The amorphous to gamma transformation in ion implanted Al2O3

Amorphous surface layers, free of implanted impurities, were produced on single crystal alpha--Al2O3 substrates by implantation at -185 C with 2 x 10(16) Al/sq cm at 90 keV and 3 x 10(16)C/sq cm at 55 keV. The response of these amorphous layers to post-implantation annealing in the temperature range from 700 C to 800 C was investigated by in situ Transmission Electron Microscopy (TEM) and conventional TEM of specimens annealed in bulk form. It was found that the amorphous Al2O3 transformed to the transitional gamma phase along an irregular front. The recrystallized material is columnar with the individual columns or domains irregular in size and shape. The alpha-Al2O3 is composed of two variants which are twin related and have the following orientation relationship with the substrate: <110> gamma <10 bar 1 0> alpha and 111 gamma 0001 alpha . The temperature dependence velocity of the amorphous-to-gamma transformation front was measured by in situ TEM. Assuming a thermally activated process, the activation energy has been determined from an Arrhenius plot of velocity versus T(-1). These results are discussed in terms of possible rate controlling processes of the transformation.