Interaction of Solute Impurity with Grain Boundary: The Impurity Drag Effect

An equation of grain boundary motion in a binary polycrystal is derived. The derivation is based on minimization of free energy of the total systems. The equation takes into account an impurity segregation at the grain boundary, grain boundary curvature and energy.As an example, we apply this equation to the analysis of the impurity drag effect problem. It is shown, that the sign of the impurity effect on grain boundary velocity (delay or acceleration) does not depend on kinetic coefficients. The sign of the effect is determined by a thermodynamic function which combines the grain boundary segregation coefficient, the derivative of grain boundary energy with respect to absorbed impurity concentration, and the derivative of bulk free energy with respect to bulk impurity concentration.     

Atomic and electronic structures of a grain boundary in iron with impurity segregation

We present a short review on our current investigations of the atomic and electronic structures of a grain boundary in iron. Atomic structures of grain boundaries were simulated and the local electronic densities of states were calculated in the simulated structure. When phosphorus impurity atoms segregated at the grain boundaries in iron, trigonal prismatic FeP clusters were formed. Segregated boron atoms tended to stay at the central site of polyhedra constructed by host atoms in the grain boundaries. The non-bonding states of the iron atom at the grain boundary disappear by forming a strong bonding orbital with the orbital of the segregated impurity atom. This bonding orbital is formed in a Fe3d host band in the case of a boron impurity. On the other hand, the bonding orbital is formed at lower energies for the phosphorus impurity and is less-mixed with the Fe3d host band. Non-bonding states are formed around the Fe₉P clusters. These can give a qualitative explanation for the embrittlement of the impurity segregated grain boundary. Finally, we can explain from the viewpoint of the electronic structure why the interstitial impurity is the only cohesive enhancer.     

Elastic fields around a nanosized elliptic hole in decagonal quasicrystals

Based on the variational principle, a continuum theory of surface elasticity and new boundary conditions for qua- sicrystals is proposed. The effect of the residual surface stress on a decagonal quasicrystal that is weakened by a nanoscale elliptical hole is considered. The explicit expressions for the hoop stress along the edge of the hole are obtained using the Stroh formalism. The results show that the residual surface stress and the shape of the hole have a significant effect on the elastic state around the hole.     

Statistical description of the motion of dislocation kinks in a random field of impurities adsorbed by a dislocation

A model has been proposed for describing the influence of impurities adsorbed by dislocation cores on the mobility of dislocation kinks in materials with a high crystalline relief (Peierls barriers). The delay time spectrum of kinks at statistical fluctuations of the impurity density has been calculated for a sufficiently high energy of interaction between impurities and dislocations when the migration potential is not reduced to a random Gaussian potential. It has been shown that fluctuations in the impurity distribution substantially change the character of the migration of dislocation kinks due to the slow decrease in the probability of long delay times. The dependences of the position of the boundary of the dynamic phase transition to a sublinear drift of kinks x ∝ t^δ (δ σ 1) and the characteristics of the anomalous mobility on the physical parameters (stress, impurity concentration, experimental temperature, etc.) have been calculated.     

Colloid-Facilitated Impurity Leaching in Statistically Homogeneous Dual-Porosity Media

A model for the leaching of an impurity from a statistically homogeneous dual-porosity medium is proposed. The region of partial leaching grows according to a diffusion law on moderate time scales. A region completely cleaned of the impurity whose boundary moves with a constant velocity arises on long time scales. A transition region is formed at the leaching front. Its size increases proportionally to the square root of time. The possibility of a significant acceleration of the leaching process by the introduction of colloidal particles capable of adsorbing the impurity into the fluid is shown.     

实验物理与工业控制系统平台在HL-2A主机测控系统中的应用

描述了利用实验物理与工业控制系统(EPICS)平台对HL-2A主机运行参数进行的集中测控系统开发。系统采用s7nodave设备驱动模块,实现了SoftIOC与各PLC的通讯,将各子系统的PLC集成到了EPICS控制系统中。通过对关系型数据库和应用层软件CSS中数据归档及报警等组件的扩展与配置,实现OPI层与各个子系统的实时通信。测控系统成功地将HL-2A主机参数集中在EPICS平台下运行,为下一代装置的主机集中测控系统设计打下了基础。     

HL-2A等离子体边缘极向剩余胁强的研究

首次报导了托卡马克等离子体边缘与湍流相关的极向剩余胁强剖面的测量结果。采用外中平面往复式静电探针阵列对HL-2A托卡马克边缘的极向湍流动量输运进行研究。在没有外部动量注入的欧姆放电下,剩余胁强为有限值、且其空间剖面在等离子体边缘具有明显的径向梯度,表明托卡马克等离子体边缘存在极向动量源。由动量源产生的动量主要以扩散形式向与剩余胁强相反的方向传播,最终的结果是等离子体边缘存在有限的雷诺胁强。在最后闭合磁面以内0.5~2cm区域,剩余胁强的梯度提供自发旋转的力矩,由该力矩引起的动量产生与由速度梯度引起的动量扩散共同导致了雷诺胁强出现负梯度,造成动量沉积,从而驱动极向平衡流。     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

Two hardening mechanisms in single crystal thin films studied by discrete dislocation plasticity

Two-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate are used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling is analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density is found close to the impenetrable film-substrate interface. There is a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state are independent of film thickness. In such films the stress outside the boundary layer is on average very low, so that the film-thickness-independent boundary layer is responsible for the size effect. A larger size effect is found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreases while the stress in the film increases. The size dependence is only described by a Hall–Petch type relation for films thicker than the threshold value.     

The Effect of Impurities in a Three-Qubit Ising Spin Model

The dynamics of genuine three-qubit entanglement in Ising spin model is investigated under the effect of impurities with different sites. The analytic expression for the three-tangle measurement is obtained. Results show that the impurity parameters exert great effect on the entanglement value. When the impurity on the boundary sit of the spin chain, the positive contribution of the impurity coefficient on the exchange interaction is greater, the stay time in the greater entanglement is longer. When the impurity sitting in the spin chain, the entanglement variation is periodic with the evolution time only at some given impurity coefficient. The realization of efficient control of the entanglement by appropriate combinations of the tunable parameters should be possible.     

Analyses of edge plasma characteristics in HL-2A

The edge plasma characteristics are studied by both a movable array of Mach/Reynolds stress/Langmuir 10-probes in the boundary region and the fixed flush probe arrays on the 4 divertor neutralization plates at the same toroidal cross-section in the HL-2A tokamak. The dependence of the Reynolds stress on poloidal flow in the edge plasma is analysed. The result indicates that the sheared poloidal flow in tokamak plasma can be induced by the radial gradient of Reynolds stress. In the divertor experiments of HL-2A, the profiles of the electron temperature, density and floating potential on divertor plates are measured by the flush probe arrays. The edge electron temperature in divertor configuration is higher than that in limiter configuration. The temperature asymmetry between outer and inner target plates is observed. The result of magnetic surface reconstructed from 18 Mirnov coils signals is presented. Both the particle recycling and the impurity flux in the bulk plasma during divertor discharges are discussed. Neutral gas pressure in divertor chamber, measured by fast ionization gauge during divertor discharge, is given.     

Morphological stability of two-dimensional new phase precipitate located at the grain boundary

The diffusion growth of two-dimensional new phase precipitate located at the grain boundary of a finite thickness is considered taking into account diffusion fluxes of impurity atoms from the bulk of a grain depth to the grain boundary. The stability of the growing precipitate of a relatively small radius to small random perturbations of its shape is analyzed taking into account the surface kinetics of the impurity atoms on the grain boundary-precipitate interface. The threshold radii of the precipitate are found, above which the amplitude of the shape distortion and the amplitude of the relative shape deformation of the precipitate rise.     

Millimetric response of stressed gallium doped germanium photoconductor

Conclusion Conventional Ge:Ga photoconductors presents a residual responsivity at millimetric wavelengths, well below the cut-off wavelength due to the gallium impurity ionization energy. At constant bias voltage, this millimetric responsivity is greatly enhanced when a strong uniaxial stress is applied. We suggest that this response is mainly due to a change in the free hole lifetime when the hole gas is heated by the absorbed microwave photons. This photoconductive process has a shorter response time than the InSb Putley mechanism. Thus optimization of this millimetric Ge:Ga detector could find an application for example for heterodyne detection with larger spectral width than InSb.     

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given te...     

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time. Fiz. Tverd. Tela (St. Petersburg) 40, 251–253 (February 1998)     

Charge Transfer in the Lattice with an Impurity Site. Reflection and Transmission of the Wave Packet

The quantum dynamics of the propagation of the charge wave function in a uniform lattice containing a single impurity site is considered. A nonstationary problem is solved in the tight-binding approximation. The initial state is the wave function fully localized at one of the lattice sites. The coefficients of transmission of the wave packet through the impurity site and reflections from it are calculated as a function of the parameters of the problem, that is, the additional energy on the impurity and the distance between the impurity and the initial position of the charge. The problem is solved for two types of boundary conditions: an infinite and a semi-infinite lattice. Good agreement with numerical simulation is obtained.     

The effect of intragranular microstress in Al₂O₃-SiC nanocomposites

A theoretical model is established to investigate the intragranular particle residual stress in Al2O3-SiC nanocomposites.Using this model,we calculate the average compressive stress on the Al2O3 grain boundary(GB) and the average tensile stress within Al2O3 grains caused by SiC nanoparticles.The normal compressive stress strengthens the GB,and the average tensile stress weakens the grains.The model gives a reasonable interpretation of the strength changes of Al2O3-SiC nanocomposites with the number of SiC particles.     

Co-Segregation Effects on Boundary Migration

We analyze the effect of co-segregation on the mobility of grain boundaries within the framework of the impurity drag theory originally proposed by Cahn and Lücke and Stüwe for an ideal solution. The new derivation extends this model to the case where there are two types of impurities (or three components in the alloy). Since the resultant expression for the boundary mobility is complicated, numerical solutions were obtained for several cases to show how co-segregation affects the boundary mobility. Depending on the relative diffusivities of the two impurities which are both attracted to the boundary, the mobility may either increase or decrease with increasing concentration of one of the impurities. When one of the impurities is attracted to the boundary and the other repelled from the boundary, increasing the concentration of the attractive impurity can lead to a sharp decrease in the boundary mobility.     

杂质离子对等离子体边界参量的影响

采用一维流体模型研究了含有杂质离子的等离子体与器壁材料相互作用给边界等离子体参量带来的影响.通过数值模拟,研究了分别选用碳和钨作为器壁材料时,器壁温度不同情形下热发射产生的电子对等离子体器壁电势、电场强度、热发射电子流以及沉积器壁离子动能流的影响.研究结果发现,当面向等离子体材料表面温度升高时,器壁电势和热发射产生的电流将增加,器壁电场强度和离子沉积器壁动能流则会减小,并且钨作为器壁材料要比碳作为器壁材料对于等离子体边界参量影响更明显.此外,研究了钨作为器壁材料时,碳杂质离子（浓度和电荷数）对等离子体器壁参量的影响.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)