Effect of solute concentration on Portevin-Le Chatelier effect in Al-Cu alloys

The solute concentration and precipitated phase content in alloys were altered by solution treatment (ST) at different temperatures. A series of serrated load curves was obtained in tensile tests at a constant applied strain rate. The influences of solute concentration and precipitated phase content on dislocation movement were analyzed with tensile test results, and the micro-mechanism of the Portevin-Le Chatelier (PLC) effect is discussed in this paper. In the tests, when ST temperature was reduced from 500°C, the amplitude of serrated flow decreased. It reached the minimum when ST temperature was reduced to 300°C. On the other hand, the amplitude of serrated flow increased with ST temperature decreasing from 300 to 100°C. Experimental results show that when ST temperature is higher than 300°C, solute concentration is a governing factor to PLC effect; whereas when ST temperature is lower than 300°C, the precipitated phase content significantly affects the PLC effect. Translated from Acta Metallurgica Sinica, 2006, 42(12): 1248–1252 (in Chinese)     

The influence of nanosized precipitates on Portevin-Le Chatelier bands and surface roughness in AlMgScZr alloy

The influence of different precipitate-dislocation interactions,namely dislocation shearing and bypass-ing mechanisms,on PLC bands and the resultant surface roughness in AlMgScZr alloy was investigated.Three-dimensional surface roughness was quantitatively measured by confocal microscopy.We find that the introduction of shearable precipitates increases the stress amplitude,decreases the PLC bands number and surface roughness.However,the stress amplitude decreases,the PLC bands number and sur-face roughness increase with shearable precipitates turning to nonshearable precipitates.By analyzing the precipitation strengthening mechanisms quantitatively,the influence of precipitates on PLC bands and the resultant surface roughness was explored.Furthermore,our study demonstrates that the shear-able precipitates can decrease the surface roughness by decreasing the number of PLC bands,which is instructive for designing structural materials with desirable mechanical property and surface quality.     

Effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet

The effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet was investigated. Prestrain magnitudes between 0% and 12% were applied in plane strain in either the transverse or longitudinal (rolling) material direction. Samples were then loaded in uniaxial tension in the longitudinal direction at strain rates of 0.001/s and 0.1/s. Results show that when a path change is involved between prestrain and subsequent uniaxial loading, the strain rate sensitivity of the hardening rate at 0.1/s compared to 0.001/s is reduced. The rate sensitivity of the yield stress remains constant with increasing magnitudes of prestrain, while the rate sensitivity of the elongation to failure decreases with increasing prestrain. A permanent softening of the flow stress is also observed, which is greater when the path change is combined with a change in orientation.     

Deformation behavior and the Portevin-Le Chatelier effect in a modified 18Cr-8Ni stainless steel

Mechanical behavior and microstructural evolution of a 2.5%W + 0.4%Nb + 0.3%V + 0.17%N modified 18Cr-8Ni austenitic steel was studied in the temperature interval 410-740 °C at initial strain rates ranging from 6.7 × 10⁻⁶ to 1.3 × 10⁻² s⁻¹. The Portevin-Le Chatelier (PLC) effect attributed to dynamic strain aging (DSA) was found to occur in the temperature range 530-680 °C. The PLC effect is manifested in the form of serrated (jerky) flow and planar slip. DSA increases yield strength, σ_0.2, and ultimate tensile strength, σ_UTS, in comparison with conventional SUS304 steel due to the additional alloying that provides the formations of the clouds of solutes around the dislocations that exert the solute drag force.     

Mg-3Al-6Zn-2Y合金在不同应变率加载下的力学性能研究

在不同应变率压缩与拉伸下,研究了Mg-3Al-6Zn-2Y合金的力学性能,发现两种条件下合金力学性能变化规律不同.压缩情况下,随应变率增大,合金的流变应力增大,极限强度、屈服强度、破坏应变先增大后减小,塑性先增大后减小;拉伸情况下,随应变率增大,合金的流变应力、极限强度、屈服强度先增大后减小,破坏应变减小,塑性减小.压缩情况下合金流变应力的应变率敏感性高于拉伸情况.     

Acoustic emission inspection of Portevin-Le Chatelier effect and deformation mechanisms of two Mg–Li–Al alloys

In this paper, Portevin-Le Chatelier (PLC) effect of LA41 magnesium alloy is reported. With increasing strain rate, both ultimate tensile stresses (σ_b) and 0.2% proof (σ_0.2) stresses increase, while serrations become less severe. Major acoustic emission (AE) peaks appear at the end of elastic region, and transform to burst type signals in the plastic range. In another alloy containing less Li (LA11), serrations are absent and major AE peak values are considerably lowered. Additionally, PLC effect disappears in LA41 after annealing. The role of twins is discussed in details. Actual causes for AE activity during deformation are also proposed.     

AZ镁合金用于干电池负极材料的电化学性能研究

采用失重法、线性电位扫描、扣式电池放电测试等方法,研究AZ21和AZ31镁合金作为镁锰干电池负极材料时的电化学性能以及电解质添加剂Li2CrO4对其电化学性能的影响。结果表明,作为干电池的负极材料,在Mg(ClO4)2作电解质时,AZ21与AZ31相比,其自腐蚀速率大,开路电压稍高,电池容量和正极材料利用率低;添加少量的Li2CrO4能大大降低AZ21和AZ31合金的自放电速率,其缓蚀作用随Li2CrO4浓度的增加而增加,但当超过0.3%(质量分数)后反而降低;Li2CrO4的添加可提高电池的工作电压、电池容量和正极材料利用率。     

Hydrogen embrittlement behavior of Inconel 718 alloy at room temperature

Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier(PLC) effect at room temperature when hydrogen charging current density is 220 mA cm~(-2) and 590 mA cm~(-2). Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.     

镁合金疲劳的研究进展

对近年来有关镁合金疲劳的研究进行了总结,考察了材料性质、腐蚀介质、高温环境等多种因素对镁合金疲劳性能的影响,并对裂纹萌生及扩展方式进行了分析,归纳了提高镁合金疲劳性能的方法--改善材料质量及表面强化处理或涂层处理,对镁合金疲劳性能的研究进行了展望.     

生物医用镁合金腐蚀行为的研究进展EI北大核心CSCD

近年来,镁合金作为生物可降解材料受到了越来越多研究者的关注,由于其具有良好的生物相容性、力学性能及可降解吸收等特点,被誉为一种“革命性的生物材料”。然而,由于腐蚀速率过快和存在局部腐蚀的缺点,目前的生物镁合金仍达不到临床应用的要求。本文从高纯化、合金化、热处理工艺、表面改性等方面综述了最近几年生物镁合金在提高腐蚀性能方面的研究进展,并从添加无毒性合金元素,适当的表面涂覆,先进的制备技术及热处理工艺方面,对如何研制出腐蚀性能更好的生物可降解材料进行了展望。     

Strain rate effects for aluminum and magnesium alloys in finite element simulations of steering wheel armature impact tests

This paper addresses the strain rate effects for aluminum and magnesium steering wheel armatures when they are subjected to dynamic impact tests. Two geometrically different steering wheel armatures, a three spoke proprietary aluminum alloy armature and a four spoke magnesium alloy (AM50A) armature, underwent experimental impact testing. The testing conditions for each armature were different; testing with the aluminum alloy armature involved impacts with a deformable chestform and the magnesium armature experienced impact tests with a rigid plate. Finite element models of all testing apparatuses were developed for both testing conditions and numerical simulations were conducted based on the experimental method employed. Strain rate effects for the aluminum alloy were considered using the Cowper–Symonds constitutive relation and a Johnson–Cook material law was utilized for the magnesium alloy. Simulations were conducted with and without strain rate effects considered. The comparison between the experimental and numerical methods illustrate that there is only a minor change in the numerical testing results with the inclusion of strain rate effects, however, a better correlation between experimental and numerical methods occurs.     

Geometrically non-linear modeling of the Portevin–Le Chatelier effect

In this work we investigate the plastic instabilities associated with the Portevin–Le Chatelier (PLC) effect in Al alloy 2024. A semi-phenomenological approach is taken. A simple geometrically non-linear elastic–viscoplastic constitutive model is proposed for simulation of material response under various applied strain rates. Using the model we determine numerically the relation between the critical strain for the onset of discontinuous yielding and the applied strain rate. The results obtained are in very good quantitative agreement with the available experimental data (collected from tests at room temperature) and cover both the normal and the inverse behavior of the critical strain. The simulations are performed using non-linear finite element method. Additional verification of the proposed constitutive framework was carried out using statistical analysis of the simulated stress–time series. A transition from a non-linear chaotic regime to self-organized critical behavior of the localized strain bands were predicted in terms of the temporal two-point correlation function of the stress–time series. Finally we investigated the influence of different factors, such as the geometry of the specimen, its orientation with respect to the rolling direction and loading conditions (strain rate), on the type of PLC instabilities and the critical conditions for their onset.     

基体对镁合金氧化膜性能影响的研究进展

阐述了镁合金应用前景,介绍了阳极氧化特点以及影响氧化膜性能的因素,主要包括基体材料、电参数、电解液的组成及其浓度等.全面综述了基体对镁合金氧化膜成膜过程、成分、表面形貌以及耐蚀性的影响,提出了由于工业上使用的材料绝大多数为镁合金而不是纯镁,因此,开展合金元素对氧化膜性能影响的研究非常必要.     

溶胶成分对镁合金阳极氧化膜层的影响研究

研究了AZ91D镁舍金材料在普通阳极氧化条件下，通过往碱性阳极氧化溶液中加入硅-铝溶胶成分(含量为0％～5％)，在60～70V的电压条件下进行交流阳极氧化处理，获得的膜层经过厚度测量和表面、断面微观形貌观察表明；溶胶成分在镁合金氧化成膜过程中，可以有效地提高镁合金表面的阳极氧化膜层厚度和膜层的致密程度。同时由于溶液中硅-铝溶胶成分的作用，使得阳极氧化成膜速度出现阶段性快速增长和缓慢增长。而溶胶成分的加入对阳极氧化膜层的X射线衍射相结构的影响不大。     

Phase transformation occurring during the heat-treatment of magnesium rare earth alloys

This work discusses the phase transformations occurring and mechanical properties of a cast and extruded Mg-5%Y-4%Gd alloy, 90 and 20 mm in diameter bar, after various heat treatments. The influence of quenching temperature, artificial and natural ageing on microstructure, microhardness and mechanical properties is presented. It is demonstrated that the alloy strengthens during natural ageing and initially recovers during artificial ageing. An understanding of the alloy behavior leads to optimization of the heat-treatment procedures; the best combination of mechanical properties is obtained after artificial aging at 200° for 3 days.     

The effect of cerium on high-cycle fatigue properties of die-cast magnesium alloy

The effect of cerium (Ce) on high‐cycle fatigue behaviour of die‐cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up‐and‐down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi‐cleavage and dimple.     

SiCW/ AZ91 镁基复合材料及AZ91镁合金的高温变形行为

利用Gleeble-1500 对SiC_W/ AZ91 复合材料和AZ91 镁合金在温度为423～723 K、应变速率为0.002～0.25s-1 、最大应变量为60 %的条件下进行高温压缩变形行为的研究。测试了其真应力-应变曲线, 观察了变形后的显微组织。结果表明: 晶须的转动和折断导致复合材料的应变软化现象较合金明显; 复合材料和合金的应变速率敏感指数(m) 和表观激活能(Q) 均随温度的升高而增大; 晶须的加入细化了晶粒, 使复合材料的m 值比合金高; 同时晶须的加入也限制了位错交滑移和晶界的迁移, 因此复合材料的Q 值比合金高; 压缩变形过程中, 合金和复合材料发生了动态回复和动态再结晶。      

Microstructure of superplastic QE22 and EZ33 magnesium alloys

Superplastic deformation has been observed in the QE22 and EZ33 magnesium alloys prepared by the special thermomechanical procedure. The microstructure was analysed by the scanning and transmission electron microscopes before and after deformation. Samples with the grain size of ∼ 1 μm were deformed at elevated temperature at the strain rates from 1 × 10^− 4 to 1 × 10^− 3 s^− 1. Microstructure analysis showed an existence of particles in the grain boundaries. This precipitates are very stable even at high temperature deformation. These heat resistant phases are very probably responsible for the grain stability and also prominent superplastic behaviour of alloys studied.