An assessment on the stability of the eutectic phases in high entropy alloys

Abstract

To overcome the limited mechanical properties of single-phase FCC/BCC high entropy alloys, a large effort is being made towards the development of eutectic composites. The eutectic/near eutectic high entropy alloys (EHEAs) with alternate lamellar phases provide an optimum balance between strength and ductility. Therefore, CoCrFeNiNbx (0.45 < x < 0.65) and CoCrFeNiTay (0.2 < y < 0.5) EHEAs comprised of FCC and Fe2Nb/Co2Nb type Laves phase were synthesized by arc-melting and were characterized by x-ray diffraction, scanning electron microscopy and energy dispersive x-ray spectroscopy. Furthermore, a large number of EHEAs comprising of BCC/FCC solid solution phases and intermetallic compound/topologically closed packed (TCP) phases have been surveyed, as reported in the literature. The stability of the phases were assessed by considering the mixing enthalpy (ΔHmix), valence electron concentration (VEC), and atomic size difference (δr). The eutectic phases in all these EHEAs become stable when −18 ≤ ΔHmix ≤ −6, 6 ≤ VEC ≤ 8.5 and δr > 3%. The synergistic effect of ΔHmix, VEC, and δr on the phase stability and the phase prediction in EHEAs, have been explored.