Portevin-Le Chatelier Research Articles

Autowave Description of Plasticity of Materials with an Unstable Phase Structure at the Macroscale Level

We consider the regularities of a plastic flow in materials with strain-induced phase transformation in titanium nickelide and trip steel as examples. The experimental analysis of plastic deformation processes has been carried out using the technique of digital correction of speckle images, which makes it possible to describe quantitatively the behavior of plasticity fronts associated with phase transformation in the given materials. The mechanisms of formation of the Luders fronts as well as the Portevin–Le Chatelier fronts at different stages of the plastic flow are considered.

The Domain of Portevin-Le Chatelier Effect in Al-3Mg Alloy

An Al-3Mg (wt. %) alloy was studied after equal channel angular pressing and subsequent cold rolling. The mechanical behavior of the alloy in the temperature range from 223 K to 373 K (from –50°C to 125°C) at strain rates 2.1×10–1 – 5.2×10–5 s–1 was investigated. The analysis of stress-strain curves was performed to determine the conditions of manifestation of the Portevin – Le Chatelier (PLC) effect in investigated alloy. The deformation curve at a temperature of 298 K (25°C) and a strain rate of 1×10–3 s–1 is characterized by instability of plastic flow in contrast to the deformation curves obtained under other studied strain rate/temperature conditions. Stress oscillations at the necking stage were observed at high temperatures (>323 K (50°C)) and lower strain rates (1×10–4 s–1 and 5.2×10–5 s–1) forming the left border of the PLC effect domain. In general, deformation curves are characterized by the absence of stress serrations during the uniform elongation.

A comparison study of microstructure, texture and mechanical properties between two 6xxx aluminum alloys

This paper investigates a comparison study of microstructure, texture and mechanical properties between two 6xxx aluminum alloys through microstructure, texture characterization and tensile test. The results show that they exhibit different microstructure, texture and mechanical properties. In comparison with the alloy sheet with a low weight ratio of Mg to Si (Si-excess), the alloy sheet with a higher weight ratio of Mg to Si (Mg-excess) and additional Zn possesses the finer as-casting and solution treated equiaxed grain structure, less particles with larger size before solution treatment, weaker recrystallization texture mainly including Cube {001}<100> orientation and weaker mechanical properties. The low weight ratio of Mg to Si corresponds to slightly higher yield strength and ultimate tensile strength, but much higher plastic strain ration r, work hardening exponent n values and elongation. Interestingly, Portevin-Le Chatelier (PLC) effect is very prevailed in the alloy sheet with a higher weight ratio of Mg to Si and additional Zn, which is responsible for the low elongation and r value. The alloy containing a low weight ratio of Mg to Si may be beneficial to improve comprehensive mechanical property.

Influence of the non-monotonic loading and short stops on the regularities of the jerky flow in the Al-Mg alloy

The aim of the paper is to investigate the influence of the non-monotonic loading and short stops on the spatial-time inhomogeneity due to the jerky flow in the Al-Mg alloy. Experimental study on the regularities of inhomogeneous deformation and the processes of macroscopic localization of the plastic flow due to the Portevin-Le Chatelier effect was carried out. Using the digital image correlation technique the data on the initiation and propagation of the strain bands were obtained. Experimental data were obtained that show the effect of short-term stops and non-monotonic loading on the regularities of inelastic deformation of the Al-Mg alloy when the PLC effect manifests itself. Short stops decrease the level of critical strain when the jerky flow starts. Non-monotonic tests led to a change in the type of the PLC effect and a change in the regularities of the spatial-time inhomogeneity of plastic flow.

Strain rate sensitivity of the aluminium-magnesium-scandium alloy - Scalmalloy®

This work investigates the strain rate sensitivity of the aluminiummagnesium-scandium alloy Scalmalloy, which is used extensively for additive manufacturing of lightweight structures. This high strength aluminium alloy combines very good weldability, machinability and mechanical strength: it can be heat-treated to reach nominal ultimate tensile strengths in excess of 500 MPa. We report tensile tests at strain rates ranging from 10−3 /s to 103 /s at room temperature. It is well known that Al-Mg alloys exhibit a negative strain rate dependency in combination with serrated flow caused by the Portevin-Le Chatelier effect, which describes the interaction of Mg solutes with dislocation propagations. In contrast, in Al-Sc alloys, the flow stress increases with increasing strain rate and displays positive strain rate dependency. Additionally, the presence of Sc in the form of Al3-Sc provides a fine-grained microstructure which allows higher tensile and fatigue strength. This research shows how these combined effects interact in the case of Scalmalloy, which contains both Mg and Sc. Tests are performed at quasi-static, intermediate and high strain rates with a servohydraulic testing machine and a Split-Hopkinson tension bar. Local specimen strain was performed using 2D Digital Image Correlation.

Influence of the loading system stiffness on the jerky flow in Al-Mg alloy at inelastic and postcritical deformation stages

The work is devoted to studying the issues of experimental studies of the behavior of materials under conditions of inhomogeneous discontinuous flow at the stages of plastic and supercritical deformation (deformation softening). Obtaining reliable experimental data on the indicated stages of inelastic deformation of materials is important for understanding the physical processes occurring in materials during deformation and ultimately leading to fracture. The results of tests are presented, confirming the influence of the rigidity of the loading system in relation to the working part of the sample on the processes of intermittent plastic deformation. The tests were carried out on samples of Al-Mg alloy. The loading rigidity was varied using a rig with variable rigidity. Registration of displacement and deformation fields on the surface of the working part of the samples was carried out by a non-contact video system Vic-3D Correlated Solutions based on the digital image correlation (DIC) technique. The use of this system makes it possible to measure both deformation at the macro level and the distribution of its components in the working part of the specimen under conditions of inhomogeneous discontinuous deformation. As a result of tensile tests, deformation diagrams were obtained for Al-Mg alloy samples tested at different values ​​of the rigidity of the loading system: 120 MN/m (nominal value), 50 MN/m, 18 MN/m, and 5 MN/m. All the diagrams show discontinuous plastic deformation (the Portevin-Le Chatelier effect). A decrease in the rigidity of loading with respect to the working part of the specimen led to a change in the type of discontinuous yield and an increase in the magnitude of the stress drop at each tooth of the diagram. The indicated effects of inhomogeneous plastic deformation are observed both at the stage of strain hardening and at the stage of postcritical deformation.

Complex study of the Portevin-Le Chatelier effect by uniaxial tension of axisymmetric specimens with ring concentrators of various geometries

In order to assess the influence of the geometry of the concentrators on the spatio-temporal inhomogeneities of the plastic flow of aluminum-magnesium alloys, uniaxial tension tests of axisymmetric specimens with ring concentrators of various geometries were carried out. The test program includes 4 groups of concentrators: a groove with a radius of 5 mm, a groove with an angle of 157 ° and a nose radius of 5 mm, a groove with an angle of 140 ° and a nose radius of 5 mm, and a groove with an angle of 140 ° and a nose radius of 5 mm. It is of interest to detect the effect of intermittent flow, which manifests itself in the form of load discontinuities in the loading diagram. An increase in the sharpness of the annular concentrator, and, consequently, an increase in the degree of stress concentration, leads to an increase in the bearing capacity of the samples (maximum perceived load), but deformation reserves decrease (maximum deformation at the time of destruction), and also affects the manifestation of the Portevin-Le Chatelier effect.

Numerical modeling of the dynamic strain aging in steels at high strain rates and high temperatures

Carbon steels can be heat treated to produce different microstructural variations and mechanical properties. At high temperatures the material plasticity and strength can be influenced by diffusional effects like the Portevin-Le Chatelier effect, leading to a commonly observed increased strength at elevated temperatures. The diffusional effects are influenced by the chemical composition, but also the heat treatment history that affects the local composition and especially the concentrations of free solute atoms. In this work, a numerical approach was implemented to reproduce the thermomechanical behaviour of two different microstructural variants of steel grade C45. The experimental data used to calibrate the model includes information of the plastic behaviour of material subjected to dynamic compression loading at a wide range of temperatures. Special emphasis was focused to describe the effects of the dynamic strain aging (DSA) on the flow stress. A strategy based on machine learning was implemented to obtain a model that reproduces the strengthening of the material due to diffusional effects. Cubic Support Vector Machine models were trained for both microstructure variants of the steel and different surfaces were obtained to describe the topology of the flow stress as function of temperature and strain rate. The model predictions were compared to the behaviour described by the Johnson-Cook model to estimate the influence of the DSA effect on the strength of the material at high strain rates and temperatures. Furthermore, the model quantifies how the microstructure affects the strength of the material and the strength of the DSA-hardening.

The Effect of Direct Electric Current on the Plastic Behavior of AA7075 Aluminum Alloy in Different States of Hardening.

Electrically-Assisted Forming (EAF) techniques are interesting and promising for the automotive industry. Electrically-assisted tensile tests were carried out on specimens of AA7075 aluminum alloy in different states of hardening, namely T6 (the as-received state) and the supersaturated solid solution state. All the tests were carried out in quasi-static conditions under the application of direct electric current (DC) in the range of 90 to 540 A. The experimental results showed that with a DC density of 10 A/mm2 the uniform strain and strain at fracture increased when the AA7075 was in the supersaturated solid solution state. A correlation between the mechanical results and microstructural features analysed through transmission electron microscopy was assessed. An explanation of the investigated phenomena based on the electron wind theory, heterogeneous microscale Joule heating and the Portevin–Le Chatelier (PLC) effect was finally proposed.

Macroscopic and microscopic characterizations of Portevin-LeChatelier effect in austenitic stainless steel using high-temperature digital image correlation analysis

Using digital image correlation (DIC) analysis in an Fe-19Cr-13Ni-0.2C austenitic stainless steel (mass%), the tensile deformation behavior accompanied with the Portevin-LeChatelier (PLC) effect was investigated at high temperatures of 723-823 K under various applied strain rates of 10−4 s−1–5 × 10−3 s−1. With the help of the DIC analysis, the global strain in the gauge part of a tensile specimen and the local strain in a PLC band were precisely measured regardless of the lowering machine stiffness at a high temperature. The high-temperature DIC analysis could identify the critical strain and strain rate conditions where stress-strain curve indicated serrated flow. The analytical results revealed that dynamic strain aging in the austenitic stainless steel was controlled by the pipe diffusion of Cr atoms. Further, it was also confirmed that the local strain gently increased in a gauge part through a PLC band propagation, which promoted a repeatable A-type PLC band propagation in the same direction. Furthermore, the local strain was discontinuously developed by the repeatable formation of PLC bands; consequently, the local strain rate became remarkably high within a PLC band. In addition, the local strain rate in the PLC band monotonically increased with the increase in the number of PLC bands. From this viewpoint, it was discovered that necking deformation occurs suddenly at the PLC band, when the local strain rate exceeds the critical value, where dynamic strain aging is ineffective.

Mesoscopic nature of serration behavior in high-Mn austenitic steel

We have thoroughly clarified the mesoscopic nature of serration behavior in a high-Mn austenitic steel in connection with its characteristic localized deformation. A typical high-Mn steel, Fe-22Mn-0.6C (wt. %), with a face centered cubic (FCC) single-phase structure was used in the present study. After 4 cycles of repeated cold-rolling and annealing process, a specimen with a fully recrystallized microstructure having a mean grain size of 2.0 μm was obtained. The specimen was tensile tested at room temperature at an initial strain rate of 8.3 × 10−4 s−1, during which the digital image correlation (DIC) technique was applied for analyzing local strain and strain-rate distributions in the specimen. Obtained results indicated that a unique strain localization behavior characterized by the formation, propagation and annihilation of deformation localized bands, so-called Portevin–Le Chatelier (PLC) bands, determined the global mechanical response appearing as serration on the stress-strain curve. In addition, the in-situ synchrotron XRD diffraction during the tensile test was utilized to understand what was happening in the material with respect to the PLC banding. Lattice strain of (200) plane nearly perpendicular to the tensile direction dropped when every PLC band passed through the beam position, which indicated a stress relaxation occurred inside the PLC band. At the same time, the dislocation density increased drastically when the PLC band passed the beam position, which described that the material was plastically deformed and work-hardened mostly within the PLC band. All the results obtained consistently explained the serration behavior in a mesoscopic scale. The serration behavior on the stress-strain curve totally corresponded to the formation, propagation and annihilation of the PLC band in the 22Mn-0.6C steel, and the localized deformation, i.e., the PLC banding, governed the characteristic strain hardening of the material.

Mechanical properties of solution heat treated Al-Zn-Mg-Cu (7075) alloy under different cooling conditions: Analysis with full field measurement and finite element modeling

The effect of W-temper heat treatment, consisting of solution heat treatment (SHT) and rapid cooling, on the mechanical properties and fracture of high strength aluminum alloy 7075 sheet is investigated experimentally and numerically. Three different cooling conditions from the solution heat treated as-received T6 temper 7075 alloy sheet are considered: water quenching, die quenching, and air cooling. The mechanical properties are measured by uniaxial tensile tests and limiting dome height (LDH) formability tests on the heat treated samples. Moreover, the distributions of strain and strain rate are analyzed by the full-field digital image correlation (DIC) technique, which enables the quantitative investigation on the Portevin–Le Chatelier (PLC) band propagations induced by dynamic strain aging (DSA). The DIC analysis reveals the distinctive formability between uniaxial and biaxial tensile stress states of the W-temper samples. In the finite element analysis, the hardening model based on Kocks-Mecking-Estrin-McCormick (KMEC) model is employed to incorporate the DSA-originated PLC band propagation in the simulation of LDH formability test. The simulated LDHs agree well with experiments when the PLC effect along with planar anisotropy of the heat treated 7075 alloy sheets is properly implemented.

Portevin-Le Chatelier effect, twinning-detwinning and disordering in an aged Ni–Cr–Mo alloy during large plastic deformation

Abstract Portevin-Le Chatelier (PLC) effects, twinning-detwinning and disordering phenomena in long-range ordered (LRO) phase strengthened Ni–Cr–Mo alloys during hot compression are reported and investigated. The results reveal that the PLC effects with type-C serrations occur in the solution-treated and aged alloys, and that the Pt2Mo-type LRO phase can increase the serration amplitude at the early stage of deformation. Only after the early stage of deformation, the twinning of aged alloy can be activated. However, the deformation twin and the Pt2Mo-type LRO phase are not stable during the severe deformation. The potential disordering process of the LRO phase is provided as the slipping of the dislocation on the two valid {111} slip systems. The occurrence of the steps on the twin boundaries of the remanent deformation twins reveals that the detwinning mechanism is likely the dislocation-twin boundary interaction.

The Effect of Preliminary Thermomechanical Processing on the Kinetics of Localized Plasticity Autowaves in Trip Steel

The kinetics of the martensitic transformation fronts in transformation-induced plasticity (TRIP) steel was studied in relation to preliminary thermomechanical treatment using the digital image correlation method. It was found that warm rolling of steel to 40–63% reduction significantly increases the stress of the onset of strain-induced phase transformation and changes the loading curve stages. The strain-induced phase transformation in TRIP steel occurring through the formation of Lüders and Portevin–Le Chatelier bands is shown to be an autowave process of localized plasticity. The austenite → martensite transformation at the elastic-plastic transition occurs in the form of several switching localized plasticity autowaves. At the jerky flow stage, excitation autowaves of localized plasticity are generated and propagate repeatedly until the strain-induced austenite → martensite transformation is completed. It is shown for the first time that the sources of excitation autowaves in the material are the sites of nucleation or annihilation of switching autowaves.

Initiation and Suppression of the Portevin–Le Chatelier Effect in Aluminum Alloy under IR Laser Irradiation and Electric Current

The separate and joint effect of IR laser irradiation and electric current on the Portevin–Le Chatelier effect and generation of deformation bands in aluminum–magnesium alloy AlMg6 has been investigated in situ by complex methods, including acoustic emission (AE) and high-speed video recording. The nucleation and evolution of the deformation bands initiated by pulsed laser radiation focused on the surface of deformed sample have been investigated with the time resolution up to 0.1 ms. The conditions under which a single laser “puncture” with a duration of up to ~1 ms initiates discontinuous deformation during several tens of seconds and reduces significantly (by a factor of 1.5) the critical strain of the onset of Portevin–Le Chatelier effect in AlMg6 alloy are revealed. It is established that transmission of low-density (below 40 А/mm2) direct electric current suppresses the negative influence of laser irradiation and completely stabilizes the deformation behavior of the alloy. Possible applications of the obtained results are discussed.

Mechanical properties and formability of EN AW-7075 in cold forming processes

Due to a low density and high tensile strength, the aluminum alloy EN AW 7075 T6 offers a high lightweight potential for structural components. Since its formability is limited at room temperature in the T6 temper state, the potential of this alloy for automotive bodies is only utilizable by adapted deep drawing processes. In recent years, process chains suited for warm and hot forming have been researched and developed. However, warm and hot forming solutions require additional process steps and a complex tooling system in comparison to cold forming processes. Alternatively, the forming of such blanks at room temperature in the W temper state is favorable since conventional tools can be used. The W temper state is a heat treatment condition achieved after solution heat treatment and subsequent quenching, which is characterized by an increased ductility. However, this condition is unstable, due to the onset of natural ageing. With increasing time after the quenching step, the strength of the material increases, which leads to a reduction of formability. Another phenomenon that occurs after quenching is the Portevin Le-Chatelier effect. This effect causes the formation of flow lines during cold forming and results in a decrease of ductility. Hence, the objective of the investigations was to determine the formability of EN AW 7075 as a function of the natural ageing time after solution heat treatment and quenching. Therefore, tensile tests of various aged samples were carried out. The results show a relation of the formability to the natural ageing time and a dependency on the quenching rate. Furthermore, a heat treatment strategy for EN AW-7075 was developed, that considers manufacturing processes like the cathodic dip coating. The influence of the quenching rate, ageing time and temperature as well as the influence of temperature of the paint baking process after the cathodic dip coating were considered. Therefore, a design of experiments and tensile tests were carried out. Thus, the deep drawing of EN AW-7075 at room temperature is particularly promoted.

Intercritical annealing to achieve a positive strain-rate sensitivity of mechanical properties and suppression of macroscopic plastic instabilities in multi-phase medium-Mn steels.

This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe-12Mn-3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α'-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates ( to ). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved.

Effect of Lüders and Portevin–Le Chatelier localization bands on plasticity and fracture of notched steel specimens studied by DIC and FE simulations

The impact of the Portevin–Le Chatelier (PLC) effect on plasticity and fracture ahead of a severe notch is investigated by DIC field measurements for C–Mn steel. The PLC effect causes an intermittent activity of highly localized strain rate bands. This first high temperature investigation of the effect of a notch on the PLC localization bands is successfully reproduced numerically in terms of strain localization in 3D FE simulations. This thereby validates quantitatively the McCormick-type model under these complex conditions. It is found that, when the PLC effect is at play at elevated temperature (175°C), a flat to slant crack transition is observed. In contrast, at room temperature, when only Lüders bands are active, the crack remains mostly flat, i.e. normal to the loading direction of the SENT samples. This behaviour may be related to the early loss of symmetry and intermittency of the plastic zone that is found for PLC even affecting the initial Lüders bands at elevated temperature. During the slant fracture at high temperature, only half the fracture energy is absorbed compared to flat fracture at room temperature. The McCormick-type model simulations predict slant strain rate bands through the sample thickness, that are consistent with the slant fracture found at elevated temperature. Accordingly, no slant bands are found for simulations outside the PLC domain. Both experiments and simulations show PLC strain localization bands that are flip-flopping up and down during crack propagation. By combining the McCormick-type model with a Rousselier porous plasticity model, the flat fracture of the sample at room temperature and the associated macroscopic curve are reproduced successfully, but the toughness at elevated temperature is overestimated.

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.

Propagating bands of plastic deformation in a metal alloy as critical avalanches.

The plastic deformation of metal alloys localizes in the Portevin-Le Chatelier effect in bands of different types, including propagating, or type "A" bands, usually characterized by their width and a typical propagation velocity. This plastic instability arises from collective dynamics of dislocations interacting with mobile solute atoms, but the resulting sensitivity to the strain rate lacks fundamental understanding. Here, we show, by using high-resolution imaging in tensile deformation experiments of an aluminum alloy, that the band velocities exhibit large fluctuations. Each band produces a velocity signal reminiscent of crackling noise bursts observed in numerous driven avalanching systems from propagating cracks in fracture to the Barkhausen effect in ferromagnets. The statistical features of these velocity bursts including their average shapes and size distributions obey predictions of a simple mean-field model of critical avalanche dynamics. Our results thus reveal a previously unknown paradigm of criticality in the localization of deformation.