| To ensure the tritium self-sufficiency and environmental safety in fusion reactors, structural materials in the blanket and auxiliary tritium handling systems must have the lowest tritium permeability. Apart from the high reliability of the structural design of tritium confinement and tritium handling systems, coating, with a low permeability for tritium, named tritium permeation barrier (TPB) is one of the most effective methods to minimize tritium permeation through structural materials to the environment and other systems. FeAl/Al2O3 type aluminide coatings have been selected as one of the prior developed TPBs for the TBMs by Europe, China, United States and India for its high permeation reduction factor (PRF), low thermal mismatch, metallurgical bonding, excellent compatibility, and self-healing. However, the practical performance of aluminide TPBs often exhibit lower efficiency than anticipated based on the bulk coating material properties. The possible reason can be alloying effect in the aluminide TPB coatings, including substrate effect and doping effect. However, the aluminide TPB studies are mainly devoted to the preparation technique and performance optimization at present, and the coating preparations are conducted only on limited steel substrates, while the effect of substrate material characteristics including alloying elements and microstructures etc. on the coatings and the related mechanisms have not been systematically and deeply involved. For this reason, the formation process and deuterium permeation resisting performance of aluminide TPB coatings formed on different Cr steel substrates have been experimentally investigated, and the substrate effect in aluminide TPBs is conformed, and the influence factors of substrate effect are also revealed. Meanwhile, effects of the main alloying element Cr in steel substrates on the defect chemistry and existing forms and transport behaviors of H in Al2O3, the main functionally part of aluminide TPB coatings, have been theoretically explored, and the role of Cr in aluminide TPBs is revealed. The results are anticipated to be instructive for the selection, development and optimization for the preparation technique of aluminide TPB coatings on different types of structural steels, and also meaningful for studies on the preparation and performance of aluminide coatings for wider aspects. The results are summarized as follows:1) The substrate effect is conformed for aluminide TPB coatings. Minor difference exists in the microstructures and crystal grains of Al coatings deposited on different Cr steel substrates via ionic liquid plating, while obvious difference emerges for the Al coating thickness. After heat treatment at vacuum, the thickness, composition and phase constituent of aluminide layers formed on different Cr steel substrates deviate, and the alloying element Cr from steel substrates is involved in the aluminization process. For each type of Cr steel substrate, scales of γ-Al2O3 form on the surface of aluminide layers after selective oxidation, yet the film thickness differs a lot. The stable deuterium permeation fluxes of Cr steel samples are reduced up to 2 orders of magnitude, for which the reduction extent varies with different Cr steels. It can thus be seen that the substrate effect exists in the aluminide TPB coatings, which will exert significant influence on the formation of aluminide TPBs and their deuterium permeation resistance.2) The influence factors of substrate effect in aluminide TPB coatings are revealed. In the aluminization process, the stratification, growth manner of crystal grains, thickness, composition and phase of aluminized layers will be significantly influenced by heat-treating temperatures. The substrate effect is obviously affected by heat-treating temperatures, which will be firstly enhanced and then degraded with the increase of temperatures. The alloying element Cr in steel substrates has a major impact on the formation of aluminide coatings, and at relative lower heat-treating temperatures (≤700℃), Cr is unfavorable for the formation of aluminized layers; while at much higher temperatures (> 700℃), the negative effect of Cr on the formation of aluminized layer will be reduced. Moreover, the crystal and metallo graphic structures of substrate materials will also exert some effect on the substrate effect of aluminide coatings. Therefore, the substrate effect originates mainly from the alloying elements and micro structures of substrate materials.3) A new perspective on the process of intrinsic point defect in α-Al2O3 is proposed. Considering the various charge states for each intrinsic point defects, VAl3-, VO0, Ali3+, Oi2-, AlO3+, and OAl3-, not all in their fully ionized states are found to be most stable and in pure Al2O3. From the formation energies of individual point defects, the antisite atom OAl will be readily formed in α-Al2O3 in an O-rich environment. By combination of charge states and formation energies, the defect types of Schottky, Al Frenkel and antisite pair formed are found to be dependent on the O condition, and the most stable Schottky defect type is not the common considered{3VO2+:2VAl3-}. There are two types of possible O Frenkel defect under both O conditions, yet the most stable defect is{Oi1+:VO1-} rather than the common believed {Oi2+:Vo2-}. The bizarre configuration and charge state of Schottky and Frenkel defects provides a new perspective on the process of intrinsic point defects in α-Al2O3.4) The role of Cr in the process of intrinsic point defects in α-Al2O3 is revealed. Cr has significant influence on the formation, charge state, relative stability and equilibrium configuration of isolated intrinsic point defects in α-Al2O3, resulting in the variation of defect process. Specifically, depending on the O-condition, the possible defect types, the dominant defects and the defect formation energies will be altered in α-Al2O3 after Cr doping. Generally speaking, Cr is favorable for the formation of Schottky defect, Frenkel defect and antisite pair, giving a different insight to the defect process in α-Al2O3.5) The role of Cr in the hydrogen interaction with intrinsic point defects and hydrogen diffusion in α-Al2O3 is revealed. The formation, existing form, charge states and relative stability of H-related defects in α-Al2O3 will be significantly influenced by Cr. Comparing with the α-Al2O3-H case, Cr is beneficial for the formation of H-related defects in α-Al2O3, whereas unfavorable for the Hi trapping ability of VAl and Vo. Ho" will dominate among Hi-, VAl3-, VO0 and [VAl3-’-H+]2-, and only one step of Hi reorientation will be involved for the Hi diffusion in Cr-doped α-Al2O3. Hi is the dominant diffusion species in both pure and Cr-doped α-Al2O3, whereas the activation energy of H diffusion in Cr-doped α-Al2O3 is sharply reduced, which is unfavorable for H-permeation resistance of aluminide TPBs. The Cr effect on hydrogen behaviors in α-Al2O3 can be attributed to the chemical unstable electron structure of Cr3+ and relative stronger bonding interaction between H and Cr than that between H and Al or O atoms. |
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