STP942

    Creep-Fatigue Behavior of Ni-Co-Cr-Al-Y Coated PWA 1480 Superalloy Single Crystals

    Published: Jan 1988


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    Abstract

    Single-crystal specimens of a Ni-base superalloy, PWA 1480, with a low pressure plasma sprayed Ni-Co-Cr-Al-Y coating, were tested in various 0.1 Hz fatigue and creep-fatigue cycles at 1015 and 1050°C. Creep-fatigue tests of the cp, pc, and cc types were conducted with various constant total strain ranges employing creep dwells at various constant stresses. Considerable cyclic softening occurred as was evidenced particularly by rapidly increasing creep rates in the creep-fatigue tests. The cycle time in the creep-fatigue tests typically decreased by more than 80% at 0.5 Nf.

    Though cyclic life did correlate with Δεin, a better correlation existed with Δσ both the fatigue and creep-fatigue tests, and poor correlations were observed with either σmax or the average cycle time. A model containing both Δσ and Δεin, Nf = αΔεβinΔσγ, with best fit values of α for each cycle type but the same values of β and γ, was found to provide good correlations. Lifelines were not greatly different among the cycle types, differing by only a factor of about three. The cp cycle life line was lowest for both test temperatures; however, among the other three cycle types there was no consistent ranking. For all test types, failure occurred predominately by multiple internal cracking originating at porosity. The strong correlation of life with Δσ may reflect a significant crack growth period in the life of the specimens. The lack of improvement in the models when average cycle time was considered reflects both that there is no large effect of strain rate on the damage mechanisms in the single-crystal material nor any environmental effect due to the internal cracking mode of failure.

    Keywords:

    low-cycle fatigue, creep-fatigue, superalloys, single crystals, fatigue life prediction


    Author Information:

    Miner, RV
    Materials Engineer, NASA-Lewis Research Center, Cleveland, OH

    Gayda, J
    Materials Engineer, NASA-Lewis Research Center, Cleveland, OH

    Hebsur, MG
    National Research Council Research Associate, NASA-Lewis Research Center, Cleveland, OH


    Paper ID: STP24492S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP24492S


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