Structure–property relationship in Al0.25CoCr1.5FeMoXNi alloyswith varying mo content (x = 0, 0.5, 1.0): Effects of boridingtreatment

In this study, high-entropy alloys (HEAs) with the compositions Al0.25CoCr1.5FeMoxNi (x = 0, 0.5, 1.0) were synthesized and subjected to pack boriding at three different temperatures (875 °C, 950 °C, and 1025 °C) to comprehensively investigate their microstructure, mechanical properties, wear, and corrosion behavior. The study's primary aim is to determine the influence of increased compositional complexity due to Mo addition and different boriding temperatures on the surface properties in relation to boride layer formation. Microstructural characterization of the produced alloys was performed using SEM/EDS, EBSD, and XRD techniques. While the Mo-free Al0.25CoCr1.5FeNi alloy exhibited a single-phase FCC structure, the addition of Mo led to the formation of a σ phase and phase separation in the Al0.25CoCr1.5FeMo0.5Ni and Al0.25CoCr1.5FeMoNi alloys. After boriding, multilayered boride coatings parallel to the surface were formed in all samples; the thickness and phase distribution of the layers varied depending on temperature and alloy composition. XRD analyses confirmed the formation of FeB, Fe2B, CrB, and certain Mo-based boride phases. Mechanical evaluations via microhardness and nanoindentation analyses revealed a notable increase in surface hardness with both Mo addition and elevated boriding temperatures. According to the tribological test results, adhesion, delamination, and oxidative wear mechanisms were observed in unborided samples, whereas abrasive wear became dominant in borided specimens, significantly enhancing wear resistance. Electrochemical corrosion tests (OCP–Tafel) demonstrated that Mo addition and, especially boriding at higher temperatures improved the corrosion resistance of the alloys by shifting the corrosion potential toward more positive values and reducing the corrosion current density. Post-corrosion SEM/EDS analyses supported the idea that borided surfaces exhibited more stable and ion-resistant structures. In conclusion, both Mo addition and an appropriate boriding temperature optimize HEA surface performance, rendering these alloys promising candidates as surface materials under severe wear and corrosion conditions.