Portevin Le Research Articles

Nonstationary Electrochemical Response to the Intermittent Portevin–Le Chatelier Deformation in an Aluminum–Magnesium Alloy

It has been established that the Portevin–Le Chatelier effect in the AlMg6 aluminum–magnesium alloy deformed in an aqueous medium is accompanied by an intermittent electrochemical response, such as the negative jumps of the electrode potential of the sample, which occur simultaneously with the jumps of the mechanical stress on the deformation curve. Statistical and fractal analyses of the stepwise component of the electrode potential indicate the presence of long-term correlations in the structure of the electrochemical response, which are typical for the state of self-organized criticality. A possible mechanism of the appearance of jumps of the electrode potential, which is associated with the dynamics of deformation bands, is discussed.

Macroscopic consequences of Piobert–Lüders and Portevin–Le Chatelier bands during tensile deformation in Al-Mg alloys

Lüders bands and the Portevin–Le Chatelier effect are both due to plastic flow instabilities that affect a wide variety of metallic materials. The present study focuses on understanding these behaviors. Field kinematic measurements combined with the comparison of two specimen sizes are used here to characterize the Al–Mg alloy AA5086, by investigating the kinetics of individual localization bands. A unifying point of view is proposed for the classification of these bands together with the identification of a constitutive relation linking strain increments and nucleation stress, as well as evidence of the factors determining the development of serration behaviors. This leads to a novel interpretation of the spatiotemporal behavior of the bands as being the result of a competition between the influence of plastic strain and its gradient.

Влияние лазерной обработки на прочность и прерывистую деформацию Портевена-Ле Шателье алюминиевого сплава

AbstractThe effect of surface processing by laser IR radiation on strength and Portevin–Le Chatelier serrated deformation of AlMg6 aluminum–magnesium alloy is experimentally studied. Laser processing consists in local “shots” on the surface at the nodes of a square grid in the gauge part of a flat sample. The conditions under which such laser processing causes an increase in the surface microhardness, ultimate tensile strength, and the critical strain of the first stress drop in the stress–strain curve; decreases the probability of deformation band formation; suppresses necking at the prefracture stage; and changes the main crack morphology. The possible mechanism of the effect of laser processing on band formation, serrated deformation, and alloy strength are discussed.

Critical Phenomena in Portevin–Le Chatelier effect during compression of aluminium-magnesium alloy and stored energy evolution

This work was inspired by works of Gianfranco D’Anna (2000) where the Portevin-Le Chaˆtelier effect (PLC) was investigated by compressing Al-Mg alloy specimen in a very large deformation range and the results were interpreted from the viewpoint of phase transitions and critical phenomena. Two critical points where deformation process changes from “laminar” to jerky flow and back were noticed. We found that the second critical point where the rate of the serrations is decreased before the macroscopic fracture is not reachable for some alloys even with proposed geometry. We have tested 2 sets of specimens made of the same AMG6 alloy (~6% of magnesium) but with different amount of Si obtained from different producers. Specimens with a higher amount of Si tends to fracture before reaching the second point.Also, we have measured the amount of stored energy which can be recovered as heat during the annealing process. We used the differential scanning calorimeter STA "Jupiter" 449. We found that the amount of this energy sharply increases with the beginning of PLC serrations (first critical point) and drops at a second critical point or at macroscopic fracture. This can be related to a special type of critical phenomena—structural scaling transitions in mesodefect ensembles and associated structural relaxation mechanisms Naimark (2016)

Нестационарный электрохимический отклик на прерывистую деформацию Портевена-Ле Шателье алюминий-магниевого сплава

AbstractIt has been established that the Portevin–Le Chatelier effect in the AlMg6 aluminum–magnesium alloy deformed in an aqueous medium is accompanied by an intermittent electrochemical response, such as the negative jumps of the electrode potential of the sample, which occur simultaneously with the jumps of the mechanical stress on the deformation curve. Statistical and fractal analyses of the stepwise component of the electrode potential indicate the presence of long-term correlations in the structure of the electrochemical response, which are typical for the state of self-organized criticality. A possible mechanism of the appearance of jumps of the electrode potential, which is associated with the dynamics of deformation bands, is discussed.

Effect of Deformation Temperature on the Portevin-Le Chatelier Effect in Medium-Mn Steel

Experimental investigations of the plastic instability phenomenon in a hot-rolled medium manganese steel were performed. The effects of tensile deformation in a temperature range of 20–140°C on the microstructure, mechanical properties, and flow stress serrations were analyzed. The Portevin–Le Chatelier (PLC) phenomenon was observed for the specimens deformed at 60 °C, 100 °C, and 140 °C. It was found that the deformation temperature substantially affects the type and intensity of serrations. The type of serration was changed at different deformation temperatures. The phenomenon was not observed at room temperature. The plastic instability occurring for the steel deformed at 60 °C was detected for lower strain levels than for the specimens deformed at 100 °C and 140 °C. The increase of the deformation temperature to 100 °C and 140 °C results in shifting the PLC effect to a post uniform deformation range. The complex issues related to the interaction of work hardening, the transformation induced plasticity (TRIP) effect, and the PLC effect stimulated by the deformation temperature were addressed.

Origin of through-thickness serrated tensile flow behavior in Al–Cu–Li (AA2195) alloy: Effect of microstructure and texture

Al–Cu–Li alloys are known to exhibit serrated flow behavior, a typical characteristic feature of the dynamic strain aging and the Portevin–Le Chatelier (PLC) effect. In present study, the effect of microstructure and texture on serrated tensile flow behavior in a thick plate of Al–Cu–Li–Mg alloy was investigated. The microstructure consisted of coarse elongated grains with significant texture variation from a {100} <110> major component at surface to {110} <112> major component at the center layer. Samples from different locations across thickness and orientations were subjected to monotonic tensile loading at different strain rates in solutionized as well as under aged temper conditions. All specimen showed serrations in stress-strain curves, but a decreased serrated intensity from surface to center of the plate. Detailed microstructural examination and texture analysis suggested that the solute-dislocation interaction and shearing of the δ' precipitates could be the possible reasons for the PLC effect in this alloy. The through-thickness anisotropy in strength and serrated flow behavior of the alloy are attributed to the grain morphology and the texture variation in the material.

Effect of Dynamic Strain Aging on Tensile Deformation of 20MnMoNi55 Alloy

Results obtained from the uniaxial tensile tests of 20MnMoNi55 low-alloy RPV steel at different temperatures (varying from 27 to 450 °C) and straining rates (varying from 10−5 to 10−1 s−1) reveal that the material response is sensitive to both temperature and strain rate. For the range of temperature varying from 200 to 400 °C, in combination with different strain rates, the material shows negative strain rate sensitivity along with different kinds of serrated flow (between 200 and 310 °C), commonly termed as Portevin–Le Chatelier effect (types A, B and C). The material behavior, in that temperature–strain rate window, is described by dynamic strain aging (DSA) effect, and the changes in the mechanical properties of the material are studied. The DSA attribute of 20MnMoNi55 steel is characterized by calculating the activation energy. The x-ray diffraction technique and atomic percentage analysis using energy-dispersive x-ray suggest that the manganese influences the interstitial (carbon and/or nitrogen) diffusion by forming Mn-C and/or Mn-N pair, which can be a plausible cause of DSA in the material.

Temperature Effect on the Fracture of Laser Welded Joints of Aviation Aluminum Alloys

This paper describes an experimental study of a temperature effect on the fracture of laser welded joints of Mg- and Cu-containing aviation aluminum alloys. The fracture of alloys and their welded joints under a uniaxial loading at temperatures of −60, 20, and 85◦C is under study. It is revealed that the strength and ultimate strain of welded joints of Cu-containing alloys decrease as temperature rises because of the formation of fixed zones of localized plastic shears. Heating and cooling suppress the Portevin–Le Chatelier effect and significantly reduce the ultimate strain of a Mg-containing alloy, even though such reduction is not observed in a welded joint. It is shown that, the maximum limiting elongation of the welded joint of a Mg-containing alloy is achieved at a negative temperature, while the formation of secondary cracks is begins during heating.

Plastic instabilities in AA5754-O under various stress states

Spatio-temporal characteristics of plastic instabilities in the aluminium alloy sheet AA5754-O were investigated under three different stress states: uniaxial tension, plane strain and equi-biaxial tension. Tensile tests including conventional uniaxial tensile specimens as well as optimized ISO 16842 cruciform specimens for plane strain and equi-biaxial tensile deformation conditions were carried out at room temperature. Arm strength of the cruciform specimens was enhanced by laser deposition of thickening layers using Al-Mg alloy wire material in order to delay fracture and extend the equivalent plastic strain in the gauge area of the cruciform specimens from ~0.03 to ~0.13 under plane strain condition and to ~0.07 under equi-biaxial tension. The differences in serrated flow behaviour and spatio-temporal evolution of Portevin–Le Châtelier (PLC) bands under these three stress states were investigated using digital image correlation (DIC) techniques. The originality of this paper is to present different PLC phenomena under plane strain condition and equi-biaxial tension, which requires improved models or modifications of existing theoretical frameworks for uniaxial tension to account for different characteristics of PLC phenomena under plane strain and equi-biaxial tension of Al-Mg alloy sheets.

Instability of Plastic Deformation in Low-Alloy Copper Alloys

The purpose of this paper is to determine the influence of temperature of plastic deformation on the structure and mechanical properties of copper alloys of the types CuCo1NiBe (CCNB), CuCo2Be (CB4), CuNi2SiCr (CNCS), CuNi1P (CNP) and CuCr1Zr (CW106C) applied on electrodesduring a tensile test. Tensile tests were carried out on polycrystalline samples of above mentioned alloys, which confirmed the presence of inhomogeneous plastic deformation in specified temperature ranges for each alloy. The tensile test of the investigated copper alloys were realized in the temperature range of 20÷800 °C with strain rate of 1.2•10-3s–1 on the universal testing machine. Metallographic observations of the structure were carried out on a light microscope and the fractographic investigation of fracture on an electron scanning microscope. Performed experimental studies have proven that analyzed structural factors, in a range of investigated strain conditions at elevated temperature, significantly influence the phenomenon of the Portevin Le Chatelier (PLC) type instability of plastic strain, revealed in low-alloy copper alloys. Moreover, it was found that the impact of examined factors on the PLC effect should be considered comprehensively, taking into account their synergic interactions.

In Situ Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Luders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.

Local Deformation and Mn-C Short-Range Ordering in a High-Mn Fe-18Mn-0.6C Steel

The localized deformation in the high-Mn austenitic Fe-18Mn-0.6C (wt %) steel manifests itself as serrations in the stress–strain curves and Portevin–Le Chatelier (PLC) bands characterized by digital image correlation (DIC) analysis in uniaxial tensile tests. The serrated flow is correlated with the nucleation, propagation and dying away of PLC bands. The PLC band velocity decreases with increasing strain. In this present study, the Mn-C short-range ordering (SRO) was analyzed quantitatively using small angle neutron scattering (SANS) in Fe-18Mn-0.6C steel. The size and number density of the Mn-C SRO were determined as a function of engineering strain at room temperature. The mean radius of the Mn-C SRO decreases, while the number density increases when there is an increase in the engineering strain. The influence of PLC bands on Mn-C SRO in tensile tests was further discussed.

Behavior of an Austenitic–Martensitic VNS9-Sh Sheet TRIP Steel during Static and Cyclic Deformation

The properties of austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) sheet TRIP steel during static and cyclic loading are studied. The specific features of the mechanical behavior of the steel during static tension that are related to shearing, twinning, and martensite formation processes are detected. The static stress–strain curve of the steel has a developed microyield stage, a long yield plateau, and a serrated stage of strain hardening (Portevin–Le Chatelier effect). The shear mechanisms at the initial stages of cyclic deformation and fatigue crack propagation mechanisms are investigated.

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin--Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718){\thinspace}={\thinspace}2.68{\thinspace}{\textpm}{\thinspace}0.05 eV, $\tau$0{\thinspace}={\thinspace}2{\textperiodcentered}10-15 {\textpm} 1 s as well as its broadening factor $\beta${\thinspace}={\thinspace}1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superalloy

On the characteristics of Portevin–Le Chatelier bands in cold-rolled 7Mn steel showing transformation-induced plasticity

Strain localization during tensile deformation of cold-rolled and annealed 7Mn steel were investigated under various strain rates and deformation temperatures. The retained austenite grain size, strain rate and deformation temperature all have remarkable influences on the appearance of PLC bands. Higher strain rate clearly suppressed the formation of PLC bands whilst deformation temperature had a more complicated influence. During the room-temperature deformation at the quasistatic strain rate of 6.67 × 10−4/s, Lüders bands appeared in all specimens whilst PLC bands only in the specimens that were annealed above 700 °C. Digital image correlation (DIC) analysis showed the specimen annealed at 700 °C exhibited the type A PLC bands during the entire period of deformation; whilst the one annealed at 720 °C showed the more complicated type A+B bands before 600s and the type A bands afterward. All of these phenomena have been discussed in relation to the interaction of C atoms/C-Mn pairs and dislocations for sound interpretation.

On the PLC Effect in a Particle Reinforced AA2017 Alloy

The Portevin–Le Châtelier (PLC) effect often results in serrated plastic flow during tensile testing of aluminum alloys. Its magnitude and characteristics are often sensitive to a material’s heat treatment condition and to the applied strain rate and deformation temperature. In this study, we analyze the plastic deformation behavior of an age-hardenable Al-Cu alloy (AA2017) and of a particle reinforced AA2017 alloy (10 vol. % SiC) in two different conditions: solid solution annealed (W) and naturally aged (T4). For the W-condition of both materials, pronounced serrated flow is observed, while both T4-conditions do not show distinct serrations. It is also found that a reduction of the testing temperature (−60 °C, −196 °C) shifts the onset of serrations to larger plastic strains and additionally reduces their amplitude. Furthermore, compressive jump tests (with alternating strain rates) at room temperature confirm a negative strain rate sensitivity for the W-condition. The occurring PLC effect, as well as the propagation of the corresponding PLC bands in the W-condition, is finally characterized by digital image correlation (DIC) and by acoustic emission measurements during tensile testing. The formation of PLC bands in the reinforced material is accompanied by distinct stress drops as well as by perceptible acoustic emission, and the experimental results clearly show that only type A PLC bands occur during testing at room temperature (RT).

Tsallis q-triplet, intermittent turbulence and Portevin–Le Chatelier effect

In this paper, we extend a previous study concerning Portevin–LeChatelier (PLC) effect and Tsallis statistics (Iliopoulos et al., 2015). In particular, we estimate Tsallis’ q-triplet, namely {qstat, qsens, qrel} for two sets of stress serration time series concerning the deformation of Cu-15%Al alloy corresponding to different deformation temperatures and thus types (A and B) of PLC bands. The results concerning the stress serrations analysis reveal that Tsallis q- triplet attains values different from unity ({qstat, qsens, qrel} ≠ {1,1,1}). In particular, PLC type A bands’ serrations were found to follow Tsallis super-q-Gaussian, non-extensive, sub-additive, multifractal statistics indicating that the underlying dynamics are at the edge of chaos, characterized by global long range correlations and power law scaling. For PLC type B bands’ serrations, the results revealed a Tsallis sub-q-Gaussian, non-extensive, super-additive, multifractal statistical profile. In addition, our results reveal also significant differences in statistical and dynamical features, indicating important variations of the stress field dynamics in terms of rate of entropy production, relaxation dynamics and non-equilibrium meta-stable stationary states. We also estimate parameters commonly used for characterizing fully developed turbulence, such as structure functions and flatness coefficient (F), in order to provide further information about jerky flow underlying dynamics. Finally, we use two multifractal models developed to describe turbulence, namely Arimitsu and Arimitsu (A&A) [2000, 2001] theoretical model which is based on Tsallis statistics and p-model to estimate theoretical multifractal spectrums f(a). Furthermore, we estimate flatness coefficient (F) using a theoretical formula based on Tsallis statistics. The theoretical results are compared with the experimental ones showing a remarkable agreement between modeling and experiment. Finally, the results of this study verify, as well as, extend previous studies which stated that type B and type A PLC bands underlying dynamics are connected with distinct dynamical behavior, namely chaotic behavior for the first and self-organized critical (SOC) behavior for the latter, while they shed new light concerning the turbulent character of the PLC jerky flow.

Dislocation based multilevel model for elastic-plastic deformation of polycrystalline materials

The main aim of the work is to study the Portevin–Le Chatelier effect. The occurrence of the effect is closely connected to the phenomenon of dynamic strain ageing, i.e. the additional pinning of mobile dislocations by foreign atoms diffusing into the dislocation core during their arrest at obstacles, e.g. forest dislocation. The description of interaction of dislocation with impurities plays a great role, that’s why it is necessary to develop a several submodels: dislocation submodel for the interaction between dislocations and dislocation submodel for the interaction between dislocations and impurities. The multilevel approach based on using the internal variables, i.e. parameters describing the evolution of meso- and microstructure of the material, was applied to construct the model. The approach to model the dislocation structure is based on introduction of homogeneous dislocations densities on each slip system and obtaining evolutionary equations describing the mechanisms of their generation and interaction. The deformation mechanisms are evaluated and evolution of structural parameters during the process of deformation is analyzed. The dislocation submodel for the interaction between dislocations and impurities will be described in the next papers.

A constitutive model accounting for strain ageing effects on work-hardening. Application to a C–Mn steel

One of the most successful models for describing the Portevin–Le Chatelier effect in engineering applications is the Kubin–Estrin–McCormick model (KEMC). In the present work, the influence of dynamic strain ageing on dynamic recovery due to dislocation annihilation is introduced in order to improve the KEMC model. This modification accounts for additional strain hardening rate due to limited dislocation annihilation by the diffusion of solute atoms and dislocation pinning at low strain rate and/or high temperature. The parameters associated with this novel formulation are identified based on tensile tests for a C–Mn steel at seven temperatures ranging from 20 °C to 350 °C. The validity of the model and the improvement compared to existing models are tested using 2D and 3D finite element simulations of the Portevin–Le Chatelier effect in tension.