Development of 4-Component Eutectic High Entropy Alloys using Electromagnetic Levitation Technique

Abstract

Eutectic high entropy alloys combine alternating ductile and hard phases to result in superior properties for load-bearing applications. In the present study, Zr, Nb, Ti, and Ta were added one at a time in equi-molar NiCoCr alloy which had previously shown single-phase Face centered cubic (FCC) microstructure. The mixture was then melted and solidified by electromagnetic levitation technique at low undercooling to ensure proper mixing and slow solidification. No post-solidification annealing was performed, which makes the process more energy- efficient. Post- solidification annealing is not only energy- and time- consuming but can also lead to oxidation of samples. Also, reducing the number of elements from 5 to 4 can reduce the casting defects and elemental segregation. The microstructure and properties of these compositions were compared with each other and with the Fe-containing 5 component counterparts of these alloys. Most of these 4 component alloys revealed microstructures consisting of FCC and intermetallic phases. Scanning electron microscope (SEM) and x-ray diffraction (XRD) techniques were used to investigate the microstructure. Microhardness and nanoindentation testing of these alloys showed high strength and hardness of these alloys. Hardness results were then investigated, using the theoretical calculations on enthalpy of mixing, atomic size difference, entropy of mixing and valence electron concentration. The study demonstrated that the addition of Zr, Nb, Ta, and Ti to NiCoCr alloy resulted in the formation of bright intermetallic phases rich in the added elements, and varying degrees of lamellar microstructures. All the alloys exhibited high nanohardness and reduced modulus, lying in the range of 11–17 GPa and 150–225 GPa, respectively.