薄规格取向硅钢二次再结晶过程中Goss织构演变的Monte Carlo模拟

通过EBSD实验获取了薄规格取向硅钢(0.18 mm厚)初次再结晶样品表面晶粒组织的取向数据,并以此构建模拟的初始组织.采用Potts模型Monte Carlo方法对薄规格取向硅钢初次再结晶样品的二次再结晶过程进行了模拟仿真,研究了表面能对Goss织构演变的影响.模拟结果表明:Goss取向晶粒与相邻晶粒的表面能差是Goss取向晶粒异常长大的重要驱动力;表面能差存在一个临界值(约12％),只有当表面能差大于此临界值时才会发生表面能驱动Goss取向晶粒的异常长大.     

薄规格取向硅钢二次再结晶过程中Goss织构演变的Monte Carlo模拟

通过EBSD实验获取了薄规格取向硅钢(0.18mm厚)初次再结晶样品表面晶粒组织的取向数据,并以此构建模拟的初始组织。采用Potts模型Monte Carlo方法对薄规格取向硅钢初次再结晶样品的二次再结晶过程进行了模拟仿真,研究了表面能对Goss织构演变的影响。模拟结果表明:Goss取向晶粒与相邻晶粒的表面能差是Goss取向晶粒异常长大的重要驱动力;表面能差存在一个临界值(约12%),只有当表面能差大于此临界值时才会发生表面能驱动Goss取向晶粒的异常长大。     

用改进的Monte Carlo算法模拟晶粒生长

材料微观组织结构决定材料的宏观力学性能.Potts模型是实现晶粒生长过程仿真的一种重要方法.在Radhakrishnan和Zacharia提出的Monte Carlo算法的基础上提出了一种改进的Monte Carlo算法,利用该算法对晶粒生长过程进行了模拟,模拟的微观组织多为等轴晶,晶粒生长指数为0.512.模拟结果表明,该算法能够准确模拟晶粒生长过程.     

Cu纳米晶块体的制备及纳米晶粒长大动力学研究

采用高能球磨的方法制取金属纳米晶粉末，然后利用放电等离子烧结（SPS）技术制备出金属纳米晶块体材料。设计系列实验研究金属纳米晶材料的晶粒长大行为，获得了纳米晶粒长大的动力学规律。根据已有的工作基础和对实验结果的深入分析，确定动力学参数对稳定相晶粒长大行为的影响。实验发现了高能球磨配合SPS技术制备的Cu纳米晶块体发生快速晶粒长大的临界温度，并结合纳米晶界过剩体积与过剩自由能的关系，分析了纳米晶粒组织的能量因素对晶粒长大行为及动力学的影响。     

纳米镁合金中引入独立大尺寸晶粒的模拟研究

通过相场模型模拟纳米尺度晶粒生长演变过程,寻求导致晶粒异常长大的影响因素,探讨获得在纳米多晶结构AZ31镁合金中产生少量分散的大晶粒的理想混晶显微组织形成的可能性。模拟结果显示,有三个因素可以控制纳米结构中的晶粒异常长大:储存能、界面能和界面能动性。如果局域应变储能低于基体储能的1.60倍,或局域界面能高于基体的0.76倍,或局域界面能动性低于基体的3倍,则没有异常长大的晶粒出现。模拟得出,适当引入少数特定的初始纳米晶粒,可以实现预想的理想混晶组织。     

各向异性条件下晶粒长大过程的相场法模拟

引入与时间有关的取向错配场变量,模拟了晶界能各向异性条件下晶粒长大的演化行为.模拟结果表明,与晶界能各向同性系统相比,随演化时间的延长,晶界能各向异性延迟晶粒的生长,使得晶粒的平均面积呈非线性变化;在相同的演化时间下,各向异性系统的晶粒尺寸分布比各向同性系统宽;晶界边数少的晶粒所占的比例明显增加;进入准稳态后,各向异性和各向同性系统中的晶粒相对尺寸分布随时间皆无明显变化.     

溶质对单相纳米晶材料晶粒长大行为的影响

从热力学角度来看,溶质在晶界偏聚,通过降低晶界能来降低晶粒长大的驱动力,从而抑制晶粒长大。从动力学角度来看,溶质与晶界交互作用,钉扎晶界,使晶界迁移速率降低,从而抑制晶粒长大。本文从热力学和动力学两方面综述了溶质对单相纳米晶材料晶粒长大行为的影响,并展望了其发展方向。     

弥散强化Pt-10Rh合金高温微观组织结构研究

用金相显微镜、扫描电镜观测普通Pt-10Rh合金和弥散强化Pt-10Rh合金的微观组织结构,结果表明,普通Pt-10Rh合金在高温下晶粒长大趋势明显,且高温持久性低,而弥散强化Pt-10Rh合金中有强化颗粒氧化锆的存在,能减少晶界缺陷,提高晶界结合力,降低晶界的扩散速度,减缓位错攀移,有效阻止晶粒长大和晶界的滑移,从而提高材料的强度和使用寿命。     

双相合金超塑性变形晶粒长大的模式

由应变促进晶粒长大是双相(α+β)合金超塑性变形中的重要组织效应。晶粒长大的驱动力来自界面滑动而造成的界面能升高。由于双相合金各界面的易动性和迁移性的不同,致使β相以晶界迁移的方式长大;α相以相遇合并的方式长大。两相晶粒的长大规律均符合下式: d=Aε~B+d_0 本文提出一个双相合金超塑性变形晶粒长大模型。     

热处理对FGH96合金异常晶粒长大的影响

基于FGH96合金双锥体试样压缩变形实验及短时加热实验,研究热处理工艺对异常晶粒长大的影响规律,分析异常晶粒长大初始过程的晶粒组织及γ′相。结果表明:具有相同局部应变的双锥体试样经过固溶热处理时,会出现异常晶粒组织,而亚固溶热处理不会出现异常晶粒组织。当压头速率为0.1mm/s及0.008mm/s时,经过固溶热处理的双锥体试样,均会在保温时间小于30s时,局部出现具有较大晶粒的不均匀组织,最终成为异常晶粒。采用过固溶热处理,晶界上的二次γ′相会在到达固溶温度后的60s内几乎完全溶解,其对晶界的钉扎作用消失。采用亚固溶热处理,晶界上的γ′相不会发生溶解,会阻碍晶界的迁移,不会发生晶粒异常长大。     

Grain boundary character distribution and its effect on corrosion of Ni–23Cr–16Mo superalloy

Grain boundary character distribution (GBCD) of the Hastelloy C2000 alloy (Ni–23Cr–16Mo) and the effect of coincidence site lattice (CSL) grain boundaries on corrosion resistance were examined by electron backscattered diffraction and electrochemical experiments. Various deformation followed by annealing was applied to optimise the GBCD of the alloy. After grain boundary engineering (GBE) treatment, the proportion of CSL boundaries increased from 37.7% to 62.4% and the corrosion current density of the specimens decreased in NaCl solution. The results indicated that GBE treatment is responsible for preferable corrosion resistance due to the increase of the fraction of special low energy grain boundaries with perfect grain boundary atom arrangement after thermomechanical process.     

Accumulation of coincidence site lattice boundaries during grain growth

Abstract

Monte Carlo simulations were used to investigate the effect of grain growth on the coincidence site lattice (CSL) boundary content of randomly textured polycrystals. Each grain was assigned an orientation, and grain boundary properties were dependent on both the boundary misorientation and the CSL character. While low misorientation angle boundaries (LABs) increase during growth, the fraction of CSL boundaries does not change with time. Decreasing CSL boundary energy and mobility did not alter these results. In contrast with LABs, which are characterised by a scalar misorientation angle, a particular combination of three independent rotation variables is required to create a low energy CSL boundary; thus, these boundaries are unlikely to form or to persist in a random polycrystal. While texture influences boundary formation, a texture that can enhance CSL boundaries is not apparent. Boundary plane effects should not increase CSL fraction during grain growth.     

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.     

Grain Boundary Sliding at Serrated Grain Boundaries

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).     

Crystal interface and high-resolution electron microscopy—the best partner

Several contributions of HRTEM on the interface science are reviewed in chronological order. The first contribution of HRTEM is the observation of gold (113)S°11 boundary, giving experimental proof of the CSL model. An observation of the asymmetric (112)S°3 boundary follows. A SiC grain boundary is effectively assessed not by the density of CSL point but the number of dangling bonds in the boundary. A ZnO/Pd interface provides an example that a misfit dislocation does not necessarily accommodate the lattice mismatch. Segregated interface shows characteristic HRTEM image contrast, suggesting change in atomic bonding. An atomic height step in the semiconductor hetero interface is observed by the Chemical Lattice Image technique. In the diamond grain boundary a dangling bond may not elevate the boundary energy, being contradictory of the least dangling bond rule. Super–high resolution of the HVHRTEM enable us to determine atomic species in the grain boundary. Combined use of HRTEM and EELSE allows us to discuss the correlation between atomic structure and nature of the corresponding interface. It is not exaggeration to say that modern interface science does not exist witout HRTEM. On the other hand, many complicated interfaces found by HRTEM remained as unaswered questions. An innovative structural model is requested to appear on the scene.     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

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.     

Simulation on Grain Boundary Sliding during Superplastic Deformation Using Molecular Dynamics Method

Grain growth and grain boundary sliding are the two main superplastic deformation mechanisms. In the paper, simulation work is focused on the sliding of a∑3 (111) symmetric twist coincidence grain boundary, a ∑13 (110) asymmetric tilt coincidence grain boundary, and a ∑3 (110) symmetric tilt coincidence grain boundary in AI, and the energies of grain boundary for each of equilibrium configurations are computed. An embedded atom method (EAM) potential was used to simulate the atomic interactions in a bicrystal containing more than 2000 atoms. At 0 K, the relationships between total potential energy and time steps for ∑3 (111) symmetric twist coincidence grain boundary and ∑3 (110) symmetric tilt coincidence grain boundary during sliding at 2 m/s represent the periodic characteristic. However, the relationship between total potential energy and time steps for ∑13 (110) asymmetric tilt coincidence grain boundary represents the damp surge characteristic. It is found that grain boundary sliding for ∑3 (110) symmetric tilt coincidence grain boundary is coupled with apparent grain boundary migration.     

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.