Boron-modified CoCrCuFeNiTi high-entropy alloys: Microstructural design and coupled high-temperature wear and corrosion performance

This study systematically investigates the effect of boron (B) addition on the microstructural evolution, mechanical properties, high-temperature wear behavior, and corrosion resistance of CoCrCuFeNiTiB_x (x = 0–1) high-entropy alloys produced via mechanical alloying and powder metallurgy. Phase constitution and microstructural changes were analyzed using XRD and SEM, revealing progressive boride formation and microstructural refinement with increasing boron content. Mechanical characterization demonstrated a pronounced strengthening effect, with hardness increasing from approximately 500 HV in the boron-free alloy to a maximum of 670 HV and a nanohardness of 10.98 GPa for the CoCrCuFeNiTiB_0.7 composition. Tribological tests conducted under room temperature and elevated temperature (650 °C) conditions showed that boron addition fundamentally alters the wear response of the alloys. While wear at room temperature was governed primarily by mechanical abrasion and adhesion, high-temperature sliding induced an oxidation-assisted wear mechanism. Notably, CoCrCuFeNiTiB_0.7 exhibited the lowest wear rates under conditions, achieving 13.79 × 10⁻⁵ mm³/N.m at room temperature and retaining excellent wear resistance at 650 °C with a value of 15.61 × 10⁻⁵ mm³/N.m, indicating remarkable thermal stability. Electrochemical measurements further revealed that this alloy possessed the lowest corrosion current density (icorr = 3.17 × 10⁻³ A/cm²) and the minimum corrosion rate (0.315 mm/y), corresponding to an approximately 7.5-fold improvement compared to the boron-free alloy. Post-wear and post-corrosion SEM–EDX analyses confirmed that the superior performance of the B_0.7 alloy arises from the synergistic effect of boride strengthening and the formation of a dense, adherent oxide layer at elevated temperatures. Overall, the results identify an optimal boron content that enables CoCrCuFeNiTi-based high-entropy alloys to overcome the conventional trade-off between hardness, wear resistance, and corrosion performance, making them highly promising for demanding high-temperature tribological applications.