Additively manufactured Co–Cr–Ni multi-principal element alloys with excellent strength and ductility at room and cryogenic temperatures

Abstract

Additive manufacturing (AM) can facilitate the design of novel multi-principal element alloys (MPEAs), with the CoCrNi system gaining prominence for its outstanding mechanical properties. In this study, two non-equiatomic CoCrNi alloys with tailored stacking fault energies (SFE)–a high-SFE (HSFE) and a low-SFE (LSFE) variant–were fabricated using the directed energy deposition (DED) process. An equiatomic CoCrNi was also printed to validate the process parameters. Monotonic tensile tests were conducted at room (298 K) and cryogenic (77 K) temperatures, followed by detailed microstructural characterisation to understand the influence of SFE on underlying deformation mechanisms. The LSFE alloy exhibited a higher yield strength than the HSFE alloy at both temperatures, attributed to its higher Cr content and lower SFE, which increases typical strengthening contributions from solid solution, dislocation forest, and grain boundaries. At 298 K, HSFE alloy deformed mainly by dislocation slip-mediated mechanisms, while LSFE alloy additionally showed twinning, resulting in improved work hardening rate and strength-ductility trade-off. At 77 K, both alloys exhibited significant strength and work hardening rate enhancements. The LSFE alloy achieved tensile strength >1 GPa with ∼45% ductility, while the HSFE alloy maintained high ductility (∼74%) and tensile strength >800 MPa. In both alloys, multiple deformation mechanisms activated at 77 K, including dislocation slip-mediated mechanisms, twinning, and ϵ-martensite formation. These mechanisms contributed to enhanced strain hardening and strength, with LSFE showcasing more pronounced contributions due to its higher Cr content and lower SFE. Overall, two MPEAs fabricated via DED showcase superior strength-ductility combinations over comparable cast or AM alloys.