Phase and Microstructure Evolution in SPS-Processed CoCrFeMnNi High Entropy Alloy: Effects of Heat Treatment and Oxidation

Abstract

The present study investigates the evolution of microstructure, phase, mechanical properties, and high-temperature oxidation behavior of a CoCrFeMnNi high entropy alloy (HEA) synthesized via ball milling and consolidated by spark plasma sintering (SPS). The mechanical milling of elemental powders in an equiatomic ratio for 30 hours results in the formation of a predominant FCC phase accompanied by a minor BCC solid solution phase. Upon SPS, the Cr carbide phase forms in addition to the primary FCC phase. The microstructure of the SPS-processed HEA revealed two distinct contrasts, gray and white. The white region shows depletion of Cr and enrichment of Co, Ni, and Fe as confirmed by EDS. Annealing of SPS-processed CoCrFeMnNi HEA at 400 degrees C and 700 degrees C results in a decrement of the gray phase concurrent to an increase of the white phase. These two regions are represented by two FCC (FCC1 + FCC2) phases having slightly different lattice parameters. The bulk hardness of SPS-processed HEA is found to be 589 +/- 6 VHN. With annealing at 400 degrees C, the bulk hardness increases to 608 +/- 4 VHN and when the temperature is raised to 700 degrees C, the bulk hardness decreases to 592 +/- 3 VHN. The increase of bulk hardness at 400 degrees C is attributed to Cr7C3 carbide precipitation, while the decrease of bulk hardness can be explained by reduction in phase fraction of the harder gray phase. The oxidation behavior of the Cantor alloy was also studied at three isothermal temperatures (800 degrees C, 900 degrees C, and 1100 degrees C). The formation of Mn oxides with varying morphologies, influenced by temperature and oxidation time was observed along with minor Cr oxide formation. At 800 degrees C for all isothermal durations, oxidized samples exhibit globular/nodular Mn oxide layer while this morphology persists up to 64 hours at 900 degrees C, while a well-faceted Mn oxide structure forms at 100 hours at the same temperature. At 1100 degrees C, the faceted Mn oxide structure is obtained after a much shorter exposure time of 36 hours. The globular and faceted morphologies of Mn oxide can be attributed to the presence of Mn2O3 and Mn3O4, respectively.