It was recalled that high-temperature plastic flow in alumina-based ceramics was very sensitive to the type of added dopant cation: the flow stress increased upon doping with Zr4+ ions, but decreased upon doping with Mg2+ ions. The dependence of the flow stress upon dopant type could not be explained in terms of grain size differences. A detailed grain boundary analysis, using high-resolution electron microscopy and energy-dispersive X-ray spectroscopy, revealed that Mg2+ or Zr4+ ions segregated along grain boundaries, but no second phase existed at the boundaries of either material. Electron energy loss spectroscopic data indicated that the segregated ions changed the chemical bonding at the grain boundaries.

Y.Takigawa, Y.Ikuhara, T.Sakuma: Materials Science Forum, 1997, 243-245, 425-30