Physical Mesomechanics最新文献

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.

The paper examines the microstructural evolution of alloy 1565ch of the Al-Mg-Mn-Zn-Zr system during thermomechanical treatment, including severe plastic deformation by high-pressure torsion or equal channel angular pressing according to the Conform scheme and subsequent isothermal rolling at 200°C. Formation of the nanostructured and ultrafine-grained states in alloy 1565ch with the controlled distribution of the Al₃Mg₂, Al₆Mn and Al₃Zr phases both inside grains and at their boundaries allows for the effect of superplasticity at the temperatures 250 and 300°C and strain rates 5 × 10^–2, 10^–2, and 5 × 10^–3 s^–1. Microstructural analysis by transmission electron microscopy shows that superplastic deformation at the temperatures 250 and 300°C allows a homogeneous ultrafine-grained state to be preserved. The studied ultrafine-grained aluminum alloy 1565ch has a high strength and the ability to relieve stresses, and therefore it can be favorably used as the matrix material in composites reinforced with continuous boron fibers. In the paper, we use this alloy to study special features of production of a multilayer (foil–fiber–foil) metal matrix composite by isothermal pressing under low-temperature superplastic conditions. This method has a positive effect on the mechanical properties of the composite, such as ultimate strength at 200°C, impact strength at room temperature, and fracture toughness at room temperature.

Ti-Ta coatings were deposited on titanium alloy by electrospark deposition in the anode mixture of titanium granules and tantalum powder in an argon atmosphere. The cathode weight gain kinetics, tantalum concentration, structure, oxidation resistance, microhardness, and tribotechnical properties of the coatings were studied. It was shown that, with increasing tantalum concentration in the anode mixture, the net cathode gain during 10 min of treatment increased monotonically. The average thickness of the deposited coatings varied in the range from 30.9 to 39.1 µm. The concentration of tantalum in the coating composition increased with increasing tantalum powder concentration in the anode mixture. The coating structure was dense without longitudinal and transverse cracks. With an excess of tantalum powder in the anode mixture, the discharge energy was not enough to completely melt it. The phase composition included α-Ti and a bcc tantalum solid solution in β-Ti. With increasing powder concentration in the anode mixture, the intensity of the bcc-phase peaks increased relative to the α-Ti peaks. The surface hydrophobicity of Ti-Ta coatings was higher than that of uncoated Ti6Al4V titanium alloy. The developed method can be used to produce Ti-Ta coatings with up to 5.9 times higher oxidation resistance compared to Ti6Al4V alloy. The high oxidation resistance of Ti-Ta coatings is explained by the formation of a dense and durable TiO₂ layer. The surface microhardness of Ti-Ta coatings ranged from 4.72 to 4.91 GPa. The friction coefficient was in the range of 0.87–0.97. The wear resistance was 23 to 36 times higher as compared to the titanium alloy.