The effects of Si addition on the microstructures and properties of CoCrNi medium-entropy alloy (MEA) were systematically investigated. The CrCoNiSi_x MEA possesses a single face-centered cubic (FCC) phase when x is less than 0.3 and promotes solution strengthening, while the crystal structure shows a transition to the FCC+σ phase structure when x = 0.4 and the volume fraction of the σ phase increases with a microstructure evolution as the Si content increases. The Orowan mechanism from σ precipitation effectively enhances the strength, hardness, and stain hardening of CrCoNiSi_x MEA, which also exhibits superior hardness at high temperatures. Furthermore, a large amount of σ phase decreases the wear resistance because of the transformation of the main wear mechanism from abrasion wear for σ-free CrCoNiSi_x MEA to adhesion wear for σ-contained CrCoNiSi_x MEA. This work contributes to the understanding of the effect of Si addition on FCC structured alloys and provides guidance for the development of novel Si-doped alloys.

The yield strength of a CrCoNiSi0.3 medium-entropy alloy is significantly increased from 450 MPa (quasi-static, 0.001 s−1) to 1600 MPa (at a strain rate of 5000 s−1) under dynamic tension, with a considerable ductility of 60%. The high strain-rate sensitivity (SRS) of strength and work hardening is obtained, and the strength SRS reaches 0.408. The dominant deformation mechanisms are abundant multiple-twinning, increasing fractions of deformation twins and phase transformation from face-centered-cubic to hexagonal-close-packed (HCP) phases with a strain rate. A universal dislocation-hardened constitutive model considering the evolution of the twin and HCP transformation is established to predict the flow stress and microstructure evolution.

A novel Ni-Co-Cr-based medium-entropy superalloy with a high Si content (7.5 at%) strengthened by an L1₂ phase was developed. The pure L1₂ phase, characterized by an average size of 50 nm and a volume fraction of 46%, was precipitated within the FCC matrix. This alloy exhibits excellent mechanical properties over a wide range of temperatures from 77 K to 1073 K. A yield strength of 1005 MPa, an ultimate tensile strength of 1620 MPa, and a tensile elongation of 36% were achieved at 77 K, with a maximum value of 4.8 GPa at the second stage of the work-hardening rate. The alloy maintains a basically consistent yield strength of approximately 800 MPa from 298 K to 973 K, showcasing significant strain-hardening capabilities, with values of 2.5 GPa, 3.7 GPa, and 4.8 GPa at 873 K, 298 K, and 77 K, respectively. Microscopic analysis revealed that at room and cryogenic temperatures, multilayer stacking faults (SFs), SF bands, and SF networks, rather than twins, effectively stored a large number of dislocations and impeded dislocation movement, thereby enhancing the work-hardening ability of the alloy. Furthermore, at 773 K, the primary deformation mechanism involved high-density dislocation walls (HDDWs) consisting of dislocation tangles and SF lines. As the temperature rose to 973 K, the work-hardening process was influenced by the APB shearing mechanism (in the form of dislocation pairs), SF lines, and microtwins generated through atomic rearrangement. This study not only provides valuable insights for the development of new oxidation-resistant superalloys but also enhances our understanding of high-temperature deformation mechanisms.

The atomic-scale deformation mechanisms under shock compression of single-crystal CoCrNi medium entropy alloy were investigated using molecular dynamics simulations. The anisotropic crystal properties significantly influence the elastic–plastic two-wave structure and the evolution of the microstructure. The results indicate that within the impact velocity range of 0.6–1.2 km/s, the [100] orientation exhibits a single-wave structure, although the mechanisms of plastic deformation differ. The plastic deformation of the single-crystal sample along the [100] direction is primarily governed by stacking faults and dislocation slip at lower impact velocities. However, as the shock velocity increases, the deformation mechanism transitions to a phase-transformation-mediated mode. In contrast, along the [110] and [111] directions, elastic–plastic two-wave structures are distinctly observed, accompanied by a plethora of dislocations with dislocation slip and the formation of disordered structures serving as the predominant plastic deformation mechanism.