Continuous observation of cavity growth and coalescence by creep-fatigue tests in SEM

Structural components operating at high temperatures in power plants are subjected to interaction of thermal fatigue and creep which results in creep-fatigue damage. In evaluating the life of those components, it is important to understand microscopic damage evolution under creep-fatigue conditions, In this study, static creep and creep-fatigue tests with tensile hold-time were conducted on a SUS 304 stainless steel by using a high-temperature fatigue testing machine combined with a scanning electron microscope (SEM), and cavity growth and coalescence behaviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observations was made for comparison with theoretical growth models. As a result, it was found that cavities nucleate at random and grow preferentially on grain boundaries in a direction almost normal to the stress axis. Under creep condition, the cavities grew monotonously on grain boundaries while remaining an elliptical shape, On the other hand, under creep-fatigue conditions, the cavities grew due to the effect of the local strain distribution around the grain boundary due to cyclic loading and microcracks of one grain-boundary length were formed hy coalescence of the cavities. Also, cavity nucleation and growth rates under the creep-fatigue condition were more rapid than those under the static creep condition and the constrained cavity growth model coincided well with the experimental data for creep.