INTERGRANULAR AND EXFOLIATION CORROSION BEHAVIOUR OF 8090Al-Li ALLOY

The A1-2.79Li-1.3Cu-0.7Mg-0.12Zr(8090) alloy has excellent resistance to intergranularand exfoliation corrosion as naturally aged, worst as peak aged and rather better asunderaged than as overaged. The corrosion behaviour of the alloy is closely dependent on itsmicrostructure. Under the peak aging condition, the coarse T_2 phase, which is continuouslydistributed along grain boundaries, has so low corrosion potential in comparison with the ma-trix, this makes the corrosion resistance of the alloy to be deteriorated by anodic dissolutionalong grain boundaries.     

Influence of Tempering Temperature on Cyclic Viscoplastic Behaviour of 55NiCrMoV7 Steel

Low cycle fatigue behaviour of a steel 55NiCrMoV7 under tour tempered conditions is reported. One special type of total swain controlled isothermal cyclic deformation tests were performed in the temperature range 20℃ to 600℃ for the steel tempered 2h at 350℃, 460℃, 560℃ and 600℃. The influence of temperature on cyclic behaviour was investigated. Generally, the cyclic stress response shows an initial exponential softening for the first few cycles, followed by a gradual softening without cyclic softening saturatioo. At 10^-2 strain rate, σmax.Δσ／2 decrease with the test tumperature for all hardness levels. They decrease linearly with tempering temperature when testing temperature is lower than that of tempering, but rest nearly constant when test temperature is equal to or exceed tempering temperature of steel. Cyclic softening intensity increases with testing temperature from 300℃ to 600℃, but the maximal softening intensity occurs at room tumpemture. The strain rate influences notably the cyclic behavioor when T≥50℃, The time dependence of cyclic behaviour is closely related to test temperature and the tempering history of the steel.     

Superplastic deformation and crystallization behavior of Cu54Ni6Zr22Ti18 metallic-glass sheet

Superplastic deformation and crystallization behavior of a Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were investigated. A maximum elongation of 650% was obtained at 733 K at 1 × 10⁻² s⁻¹ from the sheet fabricated by squeeze copper-mold casting method. At low strain rates, the strain-rate-sensitivity exponent value was close to 1, suggesting that Newtonian-like behavior governed the plastic flow. At a high strain rate around 10⁻² s⁻¹, a transition from Newtonian to non-Newtonian behavior took place with decrease in m value. Large strain hardening by crystallization occurred during the course of deformation. The strain hardening was found to be caused by crystallization according to the analyses of the relation of true stress vs. testing time, T-T-T diagram and DSC characteristics. The time periods up to the strain before strain hardening at 733 K for the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were similar to that of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glass at 696 K as 180–300 s (3–5 min). This coincidence could be explained by comparison of their T-T-T diagrams showing that the incubation times for crystallization of the Cu BMG at 733 K and for Zr BMG at 696 K are similar.     

Cold forming processes: some examples of predictions and design optimization using numerical simulations

Predicting the behaviour of steel during a deformation process, and then under service conditions, is one of the main challenges in cold forming. The design of optimized forging schedules, by means of classical trial+errors procedures, has become increasingly heavy in terms of time and cost in a competitive environment. Simultaneously, the improvement of steel qualities requires the microstructure, constitutive behaviour and deformability to be known a priori regarding a targeted application. During the last few years, numerical simulations have become a very efficient tool to reach these goals.

In this paper, we give examples of innovating forging sequences developed by numerical simulations, including the investigation of damage in tools and forged parts. In case of specific processes with very determined geometry — such as wire drawing — we show how systematic numerical studies may lead to predictive models of force, local strains and residual stress…

However, reliable predictions from numerical simulations require reliable input data, including constitutive laws, friction conditions and propensity to ductile damage. These data must be characterized under realistic sollicitations. Typical cold forging loadings are indeed very severe: local strains up to 600%, strain rates locally greater than 1000 s⁻¹, and subsequently, plastic heating over 500°C.

To characterize the constitutive behaviour, the standard upset test between grooved dies is used along with an original methodology to derive the strain hardening curve from the experimental force-displacement recording. Tool elastic deformations, specimen strain heterogeneity… are taken into account. This enables a precise determination of the strain hardening curve up to about 100% of strain. The extrapolation of the flow stress to greater deformations is then very easy and reliable. Such a test can be performed under quasi-static and isothermal conditions (0.1 s⁻¹ but also adiabatic and rapid conditions (up to 10 s⁻¹). This procedure was adapted to a Pellini hammer, which enables very simple characterization at 800 s⁻¹. The comparison of all these flow curves lead to the formulation of an original constitutive model, which accounts for the effects of plastic heating, strain rate, dynamic aging…

In order to predict ductile fracture during the forging process, the most classical criteria were tested over a wide range of experimental loading conditions. None of them were general enough to solve all the cold forming problems. On the other hand, mesoscopic models describing the deformation of the metal matrix around inclusions or second phases have proved to be in good agreement with the various experimental observations. An original plasticity criterium, based on the recent works in porous plasticity theory, has been developed and already displays promising capabilities. Simple experimental procedures enable a reliable classification of steel qualities, heat treatments… in term of forgeability.

Finally, the friction problem is treated using different methodologies based on the forming process considered. For forging operations, a fine analysis of the force-displacement curves in direct extrusion stages may lead to a precise measurement of the friction coefficient under pressures from about 200 MPa up to 1000 MPa and for sliding rates between 1 and 100 mm/s. For wire drawing process, a model relying on an analytical approach using a significantly improved slice method has been developed: the comparison of the experimental drawing force and the predicted one gives the friction coefficient in industrial processing conditions (speed up to 5 m/s).     

金属型铸造Mg-8Zn-4Al-0.25Mn合金的凝固行为及组织特征

研究了金属型铸造Mg-8Zn-4Al-0．25Mn合金的凝固行为及组织特征。结果表明：合金的铸态组织主要由初生α-Mg、Mg32（Al，Zn）49相和Al2Mg5Zn2相组成，其中Mg32（Al，Zn）49相主要呈连续状和／或半连续状分布，Al2Mg5Zn2相主要呈孤岛状分布。此外，根据DSC结果还确定了合金的凝固温度范围、液相线及固相线温度，并阐明了合金凝固过程中的相变。     

晶粒尺寸对含Zr的Ni_3Al合金氧化行为的影响

研究了Ｎｉ－１１，８２Ａｌ－１．０３Ｚｒ－０．１Ｂ（ｗｔ－％）合金晶粒尺寸为３０和１８０μｍ两种组织结构试样，在９００和１１００℃静态空气中的等温氧化行为．发现，当氧化温度为９００℃时，氧化速率随晶粒尺寸减小而减小，而在１１００℃，二者氧化速率几乎没有区别．利用ＥＰＭＡ，ＸＲＤ分析了氧化产物，证明两种试样的氧化膜相成分与结构，均由ＮｉＯ外层和Ａｌ_２Ｏ_３，ＮｉＡｌ_２Ｏ_４内层组成．但氧化膜／合金界面形貌显著不同．９００℃氧化膜与合金界面呈平滑的波浪型，而１１００℃界面出现树根状突出氧化物伸入到合金表层内部．对上述实验结果作了理论分析，认为前一结果为合金晶界快速短路扩散传质所致、而后一现象与合金晶粒尺寸关系较小，起决定作用的为活性元素Ｚｒ的“反应元素效应（ＲＥＥ）”．     

How purchase intention consummates purchase behaviour: the stochastic nature of product valuation in electronic commerce

This study challenges the conventional assumption that online customers with high purchase intention routinely move to the purchase stage. To this end, the process of how online customers form purchase intention and behaviour is examined. On the basis of product value distribution (PVD), we propose that the hypothetically expected product value (i.e. PVD average) determines purchase intention, whereas the actual probability of attaining the expected product value (i.e. PVD variance) moderates purchase behaviour. This proposal indicates that the expected product value has significance only when repeated purchase is assumed given that most consumers do not repeatedly purchase the same product in reality. Thus, the actual probability of attaining the expected product value more critically affects customer behaviour than does its expected value. The effectiveness of the research model is verified by conducting a survey on 300 online mall shoppers in Korea. The results confirm model effectiveness.     

DOBNet: exploiting the discourse of deception behaviour to uncover online deception strategies

Online deception is fuelled by the escalated penetration of the Internet and social media. As the threat of online deception increases, understanding deception behaviour and underlying strategies is having a greater social impact. The verbal behaviour of online deception has recently been extended to the discourse level; nevertheless, discourse behaviours have been examined in isolation without referring to other behaviours in the discourse. By conceptualising the discourse of online behaviour as a social network (DOBNet), this research investigates possible impacts of deception intent on the central structures of DOBNet at three different levels: the discourse behaviour, subnetwork, and whole network. The empirical results of discourse network analysis and statistical tests provide partial support for each of the hypothesised effects. The findings not only demonstrate the efficacy of discourse in distinguishing deceivers from truth-tellers but also extend deception theories by confirming deception strategies from the perspective of discourse network and by uncovering unique characteristics of online deception strategies.     

Using adaptive e-news to improve undergraduate programming courses with hybrid format

With the growing popularity of computers and the Internet, most teachers are taking advantage of Internet functions to assist in both teaching and student learning. However, students only login to the assisted learning system once or twice a week on average to surf for relevant references, participate in discussions, or hand in assignments, even though information and interaction are greatly enhanced by these online services. Therefore, increasing the transfer rate of useful information and interaction during the course period would help students learn. To this end, we designed an e-news delivery system that sends personalized e-mails to every student’s mailbox to make information access and peer interaction more convenient and efficient. Through the daily habit of using e-mail, students can receive the latest information, new instructions or peer discussions early. To encourage usage of the system, a user friendly interface was designed. The e-news system takes advantage of webpage hyperlinks to allow students to click on interesting topics while reading e-news; doing so will cause another window to pop-up with detailed content retrieved from the web-based learning system. A mouse-click allows students to login to the web-based learning system and extract the details of the chosen topic. To enhance the efficiency of e-news, learners’ needs and interests were taken into consideration when forming the e-news content. Students’ learning status and reading preferences are assessed to determine which information or content to include in the e-news. Experiment results reveal that e-news and personalized information greatly promote the reading of new information and participation in online activities.