Hexagonal (CoCrCuTi)100-xFex multi-principal element alloys

Abstract

High-entropy and multiprincipal element alloys (HEAs/MPEAs) show considerable promise for engineering applications due to their unique microstructures and tendencies for solid solutions with multiple components. Whereas one of the core thrusts of HEA development is the search for stable single phase solid solutions, MPEA development leaves the door open for alloys with multiple phases and novel microstructures. In that vein, alloys of (CoCrCuTi)100−xFex were made via arc-melting to determine the critical Fe concentration for the formation of a hexagonal Laves C14 phase in these alloys. It was observed that with no Fe addition, the equiatomic CoCrCuTi alloy solidified into two body-centered cubic dendritic phases and an face-centered cubic interdendritic phase. In contrast, additions of 5 at. % Fe or more resulted in the formation of a primary dendritic hexagonal Laves C14 phase. Aside from the majority phases, all of the alloys also contained small Ti-rich dendrites that appeared to be randomly dispersed throughout the microstructure. At higher concentrations of Fe in the CoCrCuTi base (≥ 15 at. % Fe), the melt separated in to Cu-rich and Cu-lean liquid phases, which were characterized postmortem in the solid form as previously liquid phase separated microstructure. Vickers hardness measurements of the alloys indicated that the addition of Fe increased hardness up to 541 HV 1 for 15 at. % Fe, then decreased to 482 HV 1 for the 20 at. % Fe alloy. However due to the significant liquid phase separation that occurred for the 15 and 20 at. % Fe alloys, the microstructure present is significantly heterogeneous. Therefore, the standard deviation of the hardness values increased drastically due to this heterogeneity. Energy dispersive X-ray spectroscopy (EDS) was utilized to determine the atomic concentrations of each phase in the alloys. Using the elemental concentration obtained from the Laves C14 phase, an alloy consisting of Co23Cr22Cu3Ti28Fe24 was arc-melted and characterized for comparison to the multiphase (CoCrCuTi)100−xFex alloys. The hardness of the Co23Cr22Cu3Ti28Fe24 alloy was determined to be 784 HV 1. However, the alloy showed significant cracking from the Vickers hardness test indentations. A direct comparison of the indentation cracks between Co23Cr22Cu3Ti28Fe24 and (CoCrCuTi)90Fe10 demonstrated that the Cu-rich matrix present in the 10 at. % Fe alloy was more effective in arresting crack growth.