Spatial and Temporal Characteristics of Propagating Deformation Bands in AA5182 Alloy at Room Temperature
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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 Lüders bands were seen at all the strain rates. Jerky flow took place all along the Lüders 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 Lüders 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 Lüders 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.
Aluminum alloys are important technological materials primarily due to their attractive strength-to-weight ratio. They are used in diverse applications ranging from packaging to the aeronautical industry. Important candidates for such applications are the alloys from the 5000 series whose primary alloying element is Mg. They may be rolled into thin sheets and offer significant strength. However, their plastic deformation at room temperature is discontinuous, with the strain localizing in narrow bands that leave undesirable traces on the surface of the final product. This is the signature of the Portevin–Le Chatelier (PLC) phenomenon, which manifests itself in certain ranges of temperature and strain rate. The repeated strain localization is due to the negative strain rate sensitivity (SRS) of the material, which, in turn, is correlated with smaller scale phenomena associated with interactions between solute and dislocations, referred to as dynamic strain aging (DSA). The technological goal is to increase the SRS to positive values in the range of temperatures and strain rates relevant for industrial processes. This would ensure material stability during processing and would eliminate the PLC phenomenon. In particular, in Al-Mg alloys, it is desirable to increase the SRS and to eliminate the PLC effect at room temperature.
As proposed by Cottrell,[1,2] the unstable plastic flow observed in Al-Mg alloys can be a result of solute-dislocation interaction at the microscopic level. To date, a full understanding of the micromechanical mechanisms and the relevant factors affecting the macroscopic behavior of serrated plastic flow is still lacking. However, the cause of the PLC effect is negative steady-state SRS, which is attributed to DSA associated with conditions when point defects can diffuse toward mobile dislocations and temporarily arrest them.[3,4] The results of DSA are higher flow stress and greater work hardening at lower strain rates than for higher ones, and further serrated stress-strain curves, discontinuous plastic flow, and propagating deformation bands during plastic straining. An interesting historical presentation of the various studies of the PLC effect can be found in Reference 5.
In this article, we provide experimental data on the mechanical behavior of one of the most important commercial alloys from the 5000 series, namely, AA5182 in O temper. Using DIC and DIT, the spatiotemporal characteristics of the Lüders and PLC bands are revealed in tensile tests at different strain rates. This combined use of DIC and DIT enabled us to study the similarities and differences between the two types of localized deformation and the correlation between the strain and temperature increments induced by the deformation band. The experimental observations are discussed in relation to existing models to enhance the understanding of the underlying mechanisms responsible for Lüders and PLC bands.
Tensile Test Program
Crosshead Velocity (mm/s)
Nominal Strain Rate (s−1)
Time to Rupture (s)
Optical Gage Dimensions (mm × mm)
7 × 10−5
10.11 × 10.11
3.33 × 10−4
10.11 × 10.11
2.8 × 10−3
10.11 × 10.11
1 × 10−2
10.11 × 10.11
1 × 10−1
10.11 × 10.11
The tests were carried out at room temperature in a servohydraulic material testing system (MTS model 810, MTS, Minneapolis, MN) with a 10-kN load cell, in displacement control with crosshead velocity adjusted to the desired nominal strain rate in the range from 10–5 to 10–1 s−1. Note that all specimens were gripped and clamped at exactly the same positions. The acquisition frequency for the force and displacement was as follows in the five tests: 10 Hz in P1, 50 Hz in P2, 100 Hz in P3 and P4, and 1000 Hz in P5.
Two different techniques were used to observe and eventually characterize the discontinuous plastic flow and the deformation bands spatially and temporally, namely, DIC and DIT. The gage length of the flat specimen was entirely imaged with a fast CCD camera (Photron Ultima APX-RS, Photron USA, Inc., San Diego, CA) on one side and with an infrared camera (JADE 570M, Electrophysics, Sofradir EC, Inc., Fairfield, NJ) on the other side. The imaged zones for DIC and DIT are shown in Figure 2. Prior to the tests, one side of the specimen was decorated with finely sprayed black and white paints to enhance the image contrast for the DIC, while the other was decorated with a fully black paint in order to enhance its emissivity for the DIT. The realization of the texture for DIC is as follows: a first layer of paint is spread, black (or, respectively, white) on which microdroplets of white paint (or black, respectively) are deposited so that the distribution is homogeneous. The texture of the specimen should be heterogeneous, but the distribution of the paint should be as homogeneous as possible. Note that the quality of the speckle pattern is defined by its smoothness. Errors were estimated for both measures. The thermal resolution of the infrared camera used in this work is less than 20 mK. For DIC, an estimate of errors in strain is done by the software CorreliLMT (LMT-Cachan, Cachan, France) and was found to be around 6 × 10−3 for the five tests.
Image Sizes and Acquisition Speeds for Field Measurements Using DIC and DIT
Image Size (Pixels)
Acquisition Shutter (Frames/s)
Conversion Factor (mm/Pixel)
Image Size (Pixels)
Acquisition Shutter (Frames/s)
Conversion Factor (mm/Pixel)
128 × 656
44 × 239
128 × 656
47 × 240
128 × 656
47 × 239
128 × 656
47 × 239
128 × 656
46 × 239
4.1 Yield Point, Lüders Plateau, and Serrated Yielding
The material exhibits a sharp yield point at all strain rates tested. This is a result of the strain aging phenomenon, and the reason for the sharp yield point is pinning of dislocations by interaction with solutes that migrate to the dislocations during the aging time. In this case, the aging time is the time from manufacturing of the sheet to the execution of the tensile tests. The Lüders band is then related to unlocking of dislocations in the case of weak pinning and formation of new dislocations in the case of strong pinning. Similar observations for Al-Mg alloys were reported in several studies, e.g., References 10 through 15. Robinson and Shaw found that Lüdering occurred for an AA5182 alloy that was cold worked, annealed at 573 K (300 °C) for 30 minutes, and then air cooled. When the same alloy was cold rolled, annealed at 723 K (450 °C) for 10 minutes and water quenched, the Lüders extension was absent. The materials had similar grain sizes. Robinson and Shaw suggest that the high uniform density of dislocations in the quenched material is sufficient to remove the yield point, and that the stronger static strain aging in the air-cooled material is due to the higher ratio of solute atoms to dislocation line length and the longer time available for solute migration in the period of the air cooling. Similar conclusions were also made by Rossig et al. Lloyd et al. studied the yield point elongation in the 5182-O alloy. They conclude that the occurrence of the Lüders effect was linked to the structure of the grain boundaries. Processing histories leading to grain boundaries free of defects resulted in the Lüders effect, whereas treatments giving a high density of grain boundary dislocations removed it. The result implies that segregation to individual dislocations and segregation to grain boundaries are both important processes. The room-temperature deformation behavior of an Al-Mg6.5 alloy sheet was investigated by Romhanji et al. The material was cold rolled with reductions between 5 and 70 pct and then annealed at 593 K (320 °C) for 3 hours. The Lüders extension was suppressed for the materials with rolling reductions less than 15 to 20 pct. For these materials, the rolling and annealing resulted in well-defined subgrains of small size. The fine cell structure implies a high dislocation density and, thus, a more dilute solute atmosphere. Higher rolling reductions led to a fully recrystallized microstructure with lower dislocation density after annealing. This leads to more favourable conditions for static strain aging, and for that reason, these materials exhibited Lüders extensions.
Another important observation is that jerky flow appears as soon as the material has entered into the Lüders plateau. The serrations indicate that there is no critical strain for the onset of the PLC effect; instead, it occurs at incipient yielding and seems to coexist with the Lüders band. Since this occurs in all tests, we conclude that the material exhibits negative steady-state SRS within the actual range of strain rates at room temperature and that the critical strain is zero. These findings are in agreement with the observations by Robinson and Shaw and Picu et al. for the same alloy. Another explanation proposed by Ohtani and Inagaki is that the Lüders strain exceeds the critical strain for the PLC effect to occur, and accordingly, serrated yielding will commence in a region right behind the front of the propagating Lüders band.
The amplitude of the serrations in the stress-time curve is strongest for the lowest strain rate and decreases as the strain rate is increased. Similar observations were reported by Kang et al. for AA5754 (AlMg3) sheets. At the same time, the spatiotemporal map in Figure 8 indicates that the PLC bands get more intense for higher strain rates, in the sense that the strain increment produced by one passing of the band increases. The normalized strain-time curves at the center of the gage area presented in Figure 12 indicate that the band propagation becomes steadier as the strain rate increases. These curves also show that the strain increment produced by the PLC band (also denoted the band strain) increases markedly and the band velocity decreases as the work-hardening rate of the material decreases. Kang et al. also found that the band strain increases with the global strain for AA5754.
4.2 Work Hardening and Necking
The force-elongation curves show that the work-hardening rate decreases with strain rate, while the flow stress on the yield plateau is less affected (Figure 3). Thus, it seems that the main effect of DSA is to increase the strength of the interaction between mobile and forest dislocations, as proposed by Mulford and Kocks and later discussed by Wycliffe et al. and van den Beukel and Kocks. As discussed in Reference 20, the aging may possibly affect both the friction stress and the forest hardening. The observed reduction of the work-hardening rate is further in agreement with the more recent theories for DSA developed by Picu and Soare and Curtin. In particular, Picu proposed a mechanism for DSA based on solute clustering at forest dislocations and its effect on the strength of dislocation junctions. Using a model constructed with this mechanism as its basis, Picu et al. were able to capture several of the observed characteristics of the negative SRS of the AA5182 material in O temper and to describe the variation of the flow stress and hardening rate with temperature in the negative SRS region.
The elongation at ultimate force is determined by the onset of necking. The broken specimens hardly exhibited any evidence of diffuse necking, which indicates that local necking occurs almost immediately after reaching the maximum force. It is seen from the force-elongation curves in Figure 3 that the elongation at maximum force decreases with increasing strain rate. There seem to be two possible reasons for this observation. (1) The most obvious is the reduction of the work-hardening rate with increasing strain rate, which will tend to reduce the strain to necking, according to the Considère criterion. This tendency is at least partly counterbalanced by the lower flow stress, since the criterion states that necking occurs as the work-hardening rate equals flow stress. (2) Another explanation is that the PLC bands work as geometrical imperfections in the tensile specimen, owing to the local strain increment produced by the propagating band. This has been investigated by Kang et al. The geometrical imperfection would result in lower strain to necking. Since the strain increment produced by the PLC bands seems to increase with increasing strain rate, it could be that this effect becomes more important as the deformation rate is increased.
4.3 Comparison of Lüders Bands and PLC Bands
One important observation in the behavior of AA5182 in O temper is the apparent existence of both Lüders and PLC bands. Besides, the plateau corresponding to the Lüders behavior is serrated. Furthermore, the serrations observed during the plateaus have exactly the same characteristics as the serrations observed for usual PLC bands. Their magnitudes increase when the strain rate is decreased; their shape also changes in a similar way when the strain rate is increased. The only difference is the presence of a yield point and that the average slope of the stress-strain curve is approximately zero as for the Lüders bands. Therefore, the observed bands share some properties from the classical PLC bands and others from Lüders bands. At first sight, the question arises whether the serrations are associated to secondary PLC bands superposed on a usual Lüders band or if this is some special type of bands. It is interesting to note that serrated Lüdering was also observed in steel.
Usually, when comparing PLC bands to Lüders bands, major differences between the two types are observed: on the one hand, Lüders bands propagate only once in the specimen while PLC bands propagate repeatedly; on the other hand, the slope of the stress-strain curve is zero during the propagation of a Lüders band, while the slope of the overall stress-strain curve during the propagation of a PLC band is positive.
These differences were extensively studied by Wijler et al., who reported different types of deformation bands for a gold-copper alloy, namely, Au (14 at. pct Cu). Their reasoning was the following: during the propagation of the first PLC band, an almost uniform deformation rate is maintained in front of the band. Therefore, the band meets material, which was deformed to an increasing degree, and this requires an increasing stress for the band to propagate. The strain gradient met by the band is also enhanced by the passage of the band. Due to this strain gradient along the specimen, every fresh band will start at the end where the deformation is lowest. Following this, Wijler et al. suggested that if one prevents the buildup of this strain gradient, then the shape of the stress-strain curve should be flat as in the case of a Lüders band. When the band has passed through the specimen, the strain is increased, and due to work hardening, the next band requires a stress jump. This was then experimentally verified. The strain gradient was suppressed by a homogeneous prestraining of the material at a high strain rate followed by sufficient aging in order to anchor the dislocations, after which the material was deformed at a relatively small strain rate for DSA to occur. Wijler et al. also suggest that one can obtain a similar situation with a gradient of aging rather than a strain gradient.
However, there are also some characteristic differences between the results obtained here for AA5182-O and those of Wijler et al. The first one is related to the yield point. For our material, a usual yield point followed by the plateau is observed just like in a Lüders band if one disregards the serrations seen during the plateau. For the gold-copper alloy investigated by Wijler et al., a sharp yield point is observed, which is, however, immediately followed by a sharp increase of the flow stress above the yield point; and only after this is the serrated plateau observed (Figures 1 and 6 of Reference 24). The second one is that several plateaus were observed for the gold-copper alloy, each one followed by a stress jump and corresponding to a new band, while for AA5182-O, only one plateau is observed followed by normal PLC behavior. We note, however, that Wijler et al. mention that they also observed situations where only one plateau was seen followed by normal PLC behavior, and this was attributed to the experimental conditions (prestrain, aging temperature, and aging time).
The nature of the strain gradients in the specimens of AA5182-O can be evaluated from the curves shown in Figure 6. Except for the test P2 (note that the same figure is not available for P1), in the three other tests, the plastic strain rate is almost zero outside the band. Hence, the strain gradient is negligible, leading to the plateau behavior.
Using tensile tests at different strain rates in combination with digital image correlation (DIC) and digital infrared thermography (DIT), the spatial and temporal characteristics of Lüders and Portevin–Le Chatelier bands in the Al-Mg alloy AA5182 in O temper were studied. The tests were performed at strain rates between 10−5 and 10−1 s−1and at room temperature.
The material exhibited a sharp yield point, and Lüders band propagation was observed at all the strain rates. Jerky flow occurred all along the Lüders plateau and into the hardening region of the stress-strain curve. It thus seems that the material exhibits negative SRS at room temperature, and no critical strain is required for the onset of jerky flow. At the end of the Lüders plateau, Portevin–Le Chatelier bands immediately started to propagate across the gage length of the specimen. The work hardening of the material was found to diminish consistently with increasing strain rate, while the flow stress on the Lüders plateau was less affected by the strain rate. This indicates that the dynamic strain ageing mainly affects the strength of the interaction between mobile and forest dislocations. The strain to necking was found to decrease gradually with increasing strain rate, which is consistent with the lower work-hardening rate at the higher strain rates.
The combined use of DIC and DIT allowed a more precise investigation of the Lüders and PLC bands from the mechanical and thermal points of view. It provides, in particular, better measures of all the characteristics of the bands (the width, orientation, and velocity of the bands; the strain rate inside the bands; and the strain and temperature increments induced by the passing of the bands), and also their morphology. One particular observation was the correlation between the temperature increase and the strain increment caused by the Lüders and PLC bands and how the strain rate affects this correlation. In perspective, the thermal analysis allows for energetic considerations, which will facilitate enhanced modeling of static and DSA processes through dissipation analysis. The spatiotemporal analysis also gives direct insight into the fracture process following the DSA phenomenon.
Two of the authors (JDE and OSH) acknowledge the award of a visiting professorship at LMT–Cachan. This work was supported by Ecole Normale Supérieure de Cachan, the French–Norwegian Foundation, and the Brazilian Ministry of Education through CAPES.
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