FIELD ION MICROSCOPY AND ATOM PROBE MICROANALYSIS OF VANADIUM

Abstract
FIM specimens of vanadium have been prepared by electropolishing in a mixture of methanol and concentrated sulphuric acid (6:1) at a voltage of 6.5 V (d.c.). Imaging in Ne-He mixtures at 40-55 K produced good quality micrographs. The evaporation field in the presence of neon was 40 V/nm. Oxidation of previously field evaporated specimens was carried out in air at 1 atmosphere and temperatures of 298,373 and 473 K. Atom probe analysis through the oxide films showed that the predominant oxide formed at all three temperatures was V₂O₃. There were indications that a transition layer (corresponding to the suboxide V₉O) formed in the region between the main outer oxide film and the metallic substrate. In addition, the bulk metal (which was in the form of fine drawn, 0.1 mm dia. wire) contained approximately 1% oxygen. In one specimen, a high angle grain boundary was observed. This was decorated with brightly imaging impurity atoms. Atom probe analysis revealed enhanced concentrations of carbon and oxygen in the boundary region.