Environmentally assisted fatigue crack nucleation in Ti–6Al–2Sn–4Zr–6Mo

Abstract

An unexplained feature was observed at the fatigue crack origin of a number of α/β titanium specimens tested at 450• C in the low cycle fatigue regime. The origin was discoloured blue but this was not a result of temper colouration; this feature sometimes resulted in large reductions in fatigue lives. A number of specimens were examined to determine the cause and formation mechanism of these "blue spots." This feature was associated with elevated oxygen and chloride levels and the presence of sodium. A mechanism based on hot-salt stress-corrosion cracking is proposed and the implications for service components are discussed.

Understanding the “blue spot”

“…This brittle fractographic appearance has formerly been linked to hot-salt stress corrosion cracking in titanium alloys [11,[24][25][26]. Very similar-appearing fractography had been observed by the authors on a known case of stress-corrosion cracking that had occurred later to the ST1 and ST2 tests, which provided recognition through comparison; figure 4b.…”

“…On occasion, a nodular oxidation/ corrosion product was visible on the surface, obscuring underlying fractography -indicative of an environmentally-assisted crack nucleation system [11].…”

“…This gave rise to a "brittle structuresensitive" descriptive, though the cleaved nature of the primary α laths may be considered as "translamellar cleavage" alluded to previously [24][25][26]. For further examples and description refer to [11].…”

“…A number of the concepts presented herein have formerly been aired in [11], whilst this article explores other specimen and spinning rig results and related issues -as presented orally in the Sixth International Conference on Engineering Failure Analysis on 9th July 2014 (Lisbon, Portugal).…”

“…A collaborative PhD project was set-up between Rolls-Royce plc (Materials) and Imperial College London, which partly aimed to further understand the "blue spot" crack initiation mechanism [11]. A number of the concepts presented herein have formerly been aired in [11], whilst this article explores other specimen and spinning rig results and related issues -as presented orally in the Sixth International Conference on Engineering Failure Analysis on 9th July 2014 (Lisbon, Portugal).…”