S. G. Vadchenko

Self-sustained waves of exothermic dissolution in reactive multilayer nano-foils

The phenomenon of exothermic waves in reactive multilayer nano-foils demonstrates unusually high propagating rates (up to 100 m/s), which generate numerous applications and continuously motivate attempts for understanding its mechanism. In this work, based on the studies of the “quenched” exothermic waves, we have found that the driving mechanism of this phenomenon is a direct exothermic dissolution of one solid reactant in the molten layer of the other. The suggested mechanism of the exothermic wave in the Ni/Al multilayer nano-foil allows non-contradictory explanation of its main features.

Combustion modes in the titanium-nitrogen system at low nitrogen pressures

The macroscopic combustion regimes, as well as the dynamics of phase formation, for the titanium-nitrogen system at low nitrogen pressures (0.1 kPa < p < 50 kPa) have been studied. Several combustion modes, including planar surface (high-temperature), surface spot, and planar layer-by-layer (low-temperature) mode, have been observed. Also, a chaotic spot mode was found at pressures below 2 kPa for conditions of high heat loss. Using the time-resolved x-ray diffraction method previously developed by the authors, it was shown in situ that different mechanisms of phase formation correspond to these combustion modes. For the high-temperature mode, titanium nitride appeared in the combustion front as the initial product. However, only solid solutions of nitrogen in titanium were formed during the low-temperature mode. A complex sequence of phase formation was observed for spot combustion.

Mechanically activated SHS of NiAl: Effect of Ni morphology and mechanoactivation conditions

Explored was the effect of the morphology of Ni particles in green mixtures on the mechanoactivated SHS reactions in the Ni-Al system. In case of fluffy Ni particles (produced in a carbonyl process), the mean size of the agglomerates formed in the course of MA was higher than in case of dense Ni particles (produced in an electrolytic process). The absolute values of combustion temperature and burning velocity were higher in case of carbonyl-derived Ni, while an increment in burning velocity caused by MA was higher in case of electrolytically produced Ni.

Reactivity of mechanically activated powder blends: Role of micro and nano structures

Experimental study of reactive powder blends (largely Ni + Al mixures) was made in order to reveal a role of micro to nano structural transformations during mechanical activation in improving the reactivity of activated blends. The blends were processes in planetary ball mills, and the temperature (Tig) of reaction initiation (self-ignition) was measured. Microstructure of activated compositions was characterized by SEM and TEM. Tig was found to strongly depend on the intensity and duration of mechanoactivation. Microstructural analysis has shown that nanosized sites of highly reactive precursors are formed during the activation process, apparently as a result of dry friction of the reactants. An increase in the amount of these sites causes a gradual decrease in Tig for activated blends.

Thermal explosion in various Ni-Al Systems: Effect of mechanical activation

Thermal explosion (ignition) of various Ni-Al samples—such as powder blends, rolled and magnetron sputtered films, compacts of mechanically activated and non-activated mixtures—was explored with special emphasis on the role of mechanical activation. The critical ignition temperature (Tc) was found to diminish from 650°C for starting (non-activated) Ni-Al mixtures down to 280°C for properly activated samples. For the thin films of alternating Ni and Al layers prepared by magnetron sputtering, Tc turned out to be as low as 230°C.

Gas release during combustion of Ti + 2B films: Influence of mechanical alloying

Influence of mechanical alloying on gas release during combustion of Ti + 2B films was studied experimentally. The volume of released gas and the rate of gas evolution were measured as a function of mechanoactivation duration and ambient atmosphere. Burning velocity of Ti + 2B films and gas release rate were found to depend on a volume of gas released during combustion, type of environment, and MA conditions. The results may turn useful for optimizing combustion synthesis of titanium diboride and related materials as well as for some pyrotechnic applications.

SHS joining in the Ti-Si-C system: Structure of transition layer

Explored was SHS joining in sandwiches of (Ti + yC)/(Ti + xSi) powder compacts, and the transition layers were characterized by SEM, EDS, XRD, and optical microscopy. The transition layers were found to contain titanium carbides, titanium silicides, and Ti3SiC2. The results suggest that, in the systems under study, the weld seam is formed due to convective interpenetration of melted combustion products.

Reactivity of mechanoactivated Ni-Al blends

Mechanoactivation (MA) of Ni-Al blends was found to result in formation of nanosized diffraction-silent particulates whose content increased with increasing MA duration (τ). Upon MA, the ignition temperature for the blends under study was found to decrease by a value of 300–350°C. The main reaction stages were elucidated by time-resolved XRD. An increase in the reactivity of mechanically activated powder blends was associated with formation of nanosized diffraction-silent phases and solid solutions.

Simultaneous synthesis and joining of a Ni-Al-Based layer to a Mo foil by SHS

Explored was the simultaneous synthesis and joining of a Ni-Al-based layer to a Mo foil by SHS in Al-Ni and Al-NiO mixtures. It has been demonstrated that variation in reaction conditions can be used as a tool for regulating a thickness of a deposited coating and its phase composition. Under some certain conditions, a Mo foil can be fully consumed in the reaction with combustion products to yield a product of desired composition.

Self-propagating crystallization waves in the TiCu amorphous alloy

Self-propagating crystallization waves are detected and experimentally demonstrated in the Ti50Cu50 amorphous alloy obtained by the melt spinning (ultrafast quenching) method. High-speed thermographic recording has shown that crystallization waves can appear spontaneously at the heating of an amorphous strip to 300–350°С or at the local initiation by a hot tungsten coil of a small segment of the strip preliminarily heated to 230–250°С. In the former case, the crystallization wave propagates at a velocity of ~7 cm/s; in the latter case, the crystallization wave propagates in a self-oscillation mode at an average velocity of ~1.2 cm/s. The temperature gradient across the wavefront is about 150°С. The samples crystallized in the self-oscillation mode have a characteristic banded structure with a smaller grain in depression regions. The crystallization product in all samples is the TiCu tetragonal intermetallic phase.