Portevin-Le Chatelier Effect Research Articles

Serrated Flow Behavior in Commercial 5019 Aluminum Alloy

Serrated flow effects are visible on a metal surface even after coating. Thus, they are undesirable to manufacturers and product users. To meet the expectations of the industry, research on the conditions for serrated flow occurrence in 5019 aluminum alloy was carried out and the results were collected in the current paper. Thus, the influence of the alloy initial microstructure due to different tempers as well as plastic deformation conditions, i.e., strain rate and temperature, on the alloy stress–strain behavior was determined. Two tempers were considered: the as-fabricated F-temper and the W-temper (i.e., quenched in water after annealing at 500 °C). The synergic influence of these tempers and their tensile test conditions on the serration behavior of the stress–strain curves, i.e., the stress drop and reloading time, were also determined and categorized. Structural and X-ray diffraction studies rationalized the stress–strain characteristics according to dynamic strain aging models with positron annihilation lifetime spectroscopy providing insight into the role of lattice defects (i.e., dislocations and vacancies). The map of the serrated flow domain allowed us to obtain the activation energy of the onset of the Portevin–Le Chatelier effect equal to 56 kJ/mol. It is close to the activation energy for the pipe diffusion mechanism, obtained by applying the model formulated originally for Type B stress serration.

Realization of Friction Stir Welding of Aluminum Alloy AA5754 Using a Ceramic Tool

When engaging in the friction stir welding of aluminum/aluminum joints, the conventional use of tools made of hard metal and steel involves a complex and costly production process. These tools experience wear over welding distances and require frequent replacement to ensure the consistency of the welded seams. The exploration of silicon nitrite as a tool material emerges as a promising alternative in this scenario. The heightened hardness of non-oxide ceramics anticipates a diminished wear rate compared to traditional welding materials, translating into an extended operational lifespan. Nevertheless, the adoption of ceramics introduces challenges initially perceived as detrimental to friction stir welding. The inherent brittleness of silicon nitrite makes it susceptible to breakage under specific loads, and thermal stresses within the component can lead to failure. To mitigate these vulnerabilities, a ceramic material with high thermal shock resistance and a low proportion of sintering additives was used. Employing these accurately designed tools friction stir welding (FSW) was performed on sheets of AA5754, followed by a comprehensive examination of their microstructural and mechanical properties. It was demonstrated that a joint efficiency of 88% can be achieved, and that an increase in hardness within the stir zone occurred as a consequence of grain refinement. Furthermore, the Portevin–Le Chatelier effect, which is characteristic of this alloy, was influenced by the FSW process.

Characterization of Portevin–Le Chatelier effect in aluminum alloy AA5052 under strain-controlled tensile test via digital image correlation and acoustic emission

Characterization of Portevin–Le Chatelier effect in aluminum alloy AA5052 under strain-controlled tensile test via digital image correlation and acoustic emission

Influence of surface pre-deformation on the Portevin-Le Chatelier effect and the related multiscale complexity of plastic flow in an Al-Mg alloy

The influence of the surface pre-deformation on jerky flow caused by the Portevin-Le Chatelier (PLC) effect was investigated using flat tensile specimens of an Al-Mg alloy. Although jerky flow represents a macroscopic plastic instability, the underlying mechanisms stem from self-organization of dislocations, which pertains to deformation processes at mesoscopic scales. To provide a comprehensive approach, the investigation was carried out by coupling tensile tests, digital image correlation and acoustic emission techniques, each targeting a particular range of scales. Thin superficial layers were pre-deformed using surface mechanical attrition technique (SMAT). It was found that the observed effects depend on which surfaces are processed. Overall, the treated samples exhibited an enhanced yield strength without deterioration of ductility in comparison with the initial material. These changes in the general mechanical behavior are likely to be correlated with the changes in jerky flow. Using digital image correlation, a tendency to delocalization of bursts of plastic flow was found for both the PLC bands and smaller-scale strain heterogeneities outside the bands, the latter having been little studied so far. Unexpectedly, SMAT occurred to modify plastic flow drastically at this scale. In addition to clarification of the role played by the surface in the PLC effect, these findings provide new insights into relationships between deformation processes at distinct scales.

A novel method simultaneously eliminating Portevin-Le Chatelier effect and enhancing strength in non-heat-treatable Al-Mg alloys

This study investigates the effects of cyclic plasticity on the tensile properties of AA5083 alloys. The low strength and Portevin-Le Chatelier (PLC) effect in Al-Mg alloys means they have found limited use in automotive outer panels. This work shows that by applying cyclic strengthening approach to AA5083, a high density of Mg-Al clusters can be formed in matrix resulting in increased yield and tensile strengths. Moreover, the introduction of cyclic plasticity also effectively suppresses the PLC effect, which is due to the formation of the clusters and the consequent reduction in the number of free Mg atoms in the solid solution. This method introduces the concept of particle strengthening in Al-Mg alloys without altering composition, while simultaneously eliminating the PLC effect. It suggests a potential path for using 5xxx series alloys in applications traditionally dominated by 6xxx series alloys, with added benefits for recycling and sustainability in the automotive industry.

The nonlinear analysis of Portevin-Le Chatelier (PLC) effect: An application to medium Mn steel

The instability of plastic flow in medium manganese steel was investigated through uniaxial tensile tests conducted at room temperature across various strain rates. The Portvin-Le Chatelier (PLC) effect was analyzed using statistical methods and a multifractal approach. The findings indicate that the zigzag stress–strain curves associated with instability plastic flow exhibit typical multifractal characteristics. Furthermore, the multifractal analysis quantifies the influence of strain rate and initial phase composition on the instability characteristics of the stress–strain curves. Specifically, it was observed that lower strain rates and austenite content correspond to a chaotic state, whereas higher strain rates and austenite content result in self-organizing critical dynamics. Finally, the underlying physical mechanisms driving these differing modes of behavior are discussed, with a focus on their relationship to strain rate and the initial austenitic phase fraction.

Portevin-Le Chatelier behavior in AlMgScZr alloys: Effects of Al3(Sc,Zr) dispersoid distribution and grain structure

Plastic instability caused by the Portevin-Le Chatelier (PLC) effect remains a challenge for Al–Mg alloy sheets during shape-forming processes, as it causes unsightly stretcher-strain markings on the surface of final products. In this study, the PLC behavior in annealed AlMg and AlMgScZr alloy sheets with varying Al3(Sc,Zr) dispersoid distributions and grain structures were investigated by combining tensile tests, microstructural characterization, and DIC analysis. The results show that the presence of Al3(Sc,Zr) dispersoids can inhibit the PLC effect, which is manifested by a decrease in serration amplitude, PLC band velocity, and an increase in critical strain. The inhibition effect is shown to be highly related to the distribution of Al3(Sc,Zr) dispersoids and resultant grain structures. The alloy with the densest dispersoid distribution and finest subgrain structure showed the slightest PLC effect. The reduction in serration amplitude and PLC band velocity is ascribed to the augmented resistance to local strain softening, caused by inhibition of dislocation motion by Al3(Sc,Zr) dispersoids and subgrain boundaries. In addition, a high density of Al3(Sc,Zr) dispersoids can strongly trap vacancies, which is responsible for the increase in critical strain.

Analysis of the temperature-dependent plastic deformation of single crystals of quinary, quaternary and ternary equiatomic high- and medium-entropy alloys of the Cr-Mn-Fe-Co-Ni system

ABSTRACT Temperature-dependent plastic deformation behaviors of single crystals of quaternary and ternary equiatomic medium-entropy alloys (MEAs) belonging to the Cr-Mn-Fe-Co-Ni system were investigated in compression at temperatures in the range 9 K to 1373 K. Their critical resolved shear stresses (CRSSs) increase with decreasing temperature below room temperature. There is also a dulling of the temperature dependence of CRSS below 77 K due to dislocation inertial effects that we attribute to a decrease in the phonon drag coefficient. These behaviors were compared with those of previously investigated single crystals of the equiatomic Cr-Co-Ni and Cr-Fe-Co-Ni MEAs, and the equiatomic Cr-Mn-Fe-Co-Ni high-entropy alloy (HEA). The temperature dependence of CRSS and the apparent activation volumes below room temperature can be well described by conventional thermal activation theories of face-centered cubic (FCC) alloys. Above 673 K, there is a small increase in CRSS, which we believe is due to elastic interactions between solutes and mobile dislocations, the so-called Portevin-Le Chatelier (PL) effect. The CRSS at 0 K was obtained by extrapolation of fitted CRSS vs. temperature curves and compared with predictions from solid solution strengthening models of HEA and MEAs.

Influence of SiC particles on the microstructure and mechanical behaviors of AlMgScZr alloy processed by laser powder bed fusion

Laser powder bed fusion (LPBF) of AlMgScZr alloy has garnered significant attention as a high-strength aluminum alloy. However, its low modulus, Portevin-Le Chatelier (PLC) behavior, and weak strain hardening capability restrict its application in the aerospace sector. In this study, SiC/AlMgScZr composites with different SiC content have been successfully fabricated using LPBF. The inherent bimodal grain structure of AlMgScZr alloy was not completely altered by the addition of SiC particles. However, with increasing SiC content, heat accumulation increased, and the solidification cooling rate decreased. This led to an increase in the size of α-Al grains in the 5 wt% SiC/AlMgScZr composites. Meanwhile, movable dislocations can be efficiently preserved inside the grains, where they effectively accumulate and become entangled. Consequently, the strain hardening capability of the 5 wt% SiC/AlMgScZr composites improved by approximately 58 % compared to AlMgScZr alloy, with a strain hardening index (n) reaching 0.29. Additionally, the addition of SiC particles led to an increase in the elastic modulus (81.0 GPa). Furthermore, the reaction between Mg atoms and SiC particles resulted in the formation of Mg2Si, which altered the distribution of Mg elements and initiated the reactive precipitation of a Mg-rich phase. There is no competition between the diffusive migration of solute atoms and the motion of movable dislocations within the composites. The additional SiC particles effectively eliminated the PLC effect in the LPBF of AlMgScZr alloy. This work provides essential guidance for developing high-strength and high-stiffness aluminum matrix composites with excellent processing properties in LPBF technology.

Mechanisms of electrically assisted deformation of an Al–Mg alloy (AA5083-H111): Portevin–Le Chatelier phenotype transformation, suppression, and prolonged necking

Aluminum-magnesium wrought alloys are well known for their advantageous properties and their application in various industries. However, the occurrence of the Portevin–Le Chatelier (PLC) effect leads to the formation of bands on the surface, thereby restricting the application of parts from Al–Mg alloys, primarily due to aesthetic concerns. Applying electrical pulses during deformation may not only improve the mechanical properties, moreover, it also affects the PLC effect. In this work, the properties of AA5083-H111 were investigated using standardized and electrically assisted tensile tests. Tensile behavior was assessed at room temperature, 250 °C, cryogenic conditions, and at two distinct electrically assisted conditions. We found that electrically assisted tensile testing leads to increased fracture strain compared to standard room temperature and cryogenic conditions. Furthermore, there was a transformation in PLC phenotypes, which included a partial suppression of the PLC effect. A spatio-temporal analysis of strain rate and strain highlights considerable differences in the occurrence of PLC bands and prolonged necking compared to standard room temperature testing. Furthermore, a modified dislocation pattern was observed using transmission electron microscopy.

Deformation behavior and failure mechanism of AA7075 alloy during the cryogenic temperature-assisted incremental sheet forming process

High-strength aluminum alloys are potential structural materials for thin-walled aerospace and automotive components. However, the limited formability at room temperature (RT) hinders their wide industrial application. In this study, a cryogenic temperature-assisted incremental sheet forming (CT-ISF) process for manufacturing 3D curved thin-walled structures was proposed. An experimental platform for CT-ISF was established to form AA7075 alloys. We found that the formability of the solution-treated AA7075 alloy sheet was greatly improved under the cryogenic environment without sacrificing its strength. In combination with double pass forming process, the formability of the sheet can be further improved. The stable deformation of the sheet is promoted due to the suppression of the Portevin-Le Chatelier (PLC) effect at CT, which reduces axial force fluctuations during the forming process. The line roughness value of the meridional direction was reduced by 36.8% at CT due to the enhanced work-hardening and the utilization of MoS2 lubricant. In terms of microstructural evolution, we found that the fracture morphology of the formed parts changed from tensile fracture at RT to shear fracture at CT. The improved formability was attributed to increased Goss and E texture contents, delay in dislocation evolution, and suppression of the PLC effect. This work introduces a novel CT-ISF process that significantly enhances the formability of high-strength Al alloy. In particular, it unravels the enhancement mechanism of formability in the CT-ISF process, providing theoretical support for further research.

Tensile Deformation Behaviors and Microstructure Evolution Under Various Temperatures for MAR‐M247 Nickel‐Based Superalloy

The study aims to analyze the tensile behavior and microstructural changes in MAR‐M247 nickel‐based superalloy across different temperatures. Tensile behavior is examined under a constant strain rate of 2.5 × 10−4 s−1 at varying temperatures. Results indicate a temperature‐dependent nature of the alloy's tensile properties. At 550 °C, a Portevin–Le Chatelier effect is observed, attributed to twinning nucleation and carbon atom diffusion. Dynamic recrystallization occurs at 950 °C, manifesting in a sinusoidal stress–strain curve. At room temperature, the primary fracture mechanism involves dislocation shearing in the γ matrix, with minor dislocation shearing in the γ′ phase. At 550 °C, carbides along grain boundaries and the γ′ phase impede dislocation motion. Concurrently, dislocations shear the γ′ phase, leading to the formation of superlattice intrinsic stacking faults and Kear–Wilsdorf locks, thereby enhancing tensile strength. However, at 950 °C, dislocation motion primarily involves climb, carbides decompose, the γ′ phase softens and enlarges, resulting in a notable decrease in tensile strength.

The influence of additive manufacturing on Portevin-Le Châtelier (PLC) effect in Ni–Co-based superalloy

The microstructures and tensile properties of the Ni–Co-based superalloys made by additive manufacturing (AM) were investigated. The results revealed that both normal and inverse Portevin-Le Châtelier (PLC) effects were observed in AM Ni–Co-based superalloys. In addition, AM significantly inhibited the PLC effect, the average stress decrements for the cast and wrought (C&W) alloys with the same nominal compositions were more than ten times as large as those for the AM alloys when tested at 400 °C with a strain rate of 3 × 10−4 s−1. In the AM Ni–Co-based superalloys, type C serrations had connections with the inverse PLC effect. Transmission electron microscope (TEM) was performed to investigate the interactions of solute atoms, dislocations, stacking faults (SFs) and the γ′ phase for a deep insight into mechanisms of the PLC effects and clarifying the changes in serration types and the conversion from normal to inverse PLC effect.

Dynamic strain aging and recrystallization mechanisms in laser powder bed melt printing of highly textured Hastelloy X

Hastelloy X (HX) is used in a wide range of aerospace applications due to its excellent high-temperature properties, Dynamic strain aging (DSA) and Dynamic recrystallization (DRX) are of great interest as they affect the mechanical behavior at high temperatures. In this study, the serrated flow behavior of highly textured Hastelloy X fabricated by laser powder bed fusion (LPBF) is investigated during uniaxial straining over a wide range of temperatures (20–800 °C) and strain rates (5×10−2 s−1 to 5×10−4 s−1). The alloy exhibits different types of tensile serrated flow in the temperature range of 200–600 °C. The alloy has been shown to have a very good tensile flow behavior. The presence of the Portevin-Le Chatelier effect (PLC) due to dynamic strain failure was observed at temperatures lower than 600 °C, respectively, with an average activation energy value of 45 kJ/mol calculated using the quasi-static aging (McCormick) model, and an average activation energy value calculated using the solute resistance (Cottrell) model of 41.8 kJ/mol, which is significantly lower than the 80–130 kJ/mol of conventionally fabricated HX. The effects of temperature and strain rate on fracture morphology and microstructure were analyzed by SEM, and the dynamic recrystallization behavior and nucleation mechanism were characterized by electron backscatter diffraction (EBSD). The changes in the content of recrystallized texture Cube and deformed texture Goss at different deformation temperatures are explained, and the dynamic recrystallization mode changes, from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX) as the deformation temperature increase. Verification of the different DRX mechanisms contributes to a comprehensive understanding of the microstructural evolution of the hot deformation of HX fabricated by LPBF.

Nonlinear response to contact impact on the surface of an aluminum alloy AlMg6 exhibiting the Portevin-Le Chatelier effect

Intermittent plastic flow known as the Portevin-Le Chatelier effect is a pronounced nonlinear phenomenon in a material science. One of the traditional approaches to the study of nonlinear system is to identify and analyze its responses to an external impulsive action. In the present work, the influence of impact indentation on the evolution of the spatio-temporal patterns of localized strain in an AlMg alloy deformed under the Portevin-Le Chatelier effect is studied with using an acoustic emission technique and high-speed video recording of propagating deformation bands. Dynamic and nonlinear responses to impact indentation of the surface of an alloy deformed by uniaxial tension above the conditional yield strength are revealed; the first include high values of dynamic hardness and local strain and loading rates, while the second involve the threshold and multiple nature of the force and acoustic responses. It has been established that there is a threshold impact energy of the indenter at which an induced strain jump of minimum amplitude occurs, accompanied by the formation of bands of macrolocalized plastic deformation. The conditions are established under which the work of plastic deformation during the development of the induced strain jump significantly, by more than two orders of magnitude, exceeds the energy of the initiating impact, which acts only as a trigger for the “premature” release of the elastic energy accumulated in the material before the moment of impact. It is shown that the impact-induced bands of macrolocalized deformation represent a hidden three-dimensional type of erosion damage that reduces the mechanical stability and durability of the alloy. A phenomenological model is proposed for the formation of bands, strain jumps, and bursts of acoustic emission induced by an indenter impact, which qualitatively explains the experimental results obtained in the work.

Influence of Machine Stiffness on the Portevin–Le Chatelier Effect of Ti-12Mo Alloy Based on a Modified McCormick Constitutive Model

Influence of Machine Stiffness on the Portevin–Le Chatelier Effect of Ti-12Mo Alloy Based on a Modified McCormick Constitutive Model

Extending the mechanical property regime of laser powder bed fusion Sc- and Zr-modified Al6061 alloy by manipulating process parameters and heat treatment

Highly dense compositionally modified Al6061 alloys with Sc and Zr (Al6061MOD) were fabricated using the laser powder bed fusion. The as-built AL6061MOD alloy exhibits a bimodal grain size distribution consisting of coarse columnar grains and ultrafine grains, which is tunable by either varying the printing parameters or post-heat treatment. A high laser energy density along with a lower cooling rate, leaving more time for nucleating primary Al3Sc particles, triggers a higher area fraction of ultrafine grains. The heat treatment-induced evolution of microstructure and mechanical properties is jointly influenced by the size of grains and (Al,Si)3(Sc,Zr) particles. The conventional T6 heat treatment triggers coarsening of grains (∼5.3 μm) and (Al,Si)3(Sc,Zr) particles (∼42 nm), lowering the yield strength (∼280 MPa) compared with that of as-built counterparts. On the contrary, direct aging enhances the yield strength (∼383 MPa) due to the precipitation of nano-sized secondary Al3(Sc,Zr) particles (∼5 nm) as well as retaining the grain size (∼690 nm) of ultrafine grains. Concurrent with the tunable strength, the plasticity behaviors (Lüders band and Portevin-Le Chatelier effect) are manipulated correspondingly with varying microstructure. The broad tailoring of microstructure and mechanical property regime can shed light on designing Al alloys targeting the desired applications by manipulating the printing parameters and heat treatment.

Intermediate temperature embrittlement and Portevin-Le Châtelier effect of Inconel 625 alloy caused by carbides

The phenomena of intermediate temperature embrittlement (ITE) and Portevin-Le Châtelier (PLC) effect are common in nickel-based superalloys, which seriously restrict the machinability and serviceability of the alloys. The ITE and PLC effect of Inconel 625 alloy caused by carbides and hot ductility were studied by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscopy (TEM), and Gleeble 3500 thermal simulation equipment. The results show that hot ductility has a strong dependence on temperatures of Inconel 625 alloy. During the heating process, ductility initially decreases at 600 ℃ and reaches a minimum at 800 ℃ as the temperature increases, which was due to the formation of microcracks. Dynamic recrystallization can be observed at 900 ℃, and then ductility begins to drop until 1212 ℃ due to material melting. Meanwhile, the variation trend of the cooling process is consistent with the heating process. ITE exists in both the heating process and the cooling process, occurring at 600–900 ℃ and 600–1000 ℃ respectively of Inconel 625 alloy. The microcracks caused by the precipitation of carbides (NbC, (Nb, Ti)C, and M23C6) at grain boundaries led to the formation of ITE. The size, shape, distribution position, and type of carbides have different degrees of influence on the formation of microcracks. In addition, the serrated flow was observed throughout the test temperature range of Inconel 625 alloy. With the increase of the temperature, the type and amplitude of the serration changed, which is caused by the interaction between carbides (solute atoms) and dislocations.

Using the wavelet transform to process data from experimental studies of the discontinuous plastic deformation effect

Vast number of theoretical and experimental works has been devoted to the study of discontinuous plastic deformation (the Portevin-Le Chatelier effect), which manifests itself for most widely used alloys in certain ranges of both temperatures and strain rates. Due to the statistical nature of this phenomenon, difficulties arise in processing, qualitative analysis and quantitative comparison of test results and calculations. Using of statistical methods for these purposes makes it possible to get only some averaged characteristics of the obtained data (usually - moments of the first and second orders in amplitudes and frequencies). A possible alternative for processing of experimental and theoretical results of this effect research is wavelet analysis using. The results of experimental studies of the Portevin–Le Chatelier effect, realized during the deformation of thin-walled tubular specimens made of aluminum alloy AMg6M at certain strain rates at room temperature are presented. Diagrams of deformation under uniaxial tension, shear, proportional and disproportionate loading of specimens were obtained. The inhomogeneity of strain fields and their rates is shown, illustrating the manifestation of the Portevin – Le Chatelier effect under conditions of complex loading of thin-walled tubular specimens made of AMg6M alloy. A brief overview of existing methods and means of non-destructive testing is presented that make it possible to non-contactly record the spatial heterogeneity of plastic yielding. Some possibilities of using the wavelet transform to process certain types of non-monotonic stress-strain diagrams obtained for the specimens made of the aluminum alloy in question are discussed. Using wavelet analysis, a compact presentation of data from field experiments in the form of amplitude-frequency characteristics was obtained. The scalograms analysis of specimen loading diagrams was carried out.

Portevin-Le Chatelier characterization of annealed Al-Mg alloy sheets with different Mg contents

The stress-strain flow curves of solid solution treated and annealed Al-(2.86-9.41)Mg aluminum alloy sheets were investigated during tensile process at room temperature at a strain rate of 1 × 10−3 s−1, in order to reveal the effect of solute Mg atoms and β-phase nanoparticles in the α-Al matrix on their Portevin-Le Chatelier (PLC) characteristics. The results show that with the increase of Mg contents of alloys, the concentration of solute Mg atoms and the volume fraction of β-phase nanoparticles in the α-Al matrix of the annealed alloy sheets increase. The β-phase nanoparticles promote the PLC effect during the tensile process, causing the increase of stress drop amplitude Δσ of the tensile curves of the alloy sheets. However, the β-phase nanoparticles have little influence on the strain period ΔεTmax of rectangular waves. The flow strain difference Δεc−σ between the yield point and the beginning of PLC effect decreases monotonically from 5.4 × 10−4 to 5 × 10−5 as the Mg contents of alloy sheets increase from 2.86 wt.% to 9.41 wt.%. The strain period ΔεTmax of the rectangular waves corresponding to the high flow stress and stress drop amplitude Δσ obtained for the serrated stress-strain curves of the alloy sheets gradually increase with the increasing Mg contents of alloy sheets.