A Multi-Principal Element Alloy for Braze Joining and Repair of Nickel-Base Superalloys

In braze joining and repair of nickel-base superalloys, popular boron-suppressed filler materials used in transient liquid phase bonding processes often introduce brittle boride phases to the microstructure. Even if complete isothermal solidification is achieved and eutectic borides are avoided, many studies report the formation of borides in a ‘diffusion affected zone’, and the ductility of the resulting brazes is limited. To overcome this challenge, a novel multi-principal element alloy (MPEA) filler material with an inherently appropriate melting point for brazing certain superalloys was developed, requiring no additions of melt-point depressants. The feasibility of using this MPEA filler was demonstrated on both solid-solution strengthened Alloy 600, and gamma-prime strengthened Alloy 738LC. The MPEA composition was designed to solidify to a single, ductile FCC phase, which was experimentally confirmed through high-energy synchrotron x-ray diffraction experiments. Production of the MPEA in a laboratory environment led to the presence of oxide inclusions, highlighting the need for good control of atmospheric elements in manufacturing. The ductility of this microstructure was demonstrated in tensile tests of Alloy 600 brazed with the MPEA filler, which experienced elongation gains of an order of magnitude over brazes of the same alloy that used a conventional boron-suppressed filler. Additionally, the disordered FCC phase of the MPEA demonstrated good tolerance to accommodate composition changes and the introduction of additional elements from superalloy base materials that occur through dilution and interdiffusion during brazing and subsequent superalloy service conditions. The formation of potentially brittle intermetallic phases is suppressed. This characteristic of the MPEA filler is predicted to give rise to a microstructure with enduring ductility despite any evolution that may occur as the brazed superalloy component is returned its high-temperature operating environment.