Reduction of 3D‐Ink‐Extruded (Co ₃ O ₄ +Cr ₂ O ₃ +Fe ₂ O ₃ +NiO) Powder Blends with Carbon, Methane, and Hydrogen

ABSTRACT Filaments extruded from inks composed of an equiatomic blend of binary oxide powders (Co 3 O 4 , Cr 2 O 3 , Fe 2 O 3 , and NiO) are subjected to reduction by hydrogen (H 2 ), methane (CH 4 ), or carbon (C, graphite, added to the ink). In situ X‐ray diffraction analysis elucidates the influence of various reductant combinations on the reduction behavior, including shifts in onset temperatures of various new phases and changes in the resulting microstructures. Comparisons between CH 4 and H 2 as reducing agents, in the absence of carbon, indicate that H 2 is more effective below 700°C. For oxide blends containing carbon, reduction starts at lower temperatures under flowing pure H 2 than under pure Ar. This observation indicates that gas–solid reduction (between H 2 and oxides) is more effective than solid–solid reduction (between C and oxides). Higher carbon additions facilitate the reduction of the most stable oxide (Cr 2 O 3 ), as expected from greater amounts of CO gas being generated and becoming the dominant reducing agent above 700°C. The use of nano‐sized graphite powder promotes reduction at lower temperatures compared to micron‐sized powder, consistent with a higher contact area between the graphite and oxide particles. Based on these observations, an ink containing Co 3 O 4 , Cr 2 O 3 , Fe 2 O 3 , NiO, and nano‐sized graphite powders is 3D‐extruded into a microlattice architecture. After debinding, reducing, and sintering at 1100°C under pure H 2 , a metallic microlattice is achieved with a relative density of 62%. The homogenous and nearly‐dense struts comprise a face‐centered‐cubic CoCr 0.75 FeNi matrix containing ∼12 vol% unreduced Cr 2 O 3 inclusions, which provide a notable boost in compressive yield strength.