Effect of Cooling Rate on Phase Formation and Phase Transformation Behavior During Heating in Fe₂₀Co₂₀Ni₂₀Cr₂₀B_20-xSi_x High-Entropy Alloys

Abstract

  In this study, to obtain knowledge on the phase formation and its stability in Fe₂₀Co₂₀Ni₂₀Cr₂₀B_20-xSi_x high-entropy alloys, the effect of the cooling rate during solidification on the phase formation and the phase change during heating were investigated. Fe₂₀Co₂₀Ni₂₀Cr₂₀B_20-xSi_x alloys were prepared by high-frequency induction melting and the single-roller melt spinning method. The phases formed in each alloy and their phase transformation behavior were investigated by X-ray diffraction (XRD) measurements, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and differential scanning calorimetry (DSC). The results show that the induction-melted alloys were composed of mainly the FCC phase with trace amounts of Cr₂B and Cr₃Ni₅Si₂, and that the amount of Cr₂B decreased and the amount of Cr₃Ni₅Si₂ increased with increasing Si content. In the rapidly solidified alloy prepared at a rate of 7440℃/s, it was found that the FCC phase was mainly formed in the alloy samples with a B content above 12.5 at.%. In contrast, trace amounts of Cr₂B and Cr₃Ni₅Si₂ were formed in addition to the FCC phase in alloy samples with a B content below 10 at.%. The DSC and XRD measurements showed that the alloy solidified at a rate of 7440℃/s exhibited no phase transformation up to 700℃ and remained mostly the FCC phase. In the rapidly solidified alloy prepared at a rate of 2.4 × 10⁵℃/s, it was clarified that the almost amorphous phase was formed in all alloy samples except for that with a B content of 5 at.%. The amorphous phase first transformed into the BCC phase as the temperature rose and then transformed into the FCC phase with a further increase in temperature. The results show that the presented phase transformation behavior roughly corresponds to the results obtained from thermodynamic equilibrium calculations.