Effect of hafnium and molybdenum addition on inclusion characteristics in Co-based dual-phase high entropy alloy

Specific grades of high entropy alloys (HEA) can provide opportunities for optimization properties towards the high-temperature application. The base HEA used for this work with the chemical composition of Co47.5Cr30Fe7.5Mn7.5Ni7.5 (at.%) was selected. The refractory metals hafnium (Hf) and molybdenum (Mo) were added in small amounts (1.5 at.%) due to their well-known positive effects on high-temperature properties. Inclusion characteristics were comprehensively investigated by a two-dimensional (2D) cross-section method as well as extracted by a three-dimensional (3D) electrolytic extraction method. The results showed that the addition of Hf can reduce Al2O3 inclusions and result in the formation of more stable Hf-rich inclusions as the main phase. Mo addition did not affect the inclusion type, but it can affect the inclusion characteristics by affecting the physical parameters of HEA melt. The calculated coagulation coefficient and collision rate of Al2O3 inclusions were higher than that of HfO2 inclusions, but the inclusion amount played a larger role in the agglomeration behavior of HfO2 and Al2O3 inclusions in this study. The inclusions in HEAs highly depended on the active elements present in the alloys. The impurity level and the active elements in HEA were the key factors that caused inclusion formation.