Elucidating the critical role of tribo-oxide layer in room temperature wear resistance of AlxCoCrFeNi complex concentrated alloy

The direct positive correlation between hardness and wear is a well-known heuristic in evaluating tribological performance of metallic alloys. Complex concentrated alloys (CCAs) are at the forefront of scientific investigation owing to excellent properties shown by these alloys. The AlxCoCrFeNi are one of the most extensively studied systems owing to their excellent structural properties. In this study, AlxCoCrFeNi alloys (x = 0.3, 0.5, 0.7) were synthesized via arc melting, followed by solution treatment and aging at 500 °C and 900 °C. Microstructural characterization was performed using XRD, SEM, EBSD, and TEM. The CALPHAD analysis was performed to correlate experimental results with thermodynamic predictions. The lowest hardness of 216 ± 11 HV was observed for Al0.3CoCrFeNi alloy aged at 900 °C (Al0.3_900) and the highest hardness of 491 ± 17 HV was observed for Al0.7CoCrFeNi aged at 500 °C (Al0.7_500). Wear analysis revealed a transition in wear mechanisms, from primarily oxidative in Al0.3CoCrFeNi, to adhesive-abrasive in Al0.5CoCrFeNi, and predominantly abrasive in Al0.7CoCrFeNi. Interestingly, the sample with lowest hardness (Al0.3_900) exhibited highest wear resistance (1.2 × 10-4 mm3/Nm), owing to formation of multicomponent tribo-oxides layers. The lowest wear resistance was exhibited by the Al0.5CoCrFeNi alloy aged at 900 °C (Al0.5_900) which has intermediate hardness of 303 ± 14 HV. The low wear resistance of Al0.5_900 alloy can be attributed to severe abrasive wear due to presence of higher fraction of hard B2 nanoparticles dispersed in the matrix. The study highlights the need for microstructure-informed design of wear-resistant CCAs beyond hardness-based criterion.