Study on the structure evolution of (FeCoNiCu)Nx high-entropy thin film

In this paper, we designed a FeCoNiCu high-entropy system composed of elements with weak nitride-forming tendencies. The (FeCoNiCu)N_x high-entropy films (HEFs) were fabricated using high-power pulsed magnetron sputtering (HPPMS) by adjusting the nitrogen flow rate, and the film structure and mechanical properties were systematically investigated. Upon the introduction of N atoms, the film gradually transitioned from an FCC single-phase to a simple cubic structure, accompanied by an increase in grain size. The simple cubic phase represents a multi-component phase, where N atoms are incorporated into the FeCoNiCu lattice. XPS analysis reveals that all four elements—Fe, Co, Ni, and Cu—combine with N to form weak covalent bonds. As the N content in the film increases, the hardness of the simple cubic (FeCoNiCu)N_x film increased by 33 % compared to the FeCoNiCu alloy film, from 7.9 to 10.5 GPa, which is attributed to the solid solution strengthening effect and phase transition. This work offers new insights into the structural evolution of high-entropy alloy systems composed of weak nitride-forming elements.