Unraveling the temperature-dependent strengthening and deformation mechanisms in a Co25Cr25Mn22Fe18Ni9.4N0.6 high-entropy alloy

Interstitial doping is widely recognized as an effective route to mitigating the strength–ductility trade-off in high-entropy alloys (HEAs), yet its strengthening and toughening efficacy is highly sensitive to heat-treatment temperature. Accordingly, clarifying temperature-dependent microstructural evolution and its influence on mechanical behavior is crucial. Here, we compare the phase constitution, grain size, dislocation density, and twinning characteristics of a Co25Cr25Mn22Fe18Ni9.4N0.6 HEA after solution treatment at different temperatures. The alloy exhibits a dual-phase microstructure consisting of face-centered cubic (FCC) and body-centered cubic (BCC) phases with a minor fraction of σ phase. At 800 °C, incomplete recrystallization leads to a high dislocation density and pronounced microstructural heterogeneity, resulting in high strength but limited ductility. Solution treatment at 900 °C promotes partial recrystallization and the homogeneous formation of nanoscale BCC precipitates, which activate twinning and establish a balance between strengthening and recovery. At 1000 °C, enhanced atomic diffusion and nitrogen solid-solution strengthening stabilize the FCC matrix and produce a well-coordinated fine-grained–twin microstructure, yielding an excellent strength–ductility synergy with an ultimate tensile strength of 1065 MPa and a total elongation of 35%. Dislocation strengthening, grain-boundary strengthening, and precipitation strengthening are identified as the dominant strengthening mechanisms in the N-HEA. In addition, nitrogen-induced lattice distortion, together with the cooperative interactions among twins, dislocations, and phase boundaries, sustains pronounced work hardening. Our findings deepen the understanding of thermally induced microstructural evolution and deformation mechanisms in nitrogen-interstitial strengthened high-entropy alloys.