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     

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     

分子动力学方法研究金属Ti中He小团簇的迁移

采用分子动力学方法模拟了不同温度下He原子及He团簇在金属Ti中的迁移特性,并计算了扩散前系数和激活能. 研究发现这种扩散的各向异性非常显著,具体表现在He原子及He团簇在不同方向上扩散系数的前系数完全不同,但它们的激活能却相同. 研究表明:在预测金属中He的扩散行为时,必须采用动态模拟方法才能得到准确的前系数,仅仅考虑势垒的静态模拟方法是不行的. 另外,还发现一个不同于直觉的现象,即较低温度下He二聚物的扩散系数比单个He原子的扩散系数大;此外,在所模拟的温度范围内Arrhenius方程能够很好地描述它们的扩散. 这说明动力学模拟对预测金属中He的扩散行为具有重要的意义. 关键词： 分子动力学 扩散系数 各向异性 氦     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.     

Molecular dynamics study of helium bubble pressure in titanium

In this paper,the pressure state of the helium bubble in titanium is simulated by a molecular dynamics(MD) method.First,the possible helium/vacancy ratio is determined according to therelation between the bubble pressure and helium/vacancy ratio;then the dependences of the helium bubble pressure on the bubble radius at different temperatures are studied.It is shown that the product of the bubble pressure and the radius is approximately a constant,a result justifying the pressure-radius relation predicted by thermodynamics-based theory for gas bubble.Furthermore,a state equation of the helium bubble is established based on the MD calculations.Comparison between the results obtained by the state equation and corresponding experimental data shows that the state equation can describe reasonably the state of helium bubble and thus could be used for Monte Carlo simulations of the evolution of helium bubble in metals.     

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     

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.     

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.     

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.     

Effect of grain boundary structures on the behavior of He defects in Ni: An atomistic study

We investigated the effect of grain boundary structures on the trapping strength of He_N(N is the number of helium atoms) defects in the grain boundaries of nickel. The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane. The He_N defect is much more stable in nickel bulk than in the grain boundary plane. Besides, the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane. The binding strength between the grain boundary and the He _N defect increases with the defect size. Moreover, the binding strength of the He_N defect to the Σ3(12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.     

Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries

The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures.The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries.More interestingly,at high temperatures,the ultimate strengths of the graphene with the zigzagorientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures,which is determined by inner initial stress in grain boundaries.The results indicate that the finite temperature,especially the high one,has a significant effect on the ultimate strength of graphene with grain boundaries,which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications.     

大角度Cu晶界在升温、急冷条件下晶界结构的分子动力学研究

应用分子动力学方法模拟了大角度重位点阵数Σ=5（310）/［001］对称倾斜Cu 晶界在被升温、降温到300,800和1100 K时的晶界结构. 结果表明，随着温度的升高晶界区域的结构发生变化.在远低于块体熔点温度时，晶界就已经发生了预熔化，在晶界区域存在着晶体-熔体共存的现象. 通过对高温晶界的急冷处理，改变了晶界处的原子结构. 关键词： 分子动力学 晶界 熔化 凝固     

温度及深度对钛中氦泡释放过程影响的分子动力学研究

利用分子动力学模拟方法对温度及He泡深度给金属Ti内He泡的体积、压强和释放过程等带来的影响进行了研究. 首先, 通过研究室温下He泡在金属Ti内不同深度处的状态, 得到He泡的形状、压强、体积等物理量随其深度的变化规律. 发现He泡压强随其深度增加逐渐变大, 体积则逐渐减小, 但当He泡深度增大到2.6 nm时, 二者均维持在某个固定值附近. 然后对包含有He泡的Ti体系在温度作用下的演化过程进行了模拟, 发现不同深度处He泡从金属Ti内释放出来所需要的临界温度有很大差别, 总体来看He泡越深, 释放所需的临界温度越高. 但不同温度下He原子的释放速率没有明显差别, 释放过程几乎均为瞬间完成. 最后通过对He泡内部压强和其上方金属Ti薄层的抗张强度进行统计对比, 阐述了金属Ti 体内He泡的释放机制: 当He泡内部压强大于其上方Ti薄层抗张强度时, He泡就会将Ti 薄层撕裂, 从而使He原子得到释放.     

Molecular dynamics simulation of stress and grain evolution

A three-dimensional molecular dynamics simulation is carried out to study the evolution of grains and stresses during the deposition of atoms on the (100) plane of a fcc regular crystal, using the cubic system with x–y periodic boundary conditions. At the bottom an atomic surface and at the top a reflecting wall are assumed. Atoms in the system interact via the Lennard–Jones potential. During simulation the films grow according to the Volmer–Weber mode and exhibit specific shape of the stress curves. When the film becomes continuous, the stress during the growth possesses a maximum value, but later new grain boundaries are formed. Individual atoms in the grain boundaries generate compressive stress in the films.     

界面旋转角对双晶镁力学性质影响的分子动力学模拟

采用分子动力学模拟方法,研究了在拉伸载荷下晶界对双晶镁变形机制的影响,对不同旋转角度的模型以及对称与非对称结构的模型进行了研究.模拟结果表明:应变加载方向与晶向所成角度对双晶镁塑形变形阶段的流动应力能够产生明显的影响;对称结构的双晶镁模型的塑性性质明显优于非对称结构模型.研究结果还发现,由于晶界区域不同的位错成核及发射等运动,大角度双晶模型的塑性响应明显优于对应小角度模型的塑性响应.     

镍基单晶超合金Ni/Ni3Al晶界的分子动力学模拟

在镍基单晶超合金中，由于单晶Ni的晶格常数比单晶Ni₃Al的稍小，在Ni/Ni3Al晶界面 上必然要出现错配.采用分子动力学模拟了镍基单晶超合金的Ni/Ni₃Al晶界的结 构，考虑 了两个不同的初始模型，并进行了分子动力学弛豫.弛豫的结果均表明：由于晶格的差异形 成的错配能不是通过长程晶格错配的方式来释放，而是通过在局部区域形成位错的方式释放 的.由于Ni₃Al相周围Ni相环境的不同，形成的位错也有所不同. 关键词： 镍基单晶超合金 晶界 分子动力学模拟     

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