Grain boundary structures in superplastic SiO2-doped and undoped tetragonal polycrystalline material were investigated by means of high-resolution electron microscopy, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy, using a field emission-type transmission electron microscope. No amorphous phase was observed at any grain boundaries in either the doped or undoped samples. It was found that Y ions segregated over a width of 4 to 6nm at grain boundaries in both materials, while Si ions segregated over a width of 5 to 8nm at grain boundaries in SiO2-doped samples. The average dihedral angle between the grain boundaries in doped material could be as high as 80º. This value agreed well with the fact that none of the grain boundaries contained a glass phase. Strain energy accumulated due to the dissolution of Si ions in the tetragonal lattice. However, the grain boundary energy of the doped material was thought to be low enough to compensate for the increase in strain energy near to the grain boundaries. It was noted that O-K-edge electron energy loss spectra from grain boundaries in doped samples were shifted by 3 to 4eV, towards higher energies, as compared with the signals from the grain interior. It was suggested that the chemical bonding was strengthened at grain boundaries, due to the presence of solute Si.

Y.Ikuhara, P.Thavorniti, T.Sakuma: Acta Materialia, 1997, 45[12], 5275-84