The subsurface zone in magnesium alloy studied by positron annihilation techniques

This paper presents the positron lifetime and Doppler broadening of annihilation line studies of the subsurface region in a magnesium-based alloy which was exposed to dry sliding. The total range of the subsurface zone below the worn surface detected using these techniques was lower than 100 μm and was hardly affected by the applied load and sliding distance. The obtained results are significantly different from the results achieved for pure magnesium, thus alloying has considerable effect on the subsurface zone formation. The positron lifetime profile of the subsurface region was well correlated with the microhardness profile. The significant result was that the weak long-lived component indicated ortho-positronium formation has been found at the depth lower than 30 μm. This indicates the formation of voids below the worn surface.     

The detection of subsurface zones in aluminium-based alloys 2017A and 6101A using a positron annihilation technique

This paper presents positron lifetime studies of the subsurface region in aluminium-based alloys 2017A and 6101A, the surfaces of which were exposed to dry sliding. In the case of the 2017A alloy, the total range of the subsurface zone below the worn surface detected using the positron annihilation technique was larger than 120 μm and this value was hardly affected by load and sliding distance. For the 6101A alloy, the subsurface zone ranged from 30 to 500 μm and depended on the applied load and only to some extent on the sliding distance. The obtained results are significantly different from the results achieved for pure aluminium, thus alloying has considerable effect on the subsurface zone formation. The positron lifetime depth distribution characterizing the subsurface zone was correlated with the microhardness profile.     

A positron annihilation lifetime spectroscopic study of nanocrystals formation in a chalcogenide glass-ceramic

The positron annihilation lifetime spectroscopic technique was applied to study nanocrystals formation during controlled crystallization of a chalcogenide glass 51GeS₂-9Sb₂S₃-40PbS, which was thermally treated at 330 °C for up to 15 h. The technique provided valuable information for in depth understanding of the nature of the material as well as the crystallization processes by showing systematic change of lifetime components (τ₁, τ₂, τ₃) and intensities (I₁, I₂, I₃) with thermal treatment time. Such change defines the time frame for nucleation and crystal growth processes; the nucleation process is dominating in the first 3 h, whereas after 3 h, the crystal growth becomes the main process. The values of lifetime components and their intensities reveal change of the size and amount of nano- and micro-defects with time and can be used to guide materials engineers for efficient composition designs and improvements of chalcogenide glass-ceramics.     

Subsurface zone studied by positron lifetime measurements

Measurements of the positron lifetime in the subsurface zones of copper samples are presented. The samples were slid against a graphite cylinder and the measurements performed just at the first stages of the creation of the subsurface zone. From the positron lifetime we were able to deduce the shear stress in this zone. The depth profile of the subsurface zone measured by positron annihilation exhibited different types of open volume defects in well-annealed samples. The depth profile was not observed when imperfections were present in the samples. This revised version was published online in July 2006 with corrections to the Cover Date.     

Scale-dependent subsurface deformation of metallic materials in sliding

Using laser speckle decorrelation, TEM, optical microscopy and AFM we study deformation structures generated in subsurface layers of metals and alloys by sliding wear. Strain localization process in sliding wear as well as its effect on sliding-induced structures is considered. As shown, strain localization leads to intense fragmentation in subsurface layers and generation of shear bands in previously fragmented materials. These shear bands first exist at the microscale level under mild wear but may reveal at the mesoscale deformation level when there occurs mild-to-catastrophic wear mechanism transition. The result of such a transition is a thick (tens and hundreds of micrometers) nanocrystalline layer at the surface of metals. Hadfield steel shows another type of tribological behavior when only thin nano crystalline layer is formed. We relate such a behavior to the specificity of fragmentation in this steel. High wear resistance of high manganese steel is analyzed too.     

The subsurface stress field caused by both normal loading and tangential loading

The subsurface stress field caused by both normal loads and tangential loads has been evaluated using the rectangular patch solution. The effect of tangential loading on the subsurface stress field has been investigated in detail for both the cylinder-on-cylinder contact and a spur gear teeth contact. For the cylinder-on-cylinder contact, the subsurface stress fields are moved more to the direction of tangential loads and the positions where the maximum stress occur are getting closer to the surface with the increasing tangential loads. The subsurface stress fields of the gear teeth contact are expanded more widely to the direction of tangential loads with the increasing tangential loads. The friction coefficient of a gear teeth contact is low because they are operated in a lubricated condition, and therefore surface tractions in the EHL condition hardly affect on the subsurface stress field.     

Fundamental investigation of subsurface damage in single crystalline silicon caused by diamond machining

Single crystalline silicon was plunge-cut using diamond tools at a low speed. Cross-sectional transmission electron microscopy and laser micro-Raman spectroscopy were used to examine the subsurface structure of the machined sample. The results showed that the thickness of the machining-induced amorphous layer strongly depends on the tool rake angle and depth of cut, and fluctuates synchronously with surface waviness. Dislocation activity was observed below the amorphous layers in all instances, where the dislocation density depended on the cutting conditions. The machining pressure was estimated from the micro-cutting forces, and a subsurface damage model was proposed by considering the phase transformation and dislocation behavior of silicon under high-pressure conditions.     

Subsurface Zone in Pure Magnesium Studied by Positron Lifetime Spectroscopy

This paper presents the results of investigation of a subsurface zone created during dry sliding in pure magnesium. Positron annihilation and microhardness techniques were used to detect the defect distribution below the worn surface. Near to the surface there was found a strong gradient of dislocations and defects associated with them, which were extended more than a hundred of micrometers. The decrease of microhardness at a depth less than 30 μm from a worn surface suggested that dynamic recovery took place in pure magnesium. The defect profiles were also studied for samples exposed to compression and blasting.     

Subsurface deformation and damage accumulation in aluminum–silicon alloys subjected to sliding contact

The study of plastic deformation and damage accumulation below the contact surfaces is important in order to understand the dry sliding wear behaviour of aluminum alloys. Experimental evidence exists for the nucleation of voids and microcracks around second phase particles in the material layers adjacent to the contact surface. Propagation of these cracks at a certain depth below the surface may lead to the creation of long, thin plate-like wear debris particles. This work studied the deformation processes during sliding wear by means of metallographic observations of subsurface layers in an Al–7% Si (A356 Al) alloy and by finite element analyses. Specifically, the accumulation of subsurface stresses and strains was investigated, using a coupled structural-thermal finite element model based on the Voce-type exponential stress–strain relationship obtained from the sliding wear tests. Additionally, temperature and strain rate effects were taken into account using a constitutive equation based on Johnson–Cook and Cowper–Symonds models.Accordingly during sliding, the flow stress in subsurface layers increased rapidly and reached a saturation stress after a finite number of sliding contacts. The variation of hydrostatic pressure for different loading conditions was also determined as a function of sliding passes: as the sliding process progressed from the first to the seventh contacts, the hydrostatic pressure at the surface increased from 1150 to 1300 MPa. A total temperature increase of 45 K occurred at the surface after the seventh sliding contact. A debris formation model was proposed in which the presence of a maximum damage gradient at critical depth was considered. The model showed that, with a sliding velocity of 10 m/s, and a normal load of 150 N per unit thickness in mm, the material location where the maximum rate of damage occurred corresponded to a normalized depth (depth/counterface diameter) of 0.060. Increasing the load to 250 N/mm caused an increase in the critical depth of damage (a normalized depth of 0.085). Comparisons with the experimental subsurface crack observations indicate that the proposed damage rate calculations provide a good estimation of the subsurface crack propagation depth.     

全身肿瘤扫描:MRI弥散加权成像和PET技术的比较

继正电子放射断层(PET)技术之后,磁共振(MRI)全身弥散加权成像(WB-DWI)正逐渐成为临床上又一项重要的评估全身肿瘤的影像技术。本文将WB-DWI与较为成熟的PET全身扫描技术进行比较,综述WB-DWI在肿瘤学中的初步应用,特别是在肿瘤的诊断、鉴别诊断、分期、预后、疗效监测及筛查等方面的价值。     

毛细管电泳法同时测定糖果中5种人工合成色素的含量

建立毛细管电泳法测定糖果中5种人工合成色素的分析方法。该法用于测定市售样品得到满意结果:在浓度范围1.9~52.1g/mL之间,线性关系良好,R2小于0.9921,回收率在98.5%~102.6%之间。该法简便、快速、灵敏度高、重现性好。     

虚拟环境中公差域建模技术

在分析各类尺寸和位置公差对应的公差域形状、大小及公差域局部坐标系的基础上,提出以公差域的边界表达及其对几何要素的约束作用为核心、以几何面数学方程组形式表达公差域的边界、以约束方向向量和偏差向量表达公差域对几何要素的约束作用的尺寸公差与位置公差统一的公差域模型,并运用该模型实现了圆柱域、同轴圆柱域及两平行平面域的公差域约束几何变动范围的求解。借助ACIS几何造型工具开发了原型系统,实现了虚拟环境下的公差域构建和可视性显示,相关模型和算法也通过实例进行了验证。     

乐亭海岸带大雾统计分析及监测

利用乐亭2001~2010年大雾监测地面观测资料,对乐亭大雾日、持续时间、雾的生、消时刻等进行统计.结果显示:近10a大雾日年际变化不大,但具有明显的日、月变化,其中以10~12月大雾日数最多;一年四季大雾生、消的最多时刻随着季节的转换日出时间的不同而有所差异,春、夏季生成时刻以05:00~06:00最多,秋、冬季以06:00~07:00为多.大雾的生成是多种气象条件和大气环流形势共同作用的结果,通过分析发现乐亭沿海东到偏东南风有利于暖平流和水汽的输送,弱的均压场有利于大气层结的稳定,适宜的海温均有利于产生大雾天气.对乐亭沿海大雾气候特征的分析及监测方法的探究,目的在于为沿海大雾天气的预报、预防、监测及更好的开展雾的预警服务工作提供基本依据.     

Experimental investigation of the temperature fade in the cavitation zone of full journal bearings

This paper describes an experimental investigation with a rig for measuring the surface temperatures and pressures in single bore journal bearings. The test bearing was equipped with small bores, where the pressure was altered by means of a vacuum pump. The whole bearing arrangement can also be operated at different ambient pressures ranging from atmospheric down to −0.06 MPa. Oil temperatures at inlet and outlet are also measured. Experiments show a considerable drop in the temperature of oil undergoing a drop in pressure. The overall temperature level of the bearing system did not seem to affect the magnitude of the temperature drop. Experiments where the bearing system was operated at sub-ambient pressures revealed an increased temperature drop in the cavitation zone. Pressures higher than ambient were measured within the cavitation zone while the bearing was operated at sub-atmospheric pressure. The higher pressures are believed to be caused by evaporation of oil. It is therefore proposed that the temperature drop observed in the cavitation zone of journal bearings is primarily caused by evaporation of oil. However, isentropic decompression of the gases in the oil is also believed to play a role.     

Fast genetic algorithm for roundness evaluation by the minimum zone tolerance (MZT) method

According to ISO 1101, “A geometrical tolerance applied to a feature defines the tolerance zone within which that feature shall be contained”.The main goal of the minimum zone tolerance (MZT) method is to achieve the best estimation of the roundness error, but it is computationally intensive. This paper describes the application of a genetic algorithm (GA) to minimize the computation time in the evaluation of CMM roundness errors of a large cloud of sampled points.Computational experiments have shown that by selecting the optimal GA parameters, namely a combination of the five genetic parameters related to population size, crossover, mutation, stop condition, and search space, the computation time can be reduced by up to one order of magnitude, allowing real-time operation.Optimization has been tested using seven CMM samples, obtained from different machining features. The performance of the optimized algorithm has been validated using four benchmark samples from the literature and with certified samples.     

多分辨率快速圆检测在抑菌圈测量系统中的应用

本文提出了多分辨率快速圆检测的算法。基于多分辨率的框架,在低分辨率下快速检测圆心和半径的信息,然后映射到高分辨率中,确定圆的边缘和圆心所在的局部区域,减少处理单元的数量,并提出局部单圆验证算法获得精确的圆心和半径信息。本文提出的算法克服了圆检测精度与时空复杂度的矛盾,在高分辨率、多圆、多噪声的情况下,算法的效率下降较少。最后将圆检测算法应用于基于机器视觉的抗生素残留量检测系统中进行抑菌圈检测,结果表明处理时间短、检测精度高。     

Subsurface Zones in Steel Samples Studied by Means of Positron Annihilation

This paper presents the results of positron beam experiments performed on raw steel samples whose surfaces have been damaged during sliding contact in the conventional pin-on-disc tribotest. It was important to use raw samples, because we were examining the usefulness of the positron beam in technical applications where such samples could be available. We were able to detect a change of experimental parameters, i.e., the positron diffusion length related to the type of the lubricant used in the tribotest. The analysis of the S parameter on the surface did not suggest any characteristic changes which would prove the creation of the top layer of new properties induced by the friction process. To complete our studies, we also carried out studies by measurement of the positron annihilation characteristics in the conventional experiment.     

Influence of a ring flat zone in the point contact surface on thermal elastohydrodynamic lubrication

This paper describes a geometrical profile, an elastohydrodynamically lubricated point contact surface with a ring flat zone, aimed at building up local line contact elastohydrodynamic lubrication (EHL) in point contact conjunctions to reduce the influence of side-leakage on the central film thickness. Effects of the ring flat zone on the thermal EHL characteristics are studied. A dimensionless coefficient, r_W, is defined to represent the relative half width of the ring flat zone in a point contact EHL surface. Thermal EHL numerical simulations have been performed to investigate the influence of r_W on the film thickness as well as pressure, temperature and friction coefficients under different operating conditions. In the range of 0≤r_W≤1.0 results show that the minimum film thickness decreases with increasing r_W and the central film thickness increases with increasing r_W, and the influence of r_W on the film thickness is more pronounced than those on the maximum pressure, the maximum temperature and the friction coefficients. It is revealed that the proposed ring flat zone with appropriate width is beneficial to the thermal lubrication.     

Recrystallization phenomena in an IF steel observed by in situ EBSD experiments

In situ electron backscatter diffraction microstructural analysis of recrystallizing interstitial free steels deformed to strains of 0.75 and 1.6 has been carried out in a FEG‐SEM. The experimental procedures are discussed, and it is shown that there is no degradation of the electron backscatter diffraction patterns at temperatures up to 800°C. Analysis of the surface and interior microstructures of annealed samples shows only minor difference, which suggests that in situ annealing experiments are of value. In addition, it is shown that in situ measurements allow a detailed comparison between the same areas before and after annealing, thereby providing information about the recrystallization mechanisms. Sequential recrystallization phenomena, such as initiation and growth of new grains, are observed at temperatures over 740°C, and depending on the deformation histories, different recrystallization behaviour is observed. It is found that {111}〈123〉 recrystallized grains are preferentially formed in the highly deformed material, whereas no strong recrystallization texture is formed in the lower strained material.     

Evaluation of minimum zone flatness by means of nonlinear optimization techniques and its verification

This article deals with the application of some nonlinear optimization techniques for minimum zone flatness. The convergence criteria of the techniques, namely the downhill simplex method and the repetitive bracketing method, are considered. The least-squares method is also applied, and subsequently the three methods are compared from the viewpoint of computational accuracy. A surface profile measuring system and a noncontact sensor are used to obtain three-dimensional data. The measured data are expressed by means of perspective mapping. Subsequently, the relationship among the above three methods is clarified according to accuracy and efficiency of the computation. Furthermore, some examples of the relationship between the manufacturing method and the flatness value, and the technique of a skilled hand are described.