Designing eco-friendly alternative microstructures for magnesia-chromium aggregates

On the rising demand for eco-friendly refractories, reducing the use of likely toxic magnesia-chromium aggregates remains a challenge. Previous studies by some of the authors have proposed Cr-free alternative compositions, although the morphology of the spinel precipitates has varied across the different suggested systems. The mechanisms involved in the formation of these distinct morphologies were unclear and, therefore, are the focus of this work. In all compositions, SEM/electron backscatter diffraction revealed cube–cube orientation relationships between matrix and precipitates, indicating that their formation is influenced by the lattice parameter misfit (δ), which was measured using synchrotron X-ray diffraction. It could be concluded that coarser and spherical precipitates form to minimize their surface-to-volume ratio in compositions with high absolute δ-values. Conversely, low-misfit systems enable the spinel to form a 3D-network. The potential use of this knowledge to tailor the microstructure of novel compositions was demonstrated by a small Nb₂O₅ addition into one of the proposed compositions.