Influence of Si Content and Milling Duration on the Microstructure and Mechanical–Tribological Properties of AlCoCrFeNiSi High‐Entropy Alloys

In this study, AlCoCrFeNiSi high‐entropy alloys (HEAs) with varying Si contents (4–8 at%) were synthesized via mechanical alloying to investigate the relationship between phase evolution, microstructural refinement, and mechanical/tribological properties. XRD results revealed a clear FCC‐to‐BCC phase transformation with prolonged milling time, leading to the formation of a single‐phase BCC structure in the 4% Si alloy, while a dual‐phase FCC + BCC structure remained in the 8% Si alloy, indicating a strong dependence of phase stability on Si content. This phase evolution was accompanied by significant grain refinement, as crystallite size decreased from 23.85 ± 1 to 8.18 ± 1 nm at 50 h, together with increased dislocation density and grain boundary. A direct correlation was observed between this microstructural refinement and property enhancement. Specifically, the increase in BCC phase fraction and refined microstructure resulted in improved hardness, reaching 935 ± 20 HV 1 for the 4% Si alloy and 887 ± 20 HV 1 for the 8% Si alloy. SEM observations supported these findings, revealing improved particle homogeneity and reduced particle size with increasing milling time. Consistent with these microstructural changes, tribological performance improved with milling time, since the reduction in friction coefficient and wear rate is closely related to the increase in microhardness and structural refinement.