Direction Of Burgers Vector Research Articles

Mechanisms and rate of dislocation nucleation in aluminum-copper alloys near Guinier-Preston zones

This article is devoted to a molecular dynamics simulation study of partial dislocation loop nucleation with respect to its mechanism and rate, and its propagation process under high shear stress in aluminum-copper alloys. The mechanisms of dislocation nucleation near Guinier-Preston (GP) zones of various diameters and concentrations have been analyzed. Dislocation nucleation rates near plain GP Cu-zones with diameters of 3.5, 7.5, and 13.5 nm and at various concentrations have been calculated using the mean lifetime method with temperatures in range of 100 and 700 K. It has been found that depending on the temperature and applied stress, the dislocation can nucleate either from the edge, or from the plain area of a GP zone. The dislocation nucleation is preceded by a generation of defect clusters that are formed due to local opposite atomic shifts in two adjacent (111) planes by the half-length of a Burgers vector of a partial dislocation. The expansion of a partial dislocation loop can be accompanied by the formation of twins via a shift of the atoms in the internal region of the loop. The twin velocity along the direction of the partial dislocation Burgers vector inside the loop can achieve longitudinal sound speed. The speeds of the edge and screw segments of a partial dislocation loop as a function of a shear stress component along the Burgers vector have been estimated. The latter seems to be limited by the shear sound speed.

Empirical potential simulations of interstitial dislocation loops in uranium dioxide

Stoichiometric circular shaped interstitial dislocation loop energies are calculated in stoichiometric UO2 by empirical potential simulation. The Burgers vector directions studied are 〈110〉 and 〈111〉. The main structural properties of each type of interstitial dislocation loop are determined, including stacking fault energy. Defect energies are compared and a maximum size for stable 〈111〉 dislocation loops before transition to 〈110〉 dislocation loops is given. A model of dislocation loop energy based on elasticity theory is then fitted on the basis of these simulation results.

Comprehensive molecular dynamics simulations of the stacking fault tetrahedron interacting with a mixed dislocation at elevated temperature

The defect-free channels were frequently observed in irradiated materials, i.e. copper, as a result of the stacking fault tetrahedron (SFT) interactions with dislocations. However, the underlying mechanisms for this process are still unclear to date. To address them, a comprehensive study on the interactions between SFTs and mixed dislocations was performed using molecular dynamics simulations. In particular, eight interaction geometries were considered, in terms of the dislocation Burgers vector directions, dislocation gliding directions and intersection positions on SFT. Various interaction outcomes were revealed after dislocation detachment. (1) SFT is fully absorbed through the transformation into Lomer dislocations, and subsequently moves out of free surfaces along the dislocation. (2) SFT is partially absorbed with the absorbed SFT base moving out of free surfaces along the dislocation. (3) SFT is not absorbed but sheared with ledges left on the SFT faces. (4) SFT is unaffected by the mixed dislocation. The current simulations, especially the full SFT absorption, provide important insights into the forming mechanisms of defect-free channels in irradiated materials.

Dislocation mechanisms in a zirconium alloy in the high-temperature regime: An in situ TEM investigation

Dislocation mechanisms responsible for the high-temperature mechanical properties of a Zr alloy have been investigated using in situ straining experiments between 250°C and 450°C. At 250°C and 300°C, the results show a steady and homogeneous dislocation motion in prismatic planes, with little cross-slip in the pyramidal and/or basal planes. At 350°C, the kinetics of mobile dislocations becomes very jerky and inhomogeneous, in agreement with a dynamic strain aging mechanism. Above this temperature, the motion is again steady and homogeneous. Extensive cross-slip forms super-jogs which are efficient pinning points against the glide motion. These super-jogs move by glide along the Burgers vector direction, never by climb. The glide velocity between super-jogs is linear as a function of the total driving stress (applied stress minus line-tension stress due to dislocation curvature), in agreement with the solute dragging mechanism. The origin of the stress–strain rate dependence with an exponent larger than unity is then discussed.

An automatic and simple method for specifying dislocation features in atomistic simulations

An important aspect of atomistic simulations of imperfect crystalline materials is the characterization of dislocations. In this paper, we propose the use of singular values and singular vectors of the Nye tensor to specify the features of dislocations, including the cores, nodes, Burgers vectors and line directions. Atoms locate in dislocation cores and nodes are identified by non-trivial singular values of the Nye tensor, while Burgers vector direction and line direction are specified by those singular vectors corresponding to the biggest singular value. This method provides a powerful, intuitive and automatic tool for characterizing dislocations from simulation data. Its validity for characterizing dislocations is verified with three examples.

Photoluminescence Imaging and Discrimination of Threading Dislocations in 4H-SiC Epilayers

This paper demonstrates high-resolution photoluminescence (PL) imaging and discrimination of threading dislocations in 4H-SiC epilayers. Threading screw dislocations (TSDs) and TEDs are distinguished by differences in PL spot size and spectrum. We found that TEDs are further discriminated into six types according to their Burgers vector directions by the appearance of PL imaging. Cross-sectional PL imaging reveals inclination angles of threading dislocations across a thick epilayer.

Characterization of Threading Edge Dislocation in 4H-SiC by X-Ray Topography and Transmission Electron Microscopy

A threading dislocation (TD) in 4H-SiC, which was currently interpreted as a perfect threading edge dislocation (TED) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na2O2 additive, was performed comparative characterization using weak-beam dark-field (WBDF) and large-angle convergent-beam electron diffraction (LACBED) methods. The TD was suggested to be dissociated into a dislocation pair which can be observed in the WBDF image of g=-12-10. The TD, which was identified as b//[-12-10] by SMBXT observation, was unambiguously determined as b=1/3[-12-10] by LACBED analysis. In the case of perfect TED, it was found that the direction of Burgers vector derived from SMBXT observation corresponds to LACBED analysis.

Superconducting pairing and the pseudogap in the nematic dynamical stripe phase of La2−xSrxCuO4

Fully absorption coefficient corrected Raman spectra were obtained in La2−xSrxCuO4. The B1g spectra have a Fleury–Loudon type two-magnon peak (resonant term) whose energy decreases from 3180 cm−1 (394 meV) to 440 cm−1 (55 meV) on increasing the carrier density from x = 0 to 0.25, while the B2g spectra have a 1000–3500 cm−1 (124–434 meV) hump (hill) whose lower-edge energy increases from x = 0 to 0.115 and then stays constant to x = 0.25. The B2g hump is assigned to the electronic scattering (non-resonant term) of the spectral function with magnetic self-energy. The completely different carrier density dependence arises from anisotropic magnetic excitations of spin–charge stripes. The B1g spectra were assigned to the sum of k ∥ and k⊥ stripe excitations and the B2g spectra to k⊥ stripe excitations according to the calculation by Seibold and Lorenzana (2006 Phys. Rev. B 73 144515). The k ∥ and k⊥ stripe excitations in fluctuating spin–charge stripes were separately detected for the first time. The appearance of only k⊥ stripe excitations in the electronic scattering arises from the charge hopping perpendicular to the stripe. This is the same direction as the Burgers vector of the edge dislocation in metal. The successive charge hopping in the Burgers vector direction across the charge stripes may cause Cooper pairs as predicted by Zaanen et al (2004 Ann. Phys. 310 181). Indeed, this is supported by the experimental fact that the superconducting coherent length coincides with the inter-charge stripe distance in the wide carrier density range. The one-directional charge hopping perpendicular to the stripe causes the flat Fermi surface and the pseudogap near (π,0) and (0,π), but the states around (π/2,π/2) cannot be produced. The low-energy Raman scattering disclosed that the electronic states at the Fermi arc around (π/2,π/2) are coupled to the A1g soft phonon of the tetragonal–orthorhombic phase transition. This suggests that the Fermi arc is produced by the electron–phonon interaction. All the present Raman data suggest that Cooper pairs are formed at moving edge dislocations of dynamical charge stripes.

Fatigue crack initiation – The role of point defects

The role of point defects in the formation of surface relief and in the initiation of a fatigue crack in crystalline materials is analyzed. The dislocation interactions in the bands of intensive cyclic slip (persistent slip bands – PSBs) are specified and relations describing the formation and annihilation of interstitial and vacancy type defects in the channels of the ladder-like PSB are derived.The continuous formation, annihilation and primarily the migration of point defects are proposed to be responsible for the mass redistribution within PSB and between PSB and the PSB/matrix boundary. The redistribution of the matter results in local tensile and compressive stresses that are the sources of the principal irreversibility of slip within PSB. Local tensile and compressive stresses are relaxed by dislocation movement within PSB in the direction of the active Burgers vector and lead to the formation of characteristic surface relief in the form of extrusions and intrusions. The intrusions represent crack-like defects and fatigue cracks initiate in the tip of intrusions.

Plan-View and Cross-Sectional Photoluminescence Imaging Analyses of Threading Dislocations in 4H-SiC Epilayers

We performed a plan-view and cross-sectional photoluminescence (PL) imaging and a spectral analysis of threading dislocations in 4H-SiC epilayers in the near-infrared region. The bright PL spots of threading screw dislocations (TSDs) and threading edge dislocations (TEDs) observed in the plan-view PL imaging are compared with the grazing incidence synchrotron X-ray topography contrast, and precise discrimination of threading dislocations using the PL technique and the direct acquisition of Burgers vector directions of TEDs are demonstrated. The inclination angles of TSDs and TEDs across a thick epilayer are revealed by the cross-sectional PL imaging, and the variations in the plan-view PL appearances of the threading dislocations are confirmed to originate from the line directions of such dislocations.

Finite element investigation of size effects on the mechanical behavior of nickel single crystals

Abstract The influence of dimensions on the mechanical behavior of f.c.c. single crystals with dimensions larger than a few micrometers has been the topic of many experimental investigations and controversies during the 1970s and this question is still open. The objective of this article is to shed new light on this point by performing finite element simulations thanks to a strain gradient crystal plasticity model. Based on the model identification for nickel, several single crystal samples with various thicknesses and orientations were tested numerically in tension. The effect of dimensions was then analyzed considering the spatial distribution of dislocation densities. Near loading boundaries, dislocation density gradients appeared perpendicular to the Burgers vector direction of the primary activated slip system which modified the mechanical behavior. These gradients are discussed in terms of boundary conditions, crystal orientation and dislocation interactions with surfaces.

Characterization of Basal Plane Dislocations in 4H-SiC Substrates by Topography Analysis of Threading Edge Dislocations in Epilayers

Burgers vector directions of threading edge dislocations (TEDs) in 4H-SiC epitaxial layer are distinguished by grazing incidence high resolution topography. Based on comparison between appearance of KOH etch pits and direction of TED Burgers vector, the size difference of the TED etch pits is found to be dependent on their Burgers vector directions. Examining TEDs in the epilayer by topography, the Burgers vector direction of basal plane dislocations (BPDs) in the substrate is identified. Correspondence between the topography contrast and the sense of a BPD is also investigated.

Peierls stress for 〈110〉{001} mixed dislocation in SrTiO3 within framework of constrained path approximation

The core structure of 〈110〉{001} mixed dislocation in perovskite SrTiO3 is investigated with the modified two-dimensional Peierls–Nabarro dislocation equation considering the discreteness effect of crystals. The results show that the core structure of mixed dislocation is independent of the unstable energy in the 〈100〉 direction, but closely related to the unstable energy in the 〈110〉 direction which is the direction of total Burgers vector of mixed dislocation. Furthermore, the ratio of edge displacement to screw one nearly equals to the tangent of dislocation angle for different unstable energies in the 〈110〉 direction. Thus, the constrained path approximation is effective for the 〈110〉{001} mixed dislocation in SrTiO3 and two-dimensional equation can degenerate into one-dimensional equation that is only related to the dislocation angle. The Peierls stress for 〈110〉{001} dislocations can be expediently obtained with the one-dimensional equation and the predictive values for edge, mixed and screw dislocations are 0.17, 0.22 and 0.46 GPa, respectively.

Resolving the Burgers vector for individual GaN dislocations by electron channeling contrast imaging

We report evidence for non-destructive determination of dislocation type and Burgers vector direction in GaN using electron channeling contrast imaging (ECCI) inside a scanning electron microscope. Forescattered electron intensity fluctuations generated by threading dislocations exhibit characteristic spatial profiles indicative of dislocation type (screw, edge) and Burgers vector direction. Simulated channeling contrast features by two-beam dynamical diffraction calculations show qualitative agreement with recorded images. Forescatter ECCI sensitivity to atomic steps and low-angle grain boundaries in GaN allow for additional confirmation.

Surface Morphology and Defect Formation Mechanisms for HgCdTe (211)B Grown by Molecular Beam Epitaxy

The surface morphology and crystallinity of HgCdTe films grown by molecular beam epitaxy (MBE) on both CdZnTe and CdTe/Si (211)B substrates were characterized using atomic force microscopy (AFM), as well as scanning (SEM) and transmission (TEM) electron microscopy. Crosshatch patterns and sandy-beach-like morphologies were commonly found on MBE (211) HgCdTe epilayers grown on both CdZnTe and CdTe/Si substrates. The patterns were oriented along the $$ {\left[ {\overline{2} 13} \right]} $$ , $$ {\left[ {\overline{2} 31} \right]} $$ , and $$ {\left[ {0\overline{1} 1} \right]} $$ directions, which were associated with the intersection between the (211) growth plane and each of the eight equivalent HgCdTe slip planes. This was caused by strain-driven operation of slip in these systems with relative large Schmid factor, and was accompanied by dislocation formation as well as surface strain relief. Surface crater defects were associated with relatively high growth temperature and/or low Hg flux, whereas microtwins were associated with relatively low growth temperature and/or high Hg flux. AFM and electron microscopy were used to reveal the formation mechanisms of these defects. HgCdTe/HgCdTe superlattices with layer composition differences of less than 2% were grown by MBE on CdZnTe substrates in order to clarify the formation mechanisms of void defects. The micrographs directly revealed the spiral nature of growth, hence demonstrating that the formation of void defects could be associated with the Burton, Cabrera, and Frank (BCF) growth mode. Void defects, including microvoids and craters, were caused by screw defect clusters, which could be triggered by Te precipitates, impurities, dust, other contamination or flakes. Needle defects originated from screw defect clusters linearly aligned along the $$ {\left[ {0\overline{1} 1} \right]} $$ directions with opposite Burgers vector directions. They were visible in HgCdTe epilayers grown on interfacial superlattices. Hillocks were generated owing to twin growth of void or needle defects on (111) planes due to low growth temperature and the corresponding insufficient Hg movement on the growth surface. Therefore, in addition to nucleation and growth of HgCdTe in the normal two-dimensional layer growth mode, the BCF growth mode played an important role and should be taken into account during investigation of HgCdTe MBE growth mechanisms.

Nondestructive dislocation delineation using topographically enhanced imaging of surface morphologies in 4H-SiC epitaxial layers

The morphology of surface features generated by dislocations present at 4H-SiC epitaxial layer surfaces was investigated by forescattered electron detection (FED) inside a conventional scanning electron microscope. Various growth pit morphologies were correlated to dislocation types using molten KOH etching. Specifically, sharp-apex pits and stripe-shaped pits were consistently linked to screw and edge dislocations, respectively. The size and depth of these growth pits were measured by atomic force microscopy (AFM). Tail-like features were observed by FED emanating from sharp-apex pits and verified by Nomarski optical microscopy (NOM). A mechanism is proposed to explain the FED contrast exhibited by these tail-like features. This mechanism relates the nature of step-flow and spiral growth in the wake of a screw dislocation to the surface distortions resulting in such tail-like features. The Burgers vector direction can thus be determined based on a purely morphological analysis of these tail-like features. The results of this study illustrate the various capabilities of FED for surface imaging as compared to AFM and NOM. The potential for utilizing FED to map dislocation-associated growth pits is discussed.

High-resolution x-ray topography of dislocations in 4H-SiC epilayers

Synchrotron x-ray topography with a high-resolution setup using 1128 reflection was carried out on 4H-SiC epilayers. Four different shapes of threading-edge dislocation according to Burgers vector direction were observed. The four types of threading-edge dislocation images were calculated by computer simulation, and the experimental results correlated well with the simulation results. The detailed topographic features generated by plural screw dislocations and basal plane dislocations were also investigated.

High-Resolution X-ray Topography of Dislocations in 4H-SiC Epilayers

AbstractSilicon carbide (SiC) substrates and epilayers contain many crystal defects, such as micropipes, screw dislocations, threading edge dislocations (TEDs), basal plane dislocations (BPDs) and stacking faults. To investigate these defects, synchrotron radiation topography is frequently carried out. When the monochromatic synchrotron X-ray topography is taken by the grazing-incidence reflection geometry using 11-28 reflection, screw dislocations, TEDs and BPDs are simultaneously seen and shown as different topographic images [1]. Many studies of dislocations were reported using 11-28 reflections in 4H-SiC [1,2]. Topographic images of the dislocations have been analyzed by the ray-tracing method of computer simulation [3]. However, experimental images of dislocations were not fully matched to the fine structure of simulation images, because of a lack of resolution in recording media: conventional films and nuclear emulsion plates [3]. This time, we report obtaining high-resolution topographic images using a new recording medium, and compare results between the experiment and the computer simulation. Synchrotron topography in 11-28 reflection was carried out at SPring8 applying holography films as high-resolution recording media. The TED images are distinguished as four types, which have ribbon-like features with different rotating angles, through the use of the films. The four different TED images agree well with the computer simulated images which have been reported by Vetter et.al. taking into account of the different Burgers vector directions [3]. By comparing the three topographic images taken at g=-12-18, 11-28 and 2-1-18, we confirmed experimentally that the four types of TED images originated from the difference of Burgers vector directions. We also investigated high-resolution topographic images of elementary screw dislocations, micropipes, and BPDs in 4H-SiC epilayers. The experimental image of screw dislocation fairly matched with simulated image. The fine features in the experimental topographic images of micropipes and BPDs are also compared with the simulated images in detail. [1] T. Ohno, H. Yamaguchi, S. Kuroda, K. Kojima, T. Suzuki, K. Arai: J. cryst. Growth. Vol. 260 (2004) 209. [2] H. Tsuchida, T. Miyanagi, I. Kamata, T. Nakamura, R. Ishii, K. Nakayama and Y.Sugawara: Jpn. J. Appl. Phys. Vol. 25, (2005), L806-808. [3] W. Vetter, H. Tsuchida, I. Kamata, M. Dudley: J. Appl. Cryst. Vol. 38, (2005), 442-447.

Atomistic simulations of kinks in1/2a〈111〉screw dislocations in bcc tantalum

Two types of equilibrium core structures (denoted symmetric and asymmetric) for $1/2a〈111〉$ screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric (or polarized) cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric (nonpolarized) cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of $1/3a〈112〉$ kinks in $1/2a〈111〉$ screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum (Ta) all of which reproduce bulk properties (density, cohesive energy, and elastic constants) from quantum mechanics calculations but differ in the resulting polarization of $1/2a〈111〉$ screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations [with positive (P) and negative (N) polarization], leading to 2 types of (polarization) flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of $1/2a〈111〉$ screw dislocations is an important factor in determining the kink-pair formation energy.

Structure of dislocations and stacking faults in the complex intermetallic ξ′-(Al–Pd–Mn) phase

We present the results of an electron microscopy study of defects in plastically deformed single crystals of the intermetallic ξ′-(Al–Pd–Mn) phase. Pure edge dislocations with two different Burgers vector directions and four different Burgers vector magnitudes were found. All Burgers vector magnitudes observed can be described in terms of irrational fractions of the unit-cell parameters, and we have observed Burgers vector directions that can be indexed using irrational indices. The stacking faults observed have displacement vectors whose magnitudes and directions are incompatible with the unit cell of the ξ′ phase. A comparison of the Burgers vectors observed in this study with those reported for the corresponding icosahedral quasicrystal shows that they are equivalent with respect to their directions and lengths. This leads to the conclusion that local order rather than long-range periodic (or quasiperiodic) order governs the structure of defects in these materials.