Abstract
The sulfidation behavior of sputter-deposited niobium base Nb-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (0.6–15.2 at%) and sulfur activity (40–4000 Pa) under isothermal-isobaric conditions. It has been found that under steady-state conditions the sulfidation process follows a parabolic rate law, the slowest step of the overall reaction rate being the outward diffusion of cations. The initial transient state of the reaction was longer when the aluminum content in the alloy and the temperature of sulfidation were higher. The sulfide scales on all the alloys consisted of the NbS 2 phase doped with aluminum and with a strongly developed growth texture, characterized by long columnar crystals situated perpendicular to the substrate surface. The e-axis of the hexagonal structure of niobium sulfide has been found to be parallel to the underlying alloy, i.e. the two-dimensional sulfide planes were situated perpendicular to the alloy surface. With increasing aluminum content in the alloy the structure of the scale changes from 3s-NbS 2 to 2s-NbS 2. The surfaces of the sulfide scales on Nb-5.0A1 and Nb-15.2A1 alloys were transformed to Nb 2O 5 and A1NbO 2, respectively, as a result of interaction of the scale surface with trace amounts of oxygen.
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