Transient liquid-phase bonding in the Ni-Al-B system
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I. INTRODUCTION
AS the efficiency of turbine engines increases so does the complexity of the engine parts. In addition, the increasing size of land-based turbines results in large section components that are prone to freckle formation. One method to produce these geometrically complex shapes and to reduce freckle formation requires that the part be made in sections and then joined together by transient liquid-phase (TLP) bonding, also known as diffusion brazing. The TLP bonding process uses a thin, low-melting filler material to wet the contacting base material that subsequently solidifies isothermally via a fast diffusing element.[1,2,3] Thus, at a relatively low melting temperature, TLP bonding produces a joint that has a uniform composition profile and that is tolerant of surface oxides,[4] geometrical defects, and wide gaps (,100 mm)[5] These advantageous features of TLP bonding have been exploited in a wide range of applications, from the production and repair of turbine engines in the aerospace industry[1,5,6] to the connection of circuit lines in the microelectronic industry.[7,8] However, the possible formation of intermetallic phases, which may degrade the material properties, limits the success of TLP bonding.[1,9,10] To predict the formation of potentially deleterious phases, TLP bonding requires modeling of the thermodynamics (i.e., the phase diagrams) and the diffusivities of the constituent alloy systems. The TLP bonding process is characterized as a four-step process consisting of the melting of the filler material, the homogenization of the liquid filler material, the isothermal solidification of the joint region, and the solid homogenization of the joint region. The relationships between these four stages and a binary eutectic phase diagram are described by MacDonald and Eagar[2] and Zhou et al.[3] The same relationships exist in multicomponent systems; however, there may be additional phases present and the phase diagram is more difficult to visualize. As noted by Sinclair et al.,[11] generally, a ternary system differs from the binary in that the composition of the liquid is not fixed during solidification, but changes continuously. C.E. CAMPBELL and W.J. BOETTINGER, Metallurgists, are with the National Institute of Standards and Technology, Gaithersburg, MD 20899-8555. Manuscript submitted February 11, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Common solutions used to model TLP diffusion problems are error function solutions, which assume infinite or semiinfinite length scales, and Gaussian solutions, which assume a thin uniform filler material. However, the finite thickness of filler material, as well as the substrate, precludes an exact analytical description of the TLP bonding process, even for a binary alloy. Several studies[12–15] attempt to combine analytical solutions for the different stages of the bonding process for various binary systems. These analytical models assume an effective diffusivity through the matrix material[3] and error function solutions to determine the rate o
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