1 7 ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 optimization prior to moving forward with larger footprint bonding trials. The microstructure at these interfaces will inform phase field models to develop a multi-scale metallurgical model of the diffusion bonding process that emulates grain recrystallization and growth at the bond interface and investigates the impact of material and preparation parameters on the bond. In addition, it allows the development of material deformation crystal plasticity models that predict the creep strength of the bond based on microstructure and predict the creep, fatigue, and creepfatigue performance for long-term service conditions that cannot be achieved in a reasonable experimental time. Second, the acceptance qualification loop would involve fabrication of diffusion-bonded specimens in compliance with the size requirements specified in Section IX of the ASME BPVC. These are plates that are 200 by 200 mm with at least 50 diffusion bonds. Bonding parameters identified in the rapid iteration loop will be applied in a commercial vacuum hot press. Detailed microstructural characterization and mechanical testing will be performed on bond coupons and test specimens cut out of these larger test blocks. Bond strength would be determined through tensile, creep, fatigue, and creep-fatigue testing and complemented with models based on the microstructure that can predict long term performance. Results will be compared to starting sheet performance after thermal exposure to the optimized diffusion bonding cycle to provide direct correlation of properties in addition to comparisons with the larger existing wrought material databases used in heat exchanger design. Figure 4 shows an example of macro-scale characterization of a diffusion bonded block after creep testing to confirm failure location and strain accumulation. The goal is to use the results from the pre- and post-test microstructural characterization, mechanical testing, and multiple length scale computational models to develop acceptance criteria to ensure that the diffusion-bonded plate will perform as intended during Section III, Division 5 service. These acceptance criteria will likely focus on a performance-based approach rather than a fabrication-based approach. The proposed acceptance criteria may be a list of microstructural and mechanical property requirements that must be met. Furthermore, the models may be utilized to help identify boundary conditions and sensitivity of these of fabrication methodology for the acceptance criteria. As a final step after the acceptance criteria have been developed, bonded blocks will be fabricated with and/or without channels that meet the size requirements specified in Section IX of the ASME BPVC. Detailed microstructural characterization and mechanical testing will be performed on bond coupons and test specimens cut out of these larger test blocks. Results will be used to validate the acceptance criteria for ASME BPVC Section III, Division 5 applications. This will utilize the results from microstructural and mechanical property characterization as well as phase field and crystalplasticity modeling for the plates with and without micro-channels. SUMMARY Diffusion bonded heat exchangers represent an enabling technology for the energy transformation with great potential for high-temperature applications in advanced nuclear reactors and other technologies. However, the current research reveals a number of Fig. 3 — EBSD maps of grain size distribution around the di usion bonding line of Alloy 800H; (le ) shows grain interdi usion across the bonding line and (right) shows oxide particle trapped at the bonding line. Fig. 4 — Example of post-test analysis of a stainless steel 316 di usion bonded subject to creep testing at 750°C showing creep cavity formation (insert) and failure along the bond line. Additional measurements (mm) indicate substantial deformation (creep strain accumulation) between bond lines prior to failure and outer diameter crack initiation at bond lines (arrows).
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