Exploring the limits of Fe content in Al-Co-Cr-Fe-Ni-based eutectic high entropy alloys using CALPHAD-based alloy design

Abstract

Using AlCoCrFeNi alloy as the reference composition, a novel NiAl(FeCoCr) eutectic high entropy alloy (EHEA) system is proposed. A strong linear relationship was established between the elemental combinations of Ni, Al and (Fe+Co+Cr)-content that evolves a similar volume fraction of B2 phase to the as-cast reference composition (volume fraction of B2 phase ∼34 %) using CALPHAD. For a fixed elemental combination of Ni and Al using the proposed criteria, the equi-atomic substitution of Co and Cr with Fe in the defined (Fe+Co+Cr)-content yields a similar as-cast phase structure to the reference composition (near the melting point such as 1300 C). From this large pool of alloy compositions, a few alloys with different elemental combinations (Fe-contents from 16.4 to 55 at%) were identified and prepared by arc melting. The as-cast EHEAs exhibited a similar lamellar microstructure and tensile properties (yield strength ∼570 MPa and ultimate strength of ∼1000 MPa) as the AlCoCrFeNi reference composition. It is thereby demonstrated that in alloys based on microstructure-driven mechanical performance, it is possible to substitute costly elements in EHEAs for lower cost Fe.