Emergence of a Novel BCT Phase in Eutectic High Entropy Alloys during Laser Powder Bed Fusion

High entropy alloys, composed of a minimum of four principal elements, exhibit exceptional combinations of strength and ductility, along with impressive corrosion resistance and microstructural adaptability. Recent strides in eutectic high-entropy alloys (EHEAs), a subset within this category, reveal multi-phase structures and uniform microstructures in their as-cast state. Traditional metallurgical processing of EHEAs leads to limited improvements in strength and ductility due to the formation of a coarse lamellar structure. Additive manufacturing, particularly laser powder bed fusion (L-PBF), enables the construction of intricate three-dimensional structures by incrementally adding thin layers, overcoming geometric constraints. However, the phase transition process of EHEAs from liquid to as-deposited state is constrained, particularly during the rapid heating and cooling inherent in the L-PBF process. This can induce metastable phases under these non-equilibrium conditions, significantly affecting the properties of the printed EHEA. The implementation of an operando X-ray diffraction device at a synchrotron beamline, harnessing the high brilliance and swift detectors available, serves as the vital bridge to numerical methods. Our pre-alloyed powder, with a nominal composition of Ni₃₀Co₃₀Fe₁₁Cr₁₁Al₁₈, exhibits a lamellar structure consisting of B2 and FCC phases as predicted by thermodynamic phase equilibrium calculations. During the L-PBF process, a novel BCT phase emerges during solidification. Operando X-ray diffraction, employed during L-PBF, provides a real-time measurement of phase transitions and structural evolution in EHEA. This study not only establishes the relationship between processing and phase constitution but also illuminates how kinetics influence morphology and property control.