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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 7 2 6 typically is only used for narrow-gap brazing (joint gap ≤500 µm). For SC su- peralloy brazing, TLP bonding is partic- ularly challenging because bulk lattice diffusion of MPDs such as boron is slug- gish due to the absence of grain bound- aries as a high diffusivity path [4] . Wide-gap brazing, or WGB (joint gap ≥500 µm), has more restrictions compared with narrow-gap brazing. Capillary forces cannot be relied on to fill the gap, and the time needed to suffi- ciently diffuse enough MPD is extremely long. WGB technology uses an additive gap-filler alloy that is chemically com- parable to the base material and has similar brazing mechanics to TLP bond- ing. Aside from helping to bridge the large gap, the additive gap filler serves as a sink for MPDs and provides capil- lary pressure to help flow the molten brazing alloy. This is also helpful for SC superalloys due to the lack of grain boundaries for traditional TLP bonding. Additive gap-filler metal provides an ad- ditional pathway for MPD diffusion. The process is also known as activated diffu- sion healing, and is extensively used in the gas turbine industry, but is primar- ily limited to noncritical components. Nanomaterials have been of interest for low temperature joining due to a well-known, size-dependent reduc- tion of the melting temperature [5] . After joining, nanomaterial properties more closely resemble bulk material proper- ties. The past decade has seen a grow- ing interest in developing nanomaterial pastes (nanopastes, or NPs) for brazing and high temperature applications [6] . VACUUM BRAZING Techniques available for joining and repairing turbine parts under vac- uum include conventional brazing, TLP bonding, and WGB. TLP bonding combines the advantages of both dif- fusion bonding and conventional braz- ing techniques; a high-quality joint with similar microstructure and mechanical properties to those of the parent mate- rial can be achieved in turbine engine materials such as Ni 3 Al intermetallic al- loys [7] and SC Ni-base superalloys [8] . Studies show the potential of na- nobrazing material in superalloy join- ing. Hdz-Garcia et al. [9] used tungsten nanoparticle impregnation in con- junction with BNi-9 to braze Inco- nel 725. Ma et al. [10] investigated the feasibility of brazing Nickel 200 using Ag-nanowire paste and filler materi- al, producing a joint shear strength of ~45 MPa under 700°C with little applied pressure. The study also demonstrat- ed the effect of temperature and pres- sure on joint density and strength (Fig. 2). The melting and solidification behavior of nanomaterials when heat- ed above the nanoscale melting tem- perature is currently unknown. One hypothesis is that the nanomaterials first undergo surface melting [11] , and if the heating rate is sufficiently high (as in laser brazing), the adjacent nanopar- ticles completely melt and undergo so- lidification. If the heating rate is low, nanoparticle surfaces join together without fully melting due to a loss of size-dependent melting point depres- sion, so the brazed joint could still have some small pores remaining. However, full densification can still occur with ap- plied pressure and solid state diffusion (Fig. 3) [12] . Zhang [13] showed that brazing In- conel 718 using Ni-base nanoparticles can achieve a joint shear strength of 100 MPa. Vacuum nanobrazing of SC superalloys is largely unexplored, al- Fig. 2 — SEM cross-sectional images of joint brazed at temperatures of 300°, 500°, and 700°C: (a-c) Ag nanowires, (d-f) Ag nanopastes, (g-i) Cu-Ag core-shell nanowires [10] .