Vol 25, No 1 (2016)

Some kinetic aspects of mechanically induced self-sustaining reactions (MSRs) were investigated. The work aimed at establishing possible relationship between milling conditions, ignition time, and diffusion mechanisms in such reactions. Experimental and modeling results showed that ignition of MSR systems with high ΔH/C_p values (combustion systems) is possible upon simultaneous activation of all three diffusion mechanisms, i.e. surface diffusion, grain boundary diffusion, and lattice diffusion. When the ignition occurs, a set of diffusion mechanisms are activated, which provides proper conditions for immediate production of nanocomposite materials. Scrutiny of several combustion systems testified that there exists a fundamental difference between the self-sustaining and gradual mechanochemical reactions. Incomplete reactions, reappearance of starting reactants, intensity of XRD peaks, and grain growth can be regarded as important distinctions between ignitable (MSR-type) and non-ignitable (gradually reacting) systems.

Infiltration-mediated SHS of β-S_i6–zAl_zO_zN_8–z–BN composites was explored upon variation in applied nitrogen pressure, amount of combustible and low-melting components in green mixture, sample size, and BN content of resultant composites. Synthesized ceramic composites of different density and BN content were characterized by their strength parameters, tribological behavior, and thermal shock resistance.

Combustion of Ti–Al–C compacts in air and helium was investigated by time-resolved X-ray diffraction (TRXRD). The type of environment was found to markedly affect both SHS reaction and product structure. SHS reaction in helium yielded a product containing 80–85 wt % Ti2AlC, 15–20 wt % TiAl, and trace amounts of TiC admixture.

Influence of applied pressure P and intensity of Joule heating on electrothermal explosion (ETE) in Ti–C blends placed in a porous electroconducting envelope was studied by using a home-made experimental setup. A distinctive feature of this system is non-linear warm-up prior to initiation of ETE. A non-linear temperature growth during pre-explosion warm-up was associated with gradual decrease in the electrical resistance of the sample caused by intensification of mass transfer processes leading to an increase in current density and hence in heating rate. During the warm-up period, the extent of conversion was found to attain a value of about 15%. In classical theory of gasless combustion, chemical reaction within the warm-up zone is neglected and it is postulated that combustion reaction gets started at temperatures close to the melting point of a low-melting reagent. Our results cast some doubt on the validity of the above adoptions and thus shed new light on the mechanism of SHS reactions in heterogeneous systems.

The in-situ synthesized Al-matrix composites exhibited better mechanical properties than the composites prepared by ex-situ synthesis. Several in-situ processes for production of carbide-reinforced Al-matrix composites were comparatively discussed. The benefits of SHS process as compared to other insitu techniques are highlighted. The use of halide salts replacing elemental Ti in SHS reactions turned helpful for in-situ generation of nano particles in the Al matrix. The size, shape, and distribution of in-situ prepared reinforcing particles in Al-matrix are uniform and homogeneous. The reinforced surface is compatible with the undoped matrix surface, thus providing strong bonding. The formation of particle clusters and agglomerates can be minimized or eliminated by proper choice of in-situ processing conditions. Also addressed are the problems concerning the formation of Al₃Ti and Al₄C₃ intermetallics as byproducts.