Phase prediction and microstructure of centrifugally cast non-equiatomic Co-Cr-Fe-Mn-Ni(Nb,C) high entropy alloys

The microstructural evolutions and hardness of two non-equiatomic multi elements CoCrFeMnNi and microalloy CoCrFeMnNiNbC high entropy alloys were studied after centrifugal casting. The chemical composition, microstructural changes and hardness were analysed using XRF, Leco-combustion technique, optical microscopy, SEM-WDX, XRD and Vickers hardness testing. Moreover, the thermal behaviour of both alloys was examined by differential thermal analysis technique at heating rate of 10 degrees C/min up to their melting points. The experimental results confirmed theoretical calculations based on Calphad method and thermodynamic rules, suggesting two solid solution high entropy alloys with FCC-crystal structure after centrifugal casting. Microscopy observations and hardness analysis revealed insignificant microstructural variations along the thickness of both alloys. From the microscopy results, it was also found that the microstructure consisted of a dendritic structure with the segregation of Fe and Mn in dendritic areas, while interdendritic areas were rich in Co, Cr, Ni. Furthermore, SEM-WDX results showed Nb rich nano-scale precipitates at interdendritic areas with spherical and oval shaped morphologies. Differential thermal analysis demonstrated no peak up to melting point, suggesting the high temperature stability of solid solution structure in both alloys. (C) 2018 Elsevier B.V. All rights reserved.