Macrokinetic characteristics of the combustion of mixtures in the (100% − X)(Ti + 0.5C) + X(Ta + C) system with a variable mixing parameter X and initial temperature T ₀ of charge heating are considered. For compositions with X = 10 and 30%, an abrupt increase in the velocity U _c and temperature T _c of combustion as a result of passing two parallel chemical reactions of titanium and tantalum carbide formation is established. The U _c (T ₀) and T _c (T ₀) dependencies are linear for the mixture with X = 50%. By hardening the combustion wave, it is revealed that the primary structure formation in the combustion region starts from the selection of submicron grains of nonstoichiometric titanium carbide from the supersaturated titanium melt. In the investigated range of parameter X, synthetic products are single-phase and represent titanium-tantalum carbide. An increase in X results in a decrease in the size and microhardness of (Ti, Ta) C grains and a reduction of the relative density of compact synthetic products. The kinetics of high-temperature oxidation of alloys on the basis of carbide (Ti, Ta) C is studied. Ceramics produced at X = 10% are most heat-resistant.

To ascertain the influence of SiC nanowires on sintering kinetics of heterophase ceramics, two composite powders (TaSi₂-TaC-SiC and TaSi₂-TaC-SiC-SiC_nanowire) are fabricated by mechanically activated combustion synthesis of Ta-Si-C and Ta-Si-C-(C₂F₄) reactive mixtures. Remarkable compressibility is achieved for the TaSi₂-TaC-SiC-SiC_nanowire composition (green density up to 84% as compared with 45.2% achieved for TaSi₂-SiC-TaC) which is attributed to the lubricating effect of residual adsorbed fluorinated carbon (most likely C₄F₈). The outcomes of pressureless sintering of TaSi₂-TaC-SiC and TaSi₂-TaC-SiC-SiC_nanowire compositions are vastly different; the former experiences no significant densification or grain growth and does not attain structural integrity, whereas the latter achieves relative density up to 93% and hardness up to 11 GPa. The SiC nanowires are not retained in consolidated ceramics, but instead, act as a sintering aid and promote densification and grain growth. Sintering mechanisms of TaSi₂-TaC-SiC and TaSi₂-TaC-SiC-SiC_nanowire powders are analyzed using thermodynamic and ab initio grand potential calculations, as well as the analysis of grain size versus relative density relations. In the case of solid-state sintering, the densification and grain growth in heterophase non-oxide ceramics are governed by the same mechanisms as previously investigated single-phase oxides. The presence of SiC nanowires enhances grain-boundary related diffusion processes due to the high specific surface and aspect ratio of the nanowires. At 1500 °C, where the formation of the transient Si-based liquid phase is thermodynamically viable, only the SiC nanowire-containing composition demonstrated the intense grain coarsening and densification associated with liquid-assisted sintering. This effect can be attributed both to the presence of SiC nanowires and purification of residual oxide impurities due to C₂F₄-activated combustion synthesis employed for the in situ formation of SiC nanowires.

The influence of mechanical-activation (MA) conditions on the microstructure and phase composition of Ta–Hf–C reaction mixtures and products fabricated from them by self-propagating...

This study covers the elemental synthesis features of Hf–Ta–B–Ti–Si ceramic materials used to obtain promising high-temperature cer

Исследовано влияние схемы и среды механического активирования (МА) реакционной смеси состава Ta–Zr–C на фазовый состав и микроструктуру шихты и продуктов синтеза. Установлено, что при обработке в планетарной центробежной мельнице по разным схемам МА, отличающимся последовательностью введения компонентов, микроструктура шихты различается даже при одной и той же длительности обработки. Фазовый состав смесей одинаков, так как образования продуктов химических реакций в барабанах мельницы не обнаружено. Увеличение времени в каждой из опробованных схем МА приводит к росту микродеформации кристаллической решетки тантала и уменьшению его области когерентного рассеяния, что свидетельствует о повышении количества запасенной энергии. Исследование влияния среды активирования (воздух, аргон, вакуум) показало, что при МА на воздухе СВС-продукт представляет собой практически однофазный двойной карбид (Ta,Zr)C с содержанием ZrO2 менее 3 мас.%. При синтезе из реакционных смесей, активированных в бескислородной атмосфере, обнаружены 3 карбидные фазы ТаС, ZrC и (Ta,Zr)C без следов ZrO2. Определены оптимальные режимы МА СВС, обеспечивающие получение 100 % однофазного твердого раствора (Ta,Zr)C с параметром решетки а = = 0,4488 нм, что соответствует 15 ат.% ZrC в двойном карбиде.