In-Situ Observation of Fatigue-Creep Transition Behaviors in Hastelloy-X

Faligue creep transition in Hastelloy-X at elevated temperatures under tensile-tensile cyclic leading 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°C and crack opening displacement increased at 800°C. Thus, fatigue crack initiated and propagated at 700°C, while, creep deformation accumulated at 800°C. (3) The fatigue life of specimens tested at 800°C ere decreased under cyclic loading frequency from 30 Hz to 0.3 Hz. under these conditions, the fatigue at 800°C was dominated by time dependent creep rupture. (4) Fatigue-creep transition occurred from 700°C to 800°C. The mechanism of fracture at 700°C was fatigue and that at 800°C was creep deformation