Dislocation Core Configurations Research Articles

Dislocations in AlGaN: Core Structure, Atom Segregation, and Optical Properties.

We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.

Atomic configurations of dislocation cores in a small-angle grain boundary of 3C-SiC film

[110] images are taken for 3C-SiC/(001)Si hetero epitaxial films containing small-angle grain boundaries by using a 200 kV LaB6 filament high-resolution transmission electron microscope. Deconvolution processing is performed to transform the experimental images which do not represent intuitively the projected crystal structure into structure images. First, Si and C atomic columns with a distance of 0.109 nm are resolved in a perfect structure image region, and then recognized from each other by analyzing the image contrast change with sample thickness based on the pseudo-weak phase object approximation. Subsequently, two complex dislocation cores located in the vicinity of small-angle grain boundaries are obtained at an atomic level, and the atomic structure models are constructed and confirmed by matching the experimental images with the simulated ones. Hence, the atomic configurations of dislocation cores are derived from only a single experimental image with the average structure of perfect crystal known in advance. The formation of small-angle grain boundaries in 3C-SiC/Si with the occurence of complex dislocations in their vicinity is discussed.

Cs-Corrected Scanning Transmission Electron Microscopy Investigation of Dislocation Core Configurations at a SrTiO3/MgO Heterogeneous Interface

Heterostructures and interfacial defects in a 40-nm-thick SrTiO(3) (STO) film grown epitaxially on a single-crystal MgO (001) were investigated using aberration-corrected scanning transmission electron microscopy and geometric phase analysis. The interface of STO/MgO was found to be of the typical domain-matching epitaxy with a misfit dislocation network having a Burgers vector of ½ a(STO) <100>. Our studies also revealed that the misfit dislocation cores at the heterogeneous interface display various local cation arrangements in terms of the combination of the extra-half inserting plane and the initial film plane. The type of the inserting plane, either the SrO or the TiO(2) plane, alters with actual interfacial conditions. Contrary to previous theoretical calculations, the starting film planes were found to be dominated by the SrO layer, i.e., a SrO/MgO interface. In certain regions, the starting film planes change to the TiO(2)/MgO interface because of atomic steps at the MgO substrate surface. In particular, four basic misfit dislocation core configurations of the STO/MgO system have been identified and discussed in relation to the substrate surface terraces and possible interdiffusion. The interface structure of the system in reverse--MgO/STO--is also studied and presented for comparison.

Energetics and structure of ⟨0 0 1⟩ tilt grain boundaries in SiC

We have developed a scheme, based on molecular dynamics, that allows finding minimum energy structures of grain boundaries (GBs) with relatively large cell of non-identical displacements. This scheme has been used to study symmetric ⟨0 0 1⟩ tilt GBs in cubic SiC. We analyze atomic configurations of dislocation cores found in low-angle GBs and we report structural units found in high-angle GBs. In contrast to what had been previously assumed we find that the lowest energy structures often do not favor perfect coordination of GB atoms and that most of the analyzed GBs contain 6- and 7-atom rings. We tested the applicability of existing empirical potentials to studies of high-symmetry GB structures in SiC and we found the Tersoff potential to be most appropriate. Knowledge of detailed atomic structures of GBs is essential for future studies of GB-controlled phenomena in SiC, such as diffusion of metallic fission product through this material or GB strengthening.

TEM of C-component Dislocations Associated with Pyramidal Slip Activity in Hexagonal α2-Ti3Al

AbstractWhile only a minor phase constituent, the deformation behavior of the hexagonal α2-Ti3Al phase, significantly affects the mechanical properties of two-phase TiAl based alloys. We have used conventional and high-resolution transmission electron microscopy to investigate the fine structure of pyramidal plane glide dislocations, with Burgers vectors of the type b=&lt;2c+a&gt;, in α2-Ti3Al after room temperature compression of binary polysynthetically twinned TiAl normal to the lamellar interfaces to nominal plastic strains of 1%-7%. We report atomic resolution observations of non-co-planar dislocation core configurations for &lt;2c+a&gt;-dislocations and show that translamellar deformation twins active in the majority γ-TiAl phase play an important role in facilitating pyramidal plane slip in α2-Ti3Al in the lamellar two-phase alloys.

Characterization of room-temperature plastic deformation of ß-Si3N4 by atomic force microscopy and transmission electron microscopy

Dislocation microstructures induced by room-temperature microhardness tests have been investigated in silicon nitride. Surface analysis of the residual indent by atomic force microscopy reveals intragranular slip bands and demonstrates that room-temperature plastic deformation involves dislocation motion as well as cross-slip events. Cross-slip events have been found to occur between {1010} prismatic planes. Transmission electron microscopy shows that dislocations have a Burgers vector b = [0001] and are located along the screw direction. Based on these observations, specific dislocation core configurations are discussed.

Dislocation microstructures in Si plastically deformed at RT

Dislocation microstructures induced by plastic deformation at roomtemperature in Si have been investigated by TEM. Plastic deformation hasbeen obtained by using two types of technique: deformation under aconfining pressure of 5 GPa in an anisotropic multi-anvil apparatus and bysurface scratching. The TEM observations show common features in the twodeformation substructures which are characteristic of high stress-lowtemperature deformation. The deformation microstructures are built withdislocations with a/2⟨110⟩ Burgers vector in (111) planeswhich are undissociated. Such dislocations are mainly aligned along thescrew orientation and ⟨112⟩ orientations at 30° fromthe Burgers vectors as well as along ⟨132⟩ orientations at41° from the Burgers vector. The occurrence of those Peierls valleysconfirms that different dislocation core configurations from those usuallydealt with at higher temperatures have to be taken into account whendislocations are nucleated at very high stresses.

Atomistic simulation of dislocation core configurations in TiAl

Abstract Dislocation core structures and γ surfaces in L10 TiAl were computed using embedded-atom interatomic potentials. The γ-surface studies for the (111) plane showed that no faults other than the antiphase boundary, superlattice intrinsic stacking fault and complex stacking fault appear to be stable on the close-packed plane of TiAl. Different possibilities of perfect superdislocation dissociation were examined on the basis of the dislocation core splitting configuration and separation between partials as revealed by atomistic computer simulation. Both ordinary and superdislocations in various orientations were studied with particular focus on the mechanisms of formation of nonplanar core configurations.

Atomistic simulation of dislocation core configurations in TiAl

Abstract Dislocation core structures and γ surfaces in L10 TiAl were computed using embedded-atom interatomic potentials. The γ-surface studies for the (111) plane showed that no faults other than the antiphase boundary, superlattice intrinsic stacking fault and complex stacking fault appear to be stable on the close-packed plane of TiAl. Different possibilities of perfect superdislocation dissociation were examined on the basis of the dislocation core splitting configuration and separation between partials as revealed by atomistic computer simulation. Both ordinary and superdislocations in various orientations were studied with particular focus on the mechanisms of formation of nonplanar core configurations.

Atomistic modeling of planar faults, dislocations and twins in a non-cubic intermetallic compound

This paper describes the nature of planar faults, atomic configurations of dislocation cores and a mechanism for the nucleation of twins in a model DO22 intermetallic compound. The results are in good agreement with TEM observations of dislocations and twins in Ni3V, which crystallizes in this structure. The calculations are also helpful in understanding the general deformation features of DO22 compounds.

Atomistic modeling of dislocations in Be 12X compounds

Behavior of straight dislocations, both edge and screw, in Be 12X-type intermetallic compounds is studied. These compounds, e.g. Be 12Nb and Be 12Ta, offer a unique combination of high melting point and high strength-to-weight ratio, but have limited ductility. This study is aimed at gaining a better understanding of the deformation mechanisms that exist in these types of compounds. Recent transmission electron microscopy studies indicate the existence of both a[100]- and a/2[100]-type dislocations in these materials. Accordingly, the dislocations currently being investigated by atomic simulation models are a[100] edge, a/2[100] and a[100] screw. The model is a cylindrical array of atoms with the dislocation lying along its axis. The atomic interactions are modeled by the embedded atom method potentials. Details of the dislocation core configurations, including the rearrangement of atoms relative to the linear elastic field during dynamic relaxation, are described.

TEM study of dislocation core configurations in Ni3(Al,l%Ta) deformed at high temperatures

The Ni3Al intermetallic compound of the ordered L12 structure with various alloying elements (such as Ta or Ti) reinforces most of the industrial superalloys which are designed for high temperature applications. Therefore, the understanding of its mechanical properties is of primary importance. The yield strength, measured in constant strain rate tests, exhibits an anomalous behaviour as a function of temperature: it increases up to a peak temperature above which it finally decreases. The anomalous increase has been extensively studied during the last 30 years, including weak-beam and high resolution transmission electron microscopy (TEM) observations and computer simulations. Numerous models have been proposed to explain this anomalous variation in which the structure of the dislocation core plays a fundamental role. A review of core effects on the plasticity of ordered alloys was given by Vitek (1985). In the present investigation, the Ni3(Al,l%Ta) phase was deformed at temperatures above the yield strength peak and the detailed core configurations of dislocations have been analyzed using weak-beam and high resolution TEM techniques.

Computer simulation of dislocation cores in h.c.p. metals I. Interatomic potentials and stacking-fault stability

Abstract Four pair-wise potentials have been found to be suitable for the simulation by computer of stacking faults and dislocation core configurations in model crystals having the hexagonal-close-packed structure. The potentials define stress-free perfect crystals and have markedly different ranges and shapes, but the crystals have very similar elastic properties. The stability of translational stacking faults on basal and non-basal planes has been investigated, and it is concluded that stable faults can exist on all the planes considered. The translation vectors and volume expansions associated with the faults are reasonably well understood in terms of the hard-sphere model of the structure, irrespective of the form of the potential, and changes in atom coordination at the non-basal faults may be used to estimate the fault energy. The potentials are used in parts II and III to investigate dislocation behaviour.

Atomistic breathing shell model calculations of dislocation core configurations in ionic crystals

Abstract A study was undertaken to improve upon a previous rigid boundary atomistic calculation of the core properties of a/2〈110〉{110} edge dislocations in MgO. A flexible boundary method (Flex-II) based on linear elasticity theory and recently developed by Hoagland, Hirth and Gehlen (1976) was used to provide a more accurate determination of the boundary between the atomistic and the elasto-atomic regions. A central-force breathing shell model due to Sangster (1973) was used to take account of many-body effects in the interaction between ions. This marks the first application of the breathing shell model to the calculation of dislocation core configurations in ionic crystals. Within this model two potentials were used. One is due to Sangster (1973), the other is our previously developed Model 1 potential. The results obtained using the above potentials are analysed in terms of the stability of the dislocation symmetry type, the core displacement field, the volume of expansion and the total strain energy...

The core configuration of dislocations with large Burgers vectors

The core configuration of dislocations with large Burgers vectors