Ultrahigh-strength and ductile CoCrFeNi-based high-entropy alloys manufactured by laser powder bed fusion with multiple strengthening mechanisms

Laser powder bed fusion (L-PBF) has been proposed as a potential manufacturing process of high-entropy alloys involving rapid cooling rate and compositional element tuning. It, to some extent, compensates for the lack of strength of face-centered cubic (FCC) CoCrFeNi alloy by introducing high-density dislocations and generating hierarchical structures. In this study, the CoCrFeNi-based alloy was manufactured by L-PBF with the minor addition of B4C micro-particles and their properties and microstructure were evaluated. Nano-sized precipitates consisting of the boride and varying carbides were found in the form of a core-shell structure. An in-site transformation of carbides appeared around the core of the precipitate due to a large temperature gradient in the melt pool during manufacturing. The tensile yield and ultimate strength of CoCrFeNi-B4C alloy (1249.5 MPa and 1421.0 MPa) are double that of CoCrFeNi matrix (624.5 MPa and 691.1 MPa) at room temperature with competitive ductility. The exceptional mechanical properties can be mainly ascribed to precipitate hardening, dislocation hardening and grain refinement, as well as the particle-dislocation interaction.