PLC Bands Research Articles

Effects of Pre-Stretching on the Mechanical Behavior of Cold-Rolled 5%Mn Medium Manganese Steel.

Studies about the pre-stretching effect on the mechanical behavior of cold-rolled 5%Mn medium manganese steel have adopted optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction techniques. Results showed that pre-stretching would change the ferrite morphology from massive and lath-like to strip-like. With the pre-stretching increasing from 0% to 14%, the dislocation density and yield strength both grew gradually, which corresponded to growth from 6.49 × 1014 m-2 to 7.98 × 1014 m-2 and growth from 765 MPa to 1109 MPa, respectively. Meanwhile, the austenite volume fraction, elongation and product of strength and elongation were all reduced with the pre-stretch increase. The stabilized retained austenite with pre-stretch delayed the occurrence of the TRIP effect and improved the work hardening rate. As a result, the Lüders band disappeared at 2% pre-stretch and the PLC band vanished from the stress-strain curve at 14% pre-stretch.

Mechanisms of types B and C serrations in the tensile flow curve of C-bearing high Mn steel

In the present study, the mechanisms of types B and C serrations in the tensile curves of C-bearing high Mn steel were investigated. Many tensile tests were performed at various temperatures (313–573 K) with two different initial strain rates (ε˙ini = 1 × 10−3 and 1 × 10−4/s). Type B serrations appeared in a lower tensile temperature range compared to type C serrations. Whereas a critical strain for type B serrations (ecB) was similar to yield strain regardless of tensile temperature and strain rate, a critical strain for type C serrations (ecC) increased with increasing tensile temperature, indicating the inverse PLC behavior of ecC. The inverse PLC behavior of ecC was explained by the intensified dynamic strain aging (DSA) model. The repetition of rapid aging and easy breakaway causes the discontinuous propagation of the PLC band with flickering, thereby resulting in type B serrations.

The influence of shearable and nonshearable precipitates on the Portevin-Le Chatelier behavior in precipitation hardening AlMgScZr alloys

The well-known mechanisms of interaction between precipitates and dislocations are shearing (for shearable precipitates) and bypassing mechanisms (for nonshearable precipitates). The transition from shearable to nonshearable precipitates in precipitation hardening alloys leads to changes of dislocation motion mode and dislocation multiplication behavior, which inevitably causes different PLC behaviors. In this study, we systematically investigate the influence of shearable and nonshearable Al3(Sc, Zr) precipitates on PLC behaviors by experimental characterization for precipitation hardening AlMgScZr alloys. We analyze the onset strain, critical strain-rate range, serration amplitude, and propagation behavior of PLC bands in detail for AlMgScZr alloys with shearable and nonshearable precipitates, respectively. We find that the transition from shearable to nonshearable precipitates changes the PLC band propagation behavior, decreases the magnitude of serration amplitude, expands the strain-rate range as well as decreases the critical strain rate between normal behavior (the critical strain increases with the increase of strain rate) and inverse behavior (the critical strain decreases with the increase of strain rate) regimes due to the different dislocation-precipitate interactions. Besides, the transition from shearable to nonshearable precipitates increases the onset strain at normal behavior while decreases the onset strain at inverse behavior depending on the different roles of precipitates in the solute-dislocation interaction. Finally, we reveal the nature of influence of different dislocation-precipitate interactions on PLC behavior considering different strengthening mechanisms based on quantitative characterization on precipitates and dislocation density.

Quantitative analysis of the Portevin–Le Chatelier effect by combining digital image correlation and dead-weight-type tensile test

Abstract A quantitative evaluation method of a serration deformation behavior by a dead-weight tensile test combined with a strain measurement by a DIC method is proposed. This method applies to binary solid solution aluminum alloys and a ternary cluster strengthened aluminum alloy. In the dead-weight tensile test, the serrated flow was measured as a stress-strain curve with a stepped-shape divided into two stages of a stress rising phase and a strain burst phase. By adopting this tensile test, the serration deformation behavior can be measured with an extremely reproducibility. It was confirmed that an elastic deformation occurs predominantly in the stress rising phase, and this deformation behavior is discussed in relation to the concentration of solid solution elements. Propagation of PLC bands has occurred in the strain burst phase. An activation volume obtained from the strain rate in the PLC bands was in good agreement with a calculated value based on a distribution interval assuming a regular square distribution of solute elements. For the ternary alloy strengthened by clusters, it is also shown that this method can be applied to the qualitative estimation of the change in the residual amount of the solid solution element in the matrix during natural aging.

The influence of nanosized precipitates on Portevin-Le Chatelier bands and surface roughness in AlMgScZr alloy

The influence of different precipitate-dislocation interactions, namely dislocation shearing and bypassing mechanisms, on PLC bands and the resultant surface roughness in AlMgScZr alloy was investigated. Three-dimensional surface roughness was quantitatively measured by confocal microscopy. We find that the introduction of shearable precipitates increases the stress amplitude, decreases the PLC bands number and surface roughness. However, the stress amplitude decreases, the PLC bands number and surface roughness increase with shearable precipitates turning to nonshearable precipitates. By analyzing the precipitation strengthening mechanisms quantitatively, the influence of precipitates on PLC bands and the resultant surface roughness was explored. Furthermore, our study demonstrates that the shearable precipitates can decrease the surface roughness by decreasing the number of PLC bands, which is instructive for designing structural materials with desirable mechanical property and surface quality.

On the mechanisms of nucleation and subsequent development of the PLC bands in an AlMg alloy

The nucleation and subsequent development of the Portevin-Le Chatelier bands during tensile deformation in a binary AlMg alloy was investigated by the digital image correlation technique. The following states of the material were employed for the current investigation: coarse-grained (CG), fine-grained (FG) after equal channel angular pressing (ECAP), severely strained (SS) after ECAP followed by cold rolling. Special attention was paid to the nucleation of an embryo band by maximizing the contrast of the deformation images. Particular spatiotemporal patterns of the alloy in three different conditions, testifying to the initiation of an embryo band having a wedge shape on the lateral surface of the specimen, were obtained at high and intermediate strain rates. The evaluation of the serration amplitude and the band velocity showed that the dependence of these values on strain consists of two stages (the linear growth-up/drop stage and the steady-state stage) for CG materials and only one stage (the linear growth-up/drop stage) for FG materials. The nucleation of deformation bands always occurs according to a mixed mechanism, in which the thickening of the embryo slows down with a decrease in the strain rate.

The effect of anisotropy on PLC band observed in a thin-specimen made of SUS304 during quasi-static tensile test

The effect of anisotropy on PLC band observed in a thin-specimen made of SUS304 during quasi-static tensile test

Temperature-Dependent Macroscopic Mechanical Behaviors and Their Microscopic Explanations in a Medium Mn Steel

Plastic instability (e.g., Luders band and PLC band) is a phenomenon frequently observed in cold-rolled medium Mn steels. In this study, through a number of quasi-static uniaxial tensile tests with digital image correlation (DIC) measurement of the strain rate field, we found the medium Mn steel (Fe-0.14C-7.14Mn-0.23Si wt. pct with approximately 33 vol pct of retained austenite and 67 vol pct of ferrite) exhibited distinct competition/coordination between the Luders band and the PLC band at different deformation temperatures: (1) at 25 °C and 55 °C, a Luders band was followed by several PLC bands; (2) at 100 °C, fracture occurred right after the Luders band formation, without the presence of a PLC band; (3) at 250 °C, only PLC bands were observed. We propose that these macroscopic behaviors can be explained by the temperature-dependent conditions for the formation/appearance and propagation of Luders bands and PLC bands. The propagation of Luders bands becomes difficult when the temperature increases as less hardening is provided by martensitic transformation. In contrast, the appearance and propagation of PLC bands become easier when the temperature increases and/or the plastic strain becomes large (16.5 and 18.9 pct in this material at the deformation temperatures of 25 °C and 55 °C, respectively), as the difference in the average velocities of solute atom diffusion and dislocation gliding is reduced. Either condition for Luders bands or for PLC bands will be satisfied at different deformation temperatures or plastic strains, thus leading to the competition/coordination between them.

Interactions of negative strain rate sensitivity, martensite transformation, and dynamic strain aging in 3rd generation advanced high-strength steels

Medium Mn transformation-induced plasticity (TRIP) steels, a subgroup of third generation advanced high-strength steels (AHSS), often present a combination of both dynamic strain aging (DSA) and TRIP. It has been previously shown that TRIP coincides with the passage of strain bands due to DSA, but their interdependence has not been well-established. This study seeks to explore the interactions between the two phenomena as well as the commonly-observed negative strain rate sensitivity. Digital image correlation (DIC) and in-situ magnetic measurements of the retained austenite fraction were employed to demonstrate the variations in mechanical behavior with increasing strain rate. Strain rate jump tests provided measurements of the strain rate sensitivity and were combined with either DIC to characterize PLC bands or with magnetic measurements to measure variations in the martensite fraction during the jumps. Finally, the micromechanics defining the peculiar bursts of martensite transformation in response to strain rate jumps are discussed in terms of a competition between dislocation mobility and austenite stability.

Investigation of Portevin-LeChatelier Bands in 5754 Aluminum Alloy under Various Strain Paths

This work investigated the mechanical and microstructural effects of Portevin-LeChatelier bands in aluminum 5754 alloy under different loading conditions. For these conditions (equibiaxial and uniaxial), an in-plane biaxial test apparatus was utilized with a miniature cruciform specimen. The amount and magnitude of load jumps and drops (serrations) due to the PLC bands were monitored by load cells and strain distributions were measured by digital image correlation technique. Under biaxial stretching, PLC bands nucleated and propagated repeatedly at the same region of the sample, while their formation was random under uniaxial tension. Overall contribution of PLC strain to the total strain was higher under equibiaxial stretching, yet there were no visible shear bands as the PLC bands were suppressed. Conversely, the shear stress state in tension promoted PLC bands and evolved them into sharp shear bands. Texture analysis confirmed the emergence of shear components and cube texture with increasing strain. The eventual failure of the sample was from the shear band(s). The main benefit of biaxial loading was prevention of undesired fracture from shear bands, without changing the overall forming characteristics and the initial copper texture of the material.

Progress in digital image correlation method

Photomechanics Laboratory in the University of Science and Technology of China has concentrated on principles and applications of digital image correlation (DIC) recent years. Photomechanics Laboratory not only has improved the accuracy and efficiency of DIC but also has achieved research findings on speckle pattern standardization, compensation method for 2D DIC, biomedical engineering, and serrated yielding phenomenon. This paper provides a review: (1) a theoretical model and an estimation algorithm were presented for shape function error in local deformations, and a subset size chosen criterion was presented to ensure the measurement performance; inverse compositional Gauss-Newton method using second order shape function was presented, which is two-times faster than the traditional method. (2) The inherent nature of interpolation bias was revealed, an experimental approach was developed to measure the interpolation bias, a speckle pattern assessment that considering the effects of both the systematic error and random error was presented, and a theoretical model for the speckle pattern optimization was built. (3) Compensation methods for 2D DIC based on pre-calibration were presented. (4) A system for orthognathic surgery navigation was built; full-field pulse vibration measurement system was developed. (5) The emergence and evolution of PLC band was observed, and the phenomenon of out-of-band shrink is found. The strain localization in the width-wise direction was observed using two 2D DIC systems. (6) The vibration and strain were dynamically measurement using the speckle correlation method.

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.

Serrated Flow and Dynamic Strain Aging in Fe-Mn-C TWIP Steel

The tensile behavior, serrated flow, and dynamic strain aging of Fe-(20 to 24)Mn-(0.4 to 0.6)C twinning-induced plasticity (TWIP) steel have been investigated. A mathematical approach to analyze the DSA and PLC band parameters has been developed. For Fe-(20 to 24)Mn-(0.4 to 0.6)C TWIP steel with a theoretical ordering index (TOI) between 0.1 and 0.3, DSA can occur at the very beginning of plastic deformation and provide serrations during work hardening, while for TOI less than 0.1 the occurrence of DSA is delayed and twinning-dominant work hardening remains relatively smooth. The critical strain for the onset of DSA and PLC bands in Fe-Mn-C TWIP steels decreases as C content increases, while the numbers of serrations and bands increase. As Mn content increases, the critical strain for DSA and PLC band varies irregularly, but the numbers of serrations and bands increase. For Fe-(20 to 24)Mn-(0.4 to 0.6)C TWIP steel with grain size of about 10 to 20 μm, the twinning-induced work hardening rate is about 2.5 to 3.0 GPa, while the DSA-dominant hardening rate is about 2.0 GPa on average. With increasing engineering strain from 0.01 to 0.55 at an applied strain rate of 0.001s−1, the cycle time for PLC bands in Fe-Mn-C TWIP steel increases from 6.5 to 162 seconds, while the band velocity decreases from 4.5 to 0.5 mm s−1, and the band strain increases from 0.005 to 0.08. Increasing applied strain rate leads to a linear increase of band velocity despite composition differences. In addition, the influence of the Mn and C content on the tensile properties of Fe-Mn-C TWIP steel has been also studied. As C content increases, the yield strength and tensile strength of Fe-Mn-C TWIP steel increase, but the total elongation variation against C content is dependent on Mn content. As Mn content increases, the yield strength and tensile strength decrease, while the total elongation increases, despite C content. Taking both tensile properties and serrated flow behavior into consideration, Fe-22Mn-0.4C TWIP steel shows excellent mechanical performance with a high product of tensile strength and total elongation and a slightly serrated stress–strain response. To suppress the negative effect of DSA in Fe-Mn-C TWIP steels on the stability of tensile behavior, a TOI lower than 0.1 is strongly suggested.

Portevin Le Chatelier Effect in a Metastable Austenitic CrMnNi Steel

High-alloy metastable austenitic CrMnNi steels exhibit depending on the austenite stability a transformation induced plasticity at room temperature resulting in concurrently high strength and good ductility. The martensitic phase transformation is more pronounced as lower the austenite stability is. In addition, a steel variant with low austenite stability shows a macroscopically inhomogeneous deformation behavior during monotonic loading – the so-called Portevin Le Chatelier effect, which is commonly related to the interaction of moving dislocations and diffusing solute atoms. The serrated plastic flow is related to the formation of macroscopic bands with localized plastic deformation – so-called PLC bands, which was analyzed by in situ infrared thermography.

Study of spatial-time inhomogeneity of serrated plastic flow Al-Mg alloy: using DIC-technique

The aim of the present paper is the investigation of temporal instabilities and spatial localization due to the Lüders behavior, the Portevin–Le Chatelier effect and the shoulder or necking effect during uniaxial tension tests of aluminum-magnesium alloy. This paper presents the brief description of the test procedure and experimental results of carrying out research by the combined use of a servo-hydraulic biaxial test system Instron 8850 and a non-contact 3-D digital image correlation measurement system Vic-3D. The digital image correlation is a highly effective computer-vision-based technique, which provides estimation of the displacement and strain fields on specimen surface by matching the reference subsets in the undeformed image (before loading) with the target subsets in the deformed images (captured during test). The evolution of inhomogeneous axial strain and axial strain rate fields has been illustrated for each stage of material’s deformation. To estimate the kinematics of serrated or jerky flow due to the strain bands propagation, the strain versus time curves and strain diagrams are given here. The experimental results show the recurrence in the strain distribution leveling along the specimen gauge. The changing between the macroscopic localization of the plastic flow, namely the running of the Lüders and PLC bands and the recovery of strain field homogeneity, has been observed.

Investigation of PLC band nucleation in AA5754

Abstract The purpose of the present work is the experimental investigation of the nucleation of PLC deformation bands in the aluminium alloy AA5754. The PLC bands are investigated using both mechanical methods and infrared (IR) thermography. The latter employs a high-speed IR camera which captures local changes of radiated power resulting from mechanical dissipation and heating due to the nucleation of PLC bands. The resulting IR images are used to determine spatio-temporal power field variations via image subtraction. Furthermore, band trajectories obtained from the IR images are used to study possible correlations between the spatio-temporal evolution of stress and radiated power in the specimens and PLC band development.

On nucleation and propagation of PLC bands in an Al–3Mg alloy

The processes of nucleation and propagation of Portevin–LeChatelier deformation bands during tensile deformation of the alloy AA5754 have been investigated by a special optical technique for local strain measurements, combining both a line-scan and a CMOS camera to record the changes of surface strain, applying digital image correlation. Examples for bands of types A and B are observed and discussed with respect to the micro-mechanisms of their nucleation and propagation, emphasizing local stress concentrations in addition to the well-known effects of dynamic strain aging and dislocation interactions for explaining the PLC behavior. Finally the nucleation and propagation of PLC bands are simulated by computations based on a simple constitutive model.

Circuital and Modal Characterization of the Power-Line Network in the PLC Band

With present day measurement techniques, it is very difficult to accurately model the power- line network (PLN), as seen from its power-line terminals, both in terms of impedance and noise. In this paper, a new circuital model (that considers line-ground and neutral-ground signals) and a new modal model (that considers common and the differential mode signals) for characterizing the low voltage PLN are presented. These models, which treat impedance and noise in a unified way, are composed by a three-impedance pi-network and two voltage sources. These models, which are complete and rigorous, are very interesting to desing PLC modems matched to the PLN for maximal power transfer, to compute the energy conversion between common mode and differential mode produced by asymmetries in the PLN, and to know the levels of differential mode noise that will affect a PLC receiver. These models have been experimentally validated.

Time-resolved deformation measurements of the Portevin–Le Chatelier bands

The displacement and strain fields of the PLC band are quantitatively measured using a technique based on high-speed digital photography and digital speckle correlation (DSC) method. The results show the nucleation process of a type B band within a few milliseconds. Even the type A band intermittently propagates with steps smaller than the band width. It is verified that the elastic shrinkage deformation exists outside a PLC band during avalanche-like deformation.

OS12-2-2 Time-resolved deformation measurements of the PLC shearing bands

The displacement and strain fields of PLC bands are quantitatively measured using a technique based on high-speed digital photography and digital speckle correlation (DSC) method. The results show the nucleation process of a type B band within a few milliseconds, the propagation of a type A band during the capture process and the growth of the deformation bands. The maximum strain rate inside PLC band is over three orders of magnitude larger than the nominal strain rate. The avalanche-like deformation of a PLC band may exceed the displacement of the crosshead within very short time, thus there exists an elastic shrinkage deformation outside a PLC band during avalanche-like deformation. The propagation of type A band, which is traditionally considered to be continuous, is detected to be intermittent in time-resolved results of this paper.