High-strength State Research Articles

Study of phase transformations in 14Kh2G2NMFB steel providing high-strength state

Study of phase transformations in 14Kh2G2NMFB steel providing high-strength state

High-Strength State and Strengthening Mechanisms of Ultrafine-Grained Titanium

High-Strength State and Strengthening Mechanisms of Ultrafine-Grained Titanium

Increasing the Thermal Stability and High-Temperature Strength of Vanadium Alloys by Strengthening with Nanosized Non-Metallic Particles

Using the methods of scanning and transmission electron microscopy, the features of the structural-phase state of a vanadium alloy of the V-Cr-Ta-Zr system after a combined treatment, which consisted in cyclic alternation of thermomechanical and chemical-heat treatments, were studied. The values of yield strength and ductility of the V-Cr-Ta-Zr alloy were determined, depending on the stabilization and test temperatures. It was established that, after the combined treatment, the structural-phase state of the V-Cr-Ta-Zr alloy was composite, in which the joint implementation of dispersion and substructural strengthening ensured the formation of a gradient grain structure with a polygonal state, the elements of which were fixed by nanosized ZrO2 particles characterized by a high thermal stability. Such modification of the microstructure was accompanied by an increase in the high-temperature strength and a shift in the upper limit of the temperature stability interval towards high temperatures, of up to 900 °C. It was assumed that the polygonal state inside the grains contributed to the implementation of cooperative mechanisms of the dislocation-disclination type, which ensured the accommodation of the material in the "high-strength state" under loading.

The formation of a high-strength state in martensitic Ti Grade 4 by ECAP

This work is aimed at studying the features of initial Ti Grade 4 structure that affects the changes in its structure and properties during equal channel angular pressing (ECAP). A hot-rolled and martensite structure state, obtained in the process of water quenching from 950 °C were used as initial states. It has been established that ECAP of titanium in an initial martensitic state leads to the formation of a finer and more homogeneous structure than ECAP of a hot-rolled state. The share of high-angle boundaries reaches 80%, and the average size of structural fragments is 220 ± 80 nm. After ECAP at 400 °C, recrystallized grains are observed. As a result, the ECAP of titanium in the initial martensitic state leads to more significant hardening of up to 1100 ± 20 MPa compared to the hot-rolled state of –950 ± 15 MPa. This will make it possible to reduce the number of technological operations, as well as improve the strength resource for heat treatment aimed at ductility enhancement in ultrafine-grained titanium.

Шероховатость и микротвердость ультрамелкозернистого титана Grade 4, подвергнутого безабразивной ультразвуковой финишной обработке

Increasing the fatigue resistance of implants is an important scientific and technical problem. One of the solutions to this problem is the high-strength state formation due to the ultrafine-grained (UFG) structure. However, high-strength alloys are characterized by greater sensitivity to stress concentrators and the surface roughness parameter. In turn, implant designs, as a rule, imply the presence of concentrators in the form of various grooves, threaded elements, etc., and the manufacturing technology supposes mechanical processing with an ambiguous effect on a finished product surface. The application of additional surface finishing, for example, abrasive-free ultrasonic finishing (AFUF), is a solution to this problem. This work aims to study the effect of different AFUF modes on the microhardness and roughness of a cylindrical blank made of Grade 4 commercially pure titanium in the UFG state. During the study, the authors assessed the effect of the rotation frequency of a workpiece and the static force of pressing the tool against the processed workpiece on the surface parameters; carried out microstructural studies of the obtained samples. The results showed that processing titanium in the UFG state by the AFUF method leads to a significant increase in the surface microhardness and a decrease in its roughness. For example, depending on the mode, the increase in microhardness can reach from 2 to 3.5 times. The authors investigated the effect of a power level of ultrasonic treatment on roughness and microhardness and considered various variants of surface pretreatment. The study identified that an increase in the speed of rotation of a workpiece reduces the roughness of a machined workpiece, while the microhardness increases.

Влияние термической и термомеханической обработки на микроструктуру и механические свойства многокомпонентного сплава FeCrMnNiCo0.85C0.15

The multicomponent FeCrMnNiCo alloy, known as the Cantor alloy, has a single-phase austenitic structure (with an fcc crystal lattice) and high ductility and toughness. However, at room temperature, the yield strength of the Cantor alloy is rather low (≈200 MPa). Alloying of the FeCrMnNiCo multicomponent alloy with carbon can significantly increase its strength properties due to solid solution hardening of the austenitic phase and precipitation hardening. But for alloys with interstitial atoms, high values of the configurational entropy do not provide the formation of a single-phase structure in the cast state. The formation of large incoherent M7C3 / M23C6 carbides occurs during crystallization and their presence adversely affects the plasticity of cast carbon-alloyed multicomponent alloys. Achievement of a high-strength state without the loss of plasticity is possible by modifying the microstructure of carbon-alloyed alloys using thermal and thermomechanical treatments. We studied the structure, phase composition, and mechanical properties of the FeCrMnNiCo0.85C0.15 alloy in the cast and annealed (at 1200°C, 1 h) states, as well as after multistage thermomechanical treatments, including high-temperature annealing, hot forging, and cold rolling. It was found that alloying with carbon leads to an increase in the yield strength and a sharp drop in the ductility of the cast FeCrMnNiCo0.85C0.15 alloy (σ0.2 = 315 MPa, δ =12 %) as compared to the reference Cantor alloy containing no carbon atoms (σ0.2 =180 MPa, δ = 62 %). Cast FeCrMnNiCo0.85C0.15 alloy has an inhomogeneous structure consisting of an austenitic phase and large carbides. Annealing promotes partial dissolution of carbides, is accompanied by an increase in the yield stress, but does not contribute to an increase in the plasticity of the alloy (σ0.2 = 355 MPa, δ = 9 %). Thermomechanical treatments do not provide the formation of a single-phase austenitic structure, but contribute to a significant dissolution of carbides and an increase in the solid solution hardening of the austenite phase. Such alloys have significantly higher strength and ductility values (σ0.2 = 434 – 436 MPa, δ = 35 – 37 %) in comparison with cast alloy and Cantor alloy.

The effect of thermoplastic processing on the structure and mechanical properties of metastable austenitic steel

The patterns of structural transformations and the change in the mechanical properties of a metastable austenitic steel subjected to cold plastic deformation by drawing with high total degrees of compression and subsequent aging were studied in this work. The studies showed that after quenching, the use of intense plastic deformation by drawing revealed the high technological capabilities of the studied austenitic steel. Austenite in the steel is deformation-unstable and turns into deformation martensite during cold plastic deformation by drawing. Aging of deformed steel causes an additional increase in mechanical properties, due to the separation of the intermetallic phase NiAl from the bcc solid solution (strain martensite). It was revealed that almost carbon-free corrosion-resistant austenitic steel, as a result of correctly selected alloying, combines the advantages of three steels: metastable austenitic steels, Transformation-Induced Plasticity (TRIP)-steels and martensitic aging steels. As a result of using all possible hardening mechanisms, a high-strength state was achieved.

Structure Transformation of Dispersionstrengthened Vanadium Alloys under Conditions of High-Pressure Torsion and Room-Temperature Tensile Deformation

A comparative study of the features of microstructure transformation in dispersion strengthened vanadium alloys of the V–Me(Cr, W)–Zr system under conditions of deformation by torsion under pressure and tension at room temperature is carried out. It is found out that suppression of dislocation plasticity in the high-strength state favors the formation of an anisotropic submicrocrystalline structure both during torsion deformation and in the region of strain localization in the case of tension at room temperature. It is shown that one of the main mechanisms of crystal fragmentation under these conditions is a dislocation-disclination mechanism, whose realization requires structural states with nonzero values of the curvature rotor or a high continual density of disclinations.

Effect of Thermomechanical Treatment Parameters on Structure, Phase Composition and Mechanical Properties of Ti-3Al-5Mo-4.5V Titanium Alloy

The structure, phase composition and mechanical properties of (α+β) — titanium alloy solution treated at 850 °C, cold-rolled at the reduction ratio in the range of 0… 45%, followed by ageing at 450, 500, 550 °C for 0.5, 1.5, 3 hours was studied using XRD, microindentation and tensile testing. The influence of strain level at cold rolling and time-temperature parameters of ageing on the formation of structure and phase composition of solution treated and water quenched Ti-3Al-5Mo-4.5V alloy was investigated and discussed in terms of tensile properties and microhardness. The parameters of low temperature thermomechanical treatment (LTMT) of the (α+β) — alloy were proposed to obtain a high-strength state.

Low-temperature mechanical properties and thermally activated plasticity parameters of the CrMnFeCoNi2Cu high entropy alloy

The mechanical properties of a high-entropy CrMnFeCoNi2Cu alloy with an FCC lattice have been studied in a broad temperature range (4.2–350 K). The microhardness and Young’s modulus were measured at 300 K for two structural states. The temperature dependences of the yield strength, deforming stress, and strain rate sensitivity have been obtained. It was found that a high-strength state is realized in the alloy while maintaining high plasticity, over the entire temperature range. Empirical estimates of microscopic plasticity parameters and internal and effective stresses have been obtained using the thermally activated deformation model.

High-Strength Behavior of the Al0.3CoCrFeNi High-Entropy Alloy Single Crystals

The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (σ0.1) at a test temperature above room temperature. This restricts their practical application at high test temperatures from 773 K to 973 K. In this study, we found that a high stress level was reached at the yield point σ0.1 ≈ G/100–G/160 (G is the shear modulus) of the [001]- and [1¯44]-oriented crystals of the Co23.36Cr23.29Fe23.80Ni21.88Al7.67 (Al0.3CoCrFeNi) high-entropy alloy (HEA) within a wide temperature range of 77–973 K under tension, due to the occurrence, of nanotwins, multipoles, dislocations under plastic deformation at 77 K and the subsequent precipitation of ordered L12 and B2 particles. It was shown that grain boundaries are not formed and the samples remain in a single-crystal state after low-temperature deformation and subsequent ageing at 893 K for 50 h. Achieving a high-strength state in the Al0.3CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (τcr) at T ≥ 200 K (τcr[1¯44] > τcr[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of σ0.1 above 573 K, and reduces plasticity to 5–13% in the [1¯44] orientation and 15–20% in the [001] orientation.

The influence of heat treatment on the formation of the structure and the level of mechanical properties of high-alloyed titanium alloy

The paper investigates the possibility of increasing the strength of the experimental high-alloyed titanium alloy due to various methods of thermal action, leading to a change in its phase composition and intragrain structure. Changes in mechanical properties in correlation with the change in structure in the annealed, tempered and heat-hardened state are reviewed. It is shown that by controlling phase transformations in highalloyed two-phase titanium alloys, it is possible to realize high-strength state with satisfactory plastic characteristics. The optimal complex of mechanical properties is provided by heat treatment, which leads to the creation of a two-phase heterogeneous structure with a developed bimodal intragrain structure.

Effects of grain refinement by HPT processing in carbon steel with various cementite morphology

This article reports on the effect of the initial phase composition (martensitic, patented, spheroidized) of the middle carbon steel C45 (0.45 wt.% C) on grain refinement during high-pressure torsion (HPT) at elevated temperature. It is established that the cementite dissolution and the final grain size after HPT strongly depend on the processing regime, as well as on the cementite morphology in the initial state of the steel. As result of this study, new approaches to obtain a very high-strength state in carbon steels have been proposed.

The Effect of Low-Temperature Heat Treatment on the Superelasticity in Ti–50.6 at % Ni Single Crystals

As a result of low-temperature aging at 573 K for 1.5 h of Ti–50.6 at% Ni [001]-oriented single crystals quenched from 1253 K, a high-strength state is obtained due to the precipitation of Ti3Ni4 nanoscale particles (d < 10 nm). The aging leads to the development of the B2–B19' martensitic transformation through an intermediate R phase, a reduction in the B19' martensite formation temperatures by 40–60 K, an increase in the strength properties of the B2 phase from 730 to 2100 MPa, and an extension of the superelasticity interval ΔTSE in compression from ΔTSE = 60 to 170 K (compared to quenched crystals).

Влияние низкотемпературной термообработки на развитие сверхэластичности в монокристаллах Ti-50.6 at.&amp;#37; Ni

AbstractAs a result of low-temperature aging at 573 K for 1.5 h of Ti–50.6 at% Ni [001]-oriented single crystals quenched from 1253 K, a high-strength state is obtained due to the precipitation of Ti_3Ni_4 nanoscale particles ( d &lt; 10 nm). The aging leads to the development of the B 2– B 19' martensitic transformation through an intermediate R phase, a reduction in the B 19' martensite formation temperatures by 40–60 K, an increase in the strength properties of the B 2 phase from 730 to 2100 MPa, and an extension of the superelasticity interval Δ T _SE in compression from Δ T _SE = 60 to 170 K (compared to quenched crystals).

High-strength steel structure and mechanical properties after Q-n-P-integrated heat treatment

The article considers the effect of the newly designed Q-n-P-integrated («Quenching and Partitioning») heat treatment modes on the microstructure and mechanical properties changes of cost-saving alloyed high-strength steel 55Si3Mn2CrVMoNbА. It has been found that the application of Q-n-P-integrated technological schemes, with elongated soaking at 160°C, after previous bainitization, or thermal cycling, greatly increase the mechanical properties of 55Si3Mn2CrVMoNbА steel as compared to standard quenching and tempering (at 200-300°С). It has been found that the structure-free ferrite formation, contrary to presupposition, doesn’t increase the ductility of steel as compared to other investigated heat treatment modes. The most effective results showed the heat treatment according to TCT/Q-n-P mode (three cycles 160«250°С) which provided the formation in steel 55Si3Mn2CrVMoMbA high-strength state (TS = 1941-2116 MPa, 50-54 HRC) while maintaining high total elongation (10-16%) and impact toughness (KCU = 74 J/cm2), which results in a 5-fold increase in PSE («Product of Strength and Elongation») level (up to 31,5 GPа%) of treated steel as compared to quenching and tempering respectively. Heat treatment according to TCT/Q-n-P and В-Q-n-P modes in the investigated steel provides increased impact toughness level (up to 74-79 J/cm2) which makes it possible to use this steel with high cyclic and shock loads. Besides,according to the results shown in this article, the 55Si3Mn2CrVMoNbA steel properties in the state of Q-n-P-integrated treatment are in good agreement with the requirements for the 3rd generation high-strength steels. The study suggests that high mechanical properties are achieved due to the formation of the multiphase structure consisting of tempered martensite, lower bainite and significant amount of carbon enriched residual austenite. The obtained results demonstrate the promise of this technology for better mechanical properties of cost-saving alloyed machine-building and tool steels

Low Temperature Superplasticity of High-Strength Ultrafine-Grained Al 7050 alloy

.Ultrafine-grained (UFG) Al 7050 alloy of the 7xxx series demonstrates very high strength at room temperature with UTS=845 MPa and hardness Hv=255, which considerably exceeds the value after conventional heat treatment T73, i.e. Hv=165. It is also revealed that with increasing temperature the alloy exhibits rather high ductility with elongation to failure δ=170% at 120 °C and strain rate of 5×10−4 s−1 and typical superplastic behavior with δ=480% and δ=690% at temperatures 170 °C and 200 °C correspondingly. Such values of superplasticity (SP) temperature are considerably lower than that typical of this alloy with micron-sized grains (TSP=450-500 °C). Such reduction in superplasticity temperature made it possible to preserve ultrafine grains in the alloy structure and provide its high-strength state after SP straining.

Effect of ECAP Prior to Aging on Microstructure, Precipitation Behaviour and Mechanical Properties of an Al-Cu-Mg Alloy

Microstructure, precipitation behaviour and mechanical properties of an Al-5.64Cu-0.33Mn-0.23Mg-0.14Zr-0.11Ti (in wt. %) alloy subjected to thermomechanical processing (TMP) involving equal-channel angular pressing (ECAP) at ambient temperature to total strains (ε) of ~1 and ~2 followed by aging at 180°C for 0-28 h have been investigated and compared with conventional aging at the same temperature (T6 state). TMP led to significant increase in yield stress (YS) and ultimate tensile strength (UTS) and decrease in elongation-to-fracture as compared to the peak-aged T6 state. The YS, UTS and δ values attained after ECAP to ε ~ 2 followed by peak ageing were ~510 MPa, ~540 MPa and ~7.6%, respectively. The changes in mechanical properties were related to microstructure evolution and precipitation behaviour. TMP conditions obtaining a high-strength state of the Al-Cu-Mg alloy are discussed.

Effect of fracture mode on acoustic emission behavior in the hydrogen embrittled low-alloy steel

Abstract Hydrogen embrittlement (HE) of steels and other alloys is a long-standing scientific and engineering problem causing sudden premature failures of industrial components. Although several mechanisms of hydrogen-assisted cracking (HAC) have been proposed, there is still neither commonly accepted agreement on the exact role of hydrogen in this process nor the reliable way to recognize the operating mechanism of HAC. In the present paper, we endeavor to demonstrate that the acoustic emission (AE) technique is a promising tool for in-situ investigating of HAC mechanisms and distinguishing between them. The experiments were performed using the low-alloy steel grade 09G2S obtained in the low-strength as-received state and the high-strength state after severe plastic deformation by equal channel angular pressing (ECAP). The specimens of both kinds were tensile tested before and after hydrogen charging as well as after removing diffusible hydrogen for evaluation of the influence of hydrogen induced cracks (HICs). It is established that HICs, which are formed during hydrogen charging, do not affect the mechanical properties, fracture surface topology and the AE behavior in both as-received and ECAPed specimens, provided that diffusible hydrogen is removed from the steel. Two different kinds of HAC were observed. The first operated in the as-received low-strength steel; it produced weak AE response and was featured by the formation of fisheyes defects with quasi-cleavage morphology. The second type of HAC was observed in the ECAPed high-strength steel; it generated high-energy AE signals and was featured by the formation of mixed tearing/cleavage morphology on the fracture surface. Obtained results are discussed and explained from the viewpoint of the known mechanisms of HAC including hydrogen-enhanced localized plasticity (HELP), hydrogen enhanced decohesion (HEDE) and others.

ЗАКАЛКА С ПОСЛЕДУЮЩИМ ОБОГАЩЕНИЕМ УГЛЕРОДОМ НЕПРЕВРАЩЕННОГО АУСТЕНИТА (Q&amp;P ОБРАБОТКА) МАРТЕНСИТНОЙ КОРРОЗИОННОСТОЙКОЙ СТАЛИ AISI 414

Heat treatment consisting quenching of steel to a temperature inside the martensitic transformation interval with subsequent carbon partitioning between quenched martensite and retained austenite (Q&amp;P treatment) is applied to martensitic stainless steel AISI 414 (Fe – 0.14 % C – 12.5 % Cr – 2.3 % Ni). Q&amp;P processing was modeled using the thermomechanical simulator Gleeble 3800. Phase transformations during processing were studied using a contact dilatometer, which measures the change in the specimen diameter at the thermocouple spot. After austenitization at 1150 °C for three minutes, the temperature of the onset of the martensitic transformation of the steel under study was 270 °C. To implement Q&amp;P processing, the samples were quenched to temperatures of 250 °C, 200 °C, 150 °C and 100 °C. The carbon partitioning between the quenched martensite and untransformed austenite was carried out in the process of 3-minute exposure at 450 °C. It has been shown that quenching to temperatures of 200–250 °C with subsequent annealing at 450 °C does not ensure the enrichment of unconverted austenite with carbon, sufficient to suppress martensitic transformation upon subsequent cooling to room temperature. Lowering the quenching temperature to 150 °C with subsequent annealing at 450 °C leads to complete thermal stabilization of the untransformed austenite up to temperatures of minus 60 °C, which ensures a high lowtemperature toughness of the steel under investigation through the TRIP effect, exceeding the values laid down in the requirements for high-strength pipe steel. The conclusion is made about the prospect of Q&amp;P processing of the investigated steel for obtaining a high-strength state with a high level of low-temperature viscosity.