Plastic Flow Processes Research Articles

Microstructure and grain evolution mechanisms of copper/steel explosive welding interface

By integrating smoothed particles hydrodynamics (SPH) simulation, advanced characterization, and theoretical analysis, the interfacial transient behaviors of copper/steel composite during explosive welding were deeply studied, and the evolution mechanisms of microstructures were specifically explained. According to simulated data, the entire evolution of wave was reconstructed, and the formation mechanisms of various interface features were revealed. The multi-scale EBSD analyses showed that the grain structures were diverse in the regions adjacent to the interfaces. The evolution mechanisms of these grains were mainly governed by three processes of plastic flow, dynamic recrystallization, and solidified nucleation. Finally, the correlations between the grain structures and mechanical properties were established by nanoindentation tests.

Microstructure and formation mechanism of the transition layer at the interface of Al–Cu EMPW joint

In the dissimilar metal welding field, electromagnetic pulse welding (EMPW) is a kind of potential technology. To further reveal the bonding mechanism of EMPW, this work carried out a Cu–Al sheet EMPW experiment with a discharge voltage of 15 kV and a gap of 1.5 mm. It analyzed the microstructure and formation mechanism of the Cu–Al EMPW joint bonding interface transition layer in detail by SEM, EDS, TEM, and FIB. The transition layer was found at the vortex zone, and there was an isolated long strip Cu particle separated from the main body of the Cu sheet. The tested results showed that the transition layer contained Al-rich intermetallic compound (IMC) Al2Cu, and some bulk particles in the transition layer were Cu-rich IMCs Al4Cu9 and AlCu4. There were Cu-rich IMCs Al4Cu9, AlCu4, and AlCu between the long strip Cu particle and the IMC transition layer. In the violent impact process between the Cu sheet and Al sheet, the Cu rapidly diffused in the Al matrix to form the Al-rich IMC (Al2Cu) as the primary transition layer, and the interface was high temperature and high pressure, forming a plastic flow and vortexes swirling. Some Cu particles separated from the Cu sheet and became the vortex piece during the plastic flow process, and continued to interact with the Al2Cu primary transition layer to form the secondary transition layer. This work could provide a basis for revealing the structure composition and formation mechanism of the Cu–Al EMPW joint interface transition layer.

Effect of heat treatment on deformation inhomogeneity of carbon steel / stainless steel bimetal

The work is devoted to the study of the effect of annealing on mechanical properties and inhomogeneity of plastic deformation of a bimetallic plate made of stainless / carbon steel with the dimensions of the working part 50×7×2 mm. To develop laser technology for producing bimetals of various compositions, the contact zone of two dissimilar steels is of greatest interest. Since the performance characteristics of the entire product as a whole depend on the structure and properties of this zone, interaction of the components of the bimetal in the process of its manufacture leads to appearance of heterogeneity of various types near the interface and in the volumes adjacent to it. The research material was obtained by laser cladding of wire AISI 304 stainless steel on a plate of low-carbon steel St3. Bimetallic samples were subjected to vacuum heating at a temperature of 700 °C at various times from 2 to 8 h. The use of data on the distributions of local strains by the speckle photography method made it possible to consider the process of plastic flow in the initial section of tension diagram and to establish the effect of annealing temperature on plastic strain localization during mechanical tests. For a quantitative assessment of deformation inhomogeneity in the main and cladding layers, we used spatiotemporal distributions of local elongations and the corresponding values of the variation coefficient. It was established that the level of deformation inhomogeneity of microvolumes at the interface during tension is higher than that of the bimetal main layers. With increase in the annealing time, increase in the variation coefficient in the joint zone is noted, which is more significant on the stainless steel side, and this increases the probability of microcracks initiation. The increased level of deformation inhomogeneity of microvolumes of the cladding layer carburized zone is contingent on the increased localization of deformation in nearby microvolumes due to structural heterogeneity.

Fracturing of Solids as a Thermodynamic Process

Instead of a number of different approaches or a formal description of experimental data, a unified approach is proposed to consider failure and deformation as thermodynamic processes. Mathematical modeling of the processes is carried out using rheological models of the material. Parametric identification of structural models is carried out using minimal necessary experiments. Based on results of these experiments, the scope of applicability conditions for this material and test modes necessary for parametric identification of models are selected. One fracture criterion is used that formally corresponds to the achievement of a threshold concentration of micro-damage in any volume of the material. Calculations of durability under conditions of varying temperature and variable loads are based on the relationship of plastic flow and failure processes distributed over the volume of the material. They are performed numerically over time steps depending on the ratio of the rate of change of temperature and stresses.

Macroscopic Spatio-Temporal Patterns of Localized Plastic Flow Development under Tension of Fe-Cr-Ni Alloys

This study examines the patterns of macroscopic inhomogeneity and localization of plastic flow in Fe-Cr-Ni polycrystals subjected to uniaxial tension. By varying the test temperature, the deformation curves can be altered, allowing us to explore the behavior of the material under different conditions. The stress-strain diagrams obtained from the samples reveal the areas of elastic deformation, plastic flow, and fracture. To experimentally study plastic flow, we used speckle photography to accurately reconstruct the displacement vector fields and calculate the components of the plastic distortion tensor. The results showed that plastic flow is localized at all stages of the process for test temperatures of+65, +24, and -60 °C. The forms oflocalization are entirely determined by the dependence of the strain hardening coefficient on the deformation during the relevant stage of the process. An analysis of the spatial distributions of local shifts and local rotations was conducted. We observed spatially periodic structures that can be interpreted as autowave processes and explained in terms of the concept of self-organization of the plastic flow process.

"NUMERICAL SIMULATION OF ELASTIC-PLASTIC CONTACT WITH ISOTROPIC HARDENING "

The contacts of mechanical components transmit loads that lead to subsurface stresses developing in the contacting bodies. In an efficient tribological design, these stresses are expected to remain under the yield strength of the softer contacting material. When this condition is not met, plastic flow occurs in the softer body. Under the assumption of isotropic hardening, the yield strength increases with the development of additional plastic strains. As plastic flow processes are dissipative and therefore path dependent, the elastic-plastic problem is unsolvable through analytical endeavours, but can be approached with a numerical algorithm capable of simulating the loading history. The Betti’s reciprocal theorem provides the theoretical framework for the application of superposition principle to elastic-plastic stresses and displacement. An algorithm consisting in three nested loops is assembled from the solutions of simpler problems: (1) the purely elastic rough contact problem, (2) the inclusion problem and (3) the problem of the plastic strain increment. The numerical simulations suggest that the residual stresses decrease the intensity of the total stresses, thus delaying additional plastic flow. With increasing load, the heart-shaped plastic strain volume advances toward the surface, enveloping a plastic core near the initial point of contact. Compared to the purely elastic case, the elastic-plastic pressure shows a flatter distribution, while the contact radius is increased.

Autowave Process of Plastic Flow in Cu–Ni Alloys

Autowave Process of Plastic Flow in Cu–Ni Alloys

Study on thermal compression deformation behavior and constitutive model of homogenized Mg–5Zn–1Mn alloy

The thermal compression deformation behavior of homogenized Mg–5Zn–1Mn (wt.%) alloy was systematically investigated over the temperature and strain rate ranges of 285–345 °C and 0.001–1 s−1, respectively. The constitutive model and thermal processing diagrams were established. The microstructure evolution process and dynamic recrystallization (DRX) mechanisms were analyzed. The results demonstrate that the flow stress curves of the ZM51 alloy exhibit prominent DRX characteristics. The peak stress can be improved by decreasing the deformation temperature or increasing the strain rate. A finite element simulation is performed, and the Arrhenius constitutive model can accurately depict the plastic flow process of the alloy with strain correction, and the average activation energy is about 143.821 kJ/mol. Both the percentage DRX and the DRX grain size are found to be sensitive to the applied deformation conditions, including temperature, strain rate, and true strain. The formation of DRX grains promotes the weakness of the texture and the refinement of the grains. Grain boundaries nucleation (including discontinuous and continuous DRX) and particle-stimulated nucleation (PSN) are the main DRX mechanisms. Based on the processing map, ZM51 alloy is suitable for large plastic deformation by multi-pass forging. The optimum processing temperature is 345 °C, and as the degree of plastic deformation increases, the strain rate can be suitably increased. Local plastic flow and microcracking at grain boundaries are the primary instability mechanisms of the alloy.

Nanoscale patterning of metallic surfaces with laser patterned tools using a nanoimprinting approach

In this study, we demonstrate that metallic substrates with a nanocrystalline grain size can be structured down to the micro- and nanometre range during a room temperature nanoimprinting process. WC-Co hard metal dies are patterned by Direct Laser Interference Patterning (DLIP), generating an array of separated asperities with a height of 4.7 µm and a spacing of 11.3 µm. Additionally, Laser Induced Periodic Surface Structures (LIPSS) with a spacing of 500 nm are formed as a hierarchical substructure. The patterned dies can be used to deform metallic substrates due to their high hardness. For structure replication, the WC-Co tool is pressed onto a nanocrystalline CuZn30 model alloy at room temperature. The pattern transfer process from the hard metallic tool to the nanocrystalline alloy is discussed in detail: At low contact pressures, the asperities on the tools act as single contact points, leading to the formation of separated dimples with a depth of up to 0.8 µm in the nanocrystalline alloy. At high contact pressures, the tool pattern, including the LIPSS is almost completely transferred to the substrate. The formed asperities protrude out of the surface, with a height to width aspect ratio of 0.4. It is thereby demonstrated, that plastic flow processes during the imprinting of nanocrystalline alloys allow for the replication of the DLIP tool pattern, even down to nano-scaled LIPSS. The imprinted array of surface asperities exhibits hydrophobic properties, due to a combined topographical and chemical influence on the wetting behaviour.

Elastic-plastic invariant of autowave plasticity

It is shown in the work that plastic flow in solids develops always in localized manner. A macroscopic scale ~ 10-2 m characterizes the localization. The localized flow zones form the pattern of localized strain, which is the projection of the autowave processes of plastic flow, developing in the volume, on the observed surface of the tested specimen. One can observe the pattern with the help of speckle-photography method. The investigations of various materials allowed to establish that the pattern of localized deformation is the information source for a kinetics of deformation processes. A general characteristic of localized plastic flow in solids is the elastic-plastic invariant of deformation which couples the typical characteristics of localized plastic flow autowaves with the same for the elastic waves in crystal lattice. The quantity of the invariant ratio is defined for nearly forty various materials (BCC, FCC, HCP metals and alloys, alkali-halide crystals, ceramics, rocks) studied in the conditions of active elongation and compression at the temperature range 143-420 K. The physical considerations are presented to explain the invariant origination and its relation to other physical characteristics of crystal lattice, in particular, the Debye temperature. In the light of these considerations, it is possible to explain the meaning and the origin of the invariant, and to derive numerous consequences from them. In fact, the set of these consequences comprehend all the regularities of developed plastic flow processes and allows to consider the elastic-plastic invariant of deformation as the master equation for developing this day autowave approach to physical theory of plastic deformation.

Micromechanisms of Deformation and Fracture in Porous L-PBF 316L Stainless Steel at Different Strain Rates

The process of an unstable plastic flow associated with the strain rate sensitivity of mechanical properties was studied in porous 316L austenitic steel samples manufactured by laser powder bed fusion (L-PBF). Different micromechanisms of deformation and fracture of porous samples dependent on strain rate were found. It was found that despite the porosity, the specimens showed high strength, which increased with the loading rate. Porosity led to lower ductility of the studied specimens, in comparison with literature data for low porous 316L L-PBF samples and resulted in de-localization of plastic deformation. With an increase in strain rate, nucleation of new pores was less pronounced, so that at the highest strain rate of 8 × 10−3 s−1, only pore coalescence was observed as the dominating microscopic mechanism of ductile fracture.

FRACTAL MODEL OF ESTIMATING QUALITY OF COLD WORKED FUEL CLADDING TUBES

A possibility was considered concerning estimation of grain anisomery in the structure of fuel cladding tubes of corrosion-resistant 026Cr16Ni15Mo3Nb steel of austenitic class rolled according to two flow charts: regular and intensive technologies using fractal formalism. Role of grain boundary hardening during cold plastic deformation was analyzed by studying the effect of the fractal dimension of grains D and their boundaries Dg on 0.2, w, and 5. The best correlation among those that were considered was observed between relative elongation and fractal dimensions of the grain structure (R2 = 0.90). The smallest correlation was observed with the yield stress (R2 = 0.64). It is because of variation of plastic flow processes towards a decrease in the degree of hardening in the material rolled according to the intensive technology. Cold deformation results in refining of the average grain size from 15.50 to 15.42 µm. In this case, extent of the grain boundary length L increased by 17.62% at an iteration step  commensurate with the average grain size which is indicated by a change in the fractal dimension according to L ~ δ1-D. Degree of the grain structure inhomogeneity was estimated using ratios of self-similarity of regions of fractal dimensions of the structure. The obtained results on the level of mechanical properties of fuel cladding tubes made of austenitic steel indicate advantage of the intensive technology over regular one that was confirmed by results of fractal modeling.

Plasticity Autowave Characteristics of Metals and the Periodic Table of Elements

In this paper, the general laws of localized plastic flow development are described for the linear work hardening stages of nineteen metals. The correlations are established and discussed between their element position in the periodic table of elements and the parameters of the autowave process of localized plastic flow in these elements. Patterns of plastic flow are considered for nineteen metals from the 3rd, 4th, 5th, and 6th periods of the periodic table of elements. A conditional characteristic of plasticity is introduced and its relationship with the position of the metals in the periodic table of elements is established. Correlations between the plastic properties and other metal characteristics were analyzed. Some quantitative models are proposed for explaining the observed dependency origin by the drag of moving dislocation by electron gas in metal crystals. Observed correlations indicate the existence of a close bond between localized plastic flow in deforming medium with the lattice characteristics of the elements and their electron structures.

Power Mode of High-Speed Combined Extrusion of Flat Bimetallic Road Milling Picks

The paper presents a mathematical model developed for calculating the force effect on the punch in the process of high-speed combined hot extrusion of bimetallic road milling picks under plane deformation conditions. To solve the problem, the process is divided into two phases: acceleration phase and braking phase, which consists of two stages. A distinctive feature of the acceleration phase is that it allows the analysis of reverse extrusion, in the process of which the metal flows in the opposite direction to the punch stroke. A method for calculating the force acting on the punch at each phase of the process of plastic flow of a bimetallic workpiece into a matrix cavity with three deformation zones is presented in the paper. While solving the problem in a quasi-static formulation and proceeding from the conditions of the minimum power of internal forces, equations have been obtained for calculating the optimal field parameters aopt, bopt, gopt, depending on the elongation coefficients l and the friction coefficient m. The equations obtained within the framework of the developed model are quite correct, since they allow determining the minimum force acting on the punch. The considered calculation model and equations can be used in the development of industrial technology for high-speed combined hot extrusion of flat-step bimetallic road milling picks.

Plastic flow of isotropic rigid body at uniform deformation

The mathematical analysis of plastic flow processes under uniform plane, axisymmetric and volumetric deformation is carried out. The analysis is based on the external shape change of the body, which determines the movement of material points. It is shown that the plastic flow of an isotropic rigid-plastic body under plane deformation obeys the hyperbolic law, and for axisymmetric and volumetric deformations – the inverse square law. Spatial-geometric expressions of these laws made it possible to reveal and explain in a new way the physical essence of plastic shear. It is proved that the stressed state of a body under uniform tension-compression deformation is complex and cannot be defined as “linear”. The normal stress, which coincides with the direction of the resulting deformation force, is not the main one, since in the areas perpendicular to this direction, the shear stresses are not equal to zero. Examples of solving technological problems are given: extrusion of cylindrical billets and wire drawing, rolling of a wide strip of rectangular profile. It is shown that the problems of determining the stress-strain state of isotropic rigid-plastic bodies along the known trajectories of movement of material points are statically definable.

Modelling of a production process of irreversible strains in a material of a thick-walled cylindrical tube under the influence of inner pressure

This article considers the processes of the accumulation of irreversible strains in an elastoviscoplastic material of a thick-walled cylindrical tube in the framework of small strains. The material is deformed under the action of uniform inner pressure changing with time. Irreversible strains accumulate in the material by means of two different mechanisms, namely creep and plastic flow. Initially, whereas stresses increase in the deformed body due to mechanical influence, irreversible strains are produced as creep strains because of the viscous material properties. When the stress state reaches a loading surface, the production mechanism of irreversible strains becomes plastic. When unloading, this sequence in the production mechanisms is opposite, i.e. the fast plastic mechanism is replaced by the slow viscous one. The continuity in the growth of irreversible strains is provided by the corresponding setting of creep and plastic potentials. The possibility of the use of these potentials in the form of piecewise linear functions is investigated. It makes possible to use the developed mathematical formalism of the ideal plasticity theory. The creep and plastic flow processes under increasing and constant inner pressures are considered. The unloading process and repeated plastic flow under decreasing inner pressure, the stress relaxation after the full removal of a loading force are investigated. The reversible and irreversible strains, stresses and displacements in the medium are calculated. The laws of the motion of elastoplastic boundaries in the cylindrical layer are determined.

How the interface type manipulates the thermomechanical response of nanostructured metals: A case study on nickel

The presence of interfaces with nanoscale spacing significantly enhances the strength of materials, but also the rate controlling processes of plastic flow are subject to change. Due to the confined grain volumes, intragranular dislocation-dislocation interactions, the predominant processes at the micrometer scale, are replaced by emission of dislocations from and their subsequent accommodation at the interfaces. Both processes not only depend on the interfacial spacing, but also on the atomistic structure of the interface. Hence, a thorough understanding how these processes are affected by the interface structure is required to predict and improve the behavior of nanomaterials. The present study attempts to rationalize this effect by investigating the thermomechanical behavior of samples consisting of three different interfaces. Pure nickel samples with predominant fractions of low- and high-angle as well as twin boundaries with a similar average spacing around 150 nm are investigated using high temperature nanoindentation strain rate jump tests. Depending on the interface structure, hardness, strain rate sensitivity and apparent activation volumes evolve distinctively different with testing temperature. While in case of high-angle boundaries for all quantities a pronounced thermal dependence is found, the other two interface types behave almost athermal in the same temperature range. These differences can be rationalized based on the different interfacial diffusivity, affecting the predominant process of interfacial stress relaxation.

Binderless tungsten carbides with an increased oxygen content obtained by spark plasma sintering

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders with an increased oxygen content are investigated. It is shown that the process of shrinkage of nanopowders during SPS is limited by the rate of grain-boundary diffusion with anomalously low values of the activation energy. It has been established that a decrease in the activation energy of SPS may be due to the effect of oxygen on the diffusion permeability of grain boundaries of tungsten carbide at the stage of intense compaction, as well as anomalous grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intensive compaction is controlled by the rate of sintering of oxide particles with their simultaneous transformation into W2C particles, and then, at the stage of high-temperature sintering, by the process of plastic flow of W2C particles in the presence of tungsten particles. Ceramic samples with high density (98-99%), ultrafine-grained structure (average grain size less than 0.3 μm), having increased hardness were obtained by the SPS method. Hv = 30.5 GPa at Palmquist crack resistance ∼ 6.5 MPa · m1/2.

Molecular dynamics simulations of uniaxial deformation of bimodal polyethylene melts

Tensile deformation behavior of bimodal polyethylene (PE) was investigated by employing coarse-grained (CG) molecular dynamics simulations based on a united atom model. The bimodal PE was modeled by blending two linear polyethylene chains with different molecular weights. The mechanical response and corresponding conformational behaviors of the polymer melt during stretching were recorded as a function of strain under both high and low strain rates. We find that the tensile toughness was enhanced in an additive fashion by increasing the fraction of polymers with high molecular weight in bimodal PE. During elongation, the polymer chain extends and orients itself along the loading direction. Surprisingly, varying the bimodal distribution and the strain rates, the disentanglement does not synergistically follow the tendency of chain expansion and ordering along the tensile direction as the strain is gradually raised, implying that multiple deformation modes might exist during the plastic flow process. The molecular origin of this unique development of the entanglement network in bimodal PE is discussed in detail.

Quasi-Particle Approach to the Autowave Physics of Metal Plasticity

This paper is the first attempt to use the quasi-particle representations in plasticity physics. The de Broglie equation is applied to the analysis of autowave processes of localized plastic flow in various metals. The possibilities and perspectives of such approach are discussed. It is found that the localization of plastic deformation can be conveniently addressed by invoking a hypothetical quasi-particle conjugated with the autowave process of flow localization. The mass of the quasi-particle and the area of its localization have been defined. The probable properties of the quasi-particle have been estimated. Taking the quasi-particle approach, the characteristics of the plastic flow localization process are considered herein.