Damage development during creep in diffusion bonds for high-efficiency heat exchangers

Diffusion bonded (DB) compact heat-exchangers (CHX) are an attractive technology for applications including advanced gas cooled nuclear reactors, concentrated solar power (CSP) plants, and supercritical CO₂ (sCO₂) power cycles. The diffusion bonding processes allows for the economical production of complex microchannels to enable high efficiency heat exchange with the capability of operating at high fluid/gas temperatures and pressures. To date, most of the DB-CHX’s are utilized at temperatures below the creep regime utilizing traditional stainless steel. Furthermore, literature data on the creep performance of diffusion bonds is from laboratory scale studies which may not be representative of commercial diffusion bonding practices and generally shows diffusion bonds have reduced creep life and ductility in comparison with the parent base materials, but details on the damage mechanism(s) are not well established. In this paper, we look at two materials, austenitic stainless steel 316 and nickel-based alloy 617, and explore the damage development during elevated temperature tensile, fatigue and creep in commercially produced diffusion bonds using optical and scanning electron microscopy. The results show that depending on test condition, testing type, and material, pre-existing voids, from the diffusion bonding process, influence damage development in different ways. These findings provide a technical basis for addressing component modeling, design, and life management of DB-CMX operating in the time dependent regime.