In this study, (Al_0.5CrFeNiTi_0.25)N_x high-entropy films are prepared by a reactive direct current (DC) magnetron sputtering at different N₂ flow rates on silicon wafers. It is found that the structure of (Al_0.5CrFeNiTi_0.25)N_x high-entropy films is amorphous, with x = 0. It transforms from amorphous to a face-centered-cubic (FCC) structure with the increase of nitrogen content, while the bulk Al_0.5CrFeNiTi_0.25 counterpart prepared by casting features a body-centered-cubic (BCC) phase structure. The phase formation can be explained by the atomic size difference (δ). Lacking nitrogen, δ is approximately 6.4% for the five metal elements, which is relatively large and might form a BCC or ordered-BCC structure, while the metallic elements in this alloy system all have a trend to form nitrides like TiN, CrN, AlN, and FeN. Therefore, nitride components are becoming very similar in size and structure and solve each other easily, thus, an FCC (Al-Cr-Fe-Ni-Ti)N solid solution forms. The calculated value of δ is approximately 23% for this multicomponent nitride solid solution. The (Al_0.5CrFeNiTi_0.25)N_x films achieve a pronounced hardness and a Young’s modulus of 21.45 GPa and 253.8 GPa, respectively, which is obviously much higher than that of the as-cast Al_0.5CrFeNiTi_0.25 bulk alloys.

The HEA-CoCrFeNiAl_0.3 thin film in this study has been successfully developed by radio frequency (RF) magnetron sputtering to meet the increasing demand in engineering applications. Its microstructure and surface profile were investigated accordingly. The as-synthesized HEA film was found to have a homogeneous element distribution and ultra-smooth surface, exhibiting a typical face-centered cubic (FCC) solid solution. The film showed better mechanical properties than its bulk counterpart, with a Young’s modulus and hardness of ~201.4 GPa and ~11.5 GPa, respectively. Furthermore, corrosion tests demonstrated decreased sensitivity to localized corrosion in comparison to the commercial 304 stainless steel in NaCl solution.