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The Effect of Heat Treatment on Microstructure and Mechanical Properties of Directionally Solidified Nickel-based Superalloy GTD-111

Fallah, Parastoo | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49978 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Varahram, Naser
  7. Abstract:
  8. The Ni-based superalloy GTD-111 possesses excellent hot corrosion resistance, high stability and strength at elevated temperature; accordingly, the alloy is used in manufacturing of the first stage blades of powerful gas turbines. In this study, effect of withdrawal rate from Bridgman furnace and precipitation hardening on the microstructure and microhardness of this superalloy were investigated. To this end, several experiments were carried out under growth rates of 1, 6 and 10 mm/min using Bridgman process under vacuum. Afterwards, the effects of homogenization treatment, partial dissolution with two different cooling rates (air and water) along with aging on the microstructure were studied. SEM images illustrate that size, number and morphology of the primary γʹprecipitates are not identical at two sides of the samples. Primary γʹprecipitates have larger size and lower density at the bottom of the directionally solidified specimens in comparision to the top of it. Furthermore, primary γʹ precipitates exhibited the butterfly and cubic morphology at the bottom and top of the specimens, respectively. In addition, increasing the growth rate has lowered segregation, the size of γʹ precipitates and dendrite arm spacing. Homogenization process led to dissolution of a portion of primary γʹ precipitates. Partial dissolution also improved the morphology, size and distribution of precipitates. Nucleation and growth of secondary γʹ precipitates occurred at aging process. Higher cooling rates resulted in higher hardness after aging for directionally solidified samples with different withdrawal rates. At the end, the sample with withdrawal rate of 10 mm/min and quenched in water had the desirable microstructure in aspect of size, distribution and morphology of γʹ precipitates and also highest hardness after aging.
  9. Keywords:
  10. Precipitation Hardening ; Hardness Test ; Nickel-Base Superalloy ; Directional Solidification ; Bridgman Directional Solidification ; Gamma-Prime Precipitates

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