Effects of temperature and strain range on fatigue cracking behavior in Hastelloy X

The low cycle fatigue tests of Ni-base superalloy Hastelloy X have been carried out in the temperature range of 650 - 870 °C with various total strain ranges. A change of slope in the Coffin-Manson (C-M) plot was found at 870 °C: the fatigue life significantly decreased at the total strain range less than 0.6%. The fatigue cracks initiated at the surface of the specimens and propagated transgranularly, regardless of test condition. However, the fatigue crack initiation site on the surface shifted from grain interior to grain boundary (GB) predominantly when the discontinuity of slope in the C-M plot began to occur. The fatigue crack tended to initiate preferentially at the oxidized GB rather than grain interior at 870 °C with total strain range below 0.6%. Under this condition, cyclic stability was pronounced, while cyclic hardening occurred at the rest of test conditions. The dislocation structures responsible for the cyclic stress response may partly account for the determination of major fatigue crack initiation site.