Discontinuous precipitation leading to nano-rod intermetallic precipitates in an Al0.2Ti0.3Co1.5CrFeNi1.5 high entropy alloy results in an excellent strength-ductility combination

Abstract

The phenomenon of discontinuous precipitation (DP) leading to the formation of nano-rod FCC (γ) + L12 (γ’) colonies, has been typically considered deleterious for mechanical properties. However, the present study shows clear evidence that substantially large fractions of FCC + nano-rod L12 microstructure within a thermo-mechanically processed high entropy alloys (HEA) or complex concentrated alloys (CCA) of composition Al0.2Ti0.3Co1.5CrFeNi1.5, formed via recrystallization coupled with discontinuous precipitation, can lead to an excellent combination of room temperature strength and ductility. The extent of thermomechanical processing can be engineered to modify the phase transformation pathway from homogenous L12 precipitation to discontinuous L12 precipitation in the same HEA. This predominantly FCC + nano-rod L12 microstructure exhibits a yield stress ~1.4 GPa, ultimate tensile strength ~1.6 GPa, and tensile ductility of ~14%, making it one of the best combinations of room temperature tensile properties for FCC-based HEAs, that have been reported to date, as well as better than current generation wrought nickel base superalloys. A high yield strength of the order of ~1 GPa is also retained to a temperature of 500 °C in this alloy. However, at higher temperatures (>550 °C), the DP microstructures exhibit a rapid decline in strength and become less competitive as compared to microstructures consisting of homogeneously precipitated L12 within the FCC matrix.