Physical Mesomechanics最新文献

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: the yield stress σ_0.2 = 1140 MPa, ultimate strength σ_В = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain transition-metal carbides Fe₂C. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which appears as the Portevin–Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.

The specimens of Ti–2 wt % Fe alloy were annealed at three different temperatures, in the β-Ti, α-Ti + β-Ti and α-Ti + TiFe fields of the Ti–Fe phase diagram, then water quenched and subjected to high-pressure torsion (HPT). The X-ray diffraction analysis showed that the main phase in all annealed specimens was the α phase (more than 90%), while the main phase after HPT was the ω phase. Hardness H and Young’s modulus E were determined by nanoindentation at the center, in the middle of the radius, and near the edge of each specimen. It was found that the H and E values were different for specimens annealed at different temperatures and depended on the radial coordinate of the indentation region. The maximum H values were obtained in the middle of the radius of the specimens. The E values of all specimens decreased from the center to the edge, reaching very low values. The paper discusses structure transformations during HPT, the behavior of the radial dependences of H and E, and probable causes of a strong decrease in E values.

Comparative studies are conducted on the structure and mechanical properties of the ultrafine-grained Ti-5Al-5V-5Mo-1Cr-1Fe alloy obtained by abc pressing and radial shear rolling with subsequent aging. It is shown that the ultrafine-grained structure formed by these methods provides increased strength properties under both tension and three-point bending compared to the initial coarse-grained state. At the same time, the alloy obtained by abc pressing demonstrates a higher fracture resistance during three-point bending compared to the alloy obtained by radial shear rolling + aging due to its enhanced ductility. This also determines the ductile fracture pattern of the ultrafine-grained alloy obtained by abc pressing during three-point bending in contrast to ductile-brittle fracture of the alloy obtained by radial shear rolling + aging.

The formation of ultrafine-grained structure is very desirable in the microstructural design of magnesium alloys, in particular Mg-Zn-Ca medical alloy, for a substantial increase in their strength and corrosion resistance. However, conventional processing of these alloys by equal channel angular pressing is not easily applicable due to their low deformability, which often leads to rapid fracture of billets. In this paper, computer simulation data and principles of physical mesomechanics are used to demonstrate that preliminary deformation of Mg alloy billets by reduction at high temperatures and low strain rates significantly increases their deformation capacity and enables equal channel angular pressing at lower temperatures, resulting in billets with ultrafine-grained structure. Consideration is given to the physical nature of the established effect.

The experimental data on the deformation of amorphous alloy Vit105 (Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅) and its molecular dynamics simulation gave birth to new ideas about the mechanism of plastic deformation of disordered structures. A special method of torsion under hydrostatic pressure allows forming a developed deformation relief on the surface of polished specimens. Inspection of the relief points to the formation of shear bands on the surface, which can merge or branch, freely intersect or be arrested by an obstacle, forming a delta of small shear bands. Simulations based on the Morse pair potential made it possible to build a two-dimensional amorphous model and study its deformation at the atomic level. Under loading, material parts are displaced due to the appearance of atomic-scale vortices in the shear band layer by means of free volume, which is a structural feature of amorphous materials. A vortex causes redistribution of stress fields, which, when added to external stresses, are capable of activating similar vortices in the neighboring zones of the material, both in the direction of the applied stresses and along the vortex axis. In the latter case, a vortex tube is formed, which acts by the tornado mechanism. Shear is induced by the tube motion in the direction of principle shear stresses, and traces on the specimen surface are made by its screw component. An increase in the number of vortex tubes and their interaction causes a deformation band. Though playing the role of dislocations, vortex tubes are independent of specific crystalline planes and can move in arbitrary directions. This explains the experimentally observed features of deformation of amorphous alloys.

The paper reports on finite element simulation of extrusion of a complex-shaped billet from the ultrafine-grained Ti-6Al-4V alloy and vacuum-arc deposition of a protective coating based on the TiVZrCrAl high-entropy alloy. Temperature fields formed in the billet during extrusion are studied. Deformation heating and the necessary forming force are determined for the initial temperature-rate conditions. The strain rate distribution in the billet during extrusion is also analyzed. According to the obtained data, the chosen temperature-rate conditions allow using the ultrafine-grained titanium alloy as the initial billet without deteriorating its mechanical characteristics. Computer simulation of the coating deposition on the complex-shaped billet provides values of the temperature, chemical composition, and thickness of the high-entropy coating. Thus, the coating thickness varies within 6.5–7.5 μm, and the surface is heated during deposition to 368–597°C, which allows maintaining the ultrafine-grained structure in the alloy.

We study the effect of high-pressure torsion on the microstructure, phase composition, microhardness, and strengthening mechanisms of high-nitrogen austenitic steels with different vanadium content: Fe-23Cr-19Mn-0.2C-0.5N, Fe-19Cr-21Mn-1.3V-0.3C-0.8N, and Fe-18Cr-23Mn-2.6V-0.3C-0.8N, wt %. Regardless of the chemical composition of the steels, high-pressure torsion (HPT) causes the refinement of their microstructure due to a high density of dislocations, twin boundaries, and shear bands. Vanadium alloying decreases the stacking fault probability in the structure of the steels and changes their dominating deformation mechanism under high-pressure torsion: from planar dislocation slip and twinning in the vanadium-free steel to dislocation slip with a tendency to shear band formation in the vanadium-alloyed steels. An increase in the vanadium content forces precipitation hardening. Thus, after HPT, the V-alloyed steels have a higher microhardness as compared to the vanadium-free one. Different strengthening factors (strain hardening, solid solution hardening, and precipitation strengthening) govern the value and kinetics of growth of microhardness of the steels processed by high-pressure torsion. Vanadium alloying and increasing its content result in the growth of the contribution of precipitation hardening and decreases strain hardening of high-nitrogen steels.

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.

The present paper deals with twinning-induced plasticity (TWIP) steels with the microstructure refined by severe plastic deformation via equal channel angular pressing and explores the mechanical behavior of steel with qualitatively different microstructures formed in the temperature range 400–900°C. Mechanical characteristics of the steel in different structural states are studied in static tensile tests, biaxial and dynamic tests. Structural changes in the material during severe deformation at different temperatures are discussed, and their effect on the mechanical parameters of TWIP steel is considered. High temperatures of equal channel angular pressing allow for more homogeneous recrystallized structures, which ensure the best combination of the yield stress, formability, plasticity, and crack resistance. These findings can be important in developing high-performance steels for the automotive and hydrogen industries.

Asymmetric rolling is a high-tech method based on the principles of severe plastic deformation. In the present paper, it is shown that Cu-0.8Cr-0.1Zr alloy is highly strengthened during asymmetric rolling due to structure refinement to an ultrafine-grained state. For example, in only one pass, at the accumulated strain 0.94 ± 0.20, the strength increases from 265 to 425 MPa. During the deformation process, the structure becomes refined, with the average size of fragments reaching 235 ± 90 nm. Structure heterogeneity is also observed in the cross section of a sample, which is associated with different rotation speeds of the rolls. The shape of grains in the central zone of samples corresponds to the state after conventional symmetric rolling. However, in the zone adjacent to the roll rotating at a higher speed, mechanical texture of grains is similar to that after shear. Subsequent aging of Cu-0.8Cr-0.1Zr alloy at 450°C makes it possible to achieve the ultimate strength 560 MPa and electrical conductivity 82% IACS, which exceeds the characteristics of the strengthened steel by 10–15%. The analysis of contributions to strengthening during asymmetric rolling reveals that the main contribution comes from the refinement of the grain structure to an ultrafine-grained state, which amounts to 58%. The fractions of the dislocation and dispersion contributions comprise 15 and 20%, respectively. Compared to conventional rolling, as well as other deformation methods that provide the same level of accumulated strain and strengthening in one cycle, such as equal channel angular pressing-conform, asymmetric rolling is the most promising due to its simpler process scheme.