Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy

The influence of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base precipitation hardened superalloy was studied in air from 427° to 649 °C. The tests were conducted at 100 Hz and at load ratios of 0.1 and 0.5. The threshold ΔK values were found to increase with temperature. This behavior is attributed to oxide deposits that form on the freshly created fracture surfaces which enhance crack closure. As determined from secondary ion mass spectrometry, the oxide thickness was uniform over the crack length and was of the order of the maximum crack tip opening displacement at threshold. Oxidation kinetics were important in thickening the oxide on the fracture surfaces at elevated temperatures, whereas at room temperature, the oxide deposits at near threshold fatigue crack growth rates and at low load ratios were thickened by an oxide fretting mechanism. The effect of fracture surface roughness-induced crack closure on the near threshold fatigue crack growth behavior is also discussed. Formerly with General Electric Company, Advanced Nuclear Technology Operation, Sunnyvale, CA 94086.