Effect of oxidation on the competition between internal and external fatigue crack initiation of Ni-based single crystal superalloy

Whether surface oxidation damage can shift the crack initiation location of Ni-based single crystal superalloys from internal defects to the surface in the VHCF regime remains an open question. This study examined two experimental conditions with pre-oxidation or high oxidation rates: fatigue tests at 1000 degrees C after pre-oxidation at 1100 degrees C for 100 h, and at 1050 degrees C on unoxidized specimens. The results indicate that both severe pre-oxidation and elevated oxidation rates lead to surface crack initiation. In pre-oxidized specimens, surface recrystallization occurs early, triggering surface cracking and reducing fatigue strength at 107 cycles, though its effect at 109 cycles is minimal. At 1050 degrees C, the fatigue strength decreases markedly compared with 1000 degrees C, and cracks consistently initiate at the surface and propagation in Mode I. Thermal growth stress of oxides near the crack tip alters the local stress state, promoting gamma' rafting. Deformation accumulated at gamma/gamma' interfaces, coupled with gamma' rafting and low-angle grain boundaries, accelerates aluminum diffusion, forming Al2O3 as the dominant oxide. Additionally, oxidation lowers the effective stress intensity factor, affecting crack propagation. Overall, these findings confirm that enhancing oxidation resistance is still critical for improving VHCF performance in Ni-based single crystal superalloys.