Self-propagating High-temperature Synthesis Mode Research Articles

In Situ Formation of Titanium Diboride/Magnesium Titanate Composites by Magnesiothermic-Based Combustion Synthesis

In situ formation of TiB2–Mg2TiO4 composites was investigated by combustion synthesis involving the solid-state reaction of Ti with boron and magnesiothermic reduction of B2O3. Certain amounts of MgO and TiO2 were added to the reactant mixtures of Ti/B/Mg/B2O3 to act as the moderator of highly exothermic combustion and a portion of the precursors to form Mg2TiO4. Two combustion systems were designed to ensure that synthesis reactions were sufficiently energetic to carry on self-sustainably, that is, in the mode of self-propagating high-temperature synthesis (SHS). Consistent with thermodynamic analyses, experimental results indicated that the increase in pre-added MgO and TiO2 decreased the combustion temperature and propagation velocity of the flame front. MgO was shown to have a stronger dilution effect on combustion exothermicity than TiO2, because the extent of magnesiothermic reduction of B2O3 was reduced in the MgO-added samples. In situ formation of the TiB2–Mg2TiO4 composite was achieved from both types of samples. It is believed that, in the course of the SHS progression, Mg2TiO4 was produced through a combination reaction between MgO and TiO2, both of which were entirely or partially generated from the metallothermic reduction of B2O3. The microstructure of the products exhibited fine TiB2 crystals in the shape of short rods and thin platelets that existed within the gaps of Mg2AlO4 grains. Both constituent phases were well distributed. A novel and efficient synthesis route, which is energy- and time-saving, for producing Mg2TiO4-containing composites was demonstrated.

Effects of TiC, Si, and Al on Combustion Synthesis of Ti3SiC2/TiC/Ti5Si3 Composites.

The fabrication of Ti3SiC2 from TiC-containing reactant compacts was investigated by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS). The initial sample composition was formulated based on (3 - x)Ti + ySi + (2 - x)C + xTiC + zAl, with stoichiometric parameters of x from 0 to 0.7, y = 1.0 and 1.2, and z = 0 and 0.1. For all samples studied, combustion was sufficiently exothermic to sustain the reaction in the SHS manner. Due to the dilution effect of TiC, combustion wave velocity and reaction temperature substantially decreased with TiC content. When compared with the TiC-free sample, the TiC-containing sample facilitated the formation of Ti3SiC2 and the TiC content of x = 0.5 produced the highest yield. Excess Si (y = 1.2) to compensate for the evaporation loss of Si during combustion and the addition of Al (z = 0.1) to promote the phase conversion were effective in improving the evolution of Ti3SiC2. All final products were composed of Ti3SiC2, TiC, and Ti5Si3. For the TiC-containing samples of x = 0.5, the weight fraction of Ti3SiC2 increased from 67 wt.% in the sample without extra Si and Al to 72 wt.% in the Si-rich sample of y = 1.2 and further up to 85 wt.% in the Si-rich/Al-added sample of y = 1.2 and z = 0.1. As-synthesized Ti3SiC2 grains were in a thin plate-like shape with a thickness of 0.5-1.0 μm and length of about 10 μm. Ti3SiC2 platelets were closely stacked into a layered structure.

Relationship between Structure and Properties of Intermetallic Materials Based on γ-TiAl Hardened In Situ with Ti3Al

In this work, intermetallic materials based on γ-TiAl in situ strengthened with the Ti3Al phase have been obtained from the initial components of titanium and aluminum under the conditions of free SHS-compression in one technological step and in ten seconds. This method combines the process of the combustion of initial components in the mode of self-propagating high-temperature synthesis (SHS) with high-temperature shear deformation of the synthesized materials. The following initial compositions have been studied (mol): Ti–Al, 1.5 Ti–Al, and 3 Ti–Al. Thermodynamic calculations have been carried out and the actual combustion temperature of the compositions under study has been measured. To increase the exothermicity of the studied compositions, a “chemical furnace” based on a mixture of Ti–C powders has been used, which allows us to increase the combustion temperature and stabilize the combustion front. It has been found that the actual combustion temperature of the selected compositions increased from 890–1120 to 1000–1350 °C. The results of X-ray powder diffraction and SEM are presented, mechanical and tribological characteristics of the obtained materials are measured, and 3D images of wear grooves are given. It has been found that a decrease in Ti molar fraction and an increase in Al molar fraction in the initial mixture lead to an increase in the mechanical (hardness up to 10.2 GPa, modulus of elasticity up to 215 GPa) and tribological characteristics (wear up to 4.5 times, coefficient of friction up to 2.4 times) of intermetallic materials.

Formation of TiB2-MgAl2O4 Composites by SHS Metallurgy.

TiB2-MgAl2O4 composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO2 or B2O3 as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al2O3 or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B2O3/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO2/Al/Mg thermite and Al2O3 as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB2 and MgAl2O4. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO, both of which can be totally or partially produced from the metallothermic reduction of B2O3 or TiO2. The microstructure of the TiB2-MgAl2O4 composite exhibited fine TiB2 crystals surrounded by large densified MgAl2O4 grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl2O4-containing composites.

Optimization of Aluminum Boride Synthesis in the Self-Propagating High-Temperature Synthesis Mode to Create Waste-Free Technology

This paper is the continuation of our previous paper. In this work, we optimized the synthesis of aluminum borides by the SHS method. The purpose of the research was to develop the foundations of waste-free technology. The initial components were powders of boric anhydride (B2O3), aluminum (Al), the oxide-heating additive (KNO3), various fluxing additives, including mixed ones. The optimal ratios of the initial components for increasing the yield of aluminum boride with a high boron content and obtaining slag suitable for the production of high-alumina clinkers were determined. Studies have shown that the development of a waste-free technology for producing aluminum borides by the method of self-propagating high-temperature synthesis (SHS) is possible and yields target (alloy) and by-product (slag) products that meet the requirements for chemical and phase composition.

DIELECTRIC PROPERTIES OF CERAMICS BASED ON BN–ZrB2 AND BN–ZrO2 PRODUCED BY SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS

Dielectric properties of ceramic materials based on BN–ZrB2 and BN–ZrO2 systems synthesized in the mode of self-propagating high-temperature synthesis (SHS) from a mixture of boron B powders, boron nitride BN and zirconium dioxide ZrO2 in an atmosphere of nitrogen N2 have been studied. The experimental results showed that ceramics synthesized from the initial mixture of (30B + 10ZrO2 + 60BN, wt. %) composition have permittivity (?) and tangent of the dielectric loss angle (tg) values of 2.63 and 0.006, respectively, at room temperature.

Fe-Si Intermetallics/Al2O3 Composites Formed between Fe-20% Si and Fe-70.5% Si by SHS Metallurgy Method

Fe–Si intermetallics–Al2O3 composites were fabricated by thermite-assisted combustion synthesis. Combustion reactions were conducted with powder compacts composed of Fe2O3, Al, Fe, and Si. The starting stoichiometry of powder mixtures had an atomic Fe/Si proportion ranging from Fe-20% to Fe-70.5% Si to explore the variation of silicide phases formed with Si percentage. Combustion in the mode of self-propagating high-temperature synthesis (SHS) was achieved and the activation energy of the SHS reaction was deduced. It was found that the increase of Si content decreased the combustion temperature and combustion wave velocity. Three silicide compounds, Fe3Si, FeSi, and α-FeSi2, along with Al2O3 were identified by XRD in the final products. Fe3Si was formed as the single-phase silicide from the reactions with Si percentage from Fe-20% to Fe-30% Si. FeSi dominated the silicide compounds in the reactions with atomic Si content between Fe-45% and Fe-55% Si. As the Si percentage increased to Fe-66.7% Si and Fe-70.5% Si, α-FeSi2 became the major phase. The microstructure of the composite product showed that dispersed granular or nearly spherical iron silicides were embedded in Al2O3, which was dense and continuous. Most of the silicide grains were around 3–5 μm and the atomic ratio of silicide particles from the EDS analysis confirmed the presence of Fe3Si, FeSi, and FeSi2.

Materials based on the MAX phases of the Ti-Al-C system obtained under combustion and high-temperature shear deformation

Materials based on MAX phases with Ti2AlC and Ti3AlC2 stoichiometry have been obtained from initial titanium, aluminum, and carbon black powders under conditions combining combustion processes in the mode of self-propagating high-temperature synthesis and high-temperature shear deformation. It is shown that by controlling the green sample composition and synthesis conditions, it is possible to obtain a material with a given stoichiometry and with a controlled size of MAX phase lamellar grains, additionally hardened in situ by TiC and TiAl3. It has been established that the obtained materials have three main zones in cross section: (i) ceramic material consisting of MAX phases and hardening particles; (ii) transition region formed as a result of mutual diffusion of synthesis products and titanium; (iii) consolidated titanium. It has been established that the maximum microhardness (up to 800 HV) is achieved on the surface of the obtained materials, which decreases to 180 HV as it deepens towards titanium.

New Way of Synthesis of Few-Layer Graphene Nanosheets by the Self Propagating High-Temperature Synthesis Method from Biopolymers.

For the first time, few-layer graphene (FLG) nanosheets were synthesized by the method of self-propagating high-temperature synthesis (SHS) from biopolymers (glucose, starch, and cellulose). We suggest that biopolymers and polysaccharides, particularly starch, could be an acceptable source of native cycles for the SHS process. The carbonization of biopolymers under the conditions of the SHS process was chosen as the basic method of synthesis. Under the conditions of the SHS process, chemical reactions proceed according to a specific mechanism of nonisothermal branched-chain processes, which are characterized by the joint action of two fundamentally different process-accelerating factors—avalanche reproduction of active intermediate particles and self-heating. The method of obtaining FLG nanosheets included the thermal destruction of hydrocarbons in a mixture with an oxidizing agent. We used biopolymers as hydrocarbons and ammonium nitrate as an oxidizing agent. Thermal destruction was carried out in SHS mode, heating the mixture in a vessel up to 150–200 °C at a heating speed of 20–30 °C/min and keeping at this temperature for 15–20 min with the discharge of excess gases into the atmosphere. A combination of spectrometric research methods, supplemented by electron microscopy data, has shown that the particles of the carbonated product powder in their morphometric and physical parameters correspond to FLG nanosheets. An X-ray diffraction analysis of the indicated FLG nanosheets was carried out, which showed the absence of formations with a graphite crystal structure in the final material. The surface morphology was also studied, and the IR absorption features of FLG nanosheets were analyzed. It is shown that the developed SHS method makes it possible to obtain FLG nanosheets with linear dimensions of tens of microns and a thickness of not more than 1–5 graphene layers (several graphene layers).

Effects of Fe/Si Stoichiometry on Formation of Fe3Si/FeSi-Al2O3 Composites by Aluminothermic Combustion Synthesis

Aluminothermic combustion synthesis was conducted with Fe2O3–Al–Fe–Si reaction systems under Fe/Si stoichiometry from Fe-20 to Fe-50 at. % Si to investigate the formation Fe3Si/FeSi–Al2O3 composites. The solid-state combustion was sufficiently exothermic to sustain the overall reaction in the mode of self-propagating high-temperature synthesis (SHS). Dependence of iron silicide phases formed from SHS on Fe/Si stoichiometry was examined. Experimental evidence indicated that combustion exothermicity and flame-front velocity were affected by the Si percentage. According to the X-ray diffraction (XRD) analysis, Fe3Si–Al2O3 composites were synthesized from the reaction systems with Fe-20 and Fe-25 at.% Si. The increase of Si content led to the formation of both Fe3Si and FeSi in the final products of Fe-33.3 and Fe-40 at.% Si reaction systems, and the content of FeSi increased with Si percentage. Further increase of Si to Fe-50 at.% Si produced the FeSi–Al2O3 composite. Scanning electron microscopy (SEM) images revealed that the fracture surface morphology of the products featured micron-sized and nearly spherical Fe3Si and FeSi particles distributing over the dense and connecting substrate formed by Al2O3.

Influence of high-temperature annealing on structure of titanium aluminide materials obtained by combustion and high-temperature shear deformation

A compact material based on initial powders of titanium and aluminum (porosity <1%) was obtained by unrestricted SHS-compression. This method combines combustion processes in the mode of self-propagating high-temperature synthesis (combustion temperature – 1460 °C, combustion front propagation – 3.7 mm/s) and high-temperature shear deformation. The obtained material structure and phase composition were studied. The high-temperature shear deformation was shown to lead to textured structure of the material which consisted of alternating intermetallic phases in the following sequence: α2-Ti3Al, γ-TiAl, TiAl2 и TiAl3. Influence of high-temperature annealing at T = 1000 °C during 0.5–4 h on the material structure and phase composition was studied.

Formation Ti6Al4V alloy by hydride cycle mode and its (Ti6Al4V)H1.606 hydride in self-propagating high-temperature synthesis mode

The wide use of titanium alloys is limited by high costs of both their producing technology and processing. The expansion of area of their application depends on development of more efficient, resource-saving, economical methods providing reduction in the cost of titanium alloys and titanium made products. The hydride cycle (HC) method for synthesis of binary and multicomponent refractory alloys and hydrides can be suitable alternative to traditional metallurgy for production of the alloy in small quantities (small-sized products for technical/medical purposes). This work presents the results of study on the mechanism of Ti–6Al–4V alloy formation in HC. The synthesis parameters are evaluated and optimized. The physicochemical, structural and hydrogen-sorption characteristics of the alloy samples are described. Actually, a resource-saving, efficient technology for synthesis of Ti–6Al–4V alloy by HC is developed and proposed. The synthesized alloy interaction with hydrogen in Self-Propagating High-Temperature Synthesis mode was investigated.

Effect of Sample Preparation Pressure on Transformation Law of Low-Valent Titanium Oxide in a Multi-Stage Reduction Process

A novel method for preparing titanium powder by multi-stage reduction of TiO2 was proposed. Its core is the preparation of high-quality low-valent titanium oxide. In this paper, the effect mechanism of different sample preparation pressures on the preparation of low-valent titanium oxide by the primary reduction (self-propagating high-temperature synthesis mode, SHS) of the Mg-TiO2 system was studied. The results show that the generation of Mg thermal fluid is the key link of the self-sustaining chemical reaction of the Mg-TiO2 system. Titanium exists inα-Ti and TiO at the end of combustion, and constitutes a non-stoichiometric low-valent titanium oxide. The sample preparation pressure determines the proportion of pores reserved for Mg diffusion in the compacts and the contact area of the reactants, thereby determining the partitioning behavior and heat transfer effect of Mg thermal fluid during the combustion process. When the sample preparation pressure is 75 MPa (relative density is 0.66 ± 0.01), the combustion effect is optimal, and the low-valent titanium oxide with oxygen content of 15.1% can be obtained. It was subjected to deep reduction to obtain a titanium powder product with an oxygen content of 0.27%.

Combustion synthesis of FeAl−Al2O3 composites with TiB2 and TiC additions via metallothermic reduction of Fe2O3 and TiO2

Abstract Combustion synthesis involving metallothermic reduction of Fe2O3 and TiO2 was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate FeAl-based composites with dual ceramic phases, TiB2/Al2O3 and TiC/Al2O3. The reactant mixture included thermite reagents of 0.6Fe2O3+0.6TiO2+2Al, and elemental Fe, Al, boron, and carbon powders. The formation of xFeAl−0.6TiB2−Al2O3 composites with x=2.0−3.6 and yFeAl−0.6TiC−Al2O3 composites with y=1.8−2.75 was studied. The increase of FeAl causes a decrease in the reaction exothermicity, thus resulting in the existence of flammability limits of x=3.6 and y=2.75 for the SHS reactions. Based on combustion wave kinetics, the activation energies of Ea=97.1 and 101.1 kJ/mol are deduced for the metallothermic SHS reactions. XRD analyses confirm in situ formation of FeAl/TiB2/Al2O3 and FeAl/TiC/Al2O3 composites. SEM micrographs exhibit that FeAl is formed with a dense polycrystalline structure, and the ceramic phases, TiB2, TiC, and Al2O3, are micro-sized discrete particles. The synthesized FeAl−TiB2−Al2O3 and FeAl−TiC−Al2O3 composites exhibit the hardness ranging from 12.8 to 16.6 GPa and fracture toughness from 7.93 to 9.84 MPa·m1/2.

Self-propagating reaction mechanism of Mg–TiO2 system in preparation process of titanium powder by multi-stage reduction

The novel method for the preparation of titanium powder by multi-stage reduction was proposed. The primary reduction adopted self-propagating high-temperature synthesis (SHS) mode. This paper focuses on the primary reduction process of Mg–TiO2 system under the condition of off-balance reaction. The effects of different material ratios, material arrangement methods and reaction initiation modes on the SHS reaction process of Mg–TiO2 system and its reaction mechanism were systematically studied. SHS mode was used to Mg–TiO2 system, and non-stoichiometric low-valent titanium oxide intermediate including α-Ti (Ti2O type) and TiO was directly obtained (with oxygen content of 13.93 wt%). SHS reaction initiated by local ignition is more sufficient than by overall heating method. Compared with the loose setting materials, the compacts can increase the effective contact interface of the reactants, and SHS reaction proceeds more sufficiently, which is favorable for obtaining lower oxygen content product. The adiabatic temperatures of the Mg–TiO2 system at different initial conditions were calculated according to the improved calculation method. When the initial temperature is 298 K, the adiabatic temperature of Mg–TiO2 system is between 1363 and 2067 K at different material ratios. Therefore, unreacted or partially excess Mg at the reaction front will diffuse into the unreacted region in gas or liquid form, thereby preheating the material and initiating further SHS reaction.

Intermetallic/Ceramic Composites Synthesized from Al–Ni–Ti Combustion with B4C Addition

The fabrication of intermetallic/ceramic composites by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) was investigated in the Al–Ni–Ti system with the addition of B4C. Two reaction systems were employed: one was used to produce the composites of xNiAl–2TiB2–TiC with x = 2–7, and the other was used to synthesize yNi3Al–2TiB2–TiC with y = 2–7. The reaction mechanism of the Al–Ni–Ti system was strongly influenced by the presence of B4C. The reaction of B4C with Ti was highly exothermic, so the reaction temperature and combustion velocity decreased due to increasing levels of Ni and Al in the reactant mixture. The activation energies of Ea = 110.6 and 172.1 kJ/mol were obtained for the fabrication of NiAl- and Ni3Al-based composites, respectively, by the SHS reaction. The XRD (X-ray diffraction) analysis showed an in situ formation of intermetallic (NiAl and Ni3Al) and ceramic phases (TiB2 and TiC) and confirmed no reactions taking place between Ti and Al or Ni. The microstructure of the product revealed large NiAl and Ni3Al grains and small TiB2 and TiC particles. With the addition of TiB2 and TiC, the hardness of NiAl and Ni3Al was considerably increased and the toughness was also improved.

AlMgB14–TiB2 composite materials obtained by self-propagating high-temperature synthesis and spark plasma sintering

In this work, AlMgB14–TiB2 composite materials were obtained by thermochemical-coupled self-propagating high-temperature synthesis (SHS) and subsequent spark plasma sintering. The mechanism was proposed for the formation of the composite materials in the thermochemical-coupled SHS mode. The phase composition, microstructure, and properties (density and Vickers hardness) of the dense AlMgB14–TiB2 materials were investigated. At a sintering temperature of 1470 °C, AlMgB14 is decomposed into AlB12 and Mg. The sample sintered at 1470 °C with a holding time of 5 min had a maximum average hardness of 32.1 GPa.

Self-propagating high-temperature synthesis of ceramic composite material based on Al2O3 - SiO2 - ZrB2

The present work is devoted to the production of a ceramic composite material in the mode of self-propagating high-temperature synthesis from a mixture of powder reagents: ZrO2, Al, SiO2, B, and Zr. The adiabatic combustion temperature was calculated for the stoichiometric composition of ZrO2-Al-SiO2 and for compositions with the addition of an energy additive from powders x (Zr + 2B), where x e [10-30] wt. %. It was found that an increase in the fraction of Zr + 2B leads to the refinement of structural elements. An increase in its content to 30 wt. % leads to the formation of nanoscale structural components in the material. As a result of varying the initial composition, a ceramic composite material based on Al2O3-SiO2-ZrB2 was obtained containing an oxide matrix with particles of zirconium diboride distributed from 61 to 77 wt. %. It is shown that as a result of the SHS of the selected compositions, mullite is formed. Using SEM, EDS and XRD methods, we studied the structural features and phase composition of the materials obtained.

Formation of Mo5Si3/Mo3Si-MgAl2O4 Composites via Self-Propagating High-Temperature Synthesis.

In situ formation of intermetallic/ceramic composites composed of molybdenum silicides (Mo5Si3 and Mo3Si) and magnesium aluminate spinel (MgAl2O4) was conducted by combustion synthesis with reducing stages in the mode of self-propagating high-temperature synthesis (SHS). The SHS process combined intermetallic combustion between Mo and Si with metallothermic reduction of MoO3 by Al in the presence of MgO. Experimental evidence showed that combustion velocity and temperature decreased with increasing molar content of Mo5Si3 and Mo3Si, and therefore, the flammability limit determined for the reaction at Mo5Si3 or Mo3Si/MgAl2O4 = 2.0. Based upon combustion wave kinetics, the activation energies, Ea = 68.8 and 63.8 kJ/mol, were deduced for the solid-state SHS reactions producing Mo5Si3– and Mo3Si–MgAl2O4 composites, respectively. Phase conversion was almost complete after combustion, with the exception of trivial unreacted Mo existing in both composites and a minor amount of Mo3Si in the Mo5Si3–MgAl2O4 composite. Both composites display a dense morphology formed by connecting MgAl2O4 crystals, within which micro-sized molybdenum silicide grains were embedded. For equimolar Mo5Si3– and Mo3Si–MgAl2O4 composites, the hardness and fracture toughness are 14.6 GPa and 6.28 MPa m1/2, and 13.9 GPa and 5.98 MPa m1/2, respectively.

МОДИФІКУВАННЯ РОБОЧИХ ПОВЕРХОНЬ СТАЛЕВИХ ДЕТАЛЕЙ З ФОРМУВАННЯМ ВИСОКОВУГЛЕЦЕВИХ СТРУКТУР

The urgent task is to create on the surface of steel parts layers that are capable of long time to work reliably under conditions of friction and wear. The tool for increasing the wear resistance of the surface layers of steel parts is chosen to modify the working surfaces by their carbonization. In this case, the hardness of the metal is controlled by the change in the carbon concentration in the surface layers, the content of the alloying elements by thermal and chemical-thermal processing techniques. In the work comparative studies of the efficiency of various technologies of carburization of working surfaces of steel parts and formation of the properties of wear resistance, formation of burrs and gripping according. It has been proposed to carry out comparative studies according to the criteria of performance, cost, and applicability for dimensional parts. The analysis revealed the existence of a number of methods that use the diffusion phenomenon to form wear-resistant high-carbon coatings with metal matrix and carbide reinforcing inclusions or cemented layers with different structural components on the surfaces of steel parts, which are known as “quenching structures”, differing in different technical and economic efficiency. Particular attention is also paid to the method of forming composite high-carbon coating in the mode of self-propagating high-temperature synthesis (SHS). As a result of the carried out researches it is shown that there is a significant shortage of the overwhelming majority of the considered methods of carburizing of working surfaces of parts from low or medium carbon varieties of steel - high cost of production due to low productivity and complexity of implementation. These drawbacks stimulate the development of new high-performance methods of forming on steel details corresponding coatings of considerable depth and low cost.