Generally it is known that the grain boundary cavity nucleation and growth is the main damage for the degradation of austenitic stainless steel to reduce the creep-fatigue life at high temperature.
For such materials, Hong and Nam proposed a model for life prediction under creep-fatigue interaction in terms of the cavity nucleation and growth, after that, Yoon and Nam proposed a modified life prediction model which includes the cavity nucleation factor, P'. P' is introduced to correlate the number of cavities with plastic strain range and the number of cycles, and considered as a material's constant which is independent of experimental conditions but closely related to the distribution of grain boundary carbides. Physically the cavity nucleation factor, P', means the number of nucleated cavities per unit grain boundary area and unit plastic strain range during one cycle, namely the measure of cavity nucleation.
By the way, the the mechanical properties are generally affected by grain size, however in this study it is known that the creep-fatigue life is not affected by grain size and the cavity nucleation factor, P', is not influenced by grain size also. Only to investigate the effect of grain size on creep-fatigue behavior, the linear density of grain boundary carbide(LDC) is controlled constantly by heat-treatment. In two specimens which have same grain size but different LDC's, the life of lower LDC specimen is longer than that of higher LDC specimen. Considering this result, it is identified that the cavity nucleation factor, P', is a material's constant which is independent of grain size but closely dependent on LDC.
This result has very important meaning, because in designing a new alloy, good properties can be obtained by controlling the distribution of carbide on grain boundary.