Effect of nitrogen on mechanical properties of CoCrFeMnNi high entropy alloy at room and cryogenic temperatures

Specimens of a CoCrFeMnNi high entropy alloy with different content of nitrogen (0.5–2.0 at.%) were prepared by induction melting. Microstructure and tensile mechanical behavior of the alloys in the as-cast condition were analyzed. The alloys with a low amount of N (0.5–1.0 at.%) had a single fcc phase coarse-grained microstructure. An increase in the content of N to 2.0 at.% resulted in the precipitation of a small amount (∼1%) of Cr-rich M2N nitride particles. The yield strength of the alloys increased in proportion to the percentage of nitrogen by 117 MPa/at% N at 293 K or by 316 MPa/at% N at 77 K. The observed increase in strength was attributed to solid solution hardening. Ductility of the alloy with 0.5 or 1.0 at.% of N increased with a decrease in the testing temperature while ductility of the alloy with 2 at.% dropped from 67% at 293 K to 8% at 77 K. The plastic deformation of the alloys at both 77 K or 293 K was mostly associated with planar dislocation slip. No signs of mechanical twinning were found even at 77 K. Thermo-Calc software was used to produce a quasi-binary CoCrFeMnNi–N phase diagram for comparing the experimental and calculated results. The effect of N on strengthening and deformation mechanisms at different temperatures was discussed. •Nitrogen content strongly affected on structure and properties of CoCrFeMnNiNx alloy.•Nitrogen considerably enhanced the solid solution strengthening.•Yield strength increased by 117 MPa/at% N at 293 K or by 316 MPa/at% N at 77 K.•Plastic deformation was accompanied by planar dislocations slip both at 293 K and 77 K.•CoCrFeMnNiNx alloys demonstrated attractive cryogenic properties.