Diffusion Paths of Silicide Coatings on Nb-Si-Based Alloys During Pack Cementation Process
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er density, high melting point, and excellent high-temperature strength, Nb-Si based alloys are attractive as the most promising structural materials at high temperatures.[1–3] Nb-Si-based alloys are mainly composed of Nb solid solution (Nbss) and silicides (including Nb3Si and/or Nb5Si3), where the Nbss phase offers room-temperature toughness while the silicides supply the high-temperature strength. As widely known, poor oxidation resistance and room-temperature fracture toughness are severe problems for Nb-Si-based alloys,[4] which limit their more extensive applications. By the addition of alloying elements, the multicomponent Nb-Si-Ti-Cr-Al-Hf alloys have been developed achieving a balance in mechanical, environmental properties, and oxidation resistance.[5] However, the inferior oxidation resistance remains the major weakness of the
WEI SHAO is with the Department of Materials Science and Engineering, Beihang University, Beijing 100191, China, and with the Instituto de Ciencia de Materiales de Arago´n, 50009 Zaragoza, Spain. YUWEN CUI is with the Tech Institute for Advanced Materials & School of Materials Science and Engineering, Nanjing Tech University, Nanjing 211800, China, and with the Instituto de Ciencia de Materiales de Arago´n. Contact e-mail: [email protected] CHUNGEN ZHOU is with the Department of Materials Science and Engineering, Beihang University. Contact e-mail: [email protected] Manuscript submitted November 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
materials for high-temperature application, which deserves to be improved. Recently, oxidation-resistant coatings, mainly silicide coatings including NbSi2 and others,[6] have been developed. The oxidation resistance of the coating is maintained owing to the formation of the glassy SiO2-based layer above the compact silicide coatings during the oxidation process.[7] Furthermore, the modification of the silicide coatings by alloying elements such as Al,[8] Y,[9] B,[10] Ge,[11] and so on, has been considered an effective way to promote the oxidation resistance. The addition of B and Ge into silicide coatings can improve the oxidation resistance due to the formation of SiO2-B2O3[12] and SiO2-GeO2,[11] which possess a lower viscosity than SiO2, thus, reducing the cracks and holes. Nevertheless, the oxidation resistance of the coating is crippled during the consumption of the NbSi2 layer.[13] In both thermodynamic and kinetic points of view, the stability and the property of the coating/substrate system are both significantly governed by the underlying diffusion behavior.[14] Thereby, it is of technological importance to explore the diffusion paths in the silicide coating/substrate systems to shed light on the ultimate oxidation resistance. Accordingly, the purposes of the present work are to investigate experimentally the diffusion behaviors for the silicide coating/substrate and B-modified silicide coating/substrate systems, from which to elucidate the critical diffusion paths in the perspective of the Nb-Ti-Si-Cr quaternary and Nb-Ti-Cr-Si/Nb-Ti-B-Si
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