Strain induced grain boundary premelting in bulk copper bicrystals

In bulk bicrystals strain induced grain boundary premelting (SIGBPM) occurs when heavy screw dislocation pileup can be held up to a certain high temperature, approximately 0.6T _M, where T _M is the melting point of bulk material in Kelvin. SIGBPM occurs at grain boundaries to which new twist component is added due to the rotation of both component crystals toward opposite direction about the axis perpendicular to the grain boundary plane. At the original grain boundary, grain boundary sliding takes place due to this relative rotation. In f.c.c. metals with relatively low stacking fault energies such as copper, nickel, brass(30Zn) and silver, dislocations dissociate into partials. Therefore high density tangled dislocations introduced during plastic deformation hardly loose. If these dislocations can be held to high temperatures, SIGBPM is promoted. Formation of static or dynamic recrystallized grains suppresses SIGBPM itself and the propagation of grain boundary cracks formed by SIGBPM.     

Correlation among grain boundary character,carbide precipitation and deformation in Alloy 690

The correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715^oC for 15?h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.     

Phase-field study of spinodal decomposition under effect of grain boundary

Grain boundary directed spinodal decomposition has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The morphological selection mechanism of spinodal decomposition at grain boundaries is a major challenge to reveal, and remains elusive so far. In this work, the effect of grain boundaries on spinodal decomposition is investigated by using the phase-field model. The simulation results indicate that the spinodal morphology at the grain boundary is anisotropic bicontinuous microstructures different from the isotropic continuous microstructures of spinodal decomposition in the bulk phase. Moreover, at grain boundaries with higher energy, the decomposed phases are alternating α/β layers that are parallel to the grain boundary. On the contrary, alternating α/β layers are perpendicular to the grain boundary.     

Grain boundary misorientation changes during grain growth in pure aluminium

A new method is described for data-logging large amounts of grain boundary misorientation information from channelling patterns in the scanning electron microscope (SEM). The method relies on producing specimens where the grain size is larger than the specimen thickness and where the grain boundary planes are perpendicular to the specimen plane (the so-called columnar structure). Results for grain growth in pure aluminium at 460 and 500°C are presented. There is an increase in the proportion of low angle boundaries at the expense of high angle boundaries during growth times of up to a few hours. The reasons are thought to be partly connected with lower low angle boundary mobility compared with high angle boundaries. However, the growth kinetics appear to be normal over the entire growth time range.     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

Grain boundary interdiffusion in the case of concentration-dependent grain boundary diffusion coefficient

The grain boundary diffusion in a binary system which exhibits a grain boundary phase transition is considered in the framework of Fisher's model. The kinetic law of the growth of the grain boundary phase and the distribution of the diffusant near the grain boundary are calculated. The method of determining of the concentration dependence of the grain boundary diffusion coefficient from the experimentally measured penetration profiles of the diffusant along the grain boundaries is suggested. The experimental results on Zn diffusion in Fe(Si) bicrystals, Ni diffusion in Cu bicrystals and grain boundary grooving in Al in the presence of liquid In are discussed in light of the suggested model.     

Effect of solute atoms on grain boundary sliding in magnesium alloys

The effect of solid-solution alloying on grain boundary sliding (GBS) was investigated using pure magnesium and six kinds of Mg–X (X?=?Ag, Al, Li, Pb, Y and Zn) dilute binary solid solutions with an average grain size of 10?µm. A sharp increase in damping capacity caused by GBS was observed above a certain temperature. The temperature at which a sharp increase in damping capacity occurred depended on the alloying element. The addition of Y and Ag markedly increased the onset temperature (more than 100?K) for a sharp increase in damping capacity, whereas the addition of Zn, Al and Li slightly increased the onset temperature (less than 50?K) as compared with that for pure magnesium. Tensile tests at a temperature of 423?K revealed that the higher the onset temperature, the lower the strain rate sensitivity of the flow stress. It is suggested that the former elements (Y and Ag) are more effective in suppressing GBS in magnesium alloys than the latter ones (Zn, Al and Li). The suppression of GBS was associated with low grain boundary energy, and the extent to which the energy is reduced depended on the alloying element. It was suggested that the change in the lattice parameter (the so-called c/a ratio) affects the grain boundary energy, and thus, the occurrence of GBS.     

Theory of grain boundary motion during high-temperature DIGM

A theory of diffusion induced grain boundary migration (DIGM) is presented for high temperatures where volume diffusion of solute atoms out of the grain boundary is important. It is shown that due to the presence of a gradient term in the expression for the free energy of solid solution, even a relatively small discontinuity in the solute distribution across the gain boundary provides enough driving force for grain boundary migration. From the expression obtained for the grain boundary velocity the coefficient for the Ni diffusion across the grain boundaries in a Cu(Ni) polycrystal has been estimated.     

包含倾斜晶界的双晶ZnO的热输运行为

用嵌入位错线法和重合位置点阵法构建含有小角度和大角度倾斜角的双晶氧化锌纳米结构.用非平衡分子动力学方法模拟双晶氧化锌在不同倾斜角度下的晶界能、卡皮查热阻,并研究了样本长度和温度对卡皮查热阻和热导率的影响.模拟结果表明,晶界能在小角度区域随倾斜角线性增加,而在大角度区域达到稳定,与卡皮查热阻的变化趋势一致.热导率随样本长度的增加而增加,卡皮查热阻表现出相反的趋势.然而随着温度的增加,热导率和卡皮查热阻都减小.通过比较含5.45°和38.94°晶界样本的声子态密度,发现声子光学支对热传导的影响不大,主要由声子声学支贡献,大角度晶界对声子散射作用更强,声学支波峰向低频率移动.     

Theory and simulation of coupled grain boundary migration and grain rotation in low angle grain boundaries

Abstract

Microstructure evolution due to coupled grain boundary migration and grain rotation in low angle grain boundaries is studied through a combination of molecular dynamics and phase field modeling. We have performed two dimensional molecular dynamics simulations on a bicrystal with a circular grain embedded in a larger grain. Both size and orientation of the embedded grain are observed to evolve with time. The shrinking embedded grain is observed to have two regimes: constant dislocation density on the grain boundary followed by constant rate of increase in dislocation density. Based on these observations from the molecular dynamics simulations, a theoretical formulation of the kinetics of coupled grain rotation is developed. The grain rotation rate is derived for the two regimes of constant dislocation density and constant rate of change of dislocation density on the grain boundary during evolution. The theoretical calculation of the grain rotation rate shows strong dependence on the grain size and compares very well with the molecular dynamics simulations. A multi-order parameter based phase field model with coupled grain rotation is developed using the theoretical formulation to model polycrystalline microstructure evolution.     

Simulation of grain boundary effect on characteristics of ZnO thin film transistor by considering the location and orientation of grain boundary

The grain boundaries (GBs) have a strong effect on the electric properties of ZnO thin film transistors (TFTs). A novel grain boundary model was developed to analyse the effect. The model was characterized with different angles between the orientation of the grain boundary and the channel direction. The potential barriers formed by the grain boundaries increase with the increase of the grain boundary angle, so the degradation of the transistor characteristics increases. When a grain boundary is close to the drain edge, the potential barrier height reduces, so the electric properties were improved.     

高温塑性变形引起的P非平衡晶界偏聚

阐述了高温塑性变形引起的非平衡晶界偏聚的准热力学和动力学,并使用该模型预测了低合金结构钢中高温塑性变形导致的P在奥氏体晶界的非平衡偏聚.研究发现：当变形为20%,应变速率为1×10^-3 s^-1时,在800 ℃左右会出现一个P的晶界偏聚浓度峰值;在1000 ℃变形为20％时,晶界偏聚浓度随着应变速率的增加而增加.预测结果与现有的实验结果基本一致. 关键词： 非平衡偏聚 晶界 塑性变形 磷     

AES/STEM grain boundary analysis of stabilized zirconia ceramics

Semiquantitative Auger Electron Spectroscopy (AES) on pure monophasic (ZrO₂)_0.83(YO_1.5)_0.17 was used to determine the chemical composition of the grain boundaries. Grain boundary enrichment with Y was observed with an enrichment factor of about 1.5. The difference in activation energy of the ionic conductivity of the grain boundary compared with the bulk can be explained by the Y segregation.When Bi₂O₃ is introduced into this material and second phase appears along the grain boundaries of the cubic main phase. Energy dispersive X-ray analysis (EDS) on a scanning transmission electron microscope (STEM) shows an enrichment of bismuth at the grain boundaries of this second phase.     

Grain boundary potential and charge density at grain boundaries of chemically prepared thin silver sulfide films

The grain boundary potential and interface state charge density at the grain boundaries of silver sulfide (Ag₂S) thin films prepared by chemical conversion of cadmium sulfide (CdS) films have been determined from the dc resistance of the material and are found to be sensitive to annealing. A reduction in the grain boundary potential and the grain boundary charge density of the film has been noticed when the source CdS film is annealed at different temperatures prior to chemical conversion. The variation in the grain boundary charge density of the grown Ag₂S film with source annealing temperature has been found to be similar to that of thin cadmium sulfide film, reported earlier. An additional low temperature heat treatment of the sample results in an enhancement in the charge density at the grain boundaries. The change in the silver vacancy and/or oxygen and sulfur content of the films as revealed from the energy dispersive spectra of the films suggests possible role of film composition on the grain boundary charge density.     

Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel. The simulation results suggest that the sliding of the {001}/{110} type and {110}/{111} type grain boundary can improve the impact toughness. Then, the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding. Finally, the sliding of the grain boundary is analyzed from the standpoint of the energy. We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.     

Anisotropic rheology during grain boundary diffusion creep and its relation to grain rotation,grain boundary sliding and superplasticity

The response of periodic microstructures to deformation can be analysed rigorously and this provides guidance in understanding more complex microstructures. When deforming by diffusion creep accompanied by sliding, irregular hexagons are shown to be anisotropic in their rheology. Analytic solutions are derived in which grain rotation is a key aspect of the deformation. If grain boundaries cannot support shear stress, the polycrystal viscosity is extremely anisotropic. There are two orthogonal directions of zero strength: sliding and rotation cooperate to allow strain parallel to these directions to be accomplished without any dissolution or plating. When a linear velocity/shear stress relationship is introduced for grain boundaries, the anisotropy is less extreme, but two weak directions still exist along which polycrystal strength is controlled only by the grain boundary “viscosity”. Irregular hexagons are characterised by four parameters. A particular subset of hexagons defined by two parameters, which includes regular hexagons as well as some elongate shapes, shows singular behaviour. Grain shapes that are close to that of the subset may exhibit large grain rotation rates and have no well-defined rheology unless there is a finite grain boundary viscosity. This new analysis explains why microstructures based on irregular but near equiaxed grains show high rotation rates during diffusion creep and it provides a framework for understanding strength anisotropy during diffusion creep.     

Thermodynamics and structural aspects of grain boundary segregation

Physical and chemical properties of solid materials are strongly. influenced by the chemical composition of internal interfaces, One of the crucial parameters affecting interfacial chemistry is the atomic structure of the interface. Due to its importance. a considerable amount of work was done to elucidate the relationship between structure and chemical composition of interfaces. This article reviews the present understanding of an important and fundamental part of this relationship, namely, the structural aspects of grain boundary segregation. After a brief outline of grain boundary structure and geometry. thermodynamic approaches to describe grain boundary segregation are summarized and their application to materials is discussed. covering particular sites at a single grain boundary as well as the role of interfaces in polycrystals. Both the experimental evidence of grain boundary segregation anisotropy and the theoretical results of computer simulations of grain boundary segregation are summarized. Useful methods of predicting grain boundary segregation are presented. Finally, segregation behavior of solutes at grain boundaries is compared with that at free surfaces, and examples of chemical composition of intexphase boundaries are given.     

Modelling of grain refinement driven by negative grain boundary energy

Abstract

Grain refinement can be described by the classical kinetic equation using a negative value of the specific grain boundary Gibbs energy. A respective overview is offered reporting according observations and simulations, particularly linked to grain boundary segregation. Classical grain growth model is used in the treatment of evolution of the distribution function during refinement. The adapted model requires defining nucleation rate of new grains, which significantly influences the kinetics of the system of grains. Moreover, a jump in the distribution function is allowed at a certain value of the grain radius R_J, which separates old grains from newly nucleated ones. Evolution equation for both the critical radius R_c and separation radius R_J (jump position) as well as for the dimension-free distribution (shape) function are derived. Illustrative examples for the evolution of the system parameters under various nucleation rates of newly generated grains are presented.     

多晶材料晶粒生长粗化过程的相场方法模拟

基于采用晶体有序化程度参量ψ和晶体学取向θ来表示多晶粒结构的相场模型,利用自适应有限元方法模拟了多晶材料等温过程中的晶粒粗化现象.模拟结果显示,在曲率作用下,通过晶界迁移弯曲晶界逐渐平直化,小晶粒逐渐被大晶粒吞并,当晶界之间的取向差较小时,满足一定能量和几何条件的两晶粒在界面能作用下会发生转动,合并为单个晶粒.模拟结果与实验结果符合较好.因此,该相场模型可以很好地用来模拟固态相变中多晶材料的生长粗化等现象. 关键词： 相场 晶界迁移 晶粒转动 粗化