Biocorrosion Behavior and Biocompatibility of Al <i> _x </i> FeCoCrCuTi _0.5 High‐Entropy Alloys

High‐entropy alloys (HEAs) have attracted increasing attention as potential metallic biomaterials owing to their high strength, excellent corrosion resistance, and compositional flexibility. In this study, the effects of Al content on the microstructure, biocorrosion behavior, ion release, and in vitro cytocompatibility of Al x FeCoCrCuTi 0.5 HEAs were systematically investigated through phase and microstructural characterization, electrochemical and immersion testing, ion release analysis, and cytotoxicity assays. The results showed that the effect of Al depended strongly on its content rather than being uniformly beneficial. At x = 0.75, the alloy exhibited the best overall corrosion resistance, as indicated by a more positive corrosion potential, a lower corrosion current density, less severe corrosion damage after immersion, and reduced metal‐ion release. Microstructural analysis revealed reduced Cu segregation and pronounced Cr enrichment in the dendritic regions, with the Cr content reaching 34.09 at.%. These features favored the formation of a dense and continuous Cr‐rich passive film. In contrast, excessive Al addition deteriorated corrosion resistance, indicating the dual role of Al. In vitro cytotoxicity tests further confirmed that Al 0.75 FeCoCrCuTi 0.5 exhibited acceptable cytocompatibility, corresponding to cytotoxicity Grade 1. Overall, an optimal Al content window was identified, and Al 0.75 FeCoCrCuTi 0.5 shows promise as a corrosion‐resistant metallic biomaterial candidate with acceptable cytocompatibility.