Initial Boron Uptake and Kinetics of Transient Liquid Phase Bonding in Ni-Based Superalloys
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TRODUCTION
METALLURGICALLY, diffusion brazing (also known as transient liquid phase bonding, TLPB) generally involves the following sequential stages; melting of the filler metal, spreading and wetting of the base metal surfaces, dissolution leading widening of the initial liquid region, and the progressive isothermal solidification (IS) of the liquid phase, occasionally followed by a post-homogenization treatment.[1–8] Aside from the post-homogenization stage, the isothermal solidification (IS) stage represents the lengthiest portion of furnace brazing, typically requiring anywhere from a few hours to a day to complete; a stark contrast to the melting and gap widening (or dissolution) stages which transpire within minutes.[5,6] Many authors have attempted to quantify the process of isothermal solidification through a variety of techniques, the simplest of which involve assessing the extent of IS by metallographically measuring the thickness of any residual athermally solidified liquid as a function of brazing time.[5–12] This route is however laborious, time-consuming, and prone to experimental uncertainties, since multiple samples and extensive metallographic analysis are needed for each gap thickness and brazing time. A more efficient method, first developed by Partz and Lugscheider[13] and later employed by Arafin et al.[11]
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, Nova Scotia, B3H 4R2, Canada. Contact e-mail: [email protected] Manuscript submitted October 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and Sakamoto et al.,[14] consisted of placing a filler metal powder in a low-angle wedge, created from two plates of the desired base metal (BM), thereby creating a continuously varying braze gap dimension. Brazing of a series of samples at different times allows the determination of the maximum permissible clearance required to avoid the formation of athermally solidified structures per the given conditions (temperature, composition, time, etc.). The work ultimately provided a measure of the maximum brazing clearance (MBC), a key parameter in avoiding brittle centerline eutectic phases and achieving satisfactory TLPB. While the MBC analysis has great practical importance, it also allows interpretation of the more fundamental kinetics of TLPB, which will be utilized in the current study. Since the isothermal solidification stage is the most significant step during TLPB, it has been extensively modeled.[3,4,8,11,14,15] A widely accepted prediction for the complete IS time, tf in a generalized form is ðtf Þ1=2 ¼
2wmax ; k
½1
where 2wmax is the starting gap width and k is a rate constant. The rate constant k is dependent on the diffusivity of the melting point depressant (MPD) element into the base metal and its concentration in the liquid and solid adjacent to the interface. In conventional TLPB, the expectation is that dissolution of the base metal occurs during initial heating, such that gap widening oc
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