Microstructure and mechanical properties of titanium aluminide wide-gap, transient liquid-phase bonds prepared using a s
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NTRODUCTION
TRANSIENT liquid-phase (TLP) bonding is an offshoot of brazing. Like brazing, TLP bonding employs a liquid-forming interlayer with an initial melting temperature below that of the substrates to be bonded. However, unlike brazing, the thermal exposure employed in TLP bonding is such that there is extensive interdiffusion between constituents of the interlayer and the substrates. Thus, isothermal solidification and subsequent solid-state homogenization of the joints can be achieved, and bonds with parent-metal–like microstructures and, hence, properties are possible.[1] In the conventional process of TLP bonding, the maximum jointgap width that can reasonably be accommodated is quite limited (e.g., Reference 2), especially in cases where the melting-point depressant has a low solubility and/or diffusivity in the substrate materials. When there is a requirement for joining wide gaps (e.g., a 100 m gap width or greater), a composite interlayer, consisting of a nonmelting phase, in addition to the liquidformer, can be employed.[3] The nonmelting phase reduces the amount of liquid needed to fill the joint cavity and (when added in powder form) provides an efficient diffusional sink. Even when there is not an explicit requirement for a wide joint gap, the reduction in the amount of liquid-former required and the thermal exposure employed are attractive when bonding damage-sensitive materials with carefully tailored microstructures, such as titanium aluminide structural intermetallic compounds. In previous work by some of the present authors,[4,5,6] it was found that high-quality bonds could be produced [1]
W.F. GALE, Professor, and D.A. BUTTS and T. ZHOU, Graduate Research Assistants, are with the Materials Research and Education Center, Auburn University, Auburn, AL 36849. Contact e-mail: [email protected] M. DI RUSCIO, formerly Undergraduate Student, Materials Research and Education Center, Auburn University, Product Engineer, is with Allvac, Richburg, SC 29729. Manuscript submitted December 14, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
between Ti-48 at. pct Al-2 at. pct Cr-2 at. pct Nb (abbreviated here to “48-2-2”) substrates, using a 48-2-2 ⫹ Cu composite interlayer. However, these bonds relied on manually deposited interlayers that are unsuitable for industrial applications. Hence, the present article examines the microstructural development and mechanical properties of bonds employing interlayers produced by an automated deposition process. There are a wide range of techniques for the deposition of powder-based interlayers, including (1) a variety of paste, tape, and suspension systems, (2) sintered or hot isostatically pressed mats, (3) thermal spraying, etc. (Reference 7 provides an overview of these techniques). For the present work, a 48-2-2 ⫹ Cu slurry, formed with ethanol as a fugitive binder, was selected. The use of this slurry-based process is simple, rapid, requires minimal equipment, and offers an easy path for scale-up to industrial production rates. Also, the behavior of such a
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