Microstructure and mechanical properties of 3D ink-extruded CoCrCuFeNi microlattices

Microlattices with orthogonal 0-90° architecture are 3D-extrusion printed from inks containing a blend of oxide powders (Co3O4, CuO, Fe2O3, and NiO) and metal powder (Cr). Equiatomic CoCrCuFeNi microlattices with ∼170 µm diameter struts are then synthesized by H2-reduction of the oxides followed by sintering and interdiffusion of the resulting metals. These process steps are studied by in-situ synchrotron X-ray diffraction on single extruded microfilaments (lattice struts) with ∼250 µm diameter. After reduction and partial interdiffusion at 600 ˚C for 1 h under H2, filaments consist of lightly-sintered metallic particles with some unreduced Cr2O3. A reduced, nearly fully densified (porosity: 1.6 ± 0.7%) alloy is obtained after solid-state homogenization at 1050 ˚C for 4 h under H2, with a microstructure consisting of two face-centered-cubic phases, one Cu-poor and the other Cu-rich. When a 10 min excursion to 1150 ˚C is added to the 1050 ˚C homogenization, a Cu-rich melt forms which enhances densification (porosity: 0.3 ± 0.2%) and smooths both strut surfaces and sharp cusps at nodes in the microlattices. The liquid-sintered microlattices show higher compressive strength and ductility than the solid-sintered microlattices. These improvements are consistent with finite-element modeling results which show that smoothening of the sharp cusps at nodes by the solidified melt reduces stress concentrations. These CoCrCuFeNi microlattices can be integrated in more complex load-bearing applications, e.g., as cores of sandwich structures with an unusual combination of high specific stiffness, strength, and toughness.