Nickel-based superalloys represent the current state-of-the-art for many high-temperature, nonnuclear, power-generation applications. However, these superalloys have not been tested in creep at the combination of high temperatures and very long service times anticipated in space nuclear power generation. A baseline Brayton power-generation cycle would employ a superalloy impeller having significant centrifugal stresses at temperatures up to 1200 K for durations exceeding several years in a working environment of a helium-xenon inert gas mixture (refs. 1 and 2). Designers of this impeller need to know the creep resistance of potential impeller materials at realistic temperatures, stresses, and environments.
MAR-M 247LC is a representative of the cast superalloys currently used in impellers and rotors where the hub and blades are cast as a single unit, and was selected for the present evaluations at the NASA Glenn Research Center. Most creep tests were performed in air using conventional, uniaxial-lever-arm constant-load creep frames with resistance-heating furnaces and shoulder-mounted extensometers. However, two tests were run in a specialized creep-testing machine, where the specimens were sealed within environmental chambers containing inert helium gas of 99.999-percent purity held slightly above atmospheric pressure. All creep tests were performed according to the ASTM E139 standard.
The cast MAR-M 247LC had irregular, very coarse grains with widths near 700 μm and lengths near 800 to 12,000 μm. The grains were often longer in the direction of primary dendrite growth (see the photomicrographs). The microstructure was predominated by about 65 to 70 vol% of Ni3Al-type ordered intermetallic γ′ precipitates in a face-centered cubic γ matrix, with minor MC and M23C6 carbides. The sizes of the γ′ precipitates varied from about 0.4 μm at dendrite cores to 3.0 μm between dendrites, because of dendritic growth within grains.
MAR-M 247LC microstructure. Left: Coarse grain size. Right: Varying γ′ size.
Creep tests in air were designed to determine allowable creep stresses for 980, 1090, and 1200 K that would give 1-percent creep in 10 years of service, a typical goal for this application. This service goal represented a target strain rate of 0.1 percent/year. Creep strain rate to 0.2-percent creep is shown versus stress in the following graph. Stresses of about 475, 150, and 70 MPa were estimated to achieve the target strain rate at 980, 1090, and 1200 K, respectively. Additional creep tests and analyses are necessary, but a preliminary creep analysis using current test results indicates quite good potential for an impeller fabricated of MAR-M 247LC to meet Brayton-cycle requirements, for maximum temperatures to 1200 K (ref. 3).
Creep stress versus strain rate for MAR-M 247LC, showing estimated stresses necessary to achieve a maximum strain rate of 0.1 percent per year.
Tests to estimate the effects of air versus inert environments on creep resistance were also initiated. The results of single tests in air at 1-atm pressure and in helium at slightly above 1 atm at 1090 and 1200 K are compared in the following graphs. Creep progressed as fast or even faster in helium than in air at 1090 and 1200 K. The creep tests in air reasonably approximate response in helium to low creep strain levels near 0.1 percent, but not at high strains. More tests are needed for confirmation, but this suggests that there may be no improvement in creep resistance due to the inert environment (ref. 4).
Comparison of creep response in air versus helium. Top: 1090 K. Bottom: 1200 K.
Find out more about the research of Glenn’s Materials Division:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/
Last updated: December 14, 2007
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