The Fe-Al/Al2O3 composite coating has been established as a preferential TPB (Tritium Permeation Barrier) for structural materials of fusion reactors, with α-Al2O3 demonstrating superior hydrogen permeation resistance. To optimize coating performance and elevate α-Al2O3 content, rare earth-modified coatings have become a key research priority in international studies. This work systematically examines the effects of rare earth elements on γ-α phase transformation in Al2O3 during the oxidation of Fe-Al layers, while evaluating their effects on microstructure evolution, oxidation kinetics, and phase composition in Fe-Al/Al2O3 composite coatings. Specimens are prepared through pack cementation deposition followed by in-situ oxidation, with comparative analysis conducted between rare earth-modified and unmodified coatings.
Thermogravimetric analysis (TGA) is employed to monitor structural and morphological changes in alumina coatings under elevated temperature. Phase identification and surface characterization are performed by glancing incident X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). The oxidation kinetics are quantified through mass gain measurements, revealing significant differences between modified and unmodified systems.
The rare earth-modified Fe-Al/Al2O3 coating exhibits excellent substrate adhesion with no detectable pores or cracks through microscopic examination. Cross-sectional analysis reveals a 17.03 μm thick Fe-Al interlayer and an approximately 200 nm oxide scale. In contrast, the unmodified coating displays extensive cracking with a reduced Fe-Al layer thickness of 12.81 μm. Enhanced interdiffusion of Fe and Al is observed in the modified system, which is attributed to rare earth-induced acceleration of cation migration rates. This diffusion enhancement effectively minimizes interfacial voids and improves morphological stability during high-temperature exposure.
Kinetic analysis reveals two distinct oxidation stages in the rare earth-modified system. The initial stage (Stage Ⅰ) is characterized by rapid γ-Al2O3 formation, exhibiting an oxidation rate of 3.78×10-14 g2/(cm4∙s). This is followed by a stable growth phase (Stage Ⅱ) dominated by α-Al2O3 development, with a reduced rate of 2.72×10-15 g2/(cm4∙s). Both stages demonstrate significantly enhanced oxidation rates compared with the unmodified coating's single-phase behavior of 2.18× 10-15 g2/(cm4∙s). The accelerated kinetics are attributed to solute drag effects induced by rare earth elements during oxide scale formation. The modified system's dual-stage behavior contrasts with the unmodified coating's persistent γ-phase dominance, highlighting the critical role of rare earth additives in altering both reaction kinetics and phase evolution pathways.
Phase evolution studies reveal critical differences in alumina transformation pathways. In modified coatings, α-Al2O3 nucleation is detected after 3 hours of oxidation, with complete γ-α transformation achieved within 4 hours. Conversely, unmodified coatings retain pure γ-Al2O3 phases throughout the 4-hour oxidation period. This disparity is ascribed to rare earth elements acting as heterogeneous nucleation sites, effectively reducing the activation energy barrier for α-phase formation.
In summary, the incorporation of rare earth elements is demonstrated to significantly enhance coating performance. Improved bonding between the coating and the substrate is achieved through interdiffusion optimization. An accelerated growth rate of Al2O3 on Fe-Al layers is observed, which is attributed to rare earth-induced solute drag effects. The γ-α phase transformation is facilitated at reduced temperature, resulting in an increased α-Al2O3 phase fraction after 4 hours of oxidation. It can be inferred that the tritium resistance of the composite coating will be greatly improved then.
Key words
rare earth modification /
Fe-Al/Al2O3 composite coating /
tritium permeation barrier /
α-Al2O3 /
oxidation kinetics /
phase transformation
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