dissolution ceases. This is followed by interdiffusion of base metal and liquid phase. The diffusion of MPD elements (e.g., B and Si) from the liquid phase into the base metal and diffusion of Cr and Fe from the base metal into the liquid phase, increases the liquidus temperature (TL) of the liquid phase. Once the liquidus temperature reaches the brazing temperature, the liquid resolidifies during holding at brazing temperature (i.e., isothermal solidification starts). Due to the absence of solute rejection at the solid/liquid interface during isothermal solidification under equilibrium condition, the only solid phase that forms is the solid solution phase, and formation of other phases is basically prevented (Ref. 11). In situations where the brazing time is not sufficient to allow complete diffusion of MPD elements in the base metal, some amount of liquid remains at the joint centerline, which can undergo eutectic-type solidification during cooling . The joint microstructure is significantly affected by the above-mentioned phenomena. Figure 2A shows a backscattered electron SEM image of the brazes made at 1050°C for 10 min indicating four distinct microstructural zones in the joint region, namely: 1. Isothermal solidification zone (ISZ) in which the solidification of the liquid phase occurs at the brazing temperature. The driving force for isothermal solidification is a compositional change induced by interdiffusion between substrate and interlayer during holding at the brazing temperature. 2. Athermal solidification zone (ASZ) in which the solidification of the liquid phase occurs on cooling. The presence of intermetallic phases and eutectic-type morphologies are the main feature of this region. 3. Diffusion-affected zone (DAZ) in which the microstructure is influenced by the solid-state diffusion of MPD elements into the base metal (BM). This zone is featured by secondary phase precipitates (mainly boride in the case of B-containing filler metals). 4. Base metal in which the microstructure is not affected by diffusion of MPD elements during the brazing process. The corresponding element redistribution across the joint region is shown in Fig. 3 indicating significant chemical composition inhomogeneity across the braze region. In the following section, the microstructural evolution in each zone is described and analyzed. Isothermal Solidification Zone (ISZ) As can be seen in Fig. 2A, the microstructure of the ISZ consists of a singlephase solid solution. This zone is formed due to diffusion-induced isothermal solidification. According to the typical EDS spectrum (Fig. 4A) and typical WDS analysis (Table 1), the ISZ consists of a Nirich γ phase. Presence of elements such as Mo, Nb, Al, and Ti that were not present in the initial interlayer composition (Ni- 7Cr-3Fe-4.5Si-3.2B) indicates dissolution of the base metal. According to EPMA line scan analysis (Fig. 5), this region contains a higher amount of Ni but smaller amounts of Cr, Fe, Nb, and Mo compared to the base metal. No B was detected in the ISZ using the WDS technique (Table 1); however, Si tends to build up in the ISZ (Fig. 3) due to its relatively low diffusion coefficient compared to B. Athermal Solidified Zone (ASZ) Figure 2B shows an optical micrograph of athermal solidification products. Figure 2C, D shows the SEM images indicating the details of ASZ microconstituents. Due to their specific morphologies, it can be inferred that they were formed during eutectic type reactions. Based upon the chemical analysis of the phases, it can be inferred that the microstructure of ASZ consisted of (1) binary eutectic of Ni-rich boride (marked by M in Fig. 2C) and γ solid solution, (2) binary eutectic of Crrich boride (marked by N in Fig. 2C) and γ solid solution, and (3) ternary eutectic of Ni-rich boride (marked by R in Fig. 2D), Ni-rich silicide (marked by S in Fig. 1D), and γ solid solution. A typical EDS spec- FEBRUARY 2014, VOL. 93 62-s WELDING RESEARCH Fig. 3 — EPMA line scan across the BM, DAZ, ISZ, and ASZ in material diffusion brazed at 1050°C for 10 min. Table 1 — EPMA/WDS Based Chemical Composition of Microconstituents in the ISZ and ASZ of Diffusion Brazed IN718 at 1050 for 10 Min Element ISZ Ternary Eutectic Eutectic γ Nickel-Rich Cr Boride Boride Ni 73.55 64.82 75.3 62.26 8.59 Cr 10.95 4.55 6.74 4.54 29.57 Si 6.93 12.57 11.92 0.57 0.02 B — 12.52 — 27.28 49.47 Nb 0.34 1.2 0.42 1.5 0.89 Mo 0.26 0.06 0.18 0.07 9.14 Fe 7.52 3.88 5.2 2.97 2.13 Ti 0.19 0.32 0.12 0.69 0.19 Al 0.26 0.08 0.12 0.12 —
Welding Journal | February 2014
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