Stages Of Plastic Flow Research Articles

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Shear bands and interface-affected-zone enabled high strength and ductility in heterogeneous laminate-Cu

The rapid development of shear bands (SBs) in nanocrystalline Cu is prone to lead to structural failure, while coarse crystalline Cu with high strain hardening capacity can effectively inhibit the growth of SBs. Heterogeneous laminate (HL) has a structure of alternating soft and hard domains, which dramatically increases ductility while reducing strength loss. In this work, molecular dynamics (MD) simulations revealed how SBs in the HL-Cu during the stable plastic flow stage (the applied strain of 10 ∼ 20 %) strengthen the material. It is suggested that when SBs are uniformly and densely distributed in the structure, the strain gradient strengthening effect in the neighboring regions is significantly increased. Moreover, the efficiency of strain hardening can be maximized when the width of the interface-affected-zone (Defined as the region where a significant linear change in strain occurs, i.e., a strain gradient exists, located at the junction of the soft and hard domains.) caused by the uneven distribution of geometrically necessary dislocations (GNDs) is exactly the same as the layer thickness. The results show that the strength of HL can be increased by more than 14 % by reducing the layer thickness by half.

Al-Al3Ni In Situ Composite Formation by Wire-Feed Electron-Beam Additive Manufacturing.

The regularities of microstructure formation in samples of multiphase composites obtained by additive electron beam manufacturing on the basis of aluminum alloy ER4043 and nickel superalloy Udimet-500 have been studied. The results of the structure study show that a multicomponent structure is formed in the samples with the presence of Cr23C6 carbides, solid solutions based on aluminum -Al or silicon -Si, eutectics along the boundaries of dendrites, intermetallic phases Al3Ni, AlNi3, Al75Co22Ni3, and Al5Co, as well as carbides of complex composition AlCCr, Al8SiC7, of a different morphology. The formation of a number of intermetallic phases present in local areas of the samples was also distinguished. A large amount of solid phases leads to the formation of a material with high hardness and low ductility. The fracture of composite specimens under tension and compression is brittle, without revealing the stage of plastic flow. Tensile strength values are significantly reduced from the initial 142-164 MPa to 55-123 MPa. In compression, the tensile strength values increase to 490-570 MPa and 905-1200 MPa with the introduction of 5% and 10% nickel superalloy, respectively. An increase in the hardness and compressive strength of the surface layers results in an increase in the wear resistance of the specimens and a decrease in the coefficient of friction.

Mesoscale Computational Study of the Parabolic Hardening Stage of Plastic Flow in a Low-Carbon Steel

Mesoscale Computational Study of the Parabolic Hardening Stage of Plastic Flow in a Low-Carbon Steel

Temperature-Dependent Deformation Behavior of “γ-austenite/δ-ferrite” Composite Obtained through Electron Beam Additive Manufacturing with Austenitic Stainless-Steel Wire

Temperature dependence of tensile deformation behavior and mechanical properties (yield strength, ultimate tensile strength, and an elongation-to-failure) of the dual-phase (γ-austenite/δ-ferrite) specimens, obtained through electron-beam additive manufacturing, has been explored for the first time in a wide temperature range T = (77–300) K. The dual-phase structures with a dendritic morphology of δ-ferrite (γ + 14%δ) and with a coarse globular δ-phase (γ + 6%δ) are typical of the as-built specimens and those subjected to a post-production solid–solution treatment, respectively. In material with lower δ-ferrite content, the lower values of the yield strength in the whole temperature range and the higher elongation of the specimens at T > 250 K have been revealed. Tensile strength and stages of plastic flow of the materials do not depend on the δ-ferrite fraction and its morphology, but the characteristics of strain-induced γ→α′ and γ→ε→α′ martensitic transformations and strain-hardening values are different for two types of the specimens. A new approach has been applied for the analysis of deformation behavior of additively fabricated Cr-Ni steels. Mechanical properties and plastic deformation of the dual-phase (γ + δ) steels produced through electron beam additive manufacturing have been described from the point of view of composite materials. Both types of the δ-ferrite inclusions, dendritic lamellae and globular coarse particles, change the stress distribution in the bulk of the materials during tensile testing, assist the defect accumulation and partially suppress strain-induced martensitic transformation.

Effect of dynamic loading conditions on the dynamic performance of MP1 energy-absorbing rockbolts: Insight from laboratory drop test

Energy-absorbing rockbolts have been widely adopted in burst-prone excavation support, and their serviceability is closely related to the frequency and magnitude of seismic events. In this research, the split-tube drop test with varying impact energy was conducted to reproduce the dynamic performance of MP1 rockbolts under a wide range of seismic event magnitudes. The test results showed that the impact process could be subdivided into four distinct stages, i.e. mobilization, strain hardening, plastic flow (ductile), and rebound stage, of which strain hardening and plastic flow are the primary energy absorbing stages. As the impact energy per drop increases from 8.1 to 46.7 kJ, the strain rate of the shank varies between 1.20 and 2.70 s−1, and the average impact load is between 240 and 270 kN, which may be considered as constant. The MP1 rockbolt has a cumulative maximum energy absorption (CMEA) of 31.9–40.0 kJ/m, with an average of 35.0 kJ/m, and the elongation rate is 11.4%–14.7%, with an average of 12.7%, both of which are negatively correlated with the impact energy per drop. Regression analysis shows that energy absorption and shank elongation, as well as momentum input and impact duration, conform to the linear relationship. The complete dynamic capacity envelope of MP1 rockbolts is proposed, which reflects the dynamic bearing capacity, elongation, and distinct stages. This study is helpful to better understand the dynamic characteristics of energy-absorbing rockbolts and assist design engineers in robust reinforcement systems design to mitigate rockburst damage in seismically active underground excavations.

Effect of Sc, Hf, and Yb Additions on Superplasticity of a Fine-Grained Al-0.4%Zr Alloy

This research was undertaken to study the way deformation behaves in ultrafine-grained (UFG)-conducting Al-Zr alloys doped with Sc, Hf, and Yb. All in all, eight alloys were studied with zirconium partially replaced by Sc, Hf, and/or Yb. Doping elements (X = Zr, Sc, Hf, Yb) in the alloys totaled 0.4 wt.%. The choice of doping elements was conditioned by the possible precipitation of Al3X particles with L12 structure in the course of annealing these alloys. Such particles provide higher thermal stability of a nonequilibrium UFG microstructure. Initial coarse-grained samples were obtained by induction casting. A UFG microstructure in the alloys was formed by equal-channel angular pressing (ECAP) at 225 °C. Superplasticity tests were carried out at temperatures ranging from 300 to 500 °C and strain rates varying between 3.3 × 10−4 and 3.3 × 10−1 s−1. The highest values of elongation to failure are observed in Sc-doped alloys. A UFG Al-0.2%Zr-0.1%Sc-0.1%Hf alloy has maximum ductility: at 450 °C and a strain rate of 3.3 × 10−3 s−1, relative elongation to failure reaches 765%. At the onset of superplasticity, stress (σ)–strain (ε) curves are characterized by a stage of homogeneous (uniform) strain and a long stage of localized plastic flow. The dependence of homogeneous (uniform) strain (εeq) on test temperature in UFG Sc-doped alloys is increasing uniformly, which is not the case for other UFG alloys, with εeq(T) dependence peaking at 350–400 °C. The strain rate sensitivity coefficient of flow stress m is small and does not exceed 0.26–0.3 at 400–500 °C. In UFG alloys containing no Sc, the m coefficient is observed to go down to 0.12–0.18 at 500 °C. It has been suggested that lower m values are driven by intensive grain growth and pore formation in large Al3X particles, which develop specifically at an ingot crystallization stage.

Acoustic characteristics of deformable metals

The stages of plastic flow in Fe-18 wt.% Cr-10 wt.% Ni structural steel were studied by performing non-destructive testing. In particular, the acoustic parameters of the steel were measured via uniaxial tensile tests. According to the changes in the velocity of propagation and attenuation of ultrasound in a wide temperature range, the stages of plastic deformation and destruction were clearly distinguished. Based on the results of this study, a non-destructive testing parameter characterizing the transition to the pre-destruction stage of the material was proposed.

TEMPERATURE DEPENDENCE OF THE DEFORMATION BEHAVIOR OF HIGH-ENTROPY ALLOYS CO20CR20FE20MN20NI20, CO19CR20FE20MN20NI20С1 AND CO17CR20FE20MN20NI20С3. MECHANICAL PROPERTIES AND TEMPERATURE DEPENDENCE OF YIELD STRENGTH

This paper discusses the temperature dependence of the mechanical properties of multicomponent alloys Co20Cr20Fe20Mn20Ni20 (Cantor alloy), Co19Cr20Fe20Mn20Ni20С1 and Co17Cr20Fe20Mn20Ni20С3 under uniaxial static tension in the temperature range from 77 to 473 K. It is shown that after thermomechanical treatment all the alloys have an fcc crystal structure, but unlike single-phase Co20Cr20Fe20Mn20Ni20 and Co19Cr20Fe20Mn20Ni20С1 alloys, the alloy with 3 at % carbon exhibits large incoherent chromium carbides. Alloying with carbon causes solid solution strengthening of the austenitic phase and dispersion hardening, and promotes grain refinement in the Cantor alloy. Solid solution strengthening contributes to an increase in the athermal and thermal stress components of σ0.2, leading to higher yield stress values and stronger temperature dependences σ0.2( T ) in Co19Cr20Fe20Mn20Ni20С1 and Co17Cr20Fe20Mn20Ni20С3 alloys than in the Cantor alloy. The results of X-ray diffraction and microscopic analysis indicate that despite the differences in the total concentration of interstitial atoms in Co19Cr20Fe20Mn20Ni20С1 and Co17Cr20Fe20Mn20Ni20С3 alloys, the concentrations of carbon dissolved in the crystal lattice of the austenite phase are close. However, the higher strength properties of Co17Cr20Fe20Mn20Ni20С3 compared to Co19Cr20Fe20Mn20Ni20С1 are determined primarily by grain boundary strengthening and, to a lesser extent, by dispersion hardening. Both factors such as lowering the deformation temperature and alloying with carbon contribute to an increase in the deforming stresses of the Cantor alloy. It is shown that alloying with carbon affects the staged plastic flow of the Cantor alloy: the tensile curves of Co19Cr20Fe20Mn20Ni20С1 carbon alloy exhibit a well-defined stage of microplastic deformation, and the flow curves of Co17Cr20Fe20Mn20Ni20С3 alloy at the initial stages of plastic flow have a parabolic shape, typical of the deformation of alloys with large incoherent particles. The elongation to failure of Co20Cr20Fe20Mn20Ni20 and Co19Cr20Fe20Mn20Ni20С1 alloys increases linearly with decreasing deformation temperature, i.e., the mechanical properties of single-phase alloys are improved in the region of low test temperatures. For Co17Cr20Fe20Mn20Ni20С3 alloy, an increase in strength properties during low-temperature deformation is accompanied by a decrease in ductile characteristics, and the alloy becomes brittle from the viewpoint of macromechanical behavior.

Axial compressive behavior of concrete-filled steel tubes with GFRP-confined UHPC cores

The steel–concrete-GFRP-UHPC (SCGU) composite column is a new type of column consisting of steel, concrete, ultra-high performance concrete (UHPC) and glass fiber reinforced polymer (GFRP). The sectional form of the SCGU column is a circular steel tube as the outer layer, a GFRP-wrapped UHPC column as the core, and concrete between the core column and the steel tube. In this paper, axial compression tests were conducted on twenty-four SCGU composite columns, and their damage morphology, load–strain curves and influencing factors were analyzed. The results show that the damage modes of the specimens present multiple convex curves of the external steel tube, corresponding to the fragmentation of its plain internal concrete, the fracture of GFRP and the large axial crack of the UHPC core column; that the load–displacement curves of the composite columns can be roughly simplified to elastic, elastoplastic and plastic flow stages; that the composite columns still have high residual bearing capacity after compression damage; and that effect of tube thickness, number of GFRP layers and UHPC core column diameter of the specimen on the load bearing capacity and ductility of composite columns. Based on the mechanical analysis of each component of the SCGU composite column, a formula of axial compression bearing capacity of the SCGU composite column was presented, and agreement between the formula predicted results and the experimental results was obtained. In addition, finite element (FE) models that can simulate the SCGU composite column are developed.

Localized deformation at initial stages of plastic flow in high-manganese steel

The work is devoted to the study of macroscopic localization of plastic deformation during uniaxial tension of single crystals of the Hadfield steel (Fe – 13 % Mn – 1.03 % C). In the course of studies at the stage of easy sliding, significant differences were found in the nature of macrolocalization of plastic deformation in the single-crystal samples under study. All patterns of deformation localization observed in these cases can be divided into two types. The first type of strain localization corresponds to the initiation at the upper yield point and further propagation of the strain front, which gradually transfers the sample material from undeformed state to deformed one. This manifested itself most clearly in single crystals oriented along the tension axis [377] and [355], where the strain localization pattern appears as a single zone of localized deformation on the yield plateau. Such a deformation front passes through the sample volume only once as a Chernov-Luders band. In this case, the flow of the material is carried out without hardening until all its elements are transferred to the deformed state. Single zones of strain localization were also observed at the stages of easy sliding and yield plateau in the Hadfield steel single crystals oriented along the tension axis [123] and [012]. In the second type of localization, at the stage of easy sliding, synchronous movement along the pattern of several deformation centers occurs. Their movement can be unidirectional and counter, and the speeds are both the same and different from each other. Further deformation of the Hadfield steel single crystals oriented along the tension axis [355] or [012], led to the movement of two deformation localization centers at the stages of easy sliding. In single crystals oriented along [111], the strain localization pattern is respectively represented as four localized strain sites. Consequently, the synchronous movement of deformation fronts occurs along an already deformed, albeit to a small extent, material. As a reason for the difference between the two types of macrostrain localization at stage I (the easy sliding stage and the yield plateau), the number of active sliding systems or tensile twinning in the studied single crystals can be discussed based on crystallographic analysis and metallographic studies.

The Stress-Dilatancy Behaviour of Artificially Bonded Soils.

In this study, the results of triaxial compression tests of some naturally and artificially bonded soils presented in the literature were analysed. It was shown that the three characteristic stages of plastic flow during shear can be identified. In all stages, the stress–dilatancy behaviour could be approximated by the general linear stress–dilatancy equation of the Frictional State Concept. For many shear tests, the failure states and newly defined dilatant failure states are not identical. The points representing dilatant failure states lie on a straight line, for which the position and slope in the η-D plane depend on the soil type and the amount of cement admixture. This line defines the critical frictional state angle, and its slope for bonded soils is greater than for unbonded soils.

On the serration evolution of cellular bulk metallic glass monitored by fractal analysis

Cellular bulk metallic glasses (BMGs) with varying porosities were fabricated by laser drilling, demonstrating different modes for the mechanical energy absorption process. The evolution of the elastic energy accumulation rate (Ep) and load drop parameter (L' = |d(load)/d(t)|) during the plastic-flow stages was examined using statistical analysis. The results have shown that the distribution of load drops follows a power-law scaling, and the three-parameter Weibull moduli (m) of both the Ep and L' values have fractal nature during the deformation process. The fractal dimension (D), fractal character measurement parameter (M) and fractal iteration sequence of the Weibull moduli (mEp−εp and mL′−εp) were used to characterize the serration evolution behavior under different conditions. The differences and relationships between the Ep and L' values were also discussed. The cellular BMG with stable and more iterative fractal sequences tend to exhibit relatively-stable deformation behavior, linearly growth energy absorption process and larger macroscopic plasticity.

Micromechanics of impact damage of ductile (ZnS) and hard (MgAl2O4) ceramics

AbstractThe time‐resolved generation of the electromagnetic emission (EME) and acoustic emission (AE) excited by the point impact loading of ductile ZnS and hard MgAl2O4 ceramics was detected. It has been found that the EME generation precedes the AE activity in loaded ZnS ceramics, while the EME signal lags the AE generation from MgAl2O4. This dissimilarity of temporal patterns was explained by the difference between the mechanisms of the excitation of electrical charges in loaded ductile and hard ceramics. The EME from loaded ZnS is caused by the motion of charged dislocations, while, the EME effect in MgAl2O4 is due to the annihilation of opposite electric charges emerged on microcrack edges. The AE energy yield from growing microcracks in both ceramics as well as the EME activity in MgAl2O4 were found to be random (Poissonian‐like), which is typical of the localized fracture of solids occurring without long‐range interactions between perturbed structural localities. The energy distribution in EME time series generated in ductile ZnS prior to cracking followed a power law, which is characteristic for the correlated motion of charged dislocations at the stage of plastic flow. The temporal patterns of damage accumulation at different structural levels were specified through physical and mechanical properties of tested materials.

Microstructure and mechanical properties of low-carbon steel fabricated by electron-beam additive manufacturing

The microstructure and mechanical properties of a billet obtained by electron-beam additive manufacturing (EBAM) using an industrial welding wire of low-carbon steel were studied. After EBAM, the steel billet possesses the phase composition constant in volume (ferrite with carbides). Deformation behavior of samples of steel obtained by the additive methods depends on their position in the billet. In its lower part, which is characterized by a high cooling rate in the EBAM-process, predominantly equiaxed ferrite grains with an average size of 15 μm are formed. The mechanical properties and deformation behavior (the presence of a yield plateau, the stages of plastic flow) of this part of the billet are close to those of normalized low-carbon steel obtained by conventional methods of metallurgy and thermomechanical processing. In the middle and top parts of the billet, the coarse non-equiaxed ferritic grains (hundreds of micrometers) form. The mechanical properties in this part weakly depend on the position of the samples and their orientation relative to the building direction of the billet — the yield plateau disappears; the yield stress, the ultimate tensile strength, the elongation become lower than those of the normalized low-carbon steel obtained by the conventional method. Despite the predominance of ferritic grains (with carbides), a small portion of grains with a lamellar ferrite morphology resembling martensite or bainite is observed in all parts of the billet. The latter is the result of a complex thermal history of the billet, which can undergo multiple phase transformations during successive heating and cooling cycles.

The grain size effect on strain hardening and necking instability revisited from the dislocation density evolution approach

Revisiting the classical topic of the strain hardening behaviour from a perspective of the analytical model based on the evolution of dislocation density with strain, we extended the capacity of the single internal variable model towards predicting necking instability and re-examined the grain size dependence of the flow stress. An excellent agreement is observed between the model predictions of the necking strain and stress and the results of tensile testing of nickel polycrystals with grain sizes varied from sub-micrometres to hundred micrometres. The pivotal significance of the dynamic recovery in the occurrence of necking has been analysed and emphasised in the context of the discussion on the effect of grain size on flow stress. The most interesting corollary of the analysis made is that not only the simple modelling of the dislocation density evolution, which can be traced back from the very early stage of plastic flow in materials with different grain sizes, can be used for realistic approximation of the stress-strain behaviour during homogeneous deformation under constant plastic strain rate, but also for predicting the onset of necking instability with high confidence.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

Numerical modeling of kinetic regularities of yield plateau and linear work hardening stages during tension of mild steel samples

In this paper, an analysis of the linear work hardening stage of plastic flow of low-carbon steel is carried out numerically. Unlike the yield plateau stage of plastic flow, much less attention is given to late stages of plastic flow, e.g. linear work hardening or parabolic work hardening. Late stages of plastic flow are interesting from the point of revealing the underlying mechanisms of fracture site formation. The non-uniform distribution of plastic strain is analyzed in both yield plateau and linear work hardening stages. Plastic strain localization features on the linear hardening stage are discussed based on the kinetic diagrams. A microstructure-based finite-difference analysis is employed. It is shown that maximums of plastic strain distribution stop their motion once the linear hardening stage is entered.

Автоволновое описание пластичности материалов с нестабильной фазовой структурой на макромасштабном уровне

The regularities of plastic flow in materials with deformation-induced phase transformations on the example of titanium nickelide and TRIP steel are studied. For experimental analysis of plastic deformation processes, the method of digital image correlation has been used, which allows us to quantitatively describe the behavior of plasticity fronts associated with the course of phase transformations in the materials under study. The mechanisms of formation of Lüders bands and Portevin-Le Chatelier bands at different stages of plastic flow are considered.

Mechanical behavior of lightweight concrete structures subjected to 3D coupled static–dynamic loads

It is of great significance in the design and evaluation of anti-impact protective structures to investigate the dynamic mechanical properties of lightweight concrete structures. The present work aims to address this challenging task by the development of an improved large-diameter split Hopkinson pressure bar that can accurately predict the dynamic compressive strength and impact failure characteristics of lightweight concrete under different stress states. Our findings exposed that the impact resistance of lightweight concrete is substantially improved and the failure modes of it have altered under the 3D coupled static–dynamic loads. When the confining pressure remains at the same level and the loading strain rate is low, the unloading curve of lightweight concrete shows a significant hysteresis phenomenon. We have further found that for a higher strain rate the stress–strain curve can be divided into five stages: (i) the inertial-effect-enhanced stage, (ii) the nonlinear elastic deformation stage, (iii) the elastoplastic deformation stage, (iv) the strain-softening stage, and (v) the plastic flow stage. Based on the linear relationships among the logarithm of the strain rate, the dynamic increase factor (DIF), and the confining pressure conditions, we have constructed a two-factor equation for the DIF of lightweight concrete that depends on the confining pressure and the strain rate.