Transient-Liquid-Phase Bonding of H230 Ni-Based Alloy Using Ni-P Interlayer: Microstructure and Mechanical Properties
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THE use of supercritical CO2 (sCO2) as a working fluid in power generation results in higher thermodynamic efficiencies compared to conventional power cycles that use steam as a working fluid.[1,2] This high thermodynamic efficiency is in part due to the use of compact heat exchangers (CHX), which enhance heat transfer between the high- and low-temperature working fluid. One of the CHX designs features a microchannel architecture[3] which is fabricated by joining multiple thin sheets which have microchannel design features machined in them. These sheets are then stacked and bonded together using a diffusion-based process such as transient-liquid-phase (TLP) bonding. A primary reason for using this joining process is its ability to yield tight dimensional tolerances for the microchannels, with MONICA KAPOOR and O¨MER N. DOG˘AN are with the National Energy Technology Laboratory, U.S. Dept. of Energy, 1450 Queen Ave SW, Albany, OR 97321. Contact e-mail: monica.kapoor@ netl.doe.gov CASEY S. CARNEY is with National Energy Technology Laboratory, U.S. Dept. of Energy, and also with AECOM, 1450 Queen Ave SW, Albany, OR 97321. RAJESH V. SARANAM, PATRICK MCNEFF, and BRIAN K. PAUL are with the School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, 204 Rogers Hall, Corvallis OR 97331. Manuscript submitted February 9, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
hydraulic diameters as small as a few hundred micrometers. Achieving tight dimensional tolerances in these microchannel HXs is critical to avoid pressure drop along the flow channels and thereby cause inefficiencies in operation. These CHXs for sCO2 applications typically operate at 20 to 30 MPa pressure differential between the hot and cold sides and up to 923 K to 1023 K (650 C to 750 C) on the hot side, with an anticipated lifetime of 30 years. These operating conditions whittle down the field of currently available candidate materials to a few Ni-based alloys. H230 is one such Ni-Cr-W-Mo solid-solution-strengthened Ni-based alloy which provides excellent creep resistance and oxidation properties near the operating temperature of these heat exchangers.[4–6] TLP bonding uses a coating on the joining surfaces, which acts an interlayer between the joining surfaces and has a lower melting point than the two surfaces which are being joined. This lower melting point is achieved by adding an element which acts as a melting point depressant (MPD), such as B, P, or Cr[7–16] for joining Ni or Ni-based alloys. On heating up to the bonding temperature, the interlayer liquefies and the MPD diffuses into the base metal, lowering its melting point and resulting in the expansion of the liquid zone into the base alloy.[17] Over time, this liquid zone
homogenizes to the equilibrium composition at the bonding temperature. As diffusion of MPD into the base alloy continues, the liquid zone begins to isothermally solidify forming a monolithic microstructure between the two surfaces to be joined. The solidification is driven by a change in composition at t
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