Microstructure evolution in metal-intermetallic laminate (MIL) composites synthesized by reactive foil sintering in air
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METAL-INTERMETALLIC composites can be designed for structural use to optimize the unique properties and benefits of the constituent components, resulting in materials that have the high strength and stiffness of the intermetallic phase and the high toughness of the metal. Intermetallics have been reinforced with particles, rods, or layers of ductile metals in efforts to increase toughness.[1,2] Ductile phase reinforcement of brittle materials utilizes crack-laminate interactions to generate a zone of bridging ligaments that restrict crack opening and growth by generating closure tractions in the crack wake and utilize the work of plastic deformation in the ductile metal phase to increase fracture resistance of the composite.[3,4] The synthesis and processing of metal-intermetallic laminate (MIL) composites began with work on nickel and aluminum foils in an attempt to produce the intermetallic compounds NiAl and Ni3Al.[5,6] Due to the high thermal conductivity of the foils, the reaction between the nickel and aluminum is not self-sustaining. Therefore, the foils were embedded in a pellet of nickel and aluminum powders (chemical oven) that was ignited and provided sufficient energy from the powder reaction for the foils to react and form 100 pct NiAl. Quenching of the powder reaction before propagation through the entire sample resulted in a section of layered nickel and intermetallic. The layered nickel and intermetallic microstructure prompted further work in fabricating small scale metal-intermetallic laminate composites in a vacuum[7–13] and in an argon atmosphere.[14,15] No results DAVID J. HARACH, Postgraduate Researcher, and KENNETH S. VECCHIO, Professor, are with the Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093-0411. Manuscript submitted June 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
have been presented to date for reacting MIL composites in open air. The ability to fabricate these composites in open air is of considerable interest, as vacuum or inert atmospheres require greater apparatus cost and processing time and limit the overall size of the samples that can be produced. Fabricating MIL composites in air also facilitates the production of larger samples in a greater variety of shapes (including near net-shape). Larger sizes and increased flexibility in production enables this class of composites to be considered for use as armor and structural materials, especially in aerospace applications where light weight is necessary. In open air, the choice of heat sources is also greater, and there is increased flexibility in load frame selection. The starting foil materials are readily available in a wide range of thicknesses and composition and, as will be shown here, require only a minimum of preparation, even when reacted in open air. There is the potential for excellent microstructural control and microstructural variability, as layer thickness can be chosen with great precision. In addition, the foil stacking sequence and foil mat
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