Plastic Flow Localization Research Articles

Температурная зависимость автоволновых характеристик локализованной пластичности

The behavior of autowaves of localized plastic flow in the Fe-Ni-Cr alloy at temperatures of 143≤ T≤ 420 K is considered. The temperature dependence of the propagation velocity of the autowave is studied. It was found that for the region of low temperatures the inverse proportionality of the autowave velocity to the strain hardening coefficient and the quadratic dispersion law are fulfilled. The temperature independence of the elastoplastic deformation invariant is established.

Autowave Plasticity of Metals and their Positions in the Periodic Table of Elements

General patterns of localized plastic flow development are described at linear work hardening stages of nineteen metals. Correlations between the position of the element in the periodic table of elements and the parameters of the autowave localized plastic flow have been established and discussed. The observed correlations indicate the existence of dependences of the localized plastic flow in a deformable medium on the lattice parameters of the elements and their electronic structures.

Plasticity of Metals and Mendeleev’s Periodic Law

The laws of localized plastic flow in various metals are considered. The relations between the plasticity characteristics of a metal and its position in the periodic table and its lattice characteristics are found. A model is proposed to explain the nature of these laws using the concepts of dislocation drag by an electron gas.

A Scale Effect Accompanying Autowave Plastic Strain

The relationship between the spatial parameter of the development of a localized plastic flow (the length of the localized plasticity autowave) and the length of the deformed sample is studied. According to experiments carried out on polycrystalline samples of a zirconium alloy and commercial purity aluminum, these quantities are related to each other by a logarithmic law valid at the stage of parabolic strain hardening.

Effect of pre-solution treatment on deformation behavior of AZ80-Ag magnesium alloy

To improve the deformation uniformity and plasticity of AZ80-Ag magnesium alloy, solid solution treatment was introduced before deformation. Isothermal compression tests for both as-homogenized specimens and pre-solution treated specimens were carried out and compared systematically. The results show that the coarse discontinuous precipitates (CDP) of β-Mg17Al12 were fully dissolved by the pre-solution pre-treatment, and the flow stresses were decreased greatly by 12.4–26.3MPa. Moreover, the unstable region with the local plastic flow and cracking defects was significantly reduced. Eventually, uniform deformation is achieved for AZ80-Ag magnesium alloy by the pre-solution treatment, which shows great potential for magnesium alloy engineering application. Then, microstructure observations such as OM and EBSD were carried out and analyzed. Abundant lamellar {10–12} extension twinning appear 86.3° <11–20> relationship occurs after pre-solution treatment, rather than few needle-like {10–12} extension twinning appear 7.4° <1–210> relationship in as-homogenized specimen, is found out to be responsible for deformation plasticity improvement and defects eliminating. On the one hand, stress concentration along deformation twins are released by {10–12} extension twinning appear 86.3° <11–20> effectively, which is verified by Kernel Average Misorientation (KAM) of the as-homogenized and pre-solution treated specimen. In the other hand, twinning induced dynamic recrystallization (TDRX) inside high dense lamellar twins is the main DRX mechanism for the pre-solution treated specimen, which accelerates DRX and further releases stress concentration.

In-situ study of initiation and extension of nano-thick defect-free channels in irradiated nickel

Radiation defects-induced plastic flow localization is the origin of loss of ductility in irradiated metals. Defect-free channels (DFCs) are a typical form of strain localization that lead to crack initiation and premature failure. A comprehensive understanding of the DFC dynamics is key to managing radiation boosted property degradation. Despite great research efforts, a clear mechanism of DFC remains unknown. Here, our in-situ tests on irradiated Ni pillars provide a real-time observation of the dynamics of DFCs, including DFC initiation, extension and thickening. The merging and spreading of dislocation loops serve as an alternative mechanism of dislocation sources that emit massive dislocations and initiate nano-thick DFCs inside the grain. Nano-thick DFCs were formed through chopping up or sweeping away of loops by mobile dislocations. Annihilation of opposite loops and interactions between loops and vacancies accelerate DFC extension. Activation of multiple dislocation sources and dislocation cross-slips are the mechanisms for DFC thickening.

Hydrodynamic plastic flow in metal materials

The band structures (microbands and shearbands) were first discovered and attracted attention of researchers in connection with plastic flow localization and absence of dislocations sliding signs during deformation. Experimental investigations of structure formations during plastic deformation conducted in subsequent 70 years did not make clear the nature of these bands. Nevertheless the study of structure formation in various metals and alloys at different means of deformation revealed the bands to arise in chaotic ensemble of dislocations after destruction of previous cell structure, to become on all zoom levels and to localize the deformation. Taking into account the plastic deformation to pass far from thermodynamic equilibrium we proposed the synergetic model of these structures formation in metallic crystals. It was revealed the band structures to secure the possibility of plastic deformation without dislocations by hydrodynamic plastic flow.

Autowave Plasticity: Principles and Possibilities

Basic concepts of the autowave model of development of a localized plastic flow of solids of different natures are considered. It is shown that plastic deformation develops in a localized (at the macroscale level) way throughout the process. The form of the observed localization patterns is related to stages of strain hardening of the material. The patterns are projections of different modes of the localized plastic flow autowave on the observation surface. An elastoplastic strain invariant is introduced, which is considered as the main equation of the autowave plasticity model, and its physical nature is discussed.

Autowave process of plastic flow localization

The localized plasticity development was studied for a wide range of pure metals and alloys in single-crystal and polycrystalline state as well as nonmetallic materials, alkaline halide crystals, ceramics and rocks. Using photographic and digital versions of speckle photography, the localization of plastic flow in the test samples was investigated on specially designed units having high spatial and temporal resolution. According to the analysis given here, the different stages of plastic flow are related both qualitatively and quantitatively.

Ductile failure viewed as a final stage of the autowave process of plastic flow localization

The localized plasticity development was studied for a wide range of pure metals and alloys in single-crystal and polycrystalline state as well as nonmetallic materials, alkaline halide crystals, ceramics and rocks. Using photographic and digital versions of speckle photography, the localization of plastic flow in the test samples was investigated on specially designed units having high spatial and temporal resolution. According to the analysis given here, the different stages of plastic flow are related both qualitatively and quantitatively.

Hall–Petch Relation and the Localized Plasticity Parameters

The mechanical properties and the localized plasticity characteristics of polycrystalline aluminum are compared to explain the nature of the Hall–Petch relation for the deformation of a polycrystalline metal in two grain size ranges. The Hall–Petch relation for the flow stress is shown to be related to an elastoplastic deformation invariant, which unites the characteristics of elastic deformation and localized plastic flow.

Modeling of Plastic Flow Localization and Crack Formation in Dynamic Channel Angular Pressing

The paper studies the phenomenon of localized-deformation macrobands and crack formation in metal alloys under dynamic channel-angular pressing. The numerical calculations are carried out using a threedimensional model of plastic deformation, which combines mathematical models of dislocation kinetics and mechanics of solids. Various scenarios of strain hardening the deformable medium element and the geometry of pressing channels are considered in relation to localized-deformation macrobands and crack formation during dynamic channel angular pressing. It is shown that non-monotonous strain hardening of the deformable medium element and the angle of channel connection are the important factors that form localized-deformation macrobands and cracks.

Modes of grain growth and mechanism of dislocation reaction under applied biaxial strain: Atomistic and continuum modeling

The phase field crystal method and Continuum Modeling are applied to study the cooperative dislocation motion of the grain boundary (GB) migration, the manner of the nucleation of the grain and of the grain growth in two dimensions (2D) under the deviatoric deformation at high temperature. Three types of the nucleation modes of new finding are observed by the phase field crystal simulation: The first mode of the nucleation is generated by the GB splitting into two sub-GBs; the second mode is of the reaction of the sub-GB dislocations, such as, the generation and annihilation of a pair of partial Frank sessile dislocation in 2D. The process can be considered as the nucleation of dynamic recrystallization; the third mode is caused by two oncoming rows of the dislocations of these sub-GBs, crossing and passing each other to form new gap which is the nucleation place of the new deformed grain. The research is shown that due to the nucleation of different modes the mechanism of the grain growth by means of the sub-GB migration is different, and therefore, the grain growth rates are also different. Under the deviatoric deformation of the applied biaxial strain, the grain growth is faster than that of the grain growth without external applied stress. It is observed that the cooperative dislocation motion of the GB migration under the deviatoric deformation accompanies with local plastic flow and the state of the stress of the system changes sharply. When the system is in the process of recrystallized grain growth, the system energy is in an unstable state due to the release of the strain energy to cause that the reverse movement of the plastic flow occurs. The area growth of the deformed grain is approximately proportional to the strain square and also to the time square. The rule of the time square of the deformed grain growth can also be deduced by establishing the continuum dynamic equation of the biaxial strain-driven migration of the GB. The copper metal is taken as an example of the calculation, and the obtained result is a good agreement with that of the experiment.

LOCALIZED PLASTIC DEFORMATION AND PERIODIC TABLE

: We consider the autowave mechanism of evolution of a localized plastic deformation of crystalline solids of different origins. It is found that localization of the plastic flow is determined by the relation between elastic and plastic phenomena in deforming materials. The origin of this relation is explained using general thermodynamic concepts. It is shown that parameters of plastic flow localization at the stage of linear strain hardening in the process of solid body deformation are closely related to characteristics of the electronic structure of metals. Inverse of the product of wavelength and velocity of the localized plasticity autowave grows linearly with the number of conduction electrons per unit cell of the metal. The obtained data directly indicate that the nature of the electron-gas contribution to hindering of dislocations is more complicated. Thus, the parameters of plastic flow localization are defined by the position of metal in the Mendeleev periodic table.

Strain-hardening and suppression of shear-banding in rejuvenated bulk metallic glass.

Strain-hardening (the increase of flow stress with plastic strain) is the most important phenomenon in the mechanical behaviour of engineering alloys because it ensures that flow is delocalized, enhances tensile ductility and inhibits catastrophic mechanical failure1,2. Metallic glasses (MGs) lack the crystallinity of conventional engineering alloys, and some of their properties-such as higher yield stress and elastic strain limit3-are greatly improved relative to their crystalline counterparts. MGs can have high fracture toughness and have the highest known 'damage tolerance' (defined as the product of yield stress and fracture toughness)4 among all structural materials. However, the use of MGs in structural applications is largely limited by the fact that they show strain-softening instead of strain-hardening; this leads to extreme localization of plastic flow in shear bands, and is associated with early catastrophic failure in tension. Although rejuvenation of an MG (raising its energy to values that are typical of glass formation at a higher cooling rate) lowers its yield stress, which might enable strain-hardening5, it is unclear whether sufficient rejuvenation can be achieved in bulk samples while retaining their glassy structure. Here we show that plastic deformation under triaxial compression at room temperature can rejuvenate bulk MG samples sufficiently to enable strain-hardening through a mechanism that has not been previously observed in the metallic state. This transformed behaviour suppresses shear-banding in bulk samples in normal uniaxial (tensile or compressive) tests, prevents catastrophic failure and leads to higher ultimate flow stress. The rejuvenated MGs are stable at room temperature and show exceptionally efficient strain-hardening, greatly increasing their potential use in structural applications.

3D study of plastic flow localization at a void-sheet

A single layer of voids in a ductile material is considered to study the effect of this material imperfection on the onset of shear band localization, and to study the failure evolution in the void-sheet mechanism. The layer is inclined relative to the main tensile direction, and the initially spherical voids are taken to be uniformly spaced both along the inclined direction and transverse to it. The material is elastic-viscoplastic with isotropic hardening. When the solid is subjected to a prescribed strain-rate and the shear band width is negligible compared to specimen dimensions, an infinite ratio of the strain-rate in the band relative to the overall prescribed strain-rate defines the onset of localization. For each stress-state analyzed a limited number of initial inclinations of the void layer are considered, with an angular spacing of 5 degrees, and the angle of inclination giving the smallest critical strain is used as a good approximation of the critical band orientation. Different initial void sizes, relative to the void spacing, are considered. It is shown that increased void spacing in the transverse direction significantly delays localization. Also the effects of different levels of strain hardening are compared. The evolution of the void shape during shear localization is studied, to be able to follow the void-sheet failure mechanism. The critical strain for the onset of void coalescence is compared with the critical strain for shear bands.

Experimental study of macroscopic localization of plastic flow in al-mg alloy under complicated stress-strain state

The main objective of this work is to investigate the spatial-time inhomogeneity due to the jerky flow during plastic deformation of Al-Mg alloy under complex stress conditions. The use of specimens in the form of plates with a rigid circular rim and a reverse curvature rim was considered. Experimental data on the evolution of inhomogeneous strain and temperature fields have been obtained, illustrating the processes of initiation and propagation of deformation bands of the localized plastic flow by using the digital image correlation and infrared thermography.

Formation of bulk-laminated materials by localized deformation during ECAP of an AA6060 aluminum alloy

This paper addresses localized plastic flow during equal-channel angular pressing (ECAP) of an AA6060 aluminum alloy. We observe an alternating formation of shear bands and matrix bands during ECAP that leads to pronounced strain partitioning without cracking. Local deformation is analyzed by considering the distortion of indents along flow lines in the center of a split billet. We estimate equivalent strains of about 3.6 inside the shear bands, whereas plastic deformation in the adjacent matrix bands is almost negligible. Microstructural analysis by SEM and STEM confirms that the shear bands exhibit typical features of severely plastically deformed microstructures at the onset of forming an ultrafine-grained microstructure. We further present statistics of band widths, and we discuss the roles material hardening as well as ECAP die geometry (in terms of the inner die radius) in facilitating the recurrent localized deformation that, in the absence of crack nucleation, leads to the production of an interesting and novel type of bulk-laminated materials by ECAP.

Кинетика развития фронтов пластического течения на границе раздела металлов

Кинетика развития фронтов пластического течения на границе раздела металлов

Температурная зависимость автоволнового механизма пластического течения

The behavior of the localized plastic flow autowaves characteristics is considered in the Fe-Ni-Cr alloy for the temperature range from 143 K to 420 K (420 K is the Debye temperature). It is shown that the multiplication of the autowave length and the velocity of its propagation, in the specified interval, decreases with temperature. At the same time, the ratio of this value to the viscosity of the phonon gas in the metal in the investigated temperature range remains practically constant. This indicates that the elastic-plastic invariant of deformation is independent of temperature.