Optimising the Al and Ti compositional window for the design of γ’ (L12)-strengthened Al–Co–Cr–Fe–Ni–Ti high entropy alloys

The effect of the relative contents of Al and Ti on the equilibrium phase formation in a novel Ni 51 Co 18 Fe 5 Cr 10 Al 16-X Ti X high entropy superalloy (HESA) system was analysed using Thermo-Calc software (TCHEA-3 database). The proposed HESAs showed a broad γ+γ′ equilibrium phase field over an extended temperature range (from 660 ° C to a γ′-dissolution temperature >1184 ° C) without the presence of brittle intermetallic phases. The effect of Al/Ti ratio on the microstructure, mechanical properties and precipitation strengthening of the HESAs was determined. The proposed alloys showed a high volume fraction (∼60%) of a (Ni, Co) 3 (Al, Ti)-type ordered (γ′) phase in a disordered face-centered cubic matrix phase (γ). The microstructure and the thermal properties of the HESAs showed excellent correlation to the theoretical thermodynamic predictions. A lower Al/Ti ratio improved the mechanical properties of the HESA system due to the high lattice misfit between the γ and γ′ phases. The homogenised and aged Ni 51 Co 18 Fe 5 Cr 10 Al 8 Ti 8 HESA exhibited a γ′-volume fraction of 63%, γ′-dissolution temperature of 1200 ° C and yield strengths of 1056 MPa and 852 MPa at room-temperature and 800 ° C, respectively. The present work shows the potential to design Al–Co–Cr–Fe–Ni–Ti HESAs with a high volume fraction of γ′-phase and excellent mechanical properties. • Ni 51 Co 18 Fe 5 Cr 10 Al 16-X Ti X showed a broad γ+γ′ only equilibrium phase field over an extended temperature range (660 ° C to a γ′-dissolution temperature >1184 ° C). • Yield strength of Ni 51 Co 18 Fe 5 Cr 10 Al 16-X Ti X alloys increased with a decrease in Al/Ti ratio due to the increased the lattice misfit between γ and γ′ phases. • Ni 51 Co 18 Fe 5 Cr 10 Al 16-X Ti X alloys with a γ′-volume fraction of 63% and γ′-dissolution temperature of 1200 ° C. • Homogenised and aged Ni 51 Co 18 Fe 5 Cr 10 Al 8 Ti 8 HESA exhibited yield strengths of 1056 MPa and 852 MPa at room-temperature and 800 °C.