We justified theoretical and experimental bases of synthesis of new class of highly nanostructured composite nanomaterials based on metal matrix with titanium carbide nanowires as dispersed phase. A new combined method for obtaining of metal iron-based composite materials comprising the powder metallurgy processes and the surface design of the dispersed phase is considered. The following stages of material synthesis are investigated: (1) preparation of porous metal matrix; (2) surface structuring of the porous metal matrix by TiC nanowires; (3) pressing and sintering to give solid metal composite nanostructured materials based on iron with TiC nanostructures with size 1–50 nm. This material can be represented as the material type “frame in the frame” that represents iron metal frame reinforcing the frame of different chemical compositions based on TiC. Study of material functional properties showed that the mechanical properties of composite materials based on iron with TiC dispersed phase despite the presence of residual porosity are comparable to the properties of the best grades of steel containing expensive dopants and obtained by molding. This will solve the problem of developing a new generation of nanostructured metal (iron-based) materials with improved mechanical properties for the different areas of technology.
In the past two decades scientists have paid much attention to obtaining of composite nanomaterials based on nanoobjects with varying dimensions (nanoparticles, nanofilms, nanowhiskers, etc.) [ necessity for more precise control of nanoobject sizes (preferably in the range of 1–10 nm), for example, the most studied 3D nanoobjects (nanoparticles) which are dispersed phase for the structural composite nanomaterials. This is due to the fact that in most of the proposed methods for the synthesis there are obtained particles characterized by a fairly broad curve of particle size distribution [ necessity of development of precise methods for synthesis of a new generation of composite nanomaterials. If staying, for example, on one type of the composite nanomaterials with the metal matrix, the existing methods for producing such materials often include introduction of nanoparticles or nanowhiskers in the molten metal. Almost always particles in the solid (usually with different size) are not evenly distributed in metal matrix volume. This leads to a slight increase in the strength characteristics of composite metal nanomaterials [
Thus, when considering the preparation of new generation of nanostructured composite nanomaterials, we should talk about the material [
The solution of the above problem (synthesis of highly organized nanostructured materials) involves the creation of material with required level of structural organization. In general, we are talking about artificial “super control” in the solid as an alternative to natural regulation processes, mainly represented by the process of crystallization.
The crystal structure of a solid is characterized by two main parameters: structural units (from a single atom to big molecular objects) and ordering degree (from the amorphous state to monocrystal). Auxiliary parameters are needed for describing artificially synthesized highly organized structures and for characterizing these complex systems. As a rule, the task of any solid-phase synthesis is creation of a certain structure. There are several aspects of understanding the term “synthesis of a solid chemical compound” or its physicochemical analog “synthesis of a certain structure”: the physicochemical aspect includes consideration of composition and crystal (sometimes amorphous) structure; the topological aspect includes consideration of the spatial distribution of the composition and structure (actually, the spatial distribution of structural units of an object); the electronic aspect concerns electronic states of a solid substance under consideration.
The notion of topology implies a complex structure of matter. Therefore, we consider in more detail the notion of topology for highly organized solid compounds. This notion is related to the possibility of various spatial atomic distributions in an artificially synthesized substance [
From the viewpoint of topology, the limiting states are, on one side, a substance uniform in composition and structure (e.g., monocrystal, glass, etc.) and, on the other side, multielement (not in a chemical, but in a structural meaning) structure where each structural unit can be arbitrarily connected with others (e.g., biological objects). Artificial structures of the simplest topological organization (uniformly laminated structures, Figure
Scheme of two-dimensional nanostructures at silica surface. (a) Uniform distribution of chemical composition and structure, for example, monolayers of element-oxygen groups of a given composition: (
Further complication of topological organization is possible on organizing atoms also in the monolayer plane (for one or several monolayers) (Figure
In fact, we discuss the possibility of realizing the processes of superordering in a solid, that is, formation (synthesis) of ordered distributions of matter of a certain size. Let us compare the realization of natural ordering and artificial superordering in a solid. It is of note that macroscopically ordered distributions are sufficiently widespread in nature. As examples, macroscopic ordered distributions in plumeous clouds, in the radial structure of volcanoes, in ordered eutec-tics, and in sea sandbanks can be enumerated.
Since these examples are related to the structure of matter, the processes under consideration involve mass transfer (diffusion) and may follow the only direction to a uniform state of the system. The origin of such processes lies in that they are caused by thermodynamic driving forces and directed to extreme values of thermodynamic parameters, including the entropy maximum at equilibrium.
The closer a system to equilibrium, the more pronounced are internal self-ordering processes in the system, the easier is maintaining physicochemical conditions, and the more difficult is changing them by external action. Hence, as we approach equilibrium, the range of allowed nonuniform structures becomes narrower, while uniform structures become simpler to synthesize. The reverse situation takes place far from equilibrium: a very precise external control is needed, but almost all structures are allowed. The choice of an optimal synthetic strategy is one of the most intricate problems.
Thus, the only way to create an aperiodic order consists in changing the ordering processes described by equilibrium thermodynamics and directed to securing system stability by a synthetic program directed to obtaining highly organized solid-phase structures in the metastable state.
Because of the low mobility of atoms in a solid, a certain thermodynamically unfavorable structure (e.g., diamond) can exist for any long time.
It is now firmly established that the synthesis of solid chemical compounds by crystallization can result in formation, from the same structural units, of a number of equivalent variants of composition and structure with close energies (actually, this is an unseparable mixture of variable composition). This occurs since the solidification process always proceeds at a certain supersaturation [
It is of note that chemical processes on the atomic-molecular level should play the main role in the synthesis of nanostructures with a small period and a given topology. Then all the processes are reduced to a certain sequence of surface chemical reactions between functional groups of the solid and molecules of necessary chemical nature [
There is an important aspect to be used at realizing the solid-phase matrix synthesis. With microcrystal seeds, the main role is played by the surface of a solid, since physicochemical processes just develop at the surface. It is of note that thermodynamic and kinetic barriers are not eliminated but only diminished by introduction of a seed (substrate) in a system. Separate relative equilibrium is established for each substrate under given conditions. Traditional methods of epitaxial synthesis (deposition) are controllable processes. However, it is hard to speak about obtaining a nanometric substance layer of a given thickness by the epitaxial deposition method because epitaxial layers are formed by way of appearance and integration of crystal nuclei whose dimensions certainly exceed the monolayer thickness.
Additional difficulties in realizing the synthesis of solid substances arise from the fact that the overwhelming majority of reactions used in chemical science and technology belong to the type of “unorganized” reactions. We mean the reactions with particles (atoms, molecules, ions, or radicals) reacting at accidental meetings both in time and in space and by mutual orientation. In other words, there is no spatiotemporal molecular and solid-phase organization of chemical interaction in such reactions. At the same time, the possibility of such an organization is known from molecular biology, for example, the biosynthesis process [
The problem of a standard solid surface remains actual in synthesis of nanostructures. Let us consider the principles of synthesis of solid substances of reproducible composition [
When performing controlled solid-phase synthesis, a researcher actually carries out a number of operations to obtain a solid chemical substance possessing (a) a certain excess of free energy contained in the system of interatomic bonds, as compared with the initial reagents, and (b) a higher degree of ordering of interatomic bonds as compared with the initial reagent mixture at equilibrium.
Practically, the reproducible synthesis of nanostructured solid substances is realized via chemical assembling of structural units on appropriate matrices. The monolayer-by-monolayer chemical assembling of solid substances involves a number of preprogrammed surface reactions with at least bifunctional molecules of either one substance or another.
The principal significance of this method of synthesis of solid matter is “that, instead of spontaneous packing structural units in the crystallization process, their forced packing in a preset order is performed, that is, preset composition and structure of the solid substance are realized” [
We now turn to chemical reactions performed in the process of synthesis. The progress in the field of precise inorganic synthesis is associated with the rapid development of surface chemistry. This branch of the inorganic chemistry of solid substances studies chemical reactions in the surface atomic layer of a solid substance and also studies the ways controlling these reactions [
In order to solve our problem—finding the influence of one-dimensional (nanowires, 1–50 nm) structural heterogeneities basis on TiC, amorphous or crystalline structure on the structure, and functional (mechanical) properties of bulk metallic material (metallic matrix)—it is necessary to solve the problem of substance structuring at the nanolevel. The main theoretical and experimental positions of the chemical nanostructuring are considered in [
Practically, the reproducible synthesis of nanostructured solid substances is realized via chemical assembling of structural units on appropriate matrices. The monolayer-by-monolayer chemical assembling of solid substances involves a number of preprogrammed surface reactions with at least bifunctional molecules of either one substance or another [
The main purpose of this work is to establish experimental fundamentals of the synthesis of highly organized nanostructured composite materials with the TiC dispersed phase which will directly regulate the mechanical properties of composite material.
As noted above for the solution of our problem—finding the influence of one-dimensional (nanowires, 1–50 nm) structural inhomogeneities based on TiC on the structure of and functional (mechanical) properties of metal type (iron matrix) bulk material—we should solve the problem of structuring material at nanoscale. In this case, the most important phase of the nanostructuring of matrix is the surface chemical reaction of force structuring. Thus the basic mechanical properties of obtained material are provided by the creation of nanophase dispersion (TiC) in the volume of the metal matrix. Thus, the original metal matrix must be porous. The synthesis of nanolayers of the dispersed phase (TiC) in the pores of the matrix was based on developed in St. Petersburg State University method of directed synthesis—a method of molecular layering (ML), known abroad under the name “atomic layer deposition” (ML-ALD).
Process of preparation of nanostructured metal material includes a number of successive steps: (1) preparation of nanoparticles of iron oxide; (2) reducing iron oxides (by hydrogen or other reducing agents) in order to obtain ultrafine iron metal frame with a predetermined porosity; (3) surface nanostructuring of prepared metal porous matrix by TiC nanolayers (1–50 nm); (4) consolidation (pressing) and sintering to obtain a bulk (nonporous) material with nanostructured metal material with size of TiC nanowires (1–50 nm). The third step of the synthesis involves a fundamentally new scientific approach for the development of scientific bases of ultraprecise synthesis, that is, combined chemical and physical processes leading to the creation of a given order of the atoms in synthesized structure or nanostructured material.
Following each cycle of surface chemical reactions (SCR) carried out under strictly controlled conditions on the solid surface, there is a deposition of monolayer of new structural building units (carbides) chemically associated with the original substrate and the “thickness” of a monolayer is 2-3 Å. Carrying out a number of these reactions a layer of substance with a certain thickness can be synthesized.
Chemicals and materials were obtained from commercial suppliers: TiCl4 (99.6%), (Alfa Aesar), carbon tetrachloride (99%) (Sigma-Aldrich), FeSO4·7H2O, NaOH, HNO3 (Merck), and CH4 (gas) (Lengaz).
We presumed that for obtaining of metal particles with high dispersion need to reduce their oxides at lower temperatures [
Specific surface and average size of aggregates of fine dispersed iron reduced at different temperatures.
Initial oxide | Reducing temperature, °C |
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FeOOH dried at 25°C | 400 | 9,7 | 82 |
450 | 7,2 | 109 | |
500 | 4,5 | 173 | |
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FeOOH dried at 110°C | 400 | 8,4 | 91 |
450 | 6,6 | 116 | |
500 | 3,7 | 206 | |
550 | 2,1 | 381 |
XRD spectrum of iron samples obtained by reduction of sample 2 in hydrogen flow at 400°C.
The dependence of the specific surface of iron on reduction temperature: (1) iron powder obtained from FeOOH dried at 25°C, (2) iron powder obtained from FeOOH dried at 110°C.
Thus, it can be assumed that the production of dispersed iron hydroxides by heat treatment of initial iron hydroxides in a reducing atmosphere is an optimal process because it allows to obtain a dispersed metal in comparison with the metal reduced from oxide.
Study of uniaxial compaction of iron powders with a particle size of 60 nm obtained by reduction of iron hydroxide in a hydrogen atmosphere was carried out in a cylindrical mold having an inner diameter 15 mm; compact height was 5–7 mm. Pressure was varied from 0.05 to 1.1 GPa. The density of compacts was determined by hydrostatic weighing with an accuracy of 2%. The most intense compaction of powders occurs at pressures of up to 0.8-0.9 GPa.
As a result, the optimal pressing conditions were determined as follows for iron nanopowders weight of powder—9 g, size of obtained sample: height (
As a result, the regulation conditions of residual porosity of metallic sample were found in the range from 1 to 35%. Such porosity is required for the carrying out of surface chemical reactions on the iron particles surface.
Synthesis of titanium-carbon groups and titanium-carbon nanostructures on the iron surface was carried out on the hydroxylated surface of samples by ML-ALD. In this paper objects of study on which we investigated the surface chemical reactions were: (1) modeling samples—monocrystalline silicon wafer (KDB-7, 5 with orientation (100)) with deposited layer of
Since the surface of the iron particles is always oxidized and thickness of the oxide layer is not known, then all the investigated objects before the synthesis are treated (chlorinated) by vapor CCl4 at 350°C to remove the oxide layer. Interaction reactants with the iron powder surface by ML-ALD were performed in a flow-type reactor under dry inert gas flow (helium) which simultaneously ensures removal of gaseous reaction products from the reactor. For the analysis of chemical reactions on the silicon plates with iron layer we used thermogravimetric setting in which analytical electronic scales BP 221S were used to register the change in sample weight. Quartz cup with the test substance (silicon plate with a layer of iron) is attached to scales with quartz fiber. In some experiments method of chemical vapor deposition (CVD) is used to obtain titanium carbide from the gas phase.
To study the mechanical strength of the samples of metal (iron-based) composite materials nanostructured titanium carbide, ultimate stress limit-ultimate resistance (
Synthesis of titanium-carbon groups and titanium-carbide nanostructures was performed by ML-ALD on chemically prepared surface of iron sample, that is, on a surface which contains reactive functional groups (OH, OCH3, H, Cl, or others).
In the study objects, on which the surface chemical reactions were studied are the model samples-plates of single-crystal silicon with deposited layer of dispersed iron particles obtained by the method described above; nanoscale dispersed iron particles characterized by the following characteristics: a particle size of 40–75 nm, specific surface of the particles—6 m2/g; porous pressing from iron particles as starting intermediates for the synthesis of titanium-carbon nanolayers in the pores of the metal matrix.
To clarify the mode of deposition of titanium carbide nanolayer, model samples representing monocrystal silicon wafer with a sputtered layer of pure iron were used.
When carrying out the surface chemical reactions their behaviors are dependent on temperature and holding time in the reaction zone. Note that the reaction occurs in the range 300–700°C. Our proposed method of synthesizing of titanium carbide by ML-ALD is based on reactions of chemical condensation (
The synthesis was carried out on silicon wafers with a sputtered film of pure iron with thickness of ~10 nm. The sample was alternately treated at 500°C in a flow-type reactor by carbon tetrachloride and methane.
Cycle is repeated again when thickness increasing is needed. Then the reactor with sample was cooled in the hydrogen flow. To determine the coating continuity of functional groups (–CH3) on the iron surface the water contact angles were studied. The study of contact angle was conducted by photomicrograph method. Based on a study of water contact angle of samples (initial silicon with hydroxyl groups, silicon with methyl groups and silicon with deposited titanium-carbon layers), regular increase of water contact angle from 43° to 76° and 94° was identified which indicates the transition to higher packing density of surface groups in the case of reactions involving methyl (–CH3) group (Figure
The values of contact angles by water for silicon samples with iron layer with different functional groups: (1)
Data on the chemical analysis of titanium have concluded (Figure
The dependence of Ti content in nanolayer from synthesis temperature.
Decline of the titanium content at warming temperature of 450°C can be associated with the thermal decomposition of surface groups. Apparently, in these conditions the interaction TiCl4 with the surface leads to dissociative process: TiCl4 → TiCl3 + Cl2, resulting in chlorine chlorinating the surface to form complex C–Cl that blocks it from further reaction with TiCl4.
Macrohardness study and thin section structures of the synthesized samples (Table
Macrohardness and characterization of the microstructure of samples obtained by alternating exposures of titanium tetrachloride-methane.
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Number of cycles of ALD reactions | HV, kg/mm | Description of microstructures |
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200 | 1 | 84 | Absence of reaction by XRD, nonuniform film |
2 | 82 | ||
4 | 150 | ||
5 | 144 | ||
6 | 210 | ||
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300 | 1 | 80 | Absence of reaction by XRD, nonuniform film |
2 | 63 | ||
4 | 120 | ||
5 | 380 | ||
6 | 465 | ||
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400 | 1 | 340 | Uniform fine-grained structure |
2 | 303 | ||
4 | 690 | ||
5 | 1302 | ||
6 | 1280 | ||
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450 | 1 | 684 | Uniform fine-grained structure |
2 | 905 | ||
4 | 1010 | ||
5 | 1260 | ||
6 | 1200 | ||
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500 | 1 | 204 | The appearance of chlorine excess which is manifested in the nonuniform structure |
2 | 660 | ||
4 | 785 | ||
5 | 550 | ||
6 | 930 | ||
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550 | 1 | 340 | Increasing heterogeneity of structure centers |
2 | 280 | ||
4 | 833 | ||
5 | 535 | ||
6 | 650 |
The study of dependence of titanium content on silicon samples with metallic iron layer from treatment showed that the growth dependence of the titanium content is linear and after 5-6 cycles the titanium content increasing becomes constant. Note that the optimum concentration of the hydrogen significantly increases the rate of titanium carbide formation. This is probably due to the suppression of the reaction of free carbon formation.
Obtaining of titanium carbide on the surface of dispersed iron was carried out on particles with size 40–80 nm and specific surface area 6 m2/g. Treatment of iron powder in a quartz reactor was performed in a flow of dry argon.
To study the thickness effect of the deposited layer of titanium carbide on the mechanical properties of the samples, obtaining of ultrafine micron films of titanium carbide by chemical vapor deposition (CVD) and nanolayers of titanium carbide by atomic layer deposition (ML-ALD) was investigated. Treatment of iron powder was performed in a quartz reactor in dry argon flow.
In gas phase deposition of titanium carbide formation of continuous frame in the pores of metal workpiece begins with the formation of island structures on the surface. This character of the formation is associated with the feature of the reaction of chemical deposition of titanium carbide as surface is energetically heterogeneous. This leads to different rates of growth of the titanium carbide on different sites and forming island structures—three-dimensional nucleus of titanium carbide [
Structure of the samples after chemical deposition (CVD) of titanium carbide at sequential supplying of titanium tetrachloride and methane mixture.
Diagram of the samples after process of atomic layer deposition with the cyclic supply of titanium tetrachloride and methane. (a) Initial surface of dispersed iron particles with methyl functional groups after exposure CH4; (b) exposure of titanium tetrachloride giving titanium-chloride groups; (c) exposure of methane giving titanium-carbon groups; (d) and (e) repeated sequential exposures of titanium tetrachloride and methane, respectively (reactions (
Feature of nanolayers obtained by ML-ALD is the absence of phase-formation (nucleation) under maintenance of certain synthesis conditions which allows more fine-tune of the functional properties of the resulting material.
As a result of the synthesis there were synthesized examples of iron powder with different quantity of titanium carbide deposited on the surface. The content of titanium-carbon groups in the sample was determined by the titanium content. Analysis of titanium and carbon was performed by chemical method (photometrically). Data of the chemical composition of samples are given in Table
Chemical composition of samples based on the iron particles with titanium-carbon groups.
Sample | Number of cycles processing | Composition of the sample | Estimated thickness (nm) |
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1 | — | 100% Fe | — |
2 | 1 | 95,5% Fe + 1,5% TiC | 0,5 |
3 | 2 | 97% Fe + 3% TiC | 1 |
4 | 10 | 90,5% Fe + 12,5% TiC | 5 |
5 | CVD | 84,5% Fe+ 19,5% TiC | 15 |
6 | CVD | 74,5% Fe + 35,5% TiC | 40 |
Observance of conditions of ALD reactions allows obtaining monolayers with certain chemical composition; the number of performed reactions controlled the thickness of the deposited layer of substance. Features of layers produced by chemical assembly are the absence of phase formation (nucleation) under certain synthesis conditions which allows to regulate more fine the functional properties of the obtained material.
Figure
As it follows from Figure
From the X-ray diffraction analysis data (Figure
XRD spectrum of samples of dispersed iron with titanium-carbon nanostructures after treatment with 10 cycles of ALD reactions and sintered at 1100°C.
Analysis of the compaction and sintering of metallic iron samples nanostructured by nanowires TiC allows finding the conditions for obtaining metallic samples with low porosity. To achieve the lowest porosity (5-6%) the sintering of compacts is necessary to be carried out by heating from 500 to 800°C for 2 hours with intermediate equalizing at 800°C in hydrogen for 3 hours.
Mechanical properties of nanomaterials essentially depend on the grain size. At large grain sizes a strength and hardness growth with decreasing grain size is determined by introduction of additional grain boundaries that are obstacle to dislocation motion and at small nanoscale grains a strength growth is determined by low density of existing dislocations and the difficulty of formation of new dislocations [
To study the mechanical strength of the samples of metal (iron-based) composite materials nanostructured titanium carbide ultimate stress limit-ultimate resistance (
The study of mechanical properties of the synthesized composite materials based on iron with dispersed phase of TiC.
Sample | Composition | Tensile strength, |
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Obtained samples | ||
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MN-ALD method | ||
1 | 100% Fe | 183 |
2 | 95,5% Fe + 1,5% TiC | 800 |
3 | 97% Fe + 3% TiC | 1130 |
4 | 90,5% Fe + 9,5% TiC | 1370 |
5 | 84,5% Fe + 15,5% TiC | 1460 |
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CVD method | ||
6 | 69,5% Fe + 30,5% TiC | 890 |
7 | 61,5% Fe + 38,5% TiC | 900 |
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Literature data [ | ||
7 | Steel AS 14CGN | 1120 |
8 | Steel 50CG | 1300 |
9 | Steel 12C2N4A | 1130 |
10 | High-strength steel | 1500 |
It should be borne in mind that the samples have porosity, that allows to improve the mechanical properties of specimens.
Thus we can assume that the creation of metallic (iron-based) composite materials nanostructured titanium carbide using surface structuring process is promising for producing of new generation composite materials.
Through a nanostructuring process of metal (iron) matrix developed by authors the synthesis of samples of nanostructured metal matrix composite with nanostructural (1–50 nm) inhomogeneities on the basis of TiC with the crystal structure in the volume of the iron matrix was reformed.
The synthesis, pressing, and sintering conditions of dispersed iron nanoparticles (40–75 nm) with a surface area of 6 m2/g were investigated.
The features of the behaviour of surface chemical reactions of functional groups on the metallic iron surface with low molecular substances (TiCl4, CH4) were studied and it was found that methyl (CH3) groups are most suitable for the synthesis of carbide nanostructures in inert environment compared with hydroxyl (OH) or methoxy (OCH3) groups.
A method of synthesis of titanium carbide nanostructures on the surface of dispersed iron particles using methyl (CH3) functional groups, TiCl4, and CH4 is developed.
The sequence of synthesis steps of titanium-carbon nanostructures on the surface of the dispersed iron particles, concentration and temperature ranges at which the synthesis is carried out, and the temperature intervals of sample heating were established. Based on X-ray analysis it is shown that in a cyclic process of exposure of TiCl4 vapor and CH4 on dispersed iron titanium-carbon nanolayer with amorphous structure is formed at the surface. Crystallization and the formation of titanium carbide are carried out after calcinations at 1100°C.
Investigation of sintering and pressing conditions of dispersed iron particles with deposited titanium carbide nanostructures allows to determine the conditions for obtaining a metal (iron-based) composite material with low residual porosity with a dispersed phase based on titanium carbide.
Mechanical properties of the metal (iron-based) composite material with dispersed phase based on titanium carbide are studied and it is found that it is comparable with the best grades of steel obtained by moulding.
The results suggest that the creation of metallic (iron-based) composite materials structured by titanium carbide nanostructures using the process of surface structuring is promising for the preparation of a new generation of composite materials.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Ministry of Education and Science of the Russian Federation, the agreement of 8575 from 13.09.2012.