Intermediate Temperature, Low-Cycle Fatigue Behavior of Coated and Uncoated Nickel Base Superalloys in Air and Corrosive Sulfate Environments

Low temperature hot corrosion effects on the low cycle fatigue (LCF) life and fracture morphology of a Udimet 720 alloy and the effectiveness of a protective coating in minimizing life degradation are reported. Strain controlled, continuous cycling (1 cpm) and 1 h hold (at maximum tensile strain) tests at 1350°F (732°C) in air revealed only small changes in life that could be attributed to creep during the 1 h hold. The presence of a corrosive salt environment, however, resulted in more than an order of magnitude degradation in life. In air and salt, the fracture was initially transgranular, but in salt it quickly changed to an intergranular mode, particularly at cyclic strains above 0.65 percent. These differences are attributed to reductions of 95 and 28 percent in the respective ductility and fatigue strength coefficients in the Coffin-Manson and Basquin relationships between cyclic strain and life. At cyclic strains ≤ ± 0.5 percent, a diffusion aluminide coating was protective in 1350°F (732° C) salt, with lives equivalent to those for the coated alloy in air; but at higher strains, coating failure occurred. Insignificant differences in fatigue life in air were noted for Udimet 720 and Udimet 710 alloys.