Portevin-Le Chatelier Bands Research Articles

General framework for acoustic emission during plastic deformation

Despite the long history, so far there is no general theoretical framework for calculating the acoustic emission spectrum accompanying any plastic deformation. We set up a discrete wave equation with plastic strain rate as a source term and include the Rayleigh-dissipation function to represent dissipation accompanying acoustic emission. We devise a method of bridging the widely separated time scales of plastic deformation and elastic degrees of freedom. The efficacy of the framework is illustrated by considering three distinct cases of plastic deformation. The first one is the acoustic emission during a typical continuous yield exhibiting a smooth stress-strain curve. We first construct an appropriate set of evolution equations for two types of dislocation densities and then show that the shape of the model stress-strain curve and accompanying acoustic emission spectrum match very well with experimental results. The second and the third are the more complex cases of the Portevin-Le Chatelier bands and the L\"uders band. These two cases are dealt with in the context of the Ananthakrishna model since the model predicts the three types of the Portevin-Le Chatelier bands and also L\"uders-like bands. Our results show that for the type-C bands where the serration amplitude is large, the acoustic emission spectrum consists of well-separated bursts of acoustic emission. At higher strain rates of hopping type-B bands, the burst-type acoustic emission spectrum tends to overlap, forming a nearly continuous background with some sharp acoustic emission bursts. The latter can be identified with the nucleation of new bands. The acoustic emission spectrum associated with the continuously propagating type-A band is continuous. These predictions are consistent with experimental results. The acoustic emission spectrum of the L\"uders-like band matches with recent experiments as well.

Spatio-temporal characteristics of plastic instability in AA5182-O during biaxial deformation

Spatio-temporal characteristics of plastic instability denoted by locally extreme values in strain rate contours computed with digital image correlation are investigated in biaxial deformation of AA5182-O sheet. Temporal characteristics are similar to those of Portevin–Le Châtelier (PLC) bands in uniaxial tension. Spatial characteristics, however, dramatically differ from PLC band morphologies in uniaxial tension. Initiation occurs in a localized deformation ring (LDR) that does not include the pole, followed by a rapid transition to a circular region that includes pole, i.e. a localized deformation circle (LDC). A critical strain is required to trigger serrated flow at the pole, and a negative strain rate sensitivity of the flow stress at the pole suggests that the underlying microstructural mechanism is dynamic strain aging. Plastic instability hops between the LDR and the LDC until a critical strain is reached at which it appears in a localized deformation band, with no circular symmetry, that hops around the pole. Future model development will require modifications to existing theoretical frameworks to account for the unique spatial characteristics of plastic instability during biaxial deformation of AA5XXX alloys.

Study on the out-of-plane deformation of the Portevin–Le Chatelier band by using digital shearography

The out-of-plane deformation of the Portevin–Le Chatelier (PLC) band is investigated during tensile tests in an Al alloy using digital shearography for the first time. The fringe patterns, which reflect the out-of-plane displacement gradient, indicate that the PLC bands also appear in the out-of-plane deformation field corresponding to the serrations in the stress–strain curve. Unlike the results obtained from other methods, the true bandwidth is determined by subtracting the shearing amount from the observed bandwidth. The continuous propagation of type-A PLC bands is traced at three different shearing amounts. The out-of-plane displacement of a PLC band is obtained based on the gray-level distribution in a frame of fringe pattern. The maximum displacement is approximately 250nm in an interval of 1/15s during a continuous propagation. The displacement gradient is greater in the front of the PLC band than in the back. Moreover, the position change process of the PLC band is presented, i.e., a band disappearing and a new band forming concurrently in another place. The inclination transition process, such that the inclination direction of the PLC band turns from the “/” direction to the “⧹” direction, is also shown in detail. The results prove that the vibrations-proof and high sensitivity digital shearography is a valuable and important method in the investigation of the out-of-plane deformation of the PLC band.

Dynamic strain aging of twinning-induced plasticity (TWIP) steel in tensile testing and deep drawing

The dynamic strain aging (DSA) of metallic materials due to solute atom diffusion to mobile dislocations induce deformation instability with load fluctuations and deformation localizations, hence reducing their sheet formability. In this paper, DSA behaviors of twinning induced plasticity (TWIP) steel with and without Al during tensile testing and deep drawing are investigated in terms of strain localization and the Portevin-Le Chatelier (PLC) band. A theoretical DSA model with internal variables of dislocation density and twin volume fraction is presented for an estimation of strain localization and strain hardening behavior of TWIP steels. The simulation results of the load history and PLC bands during tensile testing and deep drawing are in good agreement with the experimental values. A serration behavior is observed in high-Mn TWIP steels and its tensile residual stress is higher than that in the Al-added TWIP steels, which results in a deformation crack or delayed fracture of deep drawn specimens.

Forming performance of an as-quenched novel aluminum-lithium alloy

The forming performance of an as-quenched (W temper) novel aluminum-lithium alloy was investigated to realize a rapid and efficient forming through the route of “solution–forming after quenching–aging”. Tensile test was mainly adopted to obtain the true stress–strain curves, which were applied to calculate the strain hardening index (n value) and plastic strain ratio (r value). The results revealed that the investigated alloy displayed the satisfied forming properties in as-quenched condition. The Portevin-Le Chatelier (PLC) bands during the tensile deformation were observed within 60 min after quenching. Moreover, the n values were isotropic at different directions, decreasing rapidly with the extension of natural aging time. Meanwhile, the r values, rarely influenced by natural aging behavior, exhibited a significant anisotropy.

Thermomechanical characterization of Portevin–Le Châtelier bands in AlMg3 (AA5754) and modeling based on a modified Estrin–McCormick approach

The purpose of the current work is the experimental investigation and modeling of Portevin–Le Châtelier (PLC) band development in AlMg3 aluminum alloys, in particular in AA5754. The experimental investigation employs both mechanical and thermal (infrared) radiation measurement methods. The former involve force, displacement, and strain measurements using strain gauges. The latter employ a high-speed infrared camera to capture PLC band trajectories and evolution. In addition, the critical strain for band nucleation, as well as band characteristics such as velocity, type, and strain state, have been determined. To model the experimental results, a modification of the Estrin–McCormick model (e.g., Estrin and McCormick, 1991) due to Böhlke et al. (2009) is employed. In this modification, the saturation value of the Cottrell–Bilby–Louat (CBL) contribution to the effective flow stress due to dynamic strain aging is not constant, but rather is assumed to be linearly dependent on the accumulated effective inelastic strain. This model is incorporated into a finite-element model for the experimental specimens constructed with the help of convergence and mesh-sensitivity studies. Using selected mechanical test data, model parameters for AA5754 are then identified. With the identified model, a number of comparisons between experimental results and model predictions are carried out for validation purposes. Attention is focused here in particular on aspects of PLC band nucleation and propagation. Nucleation behavior is studied in particular with respect to stress gradients. It was found that the stress gradient is not the main trigger for PLC band nucleation, and is less relevant with increasing strain. The comparison of experimental and simulation results for spatio-temporal strain patterns and stress distribution shows that strong stress drops are correlated with the start of longitudinal propagation of a fully evolved PLC band. Small oscillations between large stress drops during propagation of type B bands indicate the consecutive nucleation, propagation and disappearing of these bands. The transition region from normal to inverse behavior (decreasing strain rate) denotes the transition from type A to type B. A transition with increasing strain is observed both experimentally and in the simulation. For lower strain rates and higher strains, type C bands are observed as well. Generally speaking, the band velocity decreases with decreasing strain rate and increasing strain. On the other hand, the band strain is found to increase with increasing strain, and only slightly with increasing strain rate. Simulation and experimental results, generally, show very good agreement.

Investigation using digital image correlation of Portevin-Le Chatelier Effect in Hastelloy X under thermo-mechanical loading

The plastic behavior of Hastelloy X subjected to thermo-mechanical loading was investigated in this work, and the phenomenon of dynamic strain aging was studied under isothermal conditions, using digital image correlation (DIC). The material of interest, Hastelloy X a Ni-based superalloy, is widely used in high temperature applications. It was seen that Hastelloy X exhibits serrated plastic flow behavior between temperatures of 300°C and 700°C, as a consequence of the dynamic strain aging effect, which, depending on temperature, manifests as type A/B or A and B oscillations on the uniaxial tension stress–strain curve, and is characterized by strain concentration bands propagating along the length of the specimen. The characteristics of Portevin-Le Chatelier (PLC) band internal strain distribution in uniaxial tension experiments were also investigated. The bands themselves were seen to have internal (sub)bands of two or three smaller strain jumps. An analysis of the strain increments induced by these bands was performed and clearly illustrates the inhomogeneous nature of the deformation – both spatially and temporally. In addition to the uniaxial behavior, biaxial stress states were investigated by studying PLC band formation near a notch in a double notched tensile geometry. Similar macroscopic results to the uniaxial case were seen while the strain bands themselves were seen to initiate in the notch region and travel away from the notches.

Kinematics of Portevin–Le Chatelier bands in simple shear

The mechanical behavior of 5000 series aluminium alloy sheets has been investigated in simple shear, in order to highlight the influence of strain rate and temperature on the occurrence of plastic instabilities. Indeed, as in tension, Portevin–Le Châtelier effect is also observed in simple shear and the aim of this study is to investigate the characteristics and kinetics of the bands of localized deformation under such a strain/stress state. Jerky flow and localized deformation are observed from room temperature up to 100°C but disappear at 150°C or above. A critical shear strain for the occurrence of jerky flow is evidenced at 100°C but it starts from the beginning of the test at room temperature. Concerning the bands, there are initially aligned with the applied load direction and rotate as the shear strain increases, similarly to the material frame. Finally, both McCormick and Johnson–Cook models were used to perform the numerical simulation of the shear tests, in order to compare the predicted load drops and kinetics of the bands with experiments.

Digital shearography investigation on the out-plane deformation of the Portevin-Le Chatelier bands

Digital shearography is proposed to study the out-plane deformation of the Portevin-Le Chatelier (PLC) band during tensile tests in an Al alloy. The geometrical shape and propagation of the PLC bands are visualized in real time through fringe pattern in subtracted images. In 1/15 s, the maximum out-plane displacement is 245 nm within the PLC band. The displacement distribution is asymmetric while in fringe pattern, the white fringe is narrower in the front of the PLC band than in the rear. In addition, the evolution processes of position shifting and inclination transition of the PLC bands are presented in detail.

Effect of the Strain Rate on the TRIP–TWIP Transition in Austenitic Fe-12 pct Mn-0.6 pct C TWIP Steel

The strain-rate dependence of the plasticity-enhancing mechanisms in Fe-12 pct Mn-0.6 pct C-0.06 pct N steel was investigated. At low strain rates, deformation-induced ε-martensite was formed. At high strain rate, the strain-induced formation of ε-martensite was inhibited, and mechanical twinning was the dominant plasticity-enhancing deformation mechanism. This transition was associated with an increased work hardening rate and a higher total elongation. Dynamic strain aging (DSA) took place at all strain rates. While propagating type C Portevin-Le Chatelier (PLC) bands were observed at low strain rates, isolated propagating type A PLC bands were observed at high strain rates. The critical strain for the occurrence of DSA had an anomalous negative strain-rate dependence at low strain rates and a normal positive dependence at high strain rates. The transition from negative-to-positive strain-rate dependence was associated with a sharp change in the strain-rate sensitivity of the flow stress. Transmission electron microscopy was used to analyze the relationship between the stacking fault energy (SFE), the strain rate, and the plasticity-enhancing mechanisms. The SFE and critical resolved shear stress for the onset of the twinning and the ε-martensite transformation were calculated and compared with experimental results.

Effect of strain rate on spatio-temporal behavior of Portevin–Le Châtelier bands in a twinning induced plasticity steel

Uniaxial tensile tests at three strain rates are performed with the aid of the digital image correlation (DIC) technique to experimentally investigate the spatio-temporal behavior of PLC bands in a twinning induced plasticity (TWIP) steel. The whole strain fields of tensile specimens are acquired throughout the tests. Significant serration crests corresponding to band nucleation are observed on the true stress vs. true strain curves derived from DIC results beyond a critical true strain. The work hardening exponent (n-value) increases from ∼0.08 to ∼0.5 when true strain increases to the critical true strain, and beyond that, the n-value exhibits serrations with increasing true strain. Two typical nucleation modes of Type-A Portevin–Le Châtelier (PLC) bands are observed in all tests. Nucleation and propagation of PLC bands are described in details based on these two nucleation modes of Type-A PLC bands. The PLC band orientation, which indicates the angle between the normal direction of a PLC band and tensile direction, fluctuates during propagation, and the fluctuation amplitude increases during the development of a localized necking band from a PLC band before fracture. In particular, the effect of strain rate on the kinematics of Type-A PLC bands (band strain, band width and band propagating speed etc.) in the TWIP steel is quantitatively analyzed, and a new algorithm based on the DIC results is presented which includes the elongating effect of tensile specimens during deformation to show the actual kinematics of Type-A PLC bands.

Effect of Mg content on Portevin–Le Chatelier band strain in Al–Mg sheet alloys

Experimental measurements of stress drops and band strains in type-B Portevin–Le Chatelier (PLC) bands were carried out for 5xxx series Al–Mg sheets with Mg content between 1.8 and 4.5 wt%. While the stress drops increase with global strain, the band strain values in all the samples follow a common linear relationship with global strain. The results indicate that the type-B PLC band strain is independent of solute content at given strain rate.

Heterogeneous deformation in twinning-induced plasticity steel

The influence of deformation localization during tensile deformation on the development of crystallographic texture in twinning-induced plasticity (TWIP) steel is reported. Whereas Al-added TWIP steel deforms homogeneously before the critical strain of onset of serrations, plastic deformation in Al-free TWIP steel occurs by propagation of isolated Portevin–Le Chatelier bands from the early stage of plastic deformation. The crystallographic texture evolution of both steels is, however, very similar. Deformation leads to the development of a Cu-type texture characterized by the strengthening of the rotated Brass component rather than the pure Brass component.

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part II. Jerky Flow and Instantaneous Strain Rate

The jerky and smooth flow curves in high-manganese twinning induced plasticity (TWIP) steels were investigated by comparing Fe-Mn-C and Fe-Mn-Al-C systems. The pronounced serrations on the flow curves of Fe-Mn-C TWIP steel, produced during tensile testing at 300 K (27 °C) and 373 K (100 °C), were shown to be the result of localized high-temperature Portevin Le-Chatelier (PLC) bands moving across the gage length throughout the deformation. The speed of the PLC bands and their temperature effects were found to be strongly dependent on the applied strain rate, which was controlled by adjusting the cross-head speed of the tensile testing machine. The localized temperature-dependent stacking fault energy (SFE) variations resulting from the PLC effect and adiabatic heating were analyzed and compared for both slow and fast deformation rates. The instabilities in the measured logarithmic strain values caused by jerky flow could cause the local strain rate to deviate systematically from the targeted (applied) strain rate. These instabilities are better observed by calculating the instantaneous strain rate (ISR) values for each instant of deformation along the entire gage length. Finally, a new type of diagram was developed by plotting the true stress against the ISR values. From the diagram, the onset of different mechanisms, such as deformation twinning, nonpronounced, and pronounced serrations, could be marked precisely.

Thermal analyses and simulations of the type A and type B Portevin–Le Chatelier effects in an Al–Mg alloy

The process of plastic deformation is associated with a transformation of mechanical energy to heat energy. In this study, the type A and type B Portevin–Le Chatelier (PLC) effects were investigated by infrared thermography. The formation, propagation, and morphology of individual PLC bands were observed and characteristic parameters of the bands were measured. In particular, temperature increments originating from elastic shrinkage outside a deformation band were observed for the type B PLC effect, and this shrinkage was confirmed by a computation of the strain rate and plastic deformation within the band. Thus, we suggest a new criterion for distinguishing between the type A and type B PLC effects by the existence of elastic shrinkage outside deformation bands. A phenomenological model is described in detail, taking into account the effect of temperature changes, competition between mobile dislocations and solute atoms, and the influence of elastic deformation. The model has been used in numerical simulations of stress–strain curves and temperature changes associated with the PLC effect. It is shown that the numerical results are able to reproduce most of the experimentally observed phenomena with reasonable accuracy.

Real‐Time Visualisation of the Portevin–Le Chatelier Effect With Mechanoluminescent‐Sensing Film

Abstract: The use of the mechanoluminescence (ML) technique to directly observe the Portevin–Le Chatelier (PLC) effect is described. A uniaxial tensile test of an aluminium alloy plate coated with SrAl2O4 : Eu2+ (SAOE) ML‐sensing film was performed, and a high‐speed camera recorded ML images simultaneously. Strong ML light in response to the PLC bands was emitted from the coated SAOE ML‐sensing film and could be seen clearly with the naked eye. The position, the slope angle of the band front and propagation velocity of the PLC bands were measured directly from the ML images. Momentary interplay between two face‐to‐face propagating PLC bands was observed and captured in the ML images. The results show that real‐time visualisation of the PLC effect was achieved using this ML technique.

Investigations on the Portevin Le Chatelier critical strain in an aluminum alloy

The critical condition for the occurrence of serrations during Portevin–Le Chatelier (PLC) effect is investigated in this work. Some experimental tensile results on an Al–Mg aluminum alloy (AA5754) are recalled. The mechanical model for strain ageing proposed by MacCormick (1989) [1] to simulate the PLC effect is described, and the critical condition usually associated to this model is discussed. This condition is shown to fail at predicting the occurrence of PLC bands in finite element (FE) simulations based on this model. An alternative loss of stability condition is proposed, which is in accordance with simulation results. This condition is used for the identification of strain ageing model parameter for Al–Mg alloy.

Spatial and Temporal Characteristics of Propagating Deformation Bands in AA5182 Alloy at Room Temperature

The spatial and temporal characteristics of propagating deformation bands in the Al-Mg alloy AA5182 in O temper were studied experimentally at room temperature. Tensile tests were carried out on flat specimens at strain rates in the range from 10−5 to 10−1 s−1. Digital image correlation (DIC) and digital infrared thermography (DIT) were applied to monitor the propagating bands. It was found that the material exhibits a sharp yield point, and Luders bands were seen at all the strain rates. Jerky flow took place all along the Luders plateau. It thus seems that the Portevin–Le Chatelier (PLC) effect starts at incipient yielding and that there is no critical strain. At the end of the Luders plateau, PLC bands immediately started to propagate back and forth along the gage section of the specimen. The work hardening of the material decreased consistently with increasing strain rate, while the flow stress on the Luders plateau was rather unaffected by the strain rate. This indicates that the dynamic strain aging (DSA) mainly affects the strength of the interaction between mobile and forest dislocations. The strain to necking was found to decrease gradually with strain rate for this alloy, which is consistent with the lower work-hardening rate at the higher strain rates.

Portevin–Le Chatelier (PLC) instabilities and slant fracture in C–Mn steel round tensile specimens

Round tensile specimens of a C–Mn steel were tested at different strain rates and temperatures. Some of the samples tested in the Portevin–Le Chatelier (PLC) domain exhibit slant fracture surfaces. Spherical dimples were evidenced all over the slant fracture surfaces. Numerical simulations showed that stress triaxiality increases around PLC bands and that slant fracture occurs within a PLC band. In a round specimen, this band is plane and inclined, whereas some numerical results predict conical bands.

Microstructural characterization of the Portevin–Le Chatelier band in an Al-Mg alloy by X-ray diffraction line profile analysis

Al–2.5% Mg alloy exhibits the Portevin–Le Chatelier (PLC) effect at room temperature for a wide range of strain rates. Tensile test has been carried out on a flat Al–2.5% Mg alloy sample at a strain rate of 3.7×10−6 s−1. The strain rate was chosen so that the type C PLC band appears in the sample. X-ray diffraction profile has been recorded from the gauge length portion of the deformed sample to investigate the microstructure of the PLC band. Analysis revealed that the dislocation density is much higher within the band compared to the undeformed sample even at small strain. The PLC band in this alloy possesses an equal fraction of screw and edge dislocations.