ISOTHERMAL FATIGUE OF AN ALUMINIDE-COATED SINGLE-CRYSTAL SUPERALLOY .2. 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 temperatur es of 600 degrees C, 800 degrees C, and 1000 degrees C. Three types of precrack were studied: narrow precracks formed at room temperature, w ide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 degrees C was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precr acks because of crack arrest via an oxidation-induced crack closure me chanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retar dation also occurred for narrow room-temperature precracks tested at 8 00 degrees C-in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the spec imen. A significant reduction in fatigue life at 800 degrees C relativ e to non-precracked specimens was observed for wide precracks and elev ated temperature precracks. The presence of precracks bypassed the ini tiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 degrees C.