Diffusion bonded compact heat exchangers have exceptionally high heat transfer efficiency and might significantly improve the performance and reduce the cost of supercritical carbon-dioxide Brayton cycle power plants using high temperature heat sources, like high temperature nuclear reactors and concentrating solar power plants. While these heat exchangers have an excellent service history for lower temperature applications, considerable uncertainty remains on the performance of diffusion bonded material operating in the creep regime. This paper describes a microstructural modeling framework to explore the plausible mechanisms that may explain the reduced creep ductility and strength of diffusion bonded material, compared to wrought material. The crystal plasticity finite element method (CPFEM) is used to study factors affecting bond strength in polycrystals mimicking diffusion bonded microstructures. Additionally, the phase field method is also employed to simulate the grain growth and recrystallization at the bond line to model the bonding process and CPFEM is used to predict the resulting material performance to connect processing parameters to the expected creep life and ductility of the material, and to study potential means to improve the structural reliability of the material and the resulting components by optimizing the material processing parameters.

Diffusion bonding is a key manufacturing process for nucleation applications including compact heat exchangers. Accurately predicting the alloy's behavior during the diffusion bonding process presents challenges, primarily due to the intricate interplay of microstructural evolution and physical processes such as compressive loading, temperature history, and component migration. The current study develops a phase-field model designed to simulate the diffusion bonding in 316H stainless steel, a material with exceptional high-temperature strength, corrosion resistance and suitability to high-pressure conditions. Our model incorporates a multi-phase, multi-component framework that aligns the experimental observations with the grain growth and heterogeneous nucleation, where arbitrary external compressive load and temperature history are considered. The simulations focus on grain nucleation, growth, and microstructure evolutions across diffusion bonding line under a variety of temperature profiles, mechanical loads, and surface roughness conditions, mirroring experimental setups. Our model predicts consistent simulation results with experiments in terms of the grain size and distribution near the bonding area, offering a better understanding of the diffusion bonding mechanism and the manufacturing process for building reliable compact heat exchangers.