Balancing strength and ductility in ultrafine-grained FeMnCoCrN high-entropy alloy

The trade-off between strength and ductility is a longstanding key issue in research on ultrafine-grained materials. To address this issue, we employed a heavily nitrogen-doped FeMnCoCr high entropy alloy as the model system, and prepared a series of coarse-grained (d_grain = ~69.8 μm), fine-grained (d_grain = ~3.5 μm), and ultrafine-grained (d_grain = ~0.7 μm) materials with different mean grain sizes through different thermomechanical treatments. The mechanical behaviors of the three materials were systematically investigated. The results revealed that the ultrafine-grained material exhibited the best comprehensive mechanical properties, maintaining a uniform elongation of 35.2% and reaching a yield strength of 958 MPa which is approximately 1.8 times higher than that of the coarse-grained material. High-density grain boundaries promote nonuniform nucleation of precipitates at the grain boundaries, and the volume fraction of the precipitates increases with decreasing grain size. Grain refinement and precipitates significantly contributed to the improvement in the yield strength of the ultrafine-grained materials. The superior work-hardening capacity and ductility of ultrafine-grained materials arise from the synergistic effect of multiple deformation mechanisms, namely the instantaneous multiplication of dislocations and the initiation of abundant planar-slip dislocations in the early stage of plastic deformation as well as twinning-induced plasticity, strengthening from local chemical ordering (LCO) laths, and precipitation hardening during subsequent plastic deformation. Among these, LCO structures driven by interstitial nitrogen atoms evolve dynamically with deformation. This provides continuous strain hardening, overcoming the loss of ductility due to the reduction in the dislocation mean free path and twinning limitations imposed by grain refinement. This study provides new insights into overcoming the trade-off between the strength and ductility of high-entropy alloys with ultrafine grains.