Tailoring of Oxidation-Resistant Nb-base Alloys at Ultra-High Temperatures and Understanding of their Oxidation Mechanism

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Sputter-deposited Nb alloys containing both aluminum and chromium reveal high oxidation resistance above 1073 K, in contrast to less oxidation-resistant Nb-Al and Nb-Cr binary alloys. The oxidation of the ternary alloys generally follows the parabolic rate law, indicating that diffusional transport of matter is the rate-determining step. However, the high oxidation resistance of the ternary alloys containing about 40 at% niobium is not due to the formation of an alumina scale, but due to an outer chromia layer with the inner layer consisting of a mixture of alumina, chromium-niobium and/or aluminum-niobium double oxides. When the ternary alloys containing about 40 at% niobium were oxidized in O_2-SO_2 atmospheres, preferential oxidation of aluminum occurs as a consequence of a transient sulfidation of aluminum at the alloy/scale interface. However, the preferential oxidation results in internal oxidation of the alloys, not in the formation of continuous external alumina scale. Further increase in aluminum content by reducing niobium content, finally results in the formation of highly protective α-alumina scales above 1073 K. Coating of this aluminum-rich niobium alloy to niobium-base alloys with high mechanical properties at high temperatures may induce interdiffusion between the coating and substrate, leading to degradation of the oxidation resistance. The authors have successfully demonstrated that introduction of an anodic alumina layer between the coating and substrate as a diffusion barrier layer suppresses effectively the interdiffusion, thus high oxidation resistance of the coating is sustained at high temperatures.