High-cyle fatigue properties were investigated for Ti-5%Al-2.5%Sn ELI alloy with a mean α grain size of 80 μm, which had been used for liquid hydrogen turbo-pump of Japanese-built launch vehicle. The 0.2% proof stress and the ultimate tensile strength of this alloy increased with decreasing temperature. The fatigue strengths at cryogenic temperatures of 4K and 77K do not increase in proportion to increments of the ultimate tensile strength, and come to be lower than that at 293K around 10^7 cycles. Observations by optical microscopy and scanning electron microscopy reveal that fatigue cracks initiate in the specimen interior and grow transgranularly at cryogenic temperatures. At the crack initiation sites, several facet-like structures are formed at cryogenic temperatures, while there are no facet-like structures at 293 K. Since localized deformation occurs at cryogenic temperatures, the subsurface cracks form facet-like structures and are supposed to initiate in the early stage of the fatigue life. As the result, the fatigue strength deteriorates at cryogenic temperatures.
