[期刊论文][Full-length article]

出版年：2023

The joining of F82H and pure Cr metal has been demonstrated at temperatures as low as 798 K, assisted by the prior application of a thin metastable Cr interlayer to both bonding surfaces using pulsed laser deposition. Bonded F82H/Cr interfaces were characterised by SEM-EDS and TEM analysis, with respect to diffusion behaviour, chemical composition and microstructure. At higher bonding temperatures, an interface layer formed on the Cr side of the joint, which was rich in M₂₃C₆, though its structure and thickness was different depending on whether Cr interlayers were applied. As the bonding temperature was decreased stepwise, the interface layer thickness reduced to a minimum of ∼60 nm and appeared to adopt a film-like structure. Although the chemical composition of the thinner interface layers could not be determined, they were most likely M₂₃C₆ in a film-like form, which subsequently formed particles, and then columns, as the bonding temperature and interface layer thickness increased. Hardness around the interface increased as compared with the bulk F82H and Cr, though the peak hardness value decreased as the interface thickness decreased. The hardness of bulk F82H and Cr was similar before and after bonding at all temperatures, suggesting little impact on mechanical properties. Diffusion of Cr into F82H and Fe into Cr was not detected at any of the conditions tested, and the original tempered lath martensite microstructure of F82H appeared to have been retained. Introduction Japanese DEMO (JA DEMO) is a steady-state conceptual fusion power plant design being developed in Japan, with an expected fusion power output between 1.5 and 2 GW [1]. JA DEMO will utilise a water-cooled solid breeding (WCSB) blanket design, which is expected to operate under conditions analogous to pressurised water reactors, with water temperatures in the range 563–598 K and pressures of ∼15.5 MPa [2]. The reduced activation ferritic-martensitic (RAFM) steel, F82H, has been developed as the main coolant system piping material. Given the relatively low Cr concentration in F82H (∼8wt%), uniform corrosion in the coolant environment is somewhat higher than in other materials, such as stainless steels [3]. Subsequently, several studies have been performed to better understand the corrosion behaviour of F82H in the water coolant environment, and optimisation of the coolant chemistry remains an ongoing task [4], [5], [6], [7], [8], [9]. Drawing from corrosion protection coating development for water-cooled fission reactors [10], the concept of Cr-coated F82H was recently proposed by the authors of this paper as a complementary approach to minimising the corrosion of F82H in the JA DEMO breeder blanket coolant system [11,12]. These studies showed that while solid-state diffusion bonding could be used to join F82H and Cr, considerable microstructural and mechanical property change was observed at the bonded F82H/Cr interface and within F82H, particularly after heating above the Ac₁ temperature (>1089 K [13]). Bonding below Ac₁ was demonstrated down to a temperature of 964 K and over reasonably short timescales (240 min), which practically eliminated diffusion of Cr into F82H, and allowed the F82H to retain a tempered lath martensite microstructure with a comparable hardness before and after bonding. However, a thin C-rich interface region comprising hard, brittle M₂₃C₆ still formed due to the diffusion of C from F82H, which is likely to have an impact on the joint properties. The formation of M₂₃C₆ aligns with the wide experience of applying Cr diffusion coatings to different steels via gaseous methods [14], where a thin layer of M₂₃C₆ is typically formed at the steel substrate surface when the steel C content is high enough. A review published by Liu et al. provides a comprehensive overview of recent progress and challenges associated with dissimilar metal bonding involving RAFM steels [15]. A method often used to prevent formation of undesirable interface compounds and microstructures is via the application of an interlayer material. For example, Ni and Ti interlayers have been used to eliminate the formation of brittle intermetallics between ferritic steels and tungsten during solid-state diffusion bonding [16,17]. While successful in preventing undesirable intermetallic compounds, the introduction of new materials to the system brings additional complexity and uncertainty in how the interface could evolve under fusion power plant conditions. If bonding was possible at lower temperatures, it could significantly supress the formation of undesirable interface species and avoid the additional complexity of dissimilar materials as interlayers. Increasing the reactivity of a material surface by utilising it in an amorphous or nano-crystalline state may be one way to achieve lower temperature bonding. Both states are considered metastable, which at elevated temperatures will transform into the coarse-grained crystalline state, either via amorphous-to-crystalline transformation, or through grain coarsening of nano-crystals. The fabrication of thin amorphous, nano-crystalline, or even mixed amorphous/nano-crystalline coatings via pulsed laser deposition (PLD) of various metals, including Cr, has been reported previously [18], [19], [20], [21], [22]. If the cooling rate of the deposited material is high enough when it initially interacts with the substrate, a thin amorphous layer can be formed directly on the substrate [23]. As the deposition layer continues to grow it gradually heats up due to the high energy of the depositing atoms, meaning further deposition tends to form columnar nano-crystals on top of the initial amorphous layer. Utilising the approach of applying a thin amorphous/nano-crystalline Cr layer to both F82H and Cr substrates, before bonding at elevated temperatures, may promote joining by exploiting the increased reactivity of metastable Cr. This is thought to be possible because amorphous materials have a considerably higher vacancy concentration than their crystalline counterparts, which may help to facilitate both vacancy- and interstitial-type diffusion at lower temperatures, and thus lead to the formation of a metallurgical bond. Furthermore, surface roughness is another key parameter to ensure successful bonding of F82H [24]. The lower surface roughness that can be achieved by depositing a thin Cr layer on each substrate should allow better contact between bonding specimen surfaces compared with mechanochemically polished surfaces, for example, and thus increase the extent of interaction at the interface. Since the amorphous-to-crystalline transition temperature of Cr is within the range 573–673 K [19,25], it can be recrystallised at temperatures considerably lower than those required for conventional solid-state diffusion bonding [12]. Therefore, a combination of the low Cr recrystallisation temperature, lower surface roughness and increased surface reactivity could allow bonding between F82H and Cr at relatively low temperatures, via pre-application of thin amorphous/nano-crystalline Cr layers. Furthermore, it may be possible to suppress the formation of a brittle M₂₃C₆ interface layer between F82H and Cr, and at the same time the deposited Cr around the bonded interface should be fully transformed into the coarsened-crystalline state. This paper examines the bonding between F82H and Cr in the temperature range 775 to 984 K over 240 min with prior application to bonding surfaces of Cr deposited by PLD. First a comparison is made between F82H and Cr specimens bonded at 984 K for 240 min both with and without prior application of Cr interlayers, paying attention to the diffusion of C and Cr and the structure of the interface layer formed. Second, the bonding temperature was reduced stepwise in separate experiments (871, 798 and 775 K) to examine whether prior application of Cr by PLD to bonded surfaces could promote joining at lower temperatures than conventional solid-state diffusion bonding. Again, the elemental diffusion behaviour and interface structure was characterised. Control experiments were undertaken at 873 and 798 K with no prior application of Cr by PLD. For the joining process, the solid-state diffusion bonding methodology outlined in our previous work was used [11,12]. Finally, the hardness of the F82H/Cr bonded interface and interdiffusion zone was measured to examine the post-bonding mechanical properties. Section snippets Cr application by pulsed laser deposition For all experiments, specimens of the F82H-IEA steel heat #9741, hereafter F82H , were utilised in the tempered martensite form following standard normalising and tempering heat treatment as outlined previously [12,26]. A commercial plate of 99.9% pure Cr with thickness 2.5 mm was also used for bonded specimens and the pulsed laser deposition target, and the chemical composition of all materials is given in Table 1. F82H specimens were cut to the size 5_L × 5_W × 0.5_T mm³, and Cr was cut to 5_L × 5_W Deposited Cr layer microstructure A TEM bright field image cross-section of the Cr deposited on a Cr substrate by PLD over a 210 min period is shown in Fig. 1a. The total thickness of the deposited Cr is approximately 300 nm, which is clearly separated into two layers. First is a thin layer with thickness ∼50 nm in direct contact with the Cr substrate, which appears to have an amorphous microstructure. Since the Cr substrate was at room temperature before deposition began, the initial coating will have experienced an ultra-fast Discussion The SEM and TEM analysis presented in this paper have highlighted some differences between the F82H/Cr bonded interfaces formed at temperatures of 984 K with and without PLD-Cr interlayers. While the interface appears to be rich in M₂₃C₆-type carbides in both cases, the structure of the interface is vastly different. For the interface formed via conventional diffusion bonding, the interface thickness appears uneven, with a range between ∼100 and 300 nm. This suggests the growth rate of the Conclusion It has been clearly demonstrated that prior application of amorphous Cr interlayers by pulsed laser deposition has allowed bonding between F82H and Cr at lower temperatures than conventional solid-state diffusion bonding. Bonding with PLD-Cr interlayers at 984, 871 and 798 K has shown that the thickness and structure of the bonded interface varies considerably with bonding temperature. Furthermore, with reducing bonding temperature the peak hardness at the bonded interface was reduced, while CRediT authorship contribution statement Reuben Holmes: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing, Visualization. Lijuan Cui: Conceptualization, Investigation. Bo Li: Methodology, Investigation. Toshiyasu O: Investigation. Sho Kano: Conceptualization, Investigation, Supervision. Huilong Yang: Conceptualization, Supervision. Hiroaki Abe: Conceptualization, Methodology, Supervision, Resources. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are grateful to the National Institutes for Quantum Science and Technology (QST) for supplying the F82H-IEA heat material used in this research. One of the authors (R.H.) appreciates the continuous support by the Ministry of Education, Culture, Sport, Science and Technology of Japan (MEXT), scholarship recipient number 200535. This study was also carried out partly under the collaborative research project at the Nuclear Professional School, School of Engineering, The University of References (31) Y. Someya et al. Development of water-cooled blanket concept with pressure tightness against in-box LOCA for JA DEMO Fusion Eng. Des. (2019) N. Yamanouchi et al. Accumulation of engineering data for practical use of reduced activation ferritic steel: 8%Cr-2%W-0.2%V-0.04%Ta-Fe J. Nucl. Mater. (1992) J. Lapeña et al. Water corrosion of F82H-modified in simulated irradiation conditions by heat treatment J. Nucl. Mater. (2000) Y. Miwa et al. 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