Effect of nitrogen on microstructure and mechanical properties of the CoCrFeMnNi high-entropy alloy after cold rolling and subsequent annealing

The effect of nitrogen on the structure and mechanical behavior of a CoCrFeMnNi high-entropy alloy after rolling and subsequent annealing was examined. The as-cast alloys doped with 0.5, 1.0, and 2.0 at% N were cold rolled to 80% thickness reduction. Further annealing in the temperature range of 700-1000 degrees C for 1 h has resulted in (i) development of recrystallization in the fcc matrix and (ii) precipitation of the M2N type nitrides in the alloys with 1.0 and 2.0 at% N. The recrystallization onset temperature lowered with an increase in the N content. Besides, the fcc grain size in the alloy with 2.0 at% N was considerably smaller than that in the alloys with a lower nitrogen content due to a strong pinning effect of the nitride particles. The cold-rolled alloys had high strength, but very limited ductility. The recrystallized microstructure has resulted in a much better strength-ductility synergy when tested at 293 K. For example, the alloy with 2.0 at % N after annealing at 900 degrees C had the yield strength of 673 MPa, ultimate tensile strength of 1021 MPa, and ductility of 47%. A decrease in the testing temperature to 77 K has resulted in a considerable increase in strength. For example, the same alloy had the yield strength and ultimate tensile strength of 1097 and 1548 MPa, respectively. The quantitative analysis of the strengthening mechanisms has revealed that (i) grain boundary strengthening provides the highest contribution at 293 K (ii) the increase in strength at 77 K is mostly associated with the lattice friction and interstitial solid solution hardening. (c) 2021 Elsevier B.V. All rights reserved.