Structure Of Dislocation Networks Research Articles

Phase equilibria, crystal growth and characterization of the novel ferroelectric tungsten bronzes (CBN) and (CSBN)

Abstract Phase diagram analyses and crystal growth experiments of the novel ferroelectric tetragonal tungsten bronzes Ca x Ba 1 - x Nb 2 O 6 (calcium barium niobate, CBN) and Ca x Sr y Ba 1 - x - y Nb 2 O 6 (calcium strontium barium niobate, CSBN) were performed in this study. The quasi-binary phase diagram BaNb 2 O 6 – CaNb 2 O 6 was found to be similar to the related system BaNb 2 O 6 – SrNb 2 O 6 , but with a remarkably reduced existence region. In the quasi-ternary phase diagram CaNb 2 O 6 – SrNb 2 O 6 – BaNb 2 O 6 the stability field of Ca x Sr y Ba 1 - x - y Nb 2 O 6 mixed crystals was determined by X-ray powder diffraction of sintered and annealed samples. Large, transparent, and nearly colorless CBN and CSBN single crystals of high quality were grown by the Czochralski method. The dislocation network structure of the core region in the as-grown crystals was analyzed by X-ray topography employing white beam synchrotron radiation. Precise measurements of the refractive index dispersion showed a composition dependance only for the extraordinary beam. The Curie temperature of the ferroelectric phase transition 4 m mm → 4 mm determinated by calorimetric measurements exhibited a clear composition dependance. It is expected that in the complete mixed crystal system Ca x Sr y Ba 1 - x - y Nb 2 O 6 the Curie temperature could be varied between about 80 ∘ C for strontium barium niobate with x Sr = 0.61 and about 300 ∘ C for CBN with x Ca ≈ 0.20 , the lowest possible Ca content.

Regular Dislocation Networks in Si. Part II: Luminescence

Regular dislocation networks formed as a result of the direct bonding of Cz-Si wafers with oxide remnants on the pre-bonding surfaces were investigated. Besides the dislocation network, oxide precipitates were detected at the bonding interface. The precipitate density across the network was ~5×1010 cm-2, except small irregularly distributed circular areas, several mm in diameter, where the density was remarkably lower (<5×108 cm-2). The dislocation network structure was not affected by the change in the precipitate density. Photoluminescence spectroscopy (PL) and light beam induced current (LBIC) mapping were applied for characterization of such dislocation networks. For the locations with high precipitate density, PL signal from dislocations and that from the band-to-band transitions were enhanced. On the other hand, the LBIC results indicated that oxide precipitates are active recombination centers and thus should suppress the observed radiative transitions. The controversy can be explained in the assumption that the D-band PL signal increases due to scattering of excitation light by the precipitates and due to related expansion of the excitation area of the dislocation network. The light reflection from the precipitate layer also enhances the detected band-to-band PL signal. The shape of PL spectra from the samples in the range of photon energies 0.75 – 1.15 eV was not influenced by the oxide precipitates.

Misfit dislocation network in Cu/Ni multilayers and its behaviors during scratching

The structure and distribution of misfit dislocations at Cu–Ni interfaces and their effects on the tribological behavior of a Ni film are investigated with 3D Molecular Dynamic Simulations. The structure of misfit dislocation network at a Cu–Ni interface differs according to different crystallographic orientations of the film relative to the substrate: a triangle and square type of misfit dislocation network are observed at (111)Cu||(111)Ni and (001)Cu||(001)Ni interfaces respectively. They play an important role in the strengthening of Cu/Ni multilayers. During the scratching of a single asperity contact on the Ni film, the misfit dislocation network becomes a significant barrier to the glide dislocations. The plot of friction force vs. normal load when scratching on the Ni film exhibits a horizontal stage, representing the decreasing of the frictional coefficient due to the existence of the misfit dislocations network.

Pattern organization on Cu(111) in perchlorate solutions

Abstract The first in situ scanning tunneling microscopy (STM) study of Pb underpotential deposition (upd) on Cu(1 1 1) in perchlorate solutions is presented. High resolution STM shows a (4 × 4) Moire structure of a full density close-packed lead monolayer that is identical to that observed in ultrahigh vacuum. Interestingly, the STM experiments revealed nano-organization morphologies of the topmost Cu(1 1 1) layer following stripping of the lead monolayer in naturally aerated solutions. The length-scale and nature of the ordered structures was found to depend on solution pH. A Moire structure indicative of an anion adsorption-induced reconstruction was observed in pH 2 and a star-pattern type of dislocation network structure was observed in pH

Strain-rate effects on microstructural deformation in irradiated 316 SS

A series of studies have been performed to investigate the post-irradiation deformation and failure behavior of 12% cold-worked 316 stainless steel following irradiation to variety of doses and temperatures in the outer rows of the experimental breeder reactor II (EBR-II). In the current phase of these studies, three sets of samples with different radiation-induced microstructures have been characterized with transmission electron microscopy (TEM) following tensile testing to failure at a ‘fast’ strain-rate (1 × 10 −3 s −1) and a ‘slow’ strain-rate (1 × 10 −7 s −1). The samples were irradiated to doses between 9 and 41 dpa at temperatures between 383 and 443 °C. Tensile tests were conducted at a temperature of 430 °C and only regions outside of the necked region were examined. Over the parameters tested, strain-rate had a negligible effect on the deformation microstructure. In addition, there was no clear evidence of localized deformation behavior and the deformation appeared relatively homogeneous, characterized by unfaulting and incorporation of faulted dislocation loops into the general dislocation network structure. The influence of the defect microstructures and strain-rate on deformation behavior is discussed.

On the synthesis and characterization of La doped MgB2 superconductor

AbstractWe report the synthesis of La doped MgB2 superconductors with nominal compositions Mg1‐xLaxB2 (with x = 0.01, 0.03, 0.05, 0.07) by solid state reaction at ambient pressure. A special encapsulation technique has been used by us to prepare high quality superconducting MgB2 samples. The bulk polycrystalline samples possess superconducting transition temperature Tc(R=0) ranging between 36‐39 K. It has been found that transport critical current density Jc of the samples change significantly with the doping level of La. A high transport (Jc) value ∼1.9 x 103 A/cm2 at 15 K has been achieved for Mg0.97La0.03B2 sample. The XRD and TEM investigations indicate that the samples prepared by encapsulation method are devoid of MgO, which is generally found when synthesis of MgB2 is done through sintering of Mg and B powders. The detailed microstructural investigations of Mg0.97La0.03B2 specimens by transmission electron microscopy (TEM) reveal the presence of partial dislocation network, moiré fringes and superlattice structure in the as synthesized samples. The higher transport critical current density observed in Mg0.97La0.03B2 superconductor has been attributed to the partial dislocations which are capable of providing pinning centres. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STM observation of the quantum interference effect in finite-sized graphite

Superperiodic patterns were observed by STM on two kinds of finite-sized graphene sheets. One is nanographene sheets inclined from a highly oriented pyrolitic graphite (HOPG) substrate and the other is a several-layer-thick graphene sheets with dislocation-network structures against a HOPG substrate. As for the former, the in-plane periodicity increased gradually in the direction of inclination, and it is easily changed by attachment of a nanographite flake on the nanographene sheets. The oscillation pattern can be explained by the interference of electron waves confined in the inclined nanographene sheets. As for the latter, patterns and their corrugation amplitudes depended on the bias voltage and on the terrace height from the HOPG substrate. The interference effect by the perturbed and unperturbed waves in the overlayer is responsible for the patterns whose local density of states varies in space.

The dislocation microstructure of a nickel-base single-crystal superalloy after tensile fracture

The dislocation microstructure after tensile deformation for a Ni-base single-crystal superalloy has been studied by transmission electron microscopy. The samples were strained to fracture at temperatures from room temperature to 1273 K. It was found that for deformation at intermediate temperatures, the dislocation microstructure was inhomogeneous due to the formation of dislocation concentrations with high-density tangling. These dislocation concentrations lie mainly along 〈1 0 0〉 or 〈1 1 0〉 directions, and extend over a length of a few micrometers. In addition, the structure of dislocation networks at the γ/γ′ interfaces formed at high temperature was also studied. Although previous studies have proposed basic configurations for these dislocation networks, the nature of a dislocation segment along the 〈0 1 0〉 direction was unresolved (Phil. Mag. A vol. 74 (1996) 229). In this work, high-resolution electron microscopy observation was used to study these dislocation networks. It was found that most of these segments are actually composed of two pairs of a/2〈1 1 0〉 dislocations with an average spacing of 8.5 nm, while only few of them are composed of single-line a〈1 0 0〉 dislocations.

Potential-induced strain relaxation in au mono- and bilayer films on Pt(111) electrode surfaces.

In situ scanning tunneling microscopy observations of 1-2 monolayer thick Au films on Pt(111) electrodes are presented, which show a complex, potential-dependent structural phase behavior of the Au film. Starting from a pseudomorphic structure at <0.35 V(Ag/AgCl), a sequence of transitions into dislocation network structures occurs with increasing potential: (1x1)--> "striped phase" --> "hex phase" in the Au bilayer, (1x1)--> striped phase in the Au monolayer. This is explained by a reduction of the in-plane stress in the Au surface layer(s) due to anion adsorption and a strain energy minimization as described by the Frenkel-Kontorova model.

Influence of interfacial binding energy and misfit on the shape of the oxide precipitates in metals

Transmission electron microscopy revealed Mn3O4 precipitates with two types of dominant shapes in Pd-3at.%Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn3O4. These observations were compared with earlier observations in the Ag/Mn3O4 system. The octahedrons show a relatively larger (002) truncation in Pd than in Ag. Further, the second type of precipitate shape, comprising about 1/3 of all the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure. HRTEM observations revealed the presence of a square misfit dislocation network with line direction <110> and Burgers vector 1/2<110> at these interfaces with (002)Mn3O4//{200}Pd. The general conclusion of the present analysis is (i) anisotropy in the interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well defined shapes associated with the Wulff construction, (ii) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in the mismatch is present and if the matrix is sufficiently stiff, and (iii) the stronger coupling strength due to electronic binding effects across the interface in Pd compared to Ag is responsible for the formation of the dislocation network structures at larger misfit.

Influence of misfit and interfacial binding energy on the shape of the oxide precipitates in metals: ; Interfaces between Mn 3O 4 precipitates and Pd studied with HRTEM

Transmission electron microcopy (TEM) revealed Mn 3O 4 precipitates with two types of dominant shape in Pd–3 at.% Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn 3O 4. These observations were compared with earlier observations in the Ag/Mn 3O 4 system and the octahedrons show a relatively larger truncation by (002) in Pd than in Ag. Further, the second type of precipitate shape, comprising about 1/3 of all of the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure, showing an orientation relationship where the tetragonal axes of Mn 3O 4 are parallel to the cube axes of Pd, with the c-axis of Mn 3O 4 as habit plane normal. High-resolution TEM observations revealed the presence of a square misfit dislocation network with line direction 〈110〉 and Burgers vector 1/2〈110〉 at these interfaces with (002)Mn 3O 4‖{200}Pd. The general conclusions of the present analysis are: (1) the anisotropy in interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well-defined shapes associated with the Wulff construction; (2) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in mismatch is present and if the matrix is sufficiently stiff; and (3) the stronger coupling strength due to electronic binding effects across the interface in Pd compared with Ag is responsible for formation of the dislocation network structures at larger misfit.

A comparison of grain boundary topography and dislocation network structure in bulk-scale [001] tilt bicrystals of Bi 2Sr 2CaCu 2O 8+ x and YBa 2Cu 3O 7−δ

The facet topography and grain boundary dislocation content of the grain boundaries in nominally [001] tilt bicrystals of Bi 2Sr 2Ca 1Cu 2O 8+ x (2212) and YBa 2Cu 3O 7−δ (YBCO) are described and compared. Bicrystals with misorientation angles ranging from 8° to 15° were studied by polarized light microscopy, diffraction-contrast transmission electron microscopy (TEM) and high-resolution TEM. On the micron length scale of light microscopy, all of the boundaries appeared to be of pure tilt character and to form facets with dimensions on the order of fifty and several hundred micrometers in YBCO and 2212, respectively. However, TEM studies revealed a distinctly different characteristic pattern of faceting on the 100 nm length scale. In 2212, a rather irregular arrangement of nominally pure symmetrical [001] tilt facets and [001] twist facets is suggested. Very regular saw-tooth shaped arrangements of just two types of tilt facets predominate in YBCO. The nanoscale topography and structure of the tilt boundary facets show similarities in the two materials. Boundaries in both materials tend to facet further onto symmetric tilt planes on the nanoscale. This preference is most pronounced in 2212. A single set of partial edge dislocations and their associated stacking faults comprises the grain boundary dislocation network structure of the symmetric segments in both materials. Thus, the boundaries show nanoscale similarities, mesoscale (∼100 nm) differences, and a number of different possible microstructural sources for the heterogeneous electrical character that is indicated by their electromagnetic properties.

Heat treatment effects for improved creep-rupture resistance of a Fe3Al-based alloy

Abstract The iron-aluminide alloy FA-180, an Fe 3 Al-based alloy with a composition of Fe-28Al-5Cr-0.5Nb-0.8Mo-0.025Zr-0.05C-0.005B (at%), is of interest because it has improved creep-rupture resistance when compared to other Fe 3 Al and FeAl-based alloys. Previous creeprupture testing at 593 °C and 207 MPa has shown that FA-180 has a rupture life of approximately 100 h in the warm-rolled and stress relieved (l h at 700–750 °C) condition, as compared to about 20 h for the FA-129 base alloy (Fe-28Al-5Cr-0.5Nb-0.2C). Solution-annealing for 1 h at 1150 °C (followed by air cooling) dramatically improved the creeprupture life of FA-180 to about 2000 h but had little effect on the FA-129 base alloy. Transmission electron microscopy analysis showed that this strengthening was due to the precipitation of fine ZrC in the matrix and along grain boundaries. In the current study, creeprupture data showed that a further improvement in creeprupture life to over 6000 h can be produced by increasing the cooling rate (quenching in oil or water) after solution annealing at 1150 °C. Instead of ZrC precipitates, the microstructure of quenched FA-180 contained many fine dislocation loops which were determined to have a (001) habit plane, b //[001]. They are therefore most likely vacancy in nature. These fine loops evolved into a structure of larger dislocation loops and networks during creep at temperatures of 593–700 °C, acting as obstacles to network dislocation climb and resulting in the observed improvement in creep strength and rupture resistance.

Facet Topography and Dislocation Structure as a Possible Source for Electrical Heterogeneity in [001] TILT Bicrystals of YBa2Cu3O7-δ

A current theme in the study of the critical current density behavior of YBa2Cu3O7-δ (YBCO) grain boundaries is that their electromagnetic properties are heterogeneous on various length scales ranging from 10s of microns to ˜ 1 Å. Recently, combined electromagnetic and TEM studies on four flux-grown bicrystals have demonstrated a direct correlation between the length scale of the boundaries’ saw-tooth facet configurations and the apparent length scale of the electrical heterogeneity. In that work, enhanced critical current densities are observed at applied fields where the facet period is commensurate with the spacing of the Abrikosov flux vortices which must be pinned if higher critical current density values are recorded. To understand the microstructural origin of the flux pinning, the grain boundary topography and grain boundary dislocation (GBD) network structure of [001] tilt YBCO bicrystals were studied by TEM and HRTEM.

Faceting, dislocation network structure, and various scales of heterogeneity in a YBa2Cu3O7−δ low-angle [001] tilt boundary

The grain boundary topography and grain boundary dislocation network structure of a 6° [001] bicrystal of YBa2Cu3O7−δ were studied using diffraction-contrast transmission electron microscopy (TEM). Saw-tooth-shaped arrays of facets composed of facets with lengths of a few tens of nanometers were observed in each of two widely separated sections of the boundary. The facet planes were {110}, {310}, and {221}. Further subfaceting of the (130) facets into a smaller-scale (a few nanometers) saw-tooth configuration of (010) and (110) facets produced a hierarchy of facets in at least one boundary section. The dislocation content observed in each type of facet agreed well with Frank's formula. However, the dislocations within individual facets frequently were inhomogeneously distributed, contrasting the picture of evenly spaced dislocations that is derived for boundaries of infinite extent. Certain types of dislocations repeatedly were grouped near the facet centers and ends. Well-separated partial dislocations frequently were observed near the facet midsections, but not near the facet junctions. Extended (∼30 nm) strain contrast was observed at all of the facet junctions formed by facets with dimensions on the order of tens of nanometers. This long-range strain may be due to the finite extent of the individual facets. These results all suggest that structural inhomogeneities occur on various length scales ranging from macroscopic to just a few nanometers. Such structural heterogeneity is consistent with the electrical heterogeneity that is indicated for many YBa2Cu3O7−δ grain boundaries.

Why is magnesia spinel a radiation-resistant material?

Side-by-side irradiation of stoichiometric MgAl 2O 4 and α-Al 2O 3 in JOYO shows that the radiation-induced microstructural evolution to exposure of ≤ 6 dpa proceeds by very different paths in these two materials. The large difference in dislocation loop evolution appears to account for the ease of void swelling in α-Al 2O 3 and the strong resistance to void formation in MgAl 2O 4. Irradiation of MgAl 2O 4 to much higher exposure (22–230 dpa) in FFTF confirms the details of the dislocation evolution, which involves a progressive change in Burgers vector and habit plane as interstitial loops increase in size. Constraints unique to the MgAl 2O 4 crystal structure do not allow the formation of dislocation network structures that favor swelling.

On dislocation link length statistics for constant strain rate deformation

A model based on the dislocation link length statistics of the three-dimensional dislocation network structure is used to analyse high-temperature constant strain rate deformation of crystalline materials. In particular, the strain rate change during steady-state constant strain rate deformation is studied in some detail. It is shown that the corresponding change in the flow stress, Δσ, associated with an instantaneous change, Δσ, in the imposed strain rate of an otherwise constant strain rate, , at a given temperature follows the Haasen relation in that the (Δσ/Δ ln ) versus σ plot is linear with the slope being equal to the strain rate sensitivity, m (Cottrell-Stokes law). The material parameters are also examined in the light of the model results and previous mechanical measurements. It is shown that the model produces constant strain rate sensitivity values for constant strain rate deformation. Agreement between model predictions and reported data is reasonably good.

Creep resistance of CMSX-3 nickel base superalloy single crystals

Creep deformation in 〈001〉 oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of γ′ phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800–900°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between γ and γ′ phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the γ matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850°C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the γ′ precipitates, dislocations move through the γ matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the γ′ particles by dislocations entering from the γ matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the γ matrix serve to neutralize the misfit between the γ and γ′ phases.

Mechanism of the Internal Reduction of (Mg, Cu)O

Single crystals of (Mg, Cu)O have been internally reduced to produce metallic copper precipitates inside the oxide matrix. Scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy investigations of the composite materials yielded information on the morphology and composition of the precipitates and on the orientation relationship between the two phases. The structure of dislocation networks formed during the reaction and the presence of Cu2O particles in short‐time reduction experiments allow the internal reduction of(Mg, Cu)O to be modeled by a two‐step reduction mechanism.

SEM imaging of the cell structure of dislocation networks in cold worked cu single crystals

AbstractIn certain metallurgical applications of SEM, such as the observation of deformation effects, and the recovery and recrystallization processes in bulk specimens, it is of special interest to obtain information about local inhomogeneities of the dislocation densities in a sample. This can be obtained using TEM on thin foils. Previously information on distortion in cold‐worked specimens was obtained by measuring the dependence of the deterioration of the resolution of the selected area channelling patterns (SACP) on the degree of deformation. This method, however, is not applicable to highly deformed materials.Direct SEM imaging of the cell structure of the dislocation network has been achieved without resolving single dislocations by using a suitable arrangement of specimen and detector, such that only backscattered electrons (BSE) leaving the surface at a large angle to the normal contribute to the image signals. The signal‐to‐noise ratio was improved to the degree that it was possible 1) to image the cell structure of the dislocation network and 2) on heating the specimens, to record the growth of recrystallized regions with TV‐frame frequency.The imaging of the cell structure gives qualitative information about the degree of crystal deformation and the crystallographic direction of grain boundary motion during recrystallization.