Effects of Erbium Addition on Microstructure and Mechanical Properties of CoCrNi Medium Entropy Alloy

The effects of Er content on microstructures and mechanical properties of Er-doped CoCrNi medium entropy alloys (MEAs) were studied in this work. A series of Er-doped CoCrNi MEAs with 0, 0.1, 0.3, 0.5, 0.7, 1.0, and 1.3 at.% Er was fabricated by high-vacuum arc melting. It was found that Er element addition led to the formation of Er-Ni riched hexagonal phase (CaCu5 type) precipitates embedded in the original FCC matrix. The volume fraction of the HCP phase increased with the increasing Er content. The grain size decreased from 18.1 to 5.1 µm as the Er content increased from 0 to 1.3 at.%. Both the ultimate tensile strength (UTS) and Vickers hardness increased with the increasing Er content, while the ductility decreased. Compared to CoCrNi MEA, the UTS and Vickers hardness of (CoCrNi)98.7Er1.3 MEA increased from 878 to 1143 MPa and 212 to 263 HV, respectively. In addition, the nanoindentation measurements on different phases indicated that the HCP precipitates had a high hardness (~ 7.87 GPa), which was almost two times of that of the FCC matrix (~ 4.72 GPa). The strengthening mechanisms included solid-solution, grain refinement, and precipitation strengthenings.