Grain-subgrain Structure Research Articles

Features of the Grain-Subgrain Structure of Ti49.8Ni50.2 Alloy after Megaplastic Deformation by abc Pressing and Subsequent Annealing

Features of the Grain-Subgrain Structure of Ti49.8Ni50.2 Alloy after Megaplastic Deformation by abc Pressing and Subsequent Annealing

FUNCTIONAL AND MECHANICAL CHARACTERISTICS OF ULTRAFINE-GRAINED FE-MN-SI ALLOYS FOR BIOMEDICAL APPLICATIONS

This work examines the possibility of regulating the corrosion rate of Fe-Mn-Si alloys by modifying their structure via equal channel angular pressing. It is found that the formed ultrafine-grained austenitic structure of Fe-Mn-Si alloys leads to a significant increase in strength characteristics at satisfactory ductility. The presence of special twin boundaries in the structure of Fe-Mn-Si alloys improves their corrosion resistance, while a predominantly grain-subgrain structure in the absence of twin boundaries increases the corrosion rate up to 0.4 mm/year. The shape memory effect in the studied alloys manifests itself at temperatures unacceptable for medical use. Structure refinement by equal channel angular pressing in modes that ensure a completely austenitic state leads to a decrease in shape memory properties.

Structure and Properties of the "Cantor HEA Coating (Mn-Fe-Cr-Co-Ni) / Substrate (Alloy 5083)" System

Using the technology of cold metal transfer (wire-arc additive manufacturing combined with welding surfacing), a coating is formed on a 5083 alloy substrate with a high-entropy Mn-Fe-Cr-Co-Ni alloy of nonequiatomic composition. Analysis of the structure, elemental composition, and microhardness of the coating-substrate system is carried out using the methods of modern physical materials science. A significant increase (up to 9.9 GPa) in the microhardness of the material is found in the zone of contact between the coating and the substrate. The formation of lamellar inclusions (Al13F4) enriched in coating atoms in the zone of contact between the coating and the substrate is revealed. The high entropy coating in the contact zone has a submicrocrystalline grain-subgrain structure with a crystallite size ranging from 0.5 ^m to 1.1 ^m along the boundaries where nanosized particles of the second phase (Al3Ni) are revealed. The volume of grains contains a dislocation substructure in the form of randomly distributed dislocations or dislocation clusters. The scalar density of the dislocations is (0.8-1.0)-1010 cm-2. An assumption is made about the physical mechanisms of hardening of the material in the "coating-substrate" contact zone. Key words: high-entropy alloy, coating / substrate system, aluminum alloy, elemental and phase composition, microhardness, hardening.

Особенности зеренно-субзеренной структуры сплава Ti49,8Ni50,2 после мегапластической деформации методом abc прессования и последующего отжига

The features of the grain-subgrain structure of Ti49.8Ni50.2 (at. %) alloy after megaplastic deformation by multi-axial forging at 573 K and subsequent annealing were investigated by the method of backscattered electron diffraction. It is shown that the best detection of the patterns of the grain-subgrain structure of the Ti49.8Ni50.2 alloy after a given true deformation e = 9.55 and annealing at 773 K for 2 hours, it is observed in areas without martensitic relief on the surface of the samples. It was found that the clearest diffraction patterns of backscattered electrons are observed in the central part of the grains, and with distance from the center of the grain, the quality of the Kikuchi lines deteriorates. The calculation of the equivalent grain diameter showed that the average grain size is 0.203 ± 0.053 microns. The grains have mainly an ellipse shape with an axis ratio of 0.48. Elongated grains along their length have a discrete-continuous disorientation of the crystal structure, which can reach 5 degrees per 1 micrometer.

Electron Microscope Studies of the 01Cr17Ni13Mo3 Austenitic Steel Structure after Cold Rolling Combined with Hydrogen Alloying with Varying Degrees of Deformation

In this paper, we consider the effect of cold rolling and hydrogen alloying on the formation of twin boundaries of the corrosion resistance of austenitic steel 01Cr17Ni13Mo3. Using the method of transmission electronic microscopy, microdiffraction patterns were obtained. The analysis of microdiffraction patterns indicates the formation of a developed grain-subgrain structure with small-angle and large-angle misorientation. The structure has a high dislocation density, deformation twins and localized shift bands. It was established that plastic deformation by flat rolling to ε = 90 % at room temperature does not contribute to the appearance of a noticeable amount of α' and ε-martensite. At the temperature of liquid nitrogen, the samples were found to form a small fraction of the α'-martensite phase. Such a small amount of martensite can contribute to steel strengthening, and a decrease in the rolling temperature will lead to an increase in the strength properties of steel. It was detected that the density of twin boundaries under the decrease in the rolling temperature but with the same intensity of hydrogen saturation is significantly higher. A noticeable reduction in the width of the twin lamellas was revealed.

Low-temperature superplasticity of ultrafine-grained near β titanium alloy

Studies of the low-temperature superplasticity (SP) of ultrafine-grained (UFG) near β alloy Ti-5Al-5V-5Mo-1Cr-1Fe at a temperature of 823 K (∼0.42 Tm) in the range of strain rates (2.0–6.9)·10−3 s−1 have been carried out. It is shown that for an UFG alloy with an average grain size of d ∼ 0.17 µm, obtained by the method of multi-axial pressing, the value of the relative elongation to failure exceeds 950% at a strain rate of 2·10−3 s−1. Annealing of the UFG alloy at a temperature of 873 K for 1 h does not cause a noticeable increase in the average grain size (d ∼ 0.23 µm), but leads to the transition of a part of grain boundaries to a more equilibrium state and a decrease in elongation to failure by more than 3 times in the range of strain rates. The study of the evolution of the microstructure under tension at a rate of 2·10−3 s−1 showed that in both states of the alloy up to an elongation of 150%, the average size of the elements of the grain-subgrain structure practically does not change, which is due, among other things, to the appearance of new grains less than 100 nm in size during the SP deformation. It is shown that the significantly lower elongation to failure of the UFG Ti-5Al-5V-5Mo-1Cr-1Fe alloy subjected to annealing may be due to the hindered accommodation of grain boundary sliding during the SP deformation by intragranular dislocation slip because of the transition of individual grain boundaries to an equilibrium state. The preservation of the UFG structure of the alloy with high strength characteristics under conditions of low-temperature superplasticity creates good prerequisites for obtaining high-strength products of complex shape in the regime of SP forming.

Влияние насыщения водородом на структуру и механические свойства аустенитной стали 01Х17Н13М3, формируемые в процессе прокатки при разных температурах

Introduction. The development of hydrogen energy implies a decrease in the dependence of various human activities on fossil energy sources and a significant reduction in carbon dioxide emission into the atmosphere. Therefore, the requirements for the quality of structural materials, which have the prospect of being used for storage and transportation of hydrogen, as well as for the creation of infrastructure facilities for hydrogen energy, are increasing. Therefore, the scientific researches on the hydrogen-assisted microstructure and mechanical behavior of structural materials in various loading schemes are of great importance. The aim of this work is to establish the effect of chemical-deformation treatment, including rolling combined with hydrogen saturation, on the microstructure, phase composition, and mechanical properties of 316L-type austenitic stainless steel. Methods. Transmission electron microscopy and backscattered electron diffraction, X-ray diffraction, X-ray phase and magnetic phase analysis, microindentation and uniaxial static tension are utilized. Results and Discussion. It is shown experimentally that after rolling with 25 and 50 % upset, the morphology of the defect structure and the phase composition of 316L steel substantially depends on the deformation temperature (at room temperature or with the cooling of the samples in the liquid nitrogen) and on hydrogen saturation rate (for 5 hours at a current density of 200 mA/cm2). The main deformation mechanisms of the steel in rolling are slip, twinning, and microlocalization of plastic flow, which all provide the formation of ultrafine grain-subgrain structure in the samples. In addition, deformation-induced ε and α' martensitic phases are formed in the structure of the rolled samples. Regardless of the regime of chemical-deformation processing, grain-subgrain structures with a high density of deformation defects are formed in steel, but its morphologies are dependent on the processing regime. The experimental data indicate that both preliminary hydrogen saturation and a decrease in the deformation temperature contribute to the more active development of mechanical twinning and deformation-induced phase transformations during rolling. Despite the discovered effects on the influence of hydrogen saturation on the deformation mechanisms and the morphology of a defective microstructure formed during rolling, preliminary hydrogenation has little effect on the mechanical properties of steel at a fixed degree and temperature of deformation. These data indicate that irrespective of the morphology of the defective grain-subgrain structure, grain refinement, accumulation of deformation defects and an increase in internal stresses lead to an increase in the strength characteristics of the steel.

Influence of Radial Shear Rolling Regimes on the Homogeneity of Structural-Phase State, Mechanical Properties and Fatigue of near β Titanium Alloy

The studies showed that as a result of radial-shear rolling (RSR) in the temperature range of 850-650°C the formation of a quasi-globular ultrafine grain-subgrain structure with a size of structural elements of about 0.5-0.7 μm is observed in near β titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe. Subsequent annealing (aging) in the temperature range of 450°C for 5 hours leads to the decomposition of the β-phase matrix deformed during the rolling process with the formation of nanosized acicular precipitates of the martensitic α" phase along with finely dispersed inclusions of α phase particles preserved after rolling. The formation of such ultrafine-grained (UFG) structure leads to a significant (more than 25%) increase in the strength properties of the alloy compared to the initial state while maintaining sufficient ductility (~5%). Also obtained by the method UFG titanium alloy exhibits a high fatigue strength under very high cycle loading (fatigue strength based on 108 cycles exceeds 700 MPa).

Kinetics of dynamic recrystallization in low - and medium carbon steels during warm ECA pressing

In this study, electron microscopy was employed to examine the microstructure evolution mechanisms and dynamic recrystallization kinetics in low and medium carbon steels with a lamellar structure of perlite during equal channel angular pressing (ECAP) at 400 °C. Mechanisms of the subgrain structure formation and its transformation into a grain-subgrain structure with average element size of 200–500 nm were revealed. It was found that new ultrafine grains with high-angle boundaries are formed in the ferrite phase as a result of dynamic recrystallization beginning after the second ECAP pass. It was shown that the growth rate of the recrystallized fraction increases drastically after 3 ECAP passes. Fragmentation of ferrite lamellas within perlite colonies and strain induced split of cementite plates was detected suppressing the recrystallization in AISI 1045 steel. Enhanced mobility of dislocations in ferrite phase of AISI 1020 steel assisted in severe strain hardening and subsequent dynamic recrystallization.

Structure and properties of rails after extremely long-term operation

The paper reveals regularities and mechanisms of structure-phase states and properties formation of of differentially hardened 100-m rails of DT 350 category after the passed tonnage of 1411 mln. tons brutto. The formation of highly defective surface layer with nanosize (40–50 nm) grain-subgrain structure of pearlite colonies and submicrocrystal (150–250 nm) structure grains with structure free ferrite is detected. The change of hardness, microhardness, crystal lattice parameter, microdistorsion level, scalar and excess dislocation density on the rails head section are analyzed. The possible mechanisms of cementite plates’ transformation at extremely long-term operation are discussed.

Structural features of ultrafine-grained aluminum processed through accumulative roll bonding providing improved mechanical properties and thermal stability

Using transmission electron microscopy, tension and indentation tests we studied commercially pure aluminum processed by severe plastic deformation through accumulative roll bonding (ARB Al) and equal-channel angular pressing (ECAP Al) up to 8.0 von Mises equivalent strain to find the structural features providing improved mechanical properties and their enhanced thermal stability in the former case. It is revealed that the average size of the elements of the grain-subgrain structure is 23% lower and the strength properties are 19 – 27% higher in ARB Al with respect to ECAP Al. The physical reason for more refinement of the grain-subgrain structure and enhanced mechanical properties is the presence of dispersed alumina nanoparticles introduced into the bulk of ARB aluminum from the surface of the bonded stripes during ARB. The thermal stability of microhardness of ARB Al exceeds the respective value of ECAP Al in the temperature interval of 473 – 513 К, where the recrystallization starts to occur. The improved thermal stability of ARB Al is related to the higher resistance to recrystallization due to the presence of alumina nanoparticles on grain boundaries formed during ARB and precipitation of Si from the solid solution on these nanoparticles.

Исследование прочностных и пластических характеристик композиционных слоев в аустенитной нержавеющей стали, подвергнутой ионно-плазменной обработке, методом наноиндентирования

One of the main issues of austenitic stainless steel is low strength properties and low wear-resistance. It can be partially or fully eliminated by the product surface modification and the creation of hardened surface layers. The ion-plasma saturation of alloys with interstitials, which is carried out in a mixture of gases with different compositions is an available and effective method of surface hardening of complex structural parts. At the same time, the mechanical and plastic characteristics of the processed materials are determined by the complex of properties of the base alloy and the hardened surface, and it is not always possible to identify their influence on the mechanical and plastic properties of each component of the composite material. The nanoindentation method allows determining local mechanical and plastic characteristics in certain areas of hardened materials (base alloy and surface) by the dynamic loading of the local microscopic areas. In this paper, using the nanoindentation method, the authors identified the mechanical and plastic characteristics of hardened layers produced by the ion-plasma treatment of austenitic 01H17N13M3 stainless steel with the grain-subgrain and coarse-grain structures. The ion-plasma treatment of steel specimens facilitates surface hardening and the formation of a composite surface layer of ≈20-25 μm in thickness. High values of nano-hardness in a composite layer are caused by the complex hardening of specimens: solid-solution hardening of austenite with nitrogen and carbon, the dispersion hardening and the formation of different nitrides and carbonitrides and the ferrite low fraction. The experimental results show that the strength properties and plasticity characteristics of such a layer strongly depend on the base material initial microstructure - the formation of a highly-defective grain-subgrain structure promotes the formation of a more enforced surface layer compared to the coarse-grained specimens.

Influence of Annealing on the Structure and Mechanical Properties of Ultrafine-Grained VT22 Titanium Alloy

Studies of the evolution of the structural-phase state and changes in the mechanical properties of the ultrafine-grained alloy VТ22 after annealing in the temperature interval 773–1073 K have been carried out. It has been shown that after annealing at 773 K practically no decrease in the density of deformation defects and practically no changes in the average size of the elements of the grain-subgrain structure and the mechanical properties of the alloy are observed. However, the structural-phase state of the alloy under these conditions is thermally unstable. There take place a generation of new grains with sizes less than 0.1 μm and growth of individual grains up to sizes on the order of 1 μm, and also a change in the volume fractions of the α- and β-phases. At higher annealing temperatures, there is observed a decrease in the density of deformation defects, growth of existing grains, a transition of the grain boundaries to a more equilibrium state, and, as a consequence, a substantial deterioration of the mechanical properties at room temperature. Under these conditions, α–β-phase transformations actively develop, leading to an increase in the volume fraction of the β-phase with increasing temperature. Estimates of the activation energy of grain growth were performed. It is shown that its value is 145 kJ/mol.

Effect of heat treatments on the structure and properties of a small-diameter thin-walled tube of a medical nickel titanium alloy

The present study examines the effect of annealing at temperatures of 300-500°C on the structure and properties of a thin-walled nitinol tube of composition Ti49.1Ni50.9 used for the manufacture of intravascular stents. Annealing temperature dependences are obtained for the martensitic transformation temperatures, the size of the structural elements of the grain-subgrain structure, phase composition, pseudo-yield stress, and ultimate strength. The effect of the Ti3Ni4 particle precipitation in aging during annealing at different temperatures on the obtained dependences is analyzed.

Mechanical properties and thermal stability of an ultrafine-grained near β titanium alloy

Comparative studies of the structure, mechanical properties and thermal stability of an ultrafine-grained near β titanium alloy of the transition class Ti-5Al-5Mo-5V-1Cr-1Fe obtained by the methods of multiple pressing and radial-shear rolling with subsequent aging have been carried out. It is shown that as a result of multiple pressing, the formation of a homogeneous micro-duplex ultrafine-grained structure with an average grain (phase) size of ∼0.2 μm is observed. Radial shear rolling followed by aging leads to the formation of an inhomogeneous structure with precipitates of the α phase particles and a subgrain size of the β phase of about 0.5 μm inside which thin-acicular precipitates of the metastable α” phase are observed. Despite the smaller imposed plastic deformation and the larger size of grain-subgrain structure, alloy samples after radial-shear rolling and aging demonstrate higher strength properties at room temperature compared to the alloy obtained by multiple pressing (the ultimate strength of the alloy after these treatments is 1695 and 1600 MPa, respectively.) At the same time, the alloy obtained by the method of multiple pressing shows higher ductility and thermal stability of the mechanical properties due to the micro-duplex ultrafine-grained structure. The drop in strength as a result of annealing is due to the development of recrystallization in the case of the alloy after pressing and the dissolution of the strengthening α” phase in the case of the alloy after radial-shear rolling and aging.

Hydrogen Embrittlement of Austenitic Stainless Steels with Ultrafine-Grained Structures of Different Morphologies

The paper studies the effect of electrolytic hydrogen charging on the plastic flow, strength properties, ductility, and fracture mechanisms in austenitic stainless steels Cr17Ni13Mo3C0.01, Cr18Ni10TiC0.12, and Cr18Ni9C0.17 with different stacking fault energies. The investigated steels are subjected to warm ABC-pressing and thermomechanical processing, including cold rolling and annealing, to produce the ultrafine-grained structure of different morphologies, such as ultrafine-grained (submicrocrystalline), misoriented grain-subgrain and mixed (grain and subgrain) structures of submicron scale. The strength properties of the steels after warm pressing and rolling with annealing exceed 3.5–6.0 times the properties of the quenched steels with coarse-grained structure. Electrolytic hydrogen charging of the studied steels with submicron-sized structure reduces the yield strength irrespective of the grain/subgrain size, structure, steel composition, and its stacking fault energy. The formation of a highly defective grain-subgrain structure with high dislocation density suppresses the effect of hydrogen embrittlement in Cr17Ni13Mo3C0.01 and Cr18Ni10TiC0.12 steels, in which no or a small volume fraction of strain-induced α′ martensite forms in tension. The tempering of the highly defective structure and the formation of a large fraction of high-angle misorientations in the stable Cr17Ni13Mo3C0.01 steel enhances the effect of hydrogen embrittlement in the specimens as compared to the specimens with a grain-subgrain structure with a high density of dislocations and low-angle boundaries. The hydrogen embrittlement effects are most pronounced in ultrafine-grained (submicrocrystalline) Cr18Ni10TiC0.12 and Cr18Ni9C0.17 steels with predominantly grain structure, which undergo induced γ→α′ phase transformation.

The Influence of Dimesions and Phase State of Structural Elements on Mechanical Properties of Binary Alloys of the Ti–Nb and Zr–Nb Systems

The results of investigation of the influence of the structural element size and phase state of the Ti – 45 wt.% Nb and Zr – 1 wt.% Nb binary alloys on their mechanical properties are presented. The structural states of the alloys with the structural elements of different dimensions were formed from the ultrafine-grained state via annealing. The curves of dependence of the yield strength and microhardness on the average size of the structural elements, d –1/2, are obtained using the Hall–Petch relation. It is shown that for the Zr – 1 wt.% Nb alloy, the Hall–Petch relation is fulfilled in the entire range of sizes under study, from ultrafine-grained to finegrained states 0.2–1.9 μm. For the Ti – 45 wt.% Nb alloy, in the analysis of the Hall–Petch relationship within the structural element size range 0.43–45 μm, its phase composition is taken into consideration, specifically, the presence of dispersion-strengthened β-phase grains, α-phase subgrains, nonequilibrium nanosized ω- phase, and the bimodal grain-subgrain structure.

Substructural-Phase Transformations during Heat Treatments of the Ti–50.9 at % Ni Nanocrystalline Alloy

The method of transmission electron microscopy is used to study the grain-subgrain structure of the Ti–50.9 at % Ni nanocrystalline alloy after heat treatments at 300–500°C. It is found that the decomposition of the B2 solid solution of TiNi by the heterogeneous mechanism occurs in the subgrain structure, while the Ti3Ni4 particles are not observed in nanograins. The features of the recovery and recrystallization of the alloy and their relationship with the processes of dissolution and precipitation of Ti3Ni4 particles are revealed.

Субструктурно-фазовые превращения при термообработках нанокристаллического сплава Ti-50.9 at.% Ni

AbstractThe method of transmission electron microscopy is used to study the grain-subgrain structure of the Ti–50.9 at % Ni nanocrystalline alloy after heat treatments at 300–500°C. It is found that the decomposition of the B_2 solid solution of TiNi by the heterogeneous mechanism occurs in the subgrain structure, while the Ti_3Ni_4 particles are not observed in nanograins. The features of the recovery and recrystallization of the alloy and their relationship with the processes of dissolution and precipitation of Ti_3Ni_4 particles are revealed.

Structure and mechanical properties of a hybrid material with copper matrix and steel fibers after ECAP

The structure and mechanical properties of a hybrid material with a copper matrix and steel fibers subjected to ECAP were studied. C12200 pure copper and AISI 304 steel were used as hybrid material components. A mixed submicrocrystalline grain-subgrain structure with a high dislocation density is formed in the copper matrix of the hybrid material as a result of ECAP that leads to an increase in strength by ~20% while maintaining a high value of the relative elongation. The strength of the steel fibers of the hybrid material after ECAP was significantly increased primarily due to the process of intensive deformation twinning and increase in the dislocation density. At the same time, the presence of a ductile copper matrix around the steel fibers makes it difficult for them to shear strain during ECAP, which slows down the process of a developed ultrafine-grained structure formation.