Search

Numerical calculations were carried out to simulate, under conditions of close spark plasma sintering (SPS), the temperature distribution during the passage of current in dense cylindrical samples of two materials: aluminum oxide and copper located in graphite forms and clamped between cylindrical graphite punches. The investigated materials differ greatly in their electrical conductivity and other physicochemical properties. Calculations were carried out for various geometric parameters of the samples, as well as graphite molds and punches at varying heating rates from the passing current.

Copper-graphite joints are functional elements of many electronic and electromechanical devices. Unique properties of graphite, such as the ability to withstand high temperatures, a low friction coefficient and high thermal and electrical conductivities make it an attractive material for various applications. For enabling efficient heat sink or making sliding electrical contacts, graphite requires joining to metallic copper. Poor wettability between copper and graphite is a problem that needs to be overcome to form a reliable joint. In the present work, we used a Ti25Cu75 alloy to braze graphite to copper in a pulsed current-assisted process. Brazing was carried out in a Spark Plasma Sintering (SPS) apparatus under a uniaxial pressure of 13 MPa. The SPS conditions – a possibility of localized heating under an applied pressure in dynamic vacuum – are beneficial for the joining processes of conductive materials. For joining graphite and copper plates, a die-free configuration was used. The microstructure and the phase composition of the copper-graphite brazed joints were investigated. Strong joints were formed when the upper electrode temperature measured by a thermocouple reached 700 °C. At the graphite-alloy interface, a TiC layer 0.5–1 μm thick was formed; in addition, TiC particles were found in the metallic matrix in a layer 5–10 μm thick adjacent to the graphite plate. The tensile strength of the brazed joint was higher than the tensile strength of graphite (13 MPa).

Composite material with aluminum matrix reinforced by carbon nanotubes (CNT) and oxide nanofibers (NF) was made by powder metallurgy method. Powder mixtures were made by low energy milling on drum ball mill. Composite samples were manufactured by spark plasma sintering system. Microstructure analysis showed agglomeration of CNT and NF in composite materials. Correlation between flexural strength and unit mass of composites was revealed. Nevertheless, the use of low energy milling allows minimizing the damage of carbon nanotubes and oxide nanofibers during the preparation of mixtures. From this point of view it is suitable for making composites with high elastic modulus. It was shown that addition of 5-12%wt of CNT can increase flexural modulus by 30-189%. Also was shown that addition of 5%wt of oxide nanofibers increase flexural modulus by 78%

The efficiency of using the collector pressing scheme in the Spark Plasma Sintering (SPS) process has been confirmed in improving the optical, physical, and mechanical properties of yttria-stabilized zirconia (YSZ) ceramics with an increased shape factor. An approach for developing a seal surface and determining the optimal method of increasing pressure and temperature during SPS on this surface was used to optimize the consolidation modes of the materials. It has been shown that transparent/translucent YSZ ceramics with an increased shape factor (14 mm in diameter and up to 5 mm in height, h/d = 0.36) can be successfully fabricated by the SPS technique combined with the collector pressing scheme. The optical properties and microhardness of ceramics obtained using the collector scheme are better to the optical properties of ceramics obtained using the conventional uniaxial pressing scheme.

Bimetals produced by explosion welding of titanium alloy VT23 and steel 45KhNM through an intermediate layer of steel 08ps followed by a heat treatment consisting of quenching and tempering are studied. The three-layer composite is subjected to metallographic analysis prior to and after the heat treatment with the help of an optical microscope and a scanning electron microscope equipped with an energy dispersive analyzer. The chemical composition and the hardness of the welded materials and of various regions of the weld are determined, and the changes in these parameters after the quenching and tempering are assessed.

Silver oxalate Ag2C2O4 is an interesting compound from both fundamental and practical viewpoints. Decomposition of Ag2C2O4 results in the formation of metallic silver, whose size and morphology can be tailored for different applications. In this work, we focused on the morphological features of silver that forms upon decomposition of Ag2C2O4 in a spark plasma sintering (SPS) apparatus. While crystals heated in a conventional furnace decompose to produce silver in the form of a pseudomorph, decomposition induced by heating in a SPS chamber at a rate of 30 °C·min-1 via a pressureless process results in the formation of foam-like porous silver. Decomposition of Ag2C2O4 was also carried out under pressure by placing the powder between copper plates and heating the assembly in a SPS die. A feasibility study has shown that Ag2C2O4 powders can be used for joining copper plates in a fast process enabled by the SPS method. Joints formed by the SPS method at 300°C and 13 MPa were formed by silver with 500-nm crystallites; the shear strength of the joints was 45 MPa.

The corrosion resistance in alkaline solutions of Ni3Al intermetallic produced by the method of spark plasma sintering of powder mixtures with or without preliminary mechanical activation was investigated. It was found that nickel aluminide has high corrosion resistance in boiling solutions of strong alkalies, such as sodium and potassium hydroxide. The rate of corrosion in these media did not exceed 0.05 mm/year, even when corrosion activators were introduced into the solution. Solutions of weak alkalies have a more aggressive effect on the sintered material. It was shown that the corrosion rate of the intermetallic in an aqueous solution of ammonia was 0.22-0.24 mm/year, which is still 1.4 times lower than that of nickel and an order of magnitude lower than that of aluminum. In all cases, mechanical activation did not affect the corrosion rate of the material; however, it did affect the nature of the surface corrosion damage.

The results of experimental study of laser welding alloys based on aluminum and titanium an intermediate composite Al+Ti insert obtained by explosion, using nanopowder additives and plastic deformation of the aluminum seam are presented. Morphologically high-quality welds have been obtained both in the welding of the aluminum plate and of the titanium one with the corresponding parts of the insert. It was found out that breakdown of the joint in the tensile test occurred along the aluminum part of the insert, which had a lower tensile strength than the titanium. The use of the nanomodifier without plastic deformation increases the strength of the weld joint by about 10% compared to the unmodified compound. The use of nanomodifying and strain hardening can significantly increase the tensile strength, the value of which is more than 97% of the strength of alloy (AA1135). The carried out experiments on laser butt welding of plates made of aluminum alloy 1424 systems Al-Mg-Li-Zr-Sc with their subsequent plastic deformation have confirmed the effectiveness of strain hardening of weld joints of aluminum alloys.

Al3Ti-based alloys attract exceptional attention due to their high specific mechanical properties. However, their application is still insufficient due to their low ductility and fracture toughness. Several approaches were previously proposed to address these problems. The first one is stabilization of the cubic modification of titanium trialuminide by alloying. Another approach consists in fabricating metal-intermetallic laminated composites (MIL). In this study, we combined both methods to synthesize the first MIL composite with cubic Al3Ti interlayers. Copper additions were used to stabilize the cubic modification of Al3Ti and produce a novel Ti-Al5CuTi2 MIL composite. First mechanical characterization by indentation tests showed that the binary Al3Ti intermetallic tended to crack at a load of 0.2 kg while the fracture was not observed in the Al5CuTi2 layers at least at a load of 1 kg. These results are an indirect evidence of a higher ductility and fracture toughness of the composite with cubic Al3Ti compared to tetragonal one. The sequence of the phase transformations in the Al-Ti-Cu system was studied using in situ synchrotron X-ray radiation diffraction. The formation of Al5CuTi2 occurred via several intermediate stages including eutectic melting of Al and Cu and the formation of binary AlCu and Al3Ti compounds. Keywords: Sintering, Titanium aluminide, Phase transformation, Synchrotron diffraction, Fracture toughness, Ternary compound

Features of the structure of a layered material welded by explosion of high-strength titanium alloy and tool roller steel with an intermediate layer of the structural low-carbon steel have been studied. The structural transformations occurring in materials upon their dynamic interaction have been analyzed. Particular attention is paid to the structure of vortex zones formed at the interfaces of billets of various steels, as well as structural steel and titanium-based alloy. The structural analysis methods made it possible to fix stable and metastable joints appearing upon the explosive welding of various metals. To reveal features of structural transformations caused by prolonged heating, billets of titanium alloy and structural steel were also joined by diffusion welding. It has been shown that, in the course of the diffusion welding process, a continuous layer of stable brittle intermetallic compounds is formed along the entire interface of the welded materials. In the explosively welded materials, the intermetallic phases are distributed locally and, thus, they have a weaker embrittlement effect.

In recent decades, spark plasma sintering (SPS) has become a widespread technique in the production of high-quality intermetallic-based materials. In this study, we used SPS to fabricate multilayer metal-intermetallic (MIL) composites from pure nickel and aluminum foils. The effect of sintering temperature and pressure on the structure and properties of the fabricated MIL composites is discussed. The sintering temperature was changed from 900 to 1100 °C, the pressure varied from 10 to 40 MPa. The structure of the materials was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray energy-dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Depending on the sintering regimes the intermetallic layers in the composites consisted of Ni2Al3, Ni3Al, and NiAl of cubic and tetragonal modifications. Tetragonal NiAl appeared in the form of microtwinned lamellar martensite. A change in SPS temperature from 900 to 1100 °C led to an increase in the volume fraction of nickel-enriched intermetallic compounds, which was accompanied by improving composites strength. With an increase of the SPS pressure from 10 to 30 MPa, the number of pores and cracks in composites decreased. At the same time, their bending strength increased by 2 times (from 450 to 900 MPa). Fractographic studies showed that the fracture mechanism of the intermetallic layers depends on their elemental composition and varies from brittle transcrystalline to wavy “feathery”.

The effect of thermal regimes and some technological methods (the application of intermediate compact metal inserts and nanomodifying additives) on the quality of laser-welded permanent joints of porous titanium or stainless steel plates has been experimentally studied in this work. It has been established that the formation of a high-quality defectless seam requires the use of an intermediate compact (nonporous) metal insert placed butt to butt between the edges of welded plates. In this case, the position of a focal spot should be installed at a depth equal to the thickness of a plate, i.e., on its lower surface. To form a bead joint or a joint flush to the plates to be welded together, the height of an insert must exceed the thickness of the plates. The application of nanomodifying titanium nitride based additives increases the structural dispersity and uniformity of a seam, thus producing a positive effect on the mechanical characteristics of a welded joint.

A Fe-40 at.%Al loosely packed powder mixture was subjected to pressureless treatment in a vacuum chamber of a Spark Plasma Sintering facility and sintered in a hot press in an argon atmosphere with and without applying mechanical pressure. The treatment conditions were varied to determine whether the electric current and the heating rate play any roles in the formation of FeAl compacts with a high open porosity (41–51%). Rapid heating of a loosely packed Fe-Al mixture in the SPS chamber with or without an electric current passing directly through the sample ensures a fast transformation of the reaction mixture into a highly porous FeAl product, the passage of electric current through the sample not being responsible for the preservation of high open porosities. The transverse rupture strength of porous FeAl did not depend on the sintering method for compacts of the same total porosity, but increased when the total porosity decreased.

Ti/Ni alloy-based laminate composite materials were produced by explosive welding with two thin intermediate layers of tantalum and copper placed between Ti and Ni alloy layers. The influence of the thickness of the Cu interlayer (0.1–0.7 mm) on the structure and mechanical properties of the explosively welded composites was examined. Investigations carried out by optical and scanning electron microscopy showed the formation of an inhomogeneous structure in the vicinity of the interfaces with zones of local melting and cavities within these zones. The wavelength and the amplitude at the interface between the copper and tantalum interlayers changed with the thickness of the copper layer. In order to evaluate the mechanical properties of the composites containing copper interlayers of different thicknesses, microhardness, tensile, and bending tests were performed. As the thickness of the copper layer was decreased to 0.3 mm, the tensile and bending strengths of the laminate composites increased.

In the present study, a mixture of powders (87.9 at.% Ni, 12 at.% Al, 0.1 at.% B) was used as the initial material to produce sintered Ni3Al + B alloy. Spark Plasma Sintering (SPS) method was used to compact the powder. The powder mixtures were previously prepared in two ways: mixing the initial powders in a mortar (М1) and mechanical activation (М2). The microstructure was observed using optical microscope (OM). The addition of small amount of boron to the initial mixture of nickel and aluminum improves the density of the sintered Ni3Al intermetallic compound (98.8%). The results of density, bending and microhardness tests showed, that the provisional three-minute mechanical activation improves almost all properties of the sintered material. The compact obtained by SPS by M2 contributes to the formation of a homogeneous fine-grained structure of the material. It leads to further increase in flexural bending strength up to 2200 MPa. This value is almost 8 times the strength of the intermetallic Ni3Al stoichiometric composition obtained by SPS.

The paper presents the results of an experimental study of the processes of obtaining a permanent connection of porous stainless steel using laser welding and some technological methods (intermediate inserts of monolithic metal, nanomodifying additives). It was established that in order to obtain a durable defect-free joint, it is necessary to use an intermediate insert made of compact metal of the same composition as the welded parts, placed between the ends of the butt-welded parts. The use of modifying additives based on refractory joints increases the dispersion and uniformity of the weld structure, which positively affects the mechanical characteristics of the welded joint.

In this paper, the structural characteristics of a W-Ta-Mo-Nb-V-Cr-Zr-Ti non-equiatomic refractory metal alloy obtained by spark plasma sintering (SPS) of a high-energy ball-milled powder mixture are reported. High-energy ball milling resulted in the formation of particle agglomerates ranging from several tens to several hundreds of micrometers. These agglomerates were composed of micrometer and submicrometer particles. It was found that, during ball milling, a solid solution of A2 structure formed. The grains of the sintered material ranged from fractions of a micrometer to several micrometers. During SPS, the phase transformations in the alloy led to the formation of a Laves phase of C15 structure and ZrO and ZrO2 nanoparticles. The microhardness of the ball-milled alloy and sintered material was found to be 9.28 GPa &plusmn, 1.31 GPa and 8.95 GPa &plusmn, 0.42 GPa, respectively. The influence of the processing conditions on the structure, phase composition, and microhardness of the alloy is discussed.

In paper, weld joints between VT1-0 titanium and AISI 321 austenitic stainless steel using laser welding were obtained. To improve the quality and strength properties of joints, two types (SS–Cu–Nb/Ta–Ti) of explosively welded four-layered composite inserts were used. Barrier layers were different from each other by refractory metal. The effect of intermediate material inserts, in particular tantalum and niobium, on microstructure, chemical composition, strength properties, and fracture behaviour of weld joints was studied. Microstructural studies have revealed two bonding types as results of welding method combination. At copper–stainless steel interface, severely deformed zone characterized by low etch ability was observed. Technological parameter’s optimization provided high joint quality and absence of defects in the weld joints. According to results of strength tests, it was found that the composite insert material affects the strength of the joints. The highest ultimate tensile strength and yield strength were detected for joints containing niobium foil.

In this study, forty-layered Ti-Al composites were fabricated in a single-shot explosive welding process. The structure of the composites was thoroughly investigated using scanning and transmission electron microscopy. Particular attention was paid to the structure of the mixing zones (vortexes) arising at the interfaces during explosive welding. The complicated process of mixing and rapid solidification of these zones led to formation of different stable and metastable structures. The vitrification of vortexes, formation of “disordered” Ti3Al and ordered solid solution of Al in hexagonal-Ti and Ti in FCC-Al were observed and discussed with respect to the conditions of solidification. Subsequent heat treatment was carried out at 640 °C under atmospheric and at 3 MPa pressure. For comparison, there were produced reference samples by reaction sintering at the same conditions as for the heat treatment. The heat treatment and reaction sintering promoted the formation of stable Al3Ti phase between Ti and Al. It was found that preliminary explosive welding accelerated the formation of Al3Ti layer and made heat treatment duration four times shorter. The application of pressure was found to play an important role at the final stage of heat treatment after explosion welding to avoid formation of defects between the plates. Keywords: Explosive welding, Multilayer, Titanium, Aluminum, Intermetallic, Composite

In this study, the issue of reactivity of materials towards carbon of graphite foil during Spark Plasma Sintering has been addressed through a case study using Ni-15 at% W powders prepared by mechanical alloying or simple mixing. Tungsten carbide WC was found at the flat ends of the compacts obtained from both mixtures. In case of the mechanically alloyed powder, carbon diffusion into the sintered compact resulted in the formation of Ni 2 W 4 C particles in a Ni-based matrix throughout the compact volume. In contrast to the compact obtained from the mechanically alloyed powder, no carbon-containing phases were detected in the interior of the compacts produced from mixed Ni+W powders sintered in the same conditions. The uptake of carbon caused by its inward diffusion was facilitated by a much higher concentration of defects and a greater volume of grain boundaries in the mechanically alloyed powder. The compacts Spark Plasma Sintered from the mechanically alloyed powders were much harder (620 HV) than the compacts produced from the mixed powders (315–370 HV) due to the presence of submicron Ni 2 W 4 C reinforcing particles distributed in a Ni-based solid solution-strengthened matrix.

An experimental study of the effect of thermal conditions and some technological methods (intermediate inserts made of monolithic metal, nano-modifying additives) on the quality of the permanent joint of porous titanium obtained by laser welding was carried out. It has been established that to obtain a high-quality defect-free weld it is necessary to use an intermediate insert made of compact metal, butt-mounted between the ends of the plates to be welded. In this case, the position of the focal spot should be set at a depth equal to the thickness of the plate, i.e. on its bottom surface. The use of modifying additives based on titanium nitride increases the dispersion and homogeneity of structure of the weld and neighboring area, which, according to the Hall-Petch law, has a positive effect on the mechanical characteristics of the metal.

Diamond-metal composites are attractive materials for thermal management and cutting tool fabrication. In order to increase the wettability of the surface of diamond filler particles by certain metals and reduce the interface-associated porosity in sintered composites, coatings are deposited on diamond particles, or alloying additives are introduced into the metal matrices. Nanodiamonds are added to metal matrices to achieve a reinforcing effect. The subject of this chapter is processing of diamond- and nanodiamond-metal composites by spark plasma sintering (SPS). The interest to SPS as a processing method of these materials is stimulated by its capability to rapidly consolidate powder mixtures into dense compacts. In this chapter, the suitability of SPS for diamond-metal composites aimed at different applications is discussed. Recently, the SPS treatment has been used to deposit coatings on micrometer-sized diamond particles. Results of these studies are reviewed. The microstructure development of composites containing nanodiamonds as a reinforcing phase in metal matrices during SPS is analyzed. Future directions of research on the SPS processing of diamond- and nanodiamond-metal composites are suggested.

In this work we perform an investigation of the influence of duration of preliminary mechanical activation in high energy ball mills on the microhardness of Ni3A intermetallide specimens synthesized under conditions of spark plasma sintering.

One of the ways to simultaneously increase the strength and the fracture and impact toughness of structural materials is by producing multilayered materials. In this paper we discuss the structure and properties of a seven-layer composite obtained by explosive welding of high-strength titanium alloys. The structure of the composite was characterized using light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). At the interfaces between plates, formation of waves and vortices was observed. The wave formation is discussed with respect to the kinetic energy loss. The vortices consisted of a mixture of two alloys and possessed a martensitic structure comprising &alpha, &prime, and &beta, phases of titanium. Localized plastic deformation occurred along the interfaces during explosive welding by formation of shear bands. The most intensive shear banding occurred in the vicinity of the upper interfaces. The local hardness at the interfaces increased due to the formation of the quenched structures. The interfaces between titanium alloys positively influenced the impact toughness of the composite, which increased in comparison with that of bulk titanium alloys by a factor of 3.5. The strength characteristics of the composite remained at the same level as that of the bulk material (1100&ndash, 1200 MPa).

Pulsed electric current can be used for the fast sintering of powders as well as joining of macroobjects. In this work, we brazed copper plates using a silver layer that was formed in situ by the decomposition of a silver oxalate Ag2C2O4 powder placed between the plates. Joining was conducted in the chamber of a Spark Plasma Sintering (SPS) facility with and without a graphite die. In the die-assisted tooling configuration, indirect heating of the assembly from the graphite die carrying electric current occurred until the brazing layer transformed into metallic silver. The passage of electric current through a Cu/Ag2C2O4/Cu stack placed between the electrodes without a die was possible because of the formation of Cu/Cu contacts in the areas free from the Ag2C2O4 particles. Joints that were formed in the die-assisted experiments showed a slightly higher shear strength (45 MPa) in comparison with joints formed without a die (41 MPa). The shear strength of the reference sample (obtained without a die), a stack of copper plates joined without any brazing layer, was only 31 MPa, which indicates a key role of the silver in producing strong bonding between the plates. This study shows that both die-assisted tooling configurations and those without a die can be used for the SPS brazing of materials by the oxalate-derived silver interlayer.

Nanodiamond particles are known to graphitize upon annealing gradually transforming into carbon onion structures. The goal of this work was to evaluate the potential of nanodiamond powders as a raw material to produce bulk porous electrically conductive materials by means of graphitization-accompanied Spark Plasma Sintering and hot pressing. Despite consolidation that occurred in parallel to graphitization, an increase in the specific surface area (up to 510 m 2 g −1 ) relative to the nanodiamond powder (360 m 2 g −1 ) was observed in compacts with high graphitization degrees. Compacts Spark Plasma Sintered at 1200 °C and hot-pressed at 1500 °C showed high electrical conductivities, which were one order of magnitude higher than those of the powders produced by nanodiamond graphitization and only 40–50 times lower than that of bulk graphite. Based on these results, a conclusion was made that consolidation of nanodiamonds by Spark Plasma Sintering and hot pressing can be used for developing porous electrically conductive materials with tailored specific surface area. A possible influence of electric current on graphitization of nanodiamonds during Spark Plasma Sintering was suggested.

Sintered compacts fabricated by spark plasma sintering (SPS) at 1050оС were investigated. Titanium and aluminum powders in a ratio of 25 % (at.) and 75 % (at.) respectively were chosen as starting materials. Powder mixture heating up to elevated temperatures led to a partial loss of an aluminum component and to the formation of a multiphase structure consisting of Al3Ti, Al2Ti, AlTi and AlTi3. The density of sintered powder mixtures was 3.7 g/cm3; an average microhardness value was about 470 HV.

Metal-intermetallic laminate composites are considered as promising materials for application in the aerospace industry. In this study, Ti-Al3Ti composites enclosed in titanium cases were produced by reactive spark plasma sintering. Sintering was carried out at 1103 K and 1323 K (830 °C and 1050 °C) for 10 minutes. In both cases, high-quality Ti-Al3Ti composites containing thin transition layers at the interfaces were obtained. Al2Ti, AlTi, and AlTi3 intermetallic phases and a solid solution of aluminum in titanium were observed in the transition layers by scanning and transmission electron microscopy. The material sintered at 1323 K (1050 °C) had higher strength in comparison with the composite obtained at 1103 K (830 °C). However, the hardness of the intermetallic component in the sample sintered at higher temperature decreased due to the grain growth. The impact toughness values of both materials were approximately identical.

One of the most well-known and technologically attractive advantages of pulsed electric current-assisted sintering is its capability of fast and efficient consolidation of nanostructured powders into nanostructured or submicron-grained compacts. But can the application of pulsed electric current produce crystallites in the sintered material smaller than those that were present in the powder? Microscopic observations of this effect have been reported for inter-particle contacts formed during Spark Plasma Sintering and high-voltage consolidation. Our results obtained on Spark Plasma Sintered porous compacts consolidated from electrolytic copper powder suggest that finer crystallites in the sintered material than those in the raw powder can form in areas other than necks between the particles. Based on the electron microscopy and X-ray diffraction evidence as well as results obtained by other authors, we consider two possible mechanisms of crystallite size refinement during pulsed electric current-assisted sintering: rapid solidification of locally formed melt and reduction of oxide films present on the surface of powder particles in a reducing environment of the sintering die/chamber causing the in situ formation of metal particles. The possibilities of crystallite size refinement are further examined in a broader context as related to phase transitions and chemical reactions in the sintered materials.

Graphitization of amorphous carbon in the presence of nickel has been reported for various configurations of the metal–carbon interface; however, no study has been performed to evaluate a possibility of forming self-supporting networks by sintering of the in situ formed graphite. In this work, we have shown that Spark Plasma Sintering (SPS) of nickel–amorphous carbon mixtures containing 50 vol% of Ni at 1000 °C results in the formation of networks formed by sintered graphite platelets 50–200 nm thick and 0.3–2 μm in diameter. Upon selective dissolution of nickel, a self-supporting porous 3D skeleton was revealed in 20 mm-diameter compacts. Starting from the mechanically milled Ni–C mixture, porous graphite of uniform microstructure and containing submicron pores was obtained. A model study has been performed, in which a thin amorphous carbon film graphitized during annealing and formed a continuous graphite film with micron-sized grains covering an area of 2 cm×2 cm of the surface of a Ni foil. We discuss the role of the in situ formation of graphite by nickel-assisted graphitization in the formation of networks consisting of well sintered platelets during the SPS and the design possibilities of porous carbon materials produced by phase separation in nickel–graphite composites.

We report the formation and development of the interface between diamond and nickel in partially densified compacts obtained from powder mixtures by Spark Plasma Sintering, hot pressing and conventional sintering at 700 and 900 °C – temperatures, which are well below the melting point of nickel and even below that of the nickel–graphite eutectic. The nickel particles sintered among themselves and formed joints with facets of the diamond crystals. Most of these joints fractured cohesively leaving Ni-containing patches on the diamond facets. The microstructure of the patches adhered to the diamond surface in compacts sintered at 900 °C and their geometry and orientation relative to the edges of the diamond facets suggest that the formation and development of the nickel/diamond interface are associated with melting and solidification of the melt according to certain directions of the diamond crystalline lattice. A possible explanation of the formation of a liquid at such a low temperature is contact melting of a metastable eutectic between nickel and diamond.

In the majority studies of materials produced by Spark Plasma Sintering (SPS), the powders are consolidated in graphite dies using graphite punches and protective graphite foil. As a result, carbon uptake by the sintered material can occur. In this study, a Ni–15 at% W alloy was studied in this context for the first time and was chosen as a suitable model system, in which tungsten forms stable carbides whereas nickel does not, but offers a medium for carbon diffusion into the interior of the compact. In a disk-shaped 3 mm-thick compact Spark Plasma Sintered at 900 °C, carbon uptake resulted in the formation of tungsten carbide WC particles ranging from 0.2 to 2 μm in the subsurface layers of the compact (within distances 50–100 μm from the interface with the foil). The size of the WC particles varied with distance, smaller particles forming in the vicinity of the interface—in the area in which the nucleation was favoured at high carbon concentrations. However, it was not only the subsurface layer that was affected by the presence of carbon: particles of Ni2W4C were found at depths greater than 100 μm from the interface and throughout the volume of the compact. The distribution of the submicron WC particles and particles of Ni2W4C corresponded to a network of boundaries between the agglomerates of the Ni–W powder that was consolidated into a compact. These boundaries offered paths for faster diffusion of carbon from the foil when compared with the volume of the agglomerates. The carbide subsurface layer dramatically changed the interdiffusion behaviour of the sintered material in a pair with aluminum due to a significantly reduced concentration of tungsten capable of diffusing within a metallic phase.

We report the formation and phase separation response of nickel-graphite composites with variable-architecture phases by graphitization-accompanied consolidation via Spark Plasma Sintering and hot pressing. It was shown that the application of pressure during consolidation is crucial for the occurrence of graphitization and formation of 3D graphite structures. We evaluated the suitability of the synthesized composites as precursors for making porous structures. Nickel behaved as a space holder with the particle size and spatial distribution changing during consolidation with the temperature and determining the structure of porous graphite formed by phase separation by dissolution in HCl. The response of the consolidated Ni-Cgr to separation of carbon by its burnout in air was studied. The result of the carbon removal was either the formation of a dense and continuous NiO film on the surface of the compacts or oxidation through the compact thickness. The choice between these two options depended on the density of the compacts and on the presence of carbon dissolved in nickel. It was found that during the burnout of graphite from Ni-Cgr composites, sintering, rather than formation of pores, dominated.

In this work, Ni3Al–Ni composite materials were obtained by spark plasma sintering (SPS) of commercial nickel aluminide Ni3Al and metallic nickel powder mixtures at 1100 °C. The effect of the nickel content on the structure and mechanical properties of the sintered composites was investigated. Nickel filled the gaps between particles of brittle nickel aluminide and acted as a binder, which led to increased ductility of the composite materials relative to the intermetallic sintered without nickel additions. The introduction of nickel had a positive effect on the particle conjugation and the relative density of the sintered materials. Attractive mechanical properties were found in the composite sintered from a mixture containing 30 wt% of metallic nickel. This composite showed a tensile strength of 590 MPa, while its deformation at fracture was ~20%. The bending strength of this composite (1900 MPa) was twice as high as that of the material, to which no nickel was added. In this composite, Ni3Al intermetallic (γ′-phase) in the form of cubic particles 0.5–2.2 μm in size and a solid solution of aluminum in nickel (γ-phase) formed between the particles of the starting intermetallic powder. These microstructural features made the composite fracture in the mixed mode.

Laminate composites with an intermetallic component are some of the most prospective constructional and functional materials. The basic formation method of such materials consists in heating a stack composed of metallic plates reacting at elevated temperatures to form intermetallic phases. The temperature of the process is usually approximately equal to a melting point of a more easily fusible component. In this study, an alternative technology of producing a titanium – titanium aluminide composite with a laminate structure is suggested. It consists in combining metallic (titanium and aluminum) powder mixtures pre-sintered at 400 оС with titanium plates, alternate stacking of these components and subsequent spark plasma sintering (SPS) of the fabricated workpieces. Applying this technology allowed for the fabrication of metal-intermetallic laminate (MIL) materials with an inhomogeneous structure of intermetallic interlayers. The phases revealed in the composite by X-Ray diffraction (XRD) were α-Ti, Al, Al3Ti and Al2Ti. Moreover, the results of the energy-dispersive analysis gave the evidence of the formation of Ti-enriched phases in powder layers after SPS. A small number of voids were observed between the structural components of the intermetallic layers. Voids were also detected at “metal-intermetallic” interfaces; however, the quality of connection between different layers in the composite was very high. The microhardness of an intermetallic layer formed in the composite was comparable to the microhardness of the Al3Ti compound. The microhardness of titanium was equal to 1600 MPa.

Analysis of structural variations taking place at the stainless steel (09Cr18Ni10Ti) - bronze (CuBe2Ni) interface obtained by explosive welding was conducted in the current work. The produced weld joint was post heat-treated in the temperature range from 500 to 800 °С. Microstructural characterizations were carried out using optical and scanning electron microscopy. The results of the analysis revealed the presence of 2 zones at the interface: mixing zone of bronze and stainless steel and a diffusion zone. The diffusion processes in the weld joint during heating were studied by the energy-dispersive analysis (EDX). EDX studies revealed that at 800 °С copper contained in bronze completely migrated from the diffusion layer to the mixing zone whereas iron concentration, on the contrary, increased in the diffusion layer. Voids appeared in the mixing zone of stainless steel and bronze due to the difference of diffusion coefficients of basic elements in the composite.

The method of spark plasma sintering (SPS) is used for the creation of fine-grained ceramics on the basis of various nanodimensional powders of alumina and other oxides. A comparison of the microhardness of ceramic samples obtained from 11 alumina nanopowders and 2 of their composites was held. The microhardness of ceramics obtained both by SPS, and by the traditional method (at successive pressing and sintering) is compared. The dependence of ceramics microhardness on the phase composition of the initial nanopowder and the average size of its particles were investigated. Besides alumina (Al2O3) nanopowders, a comparison was made of the microhardness of ceramics from nanopowders of 13 other oxides (SiO2, ZnO, Fe3O4, CuO, Cr2O3, Gd2O3, CoO, WO3, MgO, Nb2O5, TiO2, Y2O3, ZrO2) obtained both by SPS, and by the traditional method. It is found that the microhardness of the ceramics created by the method of spark plasma sintering, is significantly higher than the microhardness of the ceramics obtained by the traditional method; at the SPS method the average size of ceramics grains decreases (to 1 micron and less).

Particularities of the multilayered “Ti – Al3Ti” composites structure formation were investigated. Spark plasma sintering of Ti and Al plates in cylindrical molds at the temperature of 830оС and pressure of 3 kN during 5 minutes was found to be the most efficient technology of formation of materials with intermetallic layers. The regime described above allows fabricating material with high-quality intermetallic layers and with microhardness level up to 3800 MPa.

The structural features of multilayered "metal - intermetallic" composite materials based on nickel and aluminum, obtained by SPS method have been investigated in this paper. The effect of sintering temperature on phase composition of the intermetallic layers was studied. The influence of intermetallic layers phase composition on the mechanical properties of the composites was estimated.

In this paper structure and mechanical properties of Ni3Al intermetallic compound was studied. The materials was fabricated according to different schemes, which combined mechanical alloying of Ni and Al powders, self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS). Relative density of all sintered samples was ~ 97 %. Microhardness of the sintered materials ranged from 6100 to 6300 MPa. SPS of 86.71 % wt. Ni and 13.29 % wt. Ni powder at 1100 °C led to formation of material with the highest level of tensile strength equal to 400 MPa.

Fine-grained ceramics (with a grain size on the order of a micron) have been produced by the spark plasma sintering (SPS) of various alumina nanopowders. We have compared the microhardness of ceramic samples prepared from 11 alumina nanopowders and that of composites based on such powders. The ceramics have been prepared by both SPS and a conventional technique (sequential pressing and sintering). We examine the effect of the phase composition and average particle size of the starting nanopowder on the microhardness of the ceramics.

In this study, new multilayer TiAl-based composites were developed and characterized. The materials were produced by spark plasma sintering (SPS) of elemental Ti and Al foils and ceramic particles (TiB2 and TiC) at 1250 &deg, C. The matrix of the composites consisted of &alpha, 2-TiAl and &gamma, TiAl lamellas and reinforcing ceramic layers. Formation of the &alpha, 2 + &gamma, structure, which occurred via a number of solid&ndash, liquid and solid&ndash, solid reactions and intermediate phases, was characterized by in situ synchrotron X-ray diffraction analysis. The combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis revealed that an interaction of TiC with Ti and Al led to the formation of a Ti2AlC Mn+1AXn (MAX) phase. No chemical reactions between TiB2 and the matrix elements were observed. The microhardness, compressive strength, and creep behavior of the composites were measured to estimate their mechanical properties. The orientation of the layers with respect to the direction of the load affected the compressive strength and creep behavior of TiC-reinforced composites. The compressive strength of samples loaded in the perpendicular direction to layers was higher, however, the creep resistance was better for composites loaded in the longitudinal direction. The microhardness of the composites correlated with the microhardness of reinforcing components.

Spark plasma sintering (SPS) was used to create a fine-grain ceramic based on nanosize powders of 14 different oxides (zinc, copper, chromium, cobalt, tungsten, magnesium, niobium, titanium, aluminum, yttrium, zirconium, and others). The microhardness of the ceramic obtained by SPS as well as by the conventional method is compared. The microhardness of the SPS ceramic is considerably greater than that of the conventionally obtained ceramic.

We present several possible microstructure development scenarios in Ti3SiC2-Cu composites during mechanical milling and Spark Plasma Sintering (SPS). We have studied the effect of in situ consolidation during milling of Ti3SiC2 and Cu powders and melting of the Cu matrix during the SPS on the hardness and electrical conductivity of the sintered materials. Under low-energy milling, (3–5) vol.%Ti3SiC2-Cu composite particles of platelet morphology formed, which could be easily SPS-ed to 92–95% relative density. Under high-energy milling, millimeter-scale (3–5) vol.%Ti3SiC2-Cu granules formed as a result of in situ consolidation and presented a challenge to be sintered into a bulk fully dense sample; the corresponding SPS-ed compacts demonstrated a finer-grained Cu matrix and more significant levels of hardening compared to composites of the same composition processed by low-energy milling. The 3 vol.% Ti3SiC2-Cu in situ consolidated and Spark Plasma Sintered granules showed an extremely high hardness of 227 HV. High electrical conductivity of the Ti3SiC2-Cu composites sintered from the granules was an indication of efficient sintering of the granules to each other. Partial melting of the Cu matrix, if induced during the SPS, compromised the phase stability and uniformity of the microstructure of the Ti3SiC2-Cu composites and thus it is not to be suggested as a pathway to enhanced densification in this system.

In this paper the ceramics obtained from various silica powders investigated. Two different methods for ceramics receiving (with consecutive pressing and sintering, and the method of hot-pressing (spark plasma sintering)) were tested. For ceramics obtained by different methods the microhardness, ultimate compression strength and grain size were compared. At a classical method of powders sintering the microhardness and durability growth is observed at reduction of the initial particles size though the observed size of grains in ceramics is approximately identical for all types of powders. It is received that vitrification of samples of ceramics created by the SPS method, happens at much smaller temperature (1100°C), than the corresponding temperature at consecutive operations of pressing and sintering (1550°C). Values of microhardness of the ceramics received by two various methods, were similar (~ 2 GPa). Ultimate compression strength value (0,25 GPa) for the samples received by a method of spark plasma sintering, only slightly exceeds the value of ultimate compression strength for the samples created in the traditional way (0,23 GPa). If at consecutive operations of pressing and sintering the ceramics with a micron size of grain turns out, at the SPS method the grains of received ceramics have the size about 300 nm. It speaks about prospects of nanopowders sintering by the method of spark plasma sintering at which nanostructural parameters of powder in the consolidated material remain

Production of welded bimetallic structures of titanium and steel using a laser beam is a very urgent and important task in the shipbuilding, airspace and power engineering. Laser welding using an intermediate insert is one of the ways to solve this problem. In this paper, we present the results of experimental studies of formation of the structure and properties of composite insert, obtained by explosion welding, after its application at laser welding steel with titanium. A study of a four-layer composite insert obtained by explosion welding showed that it has no brittle intermetallic phases and defects in the form of cracks and pores. The boundaries between the plates to be welded in the composite insert have a characteristic wavy structure with narrow zones of mutual diffusion penetration of elements of the adjacent metals. It is established that the strength of the composite insert is comparable with the maximum strength of Grade 4 alloy, and the destruction of the product during the tensile tests in m...