SYMPOSIA PAPER Published: 01 January 1988
STP24492S

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

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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.

Author Information

Miner, RV
NASA-Lewis Research Center, Cleveland, OH
Gayda, J
NASA-Lewis Research Center, Cleveland, OH
Hebsur, MG
NASA-Lewis Research Center, Cleveland, OH
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Details
Developed by Committee: E08
Pages: 371–384
DOI: 10.1520/STP24492S
ISBN-EB: 978-0-8031-5035-5
ISBN-13: 978-0-8031-0944-5