氦团簇及氦间隙原子在钨中的稳定性研究

首先采用分子动力学方法研究了在钨中预存氦-空位团簇（He_nV₂₂）后氦原子结合能与氦-空位比的关系。研究发现:当氦-空位比小于4.5时,氦原子结合能随氦-空位比呈线性减小趋势;当氦-空位比大于4.5时,氦原子的结合能随氦-空位比出现剧烈振荡的现象,这种现象是由于钨中预存氦-空位团簇随机挤出位错环使体系能量骤降所导致的。与此同时,氦-空位团簇周围出现了一些处于亚稳态的fcc结构和hcp结构的钨。为了研究氦团簇周围压强对钨基体相变的影响,本文利用第一性原理对钨的三种结构进行了高压相变计算,发现静水压力不能使钨的三种结构互相转变。另外,通过对bcc钨和fcc钨中四面体间隙氦原子和八面体间隙氦原子电荷密度差的计算,发现bcc钨中四面体间隙氦原子的稳定性高于八面体间隙氦原子的稳定性,而在fcc钨中四面体间隙氦原子的稳定性弱于八面体间隙氦原子的稳定性。     

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     

α-Fe中氦泡极限压强的分子动力学模拟

为了深入认识α-Fe中氦泡冲出位错环的微观机制,有必要研究α-Fe中氦泡冲出位错环时的极限压强特性.本文建立金属-氦泡的立方体型代表性体积单元模型,针对8种不同初始半径的球形氦泡,以初始氦空位比为变量,开展分子动力学模拟,得到了各模型中位错环开始形成时的氦泡极限压强和临界氦空位比.研究结果表明:对于无量纲半径介于2—10的氦泡,冲出位错环时的氦泡极限压强和临界氦空位比均随着氦泡初始半径的增大而非线性减小;基体中氦泡冲出位错环时的临界氦空位比具有明显的尺寸效应;初始时刻(0 ps),在经过立方体型模型中心的横截面上,氦泡周围Fe原子阵列的剪应力集中和最大剪应力出现在对角线与氦泡边界交点(即45°)处,并且关于横截面上平行于边的两条对折线对称分布,剪应力集中区的范围和最大剪应力均随着初始氦空位比的增大而增大;位错环冲出方向对应最大剪应力方向.本文的研究加深了对金属中氦泡物理特性的认识,为后续分析氦泡对材料宏观物理和力学性质的影响奠定了有益的基础.     

α-Fe中氦泡阻碍位错移动硬化的分子动力学研究

基于周期性位错阵列模型,利用分子动力学方法,研究了α-Fe中1/2a0111{110}刃位错与3nm氦泡的相互作用导致硬化的过程,重点研究了不同温度下(300 K和623 K),氦空位比例对位错与3 nm氦泡作用机制影响的差异性.研究结果表明,当氦空位比例在0～1之间变化时,氦空位比例的变化对临界剪切应力的影响微弱,当氦空位比例在1～1.5之间变化时,氦泡内氦原子的数量的增加,临界剪切应力随之降低.氦空位比例升高至1.75时,高温623 K与室温相比,临界剪切应力反常升高,原因是高温引起的位错与氦泡的排斥机制:氦泡严重超压,与位错接触瞬间,沿位错拉伸侧踢出自间隙原子团簇,被位错吸收后产生割阶;由于位错割阶与超压氦泡同为压应力场,割阶被氦泡强烈排斥反弹.     

金属钛中氦团簇生长行为的分子动力学研究

运用分子动力学方法模拟了常温下金属钛中氦团簇的生长过程.从能量的角度考察了氦团簇的生长机理.研究发现,随着氦团簇的生长,在氦团簇周围逐渐形成位错环缺陷,氦团簇与氦原子的结合能有逐渐下降的趋势,当氦团簇生长到一定尺寸时会通过发射周围缺陷以使得结合能上升,从而增强了进一步吸收氦原子的能力.模拟还发现,随着氦团簇的继续生长,氦团簇的形状由原来的不规则结构逐渐变成了较为规则的棱柱形结构,在随后的生长过程中其生长仅在（001）平面进行,沿［001］轴的厚度几乎不变. 关键词： 氦团簇 缺陷发射 分子动力学模拟     

氦、氘对纯铁辐照缺陷的影响

在核聚变堆的辐照环境中, 核嬗变产物氢、氦对结构材料的抗辐照性能将产生很大的影响. 本实验采用离子注入和电子辐照模拟研究了氦和氘对具有体心立方结构的纯铁的影响. 采用离子加速器在室温分别对纯铁注入氦离子和氘离子, 经500℃时效1 h后在高压电镜下进行电子辐照.结果表明: 室温注氦和室温注氘的纯铁在500℃时效后分别形成间隙型位错环和空位型位错环. 在电子辐照下, 间隙型位错环吸收间隙原子而不断长大, 而空位型位错环吸收间隙原子不断缩小. 通过计算位错环的变化速率发现, 空位型位错环比间隙型位错环吸收了更多的间隙原子, 即室温注氘纯铁的位错偏压比室温注氦纯铁的偏压参量大, 这意味着相同实验条件下空位型位错环对辐照肿胀的贡献大于间隙型位错环对辐照肿胀的贡献. 利用氦-空位复合体和氘-空位复合体的结构, 分析了注氦和注氘后在纯铁中形成不同类型位错环的原因. 关键词： 氦 氘 辐照损伤 位错环     

氦对钛的体胀及稳定性影响的分子动力学模拟

采用分子动力学(molecular dynamics,MD)方法研究了不同压力下氦-空位复合物的大小、氦含量对材料肿胀、稳定性的影响. 分析了影响材料体积改变及稳定性因素：随着氦含量的增加,体系内聚能减小,系统体积膨胀,稳定性降低;当氦-空位复合物较小时,随着氦-空位复合物的增大,体系内聚能增加,系统体胀减小,体系趋于稳定. 当氦泡中氦原子达到一定数量时氦泡不再继续长大. 关键词： 体胀 氦-空位复合物 钛 分子动力学     

金属铁中空位团簇演化行为的相场研究

构建了一个定量化的相场模型, 用于研究金属铁(Fe)中空位团簇的演化行为. 基于理想气体自由能函数构造了体系的总能量, 并给出了将相场模型中的计算参数与实验数据相结合的方法. 此相场模型能够定量描述空位团簇在金属Fe中生长和粗化的过程以及晶界对于空位团簇演化过程的影响. 这些结果为进一步研究金属Fe中氢/氦等杂质原子与空位之间的相互作用及其演化行为提供了途径. 关键词： 铁 空位团簇 演化 相场方法     

铁的嵌入原子模型势

采用最近导出的普适分析型嵌入原子模型 [generalembeddedatommethod(GEAM) ],对BCC结构的Fe元素进行了描述。计算了元素Fe的单空位形成能 ,双空位结合能 ,弹性常数 ,结构稳定性 ,在 (10 0 )、(110 )、(111)面上的表面能 ,以及元素的声子谱。计算结果和实验结果相符合     

第一性原理计算ZrnFe(n=2-13)团簇的基态结构及其磁性

从第一性原理出发,利用密度泛函理论中的广义梯度近似对ZrnFe(n=2-3)团簇进行了结构优化、能量和频率计算.在充分考虑自旋多重度的前提下,对每一具体尺寸的团簇,得到了多个平衡构型,并根据能量高低确定了团簇的基态结构.综合团簇的结合能、二阶能量差分以及团簇的最高占据轨道和最低未占据轨道间的能隙可知Zr5Fe,Zr7Fe和Zr12Fe团簇的稳定性相对较高,Zr12Fe团簇的结构是具有Ih对称性的正二十面体,而且Zr12Fe的稳定性在所有团簇中是最高的.另外,不仅Zr5Fe,Zr7Fe和Zr12Fe团簇的稳定性相对较高,而且它们均为磁性团簇(而Zrn团簇的磁矩在n≥5时已经发生了淬灭),由此可知通过选择合适的掺杂元素可能得到高稳定的磁性团簇.从Mulliken布居分析结果可知,除了在Zr12Fe团簇中Fe原子失去少量电荷外,其他团簇中Fe原子均从Zr原子那里得到了一定量电荷,即Fe原子在ZrnFe(n=2-3,n≠12)团簇中是电子受体.     

Atomistic simulation of kink structure on edge dislocation in bcc iron

Based on the general theory of dislocation and kink, we have constructed the three kink models corresponding to the 1/2 （111）{011} and 1/2 （111）{112} edge dislocations （EDs） in bcc Fe using the molecular dynamics method. We found that the geometric structure of a kink depends on the type of ED and the structural energies of the atom sites in the dislocation core region, as well as the geometric symmetry of the dislocation core and the characteristic of the stacking sequence of atomic plane along the dislocation line. The formation energies and widths of the kinks on the 1/2 （111）{011} and 1/2 （111）{112} EDs are calculated, the formation energies are 0.05eV and 0.04eV, and widths are 6.02b and 6.51b, respectively （b is the magnitude of the Burgers vector）. The small formation energies indicate that the formation of kink in the edge dislocation is very easy in bcc Fe.     

Shellwise Mackay transformation in iron nanoclusters

Structure and magnetism of iron clusters with up to 641 atoms have been investigated by means of density functional theory calculations including full geometric optimizations. Body-centered cubic (bcc) isomers are found to be lowest in energy when the clusters contain more than about 100 atoms. In addition, another stable conformation has been identified for magic-number clusters, which lies well within the range of thermal energies as compared to the bcc isomers. Its structure is characterized by a close-packed particle core and an icosahedral surface, while intermediate shells are partially transformed along the Mackay path between icosahedral and cuboctahedral geometry. The gradual transformation results in a favorable bcc environment for the subsurface atoms. For Fe55, the shellwise Mackay-transformed morphology is a promising candidate for the ground state.     

First-principles study of He trapping in η-Fe_2C

The distribution of He in η-Fe_2C has been studied by first-principles calculations.The formation energies of interstitial He and substitutional He(replacing Fe) are 3.76 eV and 3.49 eV,respectively,which are remarkably smaller than those in bcc Fe,indicating that He is more soluble in η-Fe_2C than in bcc Fe.The binding potencies of both a substitutionalinterstitial He pair(1.28 eV) and a substitutional-substitutional He pair(0.76 eV) are significantly weaker than those in bcc Fe.The binding energy between the two He atoms in an interstitial-interstitial He pair(0.31 eV) is the same as that in bcc Fe,but the diffusion barrier of interstitial He(0.35 eV) is much larger than that in bcc Fe,suggesting that it is more difficult for the interstitial He atom to agglomerate in η-Fe_2C than in bcc Fe.Thus,self-trapping of He in η-Fe_2C is less powerful than that in bcc Fe.As a consequence,small and dense η-Fe_2C particles in ferritic steels might serve as scattered trapping centers for He,slow down He bubble growth at the initial stage,and make the steel more swelling resistant.     

Dislocation decoration and raft formation in irradiated materials

Experimental observations of dislocation decoration with self-interstitial atom (SIA) clusters and of SIA cluster rafts are analysed to establish the mechanisms controlling these phenomena in bcc metals. The elastic interaction between SIA clusters, and between clusters and dislocations is included in kinetic Monte Carlo (KMC) simulations of damage evolution in irradiated bcc metals. The results indicate that SIA clusters, which normally migrate by 1D glide, rotate due to their elastic interactions, and that this rotation is necessary to explain experimentally-observed dislocation decoration and raft formation in neutron-irradiated pure iron. The critical dose for raft formation in iron is shown to depend on the intrinsic glide/rotation characteristics of SIA clusters. The model is compared with experimental observations for the evolution of defect cluster densities (sessile SIA clusters and nano-voids), dislocation decoration characteristics and the conditions for raft formation.     

Elastic energy of a straight dislocation and contribution from core tractions

We derive an expression of the core traction contribution to the dislocation elastic energy within linear anisotropic elasticity theory using the sextic formalism. With this contribution, the elastic energy is a state variable consistent with the work of the Peach–Koehler forces. This contribution needs also to be considered when extracting from atomic simulations core energies. The core energies thus obtained are real intrinsic dislocation properties: they do not depend on the presence and position of other defects. This is illustrated by calculating core energies of edge dislocation in bcc iron, where we show that dislocations gliding in {110} planes are more stable than those gliding in {112} planes.     

求晶体位错自能的离散弹性方案

考虑到晶体的离散点阵结构,滑移只能在原子之间进行,因此位错中心永远没有原子,位错中心附近分摊到每个原子的离散弹性能量处处有限。在刚性位错假定下,直接应用位错弹性理论解析结果,求出了晶体直奇异位错等效内切半径及其随位错中心位置的周期变化。对于简单四方晶体中奇异螺型位错,一级近似与Peierls模型结果巧合。计算了fcc和bcc两种晶系中各种位错的自能和等效位错内切半径,并初步考虑了各向异性弹性效应。结果表明:位错滑移面不是几何平面,bcc螺型位错滑移面类似于蜂巢结构。指出了用这种离散弹性方法进一步估算各种次级效应的可能。 关键词：     

bcc Fe中刃型位错的结构及能量学研究

基于位错理论，利用分子动力学方法建立了〈100〉{010}，〈100〉{011}，1/2〈111〉{011}和1/2〈111〉{112}刃型位错的芯结构，并计算了这四种刃型位错的形成能、位错芯能量和芯半径.计算结果表明：〈100〉{010}和〈100〉{011}刃型位错的形成能比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的要高，这表明〈100〉刃型位错比1/2〈111〉刃型位错更难形成.而〈100〉{010}和〈100〉{011}刃型位错的芯半径比1/2〈111〉{011}和1/2〈111〉{112}刃型位错的小，这说明在1/2〈111〉刃型位错中位于奇异区的原子数多于〈100〉刃型位错，而这些原子要比完整晶体中的原子具有更大的活性.可见，1/2〈111〉刃型位错比〈100〉刃型位错更易运动，且〈100〉刃型位错在bcc Fe中难以形成. 关键词： bcc Fe 刃型位错 分子动力学模拟     

Theory of hydrogen solubility in binary iron alloys based on ab initio calculation results

Interaction of hydrogen atoms with three substitutional impurities (X?=?Pd, Ti, Cr) in bcc iron base solid solution was modelled ab initio using the WIEN2k package. It was shown that in spite of attraction between H and X atoms, excess energy of the H atom in tetrahedral sites in the first sphere of coordination of the X atom has a significant positive value, while the lowest negative values are observed in the second (Pd, ?0.087?eV; Ti, ?0.091?eV) or the third (Cr, ?0.032?eV) sphere. A new thermodynamic theory of hydrogen solubility in dilute bcc Fe–X alloys was developed on the basis of these results. The resulting equation was used to analyze existing experimental data on H solubility in a number of Fe–X alloys, and X–H interaction energies were determined for each case. The energies determined from high-temperature solubility data for Fe–Pd, Fe–Ti and Fe–Cr are somewhat greater than those obtained in ab initio calculations. The theory gives a new basis for analyzing hydrogen behaviour in iron-base solid solutions.     

第一性原理计算ZrnFe(n=2—13)团簇的基态结构及其磁性

从第一性原理出发，利用密度泛函理论中的广义梯度近似对Zr_nFe(n=2—13)团簇进行了结构优化、能量和频率计算.在充分考虑自旋多重度的前提下，对每一具体尺寸的团簇，得到了多个平衡构型，并根据能量高低确定了团簇的基态结构.综合团簇的结合能、二阶能量差分以及团簇的最高占据轨道和最低未占据轨道间的能隙可知Zr₅Fe，Zr₇Fe和Zr₁₂Fe团簇的稳定性相对较高，Zr₁₂Fe团簇的结构是具有I_h对称性的正二十面体，而且Zr₁₂Fe的稳定性在所有团簇中是最高的.另外，不仅Zr₅Fe，Zr₇Fe和Zr₁₂Fe团簇的稳定性相对较高，而且它们均为磁性团簇(而Zr_n团簇的磁矩在n≥5时已经发生了淬灭)，由此可知通过选择合适的掺杂元素可能得到高稳定的磁性团簇.从Mulliken布居分析结果可知，除了在Zr₁₂Fe团簇中Fe原子失去少量电荷外，其他团簇中Fe原子均从Zr原子那里得到了一定量电荷，即Fe原子在Zr_nFe(n=2—13，n≠12)团簇中是电子受体.     

分子动力学方法研究钛中预存缺陷对氦融合的影响

采用分子动力学方法模拟了在有预存氦空位复合物的体系中氦的迁移和融合,研究了氦融合的各向异性以及氦空位比和氦空位簇大小对氦融合的影响.研究发现:氦迁移的各向异性与体系中是否有氦空位簇无关;氦与氦空位簇的融合主要沿[001]方向;氦二聚物与氦空位簇的融合远快于单个氦原子与氦空位簇的融合.