A Macroscopic Model for the Portevin-Le Chatelier Effect

The Portevin-Le Chatelier (PLC) effect means serrated yielding or jerky flow phenomenon in some alloys. In this paper a macroscopic model is developed to investigate the PLC effect with the emphasis on mechanical response of the structure evolution on microscopic scale. In addition to the normal work hardening effect, the model takes account of the thermal activation of dislocation moving with the aid of stress and the collective interactions between mobile dislocations and solute atoms due to dynamic strain ageing (DSA). It can satisfy the negative strain rate sensitivity of flow stress, which is believed to be one of the most special features associated with the PLC effect. The heterogeneous nature of the deformed material is also considered by introducing a nonuniform spatial distribution of some model parameters. The serrated yielding and localized deformation behavior can be successfully reproduced via numerical simulation based on this model.

Influence of solute cloud and precipitates on spatiotemporal characteristics of Portevin-Le Chatelier effect in A2024 aluminum alloys

In this paper, solute concentration and precipitate content in A2024 aluminum alloy are adjusted by solution treatment （ST） at different temperatures and tensile experiments on these treated specimens are carried out. It is found that the temperature of solution treatment （ST temperature） has a remarkable influence on the amplitude of the serrated flow and the propagation characteristics of shear bands. These results are due to the effects of solute atoms and precipitates on dynamic strain aging （DSA）. When ST temperature is higher than 300 ℃, solute concentration is relatively high and solute cloud is a key factor of DSA. When ST temperature is lower than 300 ℃, precipitate content is relatively high and the mechanism of DSA is determined by precipitates.

A critical review on the Portevin-Le Chatelier effect in aluminum alloys

The plastic deformation showing instability has been a subject receiving considerable attention for centuries due to its importance in many industrial processes.For Al alloys,the major instability is the Portevin-Le Chatelier(PLC)effect that appears within a certain region of strain,strain rate and temperature.It manifests itself on the stress−strain curve as serrations associating with the rapid accumulation of plastic deformation within inclined slip bands.The PLC effect has severe practical consequences,which damages the surface quality after the sheet metal forming process and threatens the tensile ductility.Therefore,it is crucial to investigate the fundamental mechanisms underlying the PLC effect and in particular to investigate how it can be tempered by tailoring the material microstructure.In this paper,we review the common interpretations of the PLC effect and summarize the experimental results of the effects of the precipitation and the grain refinement,two conventional strengthening methodologies in Al alloys,on the serrated plastic flow.The effectiveness of solute atom clusters in suppressing the PLC effect is emphasized.

Influence of precipitation on the Portevin-Le Chatelier effect in Al-Mg alloys

In the alloy with solute content higher than the limiting solubility,the solute atoms that have failed to dissolve will precipitate from the solid solution and form precipitations.In this study, the Portevin-Le Chatelier(PLC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation contents have been investigated under different applied strain rates.The results suggest that precipitations have significant effect on the PLC effect and the more the precipitations are, the greater the influence is.Furthermore,the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content.However,for 5456 alloy with higher precipitation content,the diffusion is no longer the case but more complex.

白光散斑相关法多尺度分析Portevin-Le Chatelier剪切带变形

在适当的温度、应变率或预变形下,合金材料的拉伸试验中,将会出现伴随雪崩式剪切变形带的锯齿形塑性失稳、即PLC(Portevin-Le Chatelier)效应。本文利用图像相关求位移场的方法,对恒定加载应变率下(10-4/s),拉伸铝合金(A2017)试件时出现的PLC效应从宏观剪切带变形到微观晶粒变形等多尺度进行了观察和定量化的分析。通过对PLC效应发生时采集的试件表面的白光散斑图进行相关运算,得到试件表面剪切变形区域各点的精细位移,并在此基础上计算出剪切带区域的应变分布及剪切带的宽度。实验结果显示,当PLC效应发生时,剪切带区域的应变曲线呈台阶型,带的前后边缘应变梯度较大,中间近似呈平台状,带外区域应变值接近零,塑性拉伸变形主要集中在带内。

Al-Mg合金中析出强化相对Portevin-Le Chatelier效应的影响

Portevin-Le Chatelier(PLC)效应作为广泛存在于各种合金材料中的重要塑性失稳现象一直受到研究者的广泛关注。本文对镁原子质量百分含量分别为2.5%和5%的两种铝合金-国标LF2和LF5铝合金进行宏观拉伸实验和透射电子显微镜观察实验。通过宏观和微观实验结果的对比分析表明,由于LF5合金中作为溶质的Mg原子的含量相对于LF2合金成倍增加导致析出相颗粒数量也成倍增加,进而引起PLC效应在强度和加工硬化时间的倍增,这些对应关系表明Al-Mg合金中析出相颗粒是产生并影响PLC效应的重要因素。实验结果显示虽然析出相颗粒数量在多方面影响PLC效应,但是对其飞行时间的影响却微乎其微。

Deformation measurements of three types of Portevin-Le Chatelier bands

In this paper a technique based on high-speed digital photography and the digital speckle correlation （DSC） method is used for the quantitative measurement of the displacement and strain fields of various Portevin Le Chatelier （PLC） bands （types A, B, and C）. The experimental results clearly show the nucleation process of a type-B band and the propagation of a type-A band. The results also reveal that there exists an elastic shrinkage deformation outside a PLC band during a large avalanche-like deformation inside the PLC band.

Complexity analysis of the Portevin-Le Chatelier in an Al alloy atdifferent tem peratures

The complexity of the Portevin-Le Chatelier(PLC)effect in an Al alloy at different temperatures was ana-lyzed by modified multiscale entropy.The results show that three evolutions of entropy with scale factor,i.e.,near zero,monotonically increasing and peak-shape,were observed corresponding to the smoothcurves,type-A serrations and type-B/-C serrations,respectively.The scale factor at the peak was one-third of the average serration period.The sample entropy increased initially and then decreased withtemperature,which was opposite to the critical strain.It is also suggested that the type-A serrations cor-responded to self-organized criticality and the type-B/-C serrations corresponded to chaos through theevolutions of entropy with scale factor.

Effect of alloying elements and processing parameters on the Portevin–Le Chatelier effect of Al–Mg alloys

The effects of alloying elements and processing parameters on the mechanical properties and Portevin-Le Chatelier effect of A1-Mg alloys developed for inner auto body sheets were investigated in detail. Tensile testing was performed in various Zn and Mg contents under different annealing and cold-rolling conditions. In the results, the stress drop and reloading time of serrations increase with increasing plastic strain and exhibit a common linear relationship. The increase rates of stress drop and reloading time increase with increasing Mg or Zn content. The alloys with a greater intensity of serrated yielding generally exhibit a greater elongation. The stress drop and reloading time of serrations decrease with increasing grain size in the case of the annealed samples. The cold-rolled sample exhibits the most severe serra- tion because it initially contains a large number of grain boundaries and dislocations.

High-Temperature Stability of Mg-1Al-12Y Alloy Containing LPSO Phase and Mechanism of Its Portevin-Le Chatelier(PLC)Effect

In this study,the high-temperature stability and the generation mechanism of the Portevin-Le Chatelier(PLC)effect in solid-solution Mg-1Al-12Y alloy with different heat treatment processes were investigated by adjusting the content of long-period stacking ordered(LPSO)phases.It was found that the content of LPSO phases in the alloys differed the most after heat treatment at 530℃for 16 h and 24 h,with values of 13.56%and 3.93%respectively.Subsequently,high-temperature tensile experiments were conducted on these two alloys at temperatures of 150℃,200℃,250℃,and 300℃.The results showed that both alloys exhibited the PLC effect at temperatures ranging from 150 to 250℃.However,at a temperature 300℃,only the alloy with a greater concentration of LPSO phases exhibited the PLC effect,whereas the alloy with a lower proportion of LPSO phases did not exhibit this phenomenon.Additionally,both alloys exhibited remarkable high-temperature stability,with the alloy containing a greater percentage of LPSO phases also demonstrating superior strength.The underlying mechanism for this phenomenon lies in the exceptional high-temperature stability exhibited by the second phase within the alloy.Furthermore,the LPSO phase effectively obstructs the movement of dislocations,and it also undergoing kinking to facilitate plastic deformation of the alloy.The results indicate that the PLC effect can be suppressed by reducing dislocation pile-up at grain boundaries,which leads to a decrease in alloy plasticity but an increase in strength.The presence of the PLC effect in the WA121 alloy is attributed to the abundant dispersed second phase within the alloy,which initially hinders the movement of dislocations,leading to an increase in stress,and subsequently releases the dislocations,allowing them to continue their movement and thereby reducing in stress.

Portevin-Le Chatelier Effect in Additively Manufactured and As-Cast Inconel 939 Nickel-Based Superalloy

Nickel-based superalloys, well-established in aeronautics, have recently gained significant traction in additive manufacturing. Inconel 939 is one of the alloys increasingly playing a vital role in this field. This paper examines the development of the Portevin-Le Chatelier (PLC) effect in additively manufactured Inconel 939 in comparison with cast Inconel 939. A detailed analysis of tensile test characteristics was conducted, complemented by a high-resolution scanning electron microscopy (HR-SEM) investigation. The PLC region exhibited several properties during tensile testing, such as stress-strain behavior, cycle scale, and overall stress increase. The HR-SEM analysis of Gamma prime (γ') precipitates revealed distinct morphologies, which are suggested to be linked to the features of the PLC region. Samples with a high amount of γ' precipitates showed a less pronounced PLC region, while those with fewer γ' precipitates displayed a more distinct PLC effect. A mechanism for the cyclic drop-and-rise stress behavior, based on the work of Varvenne and La-Rose, was proposed, possibly induced by the varying morphologies of γ' precipitates in the IN939 alloy. Further study is needed to deepen the understanding of the relationship between the γ' micro-(nano) structure and the PLC phenomenon.

The Portevin-Le Chatelier effect of gradient nanostructured 5182 aluminum alloy by surface mechanical attrition treatment

Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Chatelier(PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.

析出相在铝镁合金Portevin-Le Chatelier效应中的作用研究

本文研究了在相同退火热处理条件下,不同析出相含量的两种铝镁合金5456和5052在不同加载应变率下的P-LC效应.结果表明析出相对两种材料的P-LC效应有明显影响,并且析出相含量的不同所产生的影响不同.此外,析出相含量较少的5052合金的溶质原子的扩散方式是管扩散,而析出相含量较多的5456合金不再局限于管扩散的方式,而是更为复杂.

高速DSPI研究PLC剪切带成核演化过程

在适当的温度、应变率和预变形下,合金材料的拉伸试验中,将会出现伴随应力锯齿形跌落的雪崩式剪切变形带,即PLC(Portevin-Le Chatelier)效应。本文利用高速(1000 fps)数字散斑干涉法(DSPI),对不同加载应变率下,铝合金(LY10)试件中出现的连续(A-type)、间断(B-type)传播和随机(C-type)出现的三种PLC剪切带的瞬态成核与演化过程进行捕捉,通过对捕捉的试件表面系列散斑干涉图进行顺序相减处理,得到试件表面变形过程中位移的分布和演化系列条纹图。实验结果显示,对应于三种类型的剪切带,有两种主要的成核演化形式:1.A型剪切带的主要成核形式为先形成一个在观察平面上与拉伸轴方向成60°角并且横跨试件宽度的窄带,伴随着应力下落,剪切带开始沿自身宽度方向高速扩展达到最终宽度;2.对应于B、C型剪切带,先在观察平面上试件的一侧形成沿与拉伸轴方向呈60°角、长度约为最终长度一半的窄带。随着应力下落,带的前沿开始向试件的另一侧面沿自身长度方向传播,在传播过程中带逐渐沿自身宽度方向膨胀。带的前沿贯穿试件后,带开始高速膨胀,导致雪崩式剪切变形发生。

Serrated flow accompanied with dynamic type transition of the Portevin-Le Chatelier effect in austenitic stainless steel

In order to further understand high-temperature deformation behavior and its-related mechanical prop-erties,the Portevin-Le Chatelier(PLC)effect in an Fe-19Cr-13Ni-0.2C austenitic stainless steel was in-vestigated using high-temperature digital image correlation(DIC)analysis.Under the tensile testing at temperatures from 473 to 623 K,different types of serrated flow appeared even at a constant applied strain rate,and the type transition took place dynamically in a certain order with deformation time.DIC analysis revealed that the dynamic type transition of the serrated flow obeys the PLC band propagation behavior,and that the transition of the PLC band propagation behavior could be attributed to the PLC band nucleation manner.Numerical modeling also proved that the nucleation manner of PLC band nu-cleation is determined by the spatial-temporal coupling effect.

2024铝合金中沉淀相对PLC效应的影响

对2024铝合金进行不同温度下的热处理,得到了一系列具有不同溶质原子浓度的基体及析出相含量的铝合金试样.将这些试样分别在低温(-100℃)和室温(25℃)下进行拉伸实验,分析其锯齿幅值和临界应变随应变率的变化趋势.结果表明,溶质原子是Portevin-Le Chatelier(PLC)效应的必要因素,单纯的位错切割沉淀相不足以产生PLC效应,只能影响PLC效应,并且沉淀相对PLC效应的影响在中等拉伸应变率时表现明显.

析出相对AZ91D挤压镁合金PLC效应的影响

研究了具有不同析出相含量的AZ91D挤压镁合金在拉伸变形中出现的Portevin-Le Chatelier(PLC)现象,讨论了析出相对其产生PLC效应的影响规律,指出AZ91D挤压镁合金产生PLC效应的微观机理。结果表明:Al溶质原子的浓度是影响AZ91D挤压镁合金产生PLC效应的主要因素,β-Mg_(17)Al_(12)相的析出抑制了AZ91D挤压镁合金不稳定塑性行为。当β-Mg_(17)Al_(12)相析出量较高时,AZ91D挤压镁合金表现为稳定的塑性流变过程,其对应的应力-应变曲线为光滑的曲线;随着β-Mg_(17)Al_(12)相析出量的降低,固溶于Mg基体中的Al原子量增加,固溶体内溶质原子浓度提高,拉伸曲线表现出明显的锯齿屈服并逐渐剧烈,即AZ91D挤压镁合金的PLC效应随β-Mg_(17)Al_(12)相析出量的减少和Al溶质原子浓度的提高而增强;当β-Mg_(17)Al_(12)相含量为1.0mass%,对应的曲线上只出现一个锯齿抖动,这是AZ91D挤压镁合金产生PLC效应的临界状态。

Effect of solute concentration on the serrated flow in solution-treated Al-4%Cu alloys

In order to compare with annealed Al-4%Cu alloys, the influence of solute concentration on the serrated flow is investigated by solution treatment. In this paper, some dynamic parameters, such as critical time and the ultimate tensile strength are obtained. Moreover, the change of the strain rate range for serrated flow in Al-4%Cu alloys prepared by annealing and quenching heat treatments is reported too. The difference between them is attributed to the increasing solute concentration in the bulk of the solution treated materials.

Effects of Ca and Nd addition on plastic instability in extruded Mg-Mn alloy deformed under various conditions

Plastic instability, called Portevin-Le-Chatelier(PLC) effect, manifests itself as an unstable plastic flow during tensile tests of structural materials. This phenomenon has a strong influence on diverse properties, leading to unexpected vulnerabilities in the service environment.Among various magnesium-based alloys, PLC phenomenon is most prominently observed in the Mg-Mn-Nd alloy under elevated temperature and low strain rate conditions. An important aim of the study is to clarify and compare the significance of the RE and Ca addition, which are known to cause a formation of a largely weakened non-basal type texture, in the occurrence of plastic instability. Due to the PLC phenomenon, there is a risk of weakening texture and formability improvement by the addition of RE and Ca elements in Mg alloys. Based on the understanding of the role of Nd to the PLC phenomenon in Mg-Mn alloy identified in previous studies, the PLC characteristics according to alloying elements and deformation conditions were compared and analyzed. To identify the micromechanical mechanisms of the PLC phenomenon, varies in the microstructure and mechanical properties during deformation of Mg-Mn binary and Ca or Nd-containing Mg-Mn-based ternary alloys in various conditions were systemically analyzed. The addition of Ca did not show a marked PLC effect due to the formation of low number density Mn-Ca and Ca-Ca solute clusters and an unbalanced Mn:Ca ratio. In contrast, the addition of Nd leads to the formation of a higher number density of Nd-Nd and Mn-Nd solute clusters than that of Ca-Ca and Mn-Ca solute clusters of the Mg-Mn-Ca alloy, resulting in a stable solute-dislocation interaction atmosphere under specific ranges of deformation temperature and strain rate. The deformation in the regime of PLC phenomenon, results in a decrease in ductility and an increase in strength, despite deformation at elevated temperatures with maintaining the weakened texture.

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 Mg Zn 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 Mg Zn precipitates on mobile dislocations,the tensile strength of T6 samples was relatively higher than those of the other two conditions.