Effect of Aluminum on the Friction and Wear Behavior of Al_xCrFeNi Medium-Entropy Alloys

Several high-entropy alloys (HEAs) and medium-entropy alloys (MEAs) are promising for wear applications due to their excellent strength and fracture toughness. Herein, a series of (Formula presented.) MEAs (x = 0.3, 0.5, 0.8, 1.0, 1.2) are prepared by a vacuum arc-melting techniques. The effects of Al content on the crystal structures and mechanical properties are evaluated in terms of hardness and wear resistance. With an increase in Al content, the microstructure changes from face-centered cubic (FCC) + body-centered cubic (BCC)/B2 phases to B2/BCC phases. A higher volume fraction of the BCC phase with increasing Al content favors spinodal decomposition. The Vickers microhardness increases from 345HV (x = 0.3) to 486HV (x = 1.2) with increasing Al content; a nanohardness size effect is revealed by nanoindentation tests. For high Al contents (x = 1.2), the coefficient of friction and wear rate reach the lowest value when the worn surface is smooth with smaller debris, indicating improved wear resistance. Furthermore, this series of MEAs exhibits wear related to abrasion, adhesion, and oxidation; the dominant wear mechanism is influenced by the Al content.