Application of Diffusion Path Analysis to Understand the Mechanisms of Transient Liquid-Phase Bonding in the Ni-Si-B Sys
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THE strict performance requirements of aerospace applications typically warrant the use of advanced joining techniques such as vacuum diffusion brazing, as it offers several advantages over other brazing and welding methods. Joining is accomplished through the melting of a low-melting point filler metal (FM) alloy and subsequent diffusionally induced solidification; hence the name Transient Liquid-Phase Bonding (TLPB).[1,2] Most often TLPB of superalloys is achieved using nickel-based brazing alloys containing either Boron, Phosphorous, and/or Silicon in addition to other alloying additions. These alloys generally exhibit an excellent combination of wetting, spreading, low fusion temperatures, and relatively low joining times, similar to the prohibitively expensive Ni-Au (or Pd) alloys.[3] Despite their extensive service, considerable uncertainty remains with regards to the exact mechanism by which TLPB and isothermal solidification occurs in boron-containing systems, since they deviate from classic TLPB theory.[4–7] In classic TLPB as described by Eager, Tuah-Poku et al.,[1,2,8–11] the filler metal is an ideal eutectic with melting point depressant
E.D. MOREAU and S.F. CORBIN are with the Department of Mechanical Engineering, Dalhousie University, 1360 Barrington Street, P.O. Box 15,000, Halifax, NS, B3H 4R2, Canada. Contact e-mail: [email protected] Manuscript submitted April 12, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(MPD) elements displaying appreciable solid solubility in the base metal (Figure 1(a)). During brazing the MPD solute atoms diffuse from the liquid to the solid solution (SS) matrix and subsequently redistribute and homogenize in the base metal (BM). The high soluble MPD allows for isothermal solidification and maintains a single-phase microstructure in the BM. The solute composition is assumed constant in the liquid phase, but has a decreasing concentration with increasing distance from the solid/liquid interface within the BM. Conversely, the immiscibility of B in Ni (0.015 wt pct max.) and its rapid interstitial diffusion in Ni[12–14] results in the heterogeneous distribution of embrittling borides in the vicinity of the joint (Figure 1(b)). The region of boride precipitates in the BM is known as the diffusionally affected zone (DAZ) and has been widely reported for Ni-base superalloys, in which segregation and progressive coarsening of metal borides adjacent to the original solid/liquid interface occurs—the exact type and composition governed by the materials involved.[15–19] A (c-Ni) solid solution single-phase layer forms on the opposite side of the original solid/liquid interface and is known as the isothermally solidified zone (ISZ). During brazing, the DAZ and ISZ layers grow in thickness while the liquid layer deceases and eventually removed through diffusional solidification. The compositional profile of elements in the braze that are soluble in the (c-Ni) have a similar distribution as for the classic case of Figure 1(a). However, the B profile is markedly different, going to near zero in
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