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

Abstract

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.