Laser-powder bed fusion of Y2O3 nanoparticle modified CoCrNi matrix composites: Parameter optimization, microstructural evolution and strengthening mechanisms

Laser-powder bed fusion (L-PBF) delivers significant advantages in manufacturing metal matrix composites due to its micro-sized molten pool and fast cooling rate. In this work, a simple two-step process including the ultrasonic dispersion of ceramic particles and the volatilization of liquid medium by stirring was employed to synthesize Y2O3 nanoparticle modified CoCrNi matrix (Y2O3/CoCrNi) composite powders, which were subsequently processed for L-PBF to make samples. The effects of laser power (P) and hatch spacing (h) on the microstructures and mechanical properties of the L-PBF processed (L-PBFed) Y2O3/CoCrNi composites were investigated comparatively. The results confirm that the high P (350 W) and low h (60 mu m) promoted the uniform dispersion of Y2O3 particles (similar to 50 nm) by altering the Marangoni flow, reducing the dynamic viscosity and increasing the remelting area. The L-PBFed Y2O3/CoCrNi composites under optimized condition exhibited a superior strength-ductility synergy, in which the yield strength, ultimate tensile strength and fractured strain were 647 MPa, 858 MPa and 42.2 %, respectively. It is demonstrated that a metallurgical defect-free hierarchical microstructure that involved dislocation-formed cellular structures, dispersion of Y2O3 particles and various crystallize defects (i.e. stacking faults, Lomer-Cottrell locks, and deformation twins) were responsible for the superb mechanical properties.