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.     

Multiscaling of lattice curvature on friction surfaces of metallic materials as a basis of their wear mechanism

A comprehensive structural study has been performed to explore deformation and wear debris formation on friction surfaces of metallic materials. A hierarchy of structural scales of plastic deformation and failure during wear has been established. The nanoscale plays the major role in the hierarchical self-organization of multiscale debris formation processes. On this scale, bifurcational interstitial states arise in zones of local lattice curvature, with plastic distortion and motion of nonequilibrium point defects which determine the nonlinear dynamics of structure formation and wear of surface layers. Nonequilibrium vacancies on lattice sites form microporosity through the coalescence mechanism under plastic distortion. The microporosity is a precursor of meso- and macroscale plastic shearing that defines wear debris formation.     

Scale invariance of plastic deformation of the planar and crystal subsystems of solids under superplastic conditions

The theory of structural transformations in the planar sybsystem (surface layers and internal interfaces) of solids under plastic deformation is developed. The theory is based on a consideration for local curvature of the crystal lattice, with new structural states arising in its interstices, responsible for plastic distortion. To satisfy the superplastic condition, such high-rate mechanisms should develop in both planar and 3D crystal subsystems. In a translation-invariant crystal, this condition is met by concentration fluctuations. The multiscale criterion of superplasticity is formulated based on the scale invariance of plastic deformation of the planar and crystal subsystems in a deformable solid. Beyond the criterion, superplasticity passes to the creep mode with restricted plasticity of the material.     

Effects of plastic distortion in the lattice curvature zone of a crack tip

The paper proposes a discrete-continual method of excitable cellular automata for simulating the stress-strain state at crack tips and in notches with account of lattice curvature and plastic distortion through ion motion from lattice sites to interstices. The proposed nonlinear method allows one to determine the crack type and the character of fracture, to predict the possibility of dynamic rotations and structural turbulence, and to describe the processes of nonlinear wave structural transformations in strain localization bands involved in microporosity and tearing mode cracking.     

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.     

The dynamic symmetry and plastic deformation of diamond lattice

An analysis of possible transformations of moments of force by a diamond lattice has been made. The conditions of resonance transformations of moments of force by this lattice have been found. The dynamics symmetry of the diamond lattice has been determined in terms of the Shubnikov group Fd3m'. Besides, it has been found that resonance librationally deformed configurations are oneperiodic and have cores 〈110〉 two times more deformed than the surrounding area. As a result of the action of resonance phonons of the librational mode it is possible to generate a new kind of defects-bidislocations with Burgers vector components+[110]/2 and −[110]/2. It has been shown that bidislocations can play an essential role in plastic deformation of a diamond lattice and other related lattices.     

Structural states and specific features of the deformation behavior of metals and alloys with mixed nano-and micrograin structure

A classification of the structural states of materials with a mixed nano-and microcrystalline structure is proposed. Theoretical analysis of the structural mechanisms and peculiarities of plastic flow of singlephase and two-phase nanostructured metals and alloys with a bimodal size distribution of grains and phases is performed. The effect of grain-boundary and dislocation mechanisms of plastic flow on the specific features of the deformation behavior and plasticity of nanocrystalline materials is analyzed. A microstructural model of strain hardening of a material with two-scale nano-and micrograin structure is proposed and the condition for the loss of plastic flow stability of such a material is investigated. The dependence of the yield strength and uniform strain of nanocrystalline materials with a two-scale structure on the grain size and the ratio of the volume fractions of the nano-and microstructural components is calculated.     

Lattice curvature evolution in metal materials on meso- and nanostructural scales of plastic deformation

The paper generalizes results of electron microscopy studies of structural states with high lattice curvature which arise in a wide class of materials under various conditions of severe plastic deformation: rolling, equal channel angular pressing, mechanical activation in planetary ball mills, and torsion in Bridgman anvils. The states are divided into two types: 1) a substructural state with elastoplastic lattice curvature of tens of degrees per micron due to high density of like-sign excess dislocations; and 2) a state with elastic lattice curvature up to several hundreds of degrees per micron in volumes of several nanometers. Analysis is performed to inquire into the formation of these states, peculiarities of their evolution, and their role in different mechanisms of plastic deformation and formation of nanocrystalline structures.     

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.     

Structural transformations in carbon nanoparticles induced by electron irradiation

This paper reviews the electron-irradiation effects in graphitic nanoparticles. Irradiation-induced atomic displacements cause structural defects in graphite lattice forming the basis of carbon nanoparticles such as nanotubes or carbon onions. Defects of the type of non-six-membered rings induce topological alterations of graphene layers. The generation of curvature under electron irradiation leads to the formation of new structures, such as spherical carbon onions or coalescent nanotubes. At high temperatures, the self-compression of carbon onions can promote the nucleation of diamond cores or phase transformations of foreign materials that are encapsulated by onionlike graphitic shells. Under the nonequilibrium conditions of intense irradiation, the phase equilibrium between graphite and diamond can be reversed. It is shown that graphite can be transformed into diamond even if no external pressure is applied. All electron-irradiation and electron-microscopic studies described here were carried out using in situ transmission electron microscopy.     

Three-dimensional quantification of texture heterogeneity in single-crystal aluminium subjected to equal channel angular pressing

Abstract

A three-dimensional crystal plasticity finite element method (3D CPFEM) modelling of a real equal channel angular pressing (ECAP) process for investigating the mechanical properties and texture evolutions of single-crystal aluminium has been developed for the first time. The challenge of modelling such a severe plastic deformation via 3D CPFEM is how to accurately predict the deformation mechanism under the complicated contact conditions between a billet and a die. The validation by comparison with experimental observations demonstrates that the developed 3D CPFEM ECAP model is able to precisely capture the deformation characteristics at the microscale. Furthermore, this research clarified the previously remaining disputes such as the microstructural formation mechanism in the deformed area and the deformation nature in the plastic deformation zone. It is also the first time to extensively discuss the orientation-dependent deformation feature of the ECAP-processed billets, including morphology, lattice rotation angle and grain refinement.     

Recovery of Young’s modulus upon annealing of nanostructured niobium produced through severe plastic deformation

The effect of temperature (in the range 20–500°C) on the Young’s modulus of nanostructured niobium with Ta impurity content <0.5 wt % and that of O₂ < 0.1 wt % and with a mean grain size of ?200 nm is studied. The transformation of polycrystalline niobium into a nanostructured state is performed through severe plastic deformation by equal-channel angular pressing. The Young’s modulus is found to increase in two stages as the temperature of isothermal annealing is gradually increased. The mechanisms of recovery of the elastic modulus upon annealing of the nanostructured niobium are discussed in the context of the modern concepts of the defect structure of deformed metals.     

Differential-aperture X-ray structural microscopy: a submicron-resolution three-dimensional probe of local microstructure and strain

A recently developed differential-aperture X-ray microscopy (DAXM) technique provides local structure and crystallographic orientation with submicron spatial resolution in three-dimensions; it further provides angular precision of approximately 0.01 degrees and local elastic strain with an accuracy of approximately 1.0 x 10(-4) using microbeams from high brilliance third generation synchrotron X-ray sources. DAXM is a powerful tool for inter- and intra-granular studies of lattice distortions and lattice rotations on mesoscopic length scales of tenths of microns to hundreds of microns that are largely above the range of traditional electron microscopy probes. Nondestructive, point-to-point, spatially resolved measurements of local lattice orientations in bulk materials provide direct information on geometrically necessary dislocation density distributions through measurements of the lattice curvature in plastically deformed materials. This paper reviews the DAXM measurement technique and discusses recent demonstrations of DAXM capabilities for measurements of microtexture, local elastic strain, and plastic deformation microstructure.     

Nonequilibrium Diffusional Phase Transformations in Alloys Induced by Migration of Grain Boundaries and Dislocations

Physics of the Solid State - The main scenarios of nonequilibrium diffusional transformations induced by moving defects (dislocations, grain boundaries) in alloys under severe plastic deformation...     

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).     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.     

Silver-Doping Induced Lattice Distortion in TiO2 Nanoparticles

The Ag-doping effects on Ti02 nanoparticles are investigated by means of x-ray diffraction （XRD） and Raman scattering spectroscopy. XRD and Raman results indicate that Ag-doping stabilizes the rutile phase in TiO2. We find an Ag-doping induced lattice expansion in both anatase and rutile phases. The Ag-doping has different influences on the lattice distortion for anatase and rutile phases, that is, the e/a-value for the anatase phase decreases with 0.5% Ag-doping and then increases with 1~ Ag-doping while that for the rutile phase shows a gradual increase with increasing Ag-doping. We have ascribed the different variations of lattice distortion due to Ag-doping to the change of interracial interaction between the anatase and rutile phases induced by different Ag concentrations.     

Fundamental role of crystal structure curvature in plasticity and strength of solids

In the paper, we use the nonlinear multiscale approach of physical mesomechanics to demonstrate that the scales of local crystal structure curvature in solids play a fundamental role in the generation of strain-induced defects and cracks. It is shown that strain-induced defects arise at the interfaces of 2D planar and 3D crystal subsystems by the mechanism of “laser pumping” and cracks nucleate as structural phase decay in the zones of crystal structure curvature where the nonequilibrium thermodynamic potential or so-called Gibbs energy is higher than zero. Nonlinear fracture mechanics eliminates the problem of singularity 1/r in equations of crack growth but requires accounting for local lattice curvature at the crack tip.     

Thermodynamic and kinetic aspects of the weakening of ionic crystals by pulsed magnetic fields

The effect of short (10⁻² s) magnetic-field pulses (B=7 T) on the plastic flow of ionic crystals is investigated. The stages of the transition process stimulated by a field in the system of structural defects of the lattice are distinguished. The results are discussed from the standpoint of the thermodynamics and kinetics of processes in nonequilibrium systems. Fiz. Tverd. Tela (St. Petersburg) 39, 2016–2018 (November 1997)