V. E. Panin

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.

Molecular dynamics study of cluster structure and rotational wave properties in solid-state nanostructures

Molecular dynamics simulation was performed to study the formation of cluster structure, interfaces, and surfaces with different curvature radii in a perfect nanocrystal passed through by a nonlinear wave. It is shown that this process is a type of nanostructure self-organization in response to an external energy flux with subsequent development of a strong rotational field.

Scale invariance of structural transformations in plastically deformed nanostructured solids

The scale-invariant mechanical behavior of a nanostructured solid is associated with plastic distortion as a major mechanism of nano- and microscale structural transformations. Active grain boundary sliding in a deformed material (microscale) within its highly developed planar subsystem (nanograin boundaries) causes a progressive increase in lattice curvature and plastic distortion of atoms which produces nonequilibrium vacant sites in the nanostructure. The motion of nonequilibrium point defects in nanostructure curvature zones provides conditions for noncrystallographic plastic flow, dissolution or dispersion of initial phases, and formation of nonequilibrium phases in a deformed material. The possibility of reversible structural phase transformations in the presence of high lattice curvature opens the way to greatly increase the fatigue life of surface nanostructured polycrystalline materials.

Micromechanisms of deformation and fracture in a VT6 titanium laminate under impact load

The paper studies the phase composition, microstructure, and mechanisms of plastic deformation and fracture under impact load in a laminate obtained by pressure welding of VT6 titanium alloy sheets. Under impact loading at 20 and -196°C, the material is delaminated into sheet piles with attendant changes in their fracture rate. At fracture surfaces, the initial crystal structure experiences structural phase decomposition which results in dynamic rotations. In fracture and delamination sublayers, the material is fragmented. The effects are more pronounced at Tdef = -196°C.

Meso- and microstructural features of steel 12GBA produced by different methods of thermomechanical treatment

The effect of uniform isothermal forging (UF) and warm rolling (WR) on the structure of low-carbon tube steel 12GBA has been studied. It is shown that the structures of the treated steel differ significantly by the effective grain size, density of all boundaries, percentage of density of high angle boundaries (HABs) and low angle boundaries (LABs), carbide phase morphology in the perlite zone and texture of the ferrite phase. After forging steel has the greatest degree of grain refinement, maximum boundary density, and overrepresentation of LABs. This structural state of steel is characterized by a double-component texture: (001) + (111), 〈001〉 + 〈101〉, while after warm rolling steel has a mono-component texture (111) 〈101〉. The evident differences in the steel structure treated by WR and UF may have dual effect on the strength and plasticity properties of steel and its fracture behavior.

The effect of thermal aging on the strength and the thermoelectric power of the Ti-6Al-4V alloy

When the Ti-6Al-4V alloy is overaged at 500-600°C, nanometer-sized α2 (Ti3Al) particles can be homogeneously precipitated inside a phases, thereby leading to strength improvement. Widmanstätten and equiaxed microstructures containing fine α2 (Ti3Al) particles were obtained by overaging the Ti-6Al-4V alloy. Precipitation of α2 (Ti3Al) particles was monitored using thermoelectric power measurements for different aging conditions in the Ti-6Al-4V alloy. Overaging heat treatments were conducted at 515, 545 and 575°C for different aging times. In addition, overaging samples were examined by optical microscopy, scanning electron microscopy and hardness measurements. It was found that the thermoelectric power is very sensitive to the aging process in the two studied Ti-6Al-4V structures.

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.

Scientific basis for cold brittleness of structural BCC steels and their structural degradation at below zero temperatures

The paper considers the physics of cold brittleness of structural bcc steels and methods of reducing the ductile-brittle fracture temperature. A complex study was performed to examine the degradation of structural phase state of pipe steel 09Mn2Si from the main gas pipeline of Yakutia after long-term (over 3 0 years) operation. Important regularities of degradation of pearlite colonies with carbide precipitation on ferrite grain boundaries were revealed. This phenomenon is associated with brittle fracture of gas pipelines. It is shown that the low-temperature kinetic processes in main pipelines which define the degradation of their structure and properties are related to interstitial athermal structural states in the zones of local crystal structure curvature. This is a fundamentally new, as yet unknown, mechanism. Pipe steels in warm rolling acquire a longitudinal textured band structure with alternating bands of initial ferrite grains and bands of fine grains with carbide precipitates formed during lamellar pearlite degradation. This type of structure allows for a shift of ductile-brittle transition temperature down to -80°C and ductility δ = 22% at this temperature. The production of high-curvature vortex structure in pipe steel surface layers results in a 3.5-fold increase in their service life.

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.