Abstract
Low-energy Nb/Al 2O 3 interfaces were prepared by diffusion-bonding single crystals of Nb and sapphire (S) with an orientation relationship (110) Nb|(0001) S and [001] Nb|[01⦶10] S. This orientation relationship was previously observed between Nb and Al 2O 3 precipitates produced by internal oxidation. Specimens with foil normals parallel to [001] Nb (orientation I) and [1⦶10] Nb (orientation II), suitable for high resolution transmission electron microscopy, were prepared from the bulk bonds. Lattice images in orientation I revealed misfit dislocations in the Nb with a stand-off distance from the interface of 2 to 3 (110) Nb lattice plane spacings. A preferred local matching of [001] Nb atomic columns with respect to the Al 2O 3 crystal was also observed at this interface. A detailed analysis of the interfacial contrast in orientation II, including image simulations and comparison of observed and calculated contrast, revealed the spacing between the crystals as well as the chemical composition of the interface. The Al 2O 3 crystal is terminated by an Al layer which contains more Al atoms than an Al layer in Al 2O 3. The chemical composition is explained by the local activity of O and Al in Nb at near-interface regions, which is established during cooling from the bonding temperature.
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