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：     

Extended planar boundary inclinations in fcc single crystals and polycrystals subjected to plane strain deformation

When fcc single crystals with high-symmetry crystal orientations are deformed to moderate strains by rolling, tension or channel die compression, long dislocation boundaries inclined to the extension axis form. Similarly, long dislocation boundaries are often found in grains embedded in polycrystals deformed in the same manner. These extended planar boundaries (EPBs) are characteristically -30-40° from the extension direction and contain the transverse specimen axis. The objective of the present article is to demonstrate that EPBs formed during plane strain deformation are parallel to equivalent slip planes, a pair of hypothetical slip systems used for analyses of the strain and crystal rotation components in place of the larger number of physical slip systems. The coincidence of EPBs and equivalent slip plane inclinations is shown to account for persistent observations of EPBs in the angle range -30-40° from the rolling direction, in rolled single crystals of various initial orientations. The tendency of EPBs towards tilt or twist boundary character can also be rationalized on the basis of the equivalent slip system concept and consideration of the dislocation types available to be incorporated into EPBs.     

Nodal effects in dislocation mobility

We show that, contrary to the prevailing perception, dislocations can become more mobile by zipping together to form junctions. In a series of direct atomistic simulations, the critical stress to move a junction network in a [110] plane of bcc molybdenum is found to be always smaller ( approximately 50%) than that required to move isolated dislocations. Our data support a previously proposed hypothesis about the nature of anomalous slip in bcc transition metals, yet offer a different atomistic mechanism for conservative motion of screw dislocation networks. The same data suggest a hierarchy of motion mechanisms in which lower-dimensional crystal imperfections control the rate of sliding along the low-angle twist boundaries.     

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.     

Volume density of dislocation lock sources

In this paper, we estimate the volume density of dislocation sources formed as a result of contact interaction between dislocations slipping in an octahedral plane and Lomer-Cottrell dislocation locks. Assuming that during the deformation each dislocation source forms a slip trace, we estimate the average distance between slip traces. This distance turns out to be two to three orders of magnitude less than the experimentally observed value. We draw the conclusion that dislocation lock sources may ensure plastic deformation of fcc crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 62–64, February, 1996.     

The distribution of dislocations in slip bands of Fe-3% Si alloy single crystals deformed by bending

The distribution of dislocations at the ends of slip bands was studied by etching on surfaces parallel to the slip plane. In these places the slip band is formed by groups of asymmetric dislocation loops several hundred microns wide. The long mixed-type parts of these loops running nearly equidistantly and lying in near planes, are the equilibrium arrangement of dislocations of the same sign in the shear stress gradient. From the results we can judge that the dislocation sources are at larger distances from the ends of the slip bands and that the dislocation groups at the ends of the slip bands are sources of large stress fields.     

Continuum model for dislocation dynamics in a slip plane

In this paper, we present a continuum model for dislocation dynamics in a slip plane, which accurately incorporates both the long-range interaction and the local line tension effect of dislocations. Unlike the continuum models in the literature using dislocation densities, we use the disregistry across the slip plane to represent the continuous distribution of dislocations in the slip plane, which has the advantage of including the orientation dependence of dislocations in a very simple way. The continuum dislocation dynamics model is validated by linear instability analysis of a uniform dislocation array to small perturbations and comparisons of the results with those of the discrete dislocation dynamics model. Numerical examples for the evolution of distributions of dislocations and plastic slips in a slip plane are presented.     

晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿[011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向[101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在[101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式[101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.     

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.     

The effect of shear stress on the screw dislocation core in bcc metals

The screw dislocation core structure in bcc metals under an external shear stress is investigated using the model of generalized splitting as an approximation of the Peierls-Nabarro model of screw dislocation dissociated on three {110} slip planes. The shear stress for which the core structure in this model is unstable is found.     

Dislocation structures. Part II. Slip system dependence

Part I established, via extensive transmission electron microscopy investigations, that the type of dislocation structure formed in metals of medium-to-high stacking fault energy upon deformation in tension or rolling to moderate strain levels (≤0.8) depends strongly on crystallographic grain orientation. This paper analyzes the grain orientation-dependent structures in terms of the active slip systems, focusing on the crystallographic plane of extended planar boundaries (geometrically necessary boundaries). The analysis establishes slip systems as the factor controlling the dislocation structure. Five fundamental slip classes, consisting of one to three active slip systems, have been identified. Multiple activation of these slip classes is also considered. The slip classes give rise to different types of dislocation structure, of which all except one contains geometrically necessary planar boundaries aligning with unique crystallographic planes (not necessarily slip planes). A slip class leads to the same type of structure, irrespective of the macroscopic deformation mode, as also demonstrated by successful predictions for shear deformation.     

X‐ray diffractometry and topography of lattice plane curvature in thermally deformed Si wafer

The correlation between the microscopic lattice plane curvature and the dislocation structure in thermal warpage of 200 mm‐diameter Czochralski Si (001) wafers has been investigated using high‐resolution X‐ray diffractometry and topography. It is found that the (004) lattice plane curvature is locally confined between two neighboring slip bands, with the rotation axis parallel to the slip bands. High‐resolution topography reveals that the curvature resulted from a fragmented dislocation structure. The local confinement is attributed to the multiplication of the dislocations that are generated between the two slip bands.     

Low-temperature photoluminescence determination of dislocation slip systems in CdSe single crystals

The transformation of the low-temperature emission spectrum of cadmium selenide crystals during plastic deformation by a point load and by uniaxial compression is investigated. A one-to-one correspondence is established between the occurrence of photoluminescence bands in the vicinity of 1.765 eV and the motion of dislocations in the prismatic slip system, on the one hand, and the emergence of a 1.792-eV band as a result of dislocation slip in the basal plane, on the other. Fiz. Tverd. Tela (St. Petersburg) 40, 1845–1848 (October 1998)     

Analytical estimation of dislocation distribution at the tip of arrested cracks

Computer simulation technique is used for studying the plastic flow at the tip of an arrested crack in lithium fluoride crystals. Two stages of the dislocation structure formation at the tip of a crack are analyzed: the formation of slip lines at the instant of crack arresting, and their evolution after sample unloading and partial healing of the crack. The size and the number of dislocations in a slip line are determined as functions of the loading force at the instant of crack arresting and on frictional stresses. It is shown that, during sample unloading and healing, some dislocations emerge at the plane of the crack under the action of mutual repulsion and image forces, so that the dislocation density attains its maximum value at a distance from the crack tip. A finite region free of dislocations exists in the immediate vicinity of the crack tip.     

Dislocation slip behaviors in high-quality bulk GaN investigated by nanoindentation

The dislocation slip behaviors in GaN bulk crystal are investigated by nanoindentation, the dislocation distribution patterns formed around an impress are observed by cathodoluminescence (CL) and cross-sectional transmission electron microscope (TEM). Dislocation loops, vacancy luminescence, and cross-slips show hexagonal symmetry around the <11-20> and <1-100> direction on c-plane. It is found that the slip planes of dislocation in GaN crystal are dominated in {0001} basal plane and {10-11} pyramid plane. According to the dislocation intersection theory, we come up with the dislocation formation process and the related mechanisms are discussed.     

Formation of bcc-Ni thin film on GaAs(100) substrate and phase transformation from bcc to fcc

Ni thin films are prepared on GaAs(100) single-crystal substrates at room temperature by using an ultra-high vacuum radio-frequency magnetron sputtering system. The growth behavior and the crystallographic properties are studied by in-situ refection highenergy electron diffraction and pole-figure X-ray diffraction. In an early stage of film growth, a metastable bcc Ni(100) single-crystal film is formed on GaAs(100) substrate, where the bcc structure is stabilized through hetero-epitaxial growth. With increasing the film thickness, fcc crystals coexist with the bcc(100) crystal. High-resolution cross-sectional transmission electron microscopy shows that the film consists of a mixture of bcc and fcc crystals and that a large number of planar faults exist parallel to the fcc(111) close-packed plane. The results indicate that the bcc structure starts to transform into fcc structure through atomic displacement parallel to the bcc{110} close-packed planes.     

The effect of aluminum alloying on strength properties and deformation mechanisms of the 〈123〉 Hadfield steel single crystals

The role of aluminum alloying on strength properties and deformation mechanisms (slip, twinning) of 〈123〉 single crystals of Hadfield steel under tensile loading at T = 300 K is demonstrated. It is found out that aluminum alloying suppresses twinning deformation in the 〈123〉 single crystals and, during slip, results in a dislocation structure change from a uniform dislocation distribution to a planar dislocation structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–7, October, 2007.     

间隙原子非线性应力感生扩散的简化弹性偶极子模型

为了阐明体心立方晶体中外应力、位错应力和八面体间隙原子相互作用的本质,从理论上解释S-K弛豫和位错对Snoek弛豫的影响,提出一个简化的含有可动位错的一维弹性偶极子格点模型,讨论位错应力场中间隙原子的非线性应力感生扩散,为进一步对实际晶体的从头数值计算奠定基础。对一维模型的计算机模拟计算,表明位错应力场使得间隙原子形成具有非线性扩散特征的缺陷Fermi-Dirac分布,并增强了Snoek效应,在Snoek峰高温侧出现一个非线性扩散Snoek型内耗峰。 关键词：     

Size dependence of the yield strength of fcc and bcc metallic micropillars with diameters of a few micrometers

Recent micropillar compression tests of fcc and bcc single crystals have shown that ‘Smaller is Stronger’ even in the absence of significant strain gradients, an effect that is empirically characterised by a power-law relation. When a micropillar contains a dislocation network, this power-law relation has been explained in terms of the size-dependent operation stress of the weakest single arm dislocation sources. This single arm dislocation source model has successfully captured the power-law relation for the strength of a few fcc micropillars, but a physical interpretation has not been made by comparing different materials. We applied the model, not only to fcc but also to bcc micropillars, to understand quantitatively why different materials have different power-law exponents. Here, the different power-law exponents are interpreted by comparing material parameters that are size-independent properties. Also, by rearranging these parameters such that the formulation becomes independent of material parameters, we found an alternate form of the scaling law that is a unique function of micropillar diameter. Furthermore, recent experimental studies of the effects of increasing the dislocation density, which show hardening for large micropillars and softening for small micropillars, are interpreted in terms of the statistics of dislocation source distribution. The strengths and limitations of this statistical approach are discussed. The effects of temperature on the power-law exponents are also studied.