Microstructure Evolution and Phase Formation of Fe₂₅Ni₂₅Co_xMo_y Multi-principal-Component Alloys

Fe₂₅Ni₂₅Co_xMo_y multi-principal-component alloys (MPCAs) are designed in this study; the ratio of Co and Mo is changed to study the effect on the microstructures and phase selection. Simple solid solution FCC and BCC phases are identified, as well as intermetallic σ, μ, and MoNi phases. With the increment of Mo and the decrement of Co, the phase structure undergoes the following transformation: FCC → FCC + σ → FCC + σ + μ → FCC + BCC + σ + μ → FCC + BCC + MoNi. A great trade-off of mechanical properties is achieved in the Fe₂₅Ni₂₅Co₂₅Mo₂₅ alloy, which consists of FCC + σ + μ multiphases, demonstrating typical dendrite and interdendritic eutectic structures. Furthermore, the pseudo-binary phase diagram of Fe₂₅Ni₂₅Co_xMo_y MPCAs is obtained using CALPHAD, and the phase formation is discussed in the process. The phase selection in current MPCAs is related to the atomic size differences (δ), valence electron concentration (VEC), the Allen’s electronegativity differences (Δχ_Allen), and the enthalpies of mixing (ΔH_mix). In this study, the phase selection of Fe₂₅Ni₂₅Co_xMo_y MPCAs abides by the following criterion: when δ ≤ 3.248 pct, VEC ≥ 9.0, Δχ ≤ 0.09 and ΔH ≤–2.6 kJ/mol, simple FCC phase is preferred; otherwise, multiphase can be obtained. Complex ordered σ and μ phases are stable when 7.65 ≤ VEC ≤ 8.55 and 0.106 ≤ Δχ ≤ 0.147.