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An Investigation into Effect of Post-Joining Heat Treatment on Microstructure and Mechanical Properties of Super Alloy GTD-111 Diffusion Brazed Joint

Shahsavari, Abolfazl | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58023 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba; Pouranvari, Majid
  7. Abstract:
  8. In this study, the transient liquid phase bonding of the nickel-based superalloy GTD-111 was investigated using a 75 μm thick MBF-30 interlayer. Bonding was carried out at 1150 °C for durations of 10, 60, 240, and 480 minutes, and the relationship between microstructural evolution and solidification behavior at the joint region was examined as a function of bonding time. Results indicated that, with increasing bonding time, the eutectic structure composed of γ and Ni₃B phases formed at the joint center gradually diminished, and complete isothermal solidification was achieved at 480 minutes. As bonding time increased and isothermal solidification occurred, the maximum hardness—over 1000 Vickers—attributed to the hard eutectic structure at the joint center, as well as the peak hardness (~600 Vickers) in the diffusion-affected zone due to precipitates, both decreased as these phases dissolved. Additionally, due to the formation of the γ′ phase at the joint, the local hardness drop was mitigated, resulting in a more homogeneous hardness profile. In joints where isothermal solidification was incomplete, the eutectic structure formed during athermal solidification, composed of brittle intermetallic phases, served as a preferential path for crack propagation. These brittle compounds, due to their high hardness and low ductility, facilitated crack growth, which reduced the joint's shear strength to 294 MPa in the sample bonded at 1150 °C for 60 minutes. However, once isothermal solidification was completed at 480 minutes, the shear strength increased to 692 MPa.
    Post-bonding, the effect of homogenization heat treatment at 1150 °C for durations of 4, 8, 12, and 16 hours was studied on the sample bonded at 1150 °C for 480 minutes. It was found that increasing homogenization time promoted the diffusion of Al and Ti from the base metal into the joint region, resulting in the formation of dendritic γ′ precipitates at the joint center due to supersaturation. The high temperature used for homogenization reduced the nucleation rate, allowing the dendritic γ′ precipitates to grow considerably until diffusion fields around them overlapped. In contrast to the dendritic γ′ formed during homogenization, fine cuboidal γ′ precipitates were formed during furnace cooling. These precipitates, enriched in Al and Ti due to elemental diffusion at the joint edges, evolved into spherical shapes due to increased interfacial energy. With longer homogenization durations, the entire joint region was filled with coarse spherical γ′ precipitates, and alloying elements became more uniformly distributed throughout the joint. The hardness of homogenized samples, except for a slight decrease at the joint center due to γ′ coarsening, was around 450 Vickers across the joint. Consequently, 8 hours of homogenization led to the formation of γ′ precipitates with optimal size and volume fraction, enhancing the shear strength to 826 MPa (approximately 9% higher than the bulk material). However, extending the homogenization to 16 hours resulted in γ′ coarsening and a subsequent reduction in shear strength to 566 MPa.
  9. Keywords:
  10. Transient Liquid Phase Boneling (TLPB) ; GTD-111 Superalloy ; Isothermal Solidification ; Mechanical Properties ; Athermal Solidification ; Hard Brazing ; Heat Treatment

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