Solidification mechanisms of a multi-principal-component alloy for filler applications

Abstract

Multi-principal-component alloys (MPCAs) represent an emerging material class. These alloys have garnered substantial attention for their tendencies to retain solid solution crystal structures at high temperatures, and to potentially experience sluggish diffusion. To date, the study of MPCAs has been focused on their development as structural or refractory materials. However, the same proposed characteristics of MPCAs render them an attractive class for the advent of novel fillers and interlayers used for dissimilar or advanced metallurgical joining. To accelerate the development of MPCA fillers, a comprehensive down-selection approach was developed. This approach employed a three-part process, first targeting appropriate phase selection, then solidification temperature range, and finally assessing filler-substrate interactions. Previous work has demonstrated the feasibility of an MPCA with the nominal composition Mn35Fe5Co20Ni20Cu20 as a filler for brazing Ni-base Alloy 600 at 1200°C. Examining the MnFeCoNiCu system using the down-selection model revealed that further compositional optimization is possible to decrease the brazing temperature by approximately 50°C. The solidification mechanism in Mn35Fe5Co20Ni20Cu20 filler was investigated using in-situ synchrotron radiography and diffraction methods and postmortem characterizations. Diffraction data on laser-melted pure filler revealed that Fe- and Co-rich FCC dendrites emerge from the melt slightly before Mn- and Cu-rich FCC interdendritic material solidifies. Dendrites occupy approximately 30% of the filler volume at the termination of solidification, before coarsening upon cooling to a volume fraction above 50%. These compositions do not represent two phases in equilibrium, as the filler can be readily homogenized through heat treatment at 950°C. Brazing experiments revealed similar dendritic solidification in columnar grains in joints with a fixed clearance. Joints where the clearance was not fixed displayed an equiaxed filler grain morphology with Cu and Mn concentrated at the centerline. Mn was found to diffuse into the Ni-base substrate nearly 100 times slower than its reported diffusion rate in a binary couple with Ni. Preliminary results from in-situ synchrotron experiments on a complete braze setup showed that substantial but incomplete solidification occurs during isothermal holding. Isothermal solidification is driven by element partitioning within the filler, rather than interdiffusion with the substrate.