The emergence of intergranular precipitates in TiO₂-doped α-Al₂O₃ ceramics

The minor phase accumulated at intergranular regions in ceramics is usually associated with the global doping level of additives; however, such simplification often undermines the local variation in liquid composition that leads to the emergence of precipitation. In this paper the correlation between precipitation and intergranular liquid was investigated in a TiO ₂-doped Al₂O₃ ceramic system. The discrepancy between XRD and SEM in detection of the intergranular Al₂TiO ₅ phase was rationalized by their difference in sensitivity that corresponds to a change from the casual to general mode of local liquid variation. Analytical TEM in addition revealed Ti-rich nanoprecipitates aligning along grain boundary (GB). The two intergranular phases were associated with separate processes of intergranular liquid, as clarified via a representative case that exhibited the extra grain growth from annealing. The nanoprecipitates were created from a binary liquid in local equilibration with segregation to GB, which served as transporting routes to form ternary liquid at the multigrain junctions to initiate the main intergranular phase Al₂TiO ₅.