Evolution of directionally freeze-cast Fe2O3 and Fe2O3+NiO green bodies during reduction and sintering to create lamellar Fe and Fe-20Ni foams

Directional freeze-casting (FC) of powder suspensions followed by freeze-drying and sintering is a versatile and scalable processing route for creating metallic foams with highly elongated pores. Because of the high propensity for oxidation of metal powders, the use of precursor oxide powders is studied here with an additional step of H-2-reduction of oxides to metal before sintering. However, the large volume shrinkage due to oxide reduction causes foam deformations, making it difficult to optimize the FC parameters to obtain a particular foam structure. We use quasi in situ X-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe2O3 and Fe2O3+NiO green bodies as they are reduced by H-2 at 725 degrees C to Fe and Fe-20Ni (at%), respectively, and sintered at 900 degrees C. These temperature and gas conditions result in sequential reduction and sintering steps that can be individually analyzed. Foam porosity, pore width, lamellae thickness, and macroscopic shrinkage are quantified by image analysis. Oxide green body structures match typical FC relationships: porosity increases with decreasing powder content in the FC suspension, and the lamellae spacing period, or FC wavelength, decreases with increasing freezing velocity. Upon H-2-reduction, lamellae in Fe foams buckle due to mismatch stresses from spatially-inhomogeneous reduction rates, leading to anisotropic deformation. Buckling is absent in Fe20Ni foams due to the faster reduction kinetics of Ni/NiO that lead to more spatially uniform reduction. Reduction is responsible for 73-86% of the total volumetric shrinkage, with sintering causing the remaining shrinkage, which is nearly isotropic for all foams. The observed relationships between FC parameters, green body and metal foam structure can help guide the design and optimization of metal foams for specific technological applications. (C) 2021 Elsevier B.V. All rights reserved.