Tune Al/Ti to adjust FCC+L21 hetero-structured Ni-based high-entropy alloys for improved mechanical properties and wear resistance

Abstract

Outstanding mechanical properties of Ni-based superalloy benefit from its coherent γ/γ’ structure via precipitation strengthening of γ matrix (FCC structure) by L12 Ni3Al-type γ’ phase. Back-stress strengthening is another effective strategy to further enhance the FCC+L12 structured Ni-based superalloy. In this work, we extend such approaches to high-entropy alloys (HEAs) by introducing different Al and Ti contents (5 at.% ∼18 at.%) into a Ni-based CrFe2Ni4 alloy to form FCC+L21 heterostructured AlxCrFe2Ni4Tiy HEAs. Detailed microstructural analysis indicates that L12 Ni3(Al,Ti)-type nanoparticles form in a (Ni,Fe,Cr)-rich FCC matrix. The volume fraction of L21 AlNi2Ti-type phase can be varied by adjusting the Al/Ti ratio and concentrations of Al and Ti. Higher Al and Ti contents promote L21 phase formation and higher Al/Ti ratio (>1) prohibits the high Ti-containing compounds such as D024η-Ni3Ti and C14 Laves Fe2Ti phases, which are hard but brittle. Corresponding Young's modulus, Poisson's ratio, hardness, and the bulk to shear modulus ratio (B/G) can be readily modified. Compressive tests demonstrate that Al1.5CrFe2Ni4Ti1.0 alloy with half FCC and half L21 phases possesses the optimal strength-ductility combination (with compressive yield strength of ∼1564 MPa and fracture strain of ∼28%). DFT calculations were performed to elucidate relevant mechanisms. Sliding wear tests were also performed, which demonstrate superior wear resistance of the HEAs at both room and elevated temperatures, compared with a commercial Ni-based superalloy, UHT-Nickel.