Effect of Al Addition on Microstructure and Properties of CoCrNi Medium-Entropy Alloy Prepared by Powder Metallurgy

Powder metallurgy possesses the advantages of low energy consumption, less material consumption, uniform composition, and near-final forming. In order to improve the mechanical properties and high-temperature oxidation resistance of CoCrNi medium-entropy alloy (MEA), CoCrNiAl_X (X = 0, 0.1, 0.3, 0.5, 0.7) MEAs were prepared using mechanical alloying (MA) and spark-plasma sintering (SPS). The effect of aluminum content on the microstructure and properties of the MEAs was investigated. The results show that the CoCrNi MEA is composed of face center cubic (fcc) phase and some carbides (Cr₂₃C₆). With the increase in Al content, there exists Al₂O₃ precipitation. When the Al content is increased to Al_0.5 and Al_0.7, the body center cubic (bcc) phase begins to precipitate. The addition of aluminum significantly enhances the properties of the alloys, especially those containing fcc+bcc dual-phase solid solutions. The yield strength, compressive strength, and hardness of CoCrNiAl_0.7 alloy are as high as 2083 MPa, 2498 MPa, and 646 HV, respectively. The high-temperature resistance also reaches the oxidation resistance level. Different oxides include Cr₂O₃, Al₂O₃, and (Co, Ni) Cr₂O₄ and NiCrO₃ spinel oxides formed on the surface of alloys. The formation of an Al₂O₃ oxidation film prevents the further erosion of the matrix by oxygen elements.