Le Chatelier Effect Research Articles

Chernov–Lüders bands and the Portevin–Le Chatelier effect as plastic flow instabilities

The development of macroscopic plastic flow heterogeneities in metals in the form of Chernov–Luders bands and the Portevin–Le Chatelier effect is studied. The main laws of deformation development in these two cases are found, and the kinetics of the motion of Chernov–Luders band fronts and the serrated deformation during the Portevin–Le Chatelier effect is investigated. It is shown that both versions of heterogeneities can be considered as macroscopic autowave processes of switching and excitation in deformable media.

Influence of a pulsed current on the Portevin–Le Chatelier effect in AlMg5 aluminum–magnesium alloy

We have reported on the results of experiments on the influence of current pulses on the band formation and intermittent deformation of AlMg5 alloy. It has been established that, to suppress the nucleation of deformation bands, preliminary treatment of the alloy by a current of density 60 A/mm2 for at least 1 s is required.

The tensile properties and serrated flow behavior of a thermomechanically treated CoCrFeNiMn high-entropy alloy

Serrated flow behavior of CoCrFeNiMn high entropy alloy with a single face-centered cubic phase was systematically examined over a wide range of temperature and strain rate. The alloy exhibited excellent fracture-resistance with RT elongation up to 56%. Prominent serrations occurred at the medium temperature range and evolved in the sequence of A→A+B→B→B+C→C with increasing temperature and decreasing strain rate. The initial small plasticity produced low density of dislocations piled up at grain boundaries and high density of dislocations were tangled to form cell substructures during subsequent severe deformation stage. Bowing and kinks of dislocations were frequently observed at 400 and 600°C. The critical strain for the onset of serrated flow decreased with increasing temperature and decreasing strain rate, indicating a normal and not inverse Portevin–Le Chatelier (PLC) effect due to the thermodynamic stability of the alloy. Two temperature-dependent values of activation energy for serrated flow (Q) were obtained. The estimated low value (116kJmol−1) implied that pipe diffusion is responsible for the pinning process of dislocations from 300 to 500°C. In contrast, a higher Q (295kJmol−1) between 500 and 600°C suggested that the pinning or unpinning process of dislocations is controlled by the most sluggish diffusion species (i.e. Ni) in the high-entropy alloy.

Plastic flow instability: Chernov–Lüders bands and the Portevin—Le Chatelier effect

We have studied the regularities in the evolution of macroscopic nonuniformities in the plastic flow of metals in the form of the Chernov–Luders bands and the Portevin—Le Chatelier effect. The regularities in the evolution of strain inhomogeneity in these two cases have been established, and the kinetics of motion of the Chernov–Luders fronts and abrupt deformation in the Portevin–Le Chatelier effect has been analyzed. It has been shown that the Chernov–Luders fronts and Portevin–Le Chatelier jumpwise straining can be treated as macroscopic autowave processes of switching and excitation, respectively, in deformed media of various origins.

Chernov–Luders and Portevin–Le Chatelier deformations in active deformable media of different nature

The development of macroscopic inhomogeneities in the form of Chernov–Luders bands and serrated deformation bands (Portevin–Le Chatelier effect) in plastic metal flow is studied. For these two cases, regularities in the development of deformation inhomogeneity were established and the kinetics of motion of the fronts of Chernov–Luders bands and serrated deformation bands was studied. It is shown that the Chernov–Luders fronts and the serrated Portevin–Le Chatelier deformation can be considered as macroscopic autowave switching and excitation processes, respectively, in deformable media of different nature.

Role of Self-Organization of Dislocations in the Onset and Kinetics of Macroscopic Plastic Instability

The present paper examines two aspects of the problem of critical conditions of jerky flow in alloys, or the Portevin–Le Chatelier (PLC) effect. Recent development of dynamic strain aging (DSA) models proved their capacity to qualitatively reproduce complex non-monotonic behavior of the critical strain, providing that the parameters of theory are allowed to depend on strain. Experimental measurements of such strain dependences have been realized for the first time and used to revise the predictions of the critical strain and stress relaxation kinetics upon abrupt strain-rate changes. On the other hand, it is usually omitted from consideration that the PLC stress serrations can last very short time in comparison with the characteristic time of stress transients. The development of stress drops was studied with the aid of the acoustic emission (AE) technique. It is shown that such macroscopic instabilities are caused by clustering of AE events which otherwise occur all the time, including the periods of smooth plastic flow. The role of synchronization of dislocation avalanches in the development of abrupt stress serrations and its relationship with the predictions of the local DSA models is discussed.

Effect of an aggressive medium on discontinuous deformation of aluminum–magnesium alloy AlMg6

It is experimentally shown that the molecular (chemical) process of surface etching of deformed aluminum–magnesium alloy AlMg6 causes the development of a macroscopic plastic strain step with an amplitude of a few percent. Using numerical simulation of the polycrystalline solid etching process, it is shown that the corrosion front morphology varies during etching from Euclid (flat) to fractal (rough). The results obtained show the key role of the surface state on the development of macroscopic mechanical instability of a material exhibiting the Portevin–Le Chatelier effect.

Microstructure and strain-stress analysis of the dynamic strain aging in inconel 625 at high temperature

Serrated flow is a result of unstable plastic flow, which occurs during tensile and compression tests on some dilute alloys. This phenomenon is referred as the Portevin Le-Chatelier effect (PLC effect). The aim of this research was to investigate and analyze this phenomenon in Inconel 625 solution strengthened superalloy. The tested material was subjected to tensile tests carried out within the temperature range 200-700 °C, with three different strain rates: 0.002 1/s, 0.01/s, and 0.05 1/s and additional compression tests with high deformation speeds of 0.1, 1, and 10 1/s. The tensile strain curves were analyzed in terms of intensity and the observed patterns of serrations Using a modified stress drop method proposed by the authors, the activation energy was calculated with the assumption that the stress drops’ distribution is a direct representation of an average solute atom’s interaction with dislocations. Subsequently, two models, the standard vacancy diffusion Bilby-Cottrell model and the realistic cross-core diffusion mechanism proposed by Zhang and Curtin, were compared. The results obtained show that the second one agrees with the experimental data. Additional microstructure analysis was performed to identify microstructure elements that may be responsible for the PLC effect. Based on the results, the relationship between the intensity of the phenomenon and the conditions of the tests were determined.

Influence of γ′ precipitates on the critical strain and localized deformation of serrated flow in Ni-based superalloys

Although γ′ precipitates enhance the strength of Ni-based superalloys, they can also promote an undesirable phenomenon called the Portevin–Le Chatelier (PLC) effect. Alloys designed and fabricated with various γ′ contents were subjected to uniaxial tensile tests at different temperatures (523–923 K) and strain rates (1 × 10−4–3 × 10−3 s−1). It was found that the γ′ precipitates facilitated a transition of the PLC effect from normal to inverse behavior. From the viewpoint of the critical strain, the temperature transition region shifted to low temperatures with increasing γ′ content. Digital image correlation was applied to the observations of the localization band in relation to the γ′ content at 773 K and 1 × 10−4 s−1. Surface strain mapping indicated that severe localized deformation was accompanied by high-amplitude serrations. Microscopic observations from transmission electron microscopy indicated that the deformation mode switched from dislocation motion to stacking fault (SF) formation with increasing γ′ content. With the aid of Suzuki segregation and SF intersections of different slip planes, a large number of SFs might induce high-amplitude serrations, especially those associated with the inverse PLC effect.

Investigation of Portevin–Le Chatelier Band Strain and Elastic Shrinkage in Al-Based Alloys Associated with Mg Contents

The Portevin–Le Chatelier (PLC) effect in Al–2.30wt%Mg, Al–4.57wt%Mg and Al–6.91wt%Mg alloys has been investigated at various applied strain rates at room temperature in this study. Three-dimensional digital image correlation (3D-DIC) technique was applied to obtaining the further insight into the spatiotemporal characteristics, in particular the influence of Mg content on deformation behaviors. Mg content has a pronounced effect on serration characteristics, including the serration type and amplitude; Mg content tends to weaken the spatial correlation of the propagative bands. Additionally, the serration amplitude linearly increases with the maximum PLC band strain; high Mg content generates a higher PLC band strain at a given serration amplitude compared with low Mg content. Mg content is found to be effective to enhance the serration amplitude, the maximum PLC band strain and also the amount of elastic shrinkage outside PLC bands.

Influence of Strain Rate on Tensile Properties and Dynamic Strain Aging of an Fe-24.5Mn-4Cr-0.45C Alloy

In the present study, the tensile properties and dynamic strain aging of an Fe-24.5Mn-4Cr-0.45C alloy were investigated in terms of strain rate. During tensile testing at room temperature, all the stress-strain curves exhibited serrated plastic flows related to dynamic strain aging, regardless of the strain rate. Serration appeared right after yield stress at lower strain rates, while it was hardly observed at high strain rates. On the other hand, strain-rate sensitivity, indicating a general relationship between flow stress and strain rate at constant strain and temperature, changed from positive to negative as the strain increased. The negative strain-rate sensitivity can be explained by the Portevin Le Chatelier effect, which is associated with dynamic strain aging and is dependent on the strain rate because it is very likely that the dynamic strain aging phenomenon in high-manganese steels is involved in the interaction between moving dislocations and point-defect complexes.

An analysis of the Portevin–Le Chatelier effect and cracking of CuSn6P alloy at elevated temperature of deformation applying the acoustic emission method

The paper concerns the application of the acoustic emission (AE) method for the determination of the mechanical properties of continuously cast industrial tin bronze CuSn6P, which reveals tendencies to unstable plastic flow connected particularly with the Portevin–Le Chatelier (PLC) effect. The relations between the jerky flow connected with the PLC effect, AE intensity and the evolution of a fracture of the investigated alloy subjected to the tensile and compression tests have been analyzed. It has been found that the intensity of the oscillation of stresses, observed on the curves σ–ε, coincides with the effect of the instability of plastic deformation PLC type and the recorded signals of the intensity of acoustic emissions. It has also been found that the increase of the tensile temperature in the investigated alloy involves changes in the mechanism of cracking from transcrystalline ductile to brittle intercrystalline, characteristic at the temperature of minimum plasticity (DMT).

Experimental Study on Three-Dimensional Deformation Field of Portevin–Le Chatelier Effect Using Digital Image Correlation

In view of its high precision and high efficiency, three-dimensional digital image correlation (3D-DIC) is widely used to accurately measure full-field deformation. A spatiotemporal experimental study using 3D-DIC to explore the Portevin–Le Chatelier (PLC) deformational behavior, provides a new insight into the whole 3D deformation field, including the out-of-plane displacement, and in particular the relationship between the serrations and the strain field in the deformation bands corresponding to individual serrations. Specimens 1, 2 and 3 mm thick of 5456 Al-based alloy were tested in uniaxial tension at room temperature at strain rates from 1.8 × 10−4 to 9.1 × 10−3 s −1. The spatial and temporal characteristics of the strain localization were quantitatively analyzed. The out-of-plane displacement increment field (w) of the localized bands was observed by 3D-DIC, and found to be related to the specimen thickness and the in-plane strain increment. The largest displacement increments were respectively 15, 10 and 5 μm for 3, 2 and 1 mm specimens at maximum strain increment of about 12000 μ e. The elastic shrinkage outside the deformation bands was found to be an essential characteristic of the PLC effect. The width of the PLC band (w b a n d ) increased with increasing thickness; the angle of the PLC band (𝜃 b a n d ) was not affected by either specimen thickness or serration amplitude. Temporally, the serrations in the plots both of in-plane strain and out-of-plane displacement vs. time coincided throughout the entire loading procedure. When PLC banding occurred, the serration amplitude within the bands was found to be proportional to the maximum strain increment in the direction of the applied tensile force (𝜖 m a x ).

Characterization of the deformation behaviors associated with the serrated flow of a 5456 Al-based alloy using two orthogonal digital image correlation systems

The spatio-temporal characteristics associated with the Portevin–Le Chatelier (PLC) effect in a 5456 Al-based alloy were investigated by employing two orthogonal digital image correlation systems. This investigation yielded the first-ever experimental observations of the strain localization in the thickness direction, which was measured to be homogenous along the thickness of the specimen. In fact, this localization was more severe in the thickness plane than in the widthwise plane. This confirmed that the assumption of a plane-stress state is invalid for specimens, which are susceptible to the PLC effect. With respect to the deformation mode of the PLC bands, the localized region in the thickness and widthwise planes were actually in an unequal biaxial stress state and a state of simple shear, respectively. The deformation continuity of the boundary corresponding to the widthwise and thickness planes was discussed on the basis of the experimental data. Moreover, two different fracture modes, i.e., the shearing-to-opening mode and the pure shearing mode, were proposed. The aforementioned observations can provide new insights into the deformation behaviors of the plastic instabilities, especially as it relates to the numerical modeling of the morphology and propagation of the PLC band.

Microstructural Evolution of an Al-Alloyed Duplex Stainless Steel During Tensile Deformation Between 77 K and 473 K (−196 °C and 200 °C)

Tensile deformation behavior of an Al-alloyed Fe-17Cr-6Mn-4Al-3Ni-0.45C (mass pct) duplex stainless steel containing approximately 20 vol pct ferrite was studied in the temperature range from 77 K to 473 K (−196 °C to 200 °C). While the elongation exhibited a maximum near room temperature, the yield strength continuously increased at lower tensile test temperatures. According to the microstructural examinations, the twinning-induced plasticity and the dislocation cell formation were the dominant deformation mechanisms in the austenite and ferrite, respectively. Reduction of the tensile ductility at T < 273 K (0 °C) was attributed to the ready material decohesion at the ferrite/austenite boundaries. Tensile testing at 473 K (200 °C) was associated with the serrated flow which was ascribed to the Portevin–Le Chatelier effect. Due to a rise in the stacking fault energy of austenite, the occurrence of mechanical twinning was impeded at higher tensile test temperatures. Furthermore, the evolution of microstructural constituents at room temperature was studied by interrupted tensile tests. The deformation in the austenite phase started with the formation of Taylor lattices followed by mechanical twinning at higher strains/stresses. In the ferrite phase, on the other hand, the formation of dislocation cells, cell refinement, and microbands formation occurred in sequence during deformation. Microhardness evolution of ferrite and austenite in the interrupted tensile test specimens implied a higher strain-hardening rate for the austenite as it clearly became the harder phase at higher tensile strain levels.

Effect of Grain Refinement on Jerky Flow in an Al-Mg-Sc Alloy

The influence of microstructure on the manifestations of the Portevin–Le Chatelier (PLC) effect was studied in an Al-Mg-Sc alloy with unrecrystallized, partially recrystallized, and fully recrystallized grain structures. It was found that the extensive grain refinement promotes plastic instability: the temperature–strain rate domain of the PLC effect becomes wider and the critical strain for the onset of serrations decreases. Besides, the amplitude of regular stress serrations observed at room temperature and an intermediate strain rate increases several times, indicating a strong increase of the contribution of solute solution hardening to the overall strength. Moreover, the grain refinement affects the usual sequence of the characteristic types of stress serrations, which characterize the dynamical mechanisms governing a highly heterogeneous unstable plastic flow. Finally, it reduces the strain localization and surface roughness and diminishes the difference between the surface markings detected in the necked area and in the region of uniform elongation.

Tensile Properties and Deformation Behaviors of a New Aluminum Alloy for High Pressure Die Casting

Effects of natural aging and test temperature on the tensile behaviors have been studied for a high-performance cast aluminum alloy Al–10Si–1.2Cu–0.7Mn. Based on self-strengthening mechanism and spheroidization microstructures, the alloy tested at room temperature (RT) exhibits higher 0.2% proof stress (YS) of 206 MPa, ultimate tensile strength (UTS) of 331 MPa and elongation of 10%. Increasing aging time improves the YS and UTS and reduces the ductility of the alloy. Further increasing aging time beyond 72 h does not significantly increase the tensile strengths. Increasing test temperature significantly decreases the tensile strengths and increases the ductility of the alloy. The UTS of the alloy can be estimated by using the hardness. The Portevin–Le Chatelier effect occurs at RT due to the interactions between solid solution atoms and dislocations. Similar behaviors occurring at 250 °C are attributed to dynamic strain aging mechanism. Increasing aging time leads to decrease in the strain-hardening exponent ( n ) value and increase in the strain-hardening coefficient ( k ) value. Increasing test temperature apparently decreases the n and k values. Eutectic phase particles cracking and debonding determine the fracture mechanism of the alloy. Final failure of the alloy mainly depends on the global instability (high temperature, necking) and local instability (RT, shearing). Different tensile behaviors of the alloy are mainly attributed to different matrix strengths, phase particle strengths and damage rate.

Interrelation between local strain jumps and temperature bursts due to the jerky flow in metals under the complicate loading conditions: experimental study by using the DIC-technique and the IR-analysis

The present work focused on the complex study of the influence of the stress concentrators, the loading system stiffness, and the additional cyclic impact on the behavior patterns of the Al-Mg alloy and the carbon steel during uniaxial tension tests with the use of the original samples with complicated geometry. The aim of the investigation was the complex study of the temporal instabilities and spatial localization due to the Lüder’s behavior, the Portevin–Le Chatelier effect and the shoulder or necking effect with the use of the non-contact 3-D Digital Image Correlation measurement system Vic-3D (Correlated Solutions) and the infrared system FLIR SC7700. This study contains a brief description of the test procedure and the experimental results of providing mechanical tests on the flat dog-bone specimens and the specimens with stress concentrators, with the additional deformable parts and the complicated samples with two L-shaped holes.

Improved tensile properties of a new aluminum alloy for high pressure die casting

This paper investigates the effects of strain rate and test temperature on the tensile properties and deformation behavior of a recently developed high-ductility cast aluminum alloy Al–5Mg–0.6Mn. The as-cast alloy tested at room temperature and the lowest strain rate of ~1.67×10−4s−1 shows the highest yield strength of ~212MPa, ultimate tensile strength of ~357MPa and elongation (~17.6%). Increasing strain rate reduces the ultimate tensile strength and ductility of the as-cast alloy. With the increasing of test temperature, the as-cast alloy shows significantly decreases in tensile strengths and improvements in elongation. The tensile failure of the alloy is mainly originated from the cracking and debonding eutectic particles. The Portevin–Le Chatelier effect occurs in the alloy tested at RT. Strain rate in current study ranges does not significantly affect the work-hardening behavior of the alloy. Increasing test temperature apparently reduces the strain-hardening exponent and coefficient. For the alloy tested at RT, all tensile failures occur prior to global instability, indicating the existence of localized damage. In contrast, for the alloy tested at HT, the global instability occurs at strains below the logarithmic fracture strains, suggesting that there is still a postnecking damage.

Study of the mechanisms of current-induced suppression of serrated deformation

The main results of studying the influence of electric current on the Portevin‒Le Chatelier serrated deformation in some commercial aluminum alloys of the Al‒Mg, Al‒Li-Mg, Al‒Zn‒Mg‒Cu, and Al‒Cu systems are reported. It is found that the passage of a low-density (~10–60 A/mm2) dc current leads to the suppression of serrated deformation and band formation in all alloys under study, except for the Al‒Cu alloy. Possible mechanisms of this phenomenon are discussed, basically in terms of possible influence of current on the processes of precipitation and dynamic strain aging, which are used to explain the Portevin‒Le Chatelier effect.