Oxidation behavior of Ge-modified Nb-Si alloys at 1250 °C: Self-generated NbGe2 layer with enhanced oxidation resistance
Dezhi Chen, Wotai Gong, Chao Xu, Yijie Zhang, Hao Ren, Zhenbo Yang, Ruirun Chen, Hengzhi Fu
Abstract
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.