The Role of Molybdenum Addition on the Microstructure and Nanohardness of CoCrFeNi-Based High Entropy Alloys

High entropy alloys (HEAs) have emerged as a prominent class of innovative engineering materials and are attracting increasing interest across various industries. In the aerospace industry, these alloys possess unique potential due to their ability to meet both strength and lightweight requirements. Offering superior mechanical, thermal, and chemical properties compared to conventional alloys, HEAs are therefore considered promising materials for modern aerospace applications. In this study, CoCrFeNiMox (x=0, 0.2, 0.4 at. %) HEA were produced using the vacuum arc melting method. The structural and mechanical properties of the produced HEAs were characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Vickers hardness test, compression test, and nanoindentation test. According to the results, a face centered cubic (FCC) matrix structure forms, and the lattice parameter increases with Mo addition. Microstructural investigations revealed that even the addition of 2 at. % Mo causes significant changes in the microstructure, leading to the formation of a dendritic structure. When Mo addition is increased to 4 at. %, Mo-Cr-rich regions were observed in the interdendritic regions. The hardness values were determined as 184 HV, 195 HV, and 214 HV for Mo contents of 0, 2, and 4 at. %, respectively, while the yield strengths obtained from compression tests were measured as 206 MPa, 254 MPa, and 287 MPa. Nanoindentation tests identified high hardness values (~7.9 GPa) in Cr-Mo rich regions, confirming that Mo enhances the strength of alloy through solid solution hardening and segregation mechanisms. The findings demonstrate that the addition of Mo to CoCrFeNi alloy increases strength while maintaining the ductility of the structure through solid solution strengthening and minor secondary phase precipitations. These results highlight that HEAs are strong candidates for aerospace applications, where mechanical strength and microstructural properties are critically important.