Abstract: Fe₃₅Mn₃₅Ni₁₀Cr₁₀Al₁₀ high entropy alloy was prepared by vacuum arc-melting method, followed by solution treatment at 1200 ℃ for 2 h and aging at 600 ℃ for 4 h. XRD, SEM, TEM and Vickers hardness tester were used to detect the microstructure and hardness of the high entropy alloy. The results show that the microstructure of the as-cast Fe₃₅Mn₃₅Ni₁₀Cr₁₀Al₁₀ high entropy alloy exhibits dendrite. The dendrite regions are FCC phase, while the interdendrite regions are BCC phase, in which a large number of B2 nanoparticles are uniformly exist. After solution treatment at 1200 ℃ for 2 h, the microstructure of the high entropy alloy transforms from dendrite to equiaxed grains where the intragranular orderd B2 phase is, and only a small amount of FCC phase exists at the grain boundaries. After sequential aging at 600 ℃ for 4 h, the B2 phase transform into FCC phase, resulting in the decrease of the volume fraction of B2 phase. Besides, the elliptic nanoparticles with α-Mn structure are precipitated in the B2 region. The appearance of B2 phase and the elliptic nanoparticles with α-Mn structure increase the hardness of the Fe₃₅Mn₃₅Ni₁₀Cr₁₀Al₁₀ high entropy alloy in solution treated and aged states, respectively, which are 419 and 517 HV0.5, 51% and 86% higher than that of the as-cast Fe₃₅Mn₃₅Ni₁₀Cr₁₀Al₁₀ high entropy alloy, respectively.

Key words: high entropy alloy, heat treatment, microstructure, B2 phase, α-Mn structural phase, Vickers hardness