含有烧结助剂的复相陶瓷材料烧结过程的元胞自动机模拟

基于晶界能和晶界曲率的晶粒生长驱动力理论,建立了含有烧结助剂的复相陶瓷晶粒生长的元胞自动机模型并进行了模拟。结果表明,烧结助剂对晶界有着强烈的钉扎作用,其晶粒生长指数小于未含烧结助剂时的生长指数。模拟结果与制备的含有烧结助剂的Al2O3／TiN复相陶瓷材料微观形貌组织吻合,表明所建立的模型适用于含有烧结助剂的陶瓷材料烧...     

陶瓷材料烧结致密化过程的元胞自动机模拟

为研究陶瓷材料烧结致密化过程,以晶界能和晶界曲率生长驱动力理论为基础,建立了含有气孔的二相晶粒生长的元胞自动机模型,对陶瓷材料烧结致密化过程进行了模拟,并与制备的Al2O3/TiN陶瓷材料进行对比.结果表明,模型可有效地模拟陶瓷材料烧结时晶粒的生长及气孔的湮灭情况,能较好地再现烧结致密化过程,模拟结果与制备的陶瓷材料微观形貌组织十分接近.     

Computer-aided control of the evolution of microstructure during sintering

Control of the microstructure is crucial for realizing optimum properties in advanced sintered materials. We describe a mathematical model for solid state sintering of ceramic systems, which is based on a population balance paradigm for pore shrinkage and grain growth. It incorporates the interplay between densification and grain growth and tracks the trajectories of pore and grain size spectra in a reasonably realistic manner. The model is employed in a simulation mode to identify the effect of initial grain (particle) size distribution and to optimize the time–temperature sintering cycle based on our own work on sintering of composite alumina–zirconia. The utility of this approach for computer-aided control of the evolution of microstructure is discussed.     

Monte Carlo simulation of polycrystalline microstructures and finite element stress analysis

A two-dimensional numerical model of microstructural effects is presented, with an aim to understand the mechanical performance in polycrystalline materials. The microstructural calculations are firstly carried out on a square lattice by means of a 2-D Monte Carlo (MC) simulation for grain growth, then the conventional finite element method is applied to perform stress analysis of a plane strain problem. The mean grain size and the average stress are calculated during the MC evolution. The simulation result shows that the mean grain size increases with the simulation time, which is about 3.2 at 100 Monte Carlo step (MCS), and about 13.5 at 5000 MCS. The stress distributions are heterogeneous in materials because of the existence of grains. The mechanical property of grain boundary significantly affects the average stress. As the grains grow, the average stress without grain boundary effect slightly decreases as the simulation time, while the one with strengthening effect significantly decreases, and the one with weakening effect increases. The average stress and the grain size agree well with the Hall–Petch relationship.     

材料微观组织演变的蒙特卡洛仿真原理及应用

材料宏观力学性能由材料微观组织决定.蒙特卡洛波茨模型是实现材料微观组织演变过程仿真的一种重要方法.对蒙特卡洛波茨模型的原理、算法和应用进行了综述.     

二维晶粒演变过程的模拟及分形研究

以蒙特卡罗随机模拟方法为基础,通过对原有算法进行改进,对二维晶粒的演变过程进行了计算机模拟和分形研究,获得了统计等效组织模型．分析表明,模拟的晶粒在演变过程中具有分形特性,即晶粒形态具有与时间无关的相似性,与晶粒形核和生长的物理机制相一致,证明了模拟过程的合理性．从而为后续热变形显微组织演变的模拟中晶核的空间分布和生长提供较为精确的显微组织基础．     

一种考虑晶界能各向异性模拟晶粒长大的Monte Carlo方法

一般的Monte Carlo模型都没有考虑晶界能的差异。奉研究根据晶界两侧晶粒的取向差及其与重合晶界的关系提出了晶界能分布的连续函数表达式,并且将其应用于Monte Carlo模型模拟晶粒长大过程。引入了该能量函数的Monte Carlo模型模拟的结果显示,随着晶粒长大的进行,具有低能量的重合晶界百分数增大,同时该模型模拟的重合晶界百分数明显地强于未引入该能量函数的Monte Carlo模型模拟的结果。并且采用该模型模拟的微观组织更加接近于实际观察到的平衡组织。     

正常晶粒生长形貌演化的计算机模拟研究

有关多晶材料晶体生长机理及计算机模拟方法，我们已作了详细的论述。本文以蒙特卡罗（Ｍｏｎｔｅ Ｃａｒｌｏ）方法为基础，通过对Ｑ－ｓｔｅｔｅ ｐｏｔｔｓ算法进行改进，建立快速的Ｑ－ｓｔａｔｅ ｐｏｔｔｓ算法，用Ｄｅｌｐｈｌ３．０提供的Ｏｂｊｃｅｔ Ｐａｓｃａｌ语言编写软件，在ＰＣ５８６上实现了多晶材料晶粒生长结构演化的计算机模拟及统计分析。正常晶粒演化形貌图与生产动力力学相符合，逼真度较好。生长指数的     

Implementation of a phase field model for simulating evolution of two powder particles representing microstructural changes during sintering

In this work, we present a multiphysics phase field model for capturing microstructural evolution during solid-state sintering processes. The model incorporates modifications of phase field equations to include rigid-body motion, elastic deformation, and heat conduction. The model correctly predicts consolidation of powder particles during sintering because of two competing mechanisms—neck formation and grain growth. The simulations show that the material undergoes three distinctive stages during the sintering process—stage I where neck or grain boundary between two particles is formed, stage II in which neck length stabilizes and growth or shrinkage of individual particles initiates, and finally stage III with rapid grain growth leading to disappearance of one of the grains. The driving forces corresponding to different mechanisms are found to be dependent on the radius of the particles, curvature at the neck location, surface energy, and grain boundary energy. In addition, variation in temperature is found to significantly influence the microstructure evolution by affecting the diffusivity and grain boundary mobility of the sintered material. The model is also used to compare sintering simulation results in 2D and 3D. It is observed that due to higher curvature in 3D, model predicts faster microstructural evolution in 3D when compared to 2D simulations under identical boundary conditions.     

Simulation of structure evolution in Cu films

The Monte Carlo method was used to simulate grain growth in thin Cu films. The model, based on energetic principles, was compared with the evolution of measured film structures. Surface, interface, grain boundary, and elastic strain energies were applied to determine the preferred microstructure in terms of different annealing conditions and film thicknesses. Four microstructural cases, relating to different film thicknesses, were developed in this paper. Twinning in the Cu films is simulated by arbitrary re-assignment of randomly selected crystallite lattice orientations. The observed evolution in crystallographic texture for each film thickness can be obtained from the Monte Carlo simulations.     

Three Dimension Monte Carlo Simulation of Austenite Grain Growth in CGHAZ of an Ultrafine Grain Steel

In the present research Monte Carlo technique was used to simulate the grain growth in heat-affected zone(HAZ) of an ultrafine grain steel. An experimental data based (EBD) model proposed by Gao was used to establish the relation between tMCS and real time temperature kinetics in our simulation. The simulations give out the evolution of grain structure and grain size distribution in HAZ of the ultrafine grain steel. A Microsoft Window based on computer program for the simulation of grain growth in the HAZ of weldment in three dimensions has been developed using Monte Carlo technique. For the system, inputting the temperature field data and material properties, the evolution of grain structure, both image of simulated grain structure and numerical datum reflecting grain size distribution can be produced by the program. The system was applied to the ultrafine grain steel welding, and the simulated results show that the ultrafine grain steel has large tendency of grain growth.     

Monte Carlo Simulation of Three-Dimensional Nonisothermal Grain-Microstructure Evolution: Application to LENS Rapid Fabrication

A stochastic three-dimensional (3-D) model for grain-microstructure evolution during transient nonisothermal annealing of metallic materials is developed, validated, and applied to the LENS (Laser-Engineered Net Shaping) advanced rapid fabrication process. The model is based on the assumption that the main driving force for microstructure evolution is the reduction in energy contribution arising from the grain boundaries. A temperature-dependent grain-boundary mobility factor is introduced into the expression for the transition probability in order to account for nonisothermal effects, such as those induced by the rastering laser during LENS-based manufacturing. The grain-boundary mobility factor and its temperature dependence are determined using the available experimental isothermal-annealing data. The simulation of grain growth (under nonisothermal annealing conditions encountered in the LENS process) is carried out by coupling a Monte Carlo method for microstructure evolution with a finite difference-based solution to the three-dimensional (3-D) transient energy equation. In response to the computational challenges of the simulations, a highly efficient interprocessor communications methodology is developed, which greatly reduces the simulation time on parallel computers. The results obtained show that under isothermal annealing conditions, the kinetics of grain growth is governed by a temporal power-law behavior and that, after an initial transition period, the grain-size distribution (normalized with respect to the average grain size) becomes time invariant. Furthermore, the application of the model to the LENS process is found to enable establishment of the relationships between process parameters (the laser power, beam rastering velocity, etc.) and the microstructure (grain size distribution, depth of the heat-affected region, etc.) of the deposited material.     

Linking Grain Boundaries and Grain Growth in Ceramics

The macroscopic properties of most materials are strongly influenced by grain size. In ceramic materials the microstructure usually results from the sintering process. Understanding the basic mechanisms of grain growth on an atomic length scale in ceramics would be beneficial to tailor the microstructure for improved macroscopic performance of devices. A method is presented using grain growth experiments to select samples for closer examination of grain boundaries with transmission electron microscopy. The growth experiments are used to identify temperatures were changes at grain boundaries occur at high temperature. Subsequently samples of interest are investigated using transmission electron microscopy (TEM) methods. The correlation between TEM results and changes in grain growth behavior can be used to gain closer insight into the processes occurring during grain growth at an atomic length scale. Strontium titanate is used as model system to demonstrate the combination of growth experiments with TEM results. Normal grain growth shows two distinct drops in growth rate in the temperature range between 1 300 and 1 425 °C, independent of the A‐site to B‐site stoichiometry of the perovskite. In previous studies a high preference for grain boundary planes oriented parallel to the 100 direction of one of the adjacent grains was found in the high temperature regime. This study shows that the preference does not exist in the low temperature regime possibly explaining the change in grain growth rate.     

Pore deformation and grain boundary migration during sintering in porous materials: a phase-field approach

Grain growth in porous ceramics is a complex process due to the variety of interactions between pores and grains. In this study, the pore deformation and grain boundary migration during porous ceramic sintering are simulated by the phase-field method, and the variety of diffusions during sintering is considered. Pores of different shapes and sizes are induced into the simulations to investigate the grain boundary migration and pore deformations during grain growth. Simulation results indicate that the porous microstructure is determined by the contacting mode between pore surface and grain boundaries, which is in good agreement with experimental observations. The efficiency of the grain boundary migration is analyzed via calculating the forces applied on the interfaces between grains and pores, and the mechanism of the pore deformation during grain boundary migration is discussed. Therefore, controlling the grain–pore microstructure by adjusting the synthesis process is essential to reach the desired mechanical and physical properties of sintered materials.     

蒸汽掺杂－一种新的钛酸钡基PTCR陶瓷的掺杂方法

晶界效应是陶瓷材料所固有的特性．利用某些氧化物在高温下具有较高的蒸汽压,在烧成过程对陶瓷材料进行掺杂改性,可以有效地控制晶界行为,改善材料性能．钛酸钡基半导化陶瓷中存在的PTCR效应;是一种典型的晶界效应．利用Sb2O3、Bi2O3蒸汽掺杂的钛酸钡基PTCR材料,晶粒细小、均匀致密、升阻比可以做到大于8个数量级．因而,蒸汽掺杂是一种新型高效的掺杂方法．     

烧结温度对冲击磨用Al2O3陶瓷靶材性能的影响

利用无压烧结工艺制备了陶瓷靶材,并借助扫描电子显微镜和液压机等测试设备研究了烧结温度对陶瓷靶材显微结构和力学性能的影响.结果表明,随着烧结温度的升高,掺杂Al2O3陶瓷靶材的晶粒尺寸逐渐增大,晶粒形状也由各向异性生长逐步向等轴状发育;靶材的抗弯强度也随着烧结温度的升高先升高后降低,并在1600℃时强度和耐磨性达到最大.     

Computer simulations for the design of microstructural developments in ceramics

This paper is to study the computer simulation of microstructural developments in ceramics mainly by Monte Carlo (MC) model and partly by molecular dynamics (MD). Plural mechanisms of mass transfer were introduced in the MC simulation of sintering and grain growth in ceramics at micron-size particle. The MC simulations were performed at the array of two-dimensional triangular lattices and were developed to sintering and grain growth in the complex systems involving a liquid phase and the second solid phase. The MD simulation was applied to the sintering of nano-size particles of ionic ceramics and showed the characteristic features in sintering process at atomic levels. The MC and MD simulations for sintering process are useful for microstructural design for ceramics.     

Influence of the second-phase particle size on grain growth based on computer simulation

A three-dimensional Monte Carlo simulation method for grain growth in two-phased materials is set up, basing on a micro-physical analysis of the interaction between the second-phase particle and the grain boundary. Two-phased systems containing second-phase particles with the same quantity but different sizes are designed, and the complete processes of grain growth are simulated. The influences of the particle size on grain growth are observed and studied quantitatively.