Portevin-Le Chatelier Effect Research Articles

Dynamics of deformation bands initiated by impact of an indenter on the surface of aluminum-magnesium alloy

The dynamics of macrolocalized deformation bands generated by the impact of a Vickers indenter on the surface of an AlMg6 aluminum-magnesium alloy deformed under creep conditions was studied by high-speed video recording and acoustic emission methods. It has been established that these bands are the trigger for the development of a macroscopic jump in plastic deformation on the creep curve. It is shown that in alloys exhibiting the Portevin-Le Chatelier effect it is necessary to take into account the initiation and propagation of deformation macrobands during surface erosion damage and to estimate the volume and morphology of the plastic zone during impact microindentation of a deformable alloy. Keywords: intermittent deformation, deformation band, impact, indenter, aluminum-magnesium alloy.

Studying Deformation Behaviors in Austenitic Stainless Steels within a Temperature Range of 143 K < T < 420 K

This work deals with studying staging and macroscopic strain localization in austenitic stainless steel 12Kh18N9T within a temperature range of 143 K < T < 420 K. The visualization and evolution of macroscopic localized plastic deformation bands at different stages of work hardening were carried out by the method of the double-exposure speckle photography (DESP), which allows registering displacement fields with a high accuracy by tracing changes on the surface of the material under study and then comparing the specklograms recorded during uniaxial tension. The shape of the tensile curves σ(ε) undergoes a significant change with a decreasing temperature due to the γ-α'-phase transformation induced by plastic deformation. The processing of the deformation curves of the steel samples made it possible to distinguish the following stages of strain hardening, i.e. the stage of linear hardening and jerky flow stage. A comparative analysis of the design diagrams (with the introduction of additional parameters of the Ludwigson equation) and experimental diagrams of tension of steel 12Kh18N9T for different temperatures is carried out. The analysis of local strains distributions showed that at the stage of linear work hardening, a mobile system of plastic strain localization centers is observed. The temperature dependence of the parameters of plastic deformation localization at the stages of linear work hardening has been established. Unlike the linear hardening, the jerky flow possesses the propagation of single plastic strain fronts that occur one after another through the sample due to the γ-α' phase transition and the Portevin-Le Chatelier effect. It was found that at the jerky flow stage, which is the final stage before the destruction of the sample, the centers of deformation localization do not merge, leading to the neck formation.

Selective laser melting of high-strength TiB2/AlMgScZr composites: microstructure, tensile deformation behavior, and mechanical properties

A TiB2/AlMgScZr composite was fabricated by selective laser melting (SLM) with direct powder mixing. The microstructure and mechanical properties of SLM-fabricated TiB2/AlMgScZr composites were investigated. Nearly fully dense and crack-free SLM-fabricated TiB2/AlMgScZr parts were obtained. The results showed that TiB2 formed a coherent relationship with the matrix and was swallowed by the solid–liquid interface as a nucleation point to refine grains during the SLM process. In addition, some TiB2 particles did not form a coherent relationship with the matrix, moving the solid–liquid interface at the grain boundary, which reduced the recrystallization fraction and refined the grains significantly. TiB2 also promoted the precipitation of Al3Sc and Al3Zr phases and increased the volume fraction of the Al3Sc and Al3Zr phases during the SLM process. The SLM-fabricated TiB2/AlMgScZr composites showed a very high ultimate tensile strength of ∼483 MPa and excellent ductility of ∼15.8%, which are higher than those of wrought Al alloys. The solid solubility of the Mg element slightly increased in the SLM-fabricated TiB2/AlMgScZr composites because of the high heating and cooling rate during the SLM process. The high heating and cooling rate caused discontinuous strain and Portevin–Le Chatelier effect and weakened the strengthening effect of TiB2 in SLM-fabricated TiB2/AlMgSc composites.

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.

Effect of nanoparticle fraction on Portevin-Le Chatelier effect of ZrB2 nanoparticle-reinforced AA6xxx composites

In this paper, the influence of nanoparticle fraction on Portevin-Le Chatelier effect of ZrB2 nanoparticle reinforced AA6xxx composites was investigated and friction stir processing was used to uniformize the distribution of nanoparticles. The results indicated that the population of serrations decreases with increasing the fraction of ZrB2 particles, with stress-drop serrations dominate the early stages of deformation while the stress-up serrations increase with the strain. The stress distribution complies with bimodal configuration, and 2 peaks become closing with the increasing fraction of nanoparticles. The interval for unconstrained motion of dislocation is responsible for the suppression of Portevin-Le Chatelier effect. The mean interval decreases with increasing the nanoparticles fraction. The inter-twisted dislocations in small intervals continuously dedicate to instant balance of stress-drop serrations, strengthen composites and favor the plastic deformation.

Influence of microtwins on Portevin-Le Châtelier effect of a Ni-Co based disk superalloy

Influence of microtwins (MTs) on Portevin-Le Châtelier (PLC) effect of a Ni-Co-based superalloy was investigated by tensile tests and transmission electron microscopy. Two groups of samples were pre-strained at 725 °C to produce MTs. Existence of MTs improved the alloy's yield strength, but it changed the PLC effect from type-B to type-C, and increased the serration amplitudes by ∼6 times, during tensile deformation at 400 °C. Microstructural characterizations indicate that dislocation-MT interactions lead to break of MTs in both γ matrix and γ′ precipitates, creating soft channels for dislocation glide. The high serration amplitudes were rationalized on the basis of sudden plastic flow along those soft channels and subsequently strain hardening due to dislocation multiplication. These results demonstrate that MTs influence the PLC effect much more significantly, compared with traditionally accepted dislocation-solute interactions.

Stepwise modelling method for post necking characterisation of anisotropic sheet metal

Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin–Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking hardening of a variety of anisotropic sheet metals and thereby contributes to efficient and reliable material model calibration.

Experimental study of the Portevin-Le Chatelier effect under complex loading of Al-Mg alloy: procedure issues

The aim of this work is to solve the methodological issues of the experimental study of the nucleation and propagation of deformation bands due to the Portevin-Le Chatelier effect under conditions of complex loading. It is of interest to determine the boundaries of unstable plastic deformation of the AMg6 alloy under complex loading conditions. A technique for controlling the loading process with a given rate of deformation intensity of materials has been worked out. The results showed that short-term stops and unloading during loading influence on the value of critical deformation, at which the manifestation of the jerky flow begins. The evolution of inhomogeneous strain fields and local strain rates under conditions of manifestation of jerky flow during tension with torsion tests of Al-Mg alloy samples.

Kinetics of Plastic Deformation Localization Bands in Polycrystalline Nickel

Jerky flow has recently aroused interest as an example of complex spatiotemporal dynamics resulting from the collective behavior of defects in Al- and Mg-based alloys under loading. This paper presents the results of the study of the macroscopic strain localization kinetics in Nickel 200 (99.5 wt % purity). Uniaxial tension of flat samples is monitored at room temperature in the load–unload mode at a constant strain rate and total deformation increment up to 5%. The stress–strain curves reveal jerky flow from the yield point to the formation of the neck. The digital speckle correlation method evidences the movement of localized plastic deformation bands under the conditions of the Portevin–Le Chatelier effect (PLC). It is shown that stress drops during jerky flow in Ni are accompanied by the formation of morphologically simple single PLC bands. It is established that, with an increase in total deformation, the number of PLC bands and their velocity of motion along the sample decrease, while their time period increases. Moreover, an increase in total deformation leads to an increase in the parameters of the force response (i.e., time period and stress drop magnitude). It is found that the criterion of damage for PLC bands as a function of the total strain has a sigmoidal shape.

Melt flow and grain refining in laser oscillating welding of β-21S titanium alloy

Properties and serviceability of laser welded joints depend on the defect, weld geometry, and microstructures, which in turns are affected by molten pool behaviors. In this work, beam oscillation was used to improve microstructures and mechanical properties of welded joints by changing molten pool behaviors. The 2 mm-thick β-21S titanium alloy plates were welded using three beam oscillating modes (non-oscillation, transverse oscillation, and vertical oscillation). The molten pool behaviors for three oscillation modes were observed using high-speed cameras, and the weld morphology, microstructures and mechanical properties of the corresponding joints were examined. The results showed that defect-free joint with sound weld appearances could be achieved using beam oscillation that affected molten pool behaviors. The higher probability of equiaxed grain formation and significant grain refinement were found in the joints made using beam oscillation. The average grain sizes of welded joints for non-oscillating, transverse and vertical oscillating modes are 236, 184 and 168 μm, respectively. The welded joint made using beam oscillation also showed a higher tensile strength of 900 MPa at ambient temperature and 534 MPa at 450 °C. Furthermore, the Portevin-Le Chatelier (PLC) effect was observed in the stress-strain curves of welded joint at 450 °C, which was attributed to the precipitation of brittle ω phase.

Asymmetric cold rolling of AA7075 alloy: The evolution of microstructure, crystallographic texture, and mechanical properties

In the present work, for the first time, the effect of asymmetric rolling at room temperature on the microstructural evolution, texture, hardness, tensile properties, and fracture surface of AA7075 alloy was investigated. It was found that the width of grains significantly decreased to 2.6 μm by 60 % asymmetric rolling. Also, shear bands in the RD-ND (rolling direction – normal direction) plane of AA7075 after 40 % and 60 % asymmetric rolling were evident. In addition, the size of iron-rich intermetallic particles was reduced with increasing thickness reduction. Microstructure and texture revealed that new grains through continuous dynamic recrystallization (CDRX) are created after 40 % and 60 % strain. On the one hand, after applying only 10 % thickness reduction, average hardness demonstrated 75 % enhancement. On the other hand, maximum yield (611 MPa) and ultimate tensile (644 MPa) strengths achieved in the 60 % deformed sample, demonstrating 287 % and 73 % improvement compared to the initial sample. These results were due to strain hardening, dynamic/static precipitation, and Fe-rich particle fragmentation. The Portevin-Le Chatelier (PLC) effect is reduced with increasing the strain. Interestingly, the 40 % deformed sample revealed the serrated flow. For this reason, the 40 % rolled sample has a lower total elongation (4.4 %) than the 60 % deformed sample (11.1 %). The tensile strength of the 40 % deformed sample (575 MPa) was lower than the 10 % and 20 % rolled samples (581 and 594 MPa, respectively). This result was owing to the occurrence of softening mechanisms, strengthening the Goss {110}⟨001⟩ component, and weakening the Copper {112}⟨111⟩ orientation after 40 % asymmetric rolling. By increasing the thickness reduction up to 40 %, the number of dimples decreased. However, the fracture surface of the 60 % deformed sample showed a noticeable amount of small dimples.

The Portevin-Le Chatelier Effect in Corrosion Behavior of the Mg-Zn-Sr Alloy

The as-solutionized Mg-4Zn alloys, one candidate material for biological implantation, are featured with the Portevin-Le Chatelier effect during the tensile testing with strain rate of 6.67 × 10−7 s−1, which leads to the higher corrosion rate and the acceleration of stress corrosion cracking. With the increase of the strain, new slip bands are generated and the proliferation of dislocations is very rapid in slip bands. The microgalvanic corrosion occurs between the slip bands (anode) and the low dislocation density region (cathode). The 0.1 wt% Sr addition can reduce the impact of the Portevin-Le Chatelier effect on as-solutionized Mg-4Zn alloy, resulting from the inhibition of twin, the better grain coordination deformation ability, and the lower density of geometrically necessary dislocations.

Microstructure evolution and mechanical properties in an ultralight Mg-2.76Li–3Al-2.6Zn-0.39Y alloy

A novel Mg-2.76Li–3Al-2.6Zn-0.39Y alloy was prepared by multidirectional forging (MDF) and rolling (R) to improve strength and ductility, and its microstructural evolution and mechanical properties were investigated by an optical microscope, X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile tester. After MDFR, the ultimate tensile strength was 275 ± 8 MPa, the yield strength was 207 ± 7 MPa, and the elongation was 29% at a grain size of 6.8 ± 1.1 μm. The specific modulus of this alloy was 30.04 MN m kg−1, and the specific strength was 183.58 kN m kg−1. Microstructural examination of decreased-temperature MDF revealed the presence of a bimodal microstructure, which is favorable for the enhancement of both strength and ductility. The deformation mechanisms of MDF are intense mechanically shearing fragmentation and incomplete dynamic recrystallization. XRD analysis revealed that the present alloy consists of α(Mg) phase, and Al2Y, Mg17(Al,Zn)12, Mg2Y, and AlLi intermetallic compounds. TEM analysis confirmed the existence of Mg17(Al,Zn)12 compound and dislocation wall and pile-up. Portevin–Le Chatelier (PLC) effect or serrated flow was discovered in this alloy. The relationship between the break-away stress and the experimental stress was proposed as a criterion for judging the occurrence of the PLC effect. The estimated stress is consistent with the experimental stress. The contributions of different strengthening mechanisms to tensile yield strength were estimated. The estimated tensile yield strength is in good agreement with the experimental tensile yield strength.

Hot compression behaviors and microstructural evolution of beryllium by vacuum hot pressing

The uniaxial compressive (UAC) performance of beryllium by vacuum hot pressing (VHP) was investigated in the temperature range from 250 to 550°C and strain rate from 0.01 to 10 s−1. Serrated flow and yield point showed in the stress–strain curve are caused by Portevin-Le Chatelier effect (PLCE). Arrhenius type constitutive equation based on true strain is established and the volume diffusion of Fe and C in solution is deduced by the calculated activation energy. The typical microstructure instability characters are interfacial cracking and intragranular cracking. The optimum deformation parameter is 350°C and 0.01 s−1, with uniform deformed microstructure and almost little defect increasing.

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

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

The Portevin-Le Chatelier (PLC) effect in an Al-Cu aluminum alloy fabricated by selective laser melting

An Al-Cu alloy fabricated by selective laser melting (SLM) was chosen in this study for the investigation of Portevin-Le Chatelier (PLC) effects. The effects of the process parameters, the building orientation, and the heat treatment were studied to reveal the correlation between the microstructure and the plastic instability. Serrated flow can be observed in all the as-built specimens due to the high concentration of the solute atoms and high dislocation density generated by the high cooling rate during the process. Strong anisotropy in PLC effects has been observed in the as-built samples, which is attributed to the grain structure and the texture. PLC effects changes a lot after the heat treatments. The solution treatment decreases the dislocation density. Hence, the PLC effects disappear. θ” nucleated at 446 K can delay the onset of the PLC effects. When aged at 466 K, with the absence of θ”, the onset strain of the PLC effect with the same duration is lower. As the aging duration increases from 1 h to 6 h, PLC effects become weaker and finally disappear. The concentration of the solute atom and the dislocation density decrease as the aging duration increases. Both aspects are responsible for the attenuation of PLC effects.

Microstructure and mechanical behavior of polymer-derived in-situ ceramic reinforced lightweight aluminum matrix composite

Metal/alloy matrices have been previously reinforced with ceramic nanoparticles, but such reinforcements tend to agglomerate, have weak interfaces, and are expensive. Polymer-derived ceramics (PDCs) based composites can potentially overcome these shortcomings. These composites are obtained from the preceramic polymers after pyrolysis. These polymers are inexpensive, can be fragmented to the nanoscale, and pyrolyzed in-situ. Friction stir processing (FSP) was used to fragment and disperse the polymer within an Al-Mg alloy. The PDC particles pinned the dislocations and grain boundaries. The composite exhibited a microstructure resulting from dynamic recrystallization with an average grain size of ~1 µm. It also exhibited both high strength (96% and 24% improvement in the yield and tensile strength, respectively) and good ductility. Interestingly, the serrations in the tensile curves commonly observed in Al-Mg alloys due to the Portevin-Le Chatelier (PLC) effect was reduced in the composite.

Effect of plastic anisotropy and Portevin-Le Chatelier bands on hole-expansion in AA7075 sheets in -T6 and -W tempers

Influence of tempers on the hole-expansion formability of an AA7075 aluminum sheet is investigated for -T6 (as-received) and -W tempers (super saturated by the solution heat treatment followed by water quenching). The hole is prepared by end-milling, to limit the effects of hole preparation on the results and instead highlight the effects of material behavior. A flat-headed punch is used to expand the hole, and digital image correlation captures the strain fields throughout the experiment. The results present that the hole can expand by 70 % more in -W compared to -T6 temper with the lower forming force. In contrast, thickness strain distribution around the hole shows a similar pattern in both tempers except the observation of Portevin-Le Chatelier (PLC) effect in -W temper, which causing the inhomogeneous deformation. In parallel, the numerical simulation of the hole-expansion is performed using a user material subroutine implemented for the elasto-plastic material behavior, including plastic anisotropy, of both tempers. The predictions on the thickness strain variation and average level show good agreement with the experiment for -T6 temper, but less so for -W temper. This is shown to be the effect of PLC bands on the deformation: the material in -T6 temper is mainly governed by plastic anisotropy, but -W temper shows combined effect of plastic anisotropy and PLC bands. Nevertheless, the reasonable predictions of both tempers verify that the numerical framework established in this study can be used for preliminary, computationally-efficient virtual process design with a practical purpose, despite omitting the explicit physics of the PLC effect.

Finite element simulation of the Portevin–Le Chatelier effect in highly reinforced metal matrix composites

ABSTRACT The Portevin–Le Chatelier effect in an AlCu/AlO composite with a large volume fraction (60%) of ceramic particles is investigated from a numerical point of view using a strain-ageing visco-plastic material model for the metallic matrix and several different finite element meshes based on real microstructures. Even for this very large volume fraction, some serrations are observed on the experimental tensile stress/strain curves, together with the propagation of bands of localised plasticity. The finite element model succeeds in reproducing the serrations and the propagation of bands experimentally observed, and allows a fine analysis of the spatio-temporal evolution of the bands of localised plastic strain rate. Different volume fractions are then virtually generated, tested, and compared to outline the influence of this second phase on the Portevin–Le Chatelier effect.

A method for determining r-value of aluminum sheets with the Portevin-Le Chatelier effect

Abstract Lankford coefficient (r-value) is an important parameter for metal sheet. For sheet metals with obvious Portevin-Le Chatelier (PLC) effect, the r-values are difficult to measure owing to the occurrence of inhomogeneous deformation. In this paper, the PLC bands of AA3104-H19 sheet were investigated by uniaxial tensile test with digital image correlation (DIC) technology. The result showed that inclination of the PLC band keeps invariable with respect to loading direction with straining. The constancies of PLC band angle and r-value are result from the limit of texture evolution during tensile test. Additionally, an analytic relationship between the PLC band inclined angle and r-value was derived based on the observation of zero-expansion along the PLC band direction. Accordingly, a novel r-value determining method based on the PLC band was proposed. The accuracy of the new method was evaluated by comparing with the extensometer method, DIC method and crystal plasticity simulation.