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I.

INTRODUCTION

C O M B U S T I O N synthesis is a term applied to the formation of certain compounds by using strongly exothermic reactions between powdered constituents. If the reaction is initiated at one end of the powder compact by a localized heat pulse and the exothermic reaction volume self-propagates through the free-standing pelletized mixture (Figure 1), this is referred to as selfpropagating high-temperature synthesis (SHS). If, however, the compact as a whole is heated in a furnace until the reaction occurs simultaneously throughout the compact, the process is termed thermal explosion or reactive sintering. Both modes have been successfully used to produce aluminides, such as Ni3A1, which have attractive properties for use as lightweight materials in high-temperature structural applications. An excellent review of the techniques and prospects of combustion synthesis has recently been provided by Munir and Anselmi-Tamburini. tl] Owing to the fact that it is possible to control more of the thermal processing variables in the thermal explosion method and also because the rich literature of sintering addresses many of the same problems present in studies of reactive sintering with a transient liquid phase, it is not surprising that the thermal explosion method has received much attention.[2,3,4] The experience base in reactive sintering has shed light on the influence of processing variables, such as heating rate, particle size ratios, blending methods, stoichiometry, and green density, on the details of the as-reacted microstructure. These studies indicate the following generic results: (1) The particle size ratio must be such that the liquid formed by the lower melting minor phase (A1) forms a connected network, which immediately wets and encapJ.-P. LEBRAT, Ph.D. Candidate, Department of Chemical Engineering, A. V A R M A , Arthur J. Schmitt Professor of Chemical Engineering, Department of Chemical Engineering, and A.E. MILLER, Professor, Department of Electrical Engineering, are with the University of Notre Dame, Notre Dame, IN 46556. This paper is based on a presentation made in the s y m p o s i u m "Reaction Synthesis of Materials" presented during the TMS Annual Meeting, New Orleans, LA, February 17-21, 1991, under the auspices of the TMS Powder Metallurgy Committee. METALLURGICAL TRANSACTIONS A

sulates the major phase (Ni) by capillary forces. If the particle size ratio is not appropriate, then porosity and an incomplete reaction can result. (2) Faster heating rates of the green compact yield higher transient temperatures produced by the exothermic reaction, higher product densities, and improved microstructures. (3) Stoichiometry of the mixed powders must be accurate to insure that neither an excess nor a deficiency of liquid occurs, producing slumping or porosity, respectively. (4) Highest product densities are obtained in vacuum, which removes adsorbed gases, and facilitates removal of gas trapped inside closed pores in the green state. These guidelines have had a limited test in the SHS mode