Reactive infiltration processing of aluminum-nickel intermetallic compounds
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INTRODUCTION
REACTION synthesis provides an attractive path toward the production of high-temperature ceramic and intermetallic compounds. Reactive processing methods differ substantially according to the nature and method of combination of the reactants; Dunand provides a review of reactive synthesis processes for, in particular, aluminides of titanium, iron, nickel, and niobium.[1] Reactive infiltration processing (RIP) is one such reactive process, in which a melt is made to infiltrate and react with a porous preform of a more refractory phase to form the desired compound. Advantages of RIP compared to powder-based approaches for aluminide synthesis include the following: (1) the use of liquid aluminum eliminates the oxide layer covering aluminum powders; (2) porosity, endemic to powder metallurgical techniques, is reduced or eliminated; (3) the integrity of fibrous or whisker reinforcements is conserved since solid-state preform compaction can be avoided; (4) near-net shape processing is possible despite formation of high fractions liquid, since the process makes use of a mold or crucible; and (5) the process is rapid. Reactive infiltration processing, which constitutes one of the principal processing methods for silicon carbide,[2] has been explored by several authors for the synthesis of aluminides and their composites.[3–11] There are also potential problems in RIP. Reaction choking can occur, whereby solid products formed during infiltration close the pores and ‘‘choke’’ the infiltration process prematurely. This has been observed in both capillary inC. SAN MARCHI, formerly Graduate Student, now Post-Doctoral Research Associate, and A. MORTENSEN, Professor, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, are with the Laboratory of Mechanical Metallurgy, Department of Materials, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. Manuscript submitted August 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
filtration[2,12] and pressure-assisted infiltration.[3] Macrosegregation, caused by convective solute transport is also a significant problem in systems that react during infiltration,[4] in particular, when the preform has significant solubility in the infiltrating material. Both of these problems occur when infiltration is slow in relation to the rate of reaction. When, at the opposite extreme, reaction is slow in comparison with infiltration, these problems are suppressed. The liquid then essentially permeates the unreacted preform, such that the infiltration and the reaction processes are decoupled in time. Infiltration kinetics can then be predicted independently of the reaction, and the reaction kinetics can be considered for stationary liquid and solid phases. For example, Nakae et al.[11] have found that infiltration must be complete (or nearly so) before reaction begins in order to produce uniform Ni3Al. In what follows, we investigate the slow reaction/rapid infiltration limit of reactive inf
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