Fatigue-creep transition in Hastelloy-X at elevated temperatures under tensile-tensile cyclic loading was studied by in-situ observation employing a SEM servo. The main results were as follows; (1) Fatigue limit was not found at any temperature,since oxide-induced crack closure did not occur in vacuum. (2) In-situ observation showed that surface crack length increased at 700℃ and crack opening displacement increased at 800℃. Thus, fatigue crack initiated and propagated at 700℃, while, creepdeformation accumulated at 800℃. (3) The fatigue life of specimens tested at 800℃ ere decreased under cyclic loading frequency from 30 Hz to 0.3 Hz. under these conditions, the fatigue at 800℃ was dominated by time dependent creep rupture. (4)Fatigue-creep transition occurred from 700℃ to 800℃. The mechanism of fracture at 700℃ was fatigue and that at 800℃ was creep deformation.
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