CONTINUOUS OBSERVATION OF CAVITY GROWTH AND COALESCENCE BY CREEP-FATIGUE TESTS IN SEM

Structural components operating at high temperatures in power plants a re 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 tes ts with tensile hold-time were conducted on a SUS 304 stainless steel by using a high-temperature fatigue testing machine combined with a sc anning electron microscope (SEM), and cavity growth and coalescence be haviors on surface grain boundaries were observed continuously by the SEM. Quantitative analysis of creep cavity growth based on the observa tions was made for comparison with theoretical growth models. As a res ult, it was found that cavities nucleate at random and grow preferenti ally on grain boundaries in a direction almost normal to the stress ax is. Under creep condition, the cavities grew monotonously on grain bou ndaries while remaining an elliptical shape, On the other hand, under creep-fatigue conditions, the cavities grew due to the effect of the l ocal strain distribution around the grain boundary due to cyclic loadi ng and microcracks of one grain-boundary length were formed hy coalesc ence of the cavities. Also, cavity nucleation and growth rates under t he creep-fatigue condition were more rapid than those under the static creep condition and the constrained cavity growth model coincided wel l with the experimental data for creep.