The effect of Ce addition on purification and inclusion modification of CoCrFeNiMn high entropy alloy

Abstract

This study focuses on purification and inclusion modification mechanism of equiatomic CoCrFeNiMn high entropy alloys with addition of various cerium (within the range of 0–0.0445 %) smelted by Vacuum Arc Melting (VAM), It was found that there are three types of inclusions in CoCrFeNiMn alloys: Mn(S, Se), AB2O4 spinel type, mixed type (Mn(S, Se) wraps AB2O4), and the average size of the three types of inclusions are 1.1 µm, 5.6 µm, and 4.8 µm, respectively. The addition of cerium can significantly reduce the oxygen content, as well as the sulfur content and selenium content are declined, the inclusions are refined and spheroidized simultaneously. The oxygen content, sulfur content and selenium content were reduced from 150 ppm, 46 ppm, and 105 ppm to 5–7 ppm, 22 ppm, and 80 ppm, respectively. When the cerium content is 0.0012 %, some of the inclusions are modified into three types of inclusions: Mn(S, Se) + Ce2O3, Mn(S, Se) + CeCrO3 and Mn(S, Se) + Ce2O3 + CeCrO3, while the average size of some unmodified AB2O4 spinel inclusions was reduced to 3.1 µm, and the average size of mixed inclusions was dropped to 2.6 µm. When the cerium content is 0.019 %, the inclusions in the alloy are completely modified into Ce2O2(S, Se) + Ce2(S, Se)3. Furthermore, the number density and area fraction of the inclusions are descended from 216.92 mm−2 and 0.1150 % of no cerium alloys to 121.64 mm−2 and 0.0602 %, the inclusions are well spheroidized, and the average size of cerium inclusions is 2.2 µm. Finally, the mechanism on the modification by cerium was discussed based on the composition evolution of inclusion during solidification with Factsage calculation and experimental results. The compositions of inclusions were also analyzed based on the inclusion evolution model.