The role of aging in the microstructure and mechanical properties of two multi-principal element alloys

High-entropy alloys or multi-principal element alloys (MPEAs) exhibit promising corrosion resistance in aggressive environments, making them viable candidates for high-temperature applications. However, in addition to the study of high-temperature corrosion behavior, understanding microstructural stability with respect to temperature and time is crucial, as microstructural modifications can influence the final properties of the samples. This study investigates the microstructural stability and mechanical properties of two MPEAs after aging treatments at 560 °C and 780 °C for over 1000 h. The first alloy, with an FCC-BCC eutectic microstructure (EMPEA), and the second, primarily BCC with a secondary B2 phase (MPEA6), were analyzed in as-cast and aged conditions. Upon aging at 560 °C, the hardness of EMPEA increased, likely due to the formation of nanosized coherent precipitates. However, at 780 °C, the microstructure coarsened, and the formation of a needle-like phase within the FCC matrix was observed, accompanied by a slight reduction in hardness, while the elastic modulus remained consistently high. MPEA6 exhibited an increase in both hardness and elastic modulus after aging at 560 °C, associated with the redistribution and slight alignment of the B2 precipitates. At 780 °C, these precipitates coalesced into elongated structures, particularly near grain boundaries, forming mesh-like patterns. This microstructural evolution led to a broader hardness distribution, with an overall decrease compared to the 560 °C treated sample, while the modulus showed an increase, suggesting a stiffening effect due to the precipitate's coalescence. These findings underscore the impact of thermal treatment on the microstructural evolution and mechanical properties of these MPEAs.