Portevin-Le Chatelier Phenomenon Research Articles

Effect of Pulsed Current-Assisted Tension on the Mechanical Behavior and Local Strain of Nickel-Based Superalloy Sheet

Electrically assisted (EA) forming is a plastic forming technique under the coupling action of multiple energy fields, such as force field, temperature field, and electric field. It is suitable for the forming of difficult-to-deform materials such as nickel-based superalloys. In this paper, uniaxial tensile tests on nickel-based superalloy sheets were carried out using the pulsed current assisted with different parameters. The experimental results show that the flow stress of the material decreased with the increase in the current density under a high-frequency pulsed current, and the Joule heating effect explains the flow stress drop. In the pulsed current application process, the different types of Portevin-Le Chatelier phenomena appeared with the increase in the current density. The decrease in elongation assisted by the pulsed current was explained by analyzing the inhomogeneity of the maximum Joule heating temperature distribution. In addition, the digital image correlation (DIC) analysis was used to analyze the local strain behavior of the pulsed current-assisted tensile process. Under the application of a high-frequency pulse current, the specimen exerted an inhomogeneous temperature increase and local hot pressing stress, which resulted in the inhomogeneous distribution of the local strain.

Autowave Description of the Lüders and Portevin–Le Chatelier Phenomena

Autowave Description of the Lüders and Portevin–Le Chatelier Phenomena

Observation of Portevin-Le Chatelier serrations in 6061-O aluminum alloy under ultra-low-frequency vibration-assisted tension

In this study, the evolution of Portevin-Le Chatelier (PLC) serrations was observed in 6061-O aluminum alloy under ultra-low-frequency vibration (ULFV)-assisted tension. To characterize the PLC phenomenon, the distribution of stress fluctuations in the time domain and the propagation of strain-localized bands in the space domain were recorded. The order of the transition between type A + B and type B serrations was reversed after applying 5 Hz ULFV with different amplitudes. When ULFV with amplitudes of 0.02 mm and 0.05 mm was applied, the critical nucleation time of PLC bands was delayed. However, the critical nucleation time was earlier when the amplitude reached 0.07 mm. The ULFV with an appropriate vibration mode can effectively restrain the PLC effect and modify its behavior.

Plastic instability and texture modification in extruded Mg-Mn-Nd alloy

Even though Mg alloys containing Mn and rare earth elements lead to higher ductility and lower yield asymmetry due to the weak texture after extrusion, plastic instability, commonly known as the Portevin–Le Chatelier (PLC) effect, causes unexpected fragility in the service environment. In the present study, the PLC phenomenon and texture development during the deformation of Mg-Mn and Mg-Mn-Nd extruded alloys were investigated under various temperatures and strain rates. The addition of Nd causes not only texture weakening but also severe PLC phenomenon. The PLC phenomenon was significantly affected by the temperatures and the strain rates, which causes a difference in mechanical properties and development of texture. In the conditions of high temperature and low strain rate, the strength increased while the elongation decreased significantly, and obvious PLC phenomenon with severe serration and negative strain rate sensitivity. The initial texture was maintained even after deformation only under severe PLC conditions, and this is due to the restriction of basal slip and suppression of lattice rotation in PLC conditions. The series of results indicate that the PLC phenomenon causes a reduction of formability even at high temperature.

Effects of nanosized precipitates on the Portevin-Le Chatelier behavior: Model prediction and experimental verification

The influence of homogenously-distributed nanosized Al3(Sc, Zr) precipitates on the Portevin-Le Chatelier (PLC) behavior in a model AlMgScZr alloy was investigated by experimental characterization and theoretical modeling. A remarkable advantage of the selected alloy is that the solute Mg concentration remains unchanged during the precipitation of Al3(Sc, Zr) nanoparticles, which effectively rules out the influence of solute concentration change on PLC. Transmission electron microscopy (TEM) and small angle neutron scattering (SANS) were used for quantitative characterization of precipitates and evaluation of key modeling parameters. By comparing the results for AlMg (without precipitates) and AlMgScZr (with precipitates) the effects of precipitates on the PLC phenomenon were clarified. It was found that the precipitates decrease the critical strain for the occurrence of PLC. Further, by evaluating dislocation densities of both alloys via synchrotron X-ray diffraction (XRD), the influence of precipitates on PLC was clarified by revealing the different roles of solute atoms and precipitates played in the dislocation-dislocation interaction. A model was proposed to account for these phenomena.

The serration behavior and mechanical properties of Al0.1CoCrFeNi high-entropy alloy under coupled electron-heat field

Ever-harsher service environments in the future will call for systematic studies on service behaviors of high-entropy alloys (HEAs) under multi-field coupling. Instead of focusing solely on service behaviors under conventional conditions, the promoted serration behavior of Al0.1CoCrFeNi HEA, commonly known as Portevin-Le Chatelier (PLC) effect, under coupled electron-heat field was quantitatively analyzed in terms of several characteristic parameters in this work. The obvious PLC phenomenon with severe serration can boost the decrease of tensile strength and elongation in the service environment of coupled electron-heat. Our results indicate that this macroscopic serrated behavior can be rationalized by defect-level microstructural interaction, namely the enhanced repetitive pinning and de-pinning effect of solute atoms on mobile dislocations, according to the mechanism of dynamic strain aging. This was proved by the increased kinking and bowing morphologies of dislocations, as well as abundant stacking faults under coupled electron-heat field.

Study of the Effect of the A206/1.0 wt. % γAl2O3 Nanocomposites Content on the Portevin-Le Chatelier Phenomenon in Al/0.5 wt. % Mg Alloys

The Portevin-Le Chatelier (PLC) phenomenon or dynamic strain aging in Al–0.5 wt. % Mg alloys was investigated at different strain rates. This research also examined the effect of γAl2O3 nanoparticles on the PLC phenomenon. A nanocomposite made of A206/1.0 wt. % γAl2O3 was manufactured to this purpose and then, added to an Al–0.5 wt. % Mg melt to obtain ingots of Al–0.5 wt. % Mg–20 wt. % A206/1.0 wt. % γAl2O3 and Al–0.5 wt. % Mg–10 wt. % A206/1.0 wt. % γAl2O3 with 6 mm diameter. Cold deformation allowed manufacturing 1 mm diameter wires using the 6 mm diameter ingots. A 300 °C solution treatment, followed by rapid cooling in ice water permitted to retain Mg atoms in solid solution. The tensile tests performed on the wires revealed the PLC phenomenon upon the tensile stress vs. strain plastic zone. The phenomenon was quantified using MatLab™ and statistical analysis. The results demonstrated how the alumina nanoparticles can diminish the serration amplitude of the PLC phenomenon.

Al/Niobium Diboride Nanocomposite’s Effect on the Portevin-Le Chatelier Phenomenon in Al-Mg Alloys

In Al-Mg alloys, the Portevin-Le Chatelier phenomenon, or dynamic strain aging, reveals itself as serrations upon plastic tensile deformation. This research evaluates this phenomenon when Al/NbB2 nanocomposite pellets are added to a magnesium-supersaturated Al matrix. A ball-milled 90 wt % Al and 10 wt % NbB2 nanocomposite helped inoculate an Al-Mg melt to incorporate the nanoparticles effectively. The melt was cast into rods that were cold-rolled into 1 mm diameter wires. Two sets were prepared: The first group was an as-cast set of samples, for comparison purposes, whereas the second was a solution-treated set. The solution treatment consisted of annealing followed by ice-water quenching. The results corroborating that the phenomenon was observable only in the specimens bearing the solution treatment, were used as the research baseline. Said treated alloy was compared to one containing the nanoparticles, which proved that the NbB2 particles caused a reduction of the serrated signal amplitude.

Interaction of the Portevin–Le Chatelier phenomenon with ductile fracture of a thin aluminum CT specimen: experiments and simulations

An attempt is made here to capture numerically slant ductile fracture and its early slant strain precursors via combining a dynamic strain aging (DSA) model with ductile damage models. In recent experimental studies it has been shown that in an AA2XXX alloy strain localization in slant bands preceded the onset of damage, originating slant fracture ahead of a notch. Here tensile tests are performed at different strain rates revealing some negative strain rate sensitivity which is an indication of DSA effect for AA2198-T8. A McCormick-type DSA model in conjunction with a Rousselier damage model, a reduced polycrystalline plasticity model and a Coulomb fracture criterion for slip systems have been used. Full 3D finite element simulations using this model and typical parameters for aluminum alloys capture the early strain localization in slanted bands, their intermittent activity and the final slant fracture. Prior simulation results without the DSA model and others using the von Mises plasticity or the GTN model did not capture the early slant strain localization thereby suggesting that DSA may well be the physical origin of the early slant strain localization and final slant fracture phenomena in this alloy.

Electric current–assisted deformation behavior of Al-Mg-Si alloy under uniaxial tension

The tensile deformation behavior of Al-Mg-Si alloy under a pulsed electric current has been investigated. Specimens subjected to three types of heat treatment, solution treatment, natural aging, and artificial aging, are prepared. In solution treated specimens, elongation and flow stress increase when pulsed electric current is applied during plastic deformation; also, the Portevin–Le Chatelier (PLC) phenomenon, which is observed in the tensile test without electric current, nearly disappears when the pulsed electric current is applied. In naturally aged specimens, the flow stress decreases and the elongation significantly increases when pulsed electric current is applied during the tensile test. In artificially aged specimens, both elongation and flow stress decrease under pulsed electric current. The result of XRD analysis shows that thermal and electric current–induced annealing occurs in all specimens subjected to the electric current. Especially in the solution treated specimen, the formation of early stage precipitates from a supersaturated state might be accelerated by the electric current, in an effect distinct from Joule heating; this effect would explain the observed increase in flow stress and the disappearance of the PLC phenomenon. Microstructural observation shows that electric current accelerates the formation of microvoids around the precipitates at the grain boundary, resulting in earlier fracture in the artificially aged specimen. A constitutive model based on dislocation density model and precipitation hardening model is proposed to describe the uniaxial tensile behavior for the age hardening alloys. Based on the experimental findings, the proposed constitutive model is modified to describe the upper boundary of the ratchet shape stress-strain curve under a pulsed electric current. Thermal and electric current-induced annealing with precipitation hardening is considered simultaneously in the modified constitutive model. Phenomenological descriptions of each parameter are demonstrated considering the microstructural features observed in experiments. The modified model is capable of predicting the experimental results very well.

Suppressing Effect of Heat Treatment on the Portevin–Le Chatelier Phenomenon of Mg–4%Li–6%Zn–1.2%Y Alloy

Microstructural evolution and Portevin–Le Chatelier (PLC) phenomenon of the as-extruded Mg–4%Li–6%Zn–1.2%Y alloy before and after heat treatment have been investigated. It has been demonstrated that for the as-extruded and solid solution treated (T4) samples, the PLC phenomenon could be obviously observed on tensile stress–strain curves. Moreover, the PLC phenomenon in T4 samples was more salient than that in the as-extruded condition, suggesting that the occurrence of PLC phenomenon was closely related to the super-saturation degree of solute atoms in the matrix. Since most of solute atoms were consumed for the formation of MgZn precipitates (β1′ and a little of β2′) during the subsequent ageing treatment (T6), the PLC phenomenon of T6 samples was eliminated. Meanwhile, due to the pinning effect of the formed MgZn precipitates on mobile dislocations, the tensile strength of T6 samples was relatively higher than those of the other two conditions.

Experimental studies of Portevin-Le Chatelier plastic instabilities in carbon-manganese steels by infrared pyrometry

The dynamic strain aging (DSA) phenomenon that occurs in some materials under certain temperature and strain rate conditions can cause plastic strain localization in the form of Portevin-Le Chatelier (PLC) bands. Carbon-manganese steels are used commonly and frequently in construction because of their ductility, low cost and ability to form mechanically. In these steels, the DSA phenomenon occurs for common quasi-static strain rates from 150 to 300°C, which makes band observation complicated. PLC bands on a carbon-manganese steel that was sensitive to DSA were studied using an infrared camera. Specimen heating was achieved using an induction furnace (with an adapted coil inductor), which allows for temperature recording during tensile tests. Thermography with an infrared camera was used to estimate the band characteristics and increments in band plastic strain, which is an important parameter for material behavior identification necessary for DSA phenomenon modeling. This technique had been developed only for PLC phenomenon observation at ambient temperature on aluminum alloys. Band characteristics on the carbon-manganese steels have been compared with results obtained previously on aluminum alloys.

Effect of cryorolling on the mechanical properties of AA5083 alloy and the Portevin–Le Chatelier phenomenon

The effect of cryorolling on the mechanical properties of an AA5083 alloy was investigated. The effect of rolling strain and temperature on the Portevin–Le Chatelier effect experienced by aluminium alloys with magnesium as a major constituent was also studied. Microstructure evolution during room temperature (RT) and cryorolling (LNR) and their effect on the mechanical properties of the AA5083 alloy were studied in detail. The results showed that the hardness values increased as the rolling reduction was increased. Compared to RT rolling, LNR did not have a significant influence. Deformation-induced precipitation particles of Al6Mn during RTR resulted in enhanced mechanical properties compared to LNR conditions. A negligible amount of precipitation was observed due to suppressed adiabatic heating for samples under LNR conditions. The presence of manganese (more than its solid solubility limit) in the test material could have influenced the behaviour of the AA5083 alloy material upon rolling. The Portevin–Le Chatelier effect was clearly observed, and it was noted that with smaller grain size and an increasing dislocation density, the critical strain for the onset of the PLC effect was lowered.

A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cracking

In Alloy 718, a sharp transition exists in the fracture path changing from an intergranular brittle mode to a transgranular ductile mode which is associated with a transition of flow behavior from smooth in the dynamic strain aging regime to a serrated one in the Portevin-Le Chatelier (PLC) regime. In order to better understand both deformation and rupture behavior, PLC phenomenon in a precipitation-hardened nickel-base superalloy was carefully investigated in a wide range of temperatures [573 K to 973 K (300°C to 700°C)] and strain rates (109^-5 to 3.2910^-2 s^-1 ). Distinction was made between two PLC domains characterized by different evolutions of the critical strain to the onset of the first serration namely normal and inverse behavior. The apparent activation energies associated with both domains were determined using different methods. Results showed that normal and inverse behavior domains are related to dynamic interaction of dislocations with, respectively, interstitial and substitutional solutes atoms. This analysis confirms that normal PLC regime may be associated to the diffusion of carbon atoms, whereas the substitutional species involves in the inverse regime is discussed with an emphasis on the role of Nb and Mo.

The effect of a notch on the Portevin–Le Chatelier phenomena in an Al–3Mg model alloy

The aim of this research was to investigate the influence of a geometrical notch on the Portevin–Le Chatelier effect. This phenomenon manifests itself as the serrated stress–strain curve obtained for certain materials when they undergo plastic deformation during tensile tests. The Portevin–Le Chatelier effect can often be observed in aluminum alloys especially those with an addition of magnesium. The material examined in the present experiments was a model Al–3Mg alloy. Samples were prepared with notches cut in the specimens with different depths and shapes. The results clearly indicate that the notch has a significant influence on the Portevin–Le Chatelier effect. With increasing notch depth, the stress amplitude of the serrations increases together with their frequency and the course of the serrations changes.

The Portevin –Le Chatelier Effect and Acoustic Emission of Plastic Deformation CuZn30 Monocrystals

Abstract The paper presents the investigation of the relation between the acoustic emission (AE) and instability of plastic deformation type Portevin-Le Chatelier (PLC) of single-phase brass CuZn30 monocrystals with crystallographical orientation [1¯3 9]. The monocrystals have been investigated applying the method of free compression at a constant strain rate and the temperature within the range from 200°C to 400°C, simultaneously recording PLC phenomenon by means of acoustic emission. During hot axial compression tests the correlation between work-hardening curves σ - ε, which display PLC effect and characteristic of acoustic emission signals has been found. Moreover, it was proved that in the range of the PLC effect, the acoustic signal is an impulse a character of cyclic repeatability, distinctly correlated qualitatively with the stress oscillations on the curves σ - ε. The analysis of the obtained results leads to the conclusion that in the tested monocrystals the effect PLC is probably controlled by complex processes similar to the phenomenon of dynamic strain ageing (DSA), which are described by diffusion models.

Investigation of Portevin–Le Chatelier Effect in Al-2.5 pct Mg Alloy with Different Microstructure

An experimental study is presented on the effect of microstructure change on the Portevin–Le Chatelier (PLC) effect of Al-2.5 pct Mg alloy. Tensile tests are performed on as-received and heat-treated [at 673 K (400 °C) for 16 hours] samples for a wide range of strain rates. The serrations observed in the stress–time curves are investigated on statistical analysis point of view. The microstructures of the samples are characterized by optical metallography, X-ray diffraction, and electrical resistivity measurements. It is found that the excess vacancies generated due to heat treatment have an effect on the PLC phenomenon, which leads to a decrease in the strain rate sensitivity and an increase in the number of stress drop occurrences per unit time. The microstructural parameters like domain size and dislocation density have no appreciable effect on the PLC effect as far as the statistical behaviors of the serrations are considered.

Relationship between Local Strain Jumps and Temperature Bursts Due to the Portevin-Le Chatelier Effect in an Al-Mg Alloy

Local strain and temperature of an AA5754-O aluminum alloy sheet have been full-field measured during monotonous tensile tests carried out at room temperature. Sharp strain increases and temperature bursts which are locally generated by the Portevin-Le Chatelier phenomenon have been measured at the same point for two strain rates: V2 = 1.9 × 10−3 s−1 and V10 = 9.7 × 10−3 s−1. A relationship, which is based on the underlying physical mechanisms, has been established between the strain and the temperature and experimentally verified for the highest strain rate V10. The discrepancy between the theoretical and experimental results for the lowest strain rate V2 suggests that the localized plastic deformations do not follow an adiabatic transformation. Such a set-up seems to offer a direct and experimental method to check the adiabatic character of localized plastic deformations.

Piobert–Lüders plateau and Portevin–Le Chatelier effect in an Al–Mg alloy in simple shear

The jerky flow in an Al–Mg alloy is studied during simple shear tests at room temperature and various strain rates. Direct observations of the sample surface using digital image correlation allow the study of the type and the dynamics of bands associated to plastic instabilities as a function of shear strain and shear strain rate. The paper features that both Piobert–Lüders and Portevin–Le Chatelier phenomena can be observed for a simple shear stress state at room temperature. The nucleation, growth and movement of the bands are described: it is shown that the kinematics of the bands is similar to those observed in tension but that the orientation of the bands varies with the shear strain.

The twin mechanism of Portevin Le Chatelier in Mg–5Li–3Al–1.5Zn–2RE alloy

The small serrated flow, severe Portevin Le Chatelier (PLC) phenomenon and abnormal strain rate sensitivity (SRS) were observed from the tensile curves of extruded Mg–5Li–3Al–1.5Zn–2RE alloy. The serrated curves were investigated by optical microscopy, step-tensile test and transmission electron microscopy. The small serrated flow is due to the traditional dynamic strain ageing. The positive SRS is caused by dynamic strain ageing. The severe Portevin Le Chatelier phenomenon and the negative SRS are attributed to a larger number of twins. Transmission electron microscope diffraction pattern reveals the { 10 1 ¯ 2 } twin and { 10 1 ¯ 1 } twins.