16 ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 formance of the diffusion bonded joints, there still exists a lack of fundamental knowledge and data for both the diffusion bonding process and the resultant high-temperature time dependent bond properties including: 1. The role of precipitates and oxide particles. 2. The control of pre-bond sheet surface quality. 3. The importance of base alloy composition and minor alloying elements. 4. The elevated-temperature testing methods, performance, and the need for more robust acceptance criteria. 5. The failure mechanism of the bonded joints. CURRENT RESEARCH DIRECTIONS To address those knowledge gaps, a new multi-institution project supported by the U.S. Department of Energy, Office of Nuclear Energy (DOE-NE) and led by University of Michigan Ann Arbor, aims to (1) improve diffusion bonding of alloys of interest for elevated-temperature nuclear service (Alloy 617 and type 316H stainless steel) so that the elevated-temperature mechanical performance is superior or equivalent to the wrought product form, and (2) establish and verify acceptance criteria to ensure with reasonable confidence that the diffusion-bonded material will behave as intended for the entirety of its service life. This is critical for code case qualification of diffusion bonding for Section III, Division 5 applications. The overall project structure is depicted in Fig. 2, which combines computational modeling, lab scale bonding trials, detailed material characterizations and mechanical testing in successive iterations to expand knowledge more rapidly from the labscale to commercial processing with an aim to provide new guidance for acceptance of diffusion bonding compact heat exchangers for high-temperature application. First, optimal bonding parameters will be determined in successive bonding campaigns with significant input from developed phase field and crystal plasticity models. For each alloy of interest, bond optimization may be separated into two separate cycles: both rapid iteration and acceptance qualification. This approach speeds up the optimization process and reduces material use. The rapid iteration cycle aims to quickly establish diffusion bonding parameters that yield the best microstructure at the bond interface. This may be achieved by employing a high-throughput approach to optimize the diffusion bonding parameters using small samples to reduce costs and resources. The best microstructure is that which is indistinguishable from the rest of the bulk microstructure, i.e., bearing no gross defects, minimal precipitation, and extensive grain boundary migration at the interface. Examples of bond-line characterization by electron backscatter diffraction (EBSD) analysis for two samples are shown in Fig. 3. After a good microstructure is obtained and verified, mechanical screening may be conducted on the small samples to further verify bond Fig. 2 — Illustrative sketch of the action plan for the development of di usion bonding for high-temperature applications.
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