High-cycle fatigue properties at 4 K, 77 K and 293 K were investigated in forged-INCONEL 718 nickel-based superalloy with a mean gamma () grain size of 25 mm. In the present material, plate-like delta phase precipitated at grain boundaries and niobium (Nb)-enriched MC type carbides precipitated coarsely throughout the specimens. The 0.2% proof stress and the ultimate tensile strength of this alloy increased with decreasing temperature, without decreasing elongation or reduction of area. High-cycle fatigue strengths also increased with decreasing temperature although the fatigue limit at each temperature didn't appear even around 10 7 cycles. Fatigue cracks initiated near the specimen surface and formed faceted structures around crack initiation sites. Fatigue cracks predominantly initiated from coarse Nb-enriched carbides and faceted structures mainly corresponded to these carbides. In lower stress amplitude tests, however, facets were formed through transgranular crack initiation and growth. These kinds of distinctive crack initiation behavior seem to lower the high-cycle fatigue strength below room temperature in the present material.

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High-cycle fatigue properties were investigated for Ti-5%Al-2.5%Sn extra low interstitial alloy with a mean grain size of 80 mm, 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 10 7 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.

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