Oxidation behavior of Ge-modified Nb-Si alloys at 1250 °C: Self-generated NbGe2 layer with enhanced oxidation resistance

The effects of Ge on the microstructure, oxidation kinetics, and oxidation behavior of Nb-Si alloy were systematically investigated. Although Ge has no effect on the constituent phases of the alloy, it reduces the Si content at which the eutectic reaction occurs in Nb-Si system and promotes forming coarse silicides. Experimental results indicate that Ge significantly decreases oxidation rate at 1250 °C of Nb-Si alloy and enhances both the adherence and density of the scale. The Nb-16Si-20Ti-1.5Zr-1B-1C-12Ge demonstrates superior oxidation resistance, with a weight gain of merely 82.2 mg/cm² after oxidation at 1250 °C/50 h, representing a 64.9 % reduction compared to the base alloy (234.06 mg/cm²). This enhancement can be ascribed to the generation of an in-situ formed NbGe₂ layer with enhanced oxidation resistance between the matrix and the scale in Ge-containing alloys. The NbGe₂ layer substitutes the originally porous transition layer, which significantly impedes the inward diffusion of oxygen. For alloys containing a low Ge concentration (≤3 at.%), a non-continuous NbGe₂ layer is generated, which reduces the depth of the internal oxide layer. In contrast, alloys with higher Ge content (≥6 at.%) develop a uniform and continuous NbGe₂ layer that entirely coats the matrix, completely preventing oxygen penetration, and eliminating the internal oxide layer. Furthermore, as oxidation progresses, NbGe₂ layer becomes thicker, offering sustained and reliable protection to the alloy. These findings offer valuable insights for designing advanced Nb-Si alloys with enhanced high-temperature and oxidation resistance.