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

The phenomenon of discontinuous precipitation (DP) leading to the formation of nano-rod FCC (gamma) + L1(2) (gamma') colonies, has been typically considered deleterious for mechanical properties. However, the present study shows clear evidence that substantially large fractions of FCC + nano-rod L1(2) microstructure within a thermomechanically 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 L1(2) precipitation to discontinuous L1(2) precipitation in the same HEA. This predominantly FCC + nano-rod L1(2) microstructure exhibits a yield stress similar to 1.4 GPa, ultimate tensile strength similar to 1.6 GPa, and tensile ductility of similar to 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 similar to 1 GPa is also retained to a temperature of 500 degrees C in this alloy. However, at higher temperatures (>550 degrees C), the DP microstructures exhibit a rapid decline in strength and become less competitive as compared to microstructures consisting of homogeneously precipitated L1(2) within the FCC matrix.

Automated summary

The study demonstrates that through specific thermomechanical processing methods, a microstructure with excellent room temperature strength and ductility can be formed in high entropy alloys. However, at high temperatures, the strength of the discontinuous precipitation microstructure rapidly declines.