The combustion synthesis of copper aluminides
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INTRODUCTION
THE preparation of a large number of materials through self-propagating exothermic reactions has been the objective o f numerous investigations over the past two decades. [1'2'3] Interest in this method of preparation stems from theoretical as well as practical considerations. Selfpropagating reactions are typically associated with high temperatures and extremely steep thermal gradients and thus offer the opportunity to investigate the formation of intermediate and metastable phases. Such reactions are also of interest from a practical point of view since, as has been described in numerous literature accounts, they provide the opportunity to prepare materials with unique properties. Details of this method of synthesis, commonly referred to as the self-propagating high-temperature synthesis or SHS, are given in recent reviews, tL2,3~ A requirement for this propagation of reaction in a serfsustaining mode is a high enthalpy of formation (or reaction). Reactions which are associated with relatively low enthalpy changes are not self-sustaining unless the reactants are heated prior to the initiation of the reaction. Typical among systems which belong to the latter class o f reactions are those between metals to form intermetallic compounds. Of the many metallic phases synthesized by this method, aluminides have been the most widely investigated. Aluminides o f metals such as nickel, t4-1~ copper, tH'lEA3] niobium, [14] palladium, t~Sj and zirconium t~6] have been investigated. The most common experimental approach used in the synthesis of these intermetallic compounds involves the heating of the reactants to the ignition point so that the entire specimen reacts at once. The term "thermal explosion" has been used to designate this experimental approach o f combustion synthesis. In a recent publication, Philpot e t a / . t171 studied the experimental parameters affecting the synthesis of nickel aluminides by the thermal explosion method. The study revealed that Ni-A1 phases formed through solid-state reactions prior to the onset of the major L.L. WANG, Graduate Research Assistant, and Z.A. MUNIR, Professor and Associate Dean, are with the Division of Materials Science and Engineering, College of Engineering, University of California, Davis, CA 95616. J.B. HOLT, Research Staff Member, is with the Department of Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94550. Manuscript submitted April 21, 1989. METALLURGICAL TRANSACTIONS B
liquid-phase reaction associated with the combustion reaction. The relative contribution of the two types of reactions depended on the rate of heating of the powder compacts up to the reaction temperature. The particle size of the powder was also found to affect the nature of the reaction process. In the present paper, we report the result of an investigation of the mechanisms governing the synthesis of copper aluminides. The Cu-AI binary system t18] is a relatively complex one containing six phases which are stable at temperatures below 500
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