The sulfidation and oxidation behavior of sputter-deposited amorphous Al-Nb-Si alloys at high temperatures

Abstract

In order to improve the oxidation and sulfidation resistance of binary Al-Nb alloys at high temperatures, the effect of silicon addition on the sulfidation and oxidation behavior of sputter-deposited Al-Nb-Si alloys was studied as a function of temperature (1073 to 1273 K) and alloy composition in He-S 2 and Ar-O 2 atmospheres. The sulfidation of the ternary alloys follows a parabolic rate law, although two-stage parabolic behavior is definitely observed for Al-54Nb-20Si. The steady-state sulfidation rates of this alloy are one order of magnitude lower than those of niobium at the temperatures examined. The sulfidation rate decreases with increasing alloy silicon content for alloys containing about 50 at.% niobium, but is almost independent of silicon content for high aluminum alloys. A duplex sulfide scale, comprising an outer aluminum sulfide layer containing small amounts of silicon and an inner niobium sulfide layer, is formed on all the alloys studied. It is assumed that the improved sulfidation resistance of Al-Nb alloys by addition of silicon is responsible for the decrease in the amount of aluminum in the alloy, which is preferentially sulfidized and whose ions have extremely high diffusibility in the protective niobium sulfide layer. The oxidation resistance of Al-Nb alloys is also improved by the addition of silicon, particularly when the niobium content is about 50 at.%. A ‘pest phenomenon’, observed during oxidation of Al-Nb binary alloys, is not found for the present ternary alloys. The oxidation of the alloys containing about 50 at.% niobium follows a parabolic kinetic, in contrast to the linear kinetics for the oxidation of binary alloys. The oxidation rates of the former alloys are almost comparable to that of chromia-forming alloys, though still higher than those of alumina-forming alloys due to the formation of NbAlO 4 in the scale. Rapid oxidation after an incubation period is observed for Al-Nb-Si alloys with high aluminum content.