Appendix B. Surface diffusion factorD S and the derivation of Eq.(4)

Free surface diffusion coefficient D S is used only in Eq.(4) to describe the crack-like cavity growth rate via the diffusivity ratio ofδ SD S/δD B=0.001. The condition that determines the low or high stress, as reported in34, is formulised as:
A sensitivity study was conducted at 550 ˚C to examine the effect of theδ SD S/δD Bvalue selection on the cavity growth rate. Fig. B1 shows the calculated relationship between the local normal stress σ nand cavity growth rate by using differentδ SD S/δD Bvalues. Only theδ SD S/δD Bvalues of less than 1 need to be considered for the crack-like cavity growth 34. Within the stress range of 100 to 1000 MPa, the calculated curve withδ SD S/δD B=0.001 (in red) shows the highest cavity growth rate, Fig. B1. The solid curves as predicted using Eq. are not continuous. A modification was made to the function (see Eq.), to provide a continuous curve prediction (the dashed curve). The stress range highlighted by red box indicates the magnitude facilitating the cavity growth, according to Figs. 5 and 6. For the maximum cavitation under creep-fatigue,δ SD S/δD B=0.001 has thus been selected to derive the value of D S.