Relating grain-boundary complexion to grain-boundary kinetics I: Calcia-doped alumina

Because of the complex nature of internal interfaces it has been a continual challenge to link the grain growth behavior of alumina (especially the onset of abnormal grain growth) to the internal interface structure and chemistry, and the associated atomic transport rate. The present work considers the problem of normal and abnormal grain growth development in calcia-doped alumina, a system noted for its complex abnormal grain growth behavior, in terms of the new concept of interface complexions. Calcia-doped alumina was shown to exhibit four distinct grain-boundary complexions in the temperature range of 1325 degrees-1870 degrees C. All four complexions may coexist at a single temperature. Each complexion is associated with a characteristic grain-boundary mobility, all of which enhances the grain growth kinetics relative to undoped alumina. It was found that the activation energy for the different complexions (normal and abnormal grain growth) was approximately the same in each case (similar to 450 kJ/mol). This is discussed in the context of interface- versus diffusion-controlled grain growth, and it is concluded that normal and abnormal grain growth in this system is diffusion controlled.