原子模拟钛中微孔洞的结构及其失效行为

六角金属由于其各向异性等特点,在塑性变形等过程中容易产生形状和构型都相对复杂的点缺陷团簇.这些团簇之间及其与运动位错等缺陷的相互作用直接影响材料的物理和力学性能.然而对相关问题的原子尺度、尤其是空位团簇的演化和微孔洞的形成乃至裂纹形核扩展等的理解还不全面.本文采用激发弛豫算法结合第一原理及原子间作用势,系统考察了钛中的空位团簇构型及不同构型间的相互转变,给出了不同尺寸空位团簇的稳定和亚稳构型、空位团簇合并分解和迁移的激发能垒等关键参数,发现较小的空位团簇形成稳定构型,较大的空位团簇呈现出空间对称分布趋势进而形成微孔洞;采用高通量分子动力学模拟系统研究了不同尺寸的空位团簇在拉应力作用下对变形过程的影响,发现这些空位团簇可以形成层错,并对微裂纹的形核产生影响.     

Fe-Cr合金辐照空洞微结构演化的相场法模拟

Fe-Cr合金作为包壳材料在高温高辐照强度等极端环境下服役,产生空位和间隙原子等辐照缺陷,辐照缺陷簇聚诱发空洞、位错环等缺陷团簇,引起辐照肿胀、晶格畸变,导致辐照硬化或软化致使材料失效.理解辐照缺陷簇聚和长大过程的组织演化,能更有效调控组织获得稳定服役性能.本文采用相场法研究Fe-Cr合金中空洞的演化,模型考虑了温度效...     

第一性原理研究Fe及其团簇缺陷对KDP和ADP晶体激光损伤的影响

由于金属杂质离子对晶体损伤性质有不容忽视的影响，受实验条件限制，Fe及其团簇缺陷对晶体的影响机制尚不明确。采用第一性原理的方法，对磷酸二氢钾(KDP)和磷酸二氢铵(ADP)晶体中的Fe及其团簇缺陷进行模拟研究，确定其对晶体结构及光学性质方面的影响。研究发现，Fe进入KDP和ADP晶体中主要以取代P原子形成FeO₄基团最稳定，且其稳定形式以Fe³⁺为主。磁性状态研究发现磁性条件对晶体的结构和能量影响不大，Fe对晶体的损伤主要通过引起200～300 nm范围明显的光学吸收影响损伤阈值。Fe进入晶体中形成团簇缺陷可通过电荷补偿与O空位(VO)复合，几乎不会与OH空位(VOH)复合，团簇缺陷以Fe对晶体结构和性质的影响为主。     

硅团簇自旋电子器件的理论研究

利用过渡金属掺杂的硅基团簇, 构建了一种自旋分子结; 并利用第一性原理方法, 对其电子自旋极化输运性质进行了研究. 计算表明, 通过过渡金属掺杂可以有效地产生自旋极化电流, 磁性金属Fe和非磁性金属Cr和Mn掺杂的体系呈现出较明显的自旋极化透射现象, 但分子结的自旋极化输运能力与团簇孤立状态下的磁矩无一致性.从Sc到Ni的掺杂, 体系的自旋极化透射能力先增大后迅速减小, 在Fe掺杂的Si₁₂团簇中出现最大值. 关键词： 硅团簇 自旋极化输运 密度泛函理论 非平衡格林函数     

氢团簇在飞秒强激光场中的动力学行为

利用分子动力学方法研究了氢团簇在飞秒强激光场中的动力学行为. 与库仑爆炸模型所预言的不同, 团簇的膨胀是各向异性的, 质子平均动能沿激光场极化方向上的分量要明显大于垂直于激光场极化方向上的分量. 讨论了团簇各向异性膨胀产生的原因, 分析了激光和团簇参数对各向异性程度的影响.     

NiAl中Ni空位对杂质C原子的多重俘获及温度效应的第一性原理研究

本文应用基于密度泛函理论的第一原理方法,研究了NiAl金属间化合物中Ni空位对杂质C元素的多重俘获.研究结果表明:在Ni空位存在时,单个C原子最易于存在于空位中心附近的富Ni八面体间隙位置且与邻近的Ni原子和Al原子之间存在共价键形式的相互作用.多个C原子在NiAl中倾向于以"Sequential"的方式被Ni空位俘获,进而形成CnVNi(n=1,2,3,4)团簇.通过电荷密度和差分电荷密度分析得到,当Ni空位俘获多个C原子后,C原子之间有着优先于自身成键的特性.进一步,我们应用热力学模型计算了温度对于C_nV_(Ni)(n=1,2,3,4)团簇浓度及空位浓度的影响.研究表明本征Ni空位的浓度会随着温度的升高而升高.在NiAl金属间化合物中,大多数的杂质C原子会被Ni空位俘获而不是存在于远离Ni空位的八面体间隙位置.由于C原子被Ni空位俘获的过程是一个放热过程,使得体系温度升高,因此会进一步激发更多的Ni空位产生.但是在一定的温度范围内(温度小于700 K时),Ni空位均以C_nV_(Ni)团簇的形式存在.     

嵌入原子法计算金属钚中点缺陷的能量

钚因放射性衰变而出现老化效应.钚中点缺陷的性质和行为是理解钚老化效应的一个基础和前提.运用分子动力学模拟技术,计算了金属钚中点缺陷和点缺陷团簇的形成能和结合能.其中钚-钚、钚-氦和氦-氦相互作用势分别采用嵌入原子多体势、Morse对势和Lennard-Jones对势.计算结果表明,单个自间隙原子易以〈100〉哑铃状形态存在;间隙氦原子在理想晶格的八面体间隙位置相对较为稳定;氦原子与空位的结合能较大,在钚的自辐照过程中两者易于结合并形成氦-空位团簇;氦-空位团簇的形成能随氦原子数的增加而增大,当氦与空位的数     

金属钛中氦团簇融合的分子动力学模拟

运用分子动力学方法研究了金属钛中氦的扩散聚集行为.在300—800K的温度范围内,模拟了钛基底中氦团簇之间的融合过程.研究发现,温度的升高会加快氦团簇的融合.在300—800K,融合后的氦团簇在所模拟的时间尺度内三维结构保持不变.模拟结果还表明,常温下氦团簇之间的吸引力是导致氦团簇融合的重要因素. 关键词： 氦团簇 团簇融合 分子动力学模拟     

金属钨中氦行为的分子动力学模拟

采用分子动力学方法模拟了氦在金属钨中的扩散聚集行为. 首先,建立了氦与钨原子间相互作用势,短程部分采用ZBL势形式,长程部分采用从头算法数据,实现了两者之间的平滑连接. 通过计算氦在钨中不同间隙位的形成能发现,单个氦原子更易存在于金属钨中的四面体间隙位,这与最新的研究成果是一致的. 在400-1200 K的温度范围内,考察了氦原子在金属钨中的扩散行为,获得了扩散迁移能,其值介于实验值和从头算法结果之间. 最后,研究了氦的聚集行为,从能量的角度考察了氦团簇形成初期的生长机理. 研究发现,在氦团簇形成初期,氦团簇对氦的结合能随着氦团簇的生长有逐渐增大的趋势,说明氦团簇吸收氦的能力逐渐增强. 关键词： 氦扩散 氦团簇 辐照损伤 分子动力学模拟     

Fen、Nin(n=2～100)团簇基态结构与能量的模拟淬火算法研究

采用Gupta多体势结合分子动力学模拟退火及淬火方法、分别求解了Fen及Nin(n=2～100)团簇的最低能几何结构及能量. 结果表明：在所研究尺寸范围内除少数团簇的结构不同外,两类团簇具有相似的基态几何结构,在两类团簇结构演化中皆存在类Ih、类Oh、类D5h和Ih&D5h互嵌套类构型之间的竞争;分析团簇二阶差分能和剩余能表明两类团簇共同的幻数序列为：n=13、19、23、38及55,不同处为Ni26及Fe75 也分别为相应体系幻数团簇．对两类团簇分析其平均最近邻原子间距及平均配位数均可给出幻数成因．在所研究团簇尺寸范围内二者平均结合能均表现出随团簇尺寸增大而总体增大的普遍趋势,但Ni团簇的能量演化明显快于Fe团簇,这与实验观测镍团簇磁性演化明显快于铁团簇是完全一致的．     

Two-scale modeling of adsorption processes at structured surfaces

We present an algorithm for the simulation of vicinal surface growth. It combines a lattice gas anisotropic Ising model with a phase-field model. The molecular behavior of individual adatoms is described by the lattice gas model. The microstructure dynamics on the vicinal surface are calculated using the phase-field method. In this way, adsorption processes on two different length scales can be described: nucleation processes on the terraces (lattice gas model) and step-flow growth (phase field model). The hybrid algorithm that is proposed here, is therefore able to describe an epitaxial layer-by-layer growth controlled by temperature and by deposition rate. This method is faster than kinetic Monte Carlo simulations and can take into account the stochastic processes in a comparable way.     

Phase-field study of the second phase particle effect on texture evolution of polycrystalline material

The second phase particle effect on texture evolution of polycrystalline material is studied through phase-field method. A unique field variable is introduced into the phase-field model to represent the second phase particles. Elastic interaction between particles and grains is also considered. Results indicate that in the presence of second phase particles the average particle diameter turns smaller than in the absence of these particles and retards texture formation by pinning effect. The second phase particles change the strain energy profile, which tremendously influences the pinning effect.     

Three-dimensional phase field microelasticity theory of a multivoid multicrack system in an elastically anisotropic body: Model and computer simulations

The phase field microelasticity theory of a three-dimensional, elastically anisotropic system of voids and cracks is proposed. The theory is based on the equation for the strain energy of the continuous elastically homogeneous body presented as a functional of the phase field, which is the effective stress-free strain. It is proved that the stress-free strain minimizing the strain energy of this homogeneous modulus body fully determines the elastic strain and displacement of the body with voids and/or cracks. The proposed phase field integral equation describing the elasticity of an arbitrary system of voids and cracks is exact. The geometry and evolution of multiple voids and/or cracks are described by the phase field, which is the solution of the time-dependent Ginzburg-Landau equation. Other defects, such as dislocations and precipitates, are trivially integrated into this theory. The proposed model does not impose a priori constraints on possible void and crack configurations or their evolution paths. Examples of computations of elastic equilibrium of systems with voids and/or cracks and the evolution of cracks under applied stress are considered.     

Phase-field modeling of epitaxial growth: Applications to step trains and island dynamics

In this paper, we present a new phase-field model including combined effects of edge diffusion, the Ehrlich-Schwoebel barrier, deposition and desorption to simulate epitaxial growth. A new free energy function together with a correction to the initial phase variable profile is used to efficiently capture the morphological evolution when a large deposition flux is imposed. A formal matched asymptotic analysis is performed to show the reduction of the phase-field model to the classical sharp interface Burton-Cabrera-Frank model for step flow when the interfacial thickness vanishes. The phase-field model is solved by a semi-implicit finite difference scheme, and adaptive block-structured Cartesian meshes are used to dramatically increase the efficiency of the solver. The numerical scheme is used to investigate the evolution of perturbed circularly shaped small islands. The effect of edge diffusion is investigated together with the Ehrlich-Schwoebel barrier. We also investigate the linear and nonlinear regimes of a step meandering instability. We reproduce the predicted scaling law for the growth of the meander amplitude, which was based on an analysis of a long wavelength regime. New nonlinear behavior is observed when the meander wavelength is comparable to the terrace width. In particular, a previously unobserved regime of coarsening dynamics is found to occur when the meander wavelength is comparable to the terrace width.     

Phase-Field Modelling in Extractive Metallurgy

The phase-field method has already proven its usefulness to simulate microstructural evolution for several applications, e.g., during solidification, solid-state phase transformations, fracture, etc. This wide variety of applications follows from its diffuse-interface approach. Moreover, it is straightforward to take different driving forces into account. The purpose of this paper is to give an introduction to the phase-field modelling technique with particular attention for models describing phenomena important in extractive metallurgy. The concept of diffuse interfaces, the phase-field variables, the thermodynamic driving force for microstructure evolution and the phase-field equations are discussed. Some of the possibilities to solve the equations describing microstructural evolution are also described, followed by possibilities to make the phase-field models quantitative and the phase-field modelling of the microstructural phenomena important in extractive metallurgy, i.e., multiphase field models. Finally, this paper illustrates how the phase-field method can be applied to simulate several processes taking place in extractive metallurgy and how the models can contribute to the further development or improvement of these processes.     

晶粒生长演变相场法模拟界面表达的物理模型

讨论了当前固态组织演变过程的相场法模拟模型,论证了相场法中界面的概念以及模型中界面的各种处理方法,以AZ31镁合金再结晶系统为例,研究了模型参数取值对界面特征的影响,提出了晶界作用域的概念,阐述了晶界作用域就是相场模型中界面处有序化变量的变化范围,其物理意义是界面能量的分布范围,并对应于成分界面偏析的范围.模拟得出,晶界作用域宽度主要由梯度项系数决定,晶界能则由梯度项系数和耦合项系数共同决定.对于AZ31镁合金,模拟研究了晶界作用域宽度取值的合理性和对显微组织影响的关系,得出取值为1.18 μm时,模拟符 关键词： 相场法 界面 计算机模拟 显微组织     

横向限制下凝固微观组织演化的相场法模拟

凝固过程中横向限制挡板的存在对晶体微观结构的演化存在重要的影响, 不同性质的横向挡板将产生不同的限制效应, 对最终凝固微观组织形成起着决定性作用. 本文利用非等温相场模型, 定性地模拟了纯金属Ni凝固过程中横向限制的存在对其枝晶微观形貌演化的影响, 研究了不同尺寸及性质的横向挡板对枝晶微观结构形成的影响, 讨论了横向限制对不同初始枝晶间距枝晶形貌发展的作用. 计算结果表明, 横向限制挡板的存在将直接影响凝固过程中微观组织的形貌演化过程并最终改变微观结构. 随着横向挡板间距的减小, 微观组织变化更加明显; 挡板初始温度越低, 枝晶形貌改变越明显; 初始枝晶间距越大, 形貌变化越明显; 不同挡板高度对微观结构具有基本相同的影响. 关键词： 相场模拟 微观组织演化 横向限制     

A Stefan Problem with Surface Tension as the Sharp Interface Limit of a Nonlocal System of Phase-Field Type

A model for the evolution of phase boundaries reminiscent of the phase-field model is considered. The equation related to conservation of thermal energy is diffusive and coupled to an equation for the order parameter, which contains a nonlinear convolution operator, related to the limit of an interacting particle model with Kac-potential. Under diffusive rescaling the solutions converge to solutions of the Stefan problem with kinetic undercooling and surface tension.     

Dendritic to globular morphology transition in ternary alloy solidification

The evolution of solidification microstructures in ternary metallic alloys is investigated by adaptive finite element simulations of a general multicomponent phase-field model. A morphological transition from dendritic to globular growth is found by varying the alloy composition at a fixed undercooling. The dependence of the growth velocity and of the impurity segregation in the solid phase on the composition is analyzed and indicates a smooth type of transition between the dendritic and globular growth structures.     

Coupling of the phase-field and CALPHAD methods for predicting multicomponent,solid-state phase transformations

The development of an effective microstructure design method for multicomponent alloys is of considerable importance for improving both the design of alloys and the design of processes for producing alloys with unique properties. The coupling of the phase-field method and the calculation of phase diagrams (CALPHAD) method can be used for predicting the evolution of microstructures in multicomponent alloys. Such predictions make use of CALPHAD thermodynamic information with the chemical free energy function in the phase-field method. This article reviews several of these coupling methods, focusing on solid-state phase transformations in multicomponent systems, such as phase separation and disordered or ordered phase precipitation from a matrix. When calculating disordered phase transformations, the Gibbs energy function derived from the CALPHAD database can be used directly in the phase-field method. On the other hand, when dealing with an order/disorder transition, the degrees of freedom of the element site fraction for an ordered phase in the CALPHAD method can be reduced using the Gibbs energy single formalism for constituent phases, by using a database that stores the Gibbs energy and chemical equilibrium conditions, or by obtaining the driving force calculated using the Thermo-Calc software. The current status and future directions for further development of these coupled methods are discussed.