Effect of deep cryogenic treatment on microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloy

As a typical eutectic high-entropy alloy (EHEA), AlCoCrFeNi2.1 exhibits excellent casting properties. However, the imbalance between strength and plasticity hinders its application as an advanced structural material. In order to address this challenge, deep cryogenic treatment (DCT) as a new process applied in the field of EHEAs was proposed in this study. The effects of different DCT times on the microstructure and mechanical properties of AlCoCrFeNi2.1 EHEAs were studied, mainly focusing on the flake structure of FCC+B2 layer. The experimental results suggest that with the extension of the DCT time, the dislocation density in the FCC phase increases significantly. The spherical BCC precipitate phase is generated within the B2 phase, and the average size of this newly generated precipitate phase gradually decreases. Increasing the number of dislocations and precipitate phases is of great significance to improve the mechanical properties. The AlCoCrFeNi2.1 EHEA exhibits excellent comprehensive mechanical properties after DCT for 36 h. Compared with the as-cast state, the tensile strength at room temperature reaches 1,034.51 MPa, increased by 5.74%. The plasticity reaches 21.72%, which is increased by 11.79%. The results show that the tensile strength and ductility of AlCoCrFeNi2.1 EHEAs are balanced and improved after DCT, which are more suitable as advanced structural materials. In addition, the introduction of the DCT process to EHEAs solves the problem of environmental pollution caused by traditional heat treatment process. This study provides useful guidance for using the DCT process to strengthen the mechanical properties of “lamellar + block” type EHEAs.