Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.     

Reaction synthesis of TiC–TiB<Subscript>2</Subscript>/Al composites from an Al–Ti–B<Subscript>4</Subscript>C system

The TiC–TiB₂/Al composites were fabricated by self-propagating high-temperature synthesis (SHS) from Al–Ti–B₄C compacts. The addition of Al to the Ti–B₄C reactants facilitates the ignition occurrence, lowers the reaction exothermicity, and modifies the resultant microstructure. The maximum combustion temperature and combustion wave velocity decrease with the increase in the Al amount. The B₄C particle size exerts a significant effect on the combustion wave velocity and the extent of the reaction, while that of Ti has only a limited influence. The reaction products are primarily dependent on the B₄C particle size and the Al content in the reactants. Desired products consisting of only the TiC, TiB₂, and Al phases could be obtained by a cooperative control of the B₄C particle size and the Al content.     

Combustion synthesis of titanium carbide: Theory and experiment

The combustion synthesis of titanium carbide from elemental powders has been theoretically and experimentally studied as a model system for self -propagating high temperature synthesis (SHS) of refractory compounds. Calculations of the adiabatic temperature of combustion of graphite and titanium powders to form TiC _x have been made to show the effects of stoichiometry, dilution and the initial temperature of the reactants. Experimental observations on the stability of the combustion front, combined with theoretical predictions, lead to an estimated activation energy of 117 kJ mol^–1 for the process. This value is at least a factor of about four too low to correspond to a diffusion-controlled process. The combustion of graphite and titanium powders was accompanied by the evolution of gases whose primary constituent was found to be hydrogen. This observation was attributed to the reaction of adsorbed moisture with titanium powder. The titanium carbide phase resulting from the combustion of compacted mixed powders of the elements was highly porous ( 50% porosity). It can be obtained in high density (5% porosity) when pressure is applied during the combustion process.     

Combustion Synthesis of Glass (B2O3-Al2O3-MgO)-Ceramic (TiB2) Composites

The combustion synthesis, or self-propagating high temperature synthesis (SHS), technique has been used to produce glass ceramic composites that have a glass matrix based on B₂O₃-MgO-Al₂O₃and a crystalline ceramic phase of TiB₂. Conditions for producing glassy materials by the SHS technique are discussed and the thermodynamics of these combustion reactions are analyzed. The combustion characteristics, i.e., ignition energy, combustion temperature, and wave velocity have been determined. Green density of the pellets had a significant effect on the combustion characteristics. Green pellets with low density were used to reduce heat loss, thus enabling the synthesis of those compositions having low adiabatic temperatures. The glass-forming region of these SHS glasses was found to be in relatively good agreement with that of samples produced by the traditional furnace-melting method.     

Synthesis and mechanical properties of Ti3AlC2 by spark plasma sintering

In this paper, spark plasma sintering (SPS), after hot isostatically pressing (HIP) method was reported as a new approach to prepare bulk polycrystalline samples of Ti₃AlC₂. The ternary carbide was fabricated by spark plasma sintering (SPS) at a pressure of 22 MPa and temperature of 1250°C. The raw materials, elemental powders of Ti, Al and activated carbon, were pretreated in the following different ways prior to SPS: one way was to obtain porous Ti₃AlC₂ by self-propagating high-temperature synthesis (SHS) from mixture of Ti, Al and C, and then densify the product by SPS; the second way was to synthesize Al₄C₃ from Al and C firstly, and then mix powders of Ti and C with synthesized Al₄C₃ to fabricate bulk Ti₃AlC₂ by SPS. Obtained polycrystalline Ti₃AlC₂ ceramics had excellent mechanical properties: density was 4.24 ± 0.02 g/cm³, flexural strength was 552 ± 30 MPa and fracture toughness (K _IC) was 9.1 ± 0.3 MPa · m^1/2. It could be concluded that SPS method was a useful method to synthesize bulk Ti₃AlC₂ with excellent properties in a very short time and easily sintering process. The optimal conditions to synthesize Ti₃AlC₂ were also discussed.     

Mechanical-activation-assisted combustion synthesis of SiC

Mechanical-activation-assisted combustion synthesis of SiC has been conducted with PVC as promoters. The mechanical activation of the Si-C reactants through high-energy attrition milling could result in substantial decrease of the ignition temperature and the incubation time for the Si-C combustion reaction. Ultra-fine β-SiC powders with equiaxed grains were synthesized at the preheating temperature as low as 1050 °C. The specific surface area (SSA) of the combustion synthesized SiC powders was 4.36 m²/g, and the average particle size was less than 5 μm.     

A study of combustion synthesis reaction in the Ti + C/Ti + Al system

Combustion synthesis (SHS) of the Ti + C/Ti + Al system was investigated by using titanium, graphite and aluminum powders as reactants. These powders were thoroughly mixed and pressed into cylindrical compacts, and heated in an argon atmosphere. The effects of the reactant composition and the heating rate were studied. The phase identification and morphology observation of the products were carried out by X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. XRD analysis showed that the products, in addition to the expected TiC/TiAl phases, also contained an appreciable amount of the ternary carbide (i.e., Ti _x AlC). The heating rate was found to strongly affect the extent of the combustion reaction. A possible reaction mechanism based on the experimental results was proposed to describe the whole process of the SHS reaction and the characteristic product morphology. It was considered that the ternary carbides may be formed by the peritectic reaction between TiC and the Ti–Al melt during the cooling stage after combustion.     

Synthesis of the Ti<Subscript>2</Subscript>AlC MAX Phase with a Reduction Step via Combustion of a TiO<Subscript>2</Subscript> + Mg + Al + C Mixture

Technologically viable principles have been developed for the preparation of the MAX phase Ti₂AlC by self-propagating high-temperature synthesis (SHS) with a reduction step, using titanium dioxide. We have studied the influence of synthesis conditions (starting-mixture composition and ratio of reactants) on the composition, structure, and particle size of Ti₂AlC powders. The results demonstrate that an excess of magnesium in the starting mixture leads to a decrease in the percentage of MgAl₂O₄ (spinel), and carbon deficiency in the starting mixture reduces the percentage of titanium carbide in the final product. The Ti₂AlC powders prepared by SHS consist of agglomerates of layered particles differing in size: from coarse (several microns) to ultrafine and nanometer-sized particles. The composition of the powders was confirmed by chemical analysis, microstructural examination, and X-ray diffraction.     

Formation mechanism of Ti2AlC under the self-propagating high-temperature synthesis (SHS) mode

Ti₂AlC was fabricated by the self-propagating high-temperature synthesis (SHS) technique from a compacted powder mixture consisting of Ti:Al:C = 2:1:1 (molar ratio) of mortar mixing, planetary mixing and various packing densities. A thermocouple was placed directly into the green compact body in order to monitor the combustion temperature during the SHS process. When the green compact with planetary mixing treatment and a packing density of 17% as well as 60% was used, propagation of the reaction exothermicity could occur, and the starting compact completely changed to Ti₂AlC. Formation mechanism of Ti₂AlC using a SHS technique was discussed. In addition, the melting point of the resultant sample was determined to be 1570 °C.     

Formation mechanism of titanium silicide by mechanical alloying

The syntheses of five titanium silicides (Ti₃Si, TiSi₂, Ti₅Si₄, Ti₅Si₃, and TiSi) by mechanical alloying (MA) have been investigated. Rapid, self-propagating high temperature synthesis (SHS) reactions were involved in producing the last three materials during room temperature high-energy ball-milling of elemental powders. Such reactions appeared to occur through ignition by mechanical impact in the fine powder mixture formed after a critical milling period. From in-situ thermal analyses, each critical milling period for the formation of Ti₅Si₄, Ti₅Si₃, and TiSi was observed to be 22, 35.5 and 53.5 minutes, respectively. However, the formation of Ti₃Si and TiSi₂ did not occur even after 360 minutes of milling of as-received Ti and Si powder mixture, due to the lack of homogeneity of the powder mixture. Other ball-milling procedures were employed for the syntheses of Ti₃Si and TiSi₂ using different sizes of Si powder and milling medium materials. Ti₃Si was synthesized by milling a Ti and 60 minutes premilled Si powder mixture for 240 minutes. -TiSi₂ and TiSi₂ were produced by high energy partially stabilized zirconia (PSZ) ball-milling for 360 minutes in a steel vial followed by jar-milling of a Ti and 60 min premilled Si powder mixture for 48 hr. The formation of Ti₃Si and TiSi₂ occurs through a slow solid state diffusion reaction, and the product(s) and reactants coexist for a certain period of time. The formation of titanium silicides by MA and the reaction rate appeared to depend on the homogeneity of the powder mixture, milling medium materials, and heat of formation of the product involved.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Dependence of the SHS reaction behavior and product on B4C particle size in Al-Ti-B4C and Al-TiO2-B4C systems

Dependence of the combustion reaction behavior and product on the B₄C particle size in the Al-Ti-B₄C and Al-TiO₂-B₄C systems was investigated. The results show that in both systems, the SHS reaction kinetics is primarily controlled by diffusion. Increasing the B₄C particle size not only makes the ignition and self-sustainment of the SHS reaction difficult but also decreases the wave velocity and the degree of conversion. In comparison, the dependence of the SHS reaction behavior and the resultant product on the B₄C particle size is much more significant in the Al-Ti-B₄C system than that in the Al-TiO₂-B₄C system. The reason for the different dependence behaviors was investigated based on the SHS reaction mechanisms.     

The energy required to ignite micropyretic synthesis. Part I: stable Ni + Al reaction

The progression of chemical reactions is determined by both thermodynamics and kinetics factors. Micropyretic/combustion reaction is a cascade of many chain chemical reactions and thermodynamics and kinetics of the ignition reaction are expected to greatly affect the overall reaction outcome. Furthermore, the stability of the sequential reaction and its progression are correspondingly changed once micropyretic parameters are changed. Improper ignition of micropyretic reaction provides either excessive or insufficient external energy, thus causes over-heating or extinguishing of the combustion front during propagation and therefore the heterogeneous structures. To achieve the homogeneous micropyretic reaction, it is thought possible to control ignition energy. A numerical study on the correlation of thermodynamics and kinetics factors of ignition on the stable Ni + Al reaction and the required ignition energy is reported in this study. The influences of activation energy (E), enthalpy of the micropyretic reaction (Q), pre-exponential factor (K _o), thermal conductivity (K), heat capacity (C _p ), and thermal activity of the reactants and product, on the temperature/heat loss at the ignition spot and the length of pre-heating zone are respectively studied. It is found that the activation energy and heat capacity have the most significant effects on the ignition energy. The required ignition energy is increased by 44.0% and 23.9%, respectively, when the activation energy and the heat capacity are both increased by 40.0%     

Mechanistic investigation of the field-activated combustion synthesis (FACS) of tungsten carbide with or without nickel additive

The activation of self-propagating combustion reactions in the W-C system and its composite with Ni as additive was achieved by using an electric field. The reaction mechanisms of Field-Activated Combustion Synthesis (FACS) of tungsten carbide and its composite have been investigated by using sample-quenched method. Through turning off electric field during FACS process, a series of combustion products with different phase compositions have been obtained. Layer to layer X-ray and microscopic analyses of these combustion products across quenched combustion front suggested that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. W₂C is the intermediate phase between WC and reactants (W and C). Metal additive produces liquid phase and accelerates the diffusion between solid reactants (W and C), which facilitates the formation of W₂C and the transformation from W₂C to WC phase. Moreover, melted Ni reacts with W and W₂C to form mixed compounds of type W _x C _y M _z .     

Reaction sintering and joining nickel aluminide to AISI 316 stainless steel by self-propagating high temperature synthesis

Abstract

Self-propagating high temperature synthesis (SHS) is a process whereby reactants are ignited to spontaneously transform to products in an exothermic reaction. The aim of this study is to propose a method to join nickel aluminide with AISI 316 stainless steel by SHS and to study the combustion synthesis of nickel aluminide. From the heat of combustion synthesis junctions were formed between annular AISI 316 stainless steel and a powder metallurgy compact of Ni and Al blends. The Al mole ratio for testing the joining grade in the initial powder mixture varied from 25 at.-% to 40 at.-%. In order to check the sufficiency of the SHS reaction, the test temperature was compared with the thermodynamic calculation values. The metallographic analysis indicated that NiAl and Ni₃Al were formed in the joint layer.     

Electrode materials based on a Ti–Al–C MAX phase

Long-length electrode materials based on Ti–Al–C MAX phases have been produced by self-propagating high-temperature synthesis (SHS) in combination with extrusion and their structure and performance parameters have been studied. The results demonstrate that, varying the composition of the starting mixture and parameters of the SHS extrusion process, one can obtain materials based on the MAX phases Ti₃AlC₂ and Ti₂AlC and containing intermetallic and carbide inclusions or free of carbide components. Using the SHS extrusion process, we have produced MAX phase-based rods up to 10 mm in diameter and 350 mm in length, ranging in porosity from 2 to 16%.     

Combustion synthesis of TiNi intermetallic compounds

The synthesis of the TiNi intermetallic compound using the thermal explosion mode of the combustion synthesis technique has been used to determine the heat of fusion, ΔH _m (7.77 kcal mol⁻¹), of the TiNi intermetallic and the heat capacity,C _p1 (17.96 cal mol⁻¹ K⁻¹), of the liquid-phase TiNi. The effect of heating rate and degree of dilution of the Ti + Ni powder compact reactants with previously synthesized TiNi on the ignition,T _ig, and combustion,T _c, temperatures in an argon atmosphere have been determined. It was found thatT _c was dependent on heating rate and dilution with TiNi, whereasT _ig remained unchanged with respect to these two process variables.     

场激活燃烧合成碳化钨(Ⅰ)——场激活燃烧合成

研究了场激活下燃烧合成碳化钨,研究结果显示,只有当施加的场强超过临界值(1V·cm^-1),燃烧波才能蔓延下去.燃烧产物的特性与场强有关,当场强增加时,X射线衍射圈中WC相的衍射峰强度增强,表明碳在钨中的扩散随场强的增强而增大.钨颗粒的粒径大小和样品初始相对密度对燃烧温度和燃烧波蔓延速率的影响研究表明,随着钨颗粒的缩小,燃烧温度和燃烧波蔓延速率变大,而燃烧温度和燃烧波蔓延速率的最大值出现在一个合适的相对密度处.     

The effect of the combustion channels on the compressive strength of porous NiTi shape memory alloy fabricated by SHS as implant material

Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new ignition technique “high voltage electric arc” was used to ignite the green specimens and control the orientation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19′ (NiTi) were dominant while the second phases (Ni₄Ti₃ and NiTi₂) in small amount. The undesired phases (such as pure Ni and Ni₃Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heating rate of the specimen was increased.     

Microstructure characteristics of products in Ti–Si system via combustion synthesis reaction

The products of combustion synthesis reaction in Ti–Si system with molar ratios of Ti:Si = 3:1, 5:3, 5:4, 1:1, and 1:2 were investigated. The phase composition of products and degree of completion in the reaction considerably depend on the initial stoichiometric ratios of reactants. During the SHS reaction of Ti–Si system, the degree of completion follows the order of 5Ti + 3Si > Ti + Si > Ti + 2Si > 5Ti + 4Si > 3Ti + Si. Besides, the micro-structural morphology of Ti–Si compounds, i.e., TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃ were also characterized in this study.