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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Slip-free multiplication and complexity of dislocation networks in FCC metals

Abstract During plastic deformation of crystalline solids, intricate networks of dislocation lines form and evolve. To capture dislocation density evolution, prominent theories of crystal plasticity assume that 1) multiplication is driven by slip in active slip systems and 2) pair-wise slip system interactions dominate network evolution. In this work, we analyze a massive database of over 100 discrete dislocation dynamics simulations (with cross-slip suppressed), and our findings bring both of these assumptions into question. We demonstrate that dislocation multiplication is commonly observed on slip systems with no applied stress and no plastic strain rate, a phenomenon we refer to as slip-free multiplication. We show that while the formation of glissile junctions provides one mechanism for slip-free multiplication, additional mechanisms which account for the influence of coplanar interactions are needed to fully explain the observations. Unlike glissile junction formation which results from a binary reaction between a pair of slip systems, these new multiplication mechanisms require higher order reactions that lead to complex network configurations. While these complex configurations have not been given much attention previously, they account for about 50% of the line intersections in our database.

36 MATERIALS SCIENCE↗
DOE OSTI2025

PyDislocDyn: A Python code for calculating dislocation drag and other crystal properties

PyDislocDyn is a suite of python programs designed to perform various calculations for dislocation dynamics in the continuum limit. In particular, one of its main purposes is to calculate dislocation drag from phonon wind. Additional features include the averaging of elastic constants for polycrystals, the calculation of the dislocation field including its limiting velocities, and the calculation of dislocation self-energy and line tension.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Kinetic Interaction of Uranium Vacancies and Dislocations in UO 2

Understanding how point defects and defect clusters interact with dislocations in urania is important for a number of phenomena. For example, dislocations are one (of many) trap sites in the fuel where fission gases may accumulate and ultimately nucleate fission gas bubbles. Further, some creep mechanisms are governed by the flow of point defects to dislocations. Thus, for a variety of reasons, it is important to examine how dislocations attract and accelerate the kinetics of point defects.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Low-dislocation-density epitatial layers grown by defect filtering by self-assembled layers of spheres

A method for growing low-dislocation-density material atop a layer of the material with an initially higher dislocation density using a monolayer of spheroidal particles to bend and redirect or directly block vertically propagating threading dislocations, thereby enabling growth and coalescence to form a very-low-dislocation-density surface of the material, and the structures made by this method.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Unprecedented irradiation resistance of nanocrystalline tungsten with equiaxed nanocrystalline grains to dislocation loop accumulation

Nanocrystalline metals are often postulated as irradiation tolerant materials due to higher grain boundary densities. The efficiency of these materials in mitigating irradiation damage is still under investigation. Here, we present an in-situ transmission electron microscopy with ion irradiation study on equiaxed 35 nm grained tungsten (NCW-35 nm) and compare its radiation tolerance, in terms of dislocation loop damage, to several other grades of tungsten with different grain sizes at two temperatures (RT and 1073 K). The NCW-35 nm was shown to possess significant higher radiation tolerance in terms of loop damage. As demonstrated by Kinetic Monte Carlo simulations, at least part of the higher radiation tolerance of the small grains is due to higher interstitial storage (at the grain boundaries) and defect recombination (in the grain interiors) in the small grain material. In addition, experimental observations reveal rapid and efficient dislocation loop absorption by the grain boundaries and this is considered the dominant factor for mass transport to the boundaries during irradiation, enabling the remarkable radiation tolerance of 35 nm grained tungsten. Furthermore, this study demonstrates the possibility of attaining high radiation tolerant materials, in terms of dislocation loop damage, by minimizing grain sizes in the nanocrystalline regime.

36 MATERIALS SCIENCE↗
DOE OSTI2020

STEM Characterization of Dislocation Loops in Irradiated FCC Alloys

In this study, we demonstrate the methodology systematically developed for dislocation loop (perfect and faulted loops) imaging and analysis in irradiated face-centered-cubic (FCC) alloys using scanning transmission electron microscopy (STEM). On-zone [001] STEM imaging was identified as the preferred choice for its accuracy and effectiveness based on the comparison with other dislocation loop imaging techniques including: (i) on-zone STEM imaging using other major low-index zone axes, (ii) kinematic two-beam conditions bright field imaging near the [001] zone axis in conventional TEM (CTEM) mode, and (iii) Rel-Rod CTEM dark-field (DF) imaging near the [011] zone axis. The effect of STEM collection angle on the contrast formation of dislocation loops was also investigated. The developed method was confirmed by imaging all populations of perfect and faulted loops of types a/2$\langle$110$\rangle${110} and a/3$\langle$111$\rangle${111} found in an ion irradiated Ni40Fe40Cr20 alloy. The proposed STEM-based technique can easily identify said loops with a size greater than 10 nm without any assumptions such as those commonly made using the conventional Rel-Rod CTEM-DF technique. The recommended methodology in this study is developed as a quick and convenient tool that can be generally applied to irradiated FCC-based materials due to their common crystallography.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Study of the dislocation activity in a Mg–Y alloy by differential aperture X-ray microscopy

Differential aperture X-ray microscopy (DAXM) was employed in this study to reveal the development of geometrically necessary dislocations (GNDs) when a Mg-5 wt% Y alloy is subjected to bending. The characters of the GNDs were determined from the streak direction of the diffraction peaks. Five grains were analyzed voxel by voxel for the GND content. Among those voxels with peak streaking, 77% contain basal or non-basal < a > GNDs, while the other 23% contain < c + a > GNDs. Furthermore, those < c + a > dislocations were nucleated to accommodate local stress concentration. Even though < c + a > dislocations are less mobile, they make important contribution to the high ductility of Mg-Y alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Stacking Faults Created by the Combined Deflection of Threading Dislocations of Burgers Vector c and cplusa during the Physical Vapor Transport Growth of 4H-SiC

Observations have been made, using synchrotron white beam x-ray topography, of stacking faults in 4H-SiC with fault vectors of kind 1/6 <20{bar 2}3>. A mechanism has been postulated for their formation which involves overgrowth by a macrostep of the surface outcrop of a c-axis threading screw dislocation, with two c/2-height surface spiral steps, which has several threading dislocations of Burgers vector c+a, with c-height spiral steps, which protrude onto the terrace in between the c/2-risers. Such overgrowth processes deflect the threading dislocations onto the basal plane, enabling them to exit the crystal and thereby providing a mechanism to lower their densities.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dislocation theory of steady and transient creep of crystalline solids: Predictions for olivine

In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Particle interspacing effects on the mechanical behavior of a Fe–TiB 2 metal matrix composite using FFT-based mesoscopic field dislocation mechanics

This paper presents an application to metal matrix composites (MMCs) of an enhanced elasto-viscoplastic Fast Fourier Transform (EVP-FFT) formulation coupled with a phenomenological continuum Mesoscale Field Dislocation Mechanics (MFDM) theory. Contrary to conventional crystal plasticity, which only accounts for plastic flow and hardening induced by statistically stored dislocations (SSDs), MFDM-EVP-FFT also describes the evolution of polarized geometrically necessary dislocation (GND) density and its effect on both plastic flow and hardening. Numerical results for a Fe–TiB 2 MMC made of a ferrite matrix (α-Fe) and elastic ceramic particles (TiB 2 ) are presented. Full-field simulations are performed using synthetic periodic unit cells representative of the MMC, with single-crystalline and polycrystalline matrix, for different particle interspacing distances. A strong dependence of the predicted equivalent stress, cumulated plastic strain and GND density fields with particle interspacing distance is observed, in contrast with conventional crystal plasticity. Correlations between these mechanical fields and microstructural features, and their influence on local and global mechanical behavior are examined for the different MMC microstructures.

42 ENGINEERING↗
DOE OSTI2023

Properties of Accelerating Edge Dislocations in Arbitrary Slip Systems with Reflection Symmetry

We discuss the theoretical solution to the differential equations governing accelerating edge dislocations in anisotropic crystals. This is an important prerequisite to understanding high-speed dislocation motion, including an open question about the existence of transonic dislocation speeds, and subsequently high-rate plastic deformation in metals and other crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tunable Oxygen Diffusion and Electronic Conduction in SrTiO 3 by Dislocation-Induced Space Charge Fields

Plastic strain engineering has been applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO 3 as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n-type conductivity by 50 times at oxygen pressures below 10 -5 atm at 650 °C. At higher oxygen pressures the p-type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO 3 arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro-solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Revealing the Subsurface Basal $\langle$a$\rangle$ Dislocation Activity in Magnesium Through Lattice Rotation Analysis

A method was proposed in this study to reveal the subsurface basal dislocation activity in Mg-Y alloy and determine the corresponding Burgers vector. This is achieved by correlating the slip directions of dislocations to the lattice rotation represented by the {0001} pole figure. The identified basal slip system by this approach was verified by micro-Laue diffraction. This method can be applied as a complementary method to the conventional slip trace analysis to study the dislocation behavior of Mg alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Cellular Automaton Modeling of Dynamic Recrystallization of Nimonic 80A Superalloy Based on Inhomogeneous Distribution of Dislocations Inside Grains

A modified cellular automaton (CA) model in the time and space scale is developed to simulate the dynamic recrystallization (DRX) mechanism of Nimonic 80A nickel-base superalloy during hot deformation. Some necessary thermo-mechanical parameters used as input data in CA approach such as work hardening and softening coefficient are fitted according to the previous hot compression experiments. Based on the experimental results, nucleation kinetics is modified as a function of Zener-Holloman parameter. A dislocation density model at transient recrystallization boundary is proposed, in which the dislocation density is a function of temperature, strain rate and annealed dislocation density. The simulated results are in good agreement with experimental results, proving that the modified CA model is capable of predicting both flow behavior and microstructural evolution of Nimonic 80A during hot deformation. According to the experimental and simulated results, the DRX behavior is discussed in detail. It demonstrates that the flow stress and microstructural evolution of DRX have a relationship with the deformation temperature and strain rate.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dislocation Density-Based Model for Flow Behavior of a Near-α Titanium Alloy Considering Effects of Initial Lamellar Thickness

A dislocation density-based constitutive model that considered the effects of initial lamellar thickness was proposed to predict the flow behavior of a near-α high-temperature Ti-5.4Al-3.7Sn-3.3Zr-0.5Mo-0.4Si alloy. The evolution equations of dislocation density were improved by introducing the influence of the dynamic globularization of lamellar microstructure with different initial thicknesses. The evolution equations and Hall-Petch behavior were coupled into the constitutive model, which was composed of athermal and thermal stresses. According to the established constitutive equations, the variation and contributions of corresponding stress components were analyzed. The calculated dislocation densities were compared with the experimental values obtained by electron backscattered diffraction data to verify the model. Moreover, the flow stresses of the studied alloy with different initial lamellar microstructures under various deformation conditions were predicted. Good agreements between the predicted and experimental results were obtained. Additionally, steady-state flow behavior was further analyzed and the relationship between the calculated steady-state flow stress and the equiaxed α-phase size was quantitatively obtained. The results indicated the validity of the proposed constitutive model for predicting the flow behavior of the alloy with different initial lamellar microstructures over a range of deformation temperatures and strain rates.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Helium segregation to screw and edge dislocations in α-iron and their yield strength

Helium (He) introduces one of the mayor concerns in the nuclear materials community as it modifies the mechanical properties of the system withstanding fast neutron spectra, promoting swelling and embrittlement. Ferritic/martensitic steels are one of the main candidates as structural materials for future nuclear applications. Experimentally the bubble distribution is observed to vary depending on irradiation conditions (temperature, dose rate and total dose). However, traditional atomistic models decouple the role of temperature in the mechanical properties from its effect on the bubble distribution. In this paper we study substitutional He segregation to screw and edge dislocations in α-Fe at different temperatures. We use an object kinetic Monte Carlo methodology to obtain general trends in bubble distribution and a canonical Monte Carlo algorithm, with full atomistic fidelity, to find the He distribution at the dislocation cores. Molecular dynamics has subsequently been applied to study the yield strength, which increases significantly in the presence of He, more remarkably for the edge dislocation. The total stress fits a Kocks relation. Yet, if the lattice resistance is subtracted, the relation between the critical shear stress and the temperature is non-monotonic for the screw character. To reproduce this effect, we propose to modify the Kocks relation, adding a second-order term in temperature that extends the range of applicability of the model.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dislocation mediated alignment during metal nanoparticle coalescence

Dislocation mediated alignment processes during gold nanoparticle coalescence were studied at low and high temperatures using molecular dynamics simulations and transmission electron microscopy. Particles underwent rigid body rotations immediately following attachment in both low temperature (500 K) simulated coalescence events and low temperature (~315 K) transmission electron microscopy beam heating experiments. In many low temperature simulations, some degree of misorientation between particles remained after rigid body rotations, which was accommodated by grain boundary dislocation nodes. These dislocations were either sessile and remained at the interface for the duration of the simulation or dissociated and cross-slipped through the adjacent particles, leading to improved co-alignment. Minimal rigid body rotations were observed during or immediately following attachment in high temperature (1100 K) simulations, which is attributed to enhanced diffusion at the particles' interface. However, rotation was eventually induced by {111} slip on planes parallel to the neck groove. These deformation modes led to the formation of single and multi-fold twins whose structures depended on the initial orientation of the particles. The driving force for {111} slip is attributed to high surface stresses near the intersection of low energy {111} facets in the neck region. The details of this twinning process were examined in detail using simulated trajectories, and the results reveal possible mechanisms for the nucleation and propagation of Shockley partials on consecutive planes. Deformation twinning was also observed in-situ using transmission electron microscopy, which resulted in the co-alignment of a set of the particles' {111} planes across their grain boundary and an increase in their dihedral angle. As a result, this constitutes the first detailed experimental observation of deformation twinning during nanoparticle coalescence, validating simulation results presented here and elsewhere.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Variability of non-Schmid effects in grain boundary dislocation nucleation criteria

Grain boundary dislocation nucleation is simulated in a Nickel bicrystal with Σ21b (211)/(211) grain boundaries. Using molecular dynamics, 386 different triaxial stress states are examined for their effect on dislocation nucleation on different slip systems. The approach leads to dislocation nucleation on six of the twelve partial slip systems, enabling a study of conditions leading to nucleation. The criteria for nucleation on each of the slip systems is shown to have a linear dependence on the resolved shear, normal, and co-slip stresses, though the constants for this linear dependence are unique for each slip system. The combined nucleation criteria are used to construct a theoretical nucleation surface. The surface is reminiscent of a Mohr-Coulomb yield surface, except that in the present case the facets of the surface correspond to nucleation on different slip systems.

36 MATERIALS SCIENCE↗