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.     

Supersonic dislocation stability and nano-twin formation at high strain rate

Improved understanding of the plastic deformation of metals during high-strain-rate shock loading is key to predicting their resulting material properties. This paper presents the results of molecular-dynamics simulations which address two fundamental questions related to materials deformation: the stability of supersonic dislocations and the mechanism of nano-twin formation. The results show that aluminium plastically deforms by the subsonic motion of edge dislocations when subjected to applied shear stresses of up to 600?MPa. Although higher applied stresses initially drive transonic dislocations, this motion is transient, and the dislocations decelerate to a sustained subsonic saturation velocity. Slowing of the transonic dislocation is controlled by the interaction with excited Rayleigh waves. 800?MPa marks a critical shear stress at which dislocation glide gives way to nano-twin formation via the homogeneous nucleation of Shockley partial dislocation dipoles. At still higher applied stresses, additional dislocation dipole nucleation produces a mid-stacking fault transformation of the twinned material.     

Kinematics of edge dislocations. II. Orowan-type kinematic relations

The kinematics of edge dislocations based on the model of dislocation lines located in a time-dependent equidistant Riemannian material space is considered. The distinguished material flows, called dislocation flows, are introduced in order to make a comparison between the dislocation kinematics and the macroplasticity kinematics. It is shown that the Riemannian generalization of kinematic assumptions of the perfect plasticity theory leads to Orowan-type formulas for the glide as well as for the double cross-slip types of the edge dislocation kinematics. The influence of driving stress of moving dislocations on the shear rate is discussed.     

镍铬合金中的位错运动与位错反应

对镍铬合金中单一滑移面内和两个滑移面间的位错反应,特别是动态下的反应,进行了透射电子显微镜观察,并对其中的一些位错组态进行了衍衬分析。1.六角位错网络主要是单一滑移面内柏氏矢量相差120°的两组位错间反应的结果;2.与螺型位错一样,刃型或混合型位错也能在两个滑移面间交滑移;3.两个滑移面间的位错反应有时在其截线方向生成不滑动的位错(如L.C.位错锁)并不能完全阻挡住这两个滑移面上的位错运动;4.在含铝、钛的镍铬合金中,超点阵位错的反应与不含铝、钛的合金或无序固溶体中的位错反应相似。 关键词：     

Atomic-scale plasticity in the presence of Frank loops

The different reactions between edge or screw dislocations and interstitial Frank loops were studied by means of molecular dynamics simulations. The calculations were performed at 600?K using an embedded atom method (EAM) potential describing a model FCC material with a low stacking fault energy. An interaction matrix that provides the corresponding interaction strength was determined. In an attempt to investigate the role of pile-ups, simulations with either one or two dislocations in the cell were performed. We find that screw and edge dislocations behave very differently. Edge dislocations shear Frank loops in two out of three cases, while screw dislocations systematically unfault Frank loops by mechanisms that involve cross-slip. After unfaulting, they are strongly pinned by the formation of extended helical turns. The simulations show an original unpinning effect that leads to clear band broadening. This process involves the junction of two screw dislocations around a helical turn (arm-exchange) and the transfer of a dislocation from its initial glide plane to an upper glide plane (elevator effect).     

Strongly non-local modelling of dislocation transport and pile-up

The purpose of this work is the continuum modelling of transport and pile-up of infinite discrete dislocation walls driven by non-local interaction and external loading. To this end, the underlying model for dislocation wall interaction is based on the non-singular Peierls–Nabarro (PN) model for the dislocation stress field. For simplicity, attention is restricted to walls consisting of single-sign dislocations and to continuous wall distributions on a single glide plane. In this context, the influence of strongly non-local (SNL; long-range) interaction, and its approximation as weakly non-local (WNL; short-range) are studied in the context of interaction- and external-load-driven wall pile-up at a boundary. The pile-up boundary is modelled via a spatially dependent dislocation mobility which decreases to zero at the boundary. The pile-up behaviour predicted by the current SNL-based continuous wall distribution modelling is consistent with that predicted by discrete wall distribution modelling. Both deviate substantially from the pile-up behaviour predicted by WNL-based continuous wall distribution modelling. As such, it is clearly essential to account in continuum models for the intrinsic SNL character of the interaction between same-sign dislocations ‘close’ to the boundary. Gradient-based WNL ‘approximation’ of this interaction is not justified.     

Elastic interaction between two parallel dislocations in non-parallel slip planes

The elastic interaction between two parallel dislocations which can glide in non-parallel slip planes is studied under the simplifying assumption that the dislocation glide velocity is proportional to stress. The motion of the two dislocations is represented by a motion of one reference point in a configuration plane. It is concluded that the contribution of the long-range elastic interaction between individual dislocations from different slip systems to work hardening is negligible, compared to the contribution from the formed attractive junctions. Especially, two parallel edge dislocations with mutually perpendicular Burgers vectors can co-exist in minimum energy positions, however, they can be separated by an arbitrarily small external stress.     

Dislocations jam at any density

Crystalline materials deform in an intermittent way via dislocation-slip avalanches. Below a critical stress, the dislocations are jammed within their glide plane due to long-range elastic interactions and the material exhibits plastic response, while above this critical stress the dislocations are mobile (the unjammed phase) and the material flows. We use dislocation dynamics and scaling arguments in two dimensions to show that the critical stress grows with the square root of the dislocation density. Consequently, dislocations jam at any density, in contrast with granular materials, which only jam below a critical density.     

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.     

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.     

Specific features of the dynamic behavior of a screw dislocation under excitation of transverse dislocation vibrations

The dynamic retardation of the motion of screw dislocations at point defects is investigated taking into account the excitation of transverse vibrations of dislocation elements both in the glide plane and in the plane normal to it. It is shown that the inclusion of the vibrations of the dislocation elements in the plane normal to the glide plane does not change the dependence of the retarding force on the glide rate and defect concentration but considerably increases the magnitude of this force (specifically in the case of the isotropic model, the retarding force increases by a factor of 2).     

Influence of collective effects on the dynamic behavior of a single edge dislocation in a crystal with point defects

The glide of a single edge dislocation in an elastic field of point defects randomly distributed over a crystal is investigated taking into account the influence of the phonon subsystem of the crystal. The force of retardation of the dislocation motion is calculated, and the velocities at which this force has a local maximum and a local minimum are determined. A comparative analysis of the glide of a single dislocation and the glide of a pair of edge dislocations is performed.     

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.     

Which stresses affect the glide of screw dislocations in bcc metals?

By direct application of stress in molecular statics calculations we identify the stress components that affect the glide of 1/2?111? screw dislocations in bcc tungsten. These results prove that the hydrostatic stress and the normal stress parallel to the dislocation line do not play any role in the dislocation glide. Therefore, the Peierls stress of the dislocation cannot depend directly on the remaining two normal stresses that are perpendicular to the dislocation but, instead, on their combination that causes an equibiaxial tension-compression (and thus shear) in the plane perpendicular to the dislocation line. The Peierls stress of 1/2?111? screw dislocations then depends only on the orientation of the plane in which the shear stress parallel to the Burgers vector is applied and on the magnitude and orientation of the shear stress perpendicular to the slip direction.     

Interface dislocation structures at the onset of coherency loss in nanoscale Ni–Cu bilayer films

We have performed a transmission electron microscopy study, using weak beam imaging, of the interface dislocation arrays that form initially at the (001) Ni–Cu interface during coherency loss. Interface dislocations were absent in the 2.5?nm Ni/100?nm Cu bilayers, but were present in the 3.0?nm Ni samples, indicating that the critical Ni film thickness for coherency loss is between 2.5 and 3?nm. The key features of the interface dislocation structure at the onset of coherency loss are: (i) the majority of interface dislocations are 60° dislocations, presumably formed by glide of threading dislocations in the coherently stressed Ni layer, and have Burgers vector in the {111} glide plane; (ii) the interface contained approximately 5% Lomer edge dislocations, with Burgers vector in the {001} interface plane, and an occasional Shockley partial dislocation and (iii) isolated segments of interface dislocations terminating at the surface are regularly observed. Possible mechanisms that lead to these dislocation configurations at the interface are discussed. This experimental study shows that near the critical thickness, accumulation of interface dislocations occurs in a somewhat stochastic fashion with favourable regions where coherency is first lost.     

纯物质晶界结构及运动的晶体相场法模拟

采用晶体相场模型,分别模拟了纯物质小角度晶界和大角度晶界结构及变形过程中的晶粒转动及晶界迁移.结果表明,小角度晶界迁移的主要机理是构成晶界的位错的滑移和攀移,而大角度晶界的迁移主要依靠晶界两侧原子的跳动及晶界位错等缺陷的运动. 关键词： The phase field crystal model was used to simulate the structure of the small angle and the large angle grain boundary (GB) the grain rotation and the GB migration during deformation. Simulated results show that the dislocation glide and climb are the ma     

The formation of dislocation networks

The formation of small angle boundaries consisting of dislocation networks is considered mainly on the basis of studies concerning the hot-deformation of Al-Mg alloys solidified with well developed sub-structures. It is shown that different kinds of network are built up on dislocation forests by dislocations which encounter the forest by glide and then change the mode of motion from glide to climb. Special attention is given to the mechanism of climb which enables the rapid knitting of networks during hot-deformation, and also to the annihilation of dislocations which prevents the increase in flow stress.     

Dislocation glide through non-randomly distributed point obstacles

Abstract

Classical meso-scale models for dislocation–obstacle interactions have, by and large, assumed a random distribution of obstacles on the glide plane. While a good approximation in many situations, this does not represent materials where obstacles are clustered on the glide plane. In this work, we have investigated the statistical problem of a dislocation sampling a set of clustered point obstacles in the glide plane using a modified areal-glide model. The results of these simulations show two clear regimes. For weak obstacles, the spatial distribution does not matter and the critically resolved shear stress is found to be independent of the degree of clustering. In contrast, above a critical obstacle strength determined by the degree of clustering, the critical resolved shear strength becomes constant. It is shown that this behaviour can be explained semi-analytically by considering the probability of interaction between the dislocation line and obstacles at a given level of stress. The consequences for alloys exhibiting solute clustering are discussed.     

Dislocation jamming and andrade creep

We simulate the glide motion of an assembly of interacting dislocations under the action of an external shear stress and show that the associated plastic creep relaxation follows Andrade's law. Our results indicate that Andrade creep in plastically deforming crystals involves the correlated motion of dislocation structures near a dynamic transition separating a flowing from a jammed phase. Simulations in the presence of dislocation multiplication and noise confirm the robustness of this finding and highlight the importance of metastable structure formation for the relaxation process.