Enhanced strain-hardening in newly designed Co-free austenitic high entropy alloys with an optimised nitrogen solubility

The effect of nitrogen on the microstructure and mechanical behavior of several Co-free high entropy alloys (HEAs) from the CrFeMnNi family was studied. Alloy design approach, based on thermodynamic computations, was implemented to obtain alloys with a high chromium content, a high nitrogen solubility as well as good austenite stability. An optimisation was made to meet selected criteria which led to three compositions with different nickel contents (Cr20Fe40Mn15Ni25, Cr20Fe44Mn15Ni21, and Cr20Fe47Mn15Ni18) optimised for, respectively, 0.4, 0.5 and 0.6 wt% N. Such optimised alloys were elaborated and compared with another Co-free HEA (Cr14Fe46Mn17Ni23) doped with 0.11–0.29 wt% N. It was shown that up to 0.56 wt% N could be dissolved in the matrixes in agreement with their computed high nitrogen solubility. The differences in chemical composition between the four studied matrixes did not lead to any change in behavior in presence of nitrogen: the lattice parameter expansion and mechanical resistances (yield strength and tensile strength) evolve linearly with nitrogen addition, up to at least 0.56 wt%. Especially, a significant effect of nitrogen content on strain-hardening was observed and was attributed to the formation of nanotwins in nitrogen-rich alloys. •Investigated the impact of nitrogen on the microstructure and mechanical behavior of several Co-free HEAs within the CrFeMnNi family.•Employed a thermodynamic-based approach to design alloys with high Cr content, N solubility, and stable austenite phases.•Up to 0.56 wt.% nitrogen were dissolved in the alloys leading to linear increases in lattice parameter expansion and mechanical resistances.•Observed a significant enhancement in strain-hardening due to the formation of nanotwins in nitrogen-rich alloys.