Burgers Vectors Of Type Research Articles

Orientation distribution and deviation behaviors of dislocation loops in a quenched Al-Cu alloy

Dislocation loops in quenched aluminum alloys are usually seen deviating from their nominal habit planes, while the orientation distribution and deviation mechanism are far from understanding. Based on the method of tomographic crystallography of dislocations and correlative in-situ heating experiments in transmission electron microscopy (TEM), we revisit the three-dimensional (3D) characteristics and deviation behaviors of dislocation loops in a quenched Al-Cu alloy with an aim to fully unravel the relationship between dislocation crystallography and deviation behaviors. The results show that all the dislocation loops exhibit approximately circular shapes and have a/2 < 110>-type Burgers vectors, with loop orientations deviating from their pure edge orientation {110} of 0°-17.8° and distributing mainly close to the [101]-[001] and [101]-[111] edges of the standard stereographic triangle. Three types of deviation behaviors of dislocation loops are then defined, which are characterized by loop rotation around 〈100〉, 〈110〉 and irregular axes, respectively. Theoretical calculations of strain energy variation with increasing deviation angle indicate that the relatively low energy barrier may account for a high frequency of loop deviation in the low angle range. Detailed in-situ TEM heating experiments and correlative tomographic crystallography analyses strongly demonstrate the specific roles of dislocation gliding and climbing at elevated temperatures in the shrinkage, polygonization and deviation processes of dislocation loops, and the competitive and cooperative motion of dislocation segments close to slip planes {111} are the fundamental reasons for the formation of three types of deviated dislocation loops.

Observation of threading dislocations with a c + m type Burgers vector in HVPE GaN substrates using multi-photon excitation photoluminescence and TEM

Dislocations that have a large Burgers vector are potential leakage current pathways in power devices. In this study, we analyzed the dislocations under a large pit (L-pit) produced by chemical etching in a hydride-vapor-phase-epitaxy GaN substrate by using multi-photon excitation photoluminescence (MPPL) and transmission electron microscopy (TEM). Three-dimensional dislocation images reconstructed from MPPL clearly showed that two threading dislocations (TD-1 and TD-2) approached each other laterally while propagating along the growth direction [0001]. They interacted near the sample surface and merged into a single dislocation whose large Burgers vector was responsible for the L-pit formation. A TEM observation operated in large-angle convergent-beam electron diffraction mode unambiguously identified the Burgers vectors of the two dislocations as bTD-1=13[2-1-1-3] (c + a type) and bTD-2 = [10–1–1] (c + m type). The c + m type is extremely rare in GaN. The merged dislocation appeared to constitute Burgers vectors of all three components of c, m, and a.

Hardness anisotropy and active slip systems in a (Hf-Ta-Zr-Nb)C high-entropy carbide during nanoindentation

Hardness anisotropy and deformation-induced dislocations in (Hf-Ta-Zr-Nb)C high-entropy carbide grains of low-index facets were investigated using nanoindentation and transmission electron microscopy (TEM). The (Hf-Ta-Zr-Nb)C sample was prepared by a two-step spark plasma sintering (SPS) and nanoindentation was performed on grains mapped previously by electron backscatter diffraction. To investigate the deformation zones under the indents, thin lamellas were extracted from grains of {001}, {101} and {111} orientations using a focused ion beam (FIB) technique and were studied by TEM. Nanoindentation revealed that grains of {100} orientation exhibit about 7% lower hardness (~35 GPa) compared to the {110} and {111} facets (~38 GPa) while there was no anisotropy found for the indentation modulus. TEM analysis of the lamellas revealed heavily deformed zones under the indents with dislocations of a/211¯0 type Burgers vector. TEM-based trace analysis of dislocations revealed the activation of the 1101¯11 and 1101¯10 slip systems in grains indented on the {001} and {101} facets, respectively. The hardness anisotropy of (Hf-Ta-Zr-Nb)C was attributed to the different operating slip systems, similar to that was reported for the component monocarbides in the literature.

High-Stress Compressive Creep Behavior of Ti-6Al-4V ELI Alloys with Different Microstructures

Influence of initial microstructure of Ti-6Al-4V ELI alloys on their compressive creep behavior at ambient temperature was investigated with applying compression stresses from 695 to 1092 MPa The experimental results show that the basketweave alloys have better compressive creep resistances than those duplex ones. The constitutive equations in steady-state compressive creeps of duplex or basketweave structure are calculated to be =2.77×10-15(σ-710)2.1 and =2.36×10-14(σ-740)1.7 by fitting the linear regression creep curves after uniaxial compression tests. The noticeable compressive creep strains occur when the applied compression stresses are higher than the threshold stresses, i.e. 710 MPa for duplex Ti-6Al-4V ELI alloys and 740 MPa for basketweave alloys. Microstructural analysis indicates that the creep deformation of Ti-6Al-4V ELI alloys at ambient temperature is mainly controlled by dislocation slip. The creep behavior of Ti-6Al-4V ELI alloy with duplex microstructure is controlled by dislocation slip, like slip dislocations with a-type Burgers vector sliding on the basal or prismatic planes and a few c+a type dislocation sliding on the pyramidal planes. While creep mechanism for basketweave ones is dislocation glide controlled by c+a type Burgers vector sliding on the pyramidal planes and a-type sliding on the basal or prismatic planes.

Diffusion and interaction of prismatic dislocation loops simulated by stochastic discrete dislocation dynamics

Body-centered cubic metals and alloys irradiated by energetic particles form highly mobile prismatic dislocation loops with $a/2\ensuremath{\langle}111\ensuremath{\rangle}$-type Burgers vectors. We show how to simulate thermal diffusion of prismatic loops using a discrete dislocation dynamics approach that explicitly includes the stochastic forces associated with ambient thermal fluctuations. We find that the interplay between stochastic thermal forces and internal degrees of freedom of loops, in particular the reorientation of the loop habit planes, strongly influences the observed loop dynamics. The loops exhibit three fundamental types of reactions: coalescence, repulsion, and confinement by elastic forces. The confinement reactions are highly sensitive to the internal degrees of freedom of the loops. Depending on the orientation of the loop habit planes, the barrier to enter an elastically confined bound state is lowered substantially, whereas the lifetime of the bound state increases by many orders of magnitude.

Identification of dislocation characteristics in Na-flux-grown GaN substrates using bright-field X-ray topography under multiple-diffraction conditions

Basal-plane and threading dislocations in multipoint-seed Na-flux-grown GaN single crystals are characterized in terms of Burgers vectors mainly by using bright-field X-ray topography under multiple-diffraction conditions. The technique, combined with a CMOS camera system with high spatial resolution, can provide topographic images of the dislocations with a relatively high dislocation density (up to approximately 5×105cm−2). It is possible to directly determine the Burgers vector of individual dislocations based on invisibility criteria. From the present experiment, it is found that almost all basal-plane dislocations have a-type Burgers vectors, and threading dislocations have a- and (a + c)-type Burgers vectors.

Crystalline Defects Induced during MPCVD Lateral Homoepitaxial Diamond Growth.

The development of new power devices taking full advantage of the potential of diamond has prompted the design of innovative 3D structures. This implies the overgrowth towards various crystallographic orientations. To understand the consequences of such growth geometries on the defects generation, a Transmission Electron Microscopy (TEM) study of overgrown, mesa-patterned, homoepitaxial, microwave-plasma-enhanced, chemical vapor deposition (MPCVD) diamond is presented. Samples have been grown under quite different conditions of doping and methane concentration in order to identify and distinguish the factors involved in the defects generation. TEM is used to reveal threading dislocations and planar defects. Sources of dislocation generation have been evidenced: (i) doping level versus growth plane, and (ii) methane concentration. The first source of dislocations was shown to generate <110> Burgers vector dislocations above a critical boron concentration, while the second induces <112> type Burgers vector above a critical methane/hydrogen molar ratio. The latter is attributed to partial dislocations whose origin is related to the dissociation of perfect ones by a Shockley process. This dissociation generated stacking faults that likely resulted in penetration twins, which were also observed on these samples. Lateral growth performed at low methane and boron content did not exhibit any dislocation.

Dislocation bending in GaN/step-graded (Al,Ga)N/AlN buffer layers on Si(111) investigated by STM and STEM

ABSTRACTThe distribution and bending of dislocations in GaN/step-graded (Al,Ga)N/AlN buffer layers grown on Si(111) are investigated by cross-sectional scanning tunnelling microscopy (STM) and scanning transmission electron microscopy (STEM). We observe dislocations with -type Burgers vector intersecting the m-plane cleavage surface and having line directions bent off the [0001] growth direction toward non-polar directions. The spatial distribution of dislocations intersecting the m-plane cleavage surface indicates consecutive bending of dislocations due to strain at interfaces between subsequent lattice mismatched buffer layers and at doping junctions, reducing the density of threading dislocations at the (0001) growth front. No interface misfit dislocations, v-shaped defects, or loss of crystalline quality are observed, demonstrating the high performance of the step-graded (Al,Ga)N/AlN buffer layers on Si for relaxing the lattice constant without creating large defect concentrations.

Structural defects of GaN deposited on (111) Si with Gd2O3-related buffer layers

The structural defects of GaN deposited on (111) Si with Gd2O3-related rare earth oxide buffer layers were investigated using a double-crystal x-ray rocking curve (DCXRC), high resolution transmission electron microscopy (HRTEM). By comparing the DCXRC signals of (11¯02)GaN and (12¯12)GaN asymmetric planes and (0002)GaN symmetric plane, respectively, it was found that most of the threading dislocations (TDs) in GaN of the GaN on (111) Si using Gd2O3 or Er2O3/Gd2O3 buffer layer are type a TDs in nature. Based on the results of DCXRC studies, it is believed that the full-width at half maxima of the DCXRC signals of (11¯02)GaN and (12¯12)GaN asymmetric planes are primarily due to the contributions of the 1/3⟨12¯10⟩GaN components of the Burgers vectors of type a and type c + a TDs on the interplanar spacing distortions of (11¯02)GaN and (12¯12)GaN planes in GaN of the GaN/Er2O3/Gd2O3/(111) Si samples. HRTEM observations also revealed the presence of extrinsic stacking faults and Frankel partial dislocations in GaN of the GaN on the (111) Si sample with Gd2O3 or Er2O3/Gd2O3 buffer layer.

(Invited) Atomic Scale Investigations of Extended Crystal Defects in Tetradymite-Structured Thermoelectric Materials

Dislocations and other extended defects, such as grain boundaries and stacking faults, can impart strong effects on the thermal and electronic transport properties of thermoelectric materials. Yet, our understanding of the structure of such defects, even for such widely used materials as bismuth telluride, is still quite limited. In this presentation, I will discuss our work investigating the structure of extended crystal defects in Bi2Te3 and related tetradymite-structured thermoelectric materials. As will be discussed, the anisotropic, layered crystal structure of Bi2Te3 has important implications for the resulting atomic-scale core structures of defects in this material. I will begin with a brief overview of the tetradymite system, placing the structure of Bi2Te3 in relationship to other layered chalcogenide materials and discussing the dependence of the layering sequence on the metal to chalcogen ratio. Next, I will discuss HAADF-STEM observations of the atomic structure of dislocations in Bi2Te3. I will focus on two very different types of dislocation: one with Burgers vector of type (1/3)<2 -1 -1 0>, which lies parallel with the basal plane, and the other with Burgers vector of type (1/3)<0 1 -1 -1>, which has a large dislocation component normal to the basal plane of c/3, or one full quintuple unit. The first type of dislocation possesses a stoichiometric core that is glissile on the basal plane. The second type possesses a much more complex dislocation core consisting of bismuth-rich faulted region consistent with Bi3Te4. I will also discuss our current understanding of grain boundaries in this class of materials. Arrays of discrete dislocations can accommodate small misorientations between adjacent crystallites of a few degrees. Examples will be shown for low angle boundaries composed of both (1/3)<2 -1 -1 0> and (1/3)<0 1 -1 -1> type dislocations. For large misorientations, however, simple dislocation descriptions break down and it is more physically meaningful to consider a given interface with reference to a low-energy, reference grain boundary structure. Perhaps the simplest such interface in Bi2Te3 is the (0001) basal twin. Our electron microscopic observations and density functional theory calculations have shown that the structure of the Bi2Te3 basal twin consists simply of a reversal of the basal plane stacking sequence and that this reversal occurs at the Te(1) type planes. As I will discuss it is also possible to go beyond the simple structure of an ideal (0001) twin by considering the introduction of disconnections (defects possessing both dislocation and step content) to this interface. Finally, I will discuss a more complex mode of twinning, on {1 0 -1 5} and {0 1 -1 10}, which has previously been reported in electroplated films of Bi2Te3. Our HAADF-STEM observations provide insight concerning the atomic-scale structure of such twinning, pointing to the formation of structural units consistent with local packets of rock-salt structured material. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Atomistic simulation of the segregation of alloying elements close to radiation-induced defects in irradiated Fe–Cr–Ni BCC alloys

Ferritic-martensitic steels alloyed with Cr and Ni are promising structural materials for the nuclear and thermonuclear power industry. Under the influence of neutron irradiation degradation of the plastic properties of these materials takes place as a result of the generation of extended defects such as dislocation loops, and the formation of new phases (precipitates). In this work the atomistic computer simulation of thermodynamic processes of the precipitation of alloying elements is carried out using the newest model of ternary Fe–Ni–Cr bcc (body-centered cubic) alloys and the Metropolis Monte Carlo method in combination with the method of classical Molecular Dynamics. The composition and microstructure of Cr–Ni clusters formed in defect-free alloys and alloys containing dislocation loops, depending on the temperature and concentrations of Ni and Cr, are studied. An increase in the Ni solubility limit in the presence of dislocation loops by 100–200 K, depending on the Ni concentration, is detected. The synergetic effect of Ni and Cr segregation near dislocation loops in ternary alloy is established: the presence of Ni weakens Cr segregation, whereas Cr can either attenuate or amplify Ni segregation depending on the concentration of Ni in the alloy, the temperature and the type (Burgers vector) of loop. In general, in ternary Fe–Cr–Ni alloys, the total segregation effect is less pronounced than in binary Fe–Ni and Fe–Cr alloys.

Stacking faults and the -surface on first-order pyramidal planes in -titanium

Using first principles methods, we calculated the entire -surface of the first-order pyramidal planes in -titanium. Slip on these planes involving dislocations with -type Burgers vectors is one means by which -titanium polycrystals may supplement slip on prism planes with -type Burgers vectors to maintain ductility. We find one low energy and one high energy stacking fault with energies of 163 and 681 , respectively. Contrary to previous suggestions, we do not find a stable stable stacking fault at .

Evolution of Dislocation Loops in AL-6XN Stainless Steels Irradiated by Hydrogen Ions

AL-6XN stainless steels, one of the candidate structure materials for supercritical water-cooled reactor, were irradiated from 0.5 to 5 dpa using 100 keV H2 + ions at 290 and 380 °C. Microstructures were characterized by transmission electron microscopy (TEM). Dislocation loops were the dominant radiation-induced defects. All the dislocation loops had 1/3 <111> type Burgers vector. Number density and size of the loops have been measured. Nucleation and evolution of dislocation loops were also investigated. Voids were observed only in the condition of 5 dpa at 380 °C. Different evolution mechanisms of the radiation-induced dislocation loops were discussed. Effects of hydrogen and elevated temperature on the microstructural evolution were also investigated. Besides, the formed voids have a further effect on the evolution of dislocation loops.

Twinning anisotropy of tantalum during nanoindentation

Unlike other BCC metals, the plastic deformation of nanocrystalline Tantalum (Ta) during compression is regulated by deformation twinning. Whether or not this twinning exhibits anisotropy was investigated through simulation of displacement-controlled nanoindentation test using molecular dynamics (MD) simulation. MD data was found to correlate well with the experimental data in terms of surface topography and hardness measurements. The mechanism of the transport of material was identified due to the formation and motion of prismatic dislocations loops (edge dislocations) belonging to the 1/2〈111〉 type and 〈100〉 type Burgers vector family. Further analysis of crystal defects using a fully automated dislocation extraction algorithm (DXA) illuminated formation and migration of twin boundaries on the (110) and (111) orientation but not on the (010) orientation and most importantly after retraction all the dislocations disappeared on the (110) orientation suggesting twinning to dominate dislocation nucleation in driving plasticity in tantalum. A significant finding was that the maximum shear stress (critical Tresca stress) in the deformation zone exceeded the theoretical shear strength of Ta (Shear modulus/2π~10.03GPa) on the (010) orientation but was lower than it on the (110) and the (111) orientations. In light of this, the conventional lore of assuming the maximum shear stress being 0.465 times the mean contact pressure was found to break down at atomic scale.

On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys

The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [001] and [110] directions at 1293K and 160MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [110] as compared to [001] tensile loading were identified. [110] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type 〈110〉 jointly shear the γ′ phase. The resulting 〈220〉-type superdislocation can move by glide. In contrast, during [001] tensile loading, two dislocations with different 〈110〉-type Burgers vectors must combine for γ′ phase cutting. The resulting 〈200〉-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [110] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (〈220〉 glide) in the γ′ phase.

Design and characterization of thick InxGa1-xAs metamorphic buffer layers grown by hydride vapor phase epitaxy

Thick InxGa1-xAs metamorphic buffer layers (MBLs) grown by hydride vapor phase epitaxy (HVPE) were studied. Relationships between MBL properties and growth parameters such as grading rate, cap layer thickness, final xInAs, and deposition temperature (TD) were explored. The MBLs were characterized by measurement of in-plane residual strain (妦), surface etch pit density (EPD), and surface roughness. Capping layer thickness had a strong effect on strain relaxation, with thickly capped samples exhibiting the lowest 妦. EPD was higher in samples with thicker caps, reflecting their increased relaxation through dislocation generation. 妦 and EPD were weakly affected by the grading rate, making capping layer thickness the primary structural parameter which controls these properties. MBLs graded in discrete steps had similar properties to MBLs with continuous grading. In samples with identical thickness and 10-step grading style, 妦 increased almost linearly with final xInAs, while total relaxation stayed relatively constant. Relaxation as a function of xInAs could be described by an equilibrium model in which dislocation nucleation is impeded by the energy of the existing dislocation array. EPD was constant from xInAs = 0 to 0.24 then increased exponentially, which is related to the increased dislocation interaction and blocking seen at higher dislocation densities. RMS roughness increased with xInAs above a certain strain rate (0.15%/µm); samples grown below this level possessed large surface hillocks and high roughness values. The elimination of hillocks at higher values of xInAs is attributed to increased density of surface steps and is related to the out-of-plane component of the burgers vector of the dominant type of 60° dislocation. TD did not affect 妦 for samples with a given xInAs. EPD tended to increase with TD, indicating dislocation glide likely is impeded at higher temperatures.

Configuration of superdislocations in the γ′-Pt3Al phase of a Pt-based superalloy

The configuration of superdislocations in the γ′-Pt3Al phase of a Pt-based superalloy after high temperature tensile creep deformation is evaluated. Weak-beam dark field transmission electron microscopy is used to measure dissociation width of superdislocation partials. Short, straight segments of dissociated dislocations with 〈110〉-type Burgers vectors are observed. Dissociation takes place on {111} planes with a separation of the partials in-between 3.4 and 4.5 nm, resulting in antiphase boundary energies of 344–456 mJ/m2 considering elastic anisotropy. The dislocations are of mixed edge and screw type character and are in planar configuration. First-principles antiphase boundary energy calculations in stoichiometric Pt3Al reveal that the energy of {111} boundaries is 205 mJ/m2, the {010} energy 414 mJ/m2.

Dislocation dynamics simulations of interactions between gliding dislocations and radiation induced prismatic loops in zirconium

The mechanical behavior of Pressurized Water Reactor fuel cladding tubes made of zirconium alloys is strongly affected by neutron irradiation due to the high density of radiation induced dislocation loops. In order to investigate the interaction mechanisms between gliding dislocations and loops in zirconium, a new nodal dislocation dynamics code, adapted to Hexagonal Close Packed metals, has been used. Various configurations have been systematically computed considering different glide planes, basal or prismatic, and different characters, edge or screw, for gliding dislocations with 〈a〉-type Burgers vectors. Simulations show various interaction mechanisms such as (i) absorption of a loop on an edge dislocation leading to the formation of a double super-jog, (ii) creation of a helical turn, on a screw dislocation, that acts as a strong pinning point or (iii) sweeping of a loop by a gliding dislocation. It is shown that the clearing of loops is more favorable when the dislocation glides in the basal plane than in the prismatic plane explaining the easy dislocation channeling in the basal plane observed after neutron irradiation by transmission electron microscopy.

Single- and dual-beam in situ irradiations of high-purity iron in a transmission electron microscope: Effects of heavy ion irradiation and helium injection

In order to study the effects of 14MeV neutron irradiation on materials used in the first walls of future fusion reactors, high-purity iron was ion-irradiated with and without helium in the JANNuS facility. Thin foils of high-purity iron were dual-beam irradiated in situ in a transmission electron microscope using 1MeV Fe+ and 15keV He+ ions. Several important results regarding dislocation loops and helium bubbles were obtained. For example, it was demonstrated that dislocation loops with a0 〈010〉 type Burgers vectors are glissile and can move and eliminate at the surface of the thin foil at 500°C. A comparison of irradiations with and without helium showed that helium atoms reduce the mobility of dislocation loops in pure iron irradiated at 500°C. Also, we demonstrated that the heterogeneous formation of bubbles inside dislocation loops found previously is also present for helium implantation rates of ∼80 atomic parts per million (appm) He/displacements per atom (dpa).

Dislocation instabilities and microstructure in anisotropic -Fe

We compare theoretical predictions of the shapes of dislocation shear loops in highly anisotropic crystals with TEM of -Fe irradiated at high temperatures. The sharp-cornered microstructure we observe is explained via instability of certain dislocation orientations, emerging as the temperature, and hence anisotropy, increases above a critical value. The microstructure features shear dislocations with -type Burgers vectors, which casts doubt on the traditional view that plasticity is mediated exclusively by -type dislocations in bcc crystals, at least in Fe and ferritic steels. These observations provide experimental evidence for a new mechanism for high-temperature plastic deformation, which may contribute to the severe loss of strength observed above 500C.