Abnormal grain growth in Ni-5at.%W

The growth of abnormally large grains in textured Ni-5at.%W substrates for high-temperature superconductors deteriorates the sharp texture of these materials and thus has to be avoided. Therefore the growth of abnormal grains is investigated and how it is influenced by the grain orientation and the annealing atmosphere. Texture measurements and grain growth simulations show that the grain orientation only matters so far that a high-angle grain boundary exists between an abnormally growing grain and the Cube-orientated matrix grains. The annealing atmosphere has a large influence on abnormal grain growth which is attributed to the differences in oxygen partial pressure.     

Quantitative analysis of WC grain shape in sintered WC-Co cemented carbides

The morphology of WC grains is examined by ab initio calculations, and by transmission electron microscopy in a WC-Co cemented carbide. Two compositions are studied to determine the effect of carbon chemical potential on the shape development. The WC grains are found to be truncated triangles, and the grain morphology is adequately reproduced by the ab initio calculations. The energy difference between the two types of grain facets is shown to be due to the formation of stronger bonds with more covalent character between low coordinated W and Co atoms at one type of facet.     

Abnormal reduction of coercivity on magnetic cobalt–platinum alloy films

Ultrathin Co–Pt alloy films as substrate were studied by the surface magneto-optical Kerr effect. As the growth of Ni, the films show uniquely high polar Kerr responses without any in-plane signals. The coercivity decreased until the thickness of Ni film was higher than 5 ML. A new surface structure was discovered at 7–10 ML Ni/Co–Pt films by the low-energy electron diffraction. Interestingly, polar Kerr signal and coercivity of the 10 ML Ni/Co–Pt(1 1 1) template film reduced rapidly as Co films were further deposited onto only about 1–2 ML. Then the films show a canted magnetization with a rollback hysteresis in the polar configuration during the growth of Co. Coercivity of the 7 ML Co/Ni/Co–Pt film was found unusually down to almost 100 Oe.The corresponding magic number at around 7 ML of Co in the abnormal reduction of coercivity may be attributed to the cluster formations of Co.     

Austenite grain growth kinetics and mechanism of grain growth in 12Cr ultra-super-critical rotor steel

ABSTRACT

In this paper, the austenite grain growth behaviour of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of heat treatments. The heat treatments at heating temperatures of 900°C–1250°C and holding time of 1?h–20?h were conducted in an electric box-type heating furnace. Experimental results showed that the sizes of austenite grain were affected by heating temperatures and holding time, and heating temperature was the dominant factor. In addition, the grain growth rate changed significantly before and after the turning points of 1050°C and 1250°C. Meanwhile, an austenite grain growth mathematical model was established at different heating temperature stages, and possession of the capability to accurately predict austenite grain size was confirmed. Furthermore, the microstructure of austenite grain in the heating process was observed by optical microscope (OM) and transmission electron microscopy (TEM), which revealed the mechanism of austenite grain growth. Analysis indicated that the change of quantity of precipitate particles with increasing heating temperature was the main reason for the difference in austenite grain growth.     

石墨炉原子吸收法测定粮食中的镉,钴,镍

本文详细研究了石墨炉子原子吸收法测定粮食中镉，镍的适宜条件，用于粮食样品中三元素的测定，结果令人满意。     

Scaling behavior of grain-rotation-induced grain growth

Recent investigations of grain growth in nanocrystalline materials have revealed a new growth mechanism: grain-rotation-induced grain coalescence. Based on a simple model employing a stochastic theory and using computer simulations, here we investigate the coarsening of a polycrystalline microstructure due solely to the grain-rotation coalescence mechanism. Our study demonstrates that this mechanism exhibits power-law growth with a universal scaling exponent. The value of this universal growth exponent is shown to depend on the assumed mechanism by which the grain rotations are accommodated.     

Effect of grain misorientation on the stripe domains in evaporated cobalt films

X-ray diffraction and magnetic force microscopy techniques were used to investigate the structural and the static magnetic properties of vapor-deposited cobalt films with various thicknesses t ranging from 50 to 195 nm. Texture measurements revealed that as the thickness increases, the films become predominantly c-axis oriented. Magnetic stripe domains structure was only observed for the thicker films, with t=195, 173 and 125 nm, while such a magnetic configuration was expected for all the samples based on the theoretical studies. Since the layers present increasing c-axis misorientation when the thickness decreases, we assume that this effect can prevent the stripe domains formation. This behavior is qualitatively explained by a simple model which describes the stripe domains structure taking into account the role of a small misorientation of the anisotropy axis.     

Grain boundary curvature and grain growth kinetics with particle pinning

Second-phase particles are used extensively in design of polycrystalline materials to control the grain size. According to Zener’s theory, a distribution of particles creates a pinning pressure on a moving grain boundary. As a result, a limiting grain size is observed, but the effect of pinning on the detail of grain growth kinetics is less known. The influence of the particles on the microstructure occurs in multiple length scales, established by particle radius and the grain size. In this article, we use a meso-scale phase-field model that simulates grain growth in the presence of a uniform pinning pressure. The curvature of the grain boundary network is measured to determine the driving pressure of grain growth in 2D and 3D systems. It was observed that the grain growth continues, even under conditions where the average driving pressure is smaller than the pinning pressure. The limiting grain size is reached when the maximum of driving pressure distribution in the structure is equal to the pinning pressure. This results in a limiting grain size, larger than the one predicted by conventional models, and further analysis shows consistency with experimental observations. A physical model is proposed for the kinetics of grain growth using parameters based on the curvature analysis of the grain boundaries. This model can describe the simulated grain growth kinetics.     

Mechanical grain growth in nanocrystalline copper

Nanograined materials have some unusual properties. To maintain the small size of the grains, grain growth should be avoided. But recently grain growth has been observed under an indenter at liquid-nitrogen temperatures. Such grain growth has never been reported before. How can this happen and how can it be prevented? These questions are answered here using a simple tilt boundary. It is found that high purity and nonequilibrium structure are necessary conditions for mechanical grain growth. The material must be pure enough so that free dislocations are available to move out of the boundary. But the boundary should not be in the lowest-energy state so that extra dislocations are available to be emitted by stress. Based on these conditions, methods can be devised to avoid low temperature grain growth.     

Strong electron tunneling through a small metallic grain

Electron tunneling through mesoscopic metallic grains can be treated perturbatively only under the conduction that the tunnel junction conductances are sufficiently small. If that is not the case, fluctuations of the grain charge become strong. As a result (i) the contributions of all—including high energy—charge states become important, and (ii) the excited charge states become broadened and essentially overlap. At the same time, the grain charge remains discrete and the system conductance e-periodically depends on the gate charge. We develop a non-perturbative approach which accounts for all these features and calculate the temperature-dependent conductance of the system in the strong tunneling regime at different values of the gate charge. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 953–958 (25 June 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Entangling operations and their implementation using a small amount of entanglement

We study when a physical operation can produce entanglement between two systems initially disentangled. The formalism we develop allows us to show that one can perform certain nonlocal operations with unit probability by performing local measurement on states that are weakly entangled.     

MacPherson-Srolovitz晶粒长大速率方程的仿真验证

2007年MacPherson和Srolovitz联合推导出一个三维个体晶粒长大的准确速率方程,但并未给出实验或计算机仿真的验证.采用Potts模型Monte Carlo方法对该速率方程进行了大尺度仿真验证.结果表明,仿真数据与MacPherson-Srolovitz速率方程符合很好,从而初步证实了该速率方程,即三维晶粒长大速率是晶粒棱长和晶粒平均宽度的函数. 关键词： 三维晶粒长大 速率方程 Monte Carlo仿真     

多晶材料晶粒生长的计算机模拟研究

多晶固体材料显微结构的演化与诸多因素密切相关 ,是一复杂的过程。运用适当的计算机模拟方法进行模拟 ,实现对晶粒生长的预测 ,可为材料研究提供新的方法和重要依据。此文以晶粒生长动力学为基础 ,建立晶粒生长模型 ;分析了晶粒生长过程的计算机模拟理论和方法 ;以MonteCarlo方法为基础 ,提出快速Q statepotts算法 ,简化计算量 ,在PC机上编程实现了正常晶粒生长过程的计算机模拟。得到的显微结构图逼真度较好。通过对结果进行分析处理 ,得到的生长因子为 0 .4 7。     

Two-dimensional normal grain growth: topological aspects

Normal grain growth in polycrystals is an important example of a capillarity driven coarsening phenomenon where topological structure of the system plays a major role. The process is practically important and attracts much interest, in particular in two-dimensional (2D) polycrystals because of the growing technological importance of thin polycrystalline films. In the present paper we discuss various approaches to normal grain growth in 2D polycrystals. We stay mostly within the framework of the uniform boundary model. This model provides a reasonable simplification leading to the Von Neumann-Mullins relation that relates the rate of growth of an individual grain to its local topology. Comparing different approaches—relatively simple mean-field theories, more sophisticated models incorporating real topology, and computer simulations adequately reproducing local equations of motion—we identify the principal factors responsible for different features of the phenomenon.     

Strain-induced grain growth in non-oriented electrical steels

The present work considers a method of columnar microstructure development in vacuum-degassed non-oriented electrical steels. Samples after temper rolling were annealed under special heat treatment conditions. The influence of the applied thermo-mechanical treatments on microstructure progress in the investigated non-oriented steels is studied. Columnar microstructure is obtained after combining the temper rolling and appropriate annealing conditions. The dependence of texture on the applied conditions was studied by EBSD facilities in the investigated material. It was confirmed that the obtained columnar microstructure possesses pronounced cube texture components.     

Nonequilibrium statistical mechanical formulation for grain growth

A nonequilibrium statistical mechanical theory for grain growth is presented in this paper. The fluctuation of grain growth is determined from the self similarity of grain size distribution. The grain size distribution scaled by the averaged size is derived under the condition that the volume of specimen is strictly conserved. The growth law of grain is also derived. The scaled grain size distribution are compared with the associating lognormal distribution which is usually used in the analysis of the experimental results. It has been found that the theoretical results is in good agreement with experimental results.     

Nonequilibrium statistical mechanical formulation for grain growth

A nonequilibrium statistical mechanical theory for grain growth is presented in this paper. The fluctuation of grain growth is determined from the self similarity of grain size distribution. The grain size distribution scaled by the averaged size is derived under the condition that the volume of specimen is strictly conserved. The growth law of grain is also derived. The scaled grain size distribution are compared with the associating lognormal distribution which is usually used in the analysis of the experimental results. It has been found that the theoretical results is in good agreement with experimental results.     

Recrystallization and grain growth of deformed magnesium alloy

Magnesium alloy AZ31 was cold deformed and then subjected to a wide range of annealing conditions to study the annealing response of the material. Recrystallization and grain growth kinetics during annealing were obtained from hardness testing and microstructural analysis. Results show that the annealing response of AZ31 is sensitive to the amount of deformation, temperature and annealing time, producing characteristic microstructures at each stage of the annealing process.