The effects of dopants and complexions on grain boundary diffusion and fracture toughness in α-Al2O3

Grain boundaries often play a dominant role in determining material properties and processing, which originates from their distinct local structures, chemistry, and properties. Understanding and controlling grain boundary structure-property relationships has been an ongoing challenge that is critical for engineering materials, and motivates this dissertation study. Here, α-Al2O3 is chosen as a model system due its importance as a structural, optical, and high temperature refractory ceramic whose grain boundary properties remain poorly understood despite several decades of intensive investigation. Significant controversy still surrounds two important properties of alumina that depend on its grain boundaries; diffusional transport and mechanical fracture. Previously enigmatic grain boundary behavior in alumina, such as abnormal grain growth, were found to derive from chemically or thermally induced grain boundary phase transitions or complexion transitions. This work investigates the hypothesis that such complexion transitions could also impact grain boundary diffusivity and grain boundary mechanical strength. Scanning transmission electron microscopy based energy-dispersive spectroscopy and secondary ion mass spectrometry are utilized to characterize chemical diffusion profiles and quantify lattice and grain boundary diffusivity in Mg2+ and Si4+ doped alumina. It has found that Cr3+ cation chemical tracer diffusion in both the alumina lattice and grain boundaries is insensitive to dopants and complexion type. We hypothesize that extrinsic point defects mostly form bound clusters and are immobile. This fact coupled with compensation by impurities makes the lattice diffusivity insensitive to dopant type. The lack of dopant effect on grain boundary diffusivity is difficult to rationalize, but we hypothesize that a similar mechanism as described for the lattice may be active at the boundary, although charge compensation is not necessary here. Lattice and grain boundary fracture toughness of alumina is studied by a combination in-situ transmission electron microscopy based micro-cantilever fracture and finite element simulation. These experiments allow the boundary properties to be isolated from the microstructural geometry effects that influence the measured fracture properties of polycrystals. The results suggest that samples with disordered complexions doped with either Si2+ or Y3+ at high temperature exhibit boundaries weaker than the undoped material. Whereas grain boundaries with ordered complexions doped by Y3+ are stronger than the undoped boundaries. This embrittlment phenomenon is used to address anomalous grain boundary strength versus grain size behavior that has been widely observed in the literatures