A B ering the fact the isothermal solidification is controlled by diffusion of B into the base metal, the formation of boride precipitates in the DAZ is possible. Moreover, Gale and Wallach (Ref. 29) in their work on microstructural development in transient liquid phase brazing of Ni substrate with a Ni-Si-B interlayer at 1065°C have observed formation of Ni3B particles at the original joint/base metal interface. They suggested that contrary to the predictions of currently available diffusion brazing models, a significant solid-state diffusion of B occurs in the base metal before completion of the dissolution process such that the solubility limit of B in the base metal was exceeded at brazing temperature. Due to diffusion of B into the base metal, a B-containing alloy is formed in a narrow region in the substrate zone adjacent to the ISZ. The solubility of B in this alloy is limited. This fact coupled with the presence of Cr, Mo, and Nb in the matrix that are strong boride formers can explain the formation of Cr-Mo-Nb-rich precipitates. Gale and Wallach (Ref. 29) provided some evidence confirming that these precipitates are formed at the brazing temperature not during cooling. It is interesting to note that the formation of secondary precipitates in the DAZ is not associated with the diffusion of Si, as a MPD element in filler metal. This is due to the following reasons: 1. The solubility limit of Si in nickelbased alloy is higher than that of the B in nickel-based alloys. According to the Ni-Si binary phase diagram, the solubility of Si in Ni is 8 at.-%, which is much higher than the solubility of B in Ni (0.3 at.-%, according to binary Ni-B equilibrium phase diagram (Ref. 28)). This prevents the formation of silicides in the DAZ. 2. The diffusion coefficient of Si in Ni is much lower than that of B in Si. As can be seen in the EPMA profile in Fig. 2, there is no Si buildup in the DAZ. Most of the Si is concentrated in the ISZ due to its low diffusion coefficient. Accordingly, it can be concluded that the maximum solubility of Si in the substrate region cannot be exceeded and thus formation of silicide secondary phases in the DAZ is prohibited. Formation of Cr-Mo-Nb in the DAZ leads to a significant depletion of Cr and Mo around these precipitates. This reduces the local corrosion resistance of the matrix (Cr and Mo are two key elements in corrosion behavior of IN718). Moreover, the aging behavior of IN718 strongly depends on the Nb content of the alloy (Ref. 30). Therefore, the presence of extensive Cr-Mo-Nbbased borides present in the DAZ that depletes the adjacent matrix from Nb can affect the aging behavior of this region (Refs. 31, 32). Effect of Brazing Time on Bond Microstructure and Hardness Characteristics Joint microstructure that significantly affects the joint performance depends on elemental interdiffusion between base metal and interlayer, which in turn is governed by brazing time. The average width of the isothermally solidified zone is measured at each brazing time and its variation with respect to square root of brazing time is plotted in Fig. 9A. As can be seen, there is a linear relation between ISZ size and root of brazing time. The implication is that the formation of gamma solid solution is a diffusion-controlled process. Indeed, in the case of diffusion brazing of IN718/Ni-Cr-Fe-Si-B/IN718, the isothermal solidification process is controlled by formation and growth of γ-solid solution, which is governed by MPD element diffu- WELDING JOURNAL 65-s WELDING RESEARCH Fig. 7 — Precipitation in DAZ. A — Widmanstätten-type borides; B — blocky borides. Fig. 8 — X-ray line scan across blocky secondary precipitates in the DAZ.
Welding Journal | February 2014
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