Low-density dislocation arrays at heteroepitaxial Ge/GaAs-interfaces investigated by high voltage electron microscopy

High-voltage electron microscopy in combination with a large-area thinning technique has been applied to thin epitaxial Ge layers on GaAs substrates. These layers exhibit 60° misfit dislocations along the 〈110〉 directions parallel to the interface. Various dislocation reactions are evaluated from the electron micrographs, e.g. the formation of non-glissile 90° dislocations from two nearly parallel 60° dislocations and the annihilation reaction of two crossing 60° dislocations with identical Burgers vectors. The latter reaction occasionally leads to a dislocation multiplication. The misfit dislocations in very thin layers (～0.5 μm thickness and a linear dislocation density of less than 100 dislocation lines/cm) tend to be arranged in groups rather than being equidistant. Consequences for the interpretation of x-ray topograms are discussed.     

Interaction of Lomer–Cottrell locks with screw dislocations

In fcc crystals, dislocations are dissociated into partial dislocations and, therefore, restricted to move on {111} glide planes. By junction reactions with dislocations on two intersecting {111} planes, Lomer–Cottrell dislocations along ?110? directions can be formed which are barriers for approaching screw dislocations. Treating the interaction between a dissociated screw dislocation and a LC lock conventionally, using classical continuum theory and assuming the partials to be Volterra dislocations, leads to erroneous conclusions. A realistic result can only be obtained in the framework of the Peierls model, treating the partials as Peierls dislocations and explicitly taking account of the change in atomic misfit energy in the glide plane. At even moderate stresses (at less than 3 × 10^?3 µ in Cu), the screw will combine with the LC lock to form a Hirth lock. As a result, the nature of the repulsive force will change drastically.     

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：     

Calculations of Lengths of Dislocation Combinations in FCC Crystals

In the present work, lengths of dislocation combinations formed as a result of interaction of reacting dislocations in noncoplanar glide systems are calculated. A new model is suggested in which the geometry of dislocation combination is considered for an arbitrary asymmetrical intersection of dislocation segments. The lengths of dislocation combinations are determined on the example of one dislocation reaction. The statistics of an arbitrary intersection of the reacting dislocations are considered for an arbitrary (from screw to edge) orientation of a glissile dislocation.     

A theoretical investigation of glide dislocations in BN/AlN heterojunctions

Glide dislocations with periodic pentagon-heptagon pairs are investigated within the theory of one-dimensional misfit dislocations in the framework of an improved Peierls-Nabarro (P-N) equation in which the lattice discreteness is fully considered. We find an approximate solution to handle misfit dislocations, where the second-order derivative appears in the improved P-N equation. This result is practical for periodic glide dislocations with narrow width, and those in the BN/AlN heterojunction are studied. The structure of the misfit dislocations and adhesion work are obtained explicitly and verified by first-principles calculations. Compared with shuffle dislocations, the compression force in the tangential direction of glide dislocations has a greater impact on the normal direction, and the contributions of the normal displacement to the interfacial energy cannot simply be ignored.     

Effect of H impurity on misfit dislocation in Ni-based single-crystal superalloy: molecular dynamic simulations

The effect of H impurity on the misfit dislocation in Ni-based single-crystal superalloy is investigated using the molecular dynamic simulation. It includes the site preferences of H impurity in single crystals Ni and Ni₃Al, the interaction between H impurity and the misfit dislocation and the effect of H impurity on the moving misfit dislocation. The calculated energies and simulation results show that the misfit dislocation attracts H impurity which is located at the γ/γ' interface and Ni₃Al and H impurity on the glide plane can obstruct the glide of misfit dislocation, which is beneficial to improving the mechanical properties of Ni based superalloys.     

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 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.     

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.     

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     

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.     

循环应变波形对铝疲劳过程中超声衰减的影响

研究了高纯多晶铝在三角和正弦两种波形的应变控制下的疲劳过程中的超声衰减.结果表明，在小应变振幅下，影响超声衰减变化地主要是位错与点缺陷间的相互作用，波形的变化能改变点缺陷的分布，因而影响超声衰减的变化；而在大应变振幅下，位错大量增殖，对超声衰减变化起主导作用的是位错与位错间的相互作用，波形对位错增殖和位错间相互作用影响甚小. 关键词：     

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.     

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.     

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.     

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.     

The Peierls model for dislocation rings

The energy of a shear dislocation ring is calculated in the framework of the Peierls model in which the displacement is represented by a density of infinitesimal dislocations in the glide plane. This avoids the introduction of an uncertain core cut-off radiusr ₀ to prevent divergence in the usual treatment. The atomic misfit energy in the glide plane is accounted for explicitly and the influence of the interplanar potential on the ring energy and the core structure is studied. Whereas spontaneous formation of shear rings in a homogenous stress field can be ruled out, the emission of dislocation rings from crack tips in glide planes not containing the crack front is feasible.The paper is dedicated to Dr. Frantiek Kroupa in honour of his 70^th birthday.Stimulating discussions with miss Petra Fiala are gratefully acknowledged.     

Misfit dislocations in nanocomposites with quantum dots,nanowires and their ensembles

We review theoretical concepts and experimental results on the physics of misfit dislocations in nanocomposite solids with quantum dots (QDs) and nanowires (quantum wires). Special attention is paid to thermodynamic theoretical models of formation of misfit dislocations in QDs and nanowires, including composite core–shell nanowires. The effects of misfit dislocations on the film growth mode during heteroepitaxy and phase transitions in QD systems are analysed. Experimental results and theoretical models of the ordered spatial arrangement of QDs growing on composite substrates with misfit dislocation networks are discussed. The influence of subsurface dislocations in composite substrates on the nucleation of QDs and nanowires on the substrate surface is considered. Models of misfit strain relaxation and dislocation formation in nanofilms on compliant substrates are also reviewed.     

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.     

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.