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Development of High Entropy Filler Alloy for Brazing of Precipitation Hardened Ni-Base Superalloy IN-738

Amirhossein Sajjadi | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58083 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Tavakoli, Rouhollah
  7. Abstract:
  8. Brazing is a crucial process for repairing and manufacturing high-temperature components made of precipitation-hardened superalloys. Boron-containing filler metals are amongst the most common filler metals used for repairing and joining superalloys. Although boron is an effective MPD in such filler metals, the formation of brittle boride intermetallics in the joining area (Athermal solidification zone and diffusion affected zone), due to the low solubility of boron in the matrix of superalloys, is inevitable. Isothermal Solidification is known as an effective technique for removing boride compounds in the joining area of brazed components. but as the time of isothermal solidification increases with the square of the joining gap, the completion of Isothermal solidification requires a long period of time in wide gap brazing. Hence, reaching boride-free joints are impossible in the wide gap brazing of superalloys. therefore, the development of boron-free filler metals is of great importance for the wide gap brazing of superalloys. Thanks to Unique thermodynamic properties of High Entropy alloys (effect of configurational entropy on the minimization of Gibbs free energy of mixing), these alloys can be used as boride-free filler metals in wide gap brazing of superalloys. In the current study, a high entropy filler metal is developed for wide gap brazing of precipitation-hardened nickel-base superalloy IN-738. The key parameters considered for high entropy filler alloy development are liquidus temperature, Entropy of mixing, enthalpy of mixing, wetting, and Intermetallic free microstructure. Based on these parameters a High entropy filler alloy is developed. The characterization analyses such as: XRD, SEM, OM, and DSC are performed to investigate the property and microstructure of developed filler alloy. It has been shown that the high entropy filler alloy meets the requirements of a boron-free filler alloy for the formation of Intermetallic-free joints. It has been shown that by using high entropy filler alloy, in contrast to boron-containing filler alloy, the Athermal solidification zone consists of two dendritic and interdendritic solid solution regions with no intermetallic compounds, which improves the mechanical and corrosion resistance properties of the joint. It has been shown that selecting a proper bonding system, which is bonding at 1100 °C temperature and duration of 60 min, can result in a micro-hardness profile similar to that of base metal (around 380HV) while containing no brittle intermetallic phases which can be seen in bonding system It has been shown that the shear strength of samples bonded at 1120 °C temperature and duration of 2 hours with the subsequent aging at 845 °C for 24 hours was 383 MPa
  9. Keywords:
  10. Brazing Sheets ; Intermetallic Compounds ; Inconel 738 Superalloy ; Transient Liquid Phase Boneling (TLPB) ; Precipitation-Hardened Superalloy ; High Entropy Filler Alloy ; Hard Brazing

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