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.