Combustion characteristics of the Ni 3 Ti-TiB 2 intermetallic matrix composites
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NTRODUCTION
THIS is the first of three[1,2,3] articles that describe research conducted as the combustion synthesis (SHS) of metal matrix composites (MMCs) and the role that gravity plays in such reactions. The MMC systems selected for investigation were Al reinforced with HfB2 and Ni3Ti reinforced with TiB2. These MMC systems were selected based on the significant differences in specific gravity between the matrix (Al or Ni3Ti) and the ceramic reinforcement (HfB2 or TiB2). The Ni3Ti-TiB2 MMC provides a high specific gravity matrix and low specific gravity ceramic reinforcement. The Al-HfB2 MMC provides a low specific gravity matrix and high specific gravity ceramic reinforcement. These ranges in specific gravity were selected to demonstrate the role that gravity can play in SHS reactions. Elemental reactant powders were used in each SHS (selfpropagating, high temperature) reaction.[4–7] The resulting exothermic SHS reaction produced a solid, high melting point ceramic (TiB2 or HfB2) synthesized in situ, within a large volume of the liquid metal (Ni3Ti or Al) metal matrix. In each case, the reaction stoichiometry was selected to achieve a metal matrix volume greater than 50 pct. The combustion synthesis of materials often follows a reaction sequence in which the easier ignitable compound forms first, which then triggers the formation of the compound that is more difficult to ignite.[8] From examination of the structure of the combustion zone, such reactions are often characterized by a more complex reaction process that may consist of two or more exothermic peaks.[2–5] On the other hand, owing to the highly exothermic nature of many SHS reactions, liquid phase can also form before, at, or after the combustion front. The large temperature gradient at the combustion front may induce convective flow (conH.C. YI, Engineer, and J.Y. GUIGNE´, President, are with Guigne´ International Ltd., Paradise, NF, Canada A1L 1C1. T.C. WOODGER, Graduate Student, and J.J. MOORE, Professor and Head, are with the Center for the Commercial Application of Combustion in Space (CCACS), Colorado School of Mines, Golden, CO 80401. Manuscript submitted January 8, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
ventional or unstable) of the liquid phase. The convective fluid flow can change the combustion behavior as well as the microstructure of the reacted products. When a composite consisting of two or more phases is synthesized by SHS, settling of the higher density phase will occur resulting in a nonuniform microstructure.[9,10] Processing of composite materials, either MMCs, intermetallic matrix composites (IMCs), or ceramic matrix composites (CMCs), by means of SHS has gained wide interest. While there is relatively little difficulty in producing CMCs and IMCs using SHS, synthesizing MMCs using SHS has proved to be difficult since such reactions are weakly exothermic, especially when a large amount of metal phase is present in the composite. Yi and co-workers[11,12,13] successfully produced the TiC-Al2O3-Al MMCs with a metal (Al alloy)
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