Al-TiC Composites Fabricated by a Thermally Activated Reaction Process in an Al Melt Using Al-Ti-C-CuO Powder Mixtures.
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INTRODUCTION
Al-BASED metal matrix composites reinforced with TiC particulates have been extensively studied due to their high hardness and elastic modulus, low density, and good wettability with molten aluminum with low chemical reactivity.[1–6] TiC can be introduced to the Al matrix either by adding TiC powders directly into the melt, i.e., ex situ process, or by an in situ reaction between elemental powders of Ti and C sources. In the ex situ process, infiltrated reinforcing TiC particles react with the Al matrix at high temperature and form a deteriorating interface layer on them and have poor wettability with the matrix due to surface contamination of the reinforcements. In situ TiC, on the other hand, is a thermodynamically stable phase formed by a chemical reaction and exhibits a contamination free interface with the Al matrix.[4,7–11] Among various in situ fabrication techniques suggested upto now, the self-propagating high temperature synthesis (SHS) developed by Merzhanov[12] is one of the most feasible and productive methods and involves ignition and self-sustaining combustion of reactants, which results in product formation. Earlier studies adopted halide salts such as Na3AlF6,[13,14] KxAlF6,[13,14] and K2TiF6,[15–17] that were added to molten Al to prepare Al-Ti-C grain refiners. The use of those flux agents significantly YOUNG-HEE CHO, Senior Researcher, and JUNG-MOO LEE and SU-HYEON KIM, Principal Researchers, are with the Division of Light Metals, Korea Institute of Materials Science, 797 Changwondaero, Changwon 642-831, South Korea. Contact e-mail: jmoolee@ kims.re.kr Manuscript submitted April 7, 2014. Article published online August 14, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
increases the SHS reactivity, and thereby yields far more TiC in situ synthesized.[13,14] Moreover, it has been proposed that the fluxes addition leads to a dramatic reduction in the delay time to the SHS reaction and increases the melt over-heating temperature.[13,14] K-Al-F salts generated by the reduction of K2TiF6, in particular, were suggested to clean the particle surface and to remove the oxide layer from the surface of the melt, which improves the wetting of the graphite particles, promoting the formation of TiC.[17–19] Besides shortening the SHS reaction time, lowering the temperature that initiates a combustion reaction is of importance in a low energy and cost saving in situ process. Our recent study reported a very practical process of SHS combined with a conventional casting to fabricate in situ Al/TiC composites.[20,21] It is noted that a certain amount of CuO addition thermally activates a combustion reaction of the Al-Ti-C system, and thereby enables the formation of in situ TiC with a large volume fraction in an Al melt at a reasonably low temperature range of 1023 K to 1193 K (750 C to 920 C). The microstructure evolution during SHS of Al-Ti-C, however, has not been yet clearly understood, while several mechanisms of reaction synthesis of TiC for various processes have been suggested in the literature.
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