Laser additive manufacturing of CoCrNi medium entropy alloy composites reinforced by in-situ nanoprecipitations: Microstructure formation and mechanical properties

The equiatomic CoCrNi medium entropy alloy (MEA) with a single-phase face-centered cubic (FCC) structure shows high toughness, but low yield strength, which limits its industrial applications. The incorporation of a finely distributed second phase preferable via an in-situ reaction during solidification would pose an efficient approach to strengthen the CoCrNi MEA. In this study, CoCrNi MEA nanocomposites were fabricated by laser powder bed fusion (LPBF) of the powder mixture, which consists of CoCrNi and Ti-6Al-4 V pre-alloyed powders, as feedstock material. The results showed that the nano-sized, coherent and uniformly distributed L12-Ni3(Ti, Al) precipitates formed in-situ during the LPBF resulting in the fabrication of CoCrNi MEA composites. An increasing addition of Ti-6Al-4 V causes the precipitates to progressively segregate at the grain boundaries, an intensified lattice distortion and dislocation density as well as a morphological transformation from coarse, columnar FFC grains into fine, equiaxed grains. The resulting composites showed strengthening at the expense of ductility. At the optimum addition of 5.0 at.% Ti-6Al-4 V, the resulting composite showed an excellent combination of strength and toughness with yield strength, tensile strength, and fracture elongation of ~917 MPa, ~1141 MPa, and ~ 14.65%, respectively. Compared to the CoCrNi MEA, the yield strength and tensile strength increased by ~262 MPa and 220 MPa, respectively. This work demonstrates that the fabrication of composites via an in-situ reaction leading to the formation of nano-precipitates embedded in the matrix, poses a viable tool to effectively strengthen MEA materials.