The microstructure and properties of Fe55Cr15Ni(30-x)Nbx eutectic high-entropy alloys

The emergence of eutectic high entropy alloys (EHEAs) provides new insights for the design of the next generation structural alloys. In this paper, a set of low-cost Co-free Fe55Cr15Ni(30-x)Nbx (x = 0, 3, 6, 9, 12 and 15 at.%) EHEAs was successfully designed and prepared by vacuum arc melting. With the increase of Nb content, the solidification structure of the EHEAs change from single-phase FCC (x = 0) to divorced eutectic structure (x = 3) containing FCC and Laves phase, hypoeutectic structure (x = 6 and 9) and eventually becomes hypereutectic structure (x = 12 and 15). The FCC phase is rich in Fe, Cr and Ni, and the Laves phase is rich in Nb. The addition of Nb result in the increase of the volume fraction of the hard Laves phase which improved the room temperature strength. Solid solution strengthening and second phase strengthening are the main factors affecting the properties of the alloys. In this study, Fe55Cr15Ni21Nb9 alloy shows the best comprehensive mechanical properties, the compressive fracture strength, fracture strain and yield strength are 1216 MPa, 32% and 749 MPa, respectively. Phenomenologically, when the Nb content is in the range of 0–15 at.%, the yield strength of the alloy is a function of the Nb content: σ0.2=550×ex/15.81−256. For (Co)CrFeNi-based high-entropy alloys, a new phase prediction criterion is proposed in this paper. When the d-orbital energy level of the alloy is Md ≤ 0.92, the atomic radius difference is δ ≤ 2.88 or the mixing enthalpy meets −4.16≤ΔHmix≤0, single-phase FCC will be formed. If this is not the case, the FCC + TCP phase will be formed.