M. A. Korchagin

Solid‐State Combustion in Mechanically Activated SHS Systems. I. Effect of Activation Time on Process Parameters and Combustion Product Composition

The factors responsible for the transition from reagent interaction involving a liquid phase in ordinary SHS powder mixtures to solid‐state combustion after preliminary activation of these mixtures in an energy‐intensive planetary mill were studied for Ni + 13 wt. % Al and Ni + 45 wt. % Ti compositions. The dependences of the burning rate and temperature on the duration and conditions of mechanical activation were determined. It is found that the occurrence of solid‐state SHS in a powdermixture is due to the formation of “laminated composites,” in which the reagents are ground to ultrafine size, the area of their contact increases severalfold, and the concentration of nonequilibrium defects is high. In activated samples, heat release proceeds in several stages and at lower temperature than in powder mixtures.

Solid‐State Combustion in Mechanically Activated SHS Systems. II. Effect of Mechanical Activation Conditions on Process Parameters and Combustion Product Composition

The effect of mechanical activation conditions in a planetary ball mill on the main parameters of SHS‐processes and combustion product composition was studied for Ni + 13 wt. % Al and Ni + 45 wt. % Ti compositions. From results of experiments at elevated initial temperatures, it is concluded that because of reverse quenching, the defects produced by mechanical activation are not annealed in the heating zone and are retained in the sample until the beginning of chemical interaction in the leading zone of the SHS wave. The process of annealing of the activated samples was studied in situ on a transmission electron microscope and a synchrotron radiation diffractometer. The energy stored in the samples as a result of mechanical activation is estimated by calorimetric studies.

Peculiarities of the formation of high-defect states in mechanocomposites and powders of niobium and aluminum under severe deformation in planetary ball mills

The microstructural peculiarities of Nb powder and Nb + Al powder mixtures after mechanical activation in a high-energy planetary ball mill were investigated by transmission electron microscopy. The materials revealed two-level structural states: nanograins of size from 50 to 100 nm that contain subgrains of size ∼20 nm and less with low-angle misorientation boundaries, elastic lattice curvature gCij≈100°…200°μm−1, curl or curvature gradient ∂χij/∂r > 100 μm−2 and high (up to 10E μm−1) local stress gradients (couples). An important factor in the formation of the above states is the capability of nano-objects to reach high elastic lattice curvature, high gradients of this curvature, and high local internal stress gradients at rather low absolute values of the internal stress.

Effect of severe mechanical alloying on the microstructure parameters of 3Ti + Al mechanocomposites

The microstructure of mechanocomposites produced by mechanical alloying of Ti and Al powder mixtures in high-energy planetary ball mill has been studied by the methods of transmission electron microscopy and X-ray diffraction analysis. The formation of highly defect structural states with high magnitudes of the crystal-lattice curvature and large density of disclinations at the boundaries of submicrocrystals and nanocrystals has been revealed. It is assumed that such a highly defect structural state is an important channel for the accumulation of deformation energy upon mechanical alloying and plays an essential role in phenomena of increasing reactivity of components of mixture systems, abnormal mass transfer, and solid-state interaction of reactants.

Thermal explosion in mechanoactivated 3Ni + Al mixtures

Explored was thermal explosion in mechanoactivated 3Ni + Al mixtures. Mechanoactivation was found to result in an abnormal decrease in the effective activation energy E and ignition temperature Tign for thermal explosion. Analysis of reaction thermogram allowed us to find out the kinetic function. Mechanoactivation conditions for synthesis of Ni3Al in thermal explosion mode have been optimized. SHS reaction in 3Ni + Al mixtures mechanoactivated for 180 s was found to obey the first-order kinetics.

Superadiabatic Regime of the Thermal Explosion in a Mechanically Activated Mixture of Tungsten with Carbon Black

A superadiabatic regime of the thermal explosion in mechanically activated stoichiometric mixtures of tungsten and carbon black is obtained. Regimes of preliminary mechanical activation of mixtures and the subsequent thermal explosion that allow obtaining a single-phase carbide WC with a submicron grain size are determined. The mechanical energy accumulated in the sample as a result of preliminary activation is estimated. Results of the x-ray diffraction analysis and electron microscopy of mechanically activated samples and thermal explosion products are reported.

Thermal explosion in mechanically activated low-calorific-value compositions

The effect of preliminary mechanical activation of low-calorific-value powdered formulations in a planetary ball mill on the main parameters of the subsequent thermal explosion has been studied. It has been found that in mechanically activated compositions, the initiation temperature of thermal explosion is reduced by hundreds of degrees. The maximum decrease (1300°C) is observed for the Ti + 4 wt.% C system. Regimes of preliminary mechanical activation of reaction mixtures and the subsequent thermal explosion conditions producing Ti3Al and Ni3Al single-phase intermetallic compounds with nanometer grain size were determined. For the 3Ni + Al composition, the energy accumulated during mechanical activation was evaluated. It is shown that the initiation temperature of thermal explosion in the MA compositions studied can be used to estimate the temperature that develops in the mill drums.

Anomalous decrease in the activation energy and initiation temperature of a thermal explosion in the mechanically activated composition 3Ni + Al

277 † In recent years, increasing interest in combination of the methods of self-propagating high-temperature synthesis (SHS) and mechanical activation (MA) has been observed [1–4]. This is explained by the fact that the preliminary MA of the powdered reaction mixtures provides a substantial expansion of the possibility of gasless combustion for the high-temperature synthesis of inorganic materials, specifically, to extend the concentration limits of combustion, to use for synthesis such compositions that do not combust under usual conditions, to avoid the necessity of pressing the starting samples [3, 4], and to realize the solid-phase combustion mode even in compositions with a low-melting reagent such as aluminum [4]. It should be noted that the synthesis in most of the investigated activated compositions was performed in the mode of frontal propagation of the SHS wave. However, the features of the dynamics of the self-heating process in the thermal explosion mode in activated systems and the corresponding interrelation of the processes of structure and phase formation with the MA modes are almost unknown. The thermograms of the synthesis of activated samples can give a lot of useful information with respect to the synthetic parameters and, first of all, the effective activation energy and critical conditions of interaction initiation.

Microstructural peculiarities of copper and mechanisms of its hardening after mechanical activation and torsion in Bridgman anvils

The paper presents the results of complex research on microstructural peculiarities of copper and mechanisms of its hardening after mechanical activation in planetary ball mills, high-pressure torsion, and combined treatment which includes mechanical activation and subsequent consolidation under high-pressure torsion in Bridgman anvils. The main structural factors responsible for the hardening mechanisms are discussed depending on the pattern and degree of deformation.

Thermal explosion and self-propagating high-temperature synthesis in mechanically activated SiO2-Al mixtures

A thermal explosion and self-propagating high-temperature synthesis in low-calorific SiO2-Al mixtures subjected to preliminary mechanical activation are obtained. Results of x-ray and electron-microscopic investigations of the microstructure and phase composition of activated mixtures and reaction products are reported. It is found that complete reduction of quartz by aluminum occurs only in the thermal explosion mode.