In braze assembly and repair of nickel-base superalloys, the two broad classes of available filler materials include those based in noble metals (e.g., Au, Ag), and those based in Ni with additions of melting point depressants (e.g., B, Si) to make brazing viable. Since the 1970s, employing boron-suppressed Ni-base fillers in conjunction with transient liquid phase (TLP) bonding processes has struck the best available balance between filler material cost and performance. However, TLP bonding relies on complete isothermal solidification to avoid the formation of eutectic microconstituents including brittle borides at the braze centerline, requiring long durations at the holding temperature. Even if complete isothermal solidification is achieved, a diffusion-affected zone including boride particles is frequently encountered, limiting the ductility of the resultant braze.
Multi principal element alloys (MPEAs) have been investigated for the past two decades as structural or functional materials. Flexibility in the MPEA design space to adjust the identity and concentration of constituent elements while retaining a single-phase solidification mechanism renders MPEAs a promising field in which to identify novel brazing filler compositions with inherently appropriate melting points for brazing. This characteristic eliminates the need for eutectic melting-point depressant additions. Therefore, the solidification to a single, ductile FCC solid solution renders isothermal solidification unnecessary to avoid embrittling microconstituents, allowing shorter holding durations. Using a comprehensive physics-based down-selection approach considering stability of the solid solution, melting point, and filler-substrate compatibility, an MPEA composition within the MnFeCoNiCu system was identified as a candidate brazing filler.
Brazing experiments were performed on both Ni-Alloy 600 (solid-solution strengthened) and Ni-Alloy 738LC (high volume fraction gamma-prime precipitate-strengthened). When oxygen introduction is sufficiently controlled during MPEA fabrication, brazes employing the MPEA filler demonstrated enhanced ductility in tension when compared directly with the same alloys brazed with an industrially available boron-suppressed filler as a control case. Furthermore, site-specific synchrotron X-ray diffraction analysis and supplemental microscopy evaluations revealed the disordered FCC phase of the MPEA demonstrated good tolerance to compositional changes and the introduction of additional elements from Ni-alloy 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 evolution of the composition profile as the brazed Ni-alloy component is returned to its high-temperature operating environment.