ROLE OF ETA PHASE EVOLUTION ON CREEP PROPERTIES OF NICKEL BASE SUPERALLOYS USED IN ADVANCED ELECTRIC POWER GENERATION PLANTS

Advanced fossil energy power generation plants require materials that withstand high temperatures and corrosive environments. One such material that is used in steam turbines is Nimonic 263. It is a nickel-base superalloy that is principally strengthened by gamma primephase (Ni3(Ti, Al)) and has an L12structure. At extended times and at turbine operating temperatures however, eta (Ni3Ti) phase is known to form at the expense of gamma prime. Eta has a complex DO24structure and is the stable phase between 750°C and 900°C, but with slow kinetics of formation. Little is understood about eta phase, and it may negatively impact the strength and creep resistance of Nimonic 263. The hypothesis of this project is that eta phase lowers the steady state creep rate of Nimonic 263. The aim of this project was to study the creep performance and deformation behavior of three related materials to isolate the effects of eta phase on steady state creep behavior in secondary creep regime. The three materials are: Standard commercial Nimonic 263 containing only gamma prime Standard commercial Nimonic 263 that has been heat-treated to contain both gamma primeand eta phases prior to creep testing Modified Michigan Tech alloy based on Nimonic 263 that contains only eta and no gamma prime Based on this improved understanding of creep deformation and failure mechanisms as a function of eta phase, existing creep models were modified to reflect eta phase effects in secondary creep. This modified model will improve life prediction and component design for advanced fossil energy power plant systems

Enhanced diffusion bonding of alloy 617 using electric field-assisted sintering

The development of compact heat exchangers (CHXs) has gained increasing interest in many industries owing to their high thermal efficiency and reduced size. Diffusion bonding (DB) is an advantageous technique for fabricating CHXs. Alloy 617 is a candidate for manufacturing CHXs for high-temperature advanced nuclear reactors due to its elevated-temperature properties. Previous endeavors in DB of Alloy 617 were conducted by hot pressing (HP), which reported precipitates at the diffusion-bond interface, limited grain boundary (GB) migration, and significantly reduced high-temperature mechanical properties. To overcome these challenges, this study investigated DB of Alloy 617 using electric field-assisted sintering (EFAS). Stacks composed of three sheets were bonded with EFAS using different temperatures, pressures, and hold times. DB using HP as the zero-current analog of EFAS was also performed for comparison. The result shows that Cr- and Mo-rich precipitates were formed at the interface of the hot-pressed samples. The electric current and temperature in EFAS play a significant role in precipitation and GB migration. The electric current coupled with correct temperatures can effectively prevent precipitate formation at the interface and achieve excellent GB migration. Nanoscale Al-rich oxide was formed at the interface of the samples made by both HP and EFAS, but grain boundaries can ignore the nanoscale Al-oxide and migrate across the interface. The temperature, pressure, and hold time also affected diffusion. The temperature is a prerequisite for a successful GB migration, and GB migration can be enhanced by increasing pressure and hold time