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

The trade-off between strength and ductility is a key issue that has long existed in the field of ultrafine grained materials. To address this issue, this work employs a heavily nitrogen-doped FeMnCoCr high entropy alloy as the model system, and prepares a series of coarse-grained (dgrain ~69.8 μm), fine-grained (dgrain ~3.5 μm) and ultrafine-grained (dgrain ~0.7 μm) materials with different mean grain sizes through different thermomechanical treatments. The mechanical behaviors of the three materials are systematically investigated. The results reveal that the ultrafine-grained material exhibits the most outstanding comprehensive mechanical property, with the yield strength of about 1.8 times that of coarse-grained material, reaching 958 MPa and maintaining the uniform elongation of 35.2%. High-density grain boundaries promote the non-uniform nucleation of precipitates at the grain boundaries, and as the grain size decreases, the volume fraction of precipitates increases. Grain refinement and precipitates significantly contribute to the improvement of yield strength in ultrafine-grained material. The superior work-hardening capacity and ductility of ultrafine-grained material stem from the synergistic effect of multiple deformation mechanisms, including the instantaneous multiplication of dislocations and the initiation of abundant planar-slip dislocations in the early stage of plastic deformation, as well as the twinning-induced plasticity, strengthening from local chemical ordering (LCO) laths and precipitation hardening during subsequent plastic deformation. Among them, the LCO structures driven by interstitial nitrogen atoms dynamically evolve with deformation. This provides continuous strain hardening, thus overcoming the loss of ductility due to the reduction of dislocation mean free path and the twinning limitations imposed by grain refinement. The study provides a new insight for breaking through the constraint between strength and ductility of high-entropy alloys with ultrafine grains.