Vibrational properties of straight dislocations in bcc and fcc metals within the harmonic approximation

We calculate the vibrational spectra of straight screw and edge dislocations in several body-centered cubic (bcc) (Mo and Fe) and face-centered cubic (fcc) (Cu and Al) metals within the harmonic approximation. We take advantage of the translational symmetry of straight dislocations to efficiently calculate their phonon eigenstates in the harmonic limit. This allows us to calculate the low-temperature contribution of straight screw and edge dislocations to the heat capacity of each respective metal, and show that the dominant temperature dependence below 5 K is linear. Comparison with heat capacity measurements of heavily cold-worked Cu reveals very good agreement with our calculations. At higher temperatures, the contribution from the non-linear terms becomes significant. As a result, maxima in the straight dislocation heat capacities are observed in the temperature range from 9% to 16% of the Debye temperature. We investigate the appearance of localized and resonance peaks in the vibrational spectra induced by dislocations, and study in detail their spatial spread around the dislocation cores by projecting vibrational eigenstates onto individual atoms. We study the deviation of these atomic-level vibrational free energies from that of the perfect crystal as a function of distance to the dislocation cores, and establish that, similar to the dislocation energy, the vibrational free energy of an isolated dislocation behaves logarithmically in the long-range limit. Finally, we obtain vibrational spectra for propagating waves along the dislocation line and find that the dispersion for these waves is consistent with the notion of kink formation and motion for screw dislocations.     

Dislocation and stacking fault core fields in fcc metals

Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behaviour within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields.     

In源流量与Ⅲ族流量之比对InGaN/GaN多量子阱性质的影响

利用x射线三轴晶衍射和光致发光谱研究了生长参数In源流量与Ⅲ族流量之比对InGaN/GaN多量子阱结构缺陷(如位错密度和界面粗糙度)和光致发光的影响.通过对（0002）对称和（1012）非对称联动扫描的每一个卫星峰的ω扫描，分别测量出了多量子阱的螺位错和刃位错平均密度，而界面粗糙度则由（0002）对称衍射的卫星峰半高全宽随级数的变化得出.试验发现多量子阱中的位错密度特别是刃位错密度和界面粗糙度随In源流量与Ⅲ族源流量比值的增加而增加，导致室温下光致发光性质的降低，从而也证明了刃位错在InGaN/GaN 关键词： x射线三轴晶衍射 界面粗糙度 位错 InGaN/GaN多量子阱     

Ab initio study of screw dislocations in Mo and ta: A new picture of plasticity in bcc transition metals

We report the first ab initio density-functional study of <111> screw dislocation cores in the bcc transition metals Mo and Ta. Our results suggest a new picture of bcc plasticity with symmetric and compact dislocation cores, contrary to the presently accepted picture based on continuum and interatomic potentials. Core energy scales in this new picture are in much better agreement with the Peierls energy barriers to dislocation motion suggested by experiments.     

δ掺杂对Si衬底GaN蓝光LED外延膜性能的影响研究

用X射线衍射方法通过不同晶面的ω扫描测试,分析了Si衬底GaN蓝光LED外延膜中n-型层δ掺杂Si处理对外延膜结晶性能的影响。报道了Si衬底GaN外延膜系列晶面的半峰全宽(FWHM)值。通过使用晶格旋转(Lattice-rotation)模型拟合,计算出样品的螺位错密度和刃位错密度。结果表明,δ掺杂Si处理后生长出的样品螺位错密度增大、刃位错密度减小,总位错密度有所减小。通过对未经δ掺杂处理和δ掺杂处理的GaN外延膜相应ω-2θ扫描半峰全宽值的比较,发现δ掺杂Si处理后生长出的样品非均匀应变较大;相应样品的LED电致发光光谱I、-V特性曲线显示δ掺杂后样品性能变好。     

Peculiarities of dislocation structure at surface of deformed crystals

An experimental study of the distribution of edge and screw dislocation components in deformed LiF crystals at external surface parallel to Burgers vector of mobile dislocations was performed. At the surface we observed a layer with low density of edge and screw dislocations compared to the dislocation density in the bulk. The thickness of this layer was tens of microns. The experimental results are explained on the basis of analysis of dislocation structure evolution at surface layers during the plastic deformation.     

Hydrogen diffusion in the elastic fields of dislocations in iron

The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 10¹⁴ m^–2 in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉{110}, 〈111〉{112}, 〈100〉{100}, and 〈100〉{110} are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.     

Method of an approximate solution of the Peierls-Nabarro equations for non-planar screw dislocation cores

A class of solutions of a system of three Peierls-Nabarro equations is found using the methods of complex variable. These solutions could be used for investigation of non-planar screw dislocation cores within the Peierls-Nabarro model where the cores are dissociated along three intersecting slip planes.     

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.     

Mobility of edge and screw dislocations in γ-irradiated LiF crystals

The free path lengths of ensembles of edge and screw dislocations in the stress field of a concentrated load are studied in γ-irradiated LiF crystals. The relative mobility of edge and screw dislocations is found to depend substantially on the irradiation dose and temperature. The results obtained are discussed in the context of additional retardation of screw dislocations with dislocation debris that appears during double cross slip.     

Molecular dynamics simulation of the interaction between a mixed dislocation and a stacking fault tetrahedron

A high number-density of nanometer-sized stacking fault tetrahedra are commonly found during irradiation of low stacking fault energy metals. The stacking fault tetrahedra act as obstacles to dislocation motion leading to increased yield strength and decreased ductility. Thus, an improved understanding of the interaction between gliding dislocations and stacking fault tetrahedra are critical to reliably predict the mechanical properties of irradiated materials. Many studies have investigated the interaction of a screw or edge dislocation with a stacking fault tetrahedron (SFT). However, atomistic studies of a mixed dislocation interaction with an SFT are not available, even though mixed dislocations are the most common. In this paper, molecular dynamics simulation results of the interaction between a mixed dislocation and an SFT in face-centered cubic copper are presented. The interaction results in shearing, partial absorption, destabilization or simple bypass of the SFT, depending on the interaction geometry. However, the SFT was not completely annihilated, absorbed or collapsed during a single interaction with a mixed dislocation. These observations, combined with simulation results of edge or screw dislocations, suggest that defect-free channel formation in irradiated copper is not likely by a single dislocation sweeping or destruction process, but rather by a complex mix of multiple shearing, partial absorption and defect cluster transportation that ultimately reduces the size of stacking fault tetrahedra within a localized region.     

Atomistic Simulations of the Effect of Helium on the Dissociation of Screw Dislocations in Nickel

The interactions of He with dissociated screw dislocations in face-centered-cubic(fcc) Ni are investigated by using molecular dynamics simulations based on an embedded-atom method model.The binding and formation energies of interstitial He in and near Shockley partial cores are calculated.The results show that interstitial He atoms at tetrahedral sites in the perfect fcc lattice and atoms occupying sites one plane above or below one of the two Shockley partial cores exhibit the strongest binding energy.The attractive or repulsive nature of the interaction between interstitial He and the screw dislocation depends on the relative position of He to these strong binding sites.In addition,the effect of He on the dissociation of screw dislocations are investigated.It is found that He atoms homogeneously distributed in the glide plane can reduce the stacking fault width.     

Section images of dislocations normal to the surface of a 6H-SiC single crystal

Section topographs of edge and screw dislocations with an axis along [0001] in 6H-SiC are taken and interpreted, and the image formation is explained for this case. The contrast induced by various arrangements of dislocations within the Borrmann triangle is experimentally studied. The sign of the Burgers vector of an edge or screw dislocation normal to the crystal surface is shown to be unambiguously determined from the section-topograph image of this dislocation. The sign of the Burgers vector of a screw dislocation can also be determined from its image taken with Lang projection topography. The contribution of a long-range strain field to the section images of edge and screw dislocations normal to the crystal surface is revealed. The experimental contrasts recorded using section topography and Borrmann-effect-based topography are compared.     

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.     

Polar Dependence of Threading Dislocation Density in GaN Films Grown by Metal-Organic Chemical Vapor Deposition

We investigate the threading dislocation(TD) density in N-polar and Ga-polar GaN films grown on sapphire substrates by metal-organic chemical vapor deposition.X-ray diffraction results reveal that the proportion of screw type TDs in N-polar GaN is much larger and the proportion of edge type TDs is much smaller than that in Ga-polar.Transmission electron microscope results show that the interface between the AlN nucleation layer and the GaN layer in N-polar films is smoother than that in Ga-polar films,which suggests different growth modes of GaN.This observation explains the encountered difference in screw and edge TD density.A model is proposed to explain this phenomenon.     

The gauge theory of dislocations: Static solutions of screw and edge dislocations

We investigate the T(3)-gauge theory of static dislocations in continuous solids. We use the most general linear constitutive relations in terms of the elastic distortion tensor and dislocation density tensor for the force and pseudomoment stresses of an isotropic solid. The constitutive relations contain six material parameters. In this theory, both the force and pseudomoment stresses are asymmetric. The theory possesses four characteristic lengths ?₁, ?₂, ?₃ and ?₄, which are given explicitly. We first derive the three-dimensional Green tensor of the master equation for the force stresses in the translational gauge theory of dislocations. We then investigate the situation of generalized plane strain (anti-plane strain and plane strain). Using the stress function method, we find modified stress functions for screw and edge dislocations. The solution of the screw dislocation is given in terms of one independent length ?₁ = ?₄. For the problem of an edge dislocation, only two characteristic lengths ?₂ and ?₃ arise with one of them being the same ?₂ = ?₁ as for the screw dislocation. Thus, this theory possesses only two independent lengths for generalized plane strain. If the two lengths ?₂ and ?₃ of an edge dislocation are equal, we obtain an edge dislocation, which is the gauge theoretical version of a modified Volterra edge dislocation. In the case of symmetric stresses, we recover well-known results obtained earlier.     

Peierls-Nabarro model of non-planar screw dislocation cores

The Peierls-Nabarro model originally developed for dislocations with planar cores is modified to describe the cores of screw dislocations extended along two or three intersecting slip planes, under the action of external stress. This concept generalizes the simplified concept of sessile splitting of screw dislocations into singular partials and enables an instructive interpretation of fully atomistic models of screw dislocation cores developed recently for b.c.c. metals. As an example, a numerical solution of the modified Peierls-Nabarro equation is given for the equilibrium configuration of a 1/2 [111] screw dislocation core in -Fe extended along three {110} planes.     

Evidence from numerical modelling for 3D spreading of [001] screw dislocations in Mg2SiO4 forsterite

Computer simulations have previously been used to derive the atomic scale properties of the cores of screw dislocations in Mg₂SiO₄ forsterite by direct calculation using parameterized potentials and via the Peierls–Nabarro model using density functional theory. We show that, for the [001] screw dislocation, the parameterized potentials reproduce key features of generalized stacking fault energies when compared to the density functional theory results, but that the predicted structure of the dislocation core differs between direct simulation and the Peierls–Nabarro model. The [001] screw dislocation is shown to exhibit a low-energy non-planar core. It is suggested that for this dislocation to move its core may need to change structure and form a high-energy planar structure similar to that derived from the Peierls–Nabarro model. This could lead to dislocation motion via an unlocking–locking mechanism and explain the common experimental observation of long straight screw dislocation segments in deformed olivine.     

Stable edge dislocations in finite crystals

Dislocations have been considered as mechanically unstable defects in bulk crystals, ignoring the Peierls oscillations. Eshelby [J. Appl. Phys. 24 (1953) p.176] had showed that a screw dislocation can be stable in a thin cylinder. In the current work, considering Eshelby's example of an edge dislocation in a single crystalline plate, we show that an edge dislocation can be stable in a finite crystal. Using specific examples, we also show that the position of stability of an edge dislocation can be off-centre. This shift in the stability from the centre marks the transition from a stable dislocation to an unstable one. The above-mentioned tasks are achieved by simulating edge dislocations using the finite element method.