A. S. Shchukin

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.

Self-propagating high-temperature synthesis of ceramic matrices for immobilization of actinide-containing wastes

Self-propagating high-temperature synthesis (SHS) of mineral-like matrices based on pyrochlore (Y2Ti2O7) and zirconolite (CaZrTi2O7) phases for immobilization of actinide-containing wastes (HLW) was studied. Thermodynamic analysis of the systems Ti-ZrO2-CaO-Y2O3-MenOm (Me = Mo, Fe, Ni, Cr, Mn, Cu) was performed. The possibility of preparing matrices based on the structure of titanium pyrochlore Y2Ti2O7 enriched in zirconium was demonstrated. The dependences of the formation and ratio of crystalline phases in SHS products on the charge composition and HLW concentration were revealed. The HLW components do not form intrinsic crystalline phases and are incorporated as isomorphous impurities in the crystal lattices of pyrochlore, zirconolite, and perovskite. The advantages of using Fe2O3 as oxidant were demonstrated. It prevents possible loss of HLW elements, associated with high-temperature evaporation of the oxidant. A matrix material consisting of two phases: (1) titanate pyrochlore Y2Ti2O7 containing HLW elements and enriched in zirconium and (2) Fe metal was prepared. The total content of lanthanides in separate pyrochlore crystals varies from 0.9 to 1.6 at. %, with 27 to 31% replacement of Ti atoms by Zr atoms. The study was performed with simulated HLW.

SHS of MAX compounds in the Ti-Si-C system: Influence of mechanical activation

Mechanical activation of Ti-Si-C green mixtures was found to negatively affect the composition, microstructure, and properties of synthesized Ti3SiC2.

Formation of nanolaminate structures in the Ti-Si-C system: A crystallochemical study

Combustion synthesis of laminate Ti3SiC2 structures from the elements was explored by time-resolved XRD. Crystallochemical modeling and quantum-chemical calculations suggest that the laminate structure of Ti3SiC2 arises due to regular accumulation of difference between the bond lengths of Ti3SiC2 and TiC and population of thus formed vacancies with Si atoms in the TiC lattice.

Combustion synthesis in the Ni-Al-W system: Some structural features

Explored was combustion synthesis in the Ni-Al-W system with special emphasis on structural features of the process. Using SHS in the 1500–1700°C temperature range, we observed the diffusion-induced processes at the interface between synthesized NiAl and W particles resulting in the formation of W2Ni and W-Ni intermetallics.

SHS reaction and explosive crystallization in thin films: Resemblance and distinction

The SHS waves in multilayer Ni/Al nanofilms and the explosive crystallization waves in Sb films deposited onto a thin copper substrate were comparatively explored by IR imaging and SEM. Despite a markedly lower thermal effect of crystallization, the waves of chemical reaction and crystallization were found to have much in common in their temperature–time histories: warmup rate, temperature gradient in the wave front, and duration of heat release. Observed was a micro-scintillation mode of the propagation of explosive crystallization wave in Sb films.

Self-Propagating High-Temperature Synthesis in the Ti-Si-C System: Features of Product Patterning

Nanolaminate Ti3SiC2-based material is prepared by self-propagating high-temperature synthesis (SHS) from elements, and the processes of its phase and structure formation is investigated. The Ti3SiC2 content in combustion products grows with an increase in the lifetime of the melt. The stratification observed at the level of the crystal structure results in the pronounced nanolaminate structure of Ti3SiC2 grains. The formation of the Ti3SiC2 phase occurs behind the combustion front with the interaction of stoichiometric TiC with the melt based on Ti-Si. Quantum-chemical calculations show that the formation of Ti-Si bonds is energetically preferable to Ti-C bond formation. A comparison of TiC and Ti3SiC2 crystal structures allow the assumption to be made that the laminate shape of Ti3SiC2 crystals is caused by the accumulation of structure discrepancies and hence by a lack of conjugation between the crystals.

Finely striped structure at the edges of localized deformation bands

Under conditions of high-rate loading, plastic strain localization is a result of tension in the zone of interference of unloading waves rather than of thermal softening. At stresses close to the dynamic strength of the material, the microstructure of localized strain bands consists of strongly deformed material, with a large number of incipient microdiscontinuities. At stresses below the Hugoniot elastic limit, the microstructure looks as a set of barely visible stripes. The finely striped structure at the edges of the bands of a spall damage arises as a result of the stretching of initially rounded damage centers attached to the matrix material during dynamic deformation.

SHS in preliminary structured compacts: II. Ti-2B and Ti-Al blends

Stepwise densification of Ti-2B and Ti-Al blends was used to prepare green compacts with an ordered structure. The latter ones were then used to explore combustion synthesis in these mixtures with special emphasis on the possibility of inheriting a structure of green compact by a combustion product. It has been demonstrated that, at some certain conditions, an ordered structure of green compact can be inherited by a combustion-synthesized product.

Combustion synthesis of nanosized iron oxides: The effect of precursor composition

AbsrtactExplored was the combustion synthesis of nanosized iron oxides from precursor solutions containing iron(III) nitrate or oxalate. Combustion was found to yield a mixture of α-Fe2O3 and β-Fe2O3 (the former being predominant) whose relative amount depended on precursor composition only slightly. In case of oxalate, product particles exhibited a rounded shape, smaller size, and lower extent of aggregation.