Nonlinear wave processes in a deformable solid as a hierarchically organized system

Theoretical predictions and experiments demonstrate that solid state mechanics should consider, along with a structurally equilibrium 3D crystalline subsystem, a structurally nonequilibrium planar subsystem as a complex of all surface layers and internal interfaces with broken translation invariance. Primary plastic flow of a loaded solid develops in its structurally nonequilibrium planar subsystem as channeled nonlinear waves of local structural transformations that determine the self-organization law of multiscale plastic flow. These waves initiate mesoscale rotational deformation modes, giving rise to all types of microscale strain-induced defects in the planar subsystem. The strain-induced defects are emitted into the crystalline subsystem as an inhibitor of nonlinear waves of plastic flow in the planar subsystem. Plastic deformation of solids, whatever the loading type, evolves in the field of rotational couple forces. Loss of hierarchical self-consistency by rotational deformation modes culminates in fracture of material as an uncompensated rotational deformation mode on the macroscale.     

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.     

MD simulation of plastic deformation nucleation in stressed crystallites under irradiation

The investigation of plastic deformation nucleation in metals and alloys under irradiation and mechanical loading is one of the topical issues of materials science. Specific features of nucleation and evolution of the defect system in stressed and irradiated iron, vanadium, and copper crystallites were studied by molecular dynamics simulation. Mechanical loading was performed in such a way that the modeled crystallite volume remained unchanged. The energy of the primary knock-on atom initiating a cascade of atomic displacements in a stressed crystallite was varied from 0.05 to 50 keV. It was found that atomic displacement cascades might cause global structural transformations in a region far larger than the radiation-damaged area. These changes are similar to the ones occurring in the process of mechanical loading of samples. They are implemented by twinning (in iron and vanadium) or through the formation of partial dislocation loops (in copper).     

Calculation of the effective diffusion coefficient for random wear surface migration on different scales

The paper considers the contact interaction of crystalline solids under shear deformation in the context of molecular dynamics. The interatomic interaction is specified by a potential calculated using the embedded atom method. The peculiarities of structural changes in a contact zone are studied for various materials of the contact pair. Based on the data extracted, the effective diffusion coefficient was estimated for random migration of the contact zone in a direction perpendicular to applied shear strains. The calculation results agree well with data of a microscopic contact model built around the method of movable cellular automata.     

The role of curvature of the crystal structure in the formation of micropores and crack development under fatigue fracture of commercial titanium

The multiscale mechanism of fatigue fracture of titanium with the surface layer hydrogenated under alternating bending at room temperature is studied. It is shown that the generation of the fatigue fracture occurs in the surface layer subjected to plastic deformation in conjunction with an elastically loaded substrate. The latter causes the appearance of a strong curvature of the material and the appearance of micropores in these areas along with any fatigue cracks. The emergence of the local curvature of the crystal structure plays a central role in the origin and the development of the fatigue fracture as the structural phase decomposition of the material under cyclic loading.     

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.     

Phase and structure state of titanium loaded by spherically converging shock waves

The phase and structural states of titanium spheres loaded by spherical converging shock waves of various intensities were studied layer by layer by means of X-ray diffraction, optical, and transmission electron microscopy. It was established that defects of different types (twins, dislocations, and adiabatic shear bands) are produced during high-rate deformation occurring in materials under such method of pulsed loading. The amount and distribution of the defects depend on the loading intensity. The presence of the ω-phase is revealed only in the layers near the external surface of the titanium sphere after low-intensity loading.     

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,mechanical characteristics,and deformation and fracture features of quenched structural steel under static and cyclic loading after combined strain-heat nanostructuring treatment

In the work, we studied the special features of deformation and fracture of quenched steel 50 (0.51%) under static and cyclic tension after combined strain-heat nanostructuring treatment, which includes fictional treatment with subsequent tempering at 350°C. It is shown that the combined nanostructuring treatment of quenched steel 50 changes the character of plastic flow, making it more uniform, in the loaded material. Under static tension, this shows up as disappearance of the yield plateau early in the process, and under cyclic loading, as suppression of the deformation relief formed by shear and rotational deformation modes. Despite incipient cracks, the hardened surface layer thus escapes complete fracture throughout the fatigue loading and preserves its resistance to mechanical contact action.     

Collective properties of defects,multiscale plasticity,and shock induced phenomena in solids

A statistically based approach is developed for the construction of constitutive equations that provides linkages between defect-induced mechanisms of structural relaxation, thermally activated plastic flow, and material response to extreme loading conditions. The collective properties of defects have been studied to establish the interaction of multiscale defect dynamics and plastic flow, and to explain the mechanisms leading to the universal self-similar structure of shock wave fronts. Pn explanation for structural universality of the steady-state plastic shock front (the four power law) and the self-similarity of shock wave profiles under reloading (unloading) is proposed. Structural characterization under transition from thermally activated dislocation glide to nonlinear dislocation drag effects is developed in terms of scaling invariants (effective temperatures) related to mesodefect induced morphology formed during the different stages of plastic deformation.     

Structural mechanisms of plastic deformation of amorphous alloys containing crystalline nanoparticles

Patterns of plastic deformation of amorphous nanocrystalline composites, caused by the local action of an indenter on a thin electron microscopy foil, have been experimentally investigated for the first time in structural analysis. Classification of the observed types of interaction of shear bands with crystalline nanoparticles is performed. This classification is in good agreement with the theoretically predicted interaction mechanisms.     

局部塑性变形下铁基金属玻璃的致密化和非均匀性增强

本文研究了经局部塑性变形后, Fe₇₈Si₉B₁₃金属玻璃在原子尺度上的结构演变及其对合金显微硬度的影响.借助砂纸作为传力的媒介,充分放大了作用于带材表面上的等效压力,发生塑性变形后合金表面产生了大量的剪切带.基于倒空间和实空间的同步辐射X射线衍射分析,在塑性变形后,合金结构的致密度增大,过剩自由体积被排出,并由此揭示了Fe₇₈Si₉B₁₃金属玻璃在短程及中程尺度上原子协同重排行为.结合高分辨透射电子显微镜观察的结果, Fe₇₈Si₉B₁₃金属玻璃在发生塑性变形后,结构不均匀的程度将会加剧.此外,不同于单轴加载下金属玻璃的加工软化, Fe₇₈Si₉B₁₃金属玻璃在发生局部塑性变形后,维氏硬度增大,表现出局部的加工硬化行为.从自由体积的角度看,合金表面的大量剪切带可能是由于剪切带影响区域的重叠和交叉发生相互作用,并加速原子迁移,...     

Plastic deformation in mesocomposite materials under dynamic loading as applied to their joining with metals

The paper studies the effect of the amount and distribution pattern of nanoinclusions in a high-strength mesocomposite matrix on its plastic deformation under dynamic loading. The study is performed on mesocomposite specimens shaped as hollow thick-walled cylinders subjected to combined shear/compression loading with an explosive. It is found that homogeneous strain decreases with the growing volume fraction of nanoinclusions. The mechanical texture formed by the distribution of nanoinclusions in mesocomposite bars is shown to influence the deformation and cracking mechanisms. Additionally, the influence of structure is studied by computer simulation. The simulation has revealed that plastic deformation is rotational in the mesocomposite with chaotic structural distribution.     

Phase field modeling precipitation of β-phase and mechanical properties change in Zr–Nb alloys under neutron irradiation

ABSTRACT

We study microstructure transformation in Zr–Nb system under neutron irradiation and its mechanical properties change under mechanical loads in a form of shear deformation by using phase field methodology. The developed phase field approach takes into account defects dynamics based on reaction rate theory and elastic contribution to study mechanical properties change. A numerical modeling is provided in three stages: sample preparation, irradiation of the prepared sample and mechanical loading of the irradiated sample. A precipitation of β-Niobium particles of the size of several nanometers is discussed. Results of phase field modeling indicate that β-Niobium particles grow slowly during irradiation due to point defects rearrangement. Statistical analysis of dynamics of radiation-induced microstructure transformations is provided. Simulation results of shear deformation of pre-irradiated and post-irradiated alloys are discussed. Maps of local distribution of strain and stress and strain–stress curves are obtained. Results are verified with experimental data.     

Substructure formation in high-strength dispersely strengthened alloys

Conclusions Analysis of new experimental laws of plastic flow observed in high-strength alloys with dispersional strengthening (such as the formation of substructure with high crystal-lattice curvature, high-temperature localization of deformation from the earliest stages, with reorientation of the localized-shear zones and the adjacent undeformed structural elements) leads to the conclusion that deformational point defects play an important role in the realization of collective deformational modes in the high-strength state.In conditions of high nonequilibrium concentration, deformational point defects, first, permit the inclusion of quasi-viscous diffusional mechanisms of crystal-lattice reorientation by point-defect drift in the local fields of high inhomogeneous stress and, second, by facilitating dislocational deformation mechanisms, may lead to local weakening of the shear zones, localization of the plastic flow, and stability loss, in particular, as a result of mutually consistent autocatalytic defect multiplication.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 81–92, March, 1991.     

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.     

Phase transformations and thermal stability of the structure of Ti<Subscript>3</Subscript>Al intermetallic compound at deuteration and plastic deformation

The structural transformations in Ti₃Al intermetallic compound at deuteration with concentrations x = 1.2 and 1.7, heating at 100–400°C, and shear deformation under pressure have been studied. It is established that at a given deuterium concentration deuterides with fcc and orthorhombic lattices are formed; under severe shear deformation, nanocrystalline and amorphous (or close to amorphous) deuterides arise. The reasons for the structure amorphization at deuteration and subsequent plastic deformation are discussed.     

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.     

Evolution of the defect substructure in V-4Ti-4Cr alloy under severe plastic deformation

The evolution of the defect substructure in V-4Ti-4Cr alloy under its severe plastic deformation by torsion in Bridgman anvils is studied by transmission electron microscopy. Nanoband structural states with a dipole or multipole character of misorientations and a crystallite (or nanoband) size varying from several to several tens of nanometers form in the true logarithmic strain range e ≈ 3.0−6.6. Such crystallites form inside 100-nm submicrocrystallites or coalesce (at e ≥ 6) to yield mesobands with a pronounced vortex character of their propagation. The formation of these states is related to the activation (by the flows of nonequilibrium point defects in stress fields) of quasi-viscous deformation and lattice reorientation mechanisms, which provide the generation and propagation of partial disclination nanodipoles followed by the development of collective effects in a disclination substructure. These effects lead to the group motion of nanodipoles inside the mesobands.