Influence of Si addition on the microstructure, mechanical, and wear properties of as-cast Al_0.43CoCrFeNi_2.1 high-entropy alloys and performance enhancement by cold rolling and annealing

A series of as-cast Si_xAl_0.43CoCrFeNi_2.1 (x = 0, 0.1, 0.2, and 0.3) high-entropy alloys (HEAs) was successfully fabricated by vacuum-assisted melting. The phase constituents, microstructural features, and mechanical properties (including hardness, tensile behavior, and wear behavior) of alloys with various Si contents were evaluated. The results revealed that the addition of Si promoted the precipitation of a body-centered cubic 1 (BCC1) phase enriched in Al, Ni, and Si with a B2-ordered structure. Furthermore, the secondary BCC2 phase was enriched with Cr, Fe, and Si precipitates within the BCC1 matrix. Ultimately, a multiphase face-centered cubic (FCC)/(BCC1/BCC2) structure was formed. The microstructural evolution driven by Si addition significantly enhanced the mechanical properties of the Si_xAl_0.43CoCrFeNi_2.1 HEAs. As the Si content increased, the microhardness and tensile strength improved by approximately 42% and 55%, reaching 2.359 GPa and 785 MPa, respectively. The quantitative evaluation of the various strengthening mechanisms indicated that the intrinsic hardness of the FCC matrix and hardening due to BCC1/BCC2 precipitation dominated the overall microhardness. The comparison of the energy barriers indicates that BCC2 primarily strengthens the alloy through a shear mechanism rather than an Orowan bypass mechanism. Furthermore, with increasing Si content, reduced friction and wear, together with smoother worn surfaces, reflect a greatly enhanced wear resistance. After the optimal cold-rolling and 1 h annealing at 800°C, the Si_0.3Al_0.43CoCrFeNi_2.1 alloy showed 56% and 62% increases in microhardness and tensile strength, respectively, compared to the as-cast state, reaching 3.68 GPa and 1270 MPa. The enhanced mechanical properties are attributed to the synergistic effects of residual strain hardening by FCC ordering and L1₂/BCC precipitation strengthening.