Portevin-Le Chatelier Effect Research Articles

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.

Continuous acoustic emission during intermittent plastic flow in α-brass

The Portevin–Le Chatelier effect which is usually associated with discontinuities in plastic flow has been studied in α-brass using an acoustic emission (AE) technique. It is found that intermittent plastic flow in the high-strain-rate regime corresponding to A-type serrations on the stress–strain diagram is accompanied by a continuous AE signal reflecting a smooth dynamics of emitting sources. No individual transient AE signals, which could be associated with rapid dislocation avalanches, have been resolved at different timescales.

Morphological diagram of Savart-Masson bands of macrolocalized deformation

The dynamics and morphology of Savart-Masson bands of macrolocalized deformation during a tension test of flat samples of aluminum-magnesium alloy Al-6wt.%Mg with a constant stress rate \((\dot \sigma _0 = const)\) have been investigated in situ using high-speed video recording. A phenomenological classification of the Savart-Masson bands according to the morphological and kinetic peculiarities is presented. This classification includes six types of deformation bands which replace each other with an increase in stress. A morphological diagram of Savart-Masson deformation bands is obtained for the first time. These bands differ significantly from the A, B, and C deformation bands during a tension test with a constant strain rate \((\dot \varepsilon _0 = const)\) under the conditions of the Portevin-Le Chatelier effect.

On the similarity of plastic flow processes during smooth and jerky flow in dilute alloys

The jerky flow of dilute alloys, or the Portevin–Le Chatelier effect, has a burst-like intermittent character at different fluctuation size levels. Multifractal analysis is applied to both the macroscopic stress serrations and the acoustic emission accompanying the plastic deformation. Multifractal scaling is found for both kinds of time series. The scaling range of the stress serrations is limited from below by their characteristic frequency. Unexpectedly, the scaling range for acoustic bursts not only covers this range but spreads to much shorter time scales with the same scaling exponent. This result testifies that the deformation processes revealed by the acoustic emission at a mesoscopic scale have a similar nature during both stress serrations and smooth plastic flow. The implications on the crossovers in the dynamics of jerky flow are discussed.

Effect of temperature and strain rate on serrated flow behaviour of Hastelloy X

Serrated flow behaviour of Hastelloy X has been examined over a wide range of temperature (300–1023 K) and strain rate (3 × 10−3s−1 to 3 × 10−5s−1). The alloy exhibited different types of tensile serrated flow in the intermediate temperature range of 473–923 K. Normal portevin-Le Chatelier effect (PLE) exhibiting type A and B serrations were observed at temperatures less than 823 K and inverse PLE exhibiting type C serrations was noticed at temperatures above 823 K. The average activation energy value of 106 kJ mol−1 for the A and B types of serrated flow has been evaluated. The evaluated activation energy value revealed that the migration of molybdenum in the nickel matrix has been found to be responsible for the occurrence of serrated flow in the alloy.

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.

A Revised Criterion for the Portevin–Le Châtelier Effect Based on the Strain-Rate Sensitivity of the Work-Hardening Rate

An improved analysis is presented of the stability of plastic deformation under conditions where dynamic strain aging (DSA) occurs, which leads to instabilities known as the Portevin–Le Châtelier (PLC) effect. It is shown that PLC instabilities can occur for conditions that are not covered by the currently prevailing criterion presented by Estrin and Kubin (1991), which focuses on a negative strain rate sensitivity of the flow stress, caused by interactions of solutes with thermally activated glide of mobile dislocations. The current analysis recognizes that the strain-rate sensitivity of the flow stress consists of two contributions, one associated with glide of mobile dislocations and the second with work hardening, related to storage of immobile dislocations. In this paper, an instability criterion is proposed that takes into account the possibility of a negative strain-rate sensitivity of the work-hardening rate, which is caused by diffusion of solutes to immobile dislocations. The latter contribution leads to an extended instability criterion. This criterion also provides an explanation for the existence of a critical strain above which instabilities occur. In this article, previously published tensile test data are used to show that a negative strain-rate sensitivity of the work-hardening rate, which influences significantly the occurrence of the PLC effect, can indeed occur under DSA conditions.

Fast Fourier transform on analysis of Portevin-Le Chatelier effect in Al 5052

Tensile tests were conducted with polycrystalline samples of Al5052 by deforming at room temperature on three different strain rates where the Portevin-Le Chatelier (PLC) effect was observed. The data of PLC effect serrations were obtained by subtracting moving average from the experimental stress-time data, and then analyzed by using Fast Fourier transform (FFT) to explore characteristics in frequency domain. The results indicate that the amplitude–frequency distribution is in different type from each other with different strain rate. And strain has little influence on FFT results. Moreover, through mathematic analysis on FFT results, the distribution characteristic curves have been obtained.

Strain-rate fluctuations during macroscopically uniform deformation of a solution-strengthened alloy

Using a work-hardened aluminum alloy that exhibits dynamic strain aging, oscillations in strain rate transitioning into intermittent local plastic activity are observed through an extended elastoplastic transition, before onset of the Portevin–Le Chatelier effect. Fourier analysis confirms this intermittency spans multiple scales. Wavelet analysis provides a method of quantifying transitions in the deformation behavior in both the time and frequency domains, and reveals a period doubling transition in both the local strain rate and global load signals.

On statistical behaviour of stress drops in Portevin–Le Chatelier effect

The Portevin–Le Chatelier (PLC) effect is a kind of plastic instability observed in many dilute alloys when deformed at certain ranges of strain rate and temperature. In this paper we present a comprehensive statistical analysis of the observed experimental data obtained during PLC effect and establish that the occurrence probability of the stress drops in the dynamical process responsible for PLC effect is Poisson in nature.

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.

The influence of temperature on the PLC effect in Al-Mg alloy

An experimental investigation of the influence of temperature on the Portevin-Le Chatelier (PLC) effect in Al-Mg alloy is conducted. Under a certain strain rate, the PLC effect is present in the temperature range of 223–328 K. The serration amplitude increases monotonically with increasing temperature, whereas the behaviors of the serration period and the critical strain with temperature include a descending branch under corresponding low temperature range and an ascending branch under corresponding high temperature range. The analysis in relation with dynamic strain aging (DSA) indicates that the temperature plays an important role in the PLC effect by dictating the solubility and the diffusibility of solute atoms. Both the concentration and the diffusibility of solute atoms contribute to increasing the serration amplitude. Under the descending branch temperature range, the serration period mainly depends on the preparation phase and the dominant factor of critical strain is the velocity of dislocations. However, under the ascending branch temperature range, the serration period mainly depends on the pinning phase and the dominant factor of critical strain is the applied stress required for unpinning.

Multiscale Modeling Approach to Acoustic Emission during Plastic Deformation

We address the long-standing problem of the origin of acoustic emission commonly observed during plastic deformation. We propose a framework to deal with the widely separated time scales of collective dislocation dynamics and elastic degrees of freedom to explain the nature of acoustic emission observed during the Portevin-Le Chatelier effect. The Ananthakrishna model is used as it explains most generic features of the phenomenon. Our results show that while acoustic emission bursts correlated with stress drops are well separated for the type C serrations, these bursts merge to form nearly continuous acoustic signals with overriding bursts for the propagating type A bands.

Influence of precipitation on the Portevin-Le Chatelier effect in Al-Mg alloys

In the alloy with solute content higher than the limiting solubility, the solute atoms that have failed to dissolve will precipitate from the solid solution and form precipitations. In this study, the Portevin-Le Chatelier (PLC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation contents have been investigated under different applied strain rates. The results suggest that precipitations have significant effect on the PLC effect and the more the precipitations are, the greater the influence is. Furthermore, the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content. However, for 5456 alloy with higher precipitation content, the diffusion is no longer the case but more complex.

Effect of precipitation on Portevin-Le Chateliereffect in Al-Mg alloys

In the alloy with solute content higher than the limiting solubility, the solute atoms failing to dissolve will precipitate from the solid solution and form precipitations. In this study, the Portevin-Le Chatelier (P-LC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation content are investigated under different applied strain rates respectively. The findings suggest that precipitations have a significant effect on the P-LC effect and different precipitation content has different effects. Furthermore, the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content. However, for 5456 alloy with higher precipitation content, the diffusion is no longer the case but more complex.

Computational aspects in presence of negative strain-rate sensitivity with application to aluminium alloys exhibiting the Portevin–Le Chatelier effect

Loss of ellipticity of the time-discretized boundary-value problem for elastic–viscoplastic solid materials with negative strain-rate sensitivity (SRS) is studied. The physical mechanism responsible for the negative SRS is assumed to be dynamic strain ageing. For the time-discretized problem the negative SRS translates into softening and may possibly lead to loss of ellipticity when the time step is greater than a critical value. The paper provides a method to compute the critical time step and illustrates the results for an aluminium alloy.

Temperature Dependence of the Dynamics of Portevin-Le Chatelier Effect in Al-2.5 Pct Mg Alloy

Tensile tests were carried out by deforming polycrystalline samples of Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was observed throughout the strain rate and temperature region. The mean cumulative stress drop magnitude and the mean reloading time exhibit an increasing trend with temperature which is attributed to the enhanced solute diffusion at higher temperature. The observed stress-time series data were analyzed using the nonlinear dynamical methods. From the analyses, we could establish the presence of deterministic chaos in the PLC effect throughout the temperature regime. The dynamics goes to higher dimension at a sufficiently high temperature of 425K but the complexity of the dynamics is not affected by the temperature.

Deformation behavior and the Portevin-Le Chatelier effect in a modified 18Cr–8Ni stainless steel

Mechanical behavior and microstructural evolution of a 2.5%W+0.4%Nb+0.3%V+0.17%N modified 18Cr–8Ni austenitic steel was studied in the temperature interval 410–740°C at initial strain rates ranging from 6.7×10−6 to 1.3×10−2s−1. The Portevin-Le Chatelier (PLC) effect attributed to dynamic strain aging (DSA) was found to occur in the temperature range 530–680°C. The PLC effect is manifested in the form of serrated (jerky) flow and planar slip. DSA increases yield strength, σ0.2, and ultimate tensile strength, σUTS, in comparison with conventional SUS304 steel due to the additional alloying that provides the formations of the clouds of solutes around the dislocations that exert the solute drag force.

The tensile properties and high cyclic fatigue characteristics of Mg–5Li–3Al–1.5Zn-2RE alloy

The tensile and fatigue properties of the Mg–5Li–3Al–1.5Zn-2RE (LAZ531-2RE) alloy were presented. The alloy was prepared by hot extrusion after vacuum casting. The Portevin–Le Chatelier (PLC) phenomenon was observed during tensile deformation. The PLC effect was attributed to the dynamic interaction between solute atoms (Li) and dislocation movement. The abnormal negative strain rate sensitivity (SRS) in high strain rate regime can be explained by the dynamic strain ageing (DSA) mechanism. The higher ductility may be associated with finer-grained microstructure and Li addition. Fatigue crack propagation was characterized by striation-like features. The smaller spacing of fatigue striations and larger fatigue crack propagation zone at the lower total strain amplitude gave rise to a longer fatigue life.