ISOTHERMAL FATIGUE OF AN ALUMINIDE-COATED SINGLE-CRYSTAL SUPERALLOY .1.

The isothermal fatigue behavior of a high-activity aluminide-coated si ngle-crystal superalloy was studied in air at test temperatures of 600 degrees C, 800 degrees C, and 1000 degrees C. Tests were performed us ing cylindrical specimens under strain control at similar to 0.25 Hz; total strain ranges from 0.5 to 1.6 pet were investigated. At 600 degr ees C, crack initiation occurred at brittle coating cracks, which led to a significant reduction in fatigue life compared to the uncoated al loy. Fatigue cracks grew from the brittle coating cracks initially in a stage II manner with a subsequent transition to crystallographic sta ge I fatigue. At 800 degrees C and 1000 degrees C, the coating failed quickly by a fatigue process due to the drastic reduction in strength above 750 degrees C, the ductile-brittle transition temperature. These cracks were arrested or slowed by oxidation at the coating-substrate interface and only led to a detriment in life relative to the uncoated material for total strain ranges of 1.2 pet and above 800 degrees C. The presence of the coating was beneficial at 800 degrees C for total strain ranges less than 1.2 pet. No effect of the coating was observed at 1000 degrees C. Crack growth in the substrate at 800 degrees C was similar to 600 degrees C; at 1000 degrees C, greater plasticity and o xidation were observed and cracks grew exclusively in a stage II manne r.