Transmission electron microscopy (TEM) studies of the oxide film on the sur
face of a zirconium alloy have been performed and the crystallography of in
terfaces has been analysed. Regions of a peculiar microstructure consisting
almost entirely of twins have been discovered in the zirconia film. The TE
M results have been analysed in conjunction with the known texture data, an
d the orientation relationships between monoclinic ZrO2 and the alpha-Zr su
bstrate are discussed. Computer simulation has been performed to reveal the
possible grain boundary character distributions in different regions of th
e film. The relevance of the results obtained to the diffusion properties o
f the surface zirconia layer is discussed in this paper. It is suggested th
at a high concentration of special grain boundaries hinders transport throu
gh the zirconia film.