Ceramics-Silikáty 41, (1) 13 - 20 (1997)

The experiments were carried out at 1600°C for 4-hour periods under a pressure of 1.2 bar Ar (melting of Ni-Cr, Ni-CrTi alloys and high-temperature exposure of vacuum-deposited layers), and at 1900°C or 1720°C for 25-minute periods under a pressure of 1.2 bar Ar (melting of pure chromium or titanium). The resultant interfaces were studied by scanning and transmission electron microscopy, X-ray microanalysis and X-ray phase analysis. The mechanism of high-temperature interactions of chromium- and titanium-rich metals with monocrystalline Al₂O₃ includes a redox reaction of the reactive element in the alloy with alumina, yielding Cr₂O₃ or Ti₂O₃. On the basis of calculations according to Van´t Hoff´s isotherm, the amount of aluminium in the metal required for stopping the reaction was estimated: For the reaction of chromium with Al₂O₃, this amounted to at the most tenths of weight percent, and for the reaction of titanium with Al₂O₃, it corresponded to units percent and up to tens of percent. Under the conditions of the interaction experiments, the amounts of aluminium established in the metal after the exposure were found to agree with the thermodynamic calculations, being lower than the estimated equilibrium concentrations. The interaction of the elements in question with monocrystalline Al₂O₃ was found to be more extensive in the case of the elements in pure state than in that of an admixture in Ni-alloys.