黄铜在氨水中的溶解过程对其局部塑性变形的促进作用

用激光云纹干涉法研究了黄铜在氨水溶液中应力腐蚀过程中缺口前端位移场和应变场的变化。结果表明，在应力腐蚀裂纹萌生前，溶解（腐蚀）过程本身能使缺口前端性区增大，与此同时也使塑性区中各点的塑性变形量增大，即溶解过程本身能促进塑性变形。     

ANALYSIS OF DISTRIBUTIONS OF RESIDUAL STRESS AND STRAIN BENEATH A NOTCH AFTER ROLLING

The fatigue property of the notched part can be improved significantly by introducingcompressive residual stress resulting from surface plastic deformation at the notch.Theresidual stress and strain distributions beneath the notch is predicted by using finite elementmethed (FEM) of large strain elasto-plastic analysis in this paper.It is investigatedwheather the rolling process could be replaced by a plane-strain indenting with a model inthe analysis.The effects of material strength.notch radius r.and indentation and depth Δhon the distribution of residual stress and strain are discussed.The agreement of predictedand experimental results is good enough.     

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.     

Study on ductile crack initiation limit from notch root of pipeline girth welded joints with strength mismatch

This study investigated ductile crack initiation limit of pipeline girth welded joints with different strength mismatches. The ductile crack initiation limit for the girth welded joints was evaluated by conducting three-point bending fracture toughness tests and wide plate tensile tests with a surface notch. In addition, effect of heat input on the ductile crack initiation limit of weld metal (WM) was evaluated on the assumption that a welding condition would be varied in the field in the actual pipeline construction. As the results, the equivalent plastic strain at the notch tip for the ductile crack initiation of the three-point bending tests was consistent with those of the wide plate tests, and the heat input hardly affected the ductile crack initiation limit within the range of this study. This meant that the ductile crack initiation limit of the pipeline girth welded joints with strength mis-match was able to be estimated using the equivalent plastic strain obtained from the three-point bending tests. Based on these results, we proposed a procedure to determine the rational fracture toughness requirements which took into account the difference in the plastic constraint between standard fracture toughness test and pipeline girth welded joints. This procedure was also possible to determine the required strength matching level for a strain-based design for girth welded joint containing surface notch in the centre of the WM in terms of preventing the ductile crack initiation.     

A Local Damage Approach to Predict Crack Initiation in Type AISI 316L(N) Stainless Steel

A local damage approach based on plastic strain equivalent to uniform strain and grain diameter of the material is proposed for prediction of crack initiation. Plane strain, plane stress, and 3D FEM simulations are carried out for compact tension (CT) geometry with blunt notch of different a/W ratios under mode-I loading. Elastic-plastic fracture parameters have been estimated based on certain assumptions on blunting at notch tip and micromechanisms of events leading to onset of crack. The various crack initiation parameters evaluated based on proposed local damage approach and initial assumptions have been verified by conducting experiments on CT specimens and subsequent scanning electron microscopy study on fracture surface. The laboratory scale experimental results of AISI 316L(N) stainless steel material are in good agreement with FEM-predicted fracture parameters for notch type of stress raisers. The local damage approach and FEM procedure established in the present study would be easily extendable to the analysis of stress raisers in components for the prediction of crack initiation under elastic-plastic condition.     

Effect of Poisson’s ratio on crack tip fields and fracture behavior of metallic glasses

A study of the effect of Poisson’s ratio on the stationary mode I crack tip fields in amorphous alloys is conducted. Finite element simulations under plane strain, small scale yielding conditions are performed with a Mohr–Coulomb based constitutive model that accounts for pressure sensitivity of plastic flow as well as the localization of plastic strain into discrete shear bands. Simulations are also performed with von Mises and Drucker–Prager constitutive models. While the Poisson’s ratio does not alter the crack tip fields significantly when the material obeys these two models, it has a marked influence in the Mohr–Coulomb case with softening. A higher Poisson’s ratio reduces the extent of plastic zone and plastic strain levels in front of the notch tip as well as that within the simulated shear bands around the notch root at a given level of stress intensity factor. The implications of these observations on toughness of metallic glasses are discussed.     

New insight into the pit-to-crack transition from finite element analysis of the stress and strain distribution around a corrosion pit

A finite element (FE) analysis has been undertaken to evaluate the stress and strain distribution associated with a single corrosion pit in a cylindrical steel specimen stressed remotely in tension. A key observation was the localisation of plastic strain to the pit walls (just below the surface of the specimen). Simulation of a growing pit in a static stress field indicated corresponding plastic strain rates that were commensurate with values associated with stress corrosion cracking. This observation introduces a wholly new concept in understanding of the evolution of stress corrosion cracks from pits and correlates with recent X-ray tomography measurements.     

Effect of stress on electrochemical characteristics of pre-cracked ultrahigh strength stainless steel in acid sodium sulphate solution

The electrochemical behaviors of ultrahigh strength stainless steel Cr12Ni5MoCo14 at crack tip under applied stress were studied by micro-electrochemical measurements as well as finite element analysis. The non-uniform distribution of the stress and strain induces a higher electrochemical activity at crack tip and promotes the anodic dissolution rate. The corrosion rate increases with increasing applied stress. In the elastic stress range, the effect of the applied stress on the electrochemical behaviors of Cr12Ni4Mo2Co14 steel is small. In the plastic stress range, the plastic deformation has a dramatic effect on the mechanical–electrochemical interaction and enhances the anodic activity.     

The effect of atmospheric corona treatment on AA1050 aluminium

The effect of atmospheric corona discharge on AA1050 aluminium surface was investigated using electrochemical polarization, SEM-EDX, FIB-SEM, and XPS. The corona treatment was performed with varying time (1, 5, and 15 min) in atmospheric air. A 200 nm oxide layer was generated on AA1050 after the 15 min air corona treatment. A significant reduction in anodic and cathodic reactivities was observed starting from 1 min exposure, which further decreased with prolonged exposure (15 min) and after delayed testing (after 30 days). The reduction in surface reactivity is due to the formation of thicker and denser oxide film.     

3D FEM simulations and experimental validation of plastic deformation of pure aluminum deformed by ECAP and combination of ECAP and direct extrusion

Rigid-viscoplastic 3D finite element simulations (3D FEM) of the equal channel angular pressing (ECAP), the combination of ECAP + extrusion with different extrusion ratios, and direct extrusion of pure aluminum were performed and analyzed. The 3D FEM simulations were carried out to investigate the load–displacement behavior, the plastic deformation characteristics and the effective plastic strain homogeneity of Al-1080 deformed by different forming processes. The simulation results were validated by microstructure observations, microhardness distribution maps and the correlation between the effective plastic strain and the microhardness values. The 3D FEM simulations were performed successfully with a good agreement with the experimental results. The load–displacement curves and the peak load values of the 3D FEM simulations and the experimental results were close from each other. The microhardness distribution maps were in a good conformity with the effective plastic strain contours and verifying the 3D FEM simulations results. The ECAP workpiece has a higher degree of deformation homogeneity than the other deformation processes. The microhardness values were calculated based on the average effective plastic strain. The predicted microhardness values fitted the experimental results well. The microstructure observations in the longitudinal and transverse directions support the 3D FEM effective plastic strain and microhardness distributions result in different forming processes.     

云纹干涉反转倍增技术研究Ti的应力腐蚀行为

介绍了高灵敏度的激光云纹显微倍增技术,并用其研究了工业纯钛在甲醇+0.6mol/LKCl溶液中应力腐蚀时,缺口前端位移场和应变场的变化.结果表明,在应力腐蚀裂纹萌生前,0.6mol/LKCl的甲醇溶液对工业纯钛的阳极溶解(腐蚀)过程本身能使缺口前端的塑性区增大,与此同时,塑性区中各点的塑性变形量也相应增加,即溶解过程本身促进了局部塑性变形.     

Effects of compressive stress on localized corrosion in AA2024-T3

The effect of compressive stress on intergranular corrosion (IGC) of AA2024-T3 was studied using a constant load and simultaneous electrochemical measurement. A specially designed electrochemical cell was used to compress a pillar-shaped sample and control the potential at a value that promoted IGC. The extent of IGC was assessed by metallurgical cross-sectional images. The effect of the compressive stress depended on the orientation of the stress relative to the elongated microstructure. Application of a compressive stress halfway to yield in the S or through-thickness direction significantly reduced the growth kinetics of IGC in the longitudinal direction, but did not eliminate it totally. The strain change during exposure also was used to quantify the change in radius of the cylindrical sample as a function of time during IGC growth. The effect of compression on reducing IGC was also assessed by the current density measured during potentiodynamic and potentiostatic polarization. The effects of residual compressive stress on IGC were studied using samples treated by low plastic burnishing (LPB), which produces a surface layer with high residual compressive stress. The results depended on the plane of the LPB treatment. A micro-capillary cell was used to measure corrosion behavior at different zones of the section of an LPB-treated sample. The breakdown potential was significantly higher in the zone with residual compressive stress than in the interior of the sample.     

Effect of iron-containing intermetallic particles on the corrosion behaviour of aluminium

The effect of heat treatment on the corrosion behaviour of binary Al-Fe alloys containing iron at levels between 0.04 and 0.42 wt.% was investigated by electrochemical measurements in both acidic and alkaline chloride solutions. Comparing solution heat-treated and quenched materials with samples that had been subsequently annealed to promote precipitation of Al₃Fe intermetallic particles, it was found that annealing increases both the cathodic and anodic reactivity. The increased cathodic reactivity is believed to be directly related to the increased available surface area of the iron-containing intermetallic particles acting as preferential sites for oxygen reduction and hydrogen evolution. These particles also act as pit initiation sites. Heat treatment also causes depletion in the solute content of the matrix, increasing its anodic reactivity. When breakdown occurs, crystallographic pits are formed with {1 0 0} facets, and are observed to contain numerous intermetallic particles. Fine facetted filaments also radiate out from the periphery of pits. The results demonstrate that the corrosion of aluminium is thus influenced by the presence of low levels of iron, which is one of the main impurities, and its electrochemical behaviour can be controlled by heat treatment.     

The complex corrosion system of a medieval iron rebar from the Bourges’ Cathedral. Characterization and reactivity studies

Iron reinforcements used in many medieval monuments are submitted to centuries of corrosion, and a reliable prediction of long term corrosion is needed for their conservation and restoration. This study focuses on an iron rebar from the Bourges’ Cathedral (France). The corrosion product layers formed on the rebar have been investigated through SEM–EDS, Raman microspectroscopy and electrochemical measurements. Electrochemical reductions and composition results show reactive (oxyhydr)oxides-rich corrosion systems. As the location of these phases could influence the reactivity, these macroscopic results should be considered in the light of microscopic observations to propose corrosion mechanism hypotheses.     

Ti2AlNb金属间化合物喷丸强化残余应力模拟分析与疲劳寿命预测

目的 研究经喷丸强化处理后Ti2AlNb材料表层残余应力的分布特征,并预测残余应力对材料疲劳性能的影响规律。方法 通过贴应变片逐层钻孔法,对使用喷丸强化处理后的Ti2AlNb试样进行残余应力测试分析,得到引入残余应力场各方面的测试数据,结合ABAQUS数值模拟方式,对比分析试验与模拟残余应力场结果,获取材料的最终残余应力梯度。利用FE-SAFE软件,通过叠加残余应力场的方式,预测喷丸强化前后试样的疲劳寿命。结果 在文中加工参数下,实验测试和软件模拟结果的重合度良好。喷丸强化可在Ti2AlNb金属间化合物靶材内引入300 MPa左右的最大残余压应力,深度达到了0.12 mm左右。材料表面塑性应变分布不均匀,且造成的塑性应变距表面深度可达0.1 mm。通过喷丸强化引入残余压应力,预测的Ti2AlNb材料疲劳极限可提高12%,高低周疲劳寿命均有明显的延寿效果。结论 验证了有限元数值模拟此材料喷丸强化的准确性和可靠性,得到了Ti2AlNb材料喷丸强化的残余应力场。由于塑性变形诱发机制的限制,喷丸造成塑性应变分布不均匀,塑性应变层深小于残余压应力层深。此外,强化后材料的疲劳性能显著提高,疲劳极限有可观的提升,且高低周疲劳均有较好的延寿效果。     

The effects of temperature and electric field on atmospheric corrosion behaviour of PCB-Cu under absorbed thin electrolyte layer

The effects of temperature and electric field on atmospheric corrosion behaviour of PCB-Cu under absorbed thin electrolyte layers were investigated by cathodic polarization and electrochemical impedance spectroscopy. Results indicate that the cathodic current density increases with increasing temperature, but decreases with increasing intensity of electric field. Electric field reduces the corrosion rate of PCB-Cu due to the aggressive ions migrating out from PCB-Cu electrode surface under the effect of electric field. When the ions can not freely migrate out from PCB-Cu electrode surface, local enrichment of aggressive ions under the electric field will cause serious localized corrosion of PCB-Cu.     

Influence of Desulfovibrio sp. biofilm on SAE 1018 carbon steel corrosion in synthetic marine medium

This work assessed the effect of an enriched culture medium and synthetic seawater on the growth and production of exopolymeric substances (EPS) of a Desulfovibrio sp. strain, isolated from a Mexican oil well. The EPS (mainly consisting of proteins) growth was only achieved after exposing sulfate-reducing bacteria to culture media under dissimilative conditions that predominantly promoted the growth of the biofilm and a small concentration of microorganisms. Once this EPS film was obtained, the evolution of SAE 1018 carbon steel/biofilm/synthetic seawater (VNNS medium) interface was further studied using electrochemical impedance spectroscopy technique (EIS). This study revealed strong adhesion of the biofilm during the formation of iron sulfide (pirrotite) on carbon steel surface. The biofilm inhibits the accelerated damage of the steel for some time exhibiting impedance values of 30 000 Ω. However, at longer times the chemical environment around the biofilm, as a result of microbial metabolism, may become quite corrosive to steel.     

近全层组织γ-TiAl基合金的室温拉伸断裂机理

通过对直缺口近全层组织的扫描电镜原位拉伸实验以及相应的断裂表面观察，结合有限元计算了TiAl基合金近全层组织拉伸的断裂机理。研究表明：许多裂纹在塑性变形前沿着层间起裂和扩展，断裂过程的驱动力是拉应力。在直缺口试样中，许多裂纹直接起裂于缺口根部，并且沿着层间扩展。随着拉应力的增加，主裂纹和新裂纹也可以通过障碍晶粒的穿层解理断裂来连接。通过有限元计算得沿层断裂强度大约为50MPa，穿层断裂强度大约为120MPa。     

Local additional potential model for effect of strain rate on SCC of pipeline steel in an acidic soil solution

Stress corrosion cracking (SCC) behavior of X70 pipeline steel in an acidic soil solution was investigated by slow strain rate test, surface characterization, potentiodynamic polarization curve measurement and electrochemical hydrogen permeation technique. A local additional potential model (LAPM) was developed to illustrate the critical role of strain rate in SCC of the steel. According to LAPM, both density and mobility of local active spots on the steel surface, i.e., dislocation emergence point, increase linearly with strain rate. Generation of such active spots introduces an additional negative potential locally, affecting the electrochemical reaction and, consequently, the susceptibility of the steel to SCC. It is found that a maximum of the SCC susceptibility occurs at strain rate of 10⁻⁶ s⁻¹, which is associated with an enhanced hydrogen evolution due to the local additional potential (LAP) effect. When strain rate is sufficiently high to exceed 10⁻⁶ s⁻¹, the mobility of the dislocation emergence points is so fast that the reactive species in solution cannot combine with them for cathodic reaction, resulting in a decrease of the SCC susceptibility. Similarly, a maximum of hydrogen permeation current observed at the strain rate of 10⁻⁶ s⁻¹ is also attributed to the effect of strain rate on the density and mobility of dislocations in the steel. Diffusion of hydrogen atoms in a strained steel is through both body diffusion and dislocation diffusion, with the latter enhanced by an increasing strain rate. When strain rate is so high that the dislocation mobility is sufficiently fast, hydrogen atoms become incapable of catching up with the dislocations. As a result, the hydrogen diffusion is dominated by the body diffusion mode.     

AISI-1045、1025钢的冲裁模拟

介绍了负间隙冲裁的基本原理,对不同材料刚塑性力学模型进行了分类,采用塑性分析软件DEFORM模拟金属塑性无飞边冲裁过程,基于刚塑性有限元理论建立了负间隙冲裁的几何模型.采用与实验条件完全一致的模拟参数,对金属塑性变形区域的平均应力、损伤、等效应力、等效应变等进行了仿真,得到冲裁变形区域的应力场和应变场,对等值线云图进行了一定的分析.结果表明,负间隙冲裁推迟了裂纹的发生,冲裁质量较好.