Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °, 800 °, and 1000 °. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 ° was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrestvia an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °—in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 ° relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.