Coupled effects of volumetric energy density and Y2O3 content on microstructure and tensile behavior of LPBF-fabricated CoCrFeNi high-entropy alloys

Nano-Y 2 O 3 -reinforced CoCrFeNi high-entropy alloys containing 0.5, 1.0, and 1.5 wt% Y 2 O 3 were fabricated by laser powder bed fusion (LPBF) at volumetric energy densities (VEDs) of 160, 177.8, 200, and 228.6 J/mm 3 to examine the coupled effects of energy input and Y 2 O 3 content on microstructure and tensile response. X-ray diffraction, electron backscatter diffraction, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy, tensile testing, and fracture analysis were employed. The FCC CoCrFeNi matrix remained the predominant phase within the resolution of XRD, while representative TEM regions contained crystalline Y-containing oxide particles with an average size of 48.35 ± 4.56 nm. Increasing VED from 160 to 200 J/mm 3 raised the density from approximately 7.74–7.78 to 8.06–8.16 g/cm 3 and markedly improved the tensile response; further increasing VED produced little additional densification. Grain size continued to decrease, reaching approximately 37–39 μm at 228.6 J/mm 3 , whereas the estimated GND density showed a composition-dependent response over 1.81–2.74 × 10 14 m −2 . At 200 J/mm 3 , the 1.0 wt% Y 2 O 3 composite reached an ultimate tensile strength of approximately 712 MPa and a yield strength of 615 MPa, while the 0.5 wt% composite reached a fracture strain of approximately 40.6%. Hall–Petch and GND-strengthening estimates based on measured grain size and EBSD data were approximately 25.3–37.1 and 174.9–215.2 MPa, respectively, indicating that grain refinement and GND storage contribute to strengthening but do not alone account for the full property variation. Post-deformation twinning was observed in the representative 1.0Y–200 sample and may contribute to strain hardening. The results demonstrate that the tensile response is governed by the coupled evolution of densification, grain structure, defect storage, and particle-related effects.