In-situ TiCxNy nanoparticle reinforced crack-free CoCrFeNi medium-entropy alloy matrix nanocomposites with high strength and ductility via laser powder bed fusion

Although equiatomic CoCrFeNi medium-entropy alloys (MEAs) show excellent ductility, their strength is not high enough for the use in various engineering applications. It is necessary for their strength to be improved, while maintaining their great ductility. In this study, the in-situ TiCxNy nanoparticle reinforced equiatomic CoCrFeNi MEA matrix nanocomposites were fabricated by laser powder bed fusion (LPBF) technique and the addition of sub-micron TiC particles. Interestingly, the LPBF-fabricated CoCrFeNi MEAs have been found to exhibit both poor ultimate tensile strength (UTS) (-582 MPa) and uniform elongation (delta u) (-11.7 %), attributed to the formation of hot cracks. However, the in-situ formation of TiCxNy nanoparticles could eliminate the formation of hot cracks. The near defect-free nanocomposites with substantial sub-grains were fabricated, and their UTS and delta u were increased by 74.4 % (-1015 MPa) and 92.3 % (-22.5 %), respectively. The microstructure of as-built nanocomposites was mainly composed of FCC phase and TiCxNy nanoparticles, and a few M23C6 nanoparticles were found. The high strength of as-built nanocomposites was mainly attributed to Orowan and dislocation strengthening mechanisms. To the best of our knowledge, this is the first study to introduce high strength and great ductility into CoCrFeNi MEAs via the in-situ formation of TiCxNy nanoparticles with the optimized LPBF process.