Microstructures and liquid phase separation in multicomponent CoCrCuFeNi high entropy alloys

Abstract

The microstructures of high entropy alloys of the system CoCrCuFe x Ni and CoCrCuFeNi x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe 0.5 Ni and CoCrCuFeNi 0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys ( x = 1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe 0.5 Ni and CoCrCuFeNi 0.5 alloys are likely reasons for their liquid phase separation.