Crack opening due to deformation twin shear at grain boundaries in near-γ TiAl

Microcrack nucleation has been observed at apparent deformation twin interactions with grain boundaries in a duplex near-gamma TiAl specimen deformed to surface tensile strain of about 1.4%. To prove that these microcracks are a result of twins and not dislocation slip bands, detailed characterization of the surface topography and crystallography associated with the microcracks was conducted and analyzed. Electron backscatter diffraction patterns were used in conjunction with selected area channeling patterns to determine the crystallography near the observed microcracks. Transmission electron microscopy of a selected twin, extracted using a focused ion beam, and atomic force microscopy were used to show that the observed microcracks could only have been caused by local strain heterogeneities caused by twin interactions with grain boundaries and not by dislocation slip bands.     

Voltammetric current oscillations due to general and pitting corrosion of tantalum: Implications for electrochemical-mechanical planarization

Anodic corrosion of Ta is examined for potential applications in electrochemical-mechanical planarization (ECMP) of diffusion barriers. This strategy involves electro-oxidation of Ta in the presence of (or Br⁻) to form mechanically weak surface-oxide films, followed by mechanical removal of the latter. The voltammetric currents exhibit oscillatory behaviour with frequencies that are signature attributes of localised pitting by Br⁻ or general surface corrosion by . SEM, voltammetry, and impedance spectroscopy are used to probe these corrosion mechanisms. Apart from their relevance for ECMP, the results also address certain fundamental aspects of pitting and general corrosion of valve metals.     

Monitoring Sub-Critical Crack Growth due to Stress Corrosion or Hydrogen Embrittlement by Acoustic Emission

Abstract

Acoustic emission ring-down counting has been used to monitor sub-critical crack growth by stress corrosion of mild steel in disodium hydrogen phosphate, -brass in sodium nitrite and magnesium–7% aluminium in potassium chromate/sodium chloride mixtures, and by hydrogen embrittlement in mild steel and a 7179 T651 aluminium alloy. The level of acoustic emission activity varies widely between different alloy/environment systems and is dependent upon microstructure, fracture path and crack growth rate. The practical significance of the results is discussed.     

裂纹表面接触对混合型应力下倾斜疲劳裂纹变位特性的影响

在循环载荷的作用下由中心孔向两端延伸所产生的倾斜裂纹,当载荷的应力比变化(从R=0到R=-1)时裂纹面的凸凹程度也会随之变化.对于倾斜角度比较小的疲劳裂纹(β=45°),讨论了上述的应力比所制作的裂纹其周围的压缩残留应力对Ⅰ型应力扩大系数(KⅠ)mes以及裂纹上下表面的相互接触对Ⅱ型应力扩大系数(KⅡ)mes的影响.其结果是,应力比R=0和R=-1的疲劳裂纹,应力扩大系数的实验值(K Ⅰ)mes都小于理论值KⅠ,且R=-1的(KⅠ)mes比R=0的值大.而对于Ⅱ型应力扩大系数,虽然两者的实验值(KⅡ)mes也都小于理论值KⅡ,但R=-1的(KⅡ)mes比R=0的值下降的更多.     

Regularities of the Rehbinder effect in polymers. Review

Data are analyzed that are related to particular details of manifestations of the Rehbinder effect in the case of polymer deformation in adsorption-active media (AAMs). It is shown that, as a result of such a deformation, a vitreous polymer is dispersed, forming a system of fibrillar aggregates of oriented macromolecules with a diameter of 1–20 nm separated in space by microcavities of similar size (the crazing phenomenon). It is shown that polymer crazing in AAMs is a complex multistage process. Its first stage is initiation of crazes related to the microscopic surface defect structure of the actual polymer and is controlled by the Griffith criterion. Craze growth and broadening are typical thermally activated processes of plastic deformation of the polymer. The main areas for practical application of the Rehbinder effect in polymers are pointed out.     

In situ observations of dendritic fragmentation due to local solute-enrichment during directional solidification of an aluminum alloy

High-brilliance synchrotron X-radiation microscopy and further image processing allowed in situ observations of local solute-enrichment during fragmentation of dendrite arms under normal non-forced-convection conditions during directional solidification of an Al–20 wt.% Cu alloy. An open dendritic network is exposed to solute-rich liquid which arrives from the solidification front. At grain boundaries such openness is even more pronounced, leaving higher-order branches highly exposed to the incoming solute-rich liquid stream. It was found that streaming of solute-enriched liquid at the tip of high-order branches promotes growth by local undercooling. Further solute advection and local solute rejection due to the local growth causes solute pile-up at the roots of the branches. This solute pile-up alters the local compositional balance at the solid–liquid interface at the root, causing the root to remelt, which in turn results in further branch detachment.     

Controlled crack arrest in brittle thin films: The effect of embedded voids

Controlled crack arrest is useful for controlling self-assembled crack paths in micro-fabrication, as well as limiting crack length in toughening composites. Compliant inclusions and voids can effectively attract and arrest cracks, thereby controlling the crack pattern. Analytical stress solution indicates that when the inclusion is about 10 times more compliant than its matrix, it may be effectively modeled as a void. The crack arrest capability is expressed in terms of the critical angle of the initial crack path, as the inclusion size, shape, crack origin and film properties are varied. Simple criteria are established to maximize the crack arrest ability and to determine the critical crack angle. The effectiveness of using different void patterns to arrest cracks is also explored. The results are useful for both controlling and restricting cracks in brittle thin films, through the utilization of compliant or void-like inclusions as crack arrestors.     

GH2036高温合金平板裂纹闭合效应及裂纹扩展模型

为探明GH2036高温合金的低循环疲劳裂纹扩展机理,对GH2036高温合金平板在550℃、不同应力比下的低循环疲劳裂纹扩展特性进行了试验研究,采用数字图像相关(DIC)方法确定了GH2036高温合金的张开应力强度因子。结果表明,温度550℃、应力比大于0.7时GH2036高温合金无裂纹闭合现象,在此基础上建立了以残余裂尖张开位移、应力比为参量的GH2036高温合金裂纹闭合模型。而后,断口的SEM分析表明:随着应力比的增加,裂纹扩展区由穿晶断裂向沿晶断裂转化。最后,基于GH2036高温合金的裂纹闭合模型,建立了GH2036高温合金平板的低循环疲劳裂纹扩展寿命预测方法,与试验数据吻合良好,验证了方法的准确性。     

A phase field study of microstructural changes due to the Kirkendall effect in two-phase diffusion couples

The phase field method was used to simulate the interdiffusion in ternary, two-phase diffusion couples. Model γ+γ′ alloys were designed so that changes in the microstructure could be attributed to either coarsening or the Kirkendall effect. For the model parameters adopted in the study, particle coarsening increased the average precipitate size, but it had little influence on the diffusion path. However, the Kirkendall effect, which was introduced by setting atomic mobilities to different values, changed the diffusion path slope and led to the formation of “horns” on the two-phase diffusion path. Although the results obtained were somewhat dependent on the model parameters, they demonstrate the value of the phase field method when studying complex interdiffusion problems.     

示波冲击试验中裂纹生长与扩展机理研究

通过示波冲击试验、金相分析、扫描电镜分析等方法,对P92钢室温示波冲击试验时,自裂纹萌生至断裂的全过程进行了分析,研究了裂纹的萌芽、生长和扩展规律。结果表明,冲击试样的破断过程可描述为裂纹的萌芽、生长和扩展直至断裂三个阶段。裂纹萌芽和生长过程对应宏观冲击断口的启裂区,裂纹扩展过程对应宏观冲击断口的扩展区。裂纹的生长和扩展过程因剪切裂纹和颈缩裂纹的交替出现而呈现转折的“之”字形路径。     

降低板坯三角区裂纹比率的控制措施

针对某公司炼钢厂板坯连铸生产初期产生的三角区裂纹,研究了连铸板坯三角区裂纹的形成机理,分析了二次冷却、钢种化学成分、铸机状况、工艺操作等因素的影响,提出了防止三角区裂纹产生的措施。在实际生产中实施改进措施后,三角区裂纹比率由原来的16.31%降低到目前的0.16%。     

The effect of strain-rate upon stress corrosion crack velocity in α-brass in ammoniacal solutions

Stress corrosion crack velocity in a-brass has been measured as a function of tensile crosshead speed and solution composition in aqueous solutions, mainly at pH 6.8. At low crosshead speeds the velocity was constant but at crosshead speed values high enough to cause general yielding the velocity increased with crosshead speed in a double logarithmic relationship. The maximum velocity producing 100 % intergranular fracture was ca. 30 mm/h. Potentiostatic experiments indicated an increase in crack velocity with anodic polarization but only within the potential/pH region where Cu₂O was stable. At more noble potentials the crack velocity was lower, an effect seen also with cathodic polarization and with the addition of chloride and bromide ions, and in all cases the fracture was partly transgranular. The important factor controlling both the type and amount of stress corrosion cracking is the ratio of tarnish film formation rate/crack tip strain-rate.     

The effect of the local pH in the slow relaxation of polyaniline

It is suggested that the slow relaxation phenomenon in the case of polyaniline is due to a change of structure of the double layer under polarization. The pH at the interface and along the Debye length, i.e. in the double layer, is more acidic than in the solution when the electrode is polarized below the potential of zero charge (PZC) and is more basic when the polarization is above the PZC. This is proposed as the main reason for the peak shift characteristic of the so-called relaxation effect in addition to structural changes.     

铁和铁合金的塑性变形与磁化

结晶材料通过塑性变形会发生磁物理量的变化,位错周围的磁矩变化会影响到塑性变形磁性体的磁化过程。研究了矫顽力、渐近饱和区域的磁化率与位错组织的相关关系。纯铁和Ａ533Ｂ钢的变形应力与矫顽力有着一定的相关性,矫顽力随着变形量的增加而增大形成一条曲线。Ａ533Ｂ钢...     

一种改进的Lemaitre断裂准则及其在弯曲成形裂纹预测中的应用

针对弯曲成形中的裂纹缺陷,应用Lemaitre韧性断裂准则,同时考虑应力三轴度,最大主应力比,以及塑性应变对损伤的影响,对Lemaitre准则进行改进,有效预测了弯曲成形中金属板料的成形极限。以7075-T6铝合金为研究对象,模拟得到该合金板材的裂纹产生条件,获取改进的Lemaitre准则材料参数,确定破裂阈值。对6 mm厚7075-T6铝合金板材进行三点弯曲实验,并观察其金相组织。对其产生裂纹时的压下量与断裂准则所得的理论压下量进行比较,验证了改进后断裂准则对裂纹预测的准确性。通过对比产生裂纹时的压下量,结果表明,改进后的Lemaitre断裂准则所得理论压下量为9.7 mm,与模拟和实验结果一致,证明改进后的Lemaitre准则对弯曲成形裂纹预测具有一定的准确性。     

The effect of drag reducing polymers on the rate of mass transfer in electrochemical machining

Rates of mass transfer during the diffusion controlled anodic dissolution of copper in ortho phosphoric acid solution containing polyethylene oxide drag reducing polymer were measured under turbulent flow conditions in a rectangular duct. Reynolds number and polymer concentration were varied over the range 3000–15000 and 10–200 ppm respectively. Under these conditions it was found that the polymer reduces the rate of mass transfer by a maximum of 45%. Mass transfer data in polymer solutions was correlated by the equation:     

解决17型车钩钩体尾部裂纹措施

本文论述了17型车钩钩体生产过程的工艺改进,介绍了17型车钩钩体尾部裂纹产生的原因及采取的措施。通过工艺上采用保温暗冒口提高凝固过程补缩性,同时采用空心销孔芯提高芯子的退让性,并加强车钩钩体生产过程质量控制等措施,成功地解决了17型车钩钩体尾部裂纹问题,产品废品率由4.3%降到0.23%.