The synthesis of nickel aluminides by multilayer self-propagating combustion
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I.
INTRODUCTION
INTERFACIAL reactions between vapor-deposited thin metallic layers are of theoretical as well as practical interest, t~] It has been demonstrated that such reactions are markedly different from those taking place in bulk materials. An important example of this is the recent discovery that such interactions lead to the formation of metastable phases, which can be amorphous, t2-5] quasicrystalline, [6,71or crystalline in nature, t8,9] The intriguing aspect of the formation of metastable phases in thin film reactions is that it occurs under a kinetically slow process, in contrast to fast (e.g., quenching) processes.[l~ Furthermore, there is evidence that interactions across an interface in multilayer systems can take place at room temperature.[~2] The formation of an amorphous layer as a result of the interaction between adjacent elemental thin films (solidstate amorphization) is typically observed when the elements are transition metals, with one being from the beginning and the other from the end of a period, tS1 Examples of such pairs include Zr-Cu and Hf-Ni. These pairs are typified by large differences in the diffusivities of the two elements as well as large enthalpies of mixing. Systems in which the thermodynamic driving force (approximated by the change in enthalpy) is large are characterized by reactions which can be selfsustainingJ ~3] In such systems, the reaction propagates in the form of a relatively narrow zone (or wave) leading to the complete (or partial) conversion of the reactants to the products, t~4-~7] The existence of self-propagating (or explosive) reactions in thin multilayer systems has been recently documented. {~8-21] The concept of a self-propagating reaction wave in fully dense (e.g., multilayer) systems is not predicted by the experimental observations obtained from powder T.S. DYER, Process Engineer, is with the Applied Materials Company, HDP-CVD New Product Group, Santa Clara, CA 95054. Z.A. MUNIR, Professor and Associate Dean, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Manuscript submitted June 6, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
reactantsY 2] In powder systems, the velocity of the reaction wave exhibits a maximum at a given relative density and decreases dramatically as the density is increased. Ultimately, the reaction ceases to be selfpropagating and the wave is extinguished. Additional verification of this phenomenon comes from observation on the combustion of laminated Ni/A1 foils, t23'241 The most thermodynamically energetic reaction in the A1/Ni system, that producing the equiatomic product NiAI, was not self-sustaining without the use of a "chemical oven." The difference, however, between the observations on foils (and by implication powders) and those on thin multilayer systems relates to the rates of heat generation and dissipation and, to a lesser extent, to the role of oxide impurities on the surface of foils and powders. This can be examined in light of the Fourier's on
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