Simple ordered L1₂ phase and complex ordered µ phase were found in the (FeCoNiMo)₉₀Al₁₀ alloy, while disordered FCC and µ phases were detected in the (FeCoNiMo)₉₀Cr₁₀ alloy. After annealing at 900°C, nano-size precipitates were observed in the L1₂ phase, and splitting and spheroidisation were occurred in some regions of the eutectic structures in the (FeCoNiMo)₉₀Cr₁₀ alloy. After holding for 100 h at 900°C, (FeCoNiMo)₉₀Al₁₀ alloy showed an excellent hardness resistance. The oxidation kinetics of both (FeCoNiMo)₉₀Al₁₀ and (FeCoNiMo)₉₀Cr₁₀ alloys followed a parabolic rate law at 900°C. An external layer of Al₂O₃ and an underlying MoO₂ subscale were found in the (FeCoNiMo)₉₀Al₁₀ alloy. A semi-continuous oxide layer of MoO₂ and Cr₂O₃ was observed in the (FeCoNiMo)₉₀Cr₁₀ alloy.

The as-cast CoCrFeNi high-entropy alloys show simple FCC structure and outstanding deformation ability at room temperature. Respectively, after 50% of deforming at room temperature and annealing at intermediate temperature range of 600 °C–800 °C for 2 h, CoCrFeNi alloys still show simple-phase FCC structure. It is noteworthy that CoCrFeNi high-entropy alloys can’t be strengthened by means of annealing, while deformation is an effective way to strengthen the alloys. It is investigated that the formation of sigma (σ) phase in high-entropy alloys is closely related to the difference of atomic radius (δ), the concentration of valence electron (VEC) and paired sigma-forming element (PSFE) content, and PSFE plays an crucial role. When PSFE > 40 at.%, 6.75 ≤ VEC ≤ 7.86, 4.0 ≤ δ ≤ 7.2, σ phase is prone to form in high-entropy alloys. In addition, alloying and processing route are two important factors that affect the sigma-formation in CoCrFeNi high-entropy alloys.

Disordered and ordered solid solution phases are identified in FeCoNi-based multi-component alloys. An ultimate strength of 2380 MPa was achieved in the dual face-centered cubic + body-centered cubic structural (FeCoNi)₆₀Al₁₅Cr₂₅ alloy, accompanied by a fracture strain of 38% and a hardness of 376.3 HV. After holding at 900°C for 100 h, oxidation resistance of the alloys follows by an increasing rank of (FeCoNi)₈₅Al₁₅ < (FeCoNi)₇₅Cr₂₅ < (FeCoNi)₆₀Al₁₅Cr₂₅. Moreover, the oxidation kinetic curves of the alloys follow a power-law dependence. (FeCoNi)₆₀Al₁₅Cr₂₅ alloy shows a good trade-off of mechanical properties and an excellent oxidation resistance at evaluated temperature, and can be a potential candidate for high temperature materials.

To study the effect of alloy composition on phase selection in the CoCrCu_0.1FeMoNi high-entropy alloy (HEA), Mo was partially replaced by Co, Cr, Fe, and Ni. The microstructures and phase selection behaviors of the CoCrCu_0.1FeMoNi HEA system were investigated. Dendritic, inter-dendritic, and eutectic microstructures were observed in the as-solidified HEAs. A simple face centered cubic (FCC) single-phase solid solution was obtained when the molar ratio of Fe, Co, and Ni was increased to 1.7 at the expense of Mo, indicating that Fe, Co, and Ni stabilized the FCC structure. The FCC structure was favored at the atomic radius ratio δ ≤ 2.8, valence electron concentration (VEC) ≥ 8.27, mixing entropy ΔS ≤ 13.037, local lattice distortion parameter α₂ ≤ 0.0051, and ΔS/δ² > 1.7. Mixed FCC + body centered cubic (BCC) structures occurred for 4.1 ≤ δ ≤ 4.3 and 7.71 ≤ VEC ≤ 7.86; FCC or/and BCC + intermetallic (IM) mixtures were favored at 2.8 ≤ δ ≤ 4.1 or δ > 4.3 and 7.39 < VEC ≤ 8.27. The IM phase is favored at electronegativity differences greater than 0.133. However, ΔS, α₂, and ΔS/δ² were inefficient in identifying the (FCC or/and BCC + IM)/(FCC + BCC) transition. Moreover, the mixing enthalpy cannot predict phase structures in this system.

With the increasing titanium, the volume fraction of face-centred cubic-structured dendrites decreased, and ordered B2 phase, σ, and Laves phase appeared in CoCrCu_0.5FeNiTi _x alloy when x > 0.5. With the increase of Ti content, Δ S_mix, Δ H_mix, valence electron concentration (VEC), and Λ (=Δ S_mix/ δ²) of CoCrCu_0.5FeNiTi _x alloys decreased, while atomic size difference ( δ) and electronegativity difference (Δ χ) increased. Topologically close-packed (TCP) structures are favoured when Δ S_mix > 14.53 J K⁻¹ mol⁻¹. TCP phases can be found when δ > 5.05 and Δ χ ≥ 0.124. Simple solid solution phase is favoured when VEC ≥ 8.26 but multi-phases including TCP phase coexist when VEC < 8.26. Single-disordered solid solution forms only when Λ > 0.637, solid solution and TCP phases coexist when 0.531 < Λ < 0.637, and TCP phases are favoured when Λ < 0.531.

Taking into consideration the large positive mixing enthalpy of Cu–Mo, in this work, the relative content of Cu and Mo elements was adjusted to investigate the solidification process and microstructure formation of CrCu _xFeMo _yNi high-entropy alloys. For CrCu_0.5FeMo_yNi ( y = 0.5, 1) high-entropy alloys, many Cu-rich spheres were observed in the as-solidified microstructures, while macroscopic phase separation structures were obtained in CrCuFeMo _yNi ( y = 0.5, 1) alloys. Liquid-phase separation occurred in high-entropy alloys when the enthalpy of mixing (Δ H_mix）was positive, the valence electron concentration (VEC) was in a range of 8.09–8.44 and the difference in atomic radii ( δ) was between 3.39 and 4.25. For lower positive Δ H_mix, Cu-rich spheres were observed, while for higher positive Δ H_mix, macroscopic phase-separated structures were obtained.