Microstructure and properties of Fe20Co20Ni40Al20 high-entropy alloy enhanced via deep cryogenic treatment

The Fe20Co20Ni40Al20 high-entropy alloy (HEA) exhibits a dual-phase structure composed of body-centered cubic and face-centered cubic crystal lattices. This study investigates the effects of deep cryogenic treatment (DCT) at −196°C for durations ranging from 0 to 48 hours on Fe20Co20Ni40Al20 HEAs synthesized via vacuum induction melting. The microstructure, mechanical properties, and fracture behavior of the alloy are comprehensively analyzed. The findings reveal that the dislocations proliferate and migrate during DCT, leading to the formation of dislocation tangles. High-energy dislocations generate nano-sized closed blocks at grain boundaries and within grains, thereby refining the microstructure of the alloy. This process transforms elongated grains into spherical fine grains, ultimately enhancing both the strength and hardness of the alloy. Notably, after 36 hours of DCT, the Vickers hardness of the alloy increases by 13.3% to 461.5 HV, and the tensile strength improves by 9.35% to 886.2MPa. Moreover, the elongation of the treated alloy increases by 120.8% to 6.70% compared with the untreated sample. These enhancements can be attributed to several strengthening mechanisms, including grain refinement, dislocation strengthening, nano-sized closed block reinforcement, and second-phase strengthening.