High‐Entropy Alloy Design Toward Cobalt Substitution for High Hardness and Low Wear Rate Using X–Cr–Fe–Mn–Ni System

In the aerospace, nuclear, and machining industries, alloys require high hardness and excellent wear rate. Most high‐performance alloys rely on cobalt for high‐temperature properties, despite its political, ethical, and health concerns. High‐entropy alloys (HEAs), enabled by structural hardening, lattice distortion, and sluggish diffusion, offer pathways to eliminate this critical element. This study examines cobalt substitution in HEAs to optimize hardness and wear rate. New alloys based on the Cantor system (CoCrFeMnNi) are produced by individually replacing cobalt with copper, aluminum, vanadium, or molybdenum. Four equiatomic HEAs (AlCrFeMnNi, CrFeMnNiV, CrCuFeMnNi, and CrFeMnMoNi) are compared with two literature alloys (Al _0.2 Co _1.5 CrFeNi _1.5 Ti and CoCrFeMnNi) and with the pure substituent elements, all evaluated in the same metallurgical state. All synthesized HEAs except CoCrFeMnNi are multiphased and do not mimic the structure of their corresponding pure element; CrCuFeMnNi also departs from valence electron concentration predictions. Pure cobalt shows the lowest wear rate, while the Cantor alloy exhibits a higher one. Aluminum, vanadium, and molybdenum strengthen HEAs despite limited performance in their pure state. Ultimately, pure cobalt, CrFeMnMoNi, and AlCrFeMnNi display similar and superior wear rate compared with the optimized reference alloy Al _0.2 Co _1.5 CrFeNi _1.5 Ti.