Tailored Complex Concentrated Alloys 3D Printed from Oxide Precursors

This presentation describes the fabrication of a series of compositionally complex alloys (CCAs) containing Co_,Cr_,Fe_,Mn_,and Ni in both equiatomic and near-equiatomic stoichiometries. Specimens were rapidly fabricated using an extrusion based additive manufacturing (AM) process called direct ink writing (DIW), which allows 3D printing of CCAs from a blend of oxide precursors suspended in a polymer slurry to create a greenbody part with oxide particle loadings of between 70 and 85 vol.%. These printed oxide-polymer structures are exposed to heat to remove the polymer, and then reduced in a pure hydrogen environment at ambient pressures using an environmental oven. The resulting alloys are characterized by a well annealed grain structure with well-mixed elements even at the nanoscale.<br/> <br/>Some of the most promising properties of CCAs are sensitive to small variations in elemental makeup. Flexible fabrication techniques, such as DIW, which allow for elemental modifications from batch to batch, or even from part to part, could help realize the promise of tailored materials. The chemical and structural properties of fabricated Co, Cr, Fe, Mn, and Ni containing materials will be explored as a function of varied stoichiometry, particularly as the molar concentrations of Mn and Ni are altered. X-ray diffraction (XRD) is used to calculate the lattice parameters of various stoichiometries and demonstrate the role of Mn in lattice expansion and the role of Ni in fabrication of a single-phase FCC structure in printed samples.<br/> <br/>Transmission electron microscopy (TEM) in combination with energy dispersive x-ray spectroscopy (EDS) is used to investigate potential chemical segregation within the well-annealed grains as well as to show a low dislocation density and stacking fault energy, common to the Cantor alloy. Scanning electron microscopy (SEM) in combination with EDS and electron backscattered diffraction (EBSD) is used explore the overall microstructure, local porosity from manufacturing defects, the presence of annealed twins in the FCC microstructure, and the absence of chemical segregation at the microscale. Finally, the reduction pathways of individual oxides are explored through XRD and EDS of specimens during various stages of reduction and sintering. We will highlight the complex integration of Mn into the FCC matrix, including the MnO₂to MnO to Mn reduction pathway.