Abstract: Dispersoid formation and microstructural evolution in an oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy (HEA) using a newly designed multistep sintering process are investigated. The proposed multistep sintering consists of a dispersoid preforming heat treatment of as-milled 0.1 wt% Y₂O₃-CoCrFeMnNi high-entropy alloy powders at 800 C, followed by sintering at 800-1000 C under uniaxial pressure. In the conventional single-step sintered bulk, the coarsened BCC Y₂O₃ dispersoids mainly form with an incoherent interface with the HEA matrix. In contrast, finer FCC Y₂O₃ dispersoids, an atypical form of Y₂O₃, are formed in the matrix region after multistep sintering. Nucleation of FCC Y₂O₃ dispersoids is initiated on the favorable facet, the {111} plane of the austenitic matrix, with the formation of a semi-coherent interface with the matrix during the dispersoid preforming heat treatment and it maintains its refined size even after sintering. It is found that dispersoid preforming prior to sintering appears promising to control the finer dispersoid formation and refined grain structure.

Key words: Oxide dispersion strengthening, High-Entropy alloy, Multistep sintering, Dispersoid preforming, Microstructure evolution, Interfacial structure