Effect of Silver Addition on Microstructure, Phase Composition, Microhardness and Surface Oxide Layer Formation of Al0.5CoCrCu0.5FeNi and Al0.5CoCrCuFeNi High-Entropy Alloys

High-entropy alloys (HEAs) consisting of five or more components in an equimolar ratio are attracting increasing attention due to a unique combination of various properties. Doping HEAs with small amounts of certain elements (most often rare earth, trace or noble metals) is a promising way to improve the characteristics of such alloys and to control their properties. This paper reports the results on the microstructure, phase composition, and microhardness of as-cast Ag_xAl_0.5CoCrCu_yFeNi HEAs (x = 0, 0.1; y = 0.5, 1.0). The effect of silver addition on the oxidation behavior of the studied HEAs at 700°C was determined. The morphology, phase and chemical composition of the resulting oxide film were studied. It was shown that the introduction of silver improves the mechanical characteristics of the alloys, but deteriorates the oxidation resistance due to the formation of copper-silver eutectic in the alloy microstructure, leading to a change in the morphology and phase composition of the formed oxide layer. Along with the solid solution of (Al, Cr)₂O₃ oxides and CuCr₂O₄, NiCr₂O₄ spinels, the addition of silver leads to the formation of copper oxide CuO and a small amount of silver oxide Ag₂O in the surface film.