Action Of Plastic Deformation Research Articles

Kinetics of HCP-BCC Phase Transition Boundary in Magnesium at High Pressure

Under high pressures, many crystalline metals undergo solid–solid phase transformations. In order to accurately model the behavior of materials under extreme loading conditions, it is essential to understand the kinetics of phase transition. Using molecular dynamics simulations, this work demonstrates the feasibility of characterizing the speeds of a moving phase boundary using atomistic simulations employing a suitable empirical potential for single-crystal magnesium. The model can provide temperature- and tensorial stress-dependent velocity of a moving phase boundary as a rate-limiting contribution to the kinetics of phase transformation in continuum codes. Results demonstrate that a nonlinear interaction exists between plasticity and phase transition, facilitating a jump in the velocity of a moving phase boundary, facilitated by activated plastic deformation mechanisms.

Comparison of electrical sensing and image analysis for in situ transmission electron microscopy nanomechanical testing of thin films

An in situ transmission electron microscopy (TEM) microelectromechanical system (MEMS) device has been designed to utilize TEM imaging for measuring stress and strain of thin film micro-specimens while simultaneously recording the microstructure evolution. Digital image correlation is also used to measure local normal strain values by tracking edge features of the specimens. The device performance is compared to that of a similar MEMS device that utilizes capacitive sensors for stress and strain measurements, using 100-nm thick Au thin film specimens. It is shown that there is a significant improvement in the noise levels from ∼1–2 MPa to ∼0.2 MPa and increased sensitivity with the capability of measuring small stress changes. The device can be used to perform both in situ TEM monotonic and transient tests (for activation volume measurements) to investigate the active plastic deformation mechanisms.

High-throughput evaluation of mechanical properties of biomedical Ti alloys by digital image correlation and acoustic emission

Acoustic emission and digital image correlation (DIC) were used to determine mechanical properties of six biomedical alloys from a single compression deformation experiment. Layered sample of six Ti-(20–35)Nb-7Zr-0.2O alloys was prepared by spark plasma sintering. All six layers were deformed in compression at once. Advanced use of DIC allowed us to separate the deformation in each layer and determine the flow curve of each alloy. Simultaneous use of acoustic emission provides insight into active plastic deformation mechanisms. Nb content in the studied alloys significantly affects ductility of the material as well as shape of the compression flow curve suggesting different plastic deformation mechanisms in the studied alloys. Presented methods provide high-throughput determination of mechanical properties allowing efficient optimization of alloy composition.

Nonlocal implicit gradient enhancements for strain localization informed by controllability criteria for plastic solids

Nonlocal implicit gradient enhancements are widely used to suppress mesh dependency in simulations involving strain localization. For example, nonlocality is often introduced through internal variables that account for possible material softening. This work, however, shows that this approach may become ineffective if the solid displays plastic non-normality (i.e., non-associated plastic flow). For this purpose, we consider an over-nonlocal formulation and mathematically inspect the conditions at which regularization is lost in the presence of plastic non-normality. Specifically, such loss of regularization is linked to the loss of uniqueness and/or existence of the incremental plastic response that is kinematically compatible with the development of a deformation band. By doing so, we find a lower limit for the admissibility of the parameters controlling the effectiveness of nonlocal implicit gradient regularization. Furthermore, we show that such a lower limit is regulated by a plastic modulus reflecting the loss of controllability of the constitutive response, and, hence, depends on the degree of plastic non-normality. We also derive a closed-form expression relating the thickness of the deformation band to both the controllability modulus and gradient regularization constants, which suggests that the thickness of the process zone may change in response to the prevailing plastic flow characteristics and evolve during active plastic deformation. The proposed nonlocal enhancement is applied to a non-associated elasto-plastic model for porous sedimentary rocks, which is capable of displaying both shear-dominated and compaction-dominated bands. Numerical simulations reveal that effective regularization can be enforced only when the over-nonlocal weighting coefficient is larger than the above-mentioned lower limit.

Patterns of localized deformation at pre-fracture stage in carbon steel – stainless steel bimetal

The work is devoted to the study of strain localization at macroscale level during parabolic mechanical hardening and pre-fracture under quasi-static loading of a carbon steel – stainless steel bimetal. The problem of estimating the scale of the phenomena that determine plasticity is decisive in the development of any theories of plastic deformation, in particular, dislocation theories. The main difficulty in constructing such theories is the reconciling the dislocation scales, characteristic for most deformation and mechanical hardening mechanisms, with macroscopic parameters of deformation processes. In the framework of the autowave model of localized plastic deformation, this problem can be reduced to the possibility of obtaining parameters from the results of macroscale observations of localized plastic flow development. During the experiments, it was confirmed that in a bimetal at any forming stage, a specific pattern of localization centers distribution is spontaneously generated - a pattern of localized plastic flow. The shape of such patterns is determined by the law of mechanical hardening acting in the material. It is shown that the observed localization patterns can be used as an informative feature in predicting the plasticity margin. In the process of uniaxial tension at the stage of parabolic mechanical hardening of the bimetal, the deformation mode is realized with the formation of several potential fracture centers. It was established that at the pre-fracture stage, during the time evolution of the wave pattern of deformation localization, the zone of active plastic deformation narrows, but the number of centers in it either remains the same with a decrease in the distance between them, or even increases. The result of this process is the formation of a macroscopic neck, and then fracture. At the pre-fracture stage, the collapse point indicates the place of future fracture and signals the need to stop the deformation process in order to avoid the fracture of the bimetallic material. Thus, the well-known manifestation of deformation macroscopic localization – formation of a neck – is preceded by complex phenomena of mutually coordinated motion of localized plasticity centers at the pre-fracture stage.

Influence of Plastic Deformation on the Accumulation of the Average Scalar Dislocation Density and Its Components pS and pG in Cu-Mn Alloys

The development and progress of the physical science of strength makes it possible to formulate the main aspects based on dislocation physics. This article describes the current state of this issue in the framework of a multilevel approach. It considers the patterns of accumulation of dislocations in a material after various degrees of deformation. The main mechanism of hardening of a metal polycrystal is the accumulation of dislocations in its grains, and the main hardening parameter is the average scalar dislocation density. The scalar dislocation density is divided into components: the density of statistically stored (pS) and the density of geometrically necessary (pG) dislocations. Transmission diffraction electron microscopy (TEM) is used to study the stages of the development of types of dislocation substructure (DSS) in Cu-Mn alloys depending on the concentration of the alloying element during active plastic deformation. Polycrystalline alloys were investigated over a wide concentration range from 0.4 to 25 atomic percent Mn. A number of parameters of the dislocation substructure are measured from micrographs obtained in an electron microscope: the average scalar density of dislocations <p>, the density of statistically stored (pS) and geometrically necessary (pG) dislocations, the curvature-torsion of the crystal lattice (х), the density of microbands (b), density of dangling subboundaries (Msub). A sequence of transformations of DSS types with an increase in the degree of deformation and the value of the second element to form the type of substructure and its parameters is established. The influence of the concentration of the second element and the grain size on the average scalar density of dislocations and its components is experimentally determined. The presence of misorientations in the substructure during deformation is based on the measurement of these parameters using the TEM method.

Substructure parameters in deformed Cu – Mn alloys with a FCC lattice

Development and successes of the physical science of strength and durability allow us to formulate the main aspects of dislocation physics. In this paper, the current state of this issue is considered within the framework of a multilevel approach – patterns of dislocations accumulation in the material after deformation with various degrees. The main mechanism of hardening of a metallic polycrystal is the dislocations accumulation in its grains, and the main parameter of hardening is the average scalar density of dislocations. The scalar dislocation density is divided into components: the density of statistically stored (ρS ) and the density of geometrically necessary (ρG ) dislocations. Transmission diffraction electron microscopy (TEM) is used to study the stages of development of types of dislocation substructure (DSS) in Cu – Mn alloys depending on concentration of the alloying element during active plastic deformation. Polycrystalline alloys were studied in a wide concentration range: from 0.4 to 25 % Mn (at.). A number of dislocation substructure parameters were measured using the images obtained on an electron microscope: the average scalar density of dislocations <p>, the density of statistically stored (ρS ) and geometrically necessary (ρG ) dislocations, the curvature-torsion of the crystal lattice (χ), the density of microstrips (Рstrips ), the density of ragged sub-boundaries (Мrag.bound. ). The authors established the effect of the sequence of DSS types transformations with an increase in the degree of deformation and the second element quantity on formation of the type of substructure and its parameters. Influence of concentration of the second element and the grain size on the average scalar density of dislocations and its components was experimentally determined. The presence of disorientations in the substructure during deformation is based on the measurement of these parameters by the TEM method.

Thermally activated plastic deformation of Si single crystals at temperatures above 1173 K

The effect of high-temperature heat treatment during production on the mechanical property of Si wafer, namely yield stress, was studied through tensile tests of Si single crystals at 1123–1373 K, where stress–strain curves were obtained along the and directions. The critical resolved shear stress showed a remarkable temperature dependence, and it decreased with increasing test temperature. Below 1348 K, the activation enthalpy determined through strain rate jump tests showed no significant difference between crystals made by the Chokralski and the floating zone methods. This indicated that the solute oxygen does not influence the thermally activated dislocation glides. The activation enthalpy of 4.9 eV for a dislocation glide was much higher than that reported by previous studies (2.2 eV at temperatures below 1100 K), indicating that the thermal activation of dislocation glide is different from kink-pair nucleation. It was found that self-diffusion can assist the dislocation glide at temperatures above 1130 K.

Size effect on the mechanical responses of aluminum nanoparticles under indentation: A molecular dynamics study

Micro- and nanoparticles are significant raw materials for cold spray and aerosol deposition in which the plastic deformation of particles plays an important role. The mechanical properties of the micro- and nanoparticle are greatly influenced by size. Aluminum particles of various radii under various spherical indenters are simulated with molecular dynamics method to investigate the size effect on the mechanical responses at nanoscale. The particle radius varies in a wide range from 20 nm to 400 nm, and the indenter radius is about 4 nm to 40μm. As the onset of plastic deformation, the first yielding point of the force–displacement curve is influenced by both the sizes of the particle and the indenter, which indicates that the mechanical behavior of nanoparticles depends not only on their own sizes but also on their loading conditions. The ratio between the particle size and the indenter size is found to be an essential factor, which categorizes the indentation models. Under the same ratio, the smaller particle is easier to activate plastic deformation.

Micropillar compression of single crystal tungsten carbide, Part 1: Temperature and orientation dependence of deformation behaviour

Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars in two crystal orientations, 1-5 μm in diameter, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 °C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that 101¯0 prismatic planes were the dominant slip plane in WC. Basal slip was also activated as a secondary slip system at high temperatures.

Understanding and quantifying electron beam effects during in situ TEM nanomechanical tensile testing on metal thin films

Transmission electron microscopy (TEM) imaging relies on high energy electrons for atomic scale resolution, however, the electrons themselves interact with and may alter the material being imaged. Using an in situ TEM MEMS-based nanomechanical testing technique, the effect of the electron beam (e-beam) on the deformation behavior of nanocrystalline Al and ultrafine-grained Au is investigated and quantified. We show that the e-beam enhances plastic deformation, leading to an increase in plastic strain rate and a decrease in true activation volume V* in Al (28 to 21b3, with b being the Burgers vector length). The e-beam has a much weaker effect on Au. The e-beam effect is not caused by knock-on damage, but rather an effective temperature increase due to additional atomic fluctuations provided by the e-beam. The effective temperature increase is larger for Al than Au. This e-beam effect does not change the deformation mechanisms, but instead accelerates the stress-driven, thermally activated plastic deformation. These experiments provide insight into the effects of the e-beam on plastic deformation in different metals and underscore the importance of understanding and quantifying these effects for proper interpretation of measured mechanical properties during in situ TEM experiments.

Numerical Simulation Research on Mandrel reaming of Ring Steel Ingot

The mandrel reaming forging is of great significance to the billeting of the ring ingot. Through the analysis of the reaming deformation of the ring steel ingot mandrel, it is concluded that the ring steel ingot reaming deformation exists in the radial, axial and tangential directions, which are similar to the rigid zone, active plastic deformation zone and passive plastic deformation zone in the upsetting theory. Numerical simulation methods are used to analyze the reaming of the ring steel ingot mandrel, and it is verified that the ring steel ingot reaming deformation has a rigid zone, an active plastic deformation zone and a passive plastic deformation zone in the radial, axial and tangential directions. At the same time, the reduction rate is found has a greater impact on the three regions.

بررسی اثر شدت تکسچر بر رفتار تغییر شکل آلیاژ Mg-0.8wt%Y اکسترود شده تحت بارگذاری کشش و فشار با استفاده از روش المان محدود کریستال پلاستیسیته

Magnesium alloys due to their compact hexagonal structure (HCP) usually have a yield asymmetry in tensile and compressive loadings. This is due to various active plastic deformation mechanisms under tensile and compressive loadings. Therefore, it is necessary to study the deformation behavior of magnesium alloys on the grain scale. This study focuses on the 3D Representative Volume Element (RVE) simulations of extruded magnesium alloy to investigate the effect of basal texture intensity on tensile–compressive yield asymmetry of Mg-0.8wt% Y. Polycrystalline aggregate models are built based on the theory of Voronoi diagram; then, the Slip-based crystal plasticity formulation is applied as a User Material Subroutine in ABAQUS (UMAT) to study the relationship of microstructures and stress-strain responses in Mg-0.8wt% Y alloy during tension and compression test and the results are presented. The results show that when the orientation of the grains is considered random (zero texture intensity), the activities of different deformation modes are almost the same and minimum tensile-compressive yield asymmetry happens. Also, the results reveal when all the grains are oriented in a certain direction (texture intensity 1), maximum tensile–compressive yield asymmetry occurs. Indeed, according to the Relative Activity (RA) results, in agreement with Schmid Factor (SF) results, in a strong basal texture with the intensity of 1, maximum contribution of accumulated plastic deformation is related to tensile twinning while in compressive loading, prismatic slip and contraction twinning are responsible for major plastic deformation. Therefore, due to the activation of various deformation modes in strong basal texture, the highest amount of tensile–compressive yield asymmetry occurs. The S-shape strain hardening curve which is the main characteristic of twin-dominated deformation shows a strong dependency on texture in the extruded material so that during loading at the weaker texture, s-shape hardening is no longer present due to the dominated non-basal slips in weaker texture.

Эволюция микроструктуры и ее параметров с деформацией в поликристаллических сплавах медь - алюминий с разной энергией дефекта упаковки

Evolution of the dislocation structure during active plastic deformation was carried out in copper-aluminum alloys with Al content of 0.5-14 at. % using transmission electron microscopy. Analysis of the types of the dislocation substructure as a function of the alloying element content and deformation degree was conducted. The following parameters of the defect substructure were measured: average scalar dislocation density, curvature-torsion of the crystal lattice and microtwin density. The effect of stacking fault energy on accumulation of defects in the alloys was observed and evaluated.

Product performance based on contact-condensing binders as a function of pressing parameters

The mechanism of contact-condensation solidification provides the formation of a solid rocky body in the event of contact between the microparticles of a substance of amorphous or unstable crystalline structure without changing its chemical composition as a result of physical surface phenomena. Therefore, to provide stronger contacts between the particles of such binders during their condensation, it is necessary to apply external pressure (compression).
 The implementation of such binders is not linked to the timing of grieving. The curing process is carried out immediately at the time of contact between the particles of such binders. In the future, the strength of artificial stone increases in natural conditions and in water.
 The results of studies of the dependence of the pressing parameters on the basic performance of the products based on the binder contact-condensation hardening are presented. It is established that in order to maximize the effective action of pressures, which provide plastic deformation during compaction and provide the most dense packing of powder particles, there is a limit of working pressures in the range from 40 to 150 MPa. It is shown that it is convenient to use mathematical expressions (extrusion equations) when analyzing the processes occurring during extrusion, which reveal a functional relationship between the pressure and the density of the material thus obtained. With the help of the equations of pressing, for each powder or group related to the physicochemical parameters of the powders, the most economical parameters of the pressing modes can be determined.
 The data obtained for the compression pressure characterizing the action of plastic deformation for the test mixtures is in the pressure range from 32 MPa to 158 MPa. Increasing the pressure above leads to the transition of plastic deformation into elastic deformation, in which the seal passes due to the destruction of particles under the action of high pressures.

Investigation of Structure and Properties of High-strength Bimetallic Materials Produced By Explosion Welding and Have Been Exploited in Arctic Climate

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Thermally activated plastic deformation behavior of nano oxide dispersion strengthened Fe-18Cr steel: Experiments and analysis

The objective of the present work is to investigate the ultra-high temperature (1273–1473 K) plastic deformation behavior of a nano oxide dispersion strengthened Fe-18Cr ferritic steel (n-ODS-18Cr steel) over a range of strain rates (10−2 to 10 s−1). The flow (true) stress-true strain behavior of this steel, reported for the first time, has been utilized to obtain the flow stress as a function of temperature and strain rate. From the above data, the strain rate sensitivity parameter (m) and also the activation volume have been obtained as a function of temperature (1273–1473 K). It is then demonstrated that the flow stress values obtained in the present steel over the temperature range 1273–1473 K is due to a single dominant strengthening mechanism, i.e. Orowan dispersion strengthening. Hence, the predictions of the theoretical models which assume Orowan strengthening as the dominant mechanism can be compared with the experimental flow stress data. Such a comparison indicates that the model due to Rosler and Arzt predicts the experimental data very well while the prediction of the model due to Brandes, Kovarik, Miller, Daehn and Mills is satisfactory. It is also shown that the experimentally obtained activation volume is consistent with the dislocation detachment model.

Modified Kocks–Mecking–Estrin Model to Account Nonlinear Strain Hardening

The dislocation density-based model after Kocks–Mecking–Estrin (KME) is widely used to characterize the thermally activated plastic deformation and dislocation kinetics. According to the model, the slope of the stress–strain curve decreases linearly with stress, which contradicts the experimental observation. In the current study, the evolution of dislocation density in the model is generalized to account for the nonlinearity of the slope.

Hardening mechanisms of steels with bainite and martensite structures

Martensite and bainite are the most complex structures being formed in steel in heat treatment including the quantitative interpretation. On frequent occasions, the application field of these steels includes the operation at high static and dynamic compression stresses. The thorough and comprehensive analysis of the materials’ structure after different types of treatment enables to use them competently for the manufacturing of the parts and structures providing them with the necessary complex of physical and mechanical properties. The factor determining the mechanical properties of the materials are the structure of solid solution, presence of nano-dimentional particles of the second phases, dislocation substructure, types and location of various boundaries and internal stress fields. For successful control of the formation of structural phase states and mechanical properties of the material it is necessary to know the quantitative laws and the cold hardening mechanisms of steels of different structural classes at active plastic deformation. By methods of transmission electron diffraction microscopy the analysis of cold hardening of 38CrNi3MoV steel with martensite and 30Cr2Ni-2MoV steel with bainite structures at active plastic compression deformation to 26 % and 36 %, respectively, was done in the research. The contributions caused by intraphase boundaries, dislocation substructure, carbide phases, atoms of alloying elements and long-range stress fields are considered. It is established that the substructural hardening (caused by the internal long-range stress fields) and solid solution strengthening (caused by carbon atoms) give largest contribution to cold hardening of 38CrNi3MoV hardened steel. For normalization of 30Cr2Ni2MoV steel hardening also takes place at the expense of the internal stress field’s action, at the penetration of carbon atoms to the ferrite crystal lattice as well as at the structural fragmentation with the deformation degree higher than 26 %. The dislocation substructure and the particles of carbide phase make comparatively small contribution to the hardening of these steels. It is shown that the cause of bainite steel softening at large (more than 15 %) degrees of deformation is connected with the activation of deformation microtwinning process.

Stress relaxation in ultrafine-grained copper at low homologous temperatures

The temperature and rate sensitivity of the flow stress in ultrafine-grained (UFG) copper prepared by equal-channel angular hydroextrusion was studied. Tensile tests and stress relaxation tests were carried out in the temperature range of 77 to 295 K. It was established that, with increasing temperature, the flow stress decreases monotonously, and the activation volume reaches a maximum of ∼190b3 at a temperature of ∼(200 ± 20) K. The results of the experiment are discussed in terms of two thermally activated plastic deformation mechanisms, namely the intersection of “forest” dislocations and dynamic recovery, which are capable of significantly distorting stress relaxation kinetics in UFG copper.