Linking grain boundary structure and composition to microstructure in commercial‐grade‐doped specialty Aluminas

Abstract The design of commercial‐grade specialty alumina for improved behavior relies on better understanding the complex interplay between high concentrations of intentional dopants and unintentional impurities, which are unavoidably introduced during industrial‐scale processing. In particular, dopants and impurities govern grain boundary structure/composition, grain growth, and bulk material properties. While the effects of single dopants in ultra‐high‐purity ceramics are well understood, the same concepts have not been adequately extended to commercial materials containing numerous impurities, especially when utilizing the latest atomic‐resolution characterization techniques to evaluate grain boundary structure and composition. Therefore, this work investigated the effects of varying co‐doping levels of MgO, CaO, and SiO 2 on grain boundary structure and composition in commercial‐grade alumina by applying aberration‐corrected scanning transmission electron microscopy. First, it was observed that grain growth behavior, specifically grain morphology, in doped specialty aluminas was anomalous to ultra‐high‐purity alumina; in the composition range that was investigated in this study, Ca‐doped specialty alumina exhibited equiaxed grains, whereas Si‐doped specialty alumina exhibited elongated grains. Afterward, it was determined that elemental ratios of bulk doping concentrations differed from grain boundary compositions. Grain boundary compositions, as well as grain boundary structure, were therefore determined to be the dominant metrics to predict grain growth behavior. Overall, co‐doping effects on grain boundary structure and composition are the key parameters to consider to control commercial‐grade ceramics and improve material reliability.