Dislocations and mechanical properties of icosahedral quasicrystals

In this article we interpret the mechanical properties of icosahedral quasicrystals with the dislocation theory. After having defined the concept of dislocation in a periodic crystal, we extend this notion to quasicrystals in the 6-dimensional space. We show that perfect dislocations and imperfect dislocations trailing a phason fault can be defined and observed in transmission electron microscopy (TEM). In-situ straining TEM experiments at high temperature show that dislocations move solely by climb, a non-conservative motion-requiring diffusion. This behavior at variance with that of crystals which deform mainly by glide is explained by the atypical nature of the atomic structure of icosahedral quasicrystals.     

热应力作用降低LPE层中的位错

提出由温差造成热剪切应力,引起衬底穿线位错滑移,形成<110>界面位错,从而降低LPE层中位错的模型。稳定自然对流下的温度梯度液相外延,存在衬底厚度方向的温差,能在边缘固定的衬底中造成热剪切应力。生长了厚GaAs和Ga_1-xAl_xAs层(x<0.3),估算的热剪切应力大于产生<110>暗线缺陷的临界剪切应力。表面腐蚀坑观察表明,外延层位错密度下降,或无位错。界面蚀槽和阴极荧光观察表明,衬底穿线位错在界面弯曲成<110>界面位错。透射电子显微镜观察表明,界面位错多 关键词：     

PbFe12O19单晶体中的位错蚀斑研究

应用50％HCl作为浸蚀剂,对助熔法生长的PbFe₁₂O₁₉单晶体的(0001)解理面进行了浸蚀,以显示位错蚀斑。根据蚀斑所具有的形态,将其进行了分类,并确定了各自对应的位错类型。应用Mathews等人提出的机制,解释了在(0001)基面上所观察到的位错蚀斑阵列。 关键词：     

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：     

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.     

钼单晶体中亚晶界位错结构的研究

本文应用蚀象法对电子束浮区区熔法制得的原生态钼单晶体中的亚晶界位错结构进行了直接观测。对于实验结果进行了细致的分析,并与亚晶界的Frank公式的一些预期结果比较,全面地证实了理论预测。对(111)面上平行蚀线方向的测量表明,它们大体沿着1/2〈111〉刃型位错的滑移面及攀移面的交线,从而证实了它们是这种位错所组成的一组位错倾侧型晶界。通过对蚀斑三叉亚晶界的分析,检验了推广后的Read-Shockley公式,同时表明存在着两组位错的倾侧晶界。对于(111)面上观察到的15组蚀线网络进行了分析,结果表明其中5组是1/2〈111〉/〈100〉网络,9组是〈100〉/〈110〉网络。分析中,除去应用Carrington等所发展的极图分析法以外,我们还根据Frank公式所规定的网线间距的关系式,提出了进一步定量检验的分析方法。实践证明,当极图分析不能获得唯一的结果时,这种定量检验法可以有效地确定位错网络的Burgers矢量。此外,我们还观察到奇位错和亚晶界交互作用的事例,特别是奇位错在亚晶界上引起“台阶”以及夹杂物和亚晶界交互作用的迹象。不同类型的亚晶界交接以及非平衡态的亚晶界也是经常可以观察到的。以上结果表明,蚀象法对于定量地研究原生态晶体中的亚晶界位错结构是极其有效的,其能力并不亚于电子显微镜薄膜透射法。     

掺钕钇铝石榴石单晶的位错研究

在采用引上法工艺生长掺钕钇铝石榴石(YAG:Nd)单晶时,为了要得到结构完善的激光晶体,必须设法降低晶体中的位错密度。通过一系列工作,认为在晶体生长过程中,位错走向与固液界面保持垂直的关系。在由凸的固液界面形成小晶面的晶体中,位错密度低的原因是因为凸的固液界面可使晶体中心部分的位错散失在边稜以外。因此,采用合适的工艺条件:即在生长过程中有意识地改变固液界面形状,可使晶体达到既能消除小晶面,又降低位错密度的目的。     

Positron annihilation in alpha- and beta-cyclodextrin

A new type of misfit dislocation multiplication is deduced from high-voltage electron micrographs of thin Ge layers on GaAs substrates. Two misfit dislocations with the same Burgers vectors on different glide planes cross and annihilate at the intersection point resulting in the formation of two angular dislocations. The tip of one of these dislocations may reach the growth surface by glide breaking into two separate dislocation segments. These segments may glide to form additional misfit dislocations, which may undergo the same multiplication process.     

Direct observation of misfit dislocation glide on surfaces

Using scanning tunneling microscopy we have observed thermally induced dislocation glide in monolayer Cu films on Ru(0001) at room temperature. The motion is governed by a Peierls barrier that depends on the detailed structure of the dislocations, in particular upon whether the threading dislocations that terminate them are dissociated or not. Calculations based on the Frenkel-Kontorova model reproduce the threading dislocation structure and provide estimates of the Peierls barrier and dislocation stiffness which are consistent with experiment.     

Transmission electron microscopy study of the interaction between a glide dislocation and a dislocation node

Using in-situ tensile straining in conjunction with stereo imaging in a transmission electron microscope, real-time observations have been made in thin copper foils of the interaction of glide dislocations with a dislocation node. A mechanism is observed by which a dislocation approaching the node effectively bypasses the node by exchanging segments with one of the dislocations constituting the node.     

A parametric simulation method for discrete dislocation dynamics

A new computer simulation method employed in discrete dislocation dynamics is presented. The article summarizes results of an application of the method to elementary interactions among glide dislocations and dipolar dislocation loops. The glide dislocations are represented by parametrically described curves moving in glide planes whereas the dipolar loops are treated as rigid objects. All mutual force interactions are considered in the models. As a consequence, the computational complexity rapidly increases with the number of objects considered. This difficulty is treated by advanced computational techniques such as suitable accurate numerical methods and parallel implementation of the algorithms. Therefore the method is able to simulate particular phenomena of dislocation dynamics which occur in crystalline solids deformed by single slip: generation of glide dislocations from the Frank-Read source, interaction of glide dislocations with obstacles, their encounters in channels of the bands, sweeping of dipolar loops by glide dislocations and a loop clustering.     

硅中位错增殖的实验观察

用化学侵蚀法研究了在机械应力和热应力作用下硅中位错的增殖和非均匀成核。结果表明,在使位错增殖和成核作用上,热应力同机械应力是等效的。硅中小角晶界中的位错,原生孤立位错都能成为位错源;晶体内部的缺陷及表面蚀斑处的应力集中能够引起位错成核;硅中螺型位错能够通过交叉滑移机制发生增殖。对新生位错环空间分布的研究表明,Frank-Read机制可能是位错增殖的主要形式。位错能否发生增殖,主要决定于位错源所受分切应力的数值、晶体温度、位错本身的结构特点以及钉扎情况等。     

Dislocation generation in silicon grown laterally over SiO2 by liquid phase epitaxy

Epitaxial lateral overgrowth (ELO) on thermally oxidized and patterned (111) Si is effected by liquid phase epitaxy (LPE). It produces Si layers spreading out on the amorphous SiO₂ which are either perfectly grown defect-free or, coexisting, defective layers containing dislocations. High voltage electron microscopy of the defective layers reveals regular arrangements of the dislocations which result from glide and multiplication processes governed by the elastic interactions between the dislocations. The nucleation of the first dislocations during the ELO process is attributed to a slight warping of the substrates. A corresponding bending of the epitaxial layer induces mechanical stress, which may exceed the critical value at the oxide edges of the seeding windows where the first dislocations nucleate. The characteristics of the dislocation arrangements and lattice imaging results support this model. Suggestions are made for ways to reduce stress and, thus, avoid dislocation formation.     

Simulation of the formation of a glide band

The initial stage in the development of a glide band in NaCl crystals is simulated. The process of double cross slip of segments of screw dislocations was examined according to the scheme proposed by Wiedersich. A quantitative estimate of the parameters characterizing the development of the glide band is made: structures and rate of expansion of the band, average slip distance of the dislocations, dislocation density, and others. The degree to which the different parameters affect the development of the glide band is established: ratio of the edge and screw dislocation velocities, as well as the lattice friction stress of dislocations.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 9, pp. 82–96, September, 1981.     

Direct atomic imaging of solid surfaces: IV. Dislocations on Au(100)

Direct observation of the metastable gold (100) 1 X 1 surface by high resolution electron microscopy provides evidence for surface Shockley partial dislocations. This result is correlated with the known 5 X 20 reconstruction to a hexagonal overlayer since the dislocations possess a pseudo-hexagonal core structure. This implies a dislocation mechanism for the phase transformation from 1 X 1 to 5 X 20. It is proposed that pipe diffusion along the dislocation cores or mass transport via dislocation glide or climb could explain the rapid atomic migration required during the phase transition from 1 X 1 to hexagonal.     

Modern aspects of plastic deformation in metals

After a historical survey of crystal plasticity the rôle of dislocations in plastic deformation of metals is outlined. The theory of plastic deformation in metal single crystals before the impact of transmission electron microscopy is described. Recent experimental results on glide and workhardening in single crystals of b.c.c. metals are reviewed and explained by a simple dissociation model of the screw-dislocation cores. A relative success of this explanation together with calculations of atomic structure of dislocation cores support the conclusion that different structures of screw dislocation cores are responsible for both differences and similarities between plastic behaviour of f.c.c. and b.c.c. metals. Further developments in the field of metal crystal plasticity by modern experimental techniques (transmission electron microscopy of deformed crystals in the stress-applied state, magnetic studies of dislocations in ferromagnetic crystals) and by atomic calculations of defect configurations are discussed.Na Slovance 2, Praha 8, Czechoslovakia.Invited paper presented on a plenary session of the First European Conference on the Physics of Condensed Matter organized by the Board of the Condensed Matter Division of E.P.S., in Florence on 14–17 September 1971 (Chairman: Prof. S. F. Edwards; Schuster Laboratory, University of Manchester).     

α—SiC晶体中的位错

用腐蚀法和X射线形貌术研究了α-SiC晶体中的位错。所用的腐蚀剂为熔融氢氧化钾。证实了尖底蚀坑与位错的一一对应关系。由于[0001]方向的螺型位错的Burgers矢量比刃型位错的Burgers矢量大得多,故可从蚀坑的深浅来判别螺型位错和刃型位错。给出了蚀坑形状和多型体晶体结构的对应关系。研究了表面生长蜷线的形态与SiC晶体中的位错及位错运动的关系。X射线形貌图显示了α-SiC晶体中相当数量的位错处于基面C面上。生长位错从晶体“根部”成核并随着晶体生长前沿的向前推进而延伸,因而位错线的方向常常沿[101O]和[1120]方向。将腐蚀法和X射线形貌术结合起来才能全面显示α-SiC晶体中的位错。 关键词：     

The dislocations in graphene with the correction from lattice effect

The dislocation widths and Peierls stresses of glide dislocations and shuffle dislocations in graphene have been studied by the improved Peierls-Nabarro (P-N) equation which contains the discrete correction. The discrete parameter is obtained from a simple dynamic model in which the interaction attributed to the variation of bond length and angle was considered. The restoring force in the improved P-N equation is given by the gradient of the generalized stacking fault energy surface (γ-surface). Our calculation shows that the widths of the shuffle dislocation and the glide dislocation are narrow and the width of the shuffle dislocation is about twice wider than the glide dislocation. The Peierls stress of a shuffle dislocation is one order of magnitude smaller than that of a glide dislocation. As a consequence, the shuffle dislocation moves more easily than the glide dislocation.     

Structure, stability, and motion of dislocations in double-wall carbon nanotubes

In this paper,a novel double-wall carbon nanotube(DWCNT) with both edge and screw dislocations is studied by using the molecular dynamics(MD) method.The differences between two adjacent tubule indexes of armchair and zigzag nanotubes are determined to be 5 and 9,respectively,by taking into account the symmetry,integrality,and thermal stability of the composite structures.It is found that melting first occurs near the dislocations,and the melting temperatures of the dislocated armchair and zigzag DWCNTs are around 2600 K-2700 K.At the premelting temperatures,the shrink of the dislocation loop,which is comprised of edge and screw dislocations,implies that the composite dislocation in DWCNTs has self-healing ability.The dislocated DWCNTs first fracture at the edge dislocations,which induces the entire break in axial tensile test.The dislocated DWCNTs have a smaller fracture strength compared to the perfect DWCNTs.Our results not only match with the dislocation glide of carbon nanotubes(CNTs) in experiments,but also can free from the electron beam radiation under experimental conditions observed by the high resolution transmission electron microscope(HRTEM),which is deemed to cause the motion of dislocation loop.     

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.