High-cycle fatigue properties in Ti-5% Al-2.5% Sn ELI alloy with large grain size at cryogenic temperatures

High-cycle fatigue properties were investigated for Ti-5% Al-2.5% Sn ELI alloy with a mean alpha grain size of 80 mum, which had been used for liquid hydrogen turbo-pumps of Japanese-built launch vehicles. At cryogenic temperatures, the fatigue strength in high-cycle region did not increase in proportion to increments of the ultimate tensile strength and the fatigue strengths at around 10(6) cycles were about 300 MPa independent of test temperatures. Fatigue cracks initiated in the specimen interior independent of the test temperatures of 4 K, 77 K and 293 K. At 4 K and 77 K, several crystallographic facet-like structures were formed at crack initiation sites. On the other hand, there were no facet-like structures that could be clearly identified at the crack initiation sites at 293 K. Low fatigue strengths in longer-life region at cryogenic temperatures could be attributable to the formation of large sub-surface crack initiation sites, where large facet-like structure are formed.