Microstructural analysis, hardness evaluation and wear behavior of AlCoCrFeNiV and AlCo0.25CrNi1.75V high-entropy alloys

Abstract

In this study the development and characterization of the high-entropy alloys (HEAs) AlCoCrFeNiV and AlCo_0.25CrNi_1.75V for applications requiring high hardness, elastic modulus, and wear resistance was investigated. Both compositions were designed through thermodynamic calculations and empirical parameters, produced by electric arc melting, and subjected to homogenization heat treatments. X-ray diffraction demonstrated that both alloys are biphasic, featuring A2 and B2 structures. SEM analysis revealed that AlCoCrFeNiV in the as-cast (AC) condition exhibited large columnar grains, while heat treatment (HT) results in the precipitation of the A2 phase at the grain boundaries. In contrast, AlCo_0.25CrNi_1.75V exhibits a dendritic microstructure in both as-cast and heat-treated conditions. This alloy is composed of A2 and B2 phases, formed by spinodal decomposition. These microstructural differences significantly influenced the mechanical properties and wear resistance of the alloys. The more uniform microstructure of the equimolar alloy resulted in more stable hardness and elastic modulus values in both the as-cast and heat-treated conditions. Nanoindentation tests showed that AlCoCrFeNiV maintained stable hardness and elastic modulus after heat treatment, attributed to its more uniform microstructure. Conversely, AlCo_0.25CrNi_1.75V experienced a reduction in these properties after heat treatment, linked to its basket weave morphology, which led to a less uniform distribution of phases. Tribological tests revealed contrasting behaviors between the alloys: AlCoCrFeNiV exhibited superior wear resistance and a lower coefficient of friction in ball-cratering microabrasion, attributed to its uniform microstructure. In contrast, AlCo_0.25CrNi_1.75V demonstrated better wear resistance and a reduced coefficient of friction in reciprocating nano-wear tests, probably due to the formation of protective oxide films during the wear process.