Fabrication of high-performance CoCrNi medium entropy alloy by laser powder bed fusion: The effect of grain boundary segregation

Abstract

A CoCrNi composite material with 3 wt% TiC was successfully fabricated by laser powder bed fusion (LPBF). With increasing content of TiC particles, C gradually segregated along the grain boundaries (GBs). The segregation of C improved the grain coalescence, intensified the attraction of the columnar grains, and increased the consumption of the liquid film. Furthermore, the segregation of C led to the formation of Cr23C6 and core–shell structure (TiO2/TiC) nanoprecipitates along the GBs. These precipitates reduced the mobility of the boundaries and restricted the grain growth along the direction of the maximum thermal gradient. Reinforcing the CoCrNi microstructure with 3 wt% TiC yielded optimal results, as the resultant composite microstructure was free of hot cracks, consisted of small grains, and exhibited a more random texture. The high density of wide stacking faults and Lomer–Cottrell locks hindered cross-slip during further deformation at ambient temperature, and twins dominated at elevated temperatures. These nanoprecipitates hindered dislocation propagation at all temperatures. Therefore, the synergistic strengthening effects from the different types of dislocations and nanoprecipitates endowed the CCN-3M alloy with excellent mechanical properties. Ultimate tensile strengths of approximately 1.34 and 0.88 GPa, yield strengths of approximately 1.02 and 0.75 GPa, and elongations of approximately 15.6% and 25.3% were observed at ambient and elevated temperatures (700 °C), respectively. These results indicate that GB segregation is an effective method for suppressing the evolution of hot cracks, thus expanding the scope of LPBF-fabricated components for industrial applications.