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Effect of TLP Bonding on Microstructure and Thermal Fatigue Properties of Co-Based FSX-414 Superalloy

Bakhtiari, Reza | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43290 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ekrami, Ali Akbar
  7. Abstract:
  8. In this research, transient liquid phase (TLP) bonding of FSX-414 superalloy was investigated using MBF-80 interlayer. The bonding was performed at different temperatures (1050-1200oC) for various times (1-120min). Also, different gap sizes were studied using the interlayer with various thicknesses (25-100µm). Homogenizing treatment was performed at different temperatures (1175-1225oC) and times (1-6h) for the samples with complete isothermal solidification. The microstructure of the samples were studied using the optical, scanning electron and transmission electron microscopes. Also, XRD, SEM/EDS, SEM/WDS and TEM/EDS analyses were used to analyse the observed phases at the joints. To evaluate the mechanical properties of the joints and their relation with the joints’s microstructures, the microhardness, shear, thermal shock and thermal fatigue tests were performed. Performing the bonding and homogenizing at the standard heat treatment of this superalloy resulted in complete isothermal solidification and shear strength about 93% of the base metal. Except for the 1200oC bonding temperature, there was a good correlation between the results of analytical and numerical models with the results of experiments for the complete isothermal solidification time. In this case, this time was decreased with increasing the bonding temperature up to 1150oC. Some phases, resulted of mixing the melted interlayer and liquefaction at base metal’s grain boundaries, were seen at the diffusion affected zone (DAZ) of the sample bonded at 1200oC. This temperature is higher than the critical temperature of the base metal liquefaction. For the samples bonded at different bonding temperatures with incomplete isothermal solidification, chromium-boride and nickel-boride phases were observed at the bonding zone and nickel-chromium-cobalt borides phases as the needle-like and globular ones were observed at the DAZ. The ISZ’s hardness as well as the shear strength were increased with increasing the bonding temperature from 1050 to 1150oC. But a severe reduction was observed at 1200oC bonding temperature. The time of complete isothermal solidification was increased from 1 to 30 min with increasing the gap size from 25 to 100µm. A good correlation was obtained between these results and the results of analytical models except for 75 and 100µm gap sizes. Also, the ISZ’s hardness and shear strength were decreased with increasing the gap size. For the sample bonded at 1150oC for 5 min and homogenized at 1225oC for 3h, the DAZ’s phases were removed and appropriate compositional and microstructural homogenizing were achieved. Other properties of this sample such as uniform hardness profile, the maximum shear strength (about 96% of the base metal), ductile fracture features at the shear fracture surfaces, the maximum shear strength after thermal shock test (about 98% of the base metal at the same condition), failure at the base metal during the tensile test at 600oC after applying the thermal cycles and the same fracture strength in comparison with the base metal, revealed that the optimum properties of TLP bonding of FSX-414 superalloy were obtained at this condition.

  9. Keywords:
  10. Thermal Fatigue ; Microstructure ; Transient Liquid Phase (TLP) ; FSX-414 Superalloy

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