Combustion Synthesis Reactions in Cold-Rolled Ni/Al and Ti/Al Multilayers
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COMBUSTION synthesis, also called reaction synthesis, is a high-temperature reaction process. When two or more materials with large exothermic heats of mixing, known as reactants, are heated sufficiently, they begin to spontaneously intermix on the atomic scale, release heat during the process, and form a variety of advanced materials, such as aluminides, carbides, and borides. This process offers an attractive and alternative route of preparation of intermetallics.[1] Two different reaction modes are possible in combustion synthesis: self-propagating high-temperature synthesis (SHS) and thermal explosion.[2,3] The SHS reaction mode is achieved by local heating of reactants by an external heat source and is possible only when the exothermic enthalpy of formation for the desired intermetallic product is relatively large, such as nickel aluminides or platinum aluminides.[4,5] In contrast, for the thermal explosion mode, the reactants are heated simultaneously, for example, in a furnace, until the reaction takes place over the entire sample. The final product often contains secondary product phases due to diffusion during the heating process prior to the onset of the combustion reaction.[6] XIAOTUN QIU, Student, is with the Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287. RANRAN LIU and JESSE HARRIS GRAETER, Students, and SHENGMIN GUO, Professor, are with the Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803. LASZLO KECSKES, Scientist, is with the United States Army Research Laboratory, Aberdeen Proving Ground, MD 21005. JIAPING WANG, Associate Professor, is with the Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 22, 2008. Article published online May 2, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
Combustion synthesis can be achieved both in mixed reactant powder compacts and in fully dense multilayer materials.[7–9] The reactant powders are usually pressed into a pellet of certain green density to conduct the combustion synthesis process. The multilayered systems are generally fabricated by physical vapor deposition (PVD) methods, such as magnetron sputtering or electron-beam evaporation. These methods involve creating vapors of each element, known as the source materials, and then depositing the vapors onto a substrate in an alternating manner. The rate at which the vapor is deposited is controllable, allowing the growth of multilayer films with thickness ranging from nanometers to micrometers. Alternatively, multilayer foils can be made by cold rolling.[10–13] The advantages of the cold rolling method are its simplicity, low cost, and low time-consuming nature compared with PVD methods. Previous literature usually treats the SHS and thermal explosion modes of combustion synthesis separately. Rabinovich et al. analyzed the conditions for combustion synthesis in Ni/Al nanofilms and ide
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