Effect of Nb Element on Microstructure and Mechanical Properties of (Co₃₀Cr₂₅Fe₄₀Ni₅)_100−xNb_x High-Entropy Alloys

In this study, new (Co₃₀Cr₂₅Fe₄₀Ni₅)_100-xNb_x (x = 0, 1, 2, 4, 6, 8, x values in molar ratio) high-entropy alloys (HEAs) were prepared. The effects of Nb element on microstructure and mechanical properties of Co₃₀Cr₂₅Fe₄₀Ni₅ alloy were investigated by using x-ray diffraction (XRD), scanning electron microscope (SEM), universal testing machine and Vickers microhardness tester. The results indicate that Co₃₀Cr₂₅Fe₄₀Ni₅ alloy exhibited face-centered cubic (FCC) phase. XRD patterns indicate that the inclusion of Nb causes lattice distortion. Nb-rich Laves phase precipitates in the studied alloys as the Nb content increases from 1 to 8%. When the content of Nb is 2%, Laves phase and FCC phase form eutectic structure. Additionally, the addition of Nb element can enhance hardness and yield strength. The studied alloys tended to increase and then decrease in plasticity. Strengthening mechanisms include solid solution strengthening, fine grain strengthening and precipitation strengthening. (Co₃₀Cr₂₅Fe₄₀Ni₅)₉₈Nb₂, (Co₃₀Cr₂₅Fe₄₀Ni₅)₉₆Nb₄ and (Co₃₀Cr₂₅Fe₄₀Ni₅)₉₂Nb₈ have brittle fracture characteristics. Among them, (Co₃₀Cr₂₅Fe₄₀Ni₅)₉₈Nb₂ exhibited superior mechanical properties, including a compressive strength, fracture strain and hardness of 1664.88 MPa, 44.58% and 286.28 HV, respectively. Combining theoretical analysis and experimental statistics, a criterion for the average d orbital energy level (Md) was proposed for HEAs between single-phase (FCC) and two-phase (FCC and Intermetallic phase (IM phase)). The boundary between FCC and IM phase is determined to be 0.898.