Grain Refinement of an As-Cast CoCrNi Multi-principal Element Alloy by Inducing Solute Segregation: Solute Design and Effects

Multi-principal element alloys (MPEAs) have been extensively studied in recent years. However, the as-cast MPEAs usually possess coarse columnar grains with undesirable properties, thus columnar-to-equiaxed transition (CET) and grain refinement are of great significance. According to the interdependence theory, increasing the constitutional supercooling (CS) and limiting the diffusion of solutes will result in a refined structure. Therefore, the effect of boron (B) element on grain refinement of the as-cast CoCrNi MPEAs was evaluated: B element accumulates in the region ahead of the solid-liquid (S-L) interface during solidification and has such a strong affinity with the Cr element that it can induce the segregation of Cr element and restrict the diffusion of Cr element. The results indicate that B element induces CET and grain refinement significantly. The grain refinement is attributed to CS ahead of the S-L interface caused by the interaction of solutes and the slower diffusion rate induced by the concentration of B element. This work proposed a new method to refine the microstructure of the MPEAs.