Enhancing the strength-ductility synergy in CoCrNi medium-entropy alloys via grain boundary segregation engineering

The unique solidification behaviour in metal additive manufacturing (AM), can lead to hot cracking, significantly compromising the mechanical properties of the alloys. In this study, grain boundary segregation engineering was employed, using boron (B) as a segregant, to investigate the effects of varying B concentrations on the microstructure and properties of CoCrNi medium-entropy alloys (MEAs). It was demonstrated that B gradually segregated along the grain boundaries, which improved the grain coalescence, intensified the attraction of the columnar grains, and increased the consumption of the liquid film. Furthermore, this segregation behaviour promoted the formation of the Cr2B nanoprecipitates along the grain boundaries. These precipitates reduced grain boundary mobility, hindered grain growth along the temperature gradient, and contributed to grain structure refinement. When the B content is 0.8 at. %, CoCrNi MEAs achieved a trade-off between ductility and strength, with a tensile strength of approximately 1159 MPa, a yield strength of approximately 800 MPa, and an elongation of approximately 31.4%. it also showed excellent corrosion resistance compared to commercial 316L stainless steel. These results underscore grain boundary segregation as an effective strategy for suppressing hot crack formation, thereby broadening the applicability of AM-fabricated components in industrial applications.