TEM characterization of grain boundaries in superplastic Al₂O₃-base ceramics

High-temperature plastic flow in Al₂O₃-base ceramics is very sensitive to the type of dopant cation. The flow stress increases by the doping of Zr⁴⁺ ions, but decreases by the doping of Mg²⁺ ions. The difference in flow stress with the type of dopant cannot be understood from grain size difference. Detailed grain boundary analyses using HREM (high-resolution electron microscopy) and EDS (energy-dispersive X-ray spectroscopy) reveal that Mg²⁺ or Zr⁴⁺ ions segregate along grain boundaries, but no second phase exists along the boundaries in each material. From the result of EELS (electron energy loss spectroscopy), it is found that the segregated ions change the chemical bonding in grain boundaries of Al₂O₃. The segregated ions must play an important role on the ability of grain boundary sliding by changing the chemical state of grain boundaries.