The Young's modulus, internal friction, and microplastic flow stress in Cu-Nb nanolaminate has be... more The Young's modulus, internal friction, and microplastic flow stress in Cu-Nb nanolaminate has been determined by an acoustic technique. The influence of high hydrostatic compression (1 GPa) on these elasto plastic properties of the nanolaminate has been studied.
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. Abstract The dissipation properties, evolution of the dilatation and elastic properties in sub-microcrystalline titanium Grade 4 samples loaded in HCF and VHCF regime were investigated. The dissipation properties were studied based on self-heating (Risitano) test. The mechanical properties (elastic modulus, amplitude-independent damping) and dilatation for a set of samples with different degrees of the life time exhaustion were studied based on the acoustic resonance method. The dilatation was studied by the method of hydrostatic weighing. The experimental results show the increasing of energy dissipation in submicrocrystalline titanium and absence the characteristic knee point on the graph of dependence of temperature rise versus stress amplitude for this material in self-heating test. The structural investigation shows the decreasing of elastic properties during fatigue loading and increasing of dilatation.
Mechanical stability under prolonged loading and thermostability under annealing have been studie... more Mechanical stability under prolonged loading and thermostability under annealing have been studied for nano-and microcrystalline titanium obtained by different methods of intense plastic deformation. The effect of nanoporosity and the fraction of high angle boundaries formed due to intense plastic deformation has been revealed and analyzed. It has been established that, depending on the loading or the annealing temperature, thermomechanical stability of titanium can be affected, apart from the above structural characteristics , by either twin grain boundaries or titanium-carbide disperse particles.
Results of small angle X ray scattering and high precision density measurements showed that the a... more Results of small angle X ray scattering and high precision density measurements showed that the application of counterpressure during the equal channel angular pressing (ECAP) of ultrafine grained cop per leads to a decrease in nanoporosity and an increase in mechanical properties of the ECAP processed metal.
The effect of nanoporosity and the highly dispersed carbide particles that form during screw and ... more The effect of nanoporosity and the highly dispersed carbide particles that form during screw and lengthwise rolling of VT1 0 titanium on its mechanical and, partly, thermal stability characteristics is revealed and analyzed.
The regularities of the influence of the number of passes under equal channel angular pressing on... more The regularities of the influence of the number of passes under equal channel angular pressing on the mechanical properties and defect structure of an aluminum alloy have been elucidated. It has been estab lished that the degradation of the mechanical properties (a decrease in the durability) is associated with the formation of nanoregions of an excess free volume in the course of severe plastic deformation under equal channel angular pressing. A correlation between the nucleation of excess free volume regions and the forma tion of high angle grain boundaries under equal channel angular pressing has been revealed. The nature of the influence of severe plastic deformation on the elastic modulus, the vibration decrement, and the micro plastic flow stress has been analyzed.
Transmission electron microscopy data showed evidence of the formation of structural regions corr... more Transmission electron microscopy data showed evidence of the formation of structural regions corresponding to deformation (dislocated) fragments and dynamically recrystallized grains in α-phase titanium upon torsion at high hydrostatic pressure at room and cryogenic temperatures. It is shown that the previously proposed "two-phase mixture" model is applicable to description of these defect structures. The strength of pure metals, including titanium, can be increased up to a level characteristic of heavily doped alloys by means of various extremal treatments of the bulk material, such as megaplastic deformation (MPD) and deformation at very low (cryogenic) temperatures. A widely used method of obtaining giant deformations is based on high-pressure torsion (HPT) in the Bridgman chamber [1]. Specific features of high plastic deformations in metals and alloys are the formation of fragmented structures consisting of misori-ented fragments with submicron dimensions [2] and simultaneous dynamic recrystallization processes at room temperature [3]. At present, no reliable literature data are available concerning comparative statistical analysis of the evolution of defect structure in polycrystalline α-Ti under the action of HPT at various (from room to cryogenic) temperatures. The interest in studying structural transformations in titanium and related alloys is stimulated by the demand for high-strength titanium-based materials in aerospace technologies. The present work was aimed at a detailed investigation of evolution of the defect structure in polycrystal-line α-Ti under the action of HPT in ta Bridgman chamber at various magnitudes and temperatures of deformation, based on the statistical analysis of dark-field images obtained by transmission electron microscopy (TEM). The study was performed on grade VT1-0 commercial-purity titanium (α-Ti) in which the structure formation during plastic deformation is characterized by a minimum influence of impurity atoms and/or foreign phases. The samples made from 50-μm-thick plates were deformed by HPT in the Bridgman chamber at room temperature (293 K) and at liquid nitrogen boiling temperature (77 K) at a rolling anvil rotation speed of 1 rpm under constant quasi-hydrostatic pressure of P = 6 GPa to N = 1/4, 1/2, 1, 2, 3, and 4 full continuous revolutions of the mobile anvil, which corresponded to true deformation in the interval of e = 5.5-8.3 [1]. The structure of samples was studied by TEM on JEM 200CX instrument operated at an acceleration voltage of 160 kV. All investigations of the local structure were performed in regions located at about half-radius of disk samples. The microstructure of titanium upon HPT at 293 and 77 K represents a mixture of α and ω phases. The ω phase regions have irregular shapes with the characteristic fringe contrast inside particles [4]. Detailed TEM analysis of the microstructure of α-Ti at all HPT stages at different temperatures clearly revealed separate grains shaped as almost regular hexagons (Fig. 1a, grains 1 and 2). A low density of dislocations inside these grains was obviously indicating that they were formed as a result of dynamic recrystallization during HPT. This kind of recrystallization (referred to as "continuous" [5]) can be considered as a process of relaxation under conditions of significant gradients of
It was established that when loading ultrafine-grained metallic materials under creep conditions,... more It was established that when loading ultrafine-grained metallic materials under creep conditions, the evolution of the "original" nanopores exerts a significant influence on the longevity. It is shown that in the process of creep the grains recrystallize, and the role of their boundaries in the transition from nano to microporosity is revealed.
It is established that increases in nanoporosity and the proportion of high-angle grain boundarie... more It is established that increases in nanoporosity and the proportion of high-angle grain boundaries in the process of equal-channel angular pressing are the main structural factors leading to reduction in mechanical stability (durability) of microcrystalline titanium during long-term tests under creeping conditions. The wide area of application of titanium is evoking increased interest in the study of the structure peculiarities of this metal in its high-strength microcrystal-line state formed as a result of severe plastic deformation (SPD). It is known that the high mechanical properties after SPD are determined basically by the size of the grains and condition of their boundaries. Reduction of the size of grains during SPD leads to an increase in the volume fraction of their boundaries, where high concentrations of defects (dislocations, vacancies, nanopores, etc.) and high internal stresses are localized [1, 2]. Because of this, the nano-and microcrystalline metals produced by SPD are inherently nonequilibrium, and so the problem of their mechanical stability, especially during prolonged stress, is important both for fundamental research and in terms of application [3]. In [4, 5], it was shown that a significant influence on the mechanical stability is exerted by two structural factors: nanoporosity formed during SPD and high-angle boundaries (ϕ > 15°) causing a high level of internal stress. In this paper, we investigate the contribution of these two factors to mechanical stability (durability during the test in the creeping mode) of microcrystalline titanium produced by equal channel angular pressing (ECAP). VT1-0 titanium with an impurity content ≈0.3% was selected for the study. The ECAP was performed by route B c with cyclic rotation of the workpiece around the axis of the channel by 90° after each cycle and the angle of intersection of the channels of 120° at 673 K [6]. For mechanical testing, samples were used that had been prepared after different numbers of ECAP passes. The samples had a length of the uniformly deformable part of 15 mm with a cross-sectional area of 3 × 2 mm 2. The prepared samples were tested at T = 673 K and σ = 15 MPa until rupture, and the time to failure (durability) was determined. Additionally the micro-hardness and its variation depending on the number of passes in ECAP were determined on initial (before testing in creep conditions) samples. The density of the samples and its change during ECAP caused by pore formation, among other things, were determined by triple hydrostatic weighing. The parameters of the pores were found with the help of a modified method of X-ray scattering in the field of ultralow angles using a high (1.5 GPa) hydrostatic pressure for identifying the void nature of the scattering inhomogeneities [7]. The sizes of grains and their distribution by disordering were determined using transmission and scanning electron microscopy and backscattering of electrons. Consider the obtained experimental data. It was established that the densities of samples of titanium in the original (before ECAP) state and after two, four, and eight passes were 4.5127 ± 0.0003, 4.5117 ± 0.0005, 4.5060 ± 0.0006, and 4.5100 ± 0.0005 g/cm 3 , respectively. Thus, there is a clear trend toward growth "loosening" of the titanium (determined by the level of nanoporosity, among other factors) with an increase in the number of passes. The effect of high hydrostatic pressure, as studies have shown, leads to a significant increase of density. For example, after four passes in ECAP, the density increased from 4.5065 to 4.5100 g/cm 3 because of the applied hydrostatic pressure. The action of hydrostatic pressure allowed us to identify the nature of the increased intensity of the small angle scattering that occurs after ECAP (Fig. 1, curves 1 and 2). It is seen that the intensity of the scattering is markedly reduced after the action on samples
The variation of the internal friction, Young's modulus, and electrical resistivity of two grades... more The variation of the internal friction, Young's modulus, and electrical resistivity of two grades of polycrystalline titanium (VT1-0 and Grade 4) in the area of low temperatures (100-300 K) as depending on the initial structure and subsequent severe plastic deformation converting the material into the submicrocrys-talline structural state in relation to the grain size is studied. The maximum of the internal friction is detected in submicrocrystalline titanium, which is interpreted as a Bordoni peak. All the studied characteristics are sensitive indicators for a nonequilibrium state of the grain boundaries after the deformation. The effect of the initial structure of the metal on its properties after the severe deformation is revealed.
Multiscale mechanisms of failure of metals (Armco iron, titanium, aluminum) are studied for high... more Multiscale mechanisms of failure of metals (Armco iron, titanium, aluminum) are studied for high cycle and very high cycle fatigue. By correlating with the results of structural studies, a theoretical approach is developed to describe fatigue crack kinetics in damaged material under high cycle and very high cycle fatigue loading conditions. Stages of crack nucleation and propagation are analyzed using the profilometry data from the fracture surface. The scale invariance of fracture surface roughness is established, which allows an explanation of the self-similar nature of fatigue crack kinetics under high cycle and very high cycle fatigue. Variation of elastic-plastic properties of Armco iron under very high cycle fatigue is studied using an acoustic resonance method. It is found that the material density decreases during fatigue damage accumulation, with the minimum of the material density in the bulk of the specimen.
The behavior of elastic (Young's modulus) and microplastic properties of titanium depending on th... more The behavior of elastic (Young's modulus) and microplastic properties of titanium depending on the initial structure and subsequent severe plastic deformation that transforms the material (concerning the grain size) into the submicrocrystalline structural state has been studied. It has been shown that, to a great extent, different initial structures of the metal predetermine its elastic properties after deformation.
The variation of interatomic interaction energy during allotropic transition in titanium is estab... more The variation of interatomic interaction energy during allotropic transition in titanium is established by analyzing the stress and temperature dependences of durability. This energy is ≈458 kJ/mol in body-centered β-Ti at a temperature of T > 1155 K, which agrees well with the titanium sublimation heat, and ≈305 kJ/mol in hexagonal close-packed α-Ti at T < 1155 K. Literature data confirming the conclusions drawn are discussed. At present, titanium and titanium-based alloys are finding wide application. Therefore, predicting the strength and mechanical stability of titanium is an urgent problem [1-5]. In the kinetic approach to strength [6], this problem cannot be solved without studying the dependence of durability, i.e., time τ from the instant of loading to material fracture, on stress σ and temperature T. Analysis of this dependence elucidates the physical origin of the process that leads to mechanical-stability loss and fracture. In a wide class of materials, including metals, composites, and polymers, this dependence in a certain range of external factors σ and T has the form [6] (1) where k is the Boltzmann constant; τ 0 ≈ 10-13 s for all solids, which coincides with the atomic thermal vibration period; and the factor at stress γ is the only structure sensitive coefficient. In analyzing Eq. (1), of special importance is the initial (before applying stress σ) energy barrier U 0. It was established that, in all the investigated metals, the U 0 value coincides with good accuracy with their sub-limation heats quantitatively characterizing the inter-atomic interaction. For each metal, quantity U 0 , similarly to the sublimation heat, was found to be a constant value independent of the structural state (grain size, impurity content, etc.) [6]. Titanium, similarly to other metals that are important for modern engineering (Co, Zr, and Fe), undergoes allotropic transformations upon temperature variations, which can noticeably affect its properties, including the interatomic binding energy. This Letter estimates the U 0 value and, consequently , interatomic binding energy of titanium with a body-centered cubic (bcc) lattice at T > 1155 K and hexagonal closed-packed (hcp) lattice at T < 1155 K. The investigations were carried out mainly on commercial titanium BT1-0 with an impurity content of ~0.28% and only for β-Ti. Iodide Ti of higher purity (99.9%) was additionally used. The mechanical tests were performed upon uniax-ial extension in the creep mode at the durability variation by more than six orders of magnitude (from 10-1 to 3 × 10 5 s). The technique for measuring small (<1 s) durabilities was described in [6]. It should be noted that the necessary condition for correct estimation of the activation energy using the phenomenological data is the identity of the structural elements that decisively affect the durability of the samples tested at different σ and T values [7]. Here, such a structural element is the grain size. To stabilize it, all the samples were annealed at a temperature higher than the maximum experimental temperature before the durability test. In particular , the α-Ti samples were annealed at 925 K for 40 h and the β-Ti samples at 1323 K for 1 h. Annealing and subsequent durability tests were performed in vacuum. Let us consider first the experimental data on α-Ti. Figure 1 shows the stress and temperature dependences of durability. It can be seen that in the coordinates logτ-σ at different T = const and in coordinates logτ-1/T constructed on the basis of the data presented in Fig. 1a for different σ = const, the dependences of logτ on σ and 1/T are lines converging at the extrapolation in the pole. In this case, as follows from [6], the pole in the coordinates logτ-1/T lies on the () γσ τ = τ − 0 0 exp , U kT
The work is devoted to the study of the damage accumulation in iron under gigacyclic fatigue (VHC... more The work is devoted to the study of the damage accumulation in iron under gigacyclic fatigue (VHCF) regime. The study of the mechanical properties of the samples with different state of life time existing was carried out on the base of the acoustic resonance method. The damage accumulation (porosity of the samples) was studied by the hydrostatic weighing method. The obtained results show the accumulation of porosity in the volume of the sample during fatigue loading and corresponding decrease of the elastic properties. A statistical model of damage accumulation was proposed in order to describe the damage accumulation process. The model describes the influence of the sample surface on the location of fatigue crack initiation.
This research was focused on a new low-modulus-type titanium alloy Ti 26Nb 7Mo 12Zr (wt.%). The m... more This research was focused on a new low-modulus-type titanium alloy Ti 26Nb 7Mo 12Zr (wt.%). The microstructure effects on elastic modulus (measured by the acoustic resonance method) as well as microplastic, mechanical, tribological, and corrosive properties of Ti 26Nb 7Mo 12Zr alloy after thermomechanical processing were examined. The microstruc-ture was characterized in detail by scanning electron microscopy and electron backscatter dif-fraction methods. The experimental research results have shown that formation of the fully re-crystallized structure in the titanium alloy leads to an increase in elastic modulus, microplastic flow stress and plasticity, as compared to the corresponding characteristics of the alloy having partially recrystallized and coarse-grained structures. The durability of titanium alloy was examined and compared with that of commercially pure titanium (CP Ti). It was found that, in the same creep loading conditions, the low-modulus Ti 26Nb 7Mo 12Zr alloy exhibits a longer time to creep fracture, as compared to the pure titanium.
The problem of application of physical acoustic methods to studying the mechanisms that control p... more The problem of application of physical acoustic methods to studying the mechanisms that control plastic deformation and fracture is considered using micro-and submicrocrystalline materials (Be, Al, Ti, Al-Sc alloy, Cu-Nb laminated material) as examples. The influence of grain boundaries on the acoustic (elastic, inelastic) properties of polycrystalline micro-and nanostructured metallic materials is analyzed. Experimental results are presented for a wide oscillating-stress amplitude range, from 0.2 to 50 MPa. The experimental data are discussed in terms of the theoretical concepts of oscillatory dislocation mobility, which depends on both the short-range stress fields around point defects and the long-range fields of internal stresses. It is shown that various types of discontinuities, such as pores and microcracks, noticeably influence the acoustic properties. The aspects of the relation, similarity, and difference between acoustic and mechanical (plasticity, strength) tests of polycrystalline materials with micro-and nanosized structural elements are discussed.
The influence of deformation by equal channel angular pressing (ECAP) on the durability of tita n... more The influence of deformation by equal channel angular pressing (ECAP) on the durability of tita nium (VT1 0 grade) tensile tested under creep conditions has been studied for the first time. It is established that the ECAP induced transition of titanium to a microcrystalline state leads to a decrease in the durability, while the characteristics of static strength are improved.
The effect of thermobaric treatment on the nanoporosity (excess free volume) and thus on the phys... more The effect of thermobaric treatment on the nanoporosity (excess free volume) and thus on the physical and mechanical properties of some amorphous alloys obtained by extra rapid quenching is studied.
A broad spectrum of physicomechanical properties of VT1 0 nanocrystalline titanium produced by cr... more A broad spectrum of physicomechanical properties of VT1 0 nanocrystalline titanium produced by cryomechanical fragmentation of the grain structure using rolling at a temperature close to liquid nitrogen temperature has been studied. The mechanism of grain refinement has been associated with grain fragmen tation by twins. Exactly the twin nature of internal interfaces (crystallite boundaries) provides the thermal and structural stability of nanocrystalline titanium produced by cryomechanical grain fragmentation in the tem perature range to ~500 K. It has been assumed that the observed decrease in the titanium density due to cry orolling is associated with a number of factors (high density of introduced dislocations, nanopore formation, and changes in titanium lattice parameters).
We propose a model describing the kinetics of accumulation of defects under cyclic loading in met... more We propose a model describing the kinetics of accumulation of defects under cyclic loading in metals. Analysis of experimental data on the initial defect distribution and of the role of the free surface of the sample in straining makes it possible to explain the features of generation of fatigue cracks in the bulk, which is typical of the gigacycle fatigue regime at a low stress level. The duality of the Weller curve in the gigacycle fatigue regime is attributed to the emergence of a fine grain region in the form of a dissipative peaking struc ture in the defect ensemble.
The Young's modulus, internal friction, and microplastic flow stress in Cu-Nb nanolaminate has be... more The Young's modulus, internal friction, and microplastic flow stress in Cu-Nb nanolaminate has been determined by an acoustic technique. The influence of high hydrostatic compression (1 GPa) on these elasto plastic properties of the nanolaminate has been studied.
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. Abstract The dissipation properties, evolution of the dilatation and elastic properties in sub-microcrystalline titanium Grade 4 samples loaded in HCF and VHCF regime were investigated. The dissipation properties were studied based on self-heating (Risitano) test. The mechanical properties (elastic modulus, amplitude-independent damping) and dilatation for a set of samples with different degrees of the life time exhaustion were studied based on the acoustic resonance method. The dilatation was studied by the method of hydrostatic weighing. The experimental results show the increasing of energy dissipation in submicrocrystalline titanium and absence the characteristic knee point on the graph of dependence of temperature rise versus stress amplitude for this material in self-heating test. The structural investigation shows the decreasing of elastic properties during fatigue loading and increasing of dilatation.
Mechanical stability under prolonged loading and thermostability under annealing have been studie... more Mechanical stability under prolonged loading and thermostability under annealing have been studied for nano-and microcrystalline titanium obtained by different methods of intense plastic deformation. The effect of nanoporosity and the fraction of high angle boundaries formed due to intense plastic deformation has been revealed and analyzed. It has been established that, depending on the loading or the annealing temperature, thermomechanical stability of titanium can be affected, apart from the above structural characteristics , by either twin grain boundaries or titanium-carbide disperse particles.
Results of small angle X ray scattering and high precision density measurements showed that the a... more Results of small angle X ray scattering and high precision density measurements showed that the application of counterpressure during the equal channel angular pressing (ECAP) of ultrafine grained cop per leads to a decrease in nanoporosity and an increase in mechanical properties of the ECAP processed metal.
The effect of nanoporosity and the highly dispersed carbide particles that form during screw and ... more The effect of nanoporosity and the highly dispersed carbide particles that form during screw and lengthwise rolling of VT1 0 titanium on its mechanical and, partly, thermal stability characteristics is revealed and analyzed.
The regularities of the influence of the number of passes under equal channel angular pressing on... more The regularities of the influence of the number of passes under equal channel angular pressing on the mechanical properties and defect structure of an aluminum alloy have been elucidated. It has been estab lished that the degradation of the mechanical properties (a decrease in the durability) is associated with the formation of nanoregions of an excess free volume in the course of severe plastic deformation under equal channel angular pressing. A correlation between the nucleation of excess free volume regions and the forma tion of high angle grain boundaries under equal channel angular pressing has been revealed. The nature of the influence of severe plastic deformation on the elastic modulus, the vibration decrement, and the micro plastic flow stress has been analyzed.
Transmission electron microscopy data showed evidence of the formation of structural regions corr... more Transmission electron microscopy data showed evidence of the formation of structural regions corresponding to deformation (dislocated) fragments and dynamically recrystallized grains in α-phase titanium upon torsion at high hydrostatic pressure at room and cryogenic temperatures. It is shown that the previously proposed "two-phase mixture" model is applicable to description of these defect structures. The strength of pure metals, including titanium, can be increased up to a level characteristic of heavily doped alloys by means of various extremal treatments of the bulk material, such as megaplastic deformation (MPD) and deformation at very low (cryogenic) temperatures. A widely used method of obtaining giant deformations is based on high-pressure torsion (HPT) in the Bridgman chamber [1]. Specific features of high plastic deformations in metals and alloys are the formation of fragmented structures consisting of misori-ented fragments with submicron dimensions [2] and simultaneous dynamic recrystallization processes at room temperature [3]. At present, no reliable literature data are available concerning comparative statistical analysis of the evolution of defect structure in polycrystalline α-Ti under the action of HPT at various (from room to cryogenic) temperatures. The interest in studying structural transformations in titanium and related alloys is stimulated by the demand for high-strength titanium-based materials in aerospace technologies. The present work was aimed at a detailed investigation of evolution of the defect structure in polycrystal-line α-Ti under the action of HPT in ta Bridgman chamber at various magnitudes and temperatures of deformation, based on the statistical analysis of dark-field images obtained by transmission electron microscopy (TEM). The study was performed on grade VT1-0 commercial-purity titanium (α-Ti) in which the structure formation during plastic deformation is characterized by a minimum influence of impurity atoms and/or foreign phases. The samples made from 50-μm-thick plates were deformed by HPT in the Bridgman chamber at room temperature (293 K) and at liquid nitrogen boiling temperature (77 K) at a rolling anvil rotation speed of 1 rpm under constant quasi-hydrostatic pressure of P = 6 GPa to N = 1/4, 1/2, 1, 2, 3, and 4 full continuous revolutions of the mobile anvil, which corresponded to true deformation in the interval of e = 5.5-8.3 [1]. The structure of samples was studied by TEM on JEM 200CX instrument operated at an acceleration voltage of 160 kV. All investigations of the local structure were performed in regions located at about half-radius of disk samples. The microstructure of titanium upon HPT at 293 and 77 K represents a mixture of α and ω phases. The ω phase regions have irregular shapes with the characteristic fringe contrast inside particles [4]. Detailed TEM analysis of the microstructure of α-Ti at all HPT stages at different temperatures clearly revealed separate grains shaped as almost regular hexagons (Fig. 1a, grains 1 and 2). A low density of dislocations inside these grains was obviously indicating that they were formed as a result of dynamic recrystallization during HPT. This kind of recrystallization (referred to as "continuous" [5]) can be considered as a process of relaxation under conditions of significant gradients of
It was established that when loading ultrafine-grained metallic materials under creep conditions,... more It was established that when loading ultrafine-grained metallic materials under creep conditions, the evolution of the "original" nanopores exerts a significant influence on the longevity. It is shown that in the process of creep the grains recrystallize, and the role of their boundaries in the transition from nano to microporosity is revealed.
It is established that increases in nanoporosity and the proportion of high-angle grain boundarie... more It is established that increases in nanoporosity and the proportion of high-angle grain boundaries in the process of equal-channel angular pressing are the main structural factors leading to reduction in mechanical stability (durability) of microcrystalline titanium during long-term tests under creeping conditions. The wide area of application of titanium is evoking increased interest in the study of the structure peculiarities of this metal in its high-strength microcrystal-line state formed as a result of severe plastic deformation (SPD). It is known that the high mechanical properties after SPD are determined basically by the size of the grains and condition of their boundaries. Reduction of the size of grains during SPD leads to an increase in the volume fraction of their boundaries, where high concentrations of defects (dislocations, vacancies, nanopores, etc.) and high internal stresses are localized [1, 2]. Because of this, the nano-and microcrystalline metals produced by SPD are inherently nonequilibrium, and so the problem of their mechanical stability, especially during prolonged stress, is important both for fundamental research and in terms of application [3]. In [4, 5], it was shown that a significant influence on the mechanical stability is exerted by two structural factors: nanoporosity formed during SPD and high-angle boundaries (ϕ > 15°) causing a high level of internal stress. In this paper, we investigate the contribution of these two factors to mechanical stability (durability during the test in the creeping mode) of microcrystalline titanium produced by equal channel angular pressing (ECAP). VT1-0 titanium with an impurity content ≈0.3% was selected for the study. The ECAP was performed by route B c with cyclic rotation of the workpiece around the axis of the channel by 90° after each cycle and the angle of intersection of the channels of 120° at 673 K [6]. For mechanical testing, samples were used that had been prepared after different numbers of ECAP passes. The samples had a length of the uniformly deformable part of 15 mm with a cross-sectional area of 3 × 2 mm 2. The prepared samples were tested at T = 673 K and σ = 15 MPa until rupture, and the time to failure (durability) was determined. Additionally the micro-hardness and its variation depending on the number of passes in ECAP were determined on initial (before testing in creep conditions) samples. The density of the samples and its change during ECAP caused by pore formation, among other things, were determined by triple hydrostatic weighing. The parameters of the pores were found with the help of a modified method of X-ray scattering in the field of ultralow angles using a high (1.5 GPa) hydrostatic pressure for identifying the void nature of the scattering inhomogeneities [7]. The sizes of grains and their distribution by disordering were determined using transmission and scanning electron microscopy and backscattering of electrons. Consider the obtained experimental data. It was established that the densities of samples of titanium in the original (before ECAP) state and after two, four, and eight passes were 4.5127 ± 0.0003, 4.5117 ± 0.0005, 4.5060 ± 0.0006, and 4.5100 ± 0.0005 g/cm 3 , respectively. Thus, there is a clear trend toward growth "loosening" of the titanium (determined by the level of nanoporosity, among other factors) with an increase in the number of passes. The effect of high hydrostatic pressure, as studies have shown, leads to a significant increase of density. For example, after four passes in ECAP, the density increased from 4.5065 to 4.5100 g/cm 3 because of the applied hydrostatic pressure. The action of hydrostatic pressure allowed us to identify the nature of the increased intensity of the small angle scattering that occurs after ECAP (Fig. 1, curves 1 and 2). It is seen that the intensity of the scattering is markedly reduced after the action on samples
The variation of the internal friction, Young's modulus, and electrical resistivity of two grades... more The variation of the internal friction, Young's modulus, and electrical resistivity of two grades of polycrystalline titanium (VT1-0 and Grade 4) in the area of low temperatures (100-300 K) as depending on the initial structure and subsequent severe plastic deformation converting the material into the submicrocrys-talline structural state in relation to the grain size is studied. The maximum of the internal friction is detected in submicrocrystalline titanium, which is interpreted as a Bordoni peak. All the studied characteristics are sensitive indicators for a nonequilibrium state of the grain boundaries after the deformation. The effect of the initial structure of the metal on its properties after the severe deformation is revealed.
Multiscale mechanisms of failure of metals (Armco iron, titanium, aluminum) are studied for high... more Multiscale mechanisms of failure of metals (Armco iron, titanium, aluminum) are studied for high cycle and very high cycle fatigue. By correlating with the results of structural studies, a theoretical approach is developed to describe fatigue crack kinetics in damaged material under high cycle and very high cycle fatigue loading conditions. Stages of crack nucleation and propagation are analyzed using the profilometry data from the fracture surface. The scale invariance of fracture surface roughness is established, which allows an explanation of the self-similar nature of fatigue crack kinetics under high cycle and very high cycle fatigue. Variation of elastic-plastic properties of Armco iron under very high cycle fatigue is studied using an acoustic resonance method. It is found that the material density decreases during fatigue damage accumulation, with the minimum of the material density in the bulk of the specimen.
The behavior of elastic (Young's modulus) and microplastic properties of titanium depending on th... more The behavior of elastic (Young's modulus) and microplastic properties of titanium depending on the initial structure and subsequent severe plastic deformation that transforms the material (concerning the grain size) into the submicrocrystalline structural state has been studied. It has been shown that, to a great extent, different initial structures of the metal predetermine its elastic properties after deformation.
The variation of interatomic interaction energy during allotropic transition in titanium is estab... more The variation of interatomic interaction energy during allotropic transition in titanium is established by analyzing the stress and temperature dependences of durability. This energy is ≈458 kJ/mol in body-centered β-Ti at a temperature of T > 1155 K, which agrees well with the titanium sublimation heat, and ≈305 kJ/mol in hexagonal close-packed α-Ti at T < 1155 K. Literature data confirming the conclusions drawn are discussed. At present, titanium and titanium-based alloys are finding wide application. Therefore, predicting the strength and mechanical stability of titanium is an urgent problem [1-5]. In the kinetic approach to strength [6], this problem cannot be solved without studying the dependence of durability, i.e., time τ from the instant of loading to material fracture, on stress σ and temperature T. Analysis of this dependence elucidates the physical origin of the process that leads to mechanical-stability loss and fracture. In a wide class of materials, including metals, composites, and polymers, this dependence in a certain range of external factors σ and T has the form [6] (1) where k is the Boltzmann constant; τ 0 ≈ 10-13 s for all solids, which coincides with the atomic thermal vibration period; and the factor at stress γ is the only structure sensitive coefficient. In analyzing Eq. (1), of special importance is the initial (before applying stress σ) energy barrier U 0. It was established that, in all the investigated metals, the U 0 value coincides with good accuracy with their sub-limation heats quantitatively characterizing the inter-atomic interaction. For each metal, quantity U 0 , similarly to the sublimation heat, was found to be a constant value independent of the structural state (grain size, impurity content, etc.) [6]. Titanium, similarly to other metals that are important for modern engineering (Co, Zr, and Fe), undergoes allotropic transformations upon temperature variations, which can noticeably affect its properties, including the interatomic binding energy. This Letter estimates the U 0 value and, consequently , interatomic binding energy of titanium with a body-centered cubic (bcc) lattice at T > 1155 K and hexagonal closed-packed (hcp) lattice at T < 1155 K. The investigations were carried out mainly on commercial titanium BT1-0 with an impurity content of ~0.28% and only for β-Ti. Iodide Ti of higher purity (99.9%) was additionally used. The mechanical tests were performed upon uniax-ial extension in the creep mode at the durability variation by more than six orders of magnitude (from 10-1 to 3 × 10 5 s). The technique for measuring small (<1 s) durabilities was described in [6]. It should be noted that the necessary condition for correct estimation of the activation energy using the phenomenological data is the identity of the structural elements that decisively affect the durability of the samples tested at different σ and T values [7]. Here, such a structural element is the grain size. To stabilize it, all the samples were annealed at a temperature higher than the maximum experimental temperature before the durability test. In particular , the α-Ti samples were annealed at 925 K for 40 h and the β-Ti samples at 1323 K for 1 h. Annealing and subsequent durability tests were performed in vacuum. Let us consider first the experimental data on α-Ti. Figure 1 shows the stress and temperature dependences of durability. It can be seen that in the coordinates logτ-σ at different T = const and in coordinates logτ-1/T constructed on the basis of the data presented in Fig. 1a for different σ = const, the dependences of logτ on σ and 1/T are lines converging at the extrapolation in the pole. In this case, as follows from [6], the pole in the coordinates logτ-1/T lies on the () γσ τ = τ − 0 0 exp , U kT
The work is devoted to the study of the damage accumulation in iron under gigacyclic fatigue (VHC... more The work is devoted to the study of the damage accumulation in iron under gigacyclic fatigue (VHCF) regime. The study of the mechanical properties of the samples with different state of life time existing was carried out on the base of the acoustic resonance method. The damage accumulation (porosity of the samples) was studied by the hydrostatic weighing method. The obtained results show the accumulation of porosity in the volume of the sample during fatigue loading and corresponding decrease of the elastic properties. A statistical model of damage accumulation was proposed in order to describe the damage accumulation process. The model describes the influence of the sample surface on the location of fatigue crack initiation.
This research was focused on a new low-modulus-type titanium alloy Ti 26Nb 7Mo 12Zr (wt.%). The m... more This research was focused on a new low-modulus-type titanium alloy Ti 26Nb 7Mo 12Zr (wt.%). The microstructure effects on elastic modulus (measured by the acoustic resonance method) as well as microplastic, mechanical, tribological, and corrosive properties of Ti 26Nb 7Mo 12Zr alloy after thermomechanical processing were examined. The microstruc-ture was characterized in detail by scanning electron microscopy and electron backscatter dif-fraction methods. The experimental research results have shown that formation of the fully re-crystallized structure in the titanium alloy leads to an increase in elastic modulus, microplastic flow stress and plasticity, as compared to the corresponding characteristics of the alloy having partially recrystallized and coarse-grained structures. The durability of titanium alloy was examined and compared with that of commercially pure titanium (CP Ti). It was found that, in the same creep loading conditions, the low-modulus Ti 26Nb 7Mo 12Zr alloy exhibits a longer time to creep fracture, as compared to the pure titanium.
The problem of application of physical acoustic methods to studying the mechanisms that control p... more The problem of application of physical acoustic methods to studying the mechanisms that control plastic deformation and fracture is considered using micro-and submicrocrystalline materials (Be, Al, Ti, Al-Sc alloy, Cu-Nb laminated material) as examples. The influence of grain boundaries on the acoustic (elastic, inelastic) properties of polycrystalline micro-and nanostructured metallic materials is analyzed. Experimental results are presented for a wide oscillating-stress amplitude range, from 0.2 to 50 MPa. The experimental data are discussed in terms of the theoretical concepts of oscillatory dislocation mobility, which depends on both the short-range stress fields around point defects and the long-range fields of internal stresses. It is shown that various types of discontinuities, such as pores and microcracks, noticeably influence the acoustic properties. The aspects of the relation, similarity, and difference between acoustic and mechanical (plasticity, strength) tests of polycrystalline materials with micro-and nanosized structural elements are discussed.
The influence of deformation by equal channel angular pressing (ECAP) on the durability of tita n... more The influence of deformation by equal channel angular pressing (ECAP) on the durability of tita nium (VT1 0 grade) tensile tested under creep conditions has been studied for the first time. It is established that the ECAP induced transition of titanium to a microcrystalline state leads to a decrease in the durability, while the characteristics of static strength are improved.
The effect of thermobaric treatment on the nanoporosity (excess free volume) and thus on the phys... more The effect of thermobaric treatment on the nanoporosity (excess free volume) and thus on the physical and mechanical properties of some amorphous alloys obtained by extra rapid quenching is studied.
A broad spectrum of physicomechanical properties of VT1 0 nanocrystalline titanium produced by cr... more A broad spectrum of physicomechanical properties of VT1 0 nanocrystalline titanium produced by cryomechanical fragmentation of the grain structure using rolling at a temperature close to liquid nitrogen temperature has been studied. The mechanism of grain refinement has been associated with grain fragmen tation by twins. Exactly the twin nature of internal interfaces (crystallite boundaries) provides the thermal and structural stability of nanocrystalline titanium produced by cryomechanical grain fragmentation in the tem perature range to ~500 K. It has been assumed that the observed decrease in the titanium density due to cry orolling is associated with a number of factors (high density of introduced dislocations, nanopore formation, and changes in titanium lattice parameters).
We propose a model describing the kinetics of accumulation of defects under cyclic loading in met... more We propose a model describing the kinetics of accumulation of defects under cyclic loading in metals. Analysis of experimental data on the initial defect distribution and of the role of the free surface of the sample in straining makes it possible to explain the features of generation of fatigue cracks in the bulk, which is typical of the gigacycle fatigue regime at a low stress level. The duality of the Weller curve in the gigacycle fatigue regime is attributed to the emergence of a fine grain region in the form of a dissipative peaking struc ture in the defect ensemble.
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Papers by Maria Narykova