温度对镍基单晶高温合金γ/γ'相界面上错配位错运动影响的分子动力学研究

用分子动力学方法研究了温度对镍基单晶高温合金γ/γ'相界面上错配位错运动的影响.研究结果表明:无论是在低温还是在高温下,错配位错的运动都是通过扭折的形核及扭折沿位错线的迁移来实现;在低温时错配位错的相互作用有利于错配位错的运动;然而在高温时错配位错的相互作用可以阻碍错配位错的运动,从而阻碍γ和γ'相界面的相对滑动,有利于提高镍基单晶高温合金的高温力学性能. 关键词： 镍基单晶高温合金 相界面 错配位错 分子动力学模拟     

镍基单晶高温合金界面位错网在剪切载荷作用下的演化

运用分子动力学方法,研究了镍基单晶高温合金γ/γ' 相界面错配位错网在剪切载荷作用下的演化特征.结果表明:(100),(110) 和 (111) 三种相界面形成的位错网在载荷作用下有不同形式和不同程度的损伤,其变形和损伤随温度的增加而增加.在相同的剪切载荷和温度作用下,(100) 相界面形成的正方形位错网最稳定. 关键词： 镍基单晶高温合金 界面位错网 分子动力学     

镍基单晶高温合金γ/γ′（001）相界面上原子构型的分子动力学研究

用分子动力学方法研究了镍基单晶高温合金γ/γ′（001）相界面上三种各具特征的原子堆垛结构. 能量学计算发现，存在最优构型，动力学模拟显示不同构型的界面弛豫后，在相界面上都“成对”出现刃型错配位错. 相关计算表明体系能量、界面形成能及弛豫能都依赖于界面原子堆垛特征，而几何特征则具共性，即不同原子构型的界面具有同一的应力释放模式.     

镍基单晶高温合金γ/γ′（001）相界面上原子构型的分子动力学研究

用分子动力学方法研究了镍基单晶高温合金γ/γ′（001）相界面上三种各具特征的原子堆垛结构. 能量学计算发现，存在最优构型，动力学模拟显示不同构型的界面弛豫后，在相界面上都“成对”出现刃型错配位错. 相关计算表明体系能量、界面形成能及弛豫能都依赖于界面原子堆垛特征，而几何特征则具共性，即不同原子构型的界面具有同一的应力释放模式.     

考虑相界效应的Ni基单晶合金纳米压痕模拟

利用分子动力学方法分别模拟金刚石压头压入Ni模型和Ni基单晶合金γ/γ′模型的纳米压痕过程,通过计算得到两种模型[001]晶向的弹性模量及硬度.采用中心对称参数分析不同压入深度时两种模型内部位错形核、长大过程以及Ni基单晶合金γ/γ′(001)相界面错配位错对纳米压痕过程的影响.结果显示:压入深度0.641 nm之前,两种模型的压入载荷-压入深度曲线相似,说明此时相界面处的错配位错对纳米压痕过程的影响很小;压入深度0.995 nm时,在错配位错处发生位错形核,晶体在γ相中沿着{111}面滑移,随即导致Ni基单晶合金γ/γ′模型压入载荷的下降,并在压入深度达到1.487 nm之前低于Ni模型相同压入深度时的压入载荷;压入深度从1.307 nm开始,由于相界面错配位错的阻碍作用,Ni基单晶合金γ/γ′模型压入载荷上升速度较快.     

初始压入位置对Ni基单晶合金纳米压痕影响研究

针对Ni基单晶合金建立初始压入γ 相的γ /γ' 模型和初始压入γ'相的γ'/γ 模型, 采用分子动力学方法模拟金刚石压头压入两种模型的纳米压痕过程, 计算两种模型[001]晶向硬度. 采用中心对称参数分析两种模型(001)相界面错配位错对纳米压痕过程的影响. 结果显示: 弛豫后, 两种模型(001)相界面错配位错形式不同, 其中γ'/γ 模型(001)相界面错配位错以面角位错形式存在; 压入深度在0.930 nm 之前, 两种模型(001)相界面错配位错变化不大, 压入载荷-压入深度及硬度-压入深度曲线较符合; 压入深度在0.930 nm之后, γ'/γ 模型(001)相界面错配位错长大很多, 导致相同压入深度时γ'/γ 模型比γ /γ'模型压入载荷和硬度计算结果小; 压入深度在2.055 nm之后, γ /γ'模型(001)相界面错配位错对γ 相中位错进入γ'相有阻碍作用, 但仍有部分位错越过(001) 相界面进入γ' 相中, γ'/γ 模型(001)相界面处面角位错对γ' 相中位错进入γ 相有更明显的阻碍作用, 几乎无位错越过(001) 相界面进入γ 相中, 面角位错的强化作用更明显, 所以γ'/γ 模型比γ /γ'模型压入载荷上升速度快.     

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     

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.     

中子衍射法研究单晶镍基高温合金热机械疲劳引起的应力和晶格错配

采用中子衍射法对热机械疲劳后的单晶镍基高温合金样品内部中心点进行了实验测量,数据处理采用了双相叠加峰和单峰分析两种方法,计算得到了样品材料的宏观等效应力、应力偏量、γ相和γ′相的等效应力和晶格错配度等.实验结果表明,两种分析方法得到的宏观等效应力基本一致.在热机械疲劳循环100周出现最大宏观等效应力和应力偏量,此时位错等微缺陷达到饱和,偏离中心点处γ′相应力减小显著;在热机械疲劳开始阶段负错配度明显减小,随循环周次增多基体塑性应变累积又使负错配度以每次81×10^-6的速度逐渐线形增大. 关键词： 中子衍射 单晶高温合金 等效应力 应力偏量     

镍基高温合金热机械疲劳应力的中子衍射

中子衍射法能够在不破坏样品的条件下准确地获取材料的应力状态，已用于测量某些单质单晶体的应力状态，但用于具有两相结构的镍基单晶高温合金的研究工作还很少。沉淀强化的镍基单晶高温合金主要包括两个共格合金相，一相为面心立方（fcc）结构的γ基体相，另一相为均匀镶嵌在谨体中的立方L12结构的γ＇沉淀强化相。     

单向拉伸作用下Cu(100)扭转晶界塑性行为研究

应用分子动力学方法研究了在不同扭转角度下的Cu（100）失配晶界位错结构,以及不同位错结构对晶界强度的影响.模拟结果表明：小角度扭转晶界上将形成失配位错网,失配位错密度随着晶粒之间的失配扭转角度的增加而增加.变形过程中,位错网每个单元中均产生位错形核扩展.位错之间的塞积作用影响晶界的屈服强度：随着位错网格密度的增加,位错之间的塞积作用增强,界面的屈服强度得到提高.大角度扭转晶界将形成面缺陷,在变形中位错由晶界角点处形核扩展,此时由于面缺陷位错开动应力趋于一致,因此晶界的临界屈服强度趋于定值. 关键词： 扭转晶界 失配位错网 强化机理 分子动力学     

Inhomogeneity of misfit stresses in nickel-base superalloys: Effect on propagation of matrix dislocation loops

It is shown experimentally that, during annealing and creep under low applied stresses, matrix dislocation loops frequently cross-glide. The periodic length of the zigzag dislocations deposited in the interfaces is equal to that of the γ/γ′-microstructure. Initially, the zigzag dislocations move in the (001) interface by a combination of glide and climb but then they stop near the γ′-edges and align along ?100?. Reactions of such dislocations lead to the formation of square interfacial networks consisting of ?100? oriented edge dislocations. The complex dislocation movement is explained by the inhomogeneity of the misfit stresses between γ- and γ′-lattices. The tensile components of the stress tensor drive the dislocations through the channel, whereas the shear components near the γ′-edges cause the zigzag movement and the ?100? alignment. The total effect is the most efficient relaxation of the misfit stresses. The results are relevant, especially for single-crystal superalloys of the newest generations, which have an increased γ/γ′-misfit due to the high level of refractory elements.     

Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium

Previous studies have revealed that dislocation structures in metals with medium-to-high stacking fault energy, depend on the grain orientation and therefore on the slip systems. In the present work, the dislocations in eight slip-plane-aligned geometrically necessary boundaries (GNBs) in three grains of near 45° ND rotated cube orientation in lightly rolled pure aluminium are characterized in great detail using transmission electron microscopy. Dislocations with all six Burgers vectors of the ½?1?1?0? type expected for fcc crystals were observed but dislocations from the four slip systems expected active dominate. The dislocations predicted inactive are primarily attributed to dislocation reactions in the boundary. Two main types of dislocation networks in the boundaries were identified: (1) a hexagonal network of the three dislocations in the slip plane with which the boundary was aligned; two of these come from the active slip systems, the third is attributed to dislocation reactions (2) a network of three dislocations from both of the active slip planes; two of these react to form Lomer locks. The results indicate a systematic boundary formation process for the GNBs. Redundant dislocations are not observed in significant densities.     

On the non-uniform motion of dislocations: the retarded elastic fields,the retarded dislocation tensor potentials and the Liénard–Wiechert tensor potentials

The topic of this paper is the fundamental theory of the non-uniform motion of dislocations in two and three space dimensions. We investigate the non-uniform motion of an arbitrary distribution of dislocations, a dislocation loop and straight dislocations in infinite media using the theory of incompatible elastodynamics. The equations of motion are derived for non-uniformly moving dislocations. The retarded elastic fields produced by a distribution of dislocations and the retarded dislocation tensor potentials are determined. New fundamental key formulae for the dynamics of dislocations are derived (Jefimenko type and Heaviside–Feynman type equations of dislocations). In addition, exact closed-form solutions of the elastic fields produced by a dislocation loop are calculated as retarded line integral expressions for subsonic motion. The fields of the elastic velocity and elastic distortion surrounding the arbitrarily moving dislocation loop are given explicitly in terms of the so-called three-dimensional elastodynamic Liénard–Wiechert tensor potentials. The two-dimensional elastodynamic Liénard–Wiechert tensor potentials and the near-field approximation of the elastic fields for straight dislocations are calculated. The singularities of the near-fields of accelerating screw and edge dislocations are determined.     

Formation of extended compound dislocations and barriers in the course of interdislocation reactions in FCC crystals

The behavior of compound dislocations under stress loading is considered. Dislocation configurations are onsidered for an arbitrary asymmetric intersection of reacting dislocation segments. It is demonstrated that depending on the character of dislocation segment intersection, compound dislocations of two types can be formed, one of which is destructed under increasing stress loading. In the other case, the length of the compound dislocation increases, thereby causing the formation of long extended barriers. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 25–30, March, 2009.     

Interaction of dislocation pile-up with a low-angle tilt boundary: a discrete dislocation dynamics study

Abstract

The pile-up of dislocations between two low-angle tilt boundaries (LATB) in an fcc crystal was simulated using three-dimensional discrete dislocation dynamics. The LATB was constructed using glissile edge dislocations stacked on each other. The dislocations in the pile-up were chosen such that their reactions with the dislocations in the LATB resulted in glissile junctions. Parallel pairs of dislocations were inserted to a maximum allowable value estimated from theoretical expressions. A resolved shear stress was applied and increased in steps so as to move the dislocations in the pile-up towards the boundaries. The shear stress required to break the lead dislocation from the wall was determined for varying spacings between the two boundaries. The shear stress and boundary spacing followed the Hall–Petch type relation. Dislocation pile-ups without a LATB were also simulated. The spacing of the dislocations in the pile-up with LATB was found to be closer (ie higher dislocation density) than that without LATB. It was shown through analytical expressions that LATB exerts an attractive force on the dislocations in the pile-up thereby creating a denser pile-up.     

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.     

The energy of dislocation dipoles

In conventional dislocation theory based on the concept of Volterra dislocations, the energy of coplanar dislocation dipoles cannot be obtained without introducing an ill-defined ad hoc annihilation distance. When the dislocations are described more realistically by Peierls dislocations, absolute values for the dipole energy can be obtained from physical arguments. It is found that when dislocations in a dipole come close, they may not annihilate each other but the dipole can collapse when the dislocations have reached a critical separation of several lattice constants.     

The dislocation equation as a generalization of Peierls equation

A fundamental equation is derived for the structure of dislocations in solids. With the interaction potential that can be obtained properly from the first principle, the equation provides a complete basis for a comprehensive study of the dislocation structure. In particular, the full structural feature and related properties of dislocation can be predicted theoretically, which makes it possible to compare precisely the theoretical results and what is produced by experiments or numerical simulations.     

基于连续介质位错理论的位错原子分布构造方法

基于连续介质位错理论提出一种新的位错原子分布构造方式,理论上可以构造出任意形状和任意Burgers矢量的位错结构.利用该方法,选用FCC单晶铜为模拟介质,构造Burgers矢量为b=[110]/2的刃型全位错和Burgers矢量为b=[112]/6圆环形不完全位错环,并使用分子动力学方法模拟全位错的扩展分解过程和不全位错环在自应力作用下的收缩过程,模拟结果与理论分析一致.该方法的优点在于可以方便地构造出其他传统方法难以构造的位错闭合结构——位错环,从而使位错环的细致研究成为可能.