Study on mechanical factor of grain boundary cavity nucleation under creep-fatigue condition

Creep-fatigue damage at high-temperature on an austenitic stainless steel is characterized by nucleation and growth of creep cavities on grain boundaries. We had carried out continuous observation of the cavity growth process under the creep-fatigue condition by using a high-temperature fatigue machine combined with a scanning electron microscope(SEM) and grasped the cavity growth mechanisms. In this study, in order to clarify a mechanical factor on cavity nucleation under the creep-fatigue condition, creep-fatigue tests under unsymmetrical triangle waveform were conducted on Type304 stainless steel and a cross section of the failure specimens were examined by the SEM. As a result, a volumetric density of damaged grain boundary induced by cavities, which was expressed as a function of time-dependent strain rate, was proposed based on the SEM observation results. The volumetric density of damaged grain boundary under trapezoidal waveform with tensile strain hold could be predicted by the proposal equation with good accuracy.