Mechanically Activated Self-propagating High-temperature Synthesis Research Articles

Mechanical and Oxidation Behavior of MoSi2-40wt.%Ti5Si3 Composites Produced by MASHS and SPS

Silicides, known for their exceptional oxidation resistance at high temperatures, are promising materials for advanced applications. Among them, MoSi₂ and Ti₅Si₃ stand out. In this study, a Ti29Mo18Si53 composition was selected to produce a MoSi2-40wt.%Ti5Si3 composite via mechanically activated self-propagating high-temperature synthesis (MASHS). Elemental powders were milled, synthesized in a tubular furnace, and sintered using spark plasma sintering (SPS). The resulting (Ti0.8, Mo0.2)Si2-MoSi2-Ti5Si3 composite maintained stability through sintering, achieving 99% of its theoretical density, a hardness of 10.9±0.4 GPa, and a fracture toughness of 5.11 MPa·m0.5. To evaluate processing effects a second sample, prepared under similar condition but with concurrent synthesis and sintering in the SPS, achieved 97% density, 7.1±0.5 GPa hardness, and 4.97 MPa·m0.5 fracture toughness. Additionally, pure Ti₅Si₃ and MoSi₂ were synthesized separately via MASHS, mixed to form a MoSi₂-40 wt.% Ti₅Si₃ composite, and then consolidated by SPS. This composite reached 96% density, 11 ± 0.6 GPa hardness, and 3.8 MPa·m0.5 fracture toughness. Moreover, the (Ti₀.₈Mo₀.₂)Si₂-MoSi₂-Ti₅Si₃ composite demonstrated superior oxidation resistance compared to Ti₅Si₃ and similar to monolithic MoSi₂ up to 1000°C.

Parametrization of the mechanically induced self-propagating high-temperature synthesis (MI-SHS) of Ti[formula omitted]Si[formula omitted]

Mechanically activated self-propagating high-temperature synthesis (MA-SHS) is one of the most used methodologies to synthesize titanium silicides, especially Ti5Si3. However, the problem in this methodology is to know the milling conditions needed to mechanically activate (MA) or mechanically induce (MI) the reaction of the starting materials. This information is fundamental for obtaining reproducible results. Therefore, the parametrization of the mechanically induced self-propagating high-temperature synthesis (MI-SHS) of Ti5Si3 is explored in the present study. A simple kinematic approach is used to parametrise the mechanically induced reaction as a function of the milling parameters, such as the angular velocity of the mill and the grinding time. The accumulated and transferred energy per hit needed to induce the MI-SHS of Ti5Si3 are predicted. A kinetic approach that allows the complete parametrization of mechanically induced reactions is also used.

Preparation of Ti-Si-C system and their ceramic composite coatings using gas flame spraying for microwave absorbing applications

We report the synthesis of Ti-Si-C system with MAX-phase (Ti3SiC2) by Mechanically Activated Self-propagating High-temperature Synthesis (MASHS) and preparation of their composite powders with Al2O3 in different weight fractions. In the subsequent step, the composite powders were converted into granules with desired fluidity for gas flame spraying. An in-house designed and developed Thermal Spray Coating apparatus was used in the gas flame spray process to coat the prepared composite powder granules on to Ti alloy substrate. XRD studies reveal formation of MAX-phase Ti3SiC2 primarily and SEM analysis suggests grains of layered morphology that consist of thin lamellar structures of thickness 90–130 nm. During gas flame spraying, Ti3SiC2 decomposes into TiC and their concentrations present in composites consequently influence the electromagnetic (EM) properties of the coating. Based on the EM studies, a suitable ceramic composition has been identified (Al2O3/50wt%Ti-Si-C) that offers better frequency dispersion of permittivity, with broader bandwidth of microwave absorption over 8.2–12.4 GHz. The optimized composite coating of thickness ~1.8 mm shows more than 90% microwave absorption [Reflection Loss (RL) > 10 dB] over 8.3–12.4 GHz with maximum value ~−37 dB at 9.7 GHz. In present investigation, onset of microwave absorbing properties of coating are attributed to interfacial polarization and conduction loss arises due to the presence of TiC and resistive phase Ti3SiC2 in the composite. The Al2O3/Ti-Si-C composites with optimal MAX phase Ti3SiC2 content can be a promising candidate for microwave absorbing stealth applications.

Microstructure and mechanical properties of Ti5Si3 fabricated by spark plasma sintering

The monolithic Ti5Si3 was synthesized by mechanically activated self-propagating high-temperature synthesis (MASHS) from elemental powders and sintered by Spark plasma sintering (SPS) technique. The synthesized powders were sintered at three different temperatures of 1400, 1500 and 1600 °C by SPS with 5 min holding time and reached 84, 91 and 99% of the theoretical density, respectively, while the density of the pressureless sintered sample at 1600 °C reached to 86% after 2 h. The phase composition and microstructural analyses show that titanium silicide retained its phase composition and microstructure during the sintering. The mechanical properties of the compound were measured by the Vickers hardness test and nanoindentation technique. The Young's modulus of 268 ± 8 GPa and nanohardness of 16 ± 1.7 GPa were determined by nanoindentation, while the hardness value of 12.5 ± 0.14 GPa was measured by Vickers indentation test. The crack induced by Vickers indenter was used to measure fracture toughness of the compound and the value of 4 ± 0.1 MPa m1/2 was achieved. The transgranular fracture mode can be identified from the fracture surface of the compound.

Mechanical properties and microstructure of Ti5Si3 based composites prepared by combination of MASHS and SPS in Ti-Si-Ni and Ti-Si-Ni-C systems

Abstract Mechanical properties, synthesis behavior, microstructure and oxidation resistance of Ti5Si3-based composites prepared by combination of mechanically activated self-propagating high-temperature synthesis (MASHS) and spark plasma sintering (SPS) in Ti-Si-Ni and Ti-Si-Ni-C systems were investigated. Ti5Si3-Ti2Ni composite was synthesized in Ti-Si-Ni system while Ti5Si3-TiC-Ti2Ni was formed in the Ti-Si-Ni-C. Microstructural analyses showed that Ni and C extensively distributed in Ti5Si3 phase and formed interstitial and substitutional solid solution titanium silicide. The activation energy values of 126 and 135 kJ/mol were calculated for initiation of combustion reaction in Ti-Si-Ni and Ti-Si-Ni-C systems, respectively. The isothermal oxidation behavior of the samples in air atmosphere was studied by TGA analysis and near the same oxidation resistance was detected. The fracture toughness of fabricated composites in Ti-Si-Ni and Ti-Si-Ni-C systems reached to 5 ± 0.1 and 6.1 ± 0.12 MPam1/2, respectively. The higher fracture toughness in Ti-Si-Ni-C system is related to the uniform distribution of TiC in the composite.

Influence of C addition on solid-state reaction of Ti-Si-C system

The effect of C addition on synthesis behavior, microstructure, reaction mechanism and high-temperature oxidation resistance of Ti-Si system was investigated. The Ti5Si3Cx-TiC composite was synthesized by introducing the C into Ti-Si system and using mechanically activated self-propagating high-temperature synthesis (MASHS). The addition of 1 mol C into 5Ti+3Si reduced activation energy of reaction from 207.5 kJ/mol to 189 kJ/mol. The reaction in the Ti-Si-C system was initiated by the formation of TiC and followed by synthesis of an interstitial solid solution of Ti5Si3Cx, while reaction in Ti-Si was activated by solid-state diffusion of Si into Ti. The ignition temperature of the reaction was reduced from 1000 °C to 900 °C when the combustion temperature slightly increased as a result of the C addition into the system. The wave velocity of the reaction was also declined from 55 mm/s to 48 mm/s. The microstructural observations show that dissolution of the C in Ti5Si3 crystal structure restricted microcracks formation through coarse grains. Moreover, the oxidation resistance of titanium silicide was improved by the formation of the interstitial solid solution.

A Study on the Characterization of MASHS Processed Ti-Si-Al System

Many promising titanium aluminides and silicides could be synthesized in Ti-Si-Al system via mechanically activated self-propagating high-temperature synthesis (MASHS). The influence of mechanical activation and initial composition on phase formation, microstructure and synthesis behavior of mechanically activated Ti-rich mixtures in Ti-Si-Al system is investigated. Results indicate that Al can participate in the reaction and forms Ti3Al and TiAl3 phases beside Ti5Si3 phase by using mechanically activated reactants. Si and Al could be substituted each other in titanium aluminide and silicide to form solid solution of Ti3(Si,Al) and Ti5(Si,Al)3. In Ti-Si-Al system, combustion temperature is increased by increasing Si amount in the system while, ignition temperature is decreased profoundly from 1100 to 800 ∘C by applying mechanical activation prior to reaction and increasing Al in the system. In addition, Mechanical activation prior to synthesis gives rise to narrow grain size distribution and more uniform microstructure in comparison with SHS products.

Ceramic Materials in a Ti–C–Co–Ca3(PO4)2–Ag–Mg System Obtained by MA SHS for the Deposition of Biomedical Coatings

This study aimed to obtain biocompatible ceramic materials in a Ti–C–Co–Ca3(PO4)2–Ag–Mg system by the combustion mode of mechanically activated (MA) reaction mixtures. The influence of the MA time on the reaction ability capability of the mixtures, on their structural and chemical homogeneity, on the combustion parameters and structural-phase conversions in the combustion wave, as well as on the structure and phase composition of the electrode materials has been researched. It was found that the intense treatment of powder mixtures causes plastic deformation of components, the formation of lamellar composite granules, a reduction in the sizes of coherent scattering regions, and also the formation of minor amounts of products. The influence of the activation duration of the ignition temperature and heat release during the combustion of the reaction mixtures was studied. By the method of quenching the combustion front, it was demonstrated that in a combustion wave, chemical transformations occur within the lamellar structures formed during the process of mechanoactivation. It was shown that in the combustion wave, parallel chemical reactions of Ti with C as well as Ti with Co and Ca3(PO4)2 occur, with a Ti–Co-based melt forming the reaction surface. Ceramic electrodes with different contents of Ag and Mg were synthesized by force self-propagating high-temperature synthesis (SHS)-pressing technology using the MA mixtures. The microstructure of the materials consisted of round-shaped grains of nonstoichiometric titanium carbide TiCx grains, intermetallic matrix (TiCo, TiCo2, CoTiP), inclusions of Ca and Mg oxides, and grains of the Ag-based solid solution. An increased content of Ag and Mg in the composition of the electrodes, as well as an increased MA duration, leads to an enlargement of the inclusions of the Ag-containing phase size and deterioration in the uniformity of their distribution.

Synthesis of nanometric refractory alloys powders in the Mo[sbnd]Nb[sbnd]W system

Nanometric powders of stoichiometric compositions in the MoNb, MoW, NbW binary systems and in the MoNbW ternary system were produced by highly exothermic reactions using Mechanically Induced Chemical Reaction (MICR), Self-propagating High-temperature Synthesis (SHS) and Mechanically Activated Self-propagating High-temperature Synthesis (MASHS), through the reduction of their oxides by magnesium, with sodium chloride used as a reaction moderator. Results demonstrate the possibility to obtain high purity nanostructured products in the 20–150 nm range, with an average equivalent diameter, from specific surface measurements, of 44 nm. However, in Nb containing samples, the main BCC phase always comes with one or more secondary phases, for which further developments are necessary in order either to avoid its formation or to find a way to eliminate it.

Effect of Al2O3 Addition as Diluent on Combustion Synthesis of Al2O3-ZrB2 Composite

In this study, the effect of Al2O3 addition as a diluent during mechanically activated self-propagating high temperature synthesis (MASHS) of Al2O3-ZrB2 composite was investigated. For this purpose, the thermite mixture of Al, ZrO2, * پ تابتاکم لوئسم یکینورتکلا تس : m.rajabi@nit.ac.ir

Mechanically activated combustion synthesis of molybdenum borosilicides for ultrahigh-temperature structural applications

The thermal efficiency of gas-turbine power plants could be dramatically increased by the development of new structural materials based on molybdenum silicides and borosilicides, which can operate at temperatures higher than 1300 °C with no need for cooling. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. Materials based on Mo5SiB2 (called T2) phase are promising materials that offer favorable combinations of high temperature mechanical properties and oxidation resistance. In the present paper, T2 phase based materials have been obtained using mechanically activated self-propagating high-temperature synthesis (MASHS). Upon ignition, Mo/Si/B/Ti mixtures exhibited a self-sustained propagation of a spinning combustion wave, but the products were porous, contained undesired secondary phases, and had low oxidation resistance. The “chemical oven” technique has been successfully employed to fabricate denser and stronger Mo5SiB2–TiC, Mo5SiB2–TiB2, and Mo–Mo5SiB2–Mo3Si materials. Among them, Mo5SiB2–TiB2 material exhibits the best oxidation resistance at temperatures up to 1500 °C.

Synthesis of tungsten nanopowders: Comparison of milling, SHS, MASHS and milling-induced chemical processes

Synthesis of tungsten nanopowders was studied using milling of micrometric tungsten, then using the WO3–Mg thermitic reaction, using SHS (Self-propagating High-temperature Synthesis), milling induced chemical reaction (MICR), and MASHS (Mechanically Activated SHS). Reactions are studied by measuring temperature and pressure inside the milling jar (during MICR), or by analyzing the temperature profile along the sample during the reaction propagation by infrared thermography (SHS, MASHS). After reaction, samples were analyzed by AFM or SEM, by XRD, and BET. MASHS seems to possess optimum conditions with a pre-milling before SHS of 10min, which yielded the highest tungsten purity, together with a grain size corresponding to our aim.

High frequency induction heated sintering of nanostructured Al2O3–ZrB2 composite produced by MASHS technique

Nanostructured Al2O3–ZrB2 composite was synthesized using an alternative route called Mechanically Activated Self-propagating High-temperature Synthesis (MASHS). It was demonstrated that Al2O3–ZrB2 composite with nanometric structure could be produced via a very fast combustion front in contrast with the SHS process. Ball milling for 5h with ball to powder weight ratio of 5:1 was found to be the optimum condition for activation of the reactants. Using the high frequency induction heated sintering method, the composite was consolidated within 3min. The relative density of the sintered sample was 99% of the theoretical density at an applied pressure of 20MPa and temperature of 1800°C. The average hardness and fracture toughness were 19.5GPa and 4.1±0.3MPam1/2, respectively.

Effect of substitution of Si by Al on microstructure and synthesis behavior of Ti5Si3 based alloys fabricated by mechanically activated self-propagating high-temperature synthesis

The effect of substitution of Si by Al and mechanical activation on microstructure, phase composition, ignition and combustion temperature of Ti5Si3 based alloys and composites that were prepared by mechanically activated self-propagating high-temperature synthesis (MASHS) method was investigated. For this purpose elemental powders of titanium, silicon and aluminum were mixed according to the 5Ti+3(1−X)Si+3XAl formula, where X=0, 0.2, 0.4, 0.6. The samples were characterized by X-ray diffraction (XRD) analytical technique and scanning electron microscope (SEM) equipped with an energy-dispersive spectrum (EDS) analyzer. The results have shown that formation of Ti5Si3 during milling stage is postponed by adding Al into the system. Presence of Al in the Ti–Si system have a significant effect on the phase composition of the final products. Substitutional solid solution of Ti5(Si, Al)3 and Ti5Si3–Ti3Al composite are formed by increasing Al amount in the system. Furthermore combustion temperature and crystallites size of Ti5Si3 is reduced with addition of Al into the Ti–Si system. Moreover, solubility of Al in Ti5Si3 is increased with enhancing the X up to 0.4, after that, the solubility of Al in Ti5Si3 is ceased, due to achieving the solubility limit of Al in the Ti5Si3. The average crystallites size of Ti5Si3 are decreased with increasing milling time prior to the reaction.

Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

ABSTRACTMolybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase and alloys based on this phase are promising materials.In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo−Si−B materials is independent on the concentration of Mo phase in the products. The “chemical oven” technique has been used to obtain a single Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases.

Synthesis of nanocrystalline NiAl by mechanically activated self-propagating high-temperature synthesis and mechanically activated annealing process

The mechanically activated self-propagating high-temperature synthesis (MASHS) technique and the mechanically activated annealing process (M2AP) were used to produce NiAl intermetallic compound. Rietveld analysis of X-ray diffraction data was used to characterize the mechanically activated powders, MASHS and M2AP end-products. Two-phase (B2+L12) nickel aluminide intermetallic compounds were synthesized by the mechanically activated volume combustion synthesis (MASHS). A single phase B2 NiAl was formed when mechanically activated annealing process (M2AP) was performed. Starting from a mixture of elemental pure powders, both M2AP and MASHS milling processes lead to nanostructured powders. Structural analysis deduced from the Rietveld refinement of X-ray diffraction patterns of NiAl compounds synthesized by SHS after a milling of 6 h show the formation of β-NiAl phase with a cell parameter a = (0.2885 ± 1.2618 × 10−4) nm. The average crystallite size is D = (44.6421 ± 2.4263) nm and the microstrains values are close to those of metallic alloys τ = (0.2142 ± 1.6186 × 10−2)%. Finally, scanning electron microscopy (SEM) was carried out to characterize the microstructure of end-products.

Preparation of MoSi2–Al2O3 nano-composite via MASHS route

In this study, MoSi2–Al2O3 nano-composite powder was prepared by mechanical alloying. Mixture of MoO3, Si and Al powders was exposed to high energy ball milling. Phase compositions and structural evolutions during milling were investigated by X-ray diffraction (XRD) analysis. The morphology of the milled powders was evaluated by scanning electron microscope (SEM). Within short milling times [15, 25 and 60minutes in ball to powder weight ratios (BPR) of 35:1, 20:1 and 10:1, respectively], a combustion process occurred within the milling powder and the process mechanism was characterized to be mechanically activated self-propagating high-temperature synthesis (MASHS) type. From XRD results it was found that during the combustion process, MoO3 was completely reduced with Al and the resulting Mo reacted with Si to produce molybdenum disilicide and eventually a MoSi2–Al2O3 nano-composite powder formed within a short period of time. Further milling resulted in reduced mean crystallite sizes and increased lattice micro strain in the product phases. After 30hours of milling in BPR of 35:1, the mean crystallite sizes were found to be 10, 9 and 11nm for Al2O3, α-MoSi2 and β-MoSi2, respectively. Also, effect of stearic acid addition as a Process Control Agent (PCA) was studied and the results showed that its usage postpones the reaction initiation and reduces mean crystallite sizes.

Development of nanostructured Mg 2Ni alloys for hydrogen storage applications

The Mechanically Activated Self-propagating High temperature Synthesis (MASHS) has been employed to obtain nanostructured Mg 2Ni alloys. MASHS process has been further improved by controlling the electrical parameter measurements during the combustion reaction under the thermal explosion mode. The samples were hydrogenated at 20 bar and 300 °C by means of a Pressflow Gas Controller while the dehydrogenation was conducted in a Differential Scanning Calorimetry (DSC) equipped with an H 2 detector of the purged gas. Nanostructured Mg 2Ni demonstrated hydrogen storage capacity around 3.5 wt%. The desorption temperature was about 250 °C at 3 °C/min. The activation energy for dehydrogenation, calculated by the Kissinger method, was about 100 kJ/mol.

Hydrogen Absorption/Desorption in Nanostructured Fe- and Ti-Doped Mg<sub>2</sub>Ni Alloys

Nanostructured Mg2Ni, Fe-doped and Ti-doped Mg2Ni alloys for hydrogen storage applications have been produced by means of Mechanically Activated Self-propagating High temperature Synthesis (MASHS). Different molar compositions of Fe and Ti (0.1; 0.3 and 0.5) have been studied in order to determine their influence in the hydrogen sorption properties. Different Mg-Ni based alloys have been tested in order to study their hydrogen sorption behavior. The hydrogenation was carried out in a Pressflow Gas Controller. Subsequently, the dehydrogenation process was conducted by means of a Differential Scanning Calorimetry (DSC) equipped with an H2 detector of the purged gas. The MASHS method has been demonstrated to be effective for the obtainment of nanostructured pure and doped Mg2Ni intermetallics. In addition, the materials produced showed hydrogen storage capacities superior to 4wt%, especially in the case of Fe-doped Mg2Ni and a slight reduction of desorption temperature was reached with Ti-doped Mg2Ni. Finally, the activation energy of the dehydrogenation process was evaluated and Ti-doped sample exhibited the lower activation energy value. Obtained results are promising for technological applications of Mg-based alloys.

Properties of Cu-Based Nanocomposites Produced by Mechanically-Activated Self-Propagating High-Temperature Synthesis and Spark-Plasma Sintering

One of the possible reasons for low conductivity of in-situ produced dispersion strengthened copper matrix composites may be the incompleteness of the reaction between the initial reactants that remain in a state of solid solutions in the copper matrix. We report in-situ synthesis of TiB2-Cu composites starting from the powder mixtures with the limited content of copper ensuring a high probability of contact between the particles of titanium and boron and, as a result, their full conversion into the TiB2 phase. The nanoparticles were formed in a self-propagating mode in the ball milled Ti-B-Cu powder mixture corresponding to 57 vol.% TiB2-Cu composition. Afterwards, the system was "diluted" with the required amount of the copper matrix using subsequent ball milling. Highly conductive 4.5 vol.% TiB2-Cu composites showing 82-87% IACS (International Annealed Copper Standard) conductivity were obtained by Spark Plasma Sintering (SPS) of the powders.