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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Relationship of dislocation density of silicon to solar cell current loss at low temperature

Large decreases in short circuit current of silicon solar cells have been reported to occur as temperature is decreased below -60 C. Experimental results are presented which relate high dislocation density of the silicon bulk material of cells to the large current loss effect. Solar cells were made by the same processes from a variety of silicon materials, namely low-dislocation-density, high-dislocation-density float-zone, and Czochralski silicon. All cells were etched in a manner which revealed the dislocation density of the cell bulk silicon. It was found that every cell made from any of the various low-dislocation starting materials obtained from three suppliers still had a low-dislocation bulk after cell processing, and that all such cells belonged to category good. Cells made from float-zone materials showed high dislocation densities in their bulk and either fell into category poor, or had intermediate losses of short-circuit current at low temperature.

Mandelkorn, J.↗
NASA NTRS1977

Computer simulation of the motion of a straight dislocation line in concentrated solid solutions. II

An investigation was undertaken to determine if the size and modulus interaction of a solute atom with a screw dislocation and the modulus interaction with an edge dislocation contributed to strengthening, in addition to the size interaction with an edge dislocation. The results indicate that the size interaction between solute atom and an edge dislocation accounts for most of the solid solution strengthening in f.c.c. alloys. The contribution to the yield stress from the modulus interaction with an edge dislocation is less than 15%. The interaction between a solute atom and a screw dislocation is much less than that between a solute atom and an edge dislocation.

Kuo, C. T. K.↗
NASA NTRS1994

Effects of plasma hydrogenation on trapping properties of dislocations in heteroepitaxial InP/GaAs

In previous work, we have demonstrated the effectiveness of a post-growth hydrogen plasma treatment for passivating the electrical activity of dislocations in metalorganic chemical vapor deposition (MOCVD) grown InP on GaAs substrates by a more than two order of magnitude reduction in deep level concentration and an improvement in reverse bias leakage current by a factor of approximately 20. These results make plasma hydrogenation an extremely promising technique for achieving high efficiency large area and light weight heteroepitaxial InP solar cells for space applications. In this work we investigate the carrier trapping process by dislocations in heteroepitaxial InP/GaAs and the role of hydrogen passivation on this process. It is shown that the charge trapping kinetics of dislocations after hydrogen passivation are significantly altered, approaching point defect-like behavior consistent with a transformation from a high concentration of dislocation-related defect bands within the InP bandgap to a low concentration of individual dislocation related deep levels, before and after passivation. It is further shown that the 'apparent' activation energies of dislocation related deep levels, before and after passivation, reduce by approximately 70 meV as DLTS fill pulse times are increased from 1 microsecond to 1 millisecond. A model is proposed which explains these effects based on a reduction of Coulombic interaction between individual core sites along the dislocation cores by hydrogen incorporation. Knowledge of the trapping properties in these specific structures is important to develop optimum, low loss heteroepitaxial InP cells.

Ringel, S. A.↗
NASA NTRS1994

Midfit dislocation generation mechanisms in InGaAs/GaAs heterostructures

An experimental investigation of misfit dislocation generation mechanisms at an InGaAs/GaAs heterointerface is reported. InGaAs epitaxial layers were grown by low-pressure oragnometallic vapor-phase epitaxy on patterned and unpatterned GaAs substrate having etch-pit densities (EPD) of 200, 1400, and 10,000 cm(exp -2). After epitaxial growth, the samples were annealed at temperatures between 650 and 750 C, and analyzed by optical and transmission electron microscopy. For the range of substrate EPD studied, it was found that the substrate EPD controls the onset of misfit dislocation generation for low-temperature epitaxy (less than 600 C) on unpatterned substrates. When epilayers were annealed at 750 C, the density of misfit dislocations was independent of the substrate EPD. These studies also show that the dominant misfit dislocation generation mechanism for films grown on patterned substrates is nucleation at the growth-mesa edge. The density of preexisting threading dislocations has little influence on misfit dislocation generation for films selectively deposited within 100 x 100 sq micrometer growth windows. For selective heteroepitaxy, misfit dislocation generation strongly depends on the crystallographic orientation of the growth-mesa edge.

Kui, J.↗
DOE OSTI2023

Interaction of extended dislocations with nanovoid clusters

Voids of nanoscale dimensions in irradiated metals can act as obstacles to dislocation motion and cause strengthening. In this work, nanovoid strengthening and the influences of void size, void spacing and material properties, such as stacking fault energies, on dislocation bypass mechanisms are investigated using Phase Field Dislocation Dynamics, a three-dimensional mesoscale model that predicts the minimum energy pathway taken by discrete dislocations. A broad range of face centered cubic metals (copper, nickel, silver, rhodium, and platinum) and nanovoid sizes and spacings are treated, altogether spanning void size–to–dislocation stacking fault width ratios from less than unity to ten. Material γ-surfaces, calculated from ab initio methods, are input directly into the formulation. The analysis reveals that the critical bypass stress scales linearly with the linear void fraction, effective isotropic shear modulus, and ratio of the intrinsic to unstable stacking fault energies. With only a few exceptions, the critical stress is controlled by the stress required for the leading partial to impinge the voids (to move within range of the attractive image stress field of the void). When the void diameter is nearly an order of magnitude greater than the stacking fault width, the mechanism determining critical strength shifts to the stress for the dislocation to breakaway after partially cutting the void. Furthermore, this situation corresponds to that treated by line tension models and is realized here for Pt, with a sub-nanometer stacking fault width.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoretical analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.

3D X-ray microscopy↗
DOE OSTI2015

Grain size effects on dislocation and twinning mediated plasticity in magnesium

Grain size effects on the competition between dislocation slip and {101¯2} -twinning in magnesium are investigated using discrete dislocation dynamics simulations. These simulations account for dislocation–twin boundary interactions and twin boundary migration through the glide of twinning dislocations. It is shown that twinning deformation exhibits a strong grain size effect; while dislocation mediated slip in untwinned polycrystals displays a weak one. In conclusion, this leads to a critical grain size at 2.7 μm, above which twinning dominates, and below which dislocation slip dominates.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Twist angle effect on anisotropic mobility of hexagonal dislocation networks in {110} of alpha-iron

Atomistic studies of anisotropic mobility of hexagonal dislocation networks (HDNs) in a series of twist boundaries (1 -1 0) has been performed in alpha-iron. In contrast with previous work that neglected the twist angle effect, we find when the twist angle approaches to 0, the resistance to the HDN motion could become much lower than Peierls stress of edge dislocations <1 1 1>/2 when the HDN moves along [0 0 1], but beyond Peierls stress of screw dislocations <1 1 1>/2 when the HDN moves along other directions. Vector form of Orowan equation and differential displacement map of dislocation core are used to analyse the behaviour of these boundary dislocations. This work seems favourable for understanding the absence of anomalous slip in alpha-iron.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Beyond Coherent Oxide Heterostructures: Atomic-Scale Structure of Misfit Dislocations

Nanoscale design of complex oxide heterostructures and thin films is imperative as they have significant promise in novel technological applications. A coherent interface is formed in oxide heterostructures with small mismatches, and the lattice mismatch is completely compensated by elastic strain. In semi-coherent oxide heterostructures, when an epitaxial layer is grown on the substrate above the critical thickness of the film, misfit dislocations are formed to mitigate the strain between the two materials with dissimilar lattice constants. Key properties of semi-coherent oxide heterostructures are influenced or even controlled by the presence of misfit dislocations. Thus, it is critical to understand the atomic-scale structure of semi-coherent oxide heterostructures, specifically the structure of misfit dislocations that are ubiquitous at such heterointerfaces. Numerous state-of-the-art experiments have reported emergent phenomena at semi-coherent oxide heterostructures, wherein misfit dislocations play a crucial role. Yet, their atomic-scale and nanoscale structure is not always discernable from experiments. Because of large system sizes, computational studies dedicated to examining misfit dislocations in semi-coherent oxide heterostructures are still in their infancy. This review aims to summarize the recent advancements and challenges involved in computational studies elucidating the atomic-scale structure of misfit dislocations in semi-coherent oxide heterostructures and motivate future computational efforts.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Dislocation‐Induced Structural and Luminescence Degradation in InAs Quantum Dot Emitters on Silicon

This study probes the extent to which dislocations reduce carrier lifetimes and alter growth morphology and luminescence in InAs quantum dots (QD) grown on silicon. These heterostructures are key ingredients to achieving a highly reliable monolithically integrated light source on silicon necessary for photonic-integrated circuits. Around 20%–30% shorter carrier lifetimes are found at spatially resolved individual dislocations at room temperature using time-resolved cathodoluminescence spectroscopy, highlighting the strong nonradiative impact of dislocations even against the three-dimensional confinement of QDs. Beyond these direct effects of increased nonradiative recombination, it is found that misfit dislocations in the defect filter layers employed during III–V/Si growth alter the QD growth environment to induce a crosshatch-like variation in QD emission color and intensity when the filter layer is positioned sufficiently close to the QD emitter layer. Sessile threading dislocations generate even more egregious hillock defects that also reduce emission intensities by altering layer thicknesses, as measured by transmission electron microscopy and atom probe tomography. This work presents a more complete picture of the impacts of dislocations relevant to the development of light sources for scalable silicon photonic integrated circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2017

On the origin and behavior of irradiation-induced c-component dislocation loops in magnesium

C-component dislocation loops are one of the unique defects in hexagonal close-packed (hcp) crystals that promote the accelerated growth and void formation under irradiation. In this paper, we report in situ observation of c-component dislocation loop formation in Mg under electron irradiation with emphasis on their atomic structures. Aberration-corrected scanning transmission electron microscopy imaging is utilized to reveal four possible types of double-layer loops, which were identified as different types of stacking fault and dislocation core structures. Triple- and quadruple-layer c-component dislocation loops were also observed. The formation mechanisms of the four types of double-layer loops were revealed via molecular dynamics simulations. The experimentally observed formation rate of the single- and double-layer dislocation loops is controlled by their formation energies. Finally, our direct experimental observations in combination with molecular dynamics simulations provide fundamental insight into the mechanisms governing nucleation and growth of the c-component dislocation loops as well as their interactions, which could potentially help with future development of irradiation-resistant materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Dislocation loop evolution during in-situ ion irradiation of model FeCrAl alloys

Model FeCrAl alloys of Fe-10%Cr-5%Al, Fe-12%Cr-4.5%Al, Fe-15%Cr-4%Al, and Fe-18%Cr-3%Al (in wt %) were irradiated with 1 MeV Kr++ ions in-situ with transmission electron microscopy to a dose of 2.5 displacements per atom (dpa) at 320 °C. In all cases, the microstructural damage consisted of dislocation loops with ½< 111 > and <100 > Burgers vectors. The proportion of ½< 111 > dislocation loops varied from ~50% in the Fe-10%Cr-5%Al model alloy and the Fe-18Cr%-3%Al model alloy to a peak of ~80% in the model Fe-15%Cr-4.5%Al alloy. The dislocation loop volume density increased with dose for all alloys and showed signs of approaching an upper limit. The total loop populations at 2.5 dpa had a slight (and possibly insignificant) decline as the chromium content was increased from 10 to 15 wt %, but the Fe-18%Cr-3%Al alloy had a dislocation loop population ~50% smaller than the other model alloys. As a result, the largest dislocation loops in each alloy had image sizes of close to 20 nm in the micrographs, and the median diameters for all alloys ranged from 6 to 8 nm. Nature analysis by the inside-outside method indicated most dislocation loops were interstitial type.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Quantifying dislocation density in Al-Cu coatings produced by cold spray deposition

This work quantifies the plastic deformation in terms of dislocation density for a series of cold sprayed Al-Cu coatings with copper contents from 2 mass% to 5 mass%. The success of the deposition and consolidation of the feedstock powders during the cold spray process rely on the occurrence of significant plastic deformation. Inert gas atomized Al-Cu alloy powders were sprayed onto substrates made of an Al-Cu-Mg-Mn alloy (AA2024) to produce dense coatings using a low pressure cold spray system with helium as the carrier gas. X-ray diffraction patterns were obtained from the Al-Cu feedstock powder material and the cold sprayed coatings using a monochromatic X-ray source, and the dislocation density was determined via an X-ray whole profile analysis. Increasing the Cu alloy content (from 2 mass% to 5 mass%) systematically increased the dislocation density in the Al-Cu coatings from (4.3 ± 0.5) × 10 14 m -2 to (7.5 ± 0.8) × 10 14 m -2 . The dislocation densities in the feedstock powders ranging from (0.4 ± 0.1) × 10 14 m -2 to (1.8 ± 0.2) × 10 14 m -2 were all lower than the dislocation densities in the corresponding coatings. The increasing deformation level in the Al-Cu coatings with Cu additions were confirmed by the classic and modified Williamson-Hall analyses of X-ray diffraction data, and peak breadth measurements from neutron diffraction data. Finally, a high density of dislocations was also observed in these coatings via electron backscatter diffraction and transmission electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Helical dislocation in twisted bilayer graphene

Here, we observe helical dislocation network in twisted bilayer graphene (tBLG), which is accompanied by large out-of-plane deformation. By atomistic calculations, we demonstrate two distinct out-of-plane deformation modes, breathing mode with small out-of-plane deformation and bending mode with one order larger corrugation magnitude compared to the breathing mode. The out-of-plane deformation is caused by inhomogeneous interlayer coupling resulting from the periodic stacking order of the tBLG moiré superlattice. Instead of commonly observed screw dislocation in tBLG, we demonstrate a slip-induced helical dislocation network in the bending mode tBLG. We show that bending mode deformation is more stable at low twist angles, as the energy savings due to interface energy exceeds the energy penalty due to the strain energy caused by the large out-of-plane deformation. Our work provides a detailed picture of a new helical dislocation structure in tBLG and establishes a direct connection between the dislocation and deformation. Therefore, understanding the dislocation mechanics of tBLG may open up the possibility to control the corrugation, and reveal an opportunity to tune the intriguing physical properties of twisted bilayer graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Binary dislocation junction formation and strength in hexagonal close-packed crystals

This work examines binary dislocation interactions, junction formation and junction strengths in hexagonal close-packed ( hcp ) crystals. Through a line-tension model and dislocation dynamics (DD) simulations, the interaction and dissociation of different sets of binary junctions are investigated involving one dislocation on the (011¯0) prismatic plane and a second dislocation on one of the following planes: (0001) basal, (11¯00) prismatic, (11¯01) primary pyramidal, or (2¯112) secondary pyramidal. Varying pairs of Burgers vectors are chosen from among the common types the basal type < a > 1/3 < 112¯0 >, prismatic type < c > <0001>, and pyramidal type < a+c > 1/3 < 112¯3¯ >. For binary interaction due to dislocation intersection, both the analytical results and DD-simulations indicate a relationship between symmetry of interaction maps and the relative magnitude of the Burgers vectors that constitute the junction. Using analytical formulae, a simple regressive model is also developed to represent the junction yield surface. The equation is treated as a degenerated super elliptical equation to quantify the aspect ratio and tilting angle. Lastly, the results provide analytical insights on binary dislocation interactions that may occur in general hcp metals.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A discrete dislocation dynamics study of precipitate bypass mechanisms in nickel-based superalloys

Order strengthening in nickel-based superalloys is associated with the extra stress required for dislocations to bypass the $\gamma'$ precipitates distributed in the $\gamma$ matrix. Depending on the operating conditions and microstructure, a rich variety of bypass mechanism has been identified, with various shearing and Orowan looping processes gradually giving way to climb bypass as the operating conditions change from the low/intermediate temperatures and high stress regime, to the high temperature and low stress regime. When anti phase boundary (APB) shearing and Orowan looping mechanisms operate, the classical picture is that, at for a given volume fraction, the bypass mechanism changes from shearing to looping with increased particle size and within a broad coexistence size window. Another possibility, which is supported by indirect experimental evidence, is that a third “hybrid” transition mechanism may operate. Here, in this paper we use discrete dislocation dynamics (DDD) simulations to study dislocation bypass mechanisms in Ni-based superalloys. We develop a new method to compute generalized stacking fault forces in DDD simulations, based on a concept borrowed from complex analysis and known as the winding number of a closed curve about a point. We use this method to study the mechanisms of bypass of a square lattice of spherical $\gamma'$ precipitates by $a/2\langle110\rangle$ {111} edge dislocations, as a function of the precipitates volume fraction and size. We show that not only the hybrid mechanism is possible, but also that it operates as the transition mechanism between the shearing and looping regimes over a wide range of precipitates volume fraction and radii. Based on our simulation results, we propose a simple model for the strength of this mechanism. We also consider the effects of a $\gamma$/$\gamma'$ lattice misfit on the bypass mechanisms, which we approximate by an additional precipitate stress computed according to Eshelby’s inclusion theory. We show that in the shearing and hybrid looping-shearing regimes, a lattice misfit generally results in an increased bypass stress. For sufficiently high lattice misfit, the critical bypass configuration in attractive dislocation-precipitates interactions changes dramatically, and the bypass stress is controlled by the pinning of the trailing dislocation on the exit side of the precipitates, similar to what has been reported in the high-temperature creep literature.

36 MATERIALS SCIENCE↗
DOE OSTI2021

A critical dislocation velocity for serration mechanism transition in a nickel-chromium solid solution alloy

Here, the influence of strain rate across three orders of magnitude (1.70 × 10 –5 /s to 1.43 × 10 –2 /s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600 °C. As the strain rate decreased, the critical strain for the onset of serrations transitioned from normal behavior to inverse behavior. The serrated flow was characterized as Type A+B serration at high strain rate (1.43 × 10 –2 /s). In the intermediate strain-rate regime (1.43 × 10 –3 /s and 1.45 × 10 –4 /s), Type B serrations were observed and followed by a transformation to Type C+B serrations. At the low strain rate (1.70 × 10 –5 /s), the plastic flow immediately displayed Type C serrations, which later evolved into Type C+B serrations. Regardless of the strain rate, plastic strain, or dislocation density, a critical dislocation velocity falling in the range of 1.2 × 10 –6 – 2.2 × 10 –6 m/s was identified to signify the onset of Type C serration, whereby the mobile dislocations break free from the solute cloud for short bursts of deformation. Finally, a novel model by solute rearrangement across dislocation cores was used to understand how the critical dislocation velocity is quantitatively determined by the rate at which solute atoms are able to hop across the glide plane as a partial dislocation core moves through the lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Simulation of dislocation evolution in microparticle impacts over a wide range of impact velocities

We report the dynamic loading environment created by high velocity microparticle impacts is very difficult to model. The extremely high strain rates, large deformations, and high temperatures affect the flow stress of the material in ways that standard flow stress models cannot capture. Plastic deformation and strain hardening in metals is controlled by the motion of dislocations. Dislocations can be nucleated, stored as forest dislocations, or be annihilated as loading progresses. A comprehensive accounting of dislocation density change is needed to accurately describe rate and temperature dependent dislocation glide and evolution across the wide rage of loading conditions present in microparticle impact problems. Therefore, we implement and apply a newly proposed flow stress model (Hunter and Preston, 2015, 2022) that is then coupled with mobile and immobile dislocation density evolution equations. This model is implemented in Los Alamos National Laboratory’s hydrodynamics code, FLAG, to model copper-on-copper microparticle impacts. This new strength model allows for accurate simulation of particle rebound and flattening across a wide range of impact velocities.

36 MATERIALS SCIENCE↗