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Research on Influences of Coating on Low Cycle Fatigue Properties of Ni-Base Superalloy René 80

Rahmani, Khosrow | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40257 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Nategh, Saeed
  7. Abstract:
  8. In this thesis, based on the taking-off and landing of aircraft with GE 21-J85 turbine, the low cycle fatigue (LCF) test for superalloy used in first stage blades is considered. The LCF tests were performed on both coated and uncoated Rene 80 which are used in GE 21-J85 turbine at R = (min/max) = 0, strain rate of 2 10-3 s-1 and 760oC, 871oC and 982oC. To produce specimens, an investment casting method was used so that adequate amount of polycrystalline round bars could be obtained. The specimens were all homogenized and solution treated. After the inspection, some specimens were coated with pack cementation method in CODEP-B powder in H2 atmosphere. The coated and uncoated specimens all aged. The coated specimens were qualified by investigation of microstructure, chemical composition, thickness and oxidation. In order to determination of maximum strain for preventing brittle fracture of coating, the coated specimens were strained up to maximum 1% of plastic strain. In this strain, no crack is observed in specimens. Although there was no crack visible in the coating at 1% tensile strain, to verify if any crack nucleates in cyclic strains about 1% in fatigue condition, a number of test were conducted. The LCF experimental results show that in coated specimens while the total cyclic strain is more than 1%, fatigue lifetime of the coated specimens is lower than that of uncoated specimens. TEM investigations have been performed on fractured uncoated specimens to evaluate the microstructures at different temperatures. The, hexagonal network, misfit dislocation, pair dislocations and cutting of γ' were observed at T = 871C, 982C. TEM studies also showed that at 982C stacking fault was observed in γ' particles. SEM studies of coated specimens at lower 1% of cyclic strain show that the nucleation of cracks occurs merely in substrate, but cracks start from the surfaces in uncoated specimens where it depletes from γ' phase as a result of oxidation by high temperature situations. Except for the depleted zones, surface grain boundary oxidation and oxide spikes are also considered as further crack nucleation sites for the uncoated specimens. SEM studies of coated specimens at more 1% of cyclic strain show that coating has got a low compatibility with substrate. In such condition, cracks that nucleate from outer layer and diffusion layer propagate along it while having a sharp tip. This is the result of presence of internal defects and high rate crack propagation in brittle phase leading to entire rupture and shorter LCF life.

  9. Keywords:
  10. Rene 80 Superalloy ; Extension ; Ductile-Brittle Transition Temperature (DBTT) ; CoDEP-B Coating ; Low Cycle Fatigue

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