Ignition and reaction mechanisms of thermal explosion reaction in the Ni-Ti-C system under air and Ar
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The ignition and reaction mechanisms of the thermal explosion reaction in the Ni-Ti-C system under air and Ar conditions were investigated. The reaction for the formation of TiC can be initiated at a low temperature under air. The ignition temperature under air is much lower than that under Ar. Under Ar, both the ignition and reaction mechanisms consist of dissolution, reaction, and precipitation. Under air, the ignition mechanism is confirmed to be the chemical oven mechanism, and the reaction mechanism is dissolution, reaction, and precipitation. The mechanism of gas transport plays a much more minor role in the ignition and reaction processes under air.
I. INTRODUCTION
TiC possesses many desirable properties such as high hardness, low density, high melting temperature, high modulus, and corrosion resistance. These outstanding features make it attractive for advanced structural applications.1 TiC can be produced from mixtures of titanium and carbon powders by several methods such as powder metallurgy, mechanical alloying, reactive sintering, and combustion synthesis (CS).2 Among all these fabrication technologies, CS has attracted more attention due to its lower energy consumption, shorter time, and higher product purity.3 CS reactions, however, are somewhat difficult to initiate due to the high melting temperatures of the reactants or the lack of a pre-activation reaction. On the other hand, the final products are usually poorly consolidated and contain many cracks even though a dynamic compaction is used immediately after the completion of the reactions.4 A possible solution to these problems could be the incorporation of metals with low melting point into the titanium and carbon reactants, since they can react with titanium to form low-melting intermetallics or a eutectic liquid at a low temperature (eutectic temperature). Among these metals, there is a low wetting angle between liquid Ni and solid TiC.5 Consequently, Ni is selected as the additive metal, since it not only promotes the CS reaction, but also improves the structural properties. Experimentally, a CS reaction can be started either by igniting the sample at one end (plane wave propagation, PWP) or by uniformly heating the sample to the ignition temperature (thermal explosion, TE). For a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0370 J. Mater. Res., Vol. 24, No. 10, Oct 2009
combustion synthesized TiC from the Ni-Ti-C system, earlier works concentrated almost exclusively on the use of the PWP mode under vacuum or an Ar protective atmosphere.3,6–10 At present, the mechanism of the formation of TiC under the PWP mode generally can be confirmed to be dissolution, reaction, and precipitation. In detail, this means that Ti2Ni was formed firstly by the solid reaction between Ni and Ti; subsequently the Ni-Ti liquid formed above the eutectic temperature between Ti2Ni and Ti; and finally C atoms dissolved into the Ni-Ti liquid to form TiC. The mechanism has been supported by Dunmead et al.,6 Xiao et al.,1 and
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