Microstructural evolution and mechanical properties of the in-situ La2O3 particle-reinforced CrCoNi medium entropy alloy by laser powder bed fusion

Laser powder bed fusion (LPBF) provides a powerful platform for fabricating high-performance metallic materials with tailored microstructures. In this study, a novel in-situ La₂O₃ particle-reinforced CrCoNi medium entropy alloy (MEA) is successfully fabricated via LPBF using a pre-alloyed CrCoNi MEA powder addition with 1.0 wt% LaF₃. During the LPBF process, LaF₃ decomposes under high temperatures, releasing La atoms that subsequently react with oxygen to form thermodynamically stable La₂O₃ precipitates. These precipitates, with an average diameter of 37.5 nm, are uniformly distributed within the FCC matrix. The formation of La₂O₃ precipitates is contributed to the significant grain refinement, reducing the average grain size from 25.2 to 18.2 μm. The rare earth oxide-strengthened CrCoNi MEAs exhibits significantly improved strength-ductility balance. At room temperature, the LaF₃/CrCoNi MEA achieves the yield strength (YS) of 662.59 MPa, ultimate tensile strength (UTS) of 881.35 MPa, and elongation of 52.12 %. While, the LaF₃/CrCoNi MEA maintains the YS of 584.88 MPa, UTS of 762.42 MPa and elongation of 54.83 % at elevated temperature of 800℃, demonstrating superior thermal mechanical stability. The improved mechanical performance is attributed to multiple synergistic strengthening mechanisms, including grain boundary strengthening due to refined microstructure, dispersion strengthening from uniformly distributed La₂O₃ nanoparticles and enhanced dislocation interaction. This work provides an effective strategy for designing oxide-reinforced MEAs with superior mechanical properties, offering broad potential for structural materials application in demanding environments involving both ambient and elevated temperatures.