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Formation of Exclusive Al2O3 Scale on Nb and Nb-Rich Alloys by Two-Step Oxygen–Aluminum Diffusion Process

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Abstract

External Al2O3 scale formation behavior by a diffusion aluminizing process on Nb and Nb–X (X = Mo, Re, and Ta) alloys with different oxygen solubilities was investigated. The oxygen content in Nb and Nb–X alloys was controlled by oxygen diffusion treatment at 1,100 °C using Nb/NbO mixture. Nb–aluminide, NbAl3 and Nb2Al, layers were developed on the low-oxygen Nb substrate by an aluminum diffusion treatment using Al/Al2O3/NH4Cl mixture at 1,100 °C; whereas an adhesive Al2O3 scale developed on the Nb substrate with higher oxygen content. The alloy oxygen content decreased by an addition of Ta, Mo and Re, in this order, and the decrease in alloy oxygen content resulted in the transition of Al2O3 formation from external scale to internal precipitates. This transition was not observed when low Al activity powder mixture, Ni50Al/Al2O3/NH4Cl, was used for Al diffusion process, and only external Al2O3 scale was developed on all samples. In the present study, the effects of oxygen solubility limit, oxygen content, and the activity of Al in diffusion process on the formation of Al2O3 scale will be discussed.

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Authors and Affiliations

  1. Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, N13, W8, Kitaku, Sapporo, 060-8628, Hokkaido, Japan

    Shigenari Hayashi, Shinsuke Takagi, Ryutaro Yamagata, Toshio Narita & Shigeharu Ukai

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  1. Shigenari Hayashi
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  2. Shinsuke Takagi
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  3. Ryutaro Yamagata
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  4. Toshio Narita
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  5. Shigeharu Ukai
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Correspondence to Shigenari Hayashi.

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Hayashi, S., Takagi, S., Yamagata, R. et al. Formation of Exclusive Al2O3 Scale on Nb and Nb-Rich Alloys by Two-Step Oxygen–Aluminum Diffusion Process. Oxid Met 78, 167–178 (2012). https://doi.org/10.1007/s11085-012-9298-2

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  • DOI: https://doi.org/10.1007/s11085-012-9298-2

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