Evolution of substructures under microlevel and mesolevel plastic deformation

The existence of a hierarchy of structural levels of plastic deformation can be considered to be an experimentally and theoretically proven fact [1–3]. Mescheryakov [1] showed that a noncrytallographic level of deformation arises in elastoplastic waves, manifesting itself as macrofluxes of particles of the medium; the velocity of the particles relative to each other at velocity has dispersion and the particles move in the direction of the wave propagation. Displacement of macrofragments of the crystal, which is also a manifestation of noncrystallographic structural levels of deformation, has been detected in highly excited systems [2]. The relaxation approach used increasingly to describe plastic deformation assumes that defects are created, move, and are restructured during deformation in a way so that the level of stresses inside the material drops. The nonuniformity of the stress field gives rise to nonuniform plastic deformation and local shears and rotations at points of stress concentration. These concepts make it possible to use the principles of synergetics to build specific theoretical models and to consider loaded material as a nonequilibrum dissipative structure [3]. To date, however, the construction of the theory describing multilevel plastic deformation processes has not been completed. In particular, it is not yet known what levels are added, depending on the rate and duration of the loading and on how the levels are linked.St. Petersburg Branch of the A. A. Blagonravov Institute of Mechanical Engineering. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–12, October, 1992.     

Structure,unelastic properties,and deformation behavior of ultrafine-grained titanium

The results of investigations into the structure, unelastic properties, deformation behavior, strength, and plasticity of ultrafine-grained titanium produced by equichannel angular pressing are discussed. Particlular emphasis has been placed on the grain-boundary unelasticity and the effects of external thermal and thermal-force actions on the deformation behavior and plastic deformation localization at the meso- and macroscale levels. The influence of cold plastic deformation of ultrafine-grained titanium on the grain-boundary unelasticity and temperature dependence of the mechanical properties is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 33–43, September, 2004.     

Simulation of particulate-filled composite deformation diagrams on the basis of a constitutive model of large plastic deformation for a polymer matrix

Deformation diagrams of particulate-filled polymers have been calculated on the basis of specific constitutive equations [1] for large plastic deformation of the polymer. Composite structure is represented by the Hashin polydisperse model [2]. Original finite-element (FE) code with triangular elements has been elaborated and used for the numerical solution of boundary value problems. Local achievement of a critical value by the elastic main strain was used as a fracture criterion. Engineering (force-elongation) diagrams were found to exhibit maxima for arbitrary filler fraction if the interfacial bond was perfect and for low loading at zero adhesion. Stress-strain diagrams with a yield maximum and draw minimum provide macroscopic neck-type localization. Further, the loading in the case of facilitated deboading results in the diminution of the difference between maximum and minimum drawing forces and then in the disappearance of the latter, which in turn provides the transition from localized to macrouni-form deformation. Young's modulus and the yield stress increase with filling in the case of absolute adhesion and decrease in the opposite case. Ultimate elongation sharply drops with an increase in filler fraction, and embrittlement occurs at a small fraction of inorganic particles if a perfect interfacial bond is present. Contrary, a decrease in ultimate elongation is much more gradual, and composites conserve ductile properties of the matrix up to a high portion of inclusions. The laws found qualitatively coincide with what is observed for real materials.     

Mesoscale plastic deformation of aluminum polycrystals

A study is made of the mechanism of plastic deformation at the mesoscale level in flat specimens of aluminum polycrystals. The mechanism is examined with the use of high-resolution optical-television system TOMSC-1. It is shown that a multilevel mesoscale structure is formed in the specimen as it is deformed. The formation of this structure leads to the appearance of two types of stationary waves 120 μm and 4.8 mm long. The results are interpreted within the framework of a hierarchy of mesoscale levels of deformation and are linked with the decisive role of surface oxide films in the formation of the mesoband structure and stationary waves associated plastic flow. Institute of the Physics of Strength and Materials Science, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 31–39, January, 1997.     

Plastic distortion as a fundamental mechanism in nonlinear mesomechanics of plastic deformation and fracture

Any deformed solid represents two self-consistent functional subsystems: a 3D crystal subsystem and a 2D planar subsystem (surface layers and all internal interfaces). In the planar subsystem, which lacks thermodynamic equilibrium and translation invariance, a primary plastic flow develops as nonlinear waves of structural transformations. At the nanoscale, such planar nonlinear transformations create lattice curvature in the 3D subsystem, resulting in bifurcational interstitial states there. The bifurcational states give rise to a fundamentally new mechanism of plastic deformation and fracture—plastic distortion—which is allowed for neither in continuum mechanics nor in fracture mechanics. The paper substantiates that plastic distortion plays a leading role in dislocation generation and glide, plasticity and superplasticity, plastic strain localization and fracture.     

Plasticity limit of metals during hot plastic deformation

A model of plasticity limit has been derived in the condition of hot plastic deformation, where dynamic recrystallization takes place, through the ratio between the rate of grain boundary sliding and the overall deformation rate. If fracture occurs preferentially at the grain boundaries we can replace the grain boundary deformation through the energy needed to cause fracture and express the temperature influence on the deformation stress. The plasticity limit is then the function of Zener-Hollomon parameter and deformation stress, where the exponent of deformation stress has a value of –4·3.     

Evolution of mechanisms of plastic deformation in porous metals

An experimental study was made of the structure of deformed porous iron with a porosity P ranging from 4 to 40% within the range of strains up to fracture. Measurements are made of grain-boundary slip and rotations of structural elements and their individual contributions to the total strain as a function of P and . It is established that there is a change in the dominant mechanisms of plastic deformation with an increase in porosity, this change being connected with a change in the topological characteristics of the system and corresponding to the transition from intergranular slip to a group of mechanisms that ensures movement of the grains as a whole. The established laws are analyzed on the basis of the concept of structural levels of deformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–105, January, 1996.     

Influence of plastic deformation on fracture of an aluminum single crystal under shock-wave loading

The plastic deformation and the onset of fracture of single-crystal metals under shock-wave loading have been studied using aluminum as an example by the molecular dynamics method. The mechanisms of plastic deformation under compression in a shock wave and under tension in rarefaction waves have been investigated. The influence of the defect structure formed in the compression wave on the spall strength and the fracture mechanism has been analyzed. The dependence of the spall strength on the strain rate has been obtained.     

Influence of the temperature and structural state on the systematic features of plastic deformation in Mo-Re-based alloys

The salient aspects of the formation of a dislocation structure during plastic deformation of the alloys Mo-47 wt. % Re and Mo-47 wt. % Re-1 vol. % ZrO₂ at T=300 K in previously recrystallized and polygonalized structural states have been studied with an electron microscope. The studies revealed the systematic features of rotational modes of plastic deformation in these alloys under conditions of substructural hardening and high-temperature grain-boundary slip. An analysis is made of the effect that the substructure and highly dispersed particles of the oxide phase have on the plastic deformation and mechanical properties of Mo-Re-based alloys at T=300-1500 K.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 96–104, December, 1994.     

Nonequilibrium state of grain boundaries and spontaneous grain-boundary slippage in bicrystals

Variables (order parameters) that are related to plastic deformation sites and complement the local densities of grain-boundary defects are separated out in a bicrystal being deformed under creep. By solving an evolutionary equation for the order parameters, it is shown that the nonuniformity and periodicity of spontaneous grain-boundary slippage in bicrystals and an increase in the grain-boundary strain rate under the conditions of boundary-lattice dislocation interaction may be attributed to the occurrence of local nonequilibrium regions (autosolitons).     

Structural mechanisms of fracture of nanocrystalline materials

Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.     

Distinctive features of plastic strain localization under severe plastic deformation of metals

Mechanical testing is performed and the structure of zirconium and aluminum predeformed by ∼450% using multiaxial forging (MAF) and equal channel angular pressing (ECAP) is investigated. Tensile loading tests of the severely deformed specimens exhibited their tendency to necking, with the ductility of the material in the neck, however, being superior to that in the neck of initial coarse-grained specimens. The results of the experiments imply that a fundamental stage of plastic flow of solids under severe plastic deformation (SPD) is the formation of cellular-banded structure and strain localization in the fine-grain bands. This considerably retards further deformation-induced refinement of the structure by SPD, and also results in the rapid formation of a fracture neck in the materials with this structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 43–49, November, 2007.     

Some features of the macrolocalization of the plastic deformation of copper single crystals in alternating torsion

Copper single crystals of various orientations have been plastically deformed in alternating torsion, and observations have been made of the appearance of fine slip traces in the initial stage of deformation, the longitudinal elongation, and the transformation of the circular cross section into oval-tetragonal, cruciform, triangular, elliptical, and other sections. Analysis of the slip traces has shown that, with a symmetrical arrangement of the octahedral slip planes with, respect to the axis of torsion, not all of the planes are active, but only a certain number sufficient to convert the elastic deformations into plastic slip. The axial elongation and the nature of the change in shape of the cross section depend on the number and orientation of the active slip planes.It has been found that, in addition to the accumulation of macroslips of opposite sign observed during the deformation of polycrystalline aggregates [1–3], there is in single crystals incomplete recovery of the displacements along the crystallographic slip planes. During deformation a single crystal breaks up into thin plates which are displaced by alternating rotation and translation with respect to one another. The failure of the plates to return completely to their original positions leads to an accumulation of slips of opposite sign between adjacent plates and to a corresponding change in the shape of the cross section. The elements of the ratchet mechanism [4,5] of accumulation of macroslips are regarded as a manifestation of the incomplete reversibility of dislocation movement.     

Role of local nanostructural states in plastic deformation and fracture of solids

The paper substantiates the concept of physical mesomechanics that the basis for nonlinear behavior of solids under plastic deformation and fracture is the formation of nanostructural states in local highly nonequilibrium zones. Their structural transformations and two-phase decay govern the generation of strain-induced defects and cracks. Nonlinear wave mechanisms of nanostructural states influence on plastic deformation and fracture are discussed.     

Physical nature of stages in plastic deformation

Conclusion In summary, the transition from one stage of plastic deformation to another is due to the phase transformations in the subsystem of deformation defects. These transformations are controlled by an internal parameter of the system, viz., the scalar dislocation density, whose value is determined by both the external force and processes retarding the shear and annihilation. The main structural level controlling this process is the level of the dislocation subsystem.We express our thanks to associate professors L. A. Telyakov, Yu. P. Sharkeev, and V. A. Starenchenko, Candidates G. V. Daneliya, D. V. Lychagin, and I. A. Lapsker, and scientific associates S. P. Zhukovskii, L. I. Trishkina, A. V. Paul', and T. S. Kunitsyna, with whose collaboration part of the results reported in this review were obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–106, February, 1990.     

Kinetics of localized plastic flow during deformation and fracture of a D1 alloy

The stress-strain curve of a polycrystalline duralumine (D1) is studied to find three basic deformation stages: linear hardening, parabolic hardening (n = 1/2), and prefracture (n < 1/2). The results obtained show special features of macrolocalization of the plastic flow of the alloy under review. The distribution patterns of localized plastic flow domains develop according to deformation stages. The prefracture stage is characterized by self-correlated motion of the domains to the point of subsequent fracture. It follows from an analysis of the plastic flow localization kinetics that both hardening and softening domains coexist in the specimen in the prefracture stage. The domains move with a constant velocity inherent to each of them and linearly dependent on the position of their nucleation point. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 68–73, November, 2007.     

Relaxation waves in plastic deformation

Conclusion Experimental study of distortion fields of plastically deformed solids performed on a wide range of materials including fine- and coarse-grain body- and face-centered polycrystals, as well as amorphous alloys reveals that in these materials plastic deformation develops in the form of waves having translational and rotational components. This fact is in accordance with the currently developed theory of a turbulent mechanical field, which also has translational and rotational components.The plastic deformation waves are observable at a macroscopic structural level, and their spatial period (wavelength) is determined by the dimensions of the deformed object and dimensions of the basic structural elements (for a polycrystal, the grain size). The propagation rate of these waves is significantly less than the characteristic propagation rate of an elastic excitation and the velocity of previously described plastic waves which are produced by shock deformation, which latter speed is determined by the hardening coefficient.The character of plasticity waves depends on the form of the material's deformation curve, and on the stage of the hardening curve. The distribution of plastic distortion components changes especially significantly in prefailure regions, which allows detection of the latter long before formation of a macroscopic crack. The role of rotations in forming the failure process has been established.A synergetic interpretation of plasticity wave formation has been proposed, based on synchronization of relaxation acts occurring at stress concentrators during the deformation process.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 19–35, February, 1990.     

Effect of plastic deformation and quenching on the absolute thermal emf of nickel

The effect of the degree and rate of plastic deformation of nickel on the change of its absolute thermal emf was studied. The dynamic effect of the change of thermal emf was established, the dynamic coefficient being 1.22–1.25. Plastic deformation of quenched nickel reduced its absolute thermal emf, on the basis of which conclusions are drawn about the contributions, different in sign, of holes and dislocations to the change of the absolute thermal emf of nickel. The kinetics were studied of the recovery of the thermal emf induced in nickel by plastic deformation, and the existence was established of two stages of recovery, differing in nature and with different activation energies, 0.1 and 0.3 eV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 77–81, June, 1971.