Strengthening by customizing microstructural complexity in nitrogen interstitial CoCrFeMnNi high-entropy alloys

Interstitial alloying has been proved to be a promising option to improve the mechanical properties in various commercial alloys. Herein, we systematically investigate the microstructure evolution and mechanical property change of CoCrFeMnNi high-entropy alloys (HEAs) with varied nitrogen contents as an interstitial alloying element. To equilibrate the thermal history, all the alloys are heat-treated as follows: homogenization (1100 ℃ for 20 h), cold-rolling (reduction ratio of 60%), and subsequent annealing (900 ℃ for 3 min). In N1 alloy (CoCrFeMnNi HEA with 1 at% of nitrogen doping), we could observe fully recrystallized grains with a small amount of Cr2N precipitates. As the nitrogen contents increased to 3 at% (N3 alloy), the recrystallization was significantly retarded by the formation of 3 different types of Cr2N precipitates, leading to having ~60% of non-recrystallized grains. Furthermore, the various precipitates let the alloy have a heterogeneous complex microstructure. With increasing nitrogen contents, the yield strength and ultimate tensile strength can be improved without significant reduction of ductility, which exceeds those of Cantor HEA by nearly a factor of two. The effect of each strengthening mechanism on the improved strength in heterogeneous complex microstructure is systematically discussed. These results are expected to provide a novel guideline on how to effectively control key properties of HEAs by interstitial alloying through tailoring of heterogeneous microstructure as well as the inherent complexity of HEAs.