Abstract
The effect of the relative contents of Al and Ti on the equilibrium phase formation in a novel Ni51Co18Fe5Cr10Al16-XTiX 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 Ni51Co18Fe5Cr10Al8Ti8 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.
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