THE SULFIDATION OF SPUTTER-DEPOSITED NIOBIUM-BASE ALUMINUM-ALLOYS

The sulfidation behavior of sputter-deposited niobium base Nb-Al alloy s has been studied as a function of temperature (1073-1273 K), aluminu m content (0.6-15.2 at%) and sulfur activity (40-4000 Pa) under isothe rmal-isobaric conditions. It has been found that under steady-state co nditions the sulfidation process follows a parabolic rate law, the slo west step of the overall reaction rate being the outward diffusion of cations. The initial transient state of the reaction was longer when t he aluminum content in the alloy and the temperature of sulfidation we re higher. The sulfide scales on all the alloys consisted of the NbS2 phase doped with aluminum and with a strongly developed growth texture , characterized by long columnar crystals situated perpendicular to th e substrate surface. The c-axis of the hexagonal structure of niobium sulfide has been found to be parallel to the underlying alloy, i.e. th e two-dimensional sulfide planes were situated perpendicular to the al loy surface. With increasing aluminum content in the alloy the structu re of the scale changes from 3s-NbS2 to 2s-NbS2. The surfaces of the s ulfide scales on Nb-5.0Al and Nb-15.2Al alloys were transformed to Nb2 O5 and AlNbO2, respectively, as a result of interaction of the scale s urface with trace amounts of oxygen.