Carbon reduction of 3D-ink-extruded oxide powders for synthesis of equiatomic CoCuFeNi microlattices

Equiatomic CoCuFeNi high-entropy alloy microlattices are created by 3D-extrusion printing of an ink containing a blend of binary oxides (Co₃O₄+CuO+Fe₂O₃+NiO) and graphite (C) powders. After printing, the green parts are subjected to a series of heat treatments under Ar leading to (i) carbon reduction of the oxides to form metallic particles, (ii) interdiffusion of these metallic particles to create an alloy, and (iii) sintering to remove porosity. The phase evolution in individual extruded filaments (similar to struts in the microlattices) is observed by in-situ X-ray diffraction, showing that intermediate suboxide phases (Cu₂O, CoO, Fe₃O₄, CuFeO₂, and FeO) form as the original oxides are reduced by carbon, before the final metallic alloy is formed. At 830 °C, the extruded filaments comprise a face-centered cubic CoCuNi(+Fe) alloy with unreduced FeO inclusions. After reduction and sintering at 1100 °C, homogeneous, densified, equiatomic CoCuFeNi microlattices are achieved, containing small amounts of a Cu-rich phase. At room temperature, the compressive strength of these CoCuFeNi microlattices increases as the strut diameter decreases from ∼260 to ∼130 µm, as expected from an observed drop in strut porosity resulting from more complete sintering. This is consistent with the easier escape of CO+CO₂ gas created during carbothermic oxide reduction from the thinner struts undergoing reduction and sintering.