Subsurface Fracture in High-Cycle Fatigue at Cryogenic Temperatures in Ti-5%al-2.5%Sn Extra Low Interstitial Alloy

Abstract

High-cycle fatigue properties were investigated for Ti-5%Al-2.5%Sn extra low interstitial alloy with a mean a grain size of 80 μm, which had been used for liquid hydrogen turbo-pump of Japanese-built launch vehicle. The fatigue strengths at cryogenic temperatures of 4 K and 77 K do not increase in proportion to increments of the ultimate tensile strength, and come to be lower than that at 293 K around 107 cycles. Observations by optical microscopy and scanning electron microscopy reveal that fatigue cracks initiate in the specimen interior and grow transgranularly, not depending on the test temperature. At the subsurface 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.