Reaction kinetics in the combustion synthesis of Al–Ir–Ni intermetallic compounds

The combustion synthesis of Al50Ir48Ni2 (at.%) was conducted at different heating rates in both a differential scanning calorimetry (DSC) chamber and a vacuum furnace. It was found that a higher heating rate, a sufficient amount of reactant powder, and effective control of the heat loss facilitated the complete reaction and resulted in combusted single IrAl phase products. Otherwise, multiphase products containing IrAl, unreacted Ir, and Al3Ir were synthesized. The reactions involved in different processes were discussed in terms of the thermal competition between heat generation and loss during the reaction. All ignition temperatures were below 773 K, indicating that the combustion reaction occurs at the solid–solid state. With increasing heating rate, the ignition temperature increased while the product density decreased.

Synthesis of submicrocrystalline TiCx–Al2O3 composites by mechanically-activated pressure-assisted self-propagating high-temperature synthesis technique

TiCx–Al2O3 composites have been synthesized by pressure-assisted combustion synthesis of (Ti + C + Al2O3) reactant powder. Different alumina contents (10–40 vol%) have been investigated to study the dilution effect on TiC microstructure. A mechanical method and a mixed chemical/mechanical method have been used to obtain (Ti + C + Al2O3) powder mixtures with different alumina distributions. Scanning electron microscopy (SEM) observations of these mixtures show that alumina is distributed inside micrometric (Ti + C) aggregates for the first method whereas alumina is located around (Ti + C) aggregates for the second one. X-ray diffraction (XRD) and SEM analyses of the composites indicate that TiCx is substoichiometric in carbon and mainly consists of submicrometric grains. A distribution of Al2O3 inside (Ti + C) aggregates is more efficient to reduce TiC grain size. For the 40 vol% Al2O3 diluted (Ti + C) mixture prepared from the mechanical route, TiCx nanocrystallites have been successfully stabilized, which demonstrates that the addition of Al2O3 diluent in a (Ti + C) mixture can be efficiently used to inhibit grain growth.

Combustion synthesis of mechanically activated powders in the Nb–Si system

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of niobium silicides was investigated. SHS experiments were performed on reactant powder blends of composition Nb:Si = 1:2 and Nb:Si = 5:3 pretreated for selected milling times. A self-sustaining reaction could be initiated when a sufficiently long milling time was employed. At short milling times, the reactions self-extinguished or propagated in an unsteady mode. Combustion peak temperature, wave velocity, and product composition were markedly influenced by the length of the milling treatment. Single-phase products could be obtained for sufficiently long milling times. Observation of microstructural evolution in quenched reactions together with isothermal experiments allowed clarification of the mechanism of the combustion process and the role played by the mechanical activation of the reactants.

Production of Al–Ti–C grain refiner alloys by reactive synthesis of elemental powders: Part I. Reactive synthesis and characterization of alloys

The first part of this article reports on the reactive synthesis and characterization of Al–Ti–C alloys intended as master alloys for aluminum grain refining. The alloys were produced from elemental powders by the thermal explosion mode and analyzed with x-ray diffraction, scanning electron microscopy, and differential scanning calorimetry. Parameters were the titanium concentration (15 and 30 wt%) and the Ti/C ratio (9/1, 20/1, and 120/1) in the reactant powder mixture and the cooling rate after the reactive synthesis (1 and 120 °C/min). Full conversion of titanium and carbon into Al3Ti and TiC was achieved for the 30 wt% Ti mixtures but not for the 15 wt% Ti mixtures where the reaction was not exothermic enough. The Ti/C ratio did not affect the phase composition after reactive synthesis in the 30 wt% Ti alloys and could be used to tailor the microstructure of the alloy. The formation of Al4C3 was suppressed with a high cooling rate after the exothermic formation reactions.

FGM Fabrication by Combustion Synthesis

FGMs have been fabricated using the combustion synthesis (or self-propagating high-temperature synthesis (SHS)) process by exploiting a rapid and exothermic chemical reaction, in order to synthesize some (or all) of the constituents in an FGM to simultaneously increase its density. The thermal energy required to drive the process is derived from this internal, chemical source, rather than from an external and usually expensive source (e.g., a furnace). The combustion synthesis process is a powder-based process that has been used to synthesize over 300 compounds, and is particularly useful in preparing materials such as highly refractory ceramics and high-temperature intermetallics that are difficult to prepare by other synthesis methods. In addition, the process can be used to prepare ceramic-metal and ceramic-intermetallic composite materials. As a result, only slight modifications of the combustion synthesis are required to prepare functionally gradient materials from these same combinations of materials.Sample preparation begins by the creation of a series of mixtures from the powders that will react to form the constituent materials of the FGM sample. Each of these mixtures contains a slightly different percentage of reactants, so that each mixture will yield its own (predetermined) volume fraction of each of its constituents, following the combustion synthesis process. Prior to the combustion step, the samples are assembled by stacking layers of each of the reactant powder mixtures in appropriate amounts, in such a way that the multilayered powder mixture will faithfully produce the composition gradient that is required in the resultant FGM.