Self-Diffusion Coefficient of fcc Mg: First-Principles Calculations and Semi-Empirical Predictions

First-principles calculations and semi-empirical equations are employed to determine the self-diffusion mobility of fcc Mg. All factors entering the vacancy-mediated self-diffusion coefficient, which include the equilibrium lattice parameter, the enthalpy of vacancy formation and atom migration, and the vibrational entropy of vacancy formation as well as the effective frequency, are evaluated with the local density approximation (LDA) and generalized gradient approximation (GGA) in first-principles calculations. These computed quantities are then utilized to calculate the self-diffusion coefficient of fcc Mg. For comparison, four widely used semi-empirical equations are also used to estimate the self-diffusion coefficient of fcc Mg. The comparisons show that first-principles calculations and semi-empirical equations yield values close to the activation energy Q for self-diffusion of fcc Mg from the LDA calculation, but the diffusion prefactor D ₀ predicted from the four semi-empirical equations are all about one order of magnitude larger than those from the first-principles calculations. Based on the comparison for the self-diffusion coefficients of fcc Al computed with the first-principles method and semi-empirical approaches, it is concluded that the self-diffusion coefficient of fcc Mg calculated with the LDA method is more accurate than the estimation from semi-empirical approaches.     

Simultaneous Measurement of Isotope-Free Tracer Diffusion Coefficients and Interdiffusion Coefficients in the Cu-Ni System

A new formalism recently developed by Belova et al., based on linear response theory combined with the Boltzmann–Matano method, allows determination of tracer and interdiffusion coefficients simultaneously from a single, isotope-free, traditional diffusion couple experiment. An experimental methodology with an analytical approach based on the new formalism has been carried out using the model Cu-Ni system to effectively determine tracer diffusion coefficients from an isotope-free diffusion couple experiment. Cu thin films were deposited in between several binary diffusion couples with varying terminal alloy compositions (Cu-25Ni, Cu-50Ni, Cu-75Ni, Ni). Diffusion couples were annealed at 800, 900 and 1000 °C, and the superimposed concentration profiles of thin film and interdiffusion were analyzed for the simultaneous determination of tracer and interdiffusion coefficients. Processed concentration profiles obtained from the diffusion experiments were also fitted with simple Gaussian distribution function. Results were compared to existing literature data obtained independently by radiotracer experiments, and an excellent agreement has been observed.     

Characterization and Diffusion Kinetics of borided Ni–Mg Alloys

Protection of Metals and Physical Chemistry of Surfaces - In this study, a mixture of 93% Ni–7% Mg and 97% Ni–3% Mg powders were produced by the TM method and the diffusion kinetics,...     

LF6铝合金的超塑性和扩散连接的组合工艺

研究了ＬＦ６铝合金的超塑性和扩散连接组合工艺，试验结果表明；预先采用变形再结晶的方法细化材料的晶，并有效地去除铝合金的致密氧化膜，即可成功地实现铝合金的ＳＰＦ－ＤＢ组合工艺。利用电子探针从微观观察了扩散连接接头的界面扩散行为，并首次从机理上分析珥金属的超塑性和扩散连接两种工艺之间的内部联系。     

压铸镁合金在曲轴箱上应用的材料替代研究

以摩托车曲轴箱为例,对铸造镁合金在摩托车上应用中的材料替代技术和方法进行了系统研究.研究结果表明,通过铸件服役工况及失效形式分析,铸件服役模型建立及服役状态分析,以铸件本体材料使用性能为基础的结构再设计,铸件生产工艺虚拟现实及优化,镁合金样件生产及使用性能检测等步骤,可以合理实现镁合金铸件在摩托车上应用中的材料替代.     

Self-Diffusion in Grain Boundaries and Dislocation Pipes in Al,Fe, and Ni and Application to AlN Precipitation in Steel

Diffusion along microstructural defects, such as grain boundaries or dislocation pipes, is significantly faster than diffusion through an undisturbed crystal. The ratio of diffusion enhancement is 3-4 orders of magnitude close to the melting point and reaches up to several ten orders of magnitude close to room temperature. An assessment of literature shows a large scatter in the available data and emphasizes the need for representative mean values. Applying a least mean square fit to selected experimental information delivers temperature-dependent functions for the ratio of grain boundary and dislocation pipe to bulk diffusion, respectively. We demonstrate that application of the attained results in a computational framework for the kinetics of precipitation makes the predictive simulation possible for the evolution of particles located at dislocations and grain boundaries.     

炭素回转窑内物料停留时间的实验研究

建立了回转窑实验装置,通过冷态实验研究了回转窑转速、倾角、内径、窑尾挡料圈高度等操作参数和结构参数对物料停留时间的影响。结果表明,物料在回转窑内的停留时间分布近似正态分布;物料停留时间随着回转窑转速、倾角和内径的增大而缩短;窑尾挡料圈高度对物料停留时间影响较复杂,转速小于2.5 r/min时,窑尾挡料圈高度的增加使物料停留时间延长,而当转速大于2.5 r/min时,窑尾挡料圈高度的增加反而使物料停留时间缩短;在某一固定回转窑倾角时,物料停留时间与物料质量流率的乘积与单位转速内物料质量流率近似成线性关系。     

Self-Diffusion of Iron in Molten Fe-C Alloys

Self-diffusion coefficients of iron in molten Fe-C alloys have been measured by using the capillary method. In addition, the samples have been autoradiographed and sectioned to insure that no significant convection has occurred during the diffusion. The results can be represented by the equation D = 4.3×10^?3 exp (—12.200/RT) for carbon = 4.6 pct and T = 1513° to 1633°K; and D = 1.0×10^?2 exp (—15,700/RT) for carbon = 2.5 pct and T = 1613° to 1673°K. The D values are higher and the heat of activation for diffusion lower in alloys containing more carbon. Calculation based on the Einstein-Stokes equation indicates that the diffusing species is iron ion.     

Magnaplate“协和”涂层及其在镁合金上的应用

简要介绍了协和作用和协和涂层的概念。介绍了Magnaplate公司的几种协和涂层的性能及其应用。根据Magnaplate公司的协和涂层制备方法简要说明了在镁合金上制备协和涂层的研究结果:在镁合金上先进行表面氧化处理以获得一层含有孔隙的氧化膜层,继而可控制地注入选定的工程聚合物,经交联互锁固化处理后即可获得耐中心盐雾试验超过1500 h的协和涂层。     

纯Mg扩渗合金层的冲刷磨损行为

对经过表面扩渗合金化处理的纯Mg试样进行了不同工况条件下的冲刷磨损试验,研究了不同冲刷介质条件中扩渗层的磨损特性.结果表明,纯Mg经表面扩渗处理后,其表面合金层显微硬度(HV)高达166,比基体显微硬度(HV)的73提高1倍以上;扩渗处理后的试样经介质1(油+石英砂)和介质2(水+石英砂)冲刷磨损,结果表明在两种介质条件下,纯Mg扩渗合金化试样的耐腐蚀磨损性能比未处理试样均有不同程度的提高.     

Molecular Dynamics Simulations and Experimental Investigations of Atomic Diffusion Behavior at Bonding Interface in an Explosively Welded Al/Mg Alloy Composite Plate

In this study, 6061 aluminum alloy and AZ31 B magnesium alloy composite plate was fabricated through explosive welding. Molecular dynamics(MD) simulations were conducted to investigate atomic diffusion behavior at bonding interface in the AI/Mg composite plate. Corresponding experiments were conducted to validate the simulation results. The results show that diffusion coefficient of Mg atom is larger than that of A1 atom and the difference between these two coefficients becomes smaller with increasing collision velocity. The diffusion coefficient was found to depend on collision velocity and angle. It increases linearly with collision velocity when the collision angle is maintained constant at 10° and decreases linearly with collision angle when the collision velocity is maintained constantly at 440 m/s. Based on our MD simulation results and Fick's second law, a mathematical formula to calculate the thickness of diffusion layer was proposed and its validity was verified by relevant experiments. Transmission electron microscopy and energy-dispersive system were also used to investigate the atomic diffusion behavior at the bonding interface in the explosively welded 6061/AZ31B composite plate. The results show that there were obvious Al and Mg atom diffusion at the bonding interface,and the diffusion of magnesium atoms from magnesium alloy plate to aluminum alloy plate occurs much faster than the diffusion of aluminum atoms to the magnesium alloy plate. These findings from the current study can help to optimize the explosive welding process.     

镁与镁合金广阔的应用前景

镁及其合金是迄今在工程中应用的最轻的金属结构材料,和其他金属相比,镁及其合金有很多突出的工艺及性能特点.例如,其密度小,是所有结构金属中密度最小的一种;在镁中加入不同的金属元素能够形成一系列具有各种性能的合金;熔点低,容易进行热成形,且能耗较低;用作结构件时具有质量轻、比强度高、减震和抗冲击性能好、易切削加工、不易老化;有良好的导热性、无磁、电磁屏蔽能力强、在受到冲击和摩擦时不会产生火花、阳极氧化着色能力强;镁粉在燃烧时可放出耀眼的白光并产生大量的热量;还原性强,可用于提取活性金属等.鉴于这些特点,镁及其合金很早就得到了应用,并且其应用范围由航空航天及军工领域等传统行业逐步发展,走进了千家万户,广泛应用于冶金工业、交通运输领域、3C产品、电化学、医药等行业.     

镁铝异种材料TIG焊接接头扩散行为分析

利用现代分析测试手段分别研究了TIG焊直接焊接镁铝和镀锡焊接镁铝的焊缝熔合区形貌,并对其进行了元素扩散行为的分析及比较。结果表明镁铝直接焊接时,镁与铝之间有明显的界面,镁元素向铝母材中进行大量扩散形成扩散层,而铝元素向镁母材中的扩散很少,扩散到铝母材中的镁元素与铝形成Mg17,Al126和β—AlMg金属间化合物,造成扩散层很容易发生断裂。镁铝镀锡焊接,采用熔焊一钎焊连接方法,中间过渡金属锡与母材之间形成了一个成分、组织和性能均不相同的过渡区,该区对接头性能有着较大的影响。锡层对镁铝接头产生拘束强化作用,过渡层比镁铝直接焊接要狭窄,并且尚余一些没有扩散的锡层,阻止了镁元素向铝母材中扩散。锡过渡层对镁与铝的直接接触起到了一定的阻碍作用。     

A Modified Model to Predict Self-Diffusion Coefficients in Metastable fcc, bcc and hcp Structures

A modified model was proposed to predict the self-diffusion coefficient (D) in the fcc, bcc and hcp structures with the inputs of the lattice parameter and melting point, which are determinable from first-principle calculations and calculation of phase diagram (CALPHAD) method, respectively. The anisotropy of diffusion coefficient in the hcp structure and the distinction of valence in different structures are considered in the present work. As examples, self-diffusion coefficients of Al, Nb, Zn, Au, Ag, Cr, Mo, Ru, Tc and Sn with the fcc, bcc and hcp structures were calculated, and the influencing factors to diffusion coefficient were also discussed.     

Comparison of Techniques in a Study of Zinc Self-Diffusion

Self-diffusion in zinc has been used as an instrument for comparison of the absorption and sectioning techniques as a means of studying diffusion. Single crystal as well as poly-crystal samples were used and the temperature range of diffusion extended from 200° to 415°C. For temperatures above 200°C, the data indicate that the results obtained from the absorption technique agree with those obtained from the sectioning technique. The effect on the values of the diffusion coefficient of electroplating vs evaporation as a means of applying the tracer was investigated and no significant difference observed. It was found that an excess or deficiency of tracer did not materially affect the results obtained from the sectioning technique, but invariably caused errors with the absorption technique.     

镁植入动物体内的短期效果

研究了纯镁及表面改性纯镁样品植入动物骨组织后在镁-骨界面处的降解机制以及材料对周围骨组织的影响.选择纯系日本大耳白兔,于股骨处植入样品.术后1、5周后,取出样品制作半薄切片,进行光学显微镜、扫描电镜观察.研究发现,镁棒周围新生类骨组织沉积速度很快,说明镁的腐蚀降解并没有影响骨组织修复过程.结果表明,镁在降解期间安全性较高,有较好的组织相容性.     

日本镁合金的应用与研究现状

综述了日本在汽车、“3C”产品和摩托车上镁合金的应用现状，并通过对1991年至2003年间日本国内申请的镁合金专利技术构成的分析，试图揭示日本镁合金产业的现状、研究动向和发展趋势。另外，还介绍了日本最新的镁合金回收技术。     

扩散偶制备技术发展及其应用

针对在材料科研领域已得到广泛应用的扩散偶技术,总结了扩散偶技术的特征、发展和优势,介绍了传统制备扩散偶的多种方法,以及当前较为新颖的放电等离子烧结法、激光成型、薄膜法、多元扩散节等制备高通量扩散偶的技术路线和相关技术。在此基础上总结了扩散偶技术在相图测定、扩散动力学数据测定、钎焊性能研究及高通量制备技术方面的应用。最后,结合贵金属材料体系的特点对贵金属扩散偶制备的工艺提出了具体的要求及建议。