Atomistic simulations of interactions between the 1 / 2⟨111⟩ edge dislocation and symmetric tilt grain boundaries in tungsten

The strength of polycrystals is largely controlled by the interaction between lattice dislocations and grain boundaries. The atomistic details of these interactions are difficult to discern even by advanced high-resolution microscopy methods. In this paper we present results of atomistic simulations of interactions between an edge dislocation and three symmetric tilt grain boundaries in body-centred cubic tungsten. Our simulations reveal that the outcome of the dislocation–grain-boundary interaction depends sensitively on the grain boundary structure, the geometry of the slip systems in neighbouring grains, and the precise location of the interaction within the grain boundary. A detailed analysis of the evolution of the grain boundary structures and local stress fields during dislocation absorption and transmission is provided.     

Grain boundary sliding during ambient-temperature creep in hexagonal close-packed metals

Even at ambient temperature or less, below their 0.2% proof stresses all hexagonal close-packed metals and alloys show creep behaviour because they have dislocation arrays lying on a single slip system with no tangled dislocation inside each grain. In this case, lattice dislocations move without obstacles and pile-up in front of a grain boundary. Then these dislocations must be accommodated at the grain boundary to continue creep deformation. Atomic force microscopy revealed the occurrence of grain boundary sliding (GBS) in the ambient-temperature creep region. Lattice rotation of 5° was observed near grain boundaries by electron backscatter diffraction pattern analyses. Because of an extra low apparent activation energy of 20 kJ/mol, conventional diffusion processes are not activated. To accommodate these piled-up dislocations without diffusion processes, lattice dislocations must be absorbed by grain boundaries through a slip-induced GBS mechanism.     

Micromechanistic origin of irradiation-assisted stress corrosion cracking

A multi-scale study of the micromechanics of dislocation–grain boundary interactions in proton and ion-irradiated stainless steels is presented. Interactions of dislocation channels with grain boundaries result in slip transfer, discontinuous slip without or with slip along the grain boundary. The presence of the irradiation damage enhances the importance of the magnitude of the resolved shear stress on the slip system activated by the grain boundary to transfer slip across it. However, the selected slip system is still determined by the minimization of the grain boundary strain energy density condition. These findings have implications for modelling the mechanical properties of irradiated metals as well as in establishing the mechanism for disrupting the grain boundary oxide, which is a necessary prerequisite for irradiation-assisted stress corrosion cracking.     

In situ and tomographic analysis of dislocation/grain boundary interactions in α-titanium

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system.     

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     

A theoretical and experimental study of non-perfect grain boundary dislocations

The aim of this paper is to discuss the possible appearance of non-perfect grain boundary dislocations in grain boundaries in a variety of materials. To begin with, we survey some of the different theoretical treatments which enable grain boundary dislocations and grain boundary structures to be described. The emphasis is put on more recent ideas, and on illustrating the power of group theory in identifying non-perfect grain boundary dislocations. A derivation of the geometric characteristics of interfacial dislocations is carried out in a simple and tutorial way, in a number of representative examples. It is shown that grain boundary dislocations may be divided into three classes: (1) perfect grain boundary dislocations, (2) imperfect grain boundary dislocations, and (3) partial grain boundary dislocations. Experimental transmission electron microscope evidence is then presented for boundaries in the diamond cubic structure, and it is shown that imperfect and partial grain boundary dislocations play an important role in this system. Finally, a comparison of some grain boundary dislocation types in different materials is given.     

Transformation of Slip Dislocations in Σ3 Grain Boundary

Grain boundary processes during plastic deformation of bicrystals were studied by TEM. Two methods were used. In situ straining in the electron microscope followed by post mortem examination and post mortem observation of specimens previously deformed by in situ synchrotron radiation X-ray topography. Two mechanisms governing slip propagation across a coherent twin boundary in a Fe-Si alloy bicrystal were identified. The first mechanism is a dissociation of a slip dislocation with the Burgers vector lying parallel to the boundary into three equal grain boundary dislocations. The second mechanism is a decomposition of a slip dislocation with Burgers vector inclined to the boundary into a dislocation mobile in the other grain and two screw grain boundary dislocations.     

Kinetics of Grain Boundary Recovery in Deformed Polycrystals

Annealing kinetics are studied for nonequilibrium ensembles of dislocations occurring in grain boundaries during plastic deformation. Two types of dislocation ensembles are considered: 1) walls of sessile extrinsic grain boundary dislocations (EGBDs), which cause a change of the GB misorientation angle, and 2) arrays of glissile EGBDs having a Burgers vector tangential to the grain boundary plane. For both types similar exponential relationships are obtained for the relaxation of the average EGBD density, with approximately the same characteristic time proportional to the cube of grain size.     

Multiscale modelling of dislocation/grain boundary interactions. II. Screw dislocations impinging on tilt boundaries in Al

The interaction of dislocations with grain boundaries (GBs) determines a number of important aspects of the mechanical performance of materials, including strengthening and fatigue resistance. Here, the coupled atomistic/discrete-dislocation (CADD) multiscale method, which couples a discrete dislocation continuum region to a fully atomistic region, is used to study screw-dislocations interacting with Σ3, Σ11, and Σ9 symmetric tilt boundaries in Al. The low-energy Σ3 and Σ11 boundaries absorb lattice dislocations and generate extrinsic grain boundary dislocations (GBDs). As multiple screw dislocations impinge on the GB, the GBDs form a pile-up along the GB and provide a back stress that requires increasing applied load to push the lattice dislocations into the GB. Dislocation transmission is never observed, even with large GBD pile-ups near the dislocation/GB intersection. Results are compared with experiments and previous, related simulations. The Σ9 grain boundary, composed from a more complex set of structural units, absorbs screw dislocations that remain localized, with no GBD formation. With increasing applied stress, new screw dislocations are then nucleated into the opposite grain from structural units in the GB that are nearby but not at the location where the original dislocation intersected the boundary. The detailed behaviour depends on the precise location of the incident dislocations and the extent of the pile-up. Transmission can occur on both Schmid and non-Schmid planes and can depend on the shear stresses on the GB plane. A continuum yield locus for transmission is formulated. In general, the overall dissociation and/or transmission behaviour is also determined by the Burgers vectors and associated steps of the primitive vectors of the grain boundary, and the criteria for dislocation transmission formulated by Lee et al . [Scripta Metall. 23 799 (1989); Phil. Mag. A 62 131 (1990); Metall. Trans. A 21 2437 (1990)] are extended to account for these factors.     

非对称倾侧亚晶界的晶体相场法研究

采用晶体相场法研究非对称倾侧亚晶界结构及其在应力作用下的微观运动机制.分别从温度、倾斜度角以及应力施加方向等方面对其结构及迁移过程进行分析和讨论.结果表明,非对称倾侧亚晶界由符号相同的一排刃型位错等距排列,部分出现由两个相互垂直排列的刃型位错构成的位错组;在应力作用下,非对称倾侧亚晶界迁移的微观机制包括位错的滑移和攀移、位错组分解、单个位错与位错组反应、单个位错分解以及位错湮灭;温度降低和倾斜度增大都会阻碍亚晶界的迁移过程;应力方向改变导致位错运动方向改变,从而改变晶界迁移形式.     

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.     

Interaction between Dislocation and Twinning Boundary under Incremental Loading in α-Titanium

The lattice dislocation interacting with grain boundary in the polycrystal exerts an evident influence on the materials' strength and toughness. A comprehensive study regarding the dislocation-twinning boundary(TB)interaction in a-titanium and TB migration is performed by employing molecular dynamic simulation. We analyze the interactions between dislocation and TB, under the conditions of plastic deformation and thermal stress, including the interaction between pure edge(a) dislocation and(1122) TB and the interaction between mixed type(a) dislocations and(1011) TB at 10 K/300 K. The(c + a) pyramidal transmitting slip mode is motivated in the case of edge dislocation-(1122) interaction at 300 K and then transforms into basal-dissociated dislocation after experiencing the complex dissociation and combination. The basal-dissociated pyramidal partial dislocation located in the second grain can be driven to penetrate through the second grain leaving the multiple stacking faults behind. Dissociation of incident basal dislocation on(1011) TB results in a nucleation of a(1011)twin embryo in twin crystals at room temperature. We determine the nature of the generated defects by means of the Burgers circuit analysis.     

Dislocation emission from interphase boundaries in deformed nanocomposites

A theoretical model for emission of lattice dislocations from small-angle interphase boundaries characterized by both orientational and dilatational misfit in deformed nanocomposites is proposed. With allowance for the free surface of the material, the forces acting upon the dislocation structures of the interphase boundaries are calculated, through which the dependences of the critical shear stress for dislocation emission on different parameters of the boundary are found. It is shown that the influence of dilatational misfit and proximity of the interphase boundary to the free surface on dislocation emission is insignificant. It is established that the ability of interphase boundaries to emit dislocations is not uniform: emission of certain dislocations is facilitated as compared to ordinary small-angle grain boundaries, while emission of other dislocations may be inhibited.     

晶界弛豫研究50年

文章综述了我国科学工作者５０年来关于晶界弛豫研究的早期开拓和近期发展．前者包括扭摆内耗仪的发明、晶界内耗峰的发现和无序原子群晶界模型的提出．后者包括澄清了关于晶界内耗峰来源的争论,揭示了晶界弛豫具有一个临界温度,从而提出了一个适合于各种温度的综合的晶界模型．一个最重要的进展是关于竹节晶界内耗峰的发现与其机理的阐明,从而揭示了晶界附近的位错亚结构能够影响晶界本身的性质和结构．这对于研究多晶金属的力学性质提供了一个广阔的途径．另外,晶界与邻域位错的非线性交互作用的发现,为奠定非线性滞弹性这门新学科提供了实验基础     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

Stimulation and Suppression of Grain Boundary Sliding by Intragranular Slip in Zinc Bicrystals

The influence of intragranular slip on grain boundary sliding is studied in originally compatible zinc bicrystals with symmetric tilt boundary. The experiment is designed to separate different effects of intragranular slip on the boundary sliding and establish their mechanisms. Grain boundary sliding with and without development of intragranular slip is observed. The rate of sliding accompanied by slip is more than five times of that without slip. A good correlation between the boundary sliding and intragranular slip prior to slide hardening is established. Slide hardening followed by the negative sliding near one end of the boundary and strain hardening in the boundary vicinity, are observed at the last stages of deformation. For the case of formation of slip induced glissile grain boundary dislocations of opposite signs the possibility of their contribution to total grain boundary sliding, is analyzed. The effect of the increase in the rate of sliding is explained in terms of the accommodation of sliding by slip and appearance of additional glissile grain boundary dislocations of one sign due to strain incompatibility. Contribution of these different dislocation mechanisms to the increase in the sliding rate is determined for the stage of deformation preceding slide hardening. It is supposed that the effect of slide hardening and negative sliding as well as boundary curving is created by non-smooth boundary and small degree of incompatibility caused by straining.     

Strain induced grain boundary premelting in bulk copper bicrystals

In bulk bicrystals strain induced grain boundary premelting (SIGBPM) occurs when heavy screw dislocation pileup can be held up to a certain high temperature, approximately 0.6T _M, where T _M is the melting point of bulk material in Kelvin. SIGBPM occurs at grain boundaries to which new twist component is added due to the rotation of both component crystals toward opposite direction about the axis perpendicular to the grain boundary plane. At the original grain boundary, grain boundary sliding takes place due to this relative rotation. In f.c.c. metals with relatively low stacking fault energies such as copper, nickel, brass(30Zn) and silver, dislocations dissociate into partials. Therefore high density tangled dislocations introduced during plastic deformation hardly loose. If these dislocations can be held to high temperatures, SIGBPM is promoted. Formation of static or dynamic recrystallized grains suppresses SIGBPM itself and the propagation of grain boundary cracks formed by SIGBPM.     

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.     

不同对称性下晶界结构演化及微观机理的晶体相场法研究

采用晶体相场法研究了单轴拉伸下三角相双晶变形过程及机理, 并重点分析了小角对称与非对称晶界和大角对称与非对称晶界在变形过程中的演化及微观机理, 变形过程中应力方向与初始晶界方向平行. 结果表明, 小角对称晶界由柏氏矢量夹角呈60°的两种刃型位错组成, 变形过程中不同类型的位错运动方向相反, 并各自与另一晶界上同一类型位错相互吸引以致部分位错发生湮没; 小角非对称晶界上的位错类型单一, 在应力作用下先沿水平方向攀移, 后各自分解成柏氏矢量约呈120°的两位错, 并通过位错运动和湮没最终形成理想单晶; 大角晶界在应力的作用下先保持水平状态而后锯齿化并发射位错, 伴随着位错运动和湮没, 最终大角非对称晶界发生分解, 而大角对称晶界则重新平直化, 表明大角对称晶界比大角非对称晶界更稳定, 这与实验和分子动力学模拟结果一致. 关键词： 晶体相场 双晶 晶界 对称性     

A computational approach towards modelling dislocation transmission across phase boundaries

To study the nanoscopic interaction between edge dislocations and a phase boundary within a two-phase microstructure the effect of the phase contrast on the internal stress field due to the dislocations needs to be taken into account. For this purpose a 2D semi-discrete model is proposed in this paper. It consists of two distinct phases, each with its specific material properties, separated by a fully coherent and non-damaging phase boundary. Each phase is modelled as a continuum enriched with a Peierls–Nabarro (PN) dislocation region, confining dislocation motion to a discrete plane, the glide plane. In this paper, a single glide plane perpendicular to and continuous across the phase boundary is considered. Along the glide plane bulk induced shear tractions are balanced by glide plane shear tractions based on the classical PN model. The model's ability to capture dislocation obstruction at phase boundaries, dislocation pile-ups and dislocation transmission is studied. Results show that the phase contrast in material properties (e.g. elastic stiffness, glide plane properties) alone creates a barrier to the motion of dislocations from a soft to a hard phase. The proposed model accounts for the interplay between dislocations, external boundaries and phase boundary and thus represents a suitable tool for studying edge dislocation–phase boundary interaction in two-phase microstructures.