Self-propagating High-temperature Synthesis Route Research Articles

Influence of Co content on microstructure and hardness of AlCoxCrFeNi (0 ≤ x ≤ 1) high-entropy alloys produced by self-propagating high-temperature synthesis

In an effort to reduce energy consumption and expensive Co content, herein we report the effects of Co content on microstructure and microhardness of widely studied AlCoxCrFeNi high-entropy alloys produced by cost-efficient aluminothermic self-propagating high-temperature synthesis (SHS) method. Accordingly, the feasibility of SHS technique in the production of AlCoxCrFeNi master high-entropy alloys was discussed. In order to investigate the effect of further casting, the SHS alloys were also subjected to vacuum arc melting/suction casting process. Prior to SHS; phase diagram, raw material amounts and adiabatic temperatures were calculated via thermochemical modelling. The calculated equilibrium phase diagram suggested a full BCC structure over the whole Co range. However, according to XRD results, as-cast alloys consisted of BCC and FCC phases, having microhardness values ranging in between 385 HV and 504 HV, depending on Co content. The increase in Co content, enlarged the grains, altered the spinodal decomposition morphology and led to the formation of dual-phase (FCC + BCC) alloys, which in return had a significant effect on the microhardness values. Overall hardness values were increased due to finer microstructure obtained by suction casting. Also, Widmanstätten plate-like growth of the FCC phase inside the BCC cores was kinetically suppressed due to higher cooling rate. It was concluded that AlCoxCrFeNi master high-entropy alloys can be successfully produced by SHS method and the attained dual-phase microstructures could be beneficial in tailoring mechanical properties as well as reducing the total cost of the AlCoxCrFeNi HEAs system in addition to the significant energy reduction obtained via SHS route.

Study on effect of TiB2 reinforcement on the microstructural and mechanical properties of magnesium RZ5 alloy based metal matrix composites

Abstract A self-propagating high-temperature synthesis route is adopted for the fabrication of TiB2-reinforced magnesium RZ5 alloy-based in-situ metal matrix composites. Ti-B is used according to the appropriate stoichiometry to obtain 4, 6 and 8 wt.% TiB2 reinforcements. The base alloy and cast composites are solutionised to enhance the mechanical properties of the materials. A microstructural study of the composites is carried out using optical microscopy and field emission scanning electron microscopy (FESEM) and revealed near-uniform distribution of TiB2 particles in the magnesium RZ5 alloy matrix. X-ray diffraction revealed the formation of the TiB2 reinforcement along with the transient phase TiB and MgB7. The hardness of the RZ5 alloy-based composites increases by 7.12%, 17.06% and 32.07% with the addition of 4, 6 and 8 wt.% TiB2 reinforcements, respectively. The ultimate tensile strength of the as-cast composite increases by 30.47% with the addition of 8 wt.% TiB2. The tensile strength and ductility of the materials is improved by using the solutionising heat treatment. The heat-treated composite containing 8 wt.% TiB2 results in an ultimate tensile strength of 178.7 MPa. The tensile fracture surfaces are analysed using FESEM. The wear loss of the materials decreased from 25.826 mm3 to 22.949 mm3 by the adding 8 wt.% TiB2 for the sliding distance of 2000 m. Micrographs of the worn surfaces obtained from FESEM of both the base alloy and composites are also studied which indicate delamination, wear groove and oxide formation.

Abrasive Wear Behaviour of TiB2 Reinforced In-Situ Synthesized Magnesium RZ5 Alloy based Metal Matrix Composites

Abrasive wear behaviour of TiB2 reinforced in-situ magnesium RZ5 alloy-based metal matrix composites was investigated using a pin-on-disc wear testing apparatus. The in-situ RZ5/TiB2 composite was synthesized through the self-propagating high-temperature synthesis route. The microstructure of the composite revealed a near-uniform distribution of TiB2 in magnesium matrix. The abrasive wear behaviour on the base RZ5 alloy and TiB2 reinforced composites were investigated in terms of weight loss and co-efficient of friction. The effect of varying applied load (10–30 N), sliding distance (1000–3000 m), and wt% of TiB2 content on abrasive wear properties were analysed. The results indicated a higher wear loss of the materials with increase in the applied load and sliding distance. Co-efficient of friction decreased with increase in the applied load but increased with increase in sliding distance. The morphology of the worn surface under different test conditions was analysed in terms of delamination, wear groove, oxide, and wear debris formation using field emission scanning electron microscope. Large flaky shape wear debris was noticed when the load applied is 30 N whereas equiaxed wear debris was noticed when the applied load is 10 N. Increase in sliding distance increased the contact temperature led to the formation of the oxide layer on the contact surface.

Synthesis of porous Mg-doped CeO2 powders via self-propagating high-temperature synthesis route

The aim of this study was synthesis of Mg-doped porous cerium oxide powder by self-propagating high-temperature synthesis reaction of Mg-xCeO2-O2 system. Results indicated that doping Mg in the CeO2 crystal structure increased its lattice parameter slightly, and shifted the Ce-O band in the FTIR spectra from 490 to 574 cm−1. Moreover, the EDS results revealed the distribution of Mg in the CeO2 microstructure. There were a high volume fraction of interconnected macro-pores in the microstructure of cerium oxide after the synthesis process. Increase in the x-value from 0.05 to 0.25 mol decreased the mean size and volume fraction of pores from 2.4 to 1 µm and 50 to 30 vol%, respectively. In addition, the BET surface area of porous CeO2 varied between 0.3 and 1.57 m2/g. Finally, it was inferred that the SHS technique can be introduced as a rapid and novel method for synthesis of Mg-doped porous CeO2 powders.

In situ formation of Zr2Al3C4/Al2O3 composites by combustion synthesis with PTFE and thermal activations

Abstract Preparation of Zr2Al3C4-Al2O3 in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both aluminothermic reduction of ZrO2 and chemical activation of PTFE (Teflon). The starting materials included ZrO2, Al, carbon black and PTFE. In addition to the conventional SHS method, the experiments were conducted by a chemical-oven SHS (COSHS) route to thermally assist the synthesis reaction. The threshold amount of 2% (mass fraction) PTFE was required to induce self-sustaining combustion. When the conventional SHS scheme was utilized, due to low combustion temperatures between 1152 and 1272 °C and insufficient reaction time, the dominant carbide forming in the composite was ZrC instead of Zr2Al3C4. On the other hand, the COSHS technique increased the combustion temperature of the reactant compact to about 1576 °C, lengthened the high-temperature duration for the reaction, and prevented Al vapor from escaping away. As a consequence, Zr2Al3C4-Al2O3 composites with a small amount of Zr3Al3C5 were obtained. The microstructure of the COSHS-derived product showed that plate-like Zr2Al3C4 grains were about 2 μm in thickness and 10-30 μm in length, and most of them were closely stacked into a laminated configuration.

Structure and properties of detonation gun sprayed coatings from the synthesized FeAlSi/Al2O3 powder

Structure and properties of detonation gun sprayed (DS) coatings from the synthesized FeAlSi/Al2O3 powder have been investigated. The powder was manufactured through a mechanically assisted self-propagating high-temperature synthesis route. A small addition of nanosized SiO2 particles to the reaction mixture for the synthesis of FeAl-FeAl2 eutectoid refines the thickness of B2-FeAl lamellar grains in the synthesized powder. Silicon arisen from the aluminothermic reduction of SiO2 during the synthesis is preferentially located in the B2-phase. High-quality coatings with porosity less than 1 vol% and nanocomposite structure have been obtained from the synthesized powder using a computer-aided ‘Perun-S’ detonation gun complex. Phase composition of the coatings includes B2-FeAl, hercinite FeAl2O4, FeAl2, A2-Fe-based solid-solution, Al2O3, and probably complex Fe2.95Si0.05O4 oxide. It is formation of nanocomposite structure with the mean crystallite size of main components in the range of ≈9–15 nm that seems to be responsible for the lower microhardness of the coatings from the FeAlSi/Al2O3 powder than that of the basic Fe-Al eutectoid composition. As-sprayed coatings from the synthesized FeAlSi/Al2O3 powder demonstrate good room and elevated temperature erosion behavior. Oxide dominated erosion with mixed brittle-ductile wear behavior is thought to occur in the coatings at 550 °C. The proposed powder modification is found to be beneficial for the oxidation resistance of DS coatings in air at 950 °C. Promoting of alumina formation during oxidation and the corresponding decline in Fe2O3 content in the oxide scale are considered as the main reasons for the slower oxidation kinetics of the FeAlSi/Al2O3 coatings as compared with those of the basic Fe-Al eutectoid composition.

Self-propagating high-temperature synthesis (SHS) of crystalline nanomaterials

Different silicide/PTFE compositions were processed via an SHS route. A variety of novel crystalline nanomaterials was produced, including SiC nanofibers, 1-D morphologies of other carbides and fluorides, as well as their respective micro- and nanocrystallites.

Synthesis and characterization of nano-structured TiB2 processed by milling assisted SHS route

Abstract TiB 2 was synthesized via milling assisted self-propagating high-temperature synthesis (SHS) route using TiO 2 , H 3 BO 3 and Mg powders as starting materials. The milling products were compacted and subjected to a hot filament on their top to initiate SHS. Effects of the milling time and addition of NaCl as a process diluent on the phase composition and morphology of the products were investigated. X-ray diffraction analysis indicated the formation of nano-crystalline TiB 2 with mean crystallite size of 31 nm after acid leaching process at room temperature. Addition of 2 mol of NaCl in the same sample decreases the mean crystallite size to 27 nm. SEM and TEM micrographs showed that the mean particle size of TiB 2 decreased from 200 nm in the un-milled sample to about 50 nm in the 5-hour mechanically activated (MA) sample.

A fast-pyrolysis self-propagating high temperature synthesis route to single phase of boron carbide ultrafine powders

Single phase of boron carbide B13C2 ultrafine powders was synthesized by a fast-pyrolysis-self-propagating high temperature synthesis method. The X-ray diffractometry is demonstrated a pure phase of rhombohedral structure. The field-emission scanning electron microscopy and transmission electron microscopy show that the size of particles ranged from 50 to 200 nm, with approximately 90 nm in average. The energy dispersive X-ray spectroscopy reveals that the sample has a uniform composition with the B/C atomic ratios ranging from 6.38 to 6.49. The Raman spectrum indicates the characteristic vibrations of C–B–C chain and B12 icosahedron. Fast-pyrolysis-self-propagating high temperature synthesis and vapor–solid growth mechanism ware proposed.

Cast in situ Cu–TiC composites: Synthesis by SHS route and characterization

The present investigation discusses observations pertaining to the synthesis of Cu-based composites containing TiC particles in the range of 45–50 volume % by self-propagating high temperature synthesis (SHS) process. A composite with 11–13 volume % TiC dispersion was also synthesized through remelting and dilution. The composites were observed to contain a copper matrix together with a Cu–Ti intermetallic compound, TiC dispersoid particles and partially reacted graphite. The regions showing partially reacted graphite (carbon) became less prominent in the diluted composites. Al addition led to the refinement of TiC particles, higher hardness, reduced density and improved degree of formation and better homogeneity of the distribution of TiC particles. Dilution caused reduced hardness, while the density followed a reverse trend.

Influence of stoichiometry on phase constitution, thermal behavior and magnetic properties of Ba-hexaferrite particles prepared via SHS route

Barium hexaferrite magnetic particles were synthesized via self-propagating high temperature synthesis (SHS) route by thermal initiation of compact mixed powders of iron, iron oxide and barium nitrate using various Fe (total)/Ba molar ratios of 9–12. As-SHS treated and post synthesis specimens were characterized by X-ray powder diffraction, VSM, DTA/TGA and SEM. DTA/TGA studies revealed that the formation temperature of barium hexaferrite decreased by increasing of Fe/Ba molar ratio. VSM measurement also indicated that saturation magnetization ( M s) and coercivity ( H c) of the annealed specimens increased by increasing of Fe/Ba molar ratio. XRD results confirmed by those obtained from DTA/TGA and VSM, indicated that the Fe/Ba molar ratio of 12 is favorable for the formation of single-phase barium hexaferrite.

Sintering behaviour of ultra-fine titanium diboride powder prepared by self-propagating high-temperature synthesis (SHS) technique

A systematic study on the sintering behaviour of titanium diboride (TiB 2) powder of different particle sizes prepared by a SHS route is carried out in the temperature range of 1973–2173 K. The decrease of particle size increases the densification of the TiB 2 powder compacts and a maximum densification of 93% of the theoretical density is achieved after sintering at 2173 K. The activation energies for sintering are observed to vary within ∼76 to 85 kJ/mol for the different particle sizes that indicate the boron diffusion in Ti as the possible rate-controlling process for sintering. The time dependent variation of densification under isothermal condition indicates the surface diffusion or vapour phase condensation mechanisms operative in the system. The microstructure and microhardness of different sintered samples reflect the densification behaviour of sintered pellets.

Combustion synthesis of alkaline-earth substituted lanthanum manganites; LaMnO3, La0.6Ca0.4MnO3 and La0.6Sr0.4MnO3

Alkaline-earth substituted lanthanum manganites La1 − xAxMnO3 (A = Ca, Sr; x = 0, 0.4) were synthesized in air by self-propagating high-temperature synthesis (SHS): a combustion process involving the reaction of lanthanum(III) oxide, calcium oxide (or carbonate), strontium peroxide, manganese metal powder and sodium perchlorate. A very sensitive thermal imaging method was used to study the combustion characteristics of the SHS-process. This method is based on the continuous measurement of the combustion process using an IR-camera and software developed by MIKRON Instrument Co., Inc. (M9100 Pyrovision Series – Imaging Pyrometer). X-Ray powder diffraction, scanning electron microscopy (SEM)/energy dispersive analysis by X-rays (EDAX), infrared spectroscopy and vibrating sample magnetometry were carried out on all samples. X-Ray diffraction data showed that single-phase orthorhombic LaMnO3 and monoclinic La0.6Ca0.4MnO3 and La0.6Sr0.4MnO3 were formed. Magnetic properties were found to transform from antiferromagnetic for LaMnO3 to weak ferromagnetic for La0.6Ca0.4MnO3 to strong ferromagnetic for La0.6Sr0.4MnO3. The experimental results show the SHS route is useful for the synthesis of multicomponent Mn-based oxides. The thermal imaging work enables accurate monitoring of the SHS combustion process and enabled velocity of propagation, maximum temperature, cooling rates, synthesis wave width and synthesis wave path to be determined with high precision.

Self-propagating high-temperature synthesis of chromium substituted lanthanum orthoferrites LaFe1 − xCrxO3 (0 ≤ x ≤ 1)

Pure and chromium substituted lanthanum orthoferrites LaFe1 − xCrxO3 (x = 0, 0.1, 0.25, 0.4, 0.55, 0.7, 0.85, 1) were synthesised in air by self-propagating high-temperature synthesis (SHS): a combustion process involving the reaction of lanthanum(III) oxide, iron(III) oxide, chromium(III) oxide and iron (or chromium) metal powder. Two series of samples were produced: series 1 – SHS in the absence of an external magnetic field; and series 2 – SHS in a magnetic field of 1.1 T; both processes were followed by sintering at 1400 °C for 36 h with intermediate grinding. Scanning electron microscopy, energy dispersive analysis of X-rays, X-ray powder diffraction, infra-red and ultra-violet/visible spectroscopy experiments were carried out on both series of samples. X-Ray diffraction data showed that single phase orthorhombic ferrites were produced. The ferrites showed a systematic decrease in lattice parameters and unit cell volume with chromium content (e.g. for series 1: x = 0, V = 242.4 A3; x = 1, V = 234.0 A3). 57Fe Mossbauer spectroscopy indicated that at low Cr concentration (x = 0.1 and 0.25) more than 96% of the Fe atoms experienced a perturbative disruption to their interatomic exchange interactions, consistent with the random distribution of Cr3+ ions on the B sublattice. The remaining 4% or less of Fe atoms were observed in a paramagnetic state, indicating a small degree of inhomogeneous Cr clustering in the lattice. The room temperature exchange interaction percolation limit for Cr substitution of the Fe lattice was found to lie between x = 0.4 and 0.55, and the Mossbauer spectra for the x ≥ 0.55 samples showed paramagnetic singlets. Room temperature magnetisation versus field curves showed hysteresis loops for x ≤ 0.7 (series 1) and x ≤ 0.55 (series 2), and anhysteretic curves for higher Cr contents. After an initial decrease with increasing Cr, the maximum magnetisations of the samples peaked at x = 0.25 (σmax = 2.1 emu g−1, series 1) and x = 0.4 (σmax = 1.7 emu g−1, series 2), before falling to near zero at higher concentrations. These features are consistent with a more homogeneous Cr substitution of the Fe sublattice in the series 2 samples, related to higher diffusion rates during the hotter combustion reactions achieved in the applied field SHS reactions. These results indicate that the SHS route is a potentially useful means of synthesising orthoferrites with desirable properties.

An investigation into the self-propagating high temperature synthesis (SHS) of superconducting ceramics by thermal methods

The SHS route is based on the well-known thermite reaction, in which a strongly exothermic reaction can sustain itself and propagate in the form of a combustion wave until the reactants have been completely consumed. The successful application of the method to the synthesis of superconducting ceramics of stoichiometry RBa2Cu3Oy (R=Y, Er, Yb) is reported. The 123 phase was obtained when pellets of R2O3, BaO2 and Cu metal in the correct proportions were dropped into a heater held at 800°C in an oxygen atmosphere and left there for only 10 minutes. Thermal methods (DSC and DTA) are excellent techniques with which to investigate the dependence of the reaction on heating rate, atmosphere and starting composition.