Microstructural Evolution and Mechanical Properties of Nickel-Base Superalloy Brazed Joints Using a MPCA Filler

A new multi-principal-component alloy (MPCA) filler metal with the composition Fe₅Co₂₀Ni₂₀Mn₃₅Cu₂₀ was designed for brazing Ni-base Alloy 600 (Ni-Cr-Fe). Thermodynamic calculations, including atomic size difference, mixing entropy and enthalpy, valence electron concentration, and phase diagram calculations were used to optimize the composition of the MPCA filler material, targeting a face-centered cubic (FCC) crystalline structure and a melting point appropriate for brazing. An X-ray diffraction measurement confirmed the presence of an FCC structure in the as-cast MPCA, and differential thermal analysis (DTA) results demonstrated its melting range to be 1080-1150 °C. The MPCA also exhibited mechanical properties worthy of a brazing filler candidate, with a true compressive yield stress of 286 MPa, an ultimate compressive strength of 591 MPa, and a fracture strain of 106 pct. The optimum brazing temperature was determined to be 1200 °C through a wettability test on the Alloy 600 base material, at which the MPCA exhibited a low wetting angle of 14 deg and optimal spreading behavior. The MPCA plate was cold rolled into 300 μm foils for brazing. For the full range of brazing times studied (15 to 120 minutes), no microstructural defects were observed, and electron backscatter diffraction (EBSD) results showed equiaxed grains present in the solidification microstructure of the filler material. Using data from energy-dispersive spectroscopy (EDS), a kinetic analysis was performed for the constituent elements in the MPCA. It was determined that although Mn was the fastest diffusing of the elements that diffused from the MPCA into the Alloy 600, the diffusion coefficients for all of these elements were on the same order of magnitude. This result was indicative of the sluggish diffusion theory associated with MPCAs. The effect of brazing time on the shear strength of the brazed joint was evaluated. A maximum shear strength of 530 MPa was achieved at a brazing time of 90 minutes. As brazing time increased up to 90 minutes, the increasing interdiffusion distance facilitated a stronger metallurgical bond. However, beyond 90 minutes, the formation of brittle Cr₂Mn₃ and CrMn₃ intermetallic compounds at the grain boundaries within the filler foil led to a lower shear strength and brittle fracture in the joint.