Si Powder Mixtures Research Articles

Microstructure and Mechanical Properties of Diamond–Ceramic Composites Fabricated via Reactive Spark Plasma Sintering

In order to prepare diamond composites with excellent mechanical properties under non-extreme conditions, in this study, a diamond–ceramic composite was successfully prepared via reactive spark plasma sintering using a diamond–Ti–Si powder mixture as the raw material. The microstructures and mechanical properties of the diamond–ceramic composite sintered at different temperatures were studied. When the sintering temperature was 1500 °C, the diamond–ceramic composite exhibited a volume density of 3.65 g/cm3, whereas the bending strength and fracture toughness were high at 366 MPa and 6.17 MPa·m1/2, respectively. In addition, variable-temperature sintering activated the chemical reaction at a higher temperature, whereas lowering the temperature prevented excessive graphitisation, which is conducive to optimising the microstructure and mechanical properties of the composite.

Improved mechanical properties of mechanically milled Mg2Si particles reinforced aluminum-matrix composites prepared by hot extrusion

From the viewpoints of particle refinement and homogeneous distributions of reinforcing phase, this work proposed a novel method to prepare Al/Mg2Si composites. Mg2Si particles were synthesized by solid-state reactions of Mg and Si powder mixtures at a high temperature, and then refined by mechanical milling (MM) for 12–36 h. Subsequently, the MMed Mg2Si particles were incorporated into Al powder followed by hot extrusion for consolidation. The effect of milling time on microstructure and mechanical properties of the Al/Mg2Si composites was investigated. As the milling time increased, the average size of the Mg2Si particles in the composites became smaller, but the grain sizes of Al matrix remained almost unchanged. The hot-extrusion process was found to eliminate some particle agglomerations and promote size reduction and uniform dispersion of Mg2Si particles in the Al matrix. The elastic modulus, yield strength, ultimate tensile strength (UTS), and wear resistance of the composites increased by 8%, 14%, 8.5%, and 38.2%, respectively, with increasing the milling time from 12 h to 36 h, but the elongation decreased by 41%. Furthermore, the hot-extruded Al/Mg2Si composites exhibited high strength and ductility, for example, the UTS and elongation of the composites with 15 wt% Mg2Si particles MMed for 36 h increased by 43% and 91%, respectively, compared to conventionally cast Al/Mg2Si composites. The predicated elastic modulus by the Halpin-Tsai model is close to the experimental results. Moreover, the thermal mismatch strengthening is the dominate strengthening mechanism to improve the yield strength of the Al/Mg2Si composites.

The effect of molten salt on the mechanical properties and microstructure of CuNiSi alloys with reinforced Fe

In this study, CuNiSi alloys were produced using powder metallurgy in molten salt (KBr). In the Cu, Ni, and Si powder mixture, Fe was added at a rate of 2.5%, 5 and 7.5% and mechanical alloying was carried out for 4 hours at 400 rpm. Prepared powder mixtures were cold pressed under 600 MPa pressure and sintered for 3 hours at 900 ? in an argon atmosphere. Phase formation, microstructure, microhardness, electrical conductivity, and corrosion of the produced samples were analyzed in detail. Scanning electron microscope (SEM) was used to detect the changes in the microstructure of the produced samples, and an X-ray diffractogram (XRD) was used to determine the phases formed in the internal structure of the materials. In order to determine the mechanical properties of the produced samples, hardness analyzes were made with a microhardness measuring device. The electrical conductivity properties of the produced CuNiSi and CuNiSiFe alloys were determined due to the increase in the Fe ratio. Corrosion tests of the produced samples were determined by potentiodynamic polarization curves in a 3.5% NaCl solution. Fe-reinforced CuNiSi composite materials have been successfully produced in molten salt (KBr). CuNiSi alloy, the microstructure is dominated by the typical large and small particles. Fe element is homogeneously dispersed in the CuNiSi alloy instead of being separated using the Ni element. Fe particles have decreased the hardness of produced alloys. The electrical conductivity properties changed with increasing voltages depending on the increase of Fe supplementation, and as a result, the sample containing 7.5% Fe had the best electrical conductivity values. Results showed that by increasing the amount of Fe, the mechanical properties and corrosion resistance increased.

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B4C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B4C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B4C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B4C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.

Effects of ball milling duration and sintering temperature on mechanical alloying Fe3Si

Fe3Si is under interest as a ferromagnetic electrode of magnetic tunneling junctions (MTJs). Its crystalline structure is important for achieving high device efficiency. This work focuses on mechanical alloying of 3:1 ratio of 99% pure Fe and Si powder mixtures by ball milling and sintering. The mixtures were ball-milled for various durations up to 20 h. Then, they were sintered from 400°C to 800°C for 4 h in Ar. SEM images and particle size analysis show significant reduction in average particle size of the mixtures after ball milling for 20 h. The longer duration of ball milling process promotes powder distribution. It results in agglomerated and smooth samples after sintering. XRD analysis indicates that Fe3Si phase appeared after 5 h of mechanical ball milling without sintering. More peaks of Fe3Si phase present at sintering temperatures higher than 600°C, while Fe2Si phase diminishes. However, the amount of Fe2O3 phase increases when sintering at these high temperatures, which strongly affects the magnetic properties of the samples. Magnetic hysteresis loops measured by vibrating-sample magnetometer (VSM) show lower magnetic moments of these samples. Saturation magnetization of the sample decreases more than 95% when sintered at 800°C, agreeing with high content of Fe2O3.

Functional Design to Protect TZM Alloy Against Oxidation

One of the most important disadvantages of molybdenum (Mo) and Mo-based alloys is low oxidation resistance at elevated temperatures. TZM is a well-known Mo-based alloy having a composition containing small quantities of titanium, zirconium and carbon. These additional elements provide superior properties to TZM; however, low oxidation resistance still prevents use of this alloy over 650 °C without protection. In the present work, ceramic-based layers were formed on the surface of TZM by spark plasma sintering method. B4C–Si and Al2O3–Si powder mixtures were used to form protective layers against oxidation. Ceramic–metal (TZM)–ceramic sandwich-type samples were prepared in a single step at constant temperature of 1420 °C, pressure (40 MPa) and various holding times (5–10 min) under vacuum atmosphere. Densification behavior, phase analysis, oxidation resistance, dynamic flame test performance and hardness of the samples were investigated. Ceramic-based layers were formed successfully on the surface of TZM without any crack or spallation and bonded with different kinds of diffusion layers depending on composition. Oxidation resistance and dynamic flame test performance of TZM were improved about 70%. The highest hardness values at the surface layers were measured as 32.7 GPa and 13.8 GPa for B4C–Si and Al2O3 additions, respectively.

Vapor‐mediated melt infiltration for synthesizing SiC composite matrices

AbstractA two‐step synthesis of SiC involving initial exposure of carbon surfaces to Si vapor, followed by Si melt infiltration, is investigated in this article with emphases on the mechanisms, kinetics, and microstructure evolution. Interrupted differential thermal analysis of amorphous C and Si powder mixtures and microstructure characterization are used to generate insight into the stages of the reaction. Exposure to Si vapor yields a SiC layer with nano‐scale porosity driven by the volume change associated with the reaction. This forms a continuous pore network that promotes subsequent melt access to the reaction front with the C. While the pores remain open, the vapor phase reaction proceeds at a nearly constant rate and exhibits a strong temperature sensitivity, the latter due in part to the temperature sensitivity of the Si vapor pressure. The implications for enhancing the reactive melt infiltration process and fabrication of SiC matrices for ceramic composites are discussed.

Kinetic and thermophysical phenomena in the synthesis of porous composites from (Ti+Si) and (Ti+Al+Si) powder mixtures in the reaction sintering mode

Theoretical and experimental study of the synthesis of porous composites from powder mixtures for Ti-Si and Ti-Al-Si systems in the reaction sintering mode is carried out in the present work. The thermokinetic model of reaction sintering is formulated taking into account competing physicochemical stages. The model includes the heat balance equation, the kinetic equations for the components involved in the reactions, the kinetic equation for porosity changing during heating, reaction sintering, and cooling. The melting is assumed to occur within the temperature range of liquidity and solidus. These processes lead to the appearance of local stresses and volumetric changes. The results of experimental studies show good agreement with the numerical calculations.

Characterization of porous sintered reaction-bonded silicon nitride containing three different rare-earth oxides

The effect of rare-earth oxides on the characteristics of porous sintered reaction-bonded silicon nitride (SRBSN) was investigated. Three types of raw Si powder mixtures containing different rare-earth oxide (La2O3, Er2O3, and Yb2O3) were prepared and nitrided in the form of compacts. The nitriding profiles of the respective raw powder mixtures with elevating temperature indicated that Yb2O3 clearly promoted the nitridation of Si compacts at low temperature compared with other rare-earth oxides, and the β-Si3N4 ratio after completion of the nitriding reaction was different at which temperature the major nitridation occurred. Yb2O3 was found to be the most effective additive to achieve a strong porous SRBSN, having the flexural strength of 441 MPa. The reason why Yb2O3 promotes the nitridation reaction and has excellent mechanical properties after post-sintering is that oxygen was removed during the nitriding reaction, which was supported by the analysis of oxygen content via the laser-induced breakdown spectroscopy.

Study On the Melting And Corrosion Resistance of Fe-Cr-Si Dual Phase Alloy

In this paper, Fe-Cr-Si multiphase alloy with both ductile and brittle phases was prepared by vacuum arc furnace with high purity powder mixture of Fe, Cr and Si as raw materials. The microstructure and properties of the alloy were studied by OM, SEM, and XRD. The experimental results show that Fe-Cr-Si alloy can be made into two-phase alloy, which mainly consists of ductile phase and Fe22Cr7Si10 brittle phase. It is found that the matrix has a higher iron content, lower hardness and higher chromium content in brittle phase. The silicon element is evenly distributed in the sample. The corrosion resistance of the alloy is very strong compared with 06cr19ni10 steel.

Fine and high-performance B6.5C-TiB2-SiC-BN composite fabricated by reactive hot pressing via TiCN–B–Si mixture

In this study, B6.5C-TiB2-SiC-BN composite was fabricated via reactive hot pressing from a powder mixture of TiCN, B, and Si. The reaction process of the powder mixture was investigated by heat treatment at elevated temperatures. The as-reacted powder products possessed a core-shell structure of TiB2 surrounded by h-BN. During densification, the h-BN in this structure acts as a diffusion channel and lubricant, which promoted the densification process. The extremely small size, fresh surface of in-situ formed phases, and uniform dispersion in synthesized powders also contributed to sintering. The designed reactive hot-pressing sintering effectively resolves the difficulty of obtaining uniformly-dispersed BN. The obtained bulk B6.5C-TiB2-SiC-BN composite ceramic shows high relative density (98.8%) and good comprehensive mechanical properties including hardness, bending strength, and fracture toughness of 19.6 GPa, 801 MPa, and 5.31 MPa m1/2, respectively.

Silicon-Nanographite Aerogel-Based Anodes for High Performance Lithium Ion Batteries

To increase the energy storage density of lithium-ion batteries, silicon anodes have been explored due to their high capacity. One of the main challenges for silicon anodes are large volume variations during the lithiation processes. Recently, several high-performance schemes have been demonstrated with increased life cycles utilizing nanomaterials such as nanoparticles, nanowires, and thin films. However, a method that allows the large-scale production of silicon anodes remains to be demonstrated. Herein, we address this question by suggesting new scalable nanomaterial-based anodes. Si nanoparticles were grown on nanographite flakes by aerogel fabrication route from Si powder and nanographite mixture using polyvinyl alcohol (PVA). This silicon-nanographite aerogel electrode has stable specific capacity even at high current rates and exhibit good cyclic stability. The specific capacity is 455 mAh g−1 for 200th cycles with a coulombic efficiency of 97% at a current density 100 mA g−1.

Oxidation Behaviors of Si/Al Pack Cementation Coated Mo–3Si–1B Alloys at Various Temperatures

In this study, we tried to improve the oxidation resistance of Mo–3Si–1B (wt%) alloy at intermediate (800 °C) and high temperature (1300 °C) after applying Si/Al pack cementation coatings. In addition, we examined structural changes and phase change in the intermediate and high temperature oxidizing atmospheres to observe oxidation behaviors. The diffusion pack coatings were carried out with an NH4F activator and both Si and Al powder mixtures. Mo–Si–B alloys were subjected to diffusion coatings of Si/Al powders at various temperatures in various time conditions at a temperature of 1100 °C. When the pack coatings were carried out for 48 h, Mo(Si, Al)2 and Mo3Al8 layers together with a T2 (Mo5SiB2) layer were successfully synthesized on the substrate. After oxidation tests, an Al2O3 protective oxide layer was formed by the diffusion of Al in an ambient atmosphere. The Si/Al pack cementation coatings provided excellent oxidation resistance through the formation of an Al2O3 oxide layer. The enhanced oxidation behaviors are discussed in terms of microstructural evolution at high temperatures.

Experimental Study of High Velocity Oxygen Fuel Sprayed Cr<sub>3</sub>C<sub>2</sub>- Ni-Cr-B-Si Coatings on Inconel 718 Using Design of Experiments

In the present study, D-optimal based mixture design of experiments (DoE) is used to find the optimum powder mixture for High Velocity Oxygen Fuel (HVOF) coating. Twenty five experimental trials are performed by varying the powder mixtures based on mixture design. Cr3C2, Ni, Cr, B, Si powder mixtures are deposited on Inconel 718 substrate by HVOF process.The responses porosity and hardness are determined by the cross-sectioning the specimens.The effect of powder mixtures on coated samples are characterized by X-ray diffraction, optical microscope, Vickers hardness tester and Scanning electron microscope.The optimal powder mixture is determined to obtain minimum porosity and maximum hardness and the confirmatory experiment confirms with the predicted results. From the analysis it is observed that Cr3C2, Ni and Cr are the major factor which influences the porosity and hardness followed by B and Si respectively.

Rapid reaction sintering of silicon carbide using Nd:YAG laser

Reaction sintering of silicon carbide (SiC) using a continuous-wave Nd:YAG laser as a heating source was investigated. The laser was used to irradiate pellets of a stoichiometric powder mixture of Si and C under Ar gas flow. At an appropriate laser power, a dense SiC layer was formed consisting of grains with a size of 100–500 nm; this layer was several micrometers thick. At lower laser power, fine grains of SiC were formed and dispersed in the unreacted Si–C matrix. On the other hand, at higher power, larger faceted SiC grains (1–3 µm) were formed.

Microstructure and properties of AZ31 with an Al/Si-enriched surface layer fabricated through thermochemical treatment

The surface enrichment of AZ31 magnesium alloy with Al and Si was achieved using the thermochemical treatment method. The experiments involved heating AZ31 in contact with an Al + Si powder mixture. Two mixtures acting as the source of diffusion elements were tested: 80% Al + 20% Si and 50% Al + 50% Si. The heat treatment of the AZ31 in the Al + 20% Si mixture was performed at three different temperatures: 435, 445 and 455 °C. As the best results were obtained at 445 °C, this temperature was used to heat AZ31 in contact with the Al + 50% Si mixture. The surface modification of AZ31 was attributable to the diffusion processes taking place at the substrate/powder reactive interface. The microstructure of the alloyed layers was characterized using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). An X-ray diffractometer (XRD) was employed to determine the phase composition. The examinations showed that the modified surface layer produced from the Al + 20% Si powder mixture was continuous and had uniform thickness (400–450 μm). The Al/Si-enriched layer was mainly composed of a eutectic, i.e., an Mg17Al12 intermetallic phase and a solid solution of Al in Mg, and unevenly distributed Mg2Si phase particles. The use of the Al + 50% Si powder mixture resulted in the formation of a thinner, continuous layer (about 200 μm) with a much higher volume fraction of the Mg2Si phase. In this case, the layer had a more uniform structure; the particles of the Mg2Si phase were evenly distributed over the eutectic matrix. Due to the presence of hard phases, i.e., Mg2Si and Mg17Al12, both types of Al/Si-enriched surface layers had much higher hardness than the AZ31 substrate. The tribological tests revealed that the alloyed layers had very good wear resistance under dry sliding conditions. Because of the higher volume fraction of the Mg2Si phase particles, the layer fabricated from the Al + 50% Si powder mixture had higher hardness and wear resistance than the layer fabricated from the mixture containing 20% Si.

Evaluation of properties of spark plasma sintered Ti3SiC2 and Ti3SiC2/SiC composites

Abstract The powder mixtures of Ti, Si, TiC and Al were sintered to synthesize Ti3SiC2 and Ti3SiC2/SiC by spark plasma sintering (SPS) method. Single phase Ti3SiC2 and Ti3SiC2/SiC composites were obtained by sintering Ti/1.1Si/2TiC/0.2Al and 2.2Si/3TiC/0.2Al at 1400 °C, for 15 min, under a pressure of 50 MPa, respectively. The reduction of Al and Si contents in the starting powder mixture led to the formation of TiC impurity phase. X-ray diffraction (XRD) and Rietveld analyses were used to evaluate the phase constituents. The main objective of this study was to reveal the properties of single phase Ti3SiC2 and to evaluate the development of the mechanical and tribological properties of Ti3SiC2/SiC comprehensively. The results indicated that the SiC in situ reinforced Ti3SiC2 composites exhibited higher hardness, fracture toughness and wear resistance compared to single phase Ti3SiC2.

Rapid self-sustaining consolidation of titanium silicide (Ti5Si3) via transient liquid phase reaction induced by an electric discharge

The fabrication of Ti5Si3 in the form of a solid product directly from an elemental 37.5 at.% Si and 62.5 at.% Ti powder mixture was carried out by two different powder metallurgy routes. The first was by uniaxial pressing of the reactant powder mixture with sequent vacuum-sintering, and the second was by electric discharge sintering (EDS) of reactant powder mixture. The pressing process combined with vacuum-sintering produced a porous compact with multi phases of titanium silicide such as Ti5Si3, Ti5Si4, TiSi2, and TiSi, including elemental Ti, which indicated an incomplete phase transformation into Ti5Si3. On the other hand, the EDS induced the phase transformation mostly into Ti5Si3 with a small amount of Ti5Si4 in <180 μsec, which had a sequent consolidation into a solid compact with an average crystallite size of 30.4 nm and a lattice parameter of a = 7.42 Å and c = 4.91 Å. The significantly higher hardness value of the EDS compacts can be the result of the high density and the fine microstructure stemming from the homogeneous dissolution of the elements and the constrained grain growth. The formation of Ti5Si3 solid compact from the stoichiometric Ti and Si powder mixture by EDS can be dominated by the solid to liquid phase transformation mechanism.

Zirconium modified Nb-22Ti-16Si alloys fabricated by laser additive manufacturing: Microstructure and fracture toughness

The microstructure and mechanical behavior of Nb-22Ti-16Si-xZr alloys (x = 0, 1, 5, 10, 15 at.%) were investigated by using laser solid forming (LSF) additive manufacturing from elemental powder mixture of Nb, Ti, Si and Zr. The as-deposited Nb-22Ti-16Si alloy presented a microstructure consisted of primary Nb-base solid solution (Nbss) dendrite and Nbss/(Nb)3Si eutectic dendrites. The eutectic Nbss/γ-(Nb)5Si3 appeared with a morphology of lamellar structural feature when 1 at.%Zr was added, and its content increased with Zr content. Nbss/(Nb)3Si eutectic dendrites disappeared when Zr content reaches 10 at.%. Further increasing of Zr (15 at.%) result in the appearance of a hypereutectic microstructure, which consisted by primary γ-(Nb)5Si3 and eutectic Nbss/γ-(Nb)5Si3. As the nominal Zr content increased, the fracture toughness of LSFed Nb-22Ti-16Si-xZr alloy increases first and reaches the maximum value (15.28 MPa m1/2) in LSFed Nb-22Ti-16Si-5Zr alloy, and then decreases.

Reaction synthesis of spark plasma sintered MoSi2-B4C coatings for oxidation protection of Nb alloy

MoSi2-B4C coatings with different B4C contents were prepared on Nb alloy by spark plasma sintering (SPS) process. Powder mixtures of Mo, Si and B4C were used as the coating starting materials. Besides MoSi2 and B4C phases, small amounts of SiC and MoB are also found in the coatings because of the reactions of Mo, Si and B4C powders during sintering. Compared with single MoSi2 coating, the MoSi2-B4C coatings show better oxidation resistance at 1450 ℃, and dense B2O3-SiO2 oxide scales form after 100 h oxidation. The B4C or MoB in the MoSi2-B4C coatings can serve as the B donor for the formation of B2O3. A slight degradation in the microstructure of the MoSi2-B4C coatings after oxidation is observed, which can be attributed to the presence of an NbB layer in the inter-diffusion zone of the coatings that retards the inward diffusion of Si from the coating into the substrate alloy. The microstructure development and oxidation behavior of the MoSi2-B4C coatings have been discussed.