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

溅射沉积的含有铝和铬的铌合金在1073 K以上具有较高的抗氧化性，而Nb- al和Nb- cr二元合金的抗氧化性较差。三元合金的氧化一般遵循抛物线速率规律，表明物质的扩散输运是决定氧化速率的步骤。然而，含有约40 at%铌的三元合金的高抗氧化性不是由于形成氧化铝垢，而是由于外层有铬层，内层由氧化铝、铬-铌和/或铝-铌双氧化物的混合物组成。在O_2-SO_2气氛中氧化含铌约40 at%的三元合金时，由于铝在合金/垢界面处的瞬态硫化，铝发生优先氧化。然而，优先氧化导致合金内部氧化，而不是形成连续的外部氧化铝结垢。通过降低铌含量进一步提高铝含量，最终在1073 K以上形成高保护性α-氧化铝鳞片。在高温下，将这种富铝铌合金涂覆到具有高力学性能的铌基合金上，会引起涂层与基体之间的相互扩散，导致抗氧化性能下降。作者成功地证明了在涂层和基体之间引入阳极氧化铝层作为扩散阻挡层有效地抑制了相互扩散，从而在高温下保持了涂层的高抗氧化性。

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.