Thermal analysis of self-propagating high-temperature reactions in titanium, boron, and aluminum powder compacts
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INTRODUCTION
FOR the optimum performance of many aerospace, commercial, and military systems, the use of dissimilar materials is becoming increasingly necessary.[1,2] Therefore, the joining of advanced materials, composites, and ceramics to metal structures is a prominent issue.[3] The joining of ceramics to metals presents a particularly difficult task, due to the differences in mechanical and thermal properties of these materials. These differences can lead to excessive stress at the joining interfaces, causing mechanical failure in the form of microcracks.[4] Current techniques for joining these advanced materials include the use of polymer-based adhesion, mechanical fastening, and welding. In general, polymeric adhesives do not enhance the overall performance of the structure. Also, commercially available polymeric adhesives will degrade at temperatures higher than 180 7C.[5] This severely limits the applications in which polymeric adhesives can be used. Mechanical fasteners, such as bolts and sleeves, concentrate stresses and increase the chances of brittle fracture and, ultimately, failure of the ceramic part. To alleviate these problems, ceramic parts are often metallized, which adds steps in the manufacturing process and increases the total cost of the component.[6] Also, parts must be designed to distribute stresses resulting from mechanical fastening as homogeneously as possible. Welding, on the other hand, often results in oxidization and/or recrystallization of the metal, especially when a repair is performed. Many welding compounds do not wet ceramic components. Differences in the thermal expansion of the weld material and substrate can also lead to bond failure.[6] Further, specially trained welders are usually needed, which adds to the cost of joining. The ultimate goal of this research is to investigate the joining of ceramics to metals by self-propagating high-temL.H. CHIU, formerly with the Material Science and Engineering Department, Johns Hopkins University, is with Paratek, Inc., Aberdeen, MD 21001. D.C. NAGLE is with the Material Science and Engineering Department, Johns Hopkins University, Baltimore, MD 21218. L.A. BONNEY, formerly with the Materials Science and Engineering Department, Johns Hopkins University, is with Bicron, Solon, OH 44139. Manuscript submitted January 13, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
perature synthesis (SHS) reactions. Thermite reactions involving metal-oxide reactions have been used for many decades to join steel structures.[7] In contrast to thermite reactions, the SHS reactions are primarily used to produce refractory metal compounds and ceramics from compacted mixtures of elemental powders.[8,9] Like thermite reactions, SHS reactions are typically initiated by heating a small portion of the powder compact with a hot filament or flame. Once initiated, the exothermic heat is sufficient to propagate the reaction throughout the mixture. The main advantage of using this method for joining is the generation of high temperatures (1200 7C to 4000 7C)
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