Microstructural investigations of the mechanisms of anisotropic grain growth in titania-doped alpha-alumina

Anisotropic microstructures containing needle-like or platelike grains in an equiaxed matrix have been shown to cause improvements in the fracture toughness of ceramic materials. In this study detailed microstructural characterization was used to examine the causes and mechanisms of anisotropic grain growth in titania-doped α-alumina containing some Si. α-alumina materials doped with small amounts of SiO₂ and TiO₂ added singly or together were prepared to examine the separate and cooperative effects. Upon sintering, singly doped materials developed equiaxed microstructures, but co-doped material developed an anisotropic microstructure. The development of anisotropy thus results from a cooperative effect of Si and Ti. The microstructure in the co-doped material contained platelets with 0001 faces in a matrix of equiaxed grains. Short facets at the edges of the platelets developed primarily parallel to the 101¯2 or 112¯3 planes; while other edges showed irregular, curved boundaries. Amorphous material was present at most grain boundaries in the Si-doped material while in co-doped material only boundaries exhibiting a basal facet were penetrated. The difference in the amorphous material distribution suggests that the presence of Ti at the surface of alumina grains modifies the interfacial energy changes accompanying amorphous phase penetration along grain boundaries. EDS analysis showed strong Ti enrichment at the edges of platelets. The enrichment increases the concentration of Al vacancies ($V\prime \prime \prime Al$) near the grain boundary due to substitution of Ti4+ cations on Al3+ sites, apparently leading to enhanced mobility of the edge boundaries of platelets. Microstructural evolution studies in co-doped material showed that the changes in the overall grain morphology are dominated by the rapid radial growth of highly anisotropic grains. Ti enrichment at the edge boundaries of platelets increased with growth during sintering while there was very little change at basal boundaries of platelets and at the boundaries of equiaxed grains although some growth in the perpendicular directions occurred. Overall, observations indicate that anisotropic grain growth is related to the preferential segregation of Ti that causes enhanced mobility at the edges of platelets, and the presence and distribution of amorphous material which appears to create energetically favorable conditions for the development of large basal surfaces.