Crack Progression during Sustained-Peak Low-Cycle Fatigue in Single-Crystal Ni-Base Superalloy René N5

Crack progression during compressive sustained-peak low-cycle fatigue (SPLCF) was examined in vapor phase aluminide coated single-crystal Ni-base superalloy René N5. Strain-controlled tests with a 120-second hold at compression were conducted at 1366 K (1093 °C) with A = –1 (R = –∞) and 0.35 pct total strain range, and were terminated at selected fractions of predicted life. Crack lengths on the surface and crack depth in longitudinal sections were examined for each specimen. All cracks appeared to have initiated at the coating surface. Failed specimens showed that cracks initially grew on (001), perpendicular to the stress axis, and then deflected to other crystallographic planes. Interrupted test specimens showed crevices initiated on the coating surface at less than 10 pct of the predicted life. The depths of crevices into the coating increased with cyclic exposure, but they did not penetrate into the substrate through the interdiffusion zone (IDZ) until about 80 pct of predicted life. Stress relaxation during compressive hold results in residual tension upon unloading. These results suggest that improving creep resistance of the substrate alloy and developing a coating system that can delay crack penetration into the substrate are keys for improved SPLCF life.