Bulk-alloy microstructural analogues for transient liquid-phase bonds in the NiAl/Cu/Ni system
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I. INTRODUCTION
AT first sight, transient liquid-phase (TLP) bonding is a straightforward process. Standard TLP bonding models (e.g., References 2 and 3) assume sequential dissolution, isothermal solidification, and solid-state homogenization processes. These three stages result, respectively, in widening of the liquid layer, removal of the liquid, and formation of a second-phase free bond. Modeling work has generally focused on simple eutectic-forming systems (such as pure nickel substrates bonded using an Ni-P interlayer). In simple systems, assumption of the three sequential TLP process stages allows a fairly accurate prediction of bonding rates,[4] so long as factors such as the existence of noninfinite heating rates (and, hence, diffusion during the heating stage) are considered. In complex systems, the progression of microstructural development can deviate from that predicted by standard TLP process models. For example, in Ni/Ni-Si-B/Ni bonds, the formation of borides has been observed[5] to occur at the bonding temperature, after complete melting of the interlayer. This situation is not compatible with standard models of the TLP process, which assume that local equilibrium is established at the solid-liquid interface prior to the onset of isothermal solidification. In cases where the two bulk substrate materials are significantly dissimilar, microstructural development in TLP bonds can become exceedingly complex, even when the microstructures of the substrates themselves are quite simple (e.g., Reference 6). In the present article, an investigation is presented of the use of bulk alloys as microstructural analogues for selected regions of complex microstructures formed in TLP bonds. The use of such bulk-alloy analogues offers the following potential advantages. [1]
(1) In systems with complex microstructures that evolve W.F. GALE, Associate Professor, and Z.A.M. ABDO, Graduate Student, are with the Materials Research and Education Center, Auburn University, Auburn, AL 36849. Manuscript submitted March 25, 1999 METALLURGICAL AND MATERIALS TRANSACTIONS A
during holding at the bonding temperature and/or on cooling, the use of bulk-alloy analogues allows, for example, separation of the effects of interdiffusion between different microstructural layers and aging of individual layers. Hence, the understanding of the mechanisms driving microstructural development in complex bonds could be improved. (2) This work will allow the production of a variety of bulk alloys, each of which has a microstructure and, hence, properties characteristic of a different region of a bond. In this fashion, the contribution (in terms of strength, coefficient of thermal expansion, oxidation and hot corrosion resistance, etc.) of individual microstructural regions to the overall performance of bonds can be determined. In other words, TLP bond bulk-alloy analogues would serve a function similar to that of the widely used practice of heat affected zone simulation for fusion welds (e.g., References 7 through 9). However, these capabiliti
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