Thermodynamic simulation and solidification behavior of multicomponent high entropy alloys

Abstract

Zirconium containing high entropy alloys (HEAs) such as FeCoNiCrZrx , FeCoNiCrAlZrx and FeCoNiCrCuZrx (x = 0, 2.5, 5.0, 7.5, 10) are designed with the help of CALPHAD technique by using Thermocal simulation. Phase diagram obtained from the CALPHAD technique shows formation of eutectic reaction due to addition of the zirconium. The solidification pathways of the designed HEAs are understood based upon thermodynamic simulation results. Thermodynamic and Hume-Rothery parameters that govern the morphology and composition of high entropy alloys are also calculated to identify the formation of solid solution phase(s). The high entropy alloys such as FeCoNiCrZrx, FeCoNiCrAlZrx and FeCoNiCrCuZrx (x = 0, 2.5, 5.0, 7.5, 10) are synthesized by vacuum arc melting cum suction casting technique. The detailed structural and microstructural characterization are done to identify the phases and also to understand the sequence of phase evolution during solidification. FeCoNiCrZr10 alloy exhibitsbimodal eutectic morphology while FeCoNiCrCuZr10 alloy shows hypereutectic structure with acicular structure.FeCoNiCrZr10 HEA exhibits bimodal eutectic microstructure consisting of both lamellar eutectics (i.e. L ⟶ FCC (𝞪) + Co2Zr-type Laves phases (𝛾1)) and globular eutectics (i.e. L ⟶+ FCC (𝞪) + Ni7Zr2). The new pseudo-quasiperitectic reaction i.e. L + Laves phases (𝛾1) ⟶ FCC (𝞪) + + Ni7Zr2 has been proposed for FeCoNiCrZr10 HEA. While FeCoNiCrAlZr10HEA shows eutectic microstructure consisting of BCC solid solution phase and Laves phase (i.e. L → BCC solid solution (B2) + Laves phase). FeCoNiCrCuZr10 HEA shows bimodal eutectic microstructure (i.e. L → FCC solid solution (α1) + Laves phase and L→ FCC solid solution (α2) + Ni5Zr). It is found that Zr containing HEA (i.e.bimodal eutectic morphology while FeCoNiCrCuZr10 alloy shows hypereutectic structure with acicular structure.FeCoNiCrZr10 HEA exhibits bimodal eutectic microstructure consisting of both lamellar eutectics (i.e. L ⟶ FCC (𝞪) + Co2Zr-type Laves phases (𝛾1)) and globular eutectics (i.e. L ⟶+ FCC (𝞪) + Ni7Zr2). The new pseudo-quasiperitectic reaction i.e. L + Laves phases (𝛾1) ⟶ FCC (𝞪) + + Ni7Zr2 has been proposed for FeCoNiCrZr10 HEA. While FeCoNiCrAlZr10HEA shows eutectic microstructure consisting of BCC solid solution phase and Laves phase (i.e. L → BCC solid solution (B2) + Laves phase). FeCoNiCrCuZr10 HEA shows bimodal eutectic microstructure (i.e. L → FCC solid solution (α1) + Laves phase and L→ FCC solid solution (α2) + Ni5Zr). It is found that Zr containing HEA (i.e.FeCoNiCrZr10HEA) shows better hardness (≃259.9 HV) as compared to other studied HEAs. Secondly, various experiments are done using polyol process to synthesize nanoparticles of high entropy alloys. Binary alloy system of CuNi is successfully prepared through this process. In case of FeNiCu and CoNiCu, ternary alloy systems, solid solution phase is formed as a final product along with some other organometallic compounds. While in case of CoNiCr alloy system, only Cr gets reduced leaving other two.