Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The re...

Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils

Fabrication of TiC particulate reinforced magnesium matrix composites

TiB2 and TiB2x2013;TiC composite compacts with 98x2013;99% density are prepared by high-pressure sintering (HPS) of premixed powders and by high-pressure self-combustion synthesis (HPCS) from the elemental constituents. The sintering and synthesis experiments are carried out at 3 GPa in the temperature and time ranges 2250x2013;2750 K and 5x2013;300 s, respectively. A high sintering temperature (2750 K) is required to obtain dense monolithic TiB2 compacts (98% density) by HPS. Compacts with a similar density are obtained at lower sintering temperature (2250 K) when 15 mol% TiC is added to TiB2. The composite compacts have marginally better fracture toughness than that of monolithic compacts. TiB2 and TiB2x2013;TiC compacts (99% density) are also prepared by HPCS from elemental constituents. A minimum ignition temperature of 2250 K is required to make the reaction self-sustaining.The compacts prepared by HPCS have superior fracture toughness to those prepared by HPS. The microstructures and the properties of the compacts prepared by HPS and HPCS are compared. A possible sequence of reaction during the HPCS of TiB2x2013;TiC is proposed.

Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure

In the present work, a method for fabrication of Ti 3 AlC 2 by combustion synthesis was developed. Low cost titanium, aluminum and carbon powders were used as raw materials. The effects of the initial ratio among the above-mentioned materials on the combustion behavior and phase formations in the final products were investigated. The experimental results showed that the combustion reaction usually preferred to form a mixture of titanium carbide and aluminum. However, the desired product of Ti 3 AlC 2 can be obtained through adjusting the initial ratio of the elemental powders or adding TiC as additives. The formation mechanism of ternary phases, Ti 3 AlC 2 and Ti 2 AlC, is that the previously formed crystallites of TiC dissolve into the melting Ti–Al composition, and then, ternary phases precipitate from liquid phase and grow into layered morphology.

Combustion synthesis of ternary carbide Ti3AlC2 in Ti–Al–C system

In situ synthesis of TiC/Mg composites in molten magnesium

The effect of the addition of aluminum on the ignition and self-propagating high-temperature synthesis (SHS) reaction between titanium and carbon was experimentally investigated. Although TiC was the only product compound in the final product, the reaction between titanium and aluminum was believed to occur before the ignition of the reaction between titanium and carbon, as evidenced by a hump that appeared in the temperature profile during heating. The ignition temperature of the reaction between titanium and carbon was significantly decreased by the addition of aluminum. This was explained by the fact that the aluminum provides an easier route for reactant mass transfer, thus significantly increasing the reaction rate of the reaction between titanium and carbon. As the temperature was increased to beyond the melting point of aluminum during heating, aluminum melted and titanium dissolved into it. As the temperature was increased further (1050°C), the titanium-containing aluminum spread over the carbon particles. Ignition is believed to have occurred by the reaction of titanium and carbon at the interface by the diffusion of titanium through the aluminum melt to the interface. The rate of heating and the density of the reactant compact also affected the ignition temperature. These were explained by their influence on the extent of the capillary spreading of the titanium-containing aluminum melt on the carbon particles or by the formation of a TiAlx layer at the interface between the aluminum melt and the titanium particles. Fracture surfaces of the product showed three different types of morphology, i.e., discrete particles, groups of grains similar to sintered bodies, and unreacted reactant particles. A possible reaction mechanism was proposed that describes the ignition process and explains the formation of each type of product.

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the titanium-carbon-aluminum system

The ignition phenomena and the reaction mechanism of the self-propagating high-temperature synthesis reaction of titanium and carbon powders were experimentally investigated. When using coarse graphite powders (<325 mesh) as the carbon source, the ignition temperature ranged from 1650–1720‡C and was independent of the C/Ti ratio. The ignition temperature could be significantly lowered by using finer graphite powders (e.g. 1400‡C for <1 Μm powder). When using carbon black as the carbon source, the ignition temperature ranged from 1050–1475‡C and was dependent on the C/Ti ratio. The ignition was confirmed in this study to be controlled by the rate of the surface reaction between titanium and carbon which, in turn, was determined by the contact surface area between them. The fractured surfaces of the products showed two different types of morphology, i.e. groups of grains similar to sintered bodies and agglomerated fine particles. The relative quantities of the two types of morphology depended on the type of carbon used, the C/Ti ratio, the particle size of graphite and the density of the reactant pellet. Possible reaction mechanisms have been proposed on the basis of the experimental observations of the ignition phenomena and the product morphology.

Wei Chang Lee

Papers

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the titanium-carbon-aluminum system

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the Ti+C system