of d K /d t and d a /d t to ε˙_CT and EAC, but it is inconsistent with in situ measurements of crack tip strain. Intergranular cracking is exacerbated by high ε˙_CT that destabilizes the crack tip passive film and enables hydrogen uptake from an electrochemical reaction. EAC is mitigated when ε˙_CT is insufficient in these regards.