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At least 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

The evolution with strain of the stored energy in different texture components of cold-rolled IF steel revealed by high resolution X-ray diffraction

During the deformation of low carbon steel by cold-rolling, dislocations are created and stored in grains depending on local crystallographic orientation, deformation, and deformation gradient. Orientation dependent dislocation densities have been estimated from the broadening of X-ray diffraction lines measured on a synchrotron beamline. Different cold-rolling levels (from 30% to 95% thickness reduction) have been considered. It is shown that the present measurements are consistent with the hypothesis of the sole consideration of screw dislocations for the analysis of the data. The presented evolutions show that the dislocation density first increases within the α fiber (=(hkl)<110>) and then within the γ fiber (=(111) ). A comparison with EBSD measurements is done and confirms that the storage of dislocations during the deformation process is orientation dependent and that this dependence is correlated to the cold-rolling level. If we assume that this dislocation density acts as a driving force during recrystallization, these observations can explain the fact that the recrystallization mechanisms are generally different after moderate or large strains. - Highlights: • Dislocation densities are assessed by XRD in main texture components of a steel sheet. • Dislocation densities vary with both strain and texture components. • The analysis relies on the sole presence of screw dislocations. • The measured dislocation densities include the contribution of both SSD and GND.

36 MATERIALS SCIENCE↗
DOE OSTI2020

X-ray topography characterization of gallium nitride substrates for power device development

Gallium nitride substrates grown by the hydride vapor phase epitaxy (HVPE) method using a patterned growth process have been characterized by synchrotron monochromatic beam X-ray topography in the grazing incidence geometry. Images reveal a starkly heterogeneous distribution of dislocations with areas as large as 0 3 mm 2 containing threading dislocation densities below 10 3 cm -2 in between a grid of strain centers with higher threading dislocation densities (> 10 4 cm -2 ). Basal plane dislocation densities in these areas are as low as 104 cm -2 . By comparing the recorded images of dislocations with ray tracing simulations of expected dislocations in GaN, the Burgers vectors of the dislocations have been determined. The distribution of threading screw/mixed dislocations (TSDs/TMDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) is discussed with implications for fabrication of power devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructural investigation of plastically deformed Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy by X-ray diffraction and transmission electron microscopy

The microstructure evolution in body-centered cubic (bcc) Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy during quasi-static compression test was studied by X-ray line profile analysis (XLPA) and transmission electron microscopy (TEM). The average lattice constant and other important parameters of the microstructure such as the mean crystallite size, the dislocation density and the edge/screw character of dislocations were determined by XLPA. The elastic anisotropy factor required for XLPA procedure was determined by nanoindentation. XLPA shows that the crystallite size decreased while the dislocation density increased with strain during compression, and their values reached about 39 nm and 15 × 10{sup 14} m{sup −2}, respectively, at a plastic strain of ~ 20%. It was revealed that with increasing strain the dislocation character became more screw. This can be explained by the reduced mobility of screw dislocations compared to edge dislocations in bcc structures. These observations are in line with TEM investigations. The development of dislocation density during compression was related to the yield strength evolution. - Highlights: • Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy was processed by arc-melting. • The mechanical was evaluated by RT compression test. • The microstructure evolution was studied by XLPA and TEM. • With increasing strain the dislocation character became more screw. • The yield strength was related to the development of the dislocation density.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Study of tempering behavior of lath martensite using in situ neutron diffraction

To elucidate changes in the density and substructure of dislocations during tempering of lath martensite steel, a convolutional multiple whole-profile fitting method was applied to in situ neutron diffraction profiles. With increasing tempering temperature, the dislocation density scarcely changed in the beginning and then decreased at temperatures above 473 K, whereas the dislocation arrangement drastically changed at temperatures above 673 K. The strength of the steel is speculated to depend on the density and arrangement of dislocations. - Highlights: • A convolutional multiple whole-profile fitting method was applied. • Dislocation density and dislocation arrangement changing with tempering were discussed. • Dislocation density scarcely changed in the beginning. • And then dislocation density decreased at temperatures above 473 K. • The dislocation arrangement drastically changed at temperatures above 673 K.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Data-driven analysis of neutron diffraction line profiles: application to plastically deformed Ta

Abstract Non-destructive evaluation of plastically deformed metals, particularly diffraction line profile analysis (DLPA), is valuable both to estimate dislocation densities and arrangements and to validate microstructure-aware constitutive models. To date, the interpretation of whole line diffraction profiles relies on the use of semi-analytical models such as the extended convolutional multiple whole profile (eCMWP) method. This study introduces and validates two data-driven DLPA models to extract dislocation densities from experimentally gathered whole line diffraction profiles. Using two distinct virtual diffraction models accounting for both strain and instrument induced broadening, a database of virtual diffraction whole line profiles of Ta single crystals is generated using discrete dislocation dynamics. The databases are mined to create Gaussian process regression-based surrogate models, allowing dislocation densities to be extracted from experimental profiles. The method is validated against 11 experimentally gathered whole line diffraction profiles from plastically deformed Ta polycrystals. The newly proposed model predicts dislocation densities consistent with estimates from eCMWP. Advantageously, this data driven LPA model can distinguish broadening originating from the instrument and from the dislocation content even at low dislocation densities. Finally, the data-driven model is used to explore the effect of heterogeneous dislocation densities in microstructures containing grains, which may lead to more accurate data-driven predictions of dislocation density in plastically deformed polycrystals.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Single- and dual-variant atomic ordering in GaAsP compositionally graded buffers on GaP and Si substrates

Here, we investigate the material properties of GaAsP graded buffers on both GaP and Si substrates in order to determine limitations to dislocation glide in GaAsP. Phase separation is not observed in these GaAsP buffers, but CuPt B atomic ordering is present, and has an impact on the Burgers vector distribution of gliding dislocations. Tellurium surfactant eliminates ordering in GaAsP, allowing control over the glide plane distribution while highlighting the importance of the growth surface and choice of dopant. The impact of single-variant CuPtB ordering of GaAsP buffers grown on GaP substrates is investigated by monitoring the threading dislocation density throughout the buffer in sequential steps. The dislocation density rises steadily throughout the graded buffer, implying that factors other than ordering-induced glide plane switches play a dominant role in the final dislocation density. We observe an increase in surface roughness throughout the buffer and speculate that a dislocation nucleation source on the surface has a low activation energy and may lead to increased threading dislocation density. On Si substrates, the GaAsP buffer displays dual-variant CuPt B ordering rather than single-variant ordering due to the surface step geometry. We discuss how dual-variant ordering leads to a variable glide force, and thus has a different impact on dislocation dynamics than single-variant ordering. Because the III-V on Si nucleation procedure determines the step geometry, it also influences the dislocation dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Loading direction dependence of asymmetric response of pyramidal slip in rolled AZ31 magnesium alloy

Textured magnesium alloys usually exhibit anisotropic mechanical behavior due to the asymmetric activation of different twinning and slipping modes. This work focuses on the pyramidal slip responses of rolled AZ31 magnesium alloy under two loading conditions, compressive and tensile loading along the normal direction. Under the condition where the compressive loading direction is closely parallel to the c-axis of the unit cell, tensile twinning and basal slips are prohibited, dislocations then active and tend to accumulate at grain boundaries and form dislocation walls. Meanwhile, these dislocations exhibit zigzag morphologies, which result from the cross-slip from {$10\bar{1}1$} first-order pyramidal plane to {$11\bar{2}2$} second-order pyramidal plane, then back to {$10\bar{1}1$} first-order pyramidal plane. Under the condition where tensile twins are prevalent, {$10\bar{1}1$} first-order and {$11\bar{2}2$} second-order pyramidal dislocations are favorable to be activated. Both types of dislocations behave climb-like dissociations onto the basal plane, forming zigzag dislocations. first-order pyramidal plane to second-order pyramidal plane, then back to first-order pyramidal plane. Under the condition where tensile twins are prevalent, first-order and second-order pyramidal dislocations are favorable to be activated. Both types of dislocations behave climb-like dissociations onto the basal plane, forming zigzag dislocations.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Resolving electrochemically triggered topological defect dynamics and structural degradation in layered oxides

Understanding topological defects-controlled structural degradation of layered oxides—a key cathode material for high-performance lithium-ion batteries—plays a critical role in developing next-generation cathode materials. Here, by constructing a nanobattery in an electron microscope enabling atomic-scale monitoring of electrochemcial reactions, we captured the electrochemically driven atomistic dynamics and evolution of dislocations—a most important topological defect in material. We deciphered how dislocations nucleate, move, and annihilate within layered cathodes at the atomic scale. Specifically, we found two types of dislocation configurations, i.e., single dislocations and dislocation dipoles. Both pure dislocation glide/climb and mixed motions were captured, and the dislocation glide and climb velocities were first experimentally measured. Moreover, dislocation activity-mediated structural degradation such as crack nucleation, phase transformation, and lattice reorientation was unraveled. Our work provides deep insights into the atomistic dynamics of electrochemically driven dislocation activities in layered oxides.

Wang, Chunyang (ORCID:0000000184613952)↗
DOE OSTI2012

Monte Carlo simulations of channeling spectra recorded for samples containing complex defects

The main aim of the present paper is to describe the current status of the development of McChasy, a Monte Carlo simulation code, to make it suitable for the analysis of dislocations and dislocation loops in crystals. Such factors like the shape of the bent channel and geometrical distortions of the crystalline structure in the vicinity of dislocation has been discussed. Several examples of the analysis performed at different energies of analyzing ions are presented. The results obtained demonstrate that the new procedure applied to the spectra recorded on crystals containing dislocation yields damage profiles which are independent of the energy of the analyzing beam.

Carlo↗
DOE OSTI2015

Robust Technique for Measuring and Simulating Silicon Wafer Quality Characteristics that Enable the Prediction of Solar Cell Electrical Performance of MEMC Silicon Wafer. Cooperative Research and Development Final Report, CRADA Number CRD-11-438

NREL and MEMC Electronic Materials are interested in developing a robust technique for monitoring material quality of mc-Si and mono-Si wafers -- a technique that can provide relevant data to accurately predict the performance of solar cells fabricated on them. Previous work, performed under two TSAs between NREL and MEMC, has established that dislocation clusters are the dominant performance-limiting factor in MEMC mc-Si solar cells. The work under this CRADA will go further in verifying these results on a larger data set, evaluate possibilities of faster method(s) for mapping dislocations in wafers/ingots, understanding dislocation generation during ingot casting, and helping MEMC to have an internal capability for basic characterization that will provide feedback needed for more accurate crystallization simulations. NREL has already developed dislocation mapping technique and developed a basic electronic model (called Network Model) that uses spatial distribution of dislocations to predict the cell performance. In this CRADA work, we will use these techniques to: (i) establish dislocation, grain size, and grain orientation distributions of the entire ingots (through appropriate DOE) and compare these with theoretical models developed by MEMC, (ii) determine concentrations of some relevant impurities in selected wafers, (iii) evaluate potential of using photoluminescence for dislocation mapping and identification of recombination centers, (iv) evaluate use of diode array analysis as a detailed characterization tool, and (v) establish dislocation mapping as a wafer-quality monitoring tool for commercial mc-Si production.

14 SOLAR ENERGY↗
DOE OSTI2017

The effect of carbon on the microstructures, mechanical properties, and deformation mechanisms of thermo-mechanically treated Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 high entropy alloys

We investigated the effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 high entropy alloys (HEAs). X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M 23 C 6 and M 7 C 3 carbides for the C-doped HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 μm grain-sized undoped HEA, while microbands formed in the 23 μm grain-sized C-doped HEA. Conversely, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 μm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocation-cells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Our investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Elemental segregation to lattice defects in the CrMnFeCoNi high-entropy alloy during high temperature exposures

The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2<110>-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a <110>-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2<110> dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L1 0 NiMn phase that is observed on the grain boundaries after prolonged annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Atomistic modeling of interface strengthening in Al-Si eutectic alloys

Al-Si cast alloys are usually composed of α-Al and Al-Si eutectic. Si flakes and Al matrix generally hold cube-on-cube orientation relationship with the primary interface (111) Al ∥(111) Si . Extensive experimental studies demonstrated that Si flakes cannot significantly improve mechanical properties of Al-Si cast alloys. We hypothesize that the weak strengthening effect associated with Si flakes might be attributed to thermomechanical properties of Al-Si interfaces besides their morphologies. To characterize Al-Si interfaces with a large lattice mismatch (> 30%), we proposed the quasi-coincident site lattice (Q-CSL) as reference lattice, and demonstrated that the Q-CSL Al-Si coherent interface has three characteristic coherent structures, one stable and low energy structure and two metastable and high energy structures. The translation vectors for the same type of coherent Q-CSL structures are consistent with three displacement shift complete (DSC) vectors. The two metastable structures can be obtained by shifting the low energy structure with three partial DSC vectors. Semi-coherent interface is composed of the low energy Q-CSL patches and three sets of interface misfit dislocations with Burgers vectors same as the DSC vectors. Atomistic simulations revealed that Al-Si interface exhibits low shear resistance. Ideal shear strength of the Q-CSL coherent interface is 110 MPa and semi-coherent interface is 20 MPa. The low shear resistance is attributed to the glide of interface misfit dislocations. Al-Si interface also exhibits low formation and migration energies of point defects. Owing to low shear strength and low formation and migration energies of point defects, interface sliding or shear readily happen under mechanical loading or during dislocation-interface interactions. Lattice dislocations can cross slip onto or climb along Al-Si interfaces. These reactions decrease the number of accumulated dislocation loops around Si flakes and promote nucleation and emission of lattice dislocations from Al-Si interfaces to matrix, consequently reduce the repulsive force on approaching dislocations and weaken Si flakes strengthening effect. In situ tension and compression tests in a scanning electron microscope reveal relatively weak strengthening effect due to Si flakes, consistent with the computed dislocation interaction with interfaces and shear behavior of interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Plastic deformation of a porous bcc metal containing nanometer sized voids

Nanoporous materials, can present an outstanding range of mechanical properties. Here both molecular dynamics and dislocation analysis were used to evaluate and quantify the evolution of plasticity in a porous Ta single crystal containing randomly placed voids with 3.3 nm radii and average initial porosity of 4.1%, when subjected to uniaxial compressive strain. Nanovoids act as effective sources for dislocation emission. Dislocation shear loops nucleate at the surface of the voids and expand by the advance of the edge component. The evolution of dislocation configuration and densities were predicted by the molecular dynamics calculations and successfully compared to an analysis based on Ashby’s concept of geometrically-necessary dislocations. Resolved shear stress calculations were performed for all bcc slip systems and used to identify the operating Burgers vectors in the dislocation loops. The temperature excursion during plastic deformation was used to estimate the mobile dislocation density which is found to be less than 10% of the total dislocation density.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Incubational domain characterization in lightly doped ceria

Microstructures of both Gd- and Y-doped ceria with different doping level (i.e., 10 at% and 25 at%) have been comprehensively characterized by means of high resolution transmission electron microscopy and selected area electron diffraction. Coherent nano-sized domains can be widely observed in heavily doped ceria. Nevertheless, it was found that a large amount of dislocations actually exist in lightly doped ceria instead of heavily doped ones. Furthermore, incubational domains can be detected in lightly doped ceria, with dislocations located at the interfaces. The interactions between such linear dislocations and dopant defects have been simulated accordingly. As a consequence, the formation mechanism of incubational domains is rationalized in terms of the interaction between intrinsic dislocations of doped ceria and dopant defects. This study offers the insights into the initial state and related mechanism of the formation of nano-sized domains, which have been widely observed in heavily rare-earth-doped ceria in recent years. - Graphical abstract: Interactions between dislocations and dopants lead to incubational domain formation in lightly doped ceria. Highlights: Black-Right-Pointing-Pointer Microstructures were characterized in both heavily and light Gd-/Y-doped ceria. Black-Right-Pointing-Pointer Dislocations are existed in lightly doped ceria rather than heavily doped one. Black-Right-Pointing-Pointer Interactions between dislocations and dopant defects were simulated. Black-Right-Pointing-Pointer Formation of dislocation associated incubational domain is rationalized.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Characterization of microstructure in hydrogen ion irradiated vanadium at room temperature and the microstructural evolution during post-irradiation annealing

The microstructure of pure vanadium after hydrogen ion irradiation at room temperature to a fluence of 1 × 10{sup 17} ions/cm{sup 2} (and 5 × 10{sup 16} ions/cm{sup 2}) was investigated by transmission electron microscopy (TEM). Small dislocation loops (black spots) and cavities are formed after the irradiation. The nature and Burgers vector of dislocation loops formed in vanadium was characterized using g·b technique and inside–outside method. Interstitial dislocation loops with Burgers vector of 1/2 < 111 > predominantly formed with less than 10% of 1/2 < 110 > type. No < 100 > type or vacancy type dislocation loop formed. The microstructural evolution during the annealing process was also studied. Density and size of dislocation loops changed sharply when the annealing temperature was lifted up to 450 °C. When the annealing temperature was higher than 500 °C, bubble coalescence occurred with some large hydrogen bubbles formed. - Highlights: • Interstitial dislocation loops with Burgers vector of 1/2<111> were predominant. • Less than 10% of 1/2<110> dislocation loops were present in pure vanadium. • No <100> or vacancy type loops were present in pure vanadium. • Density and size of dislocation loops changed sharply at temperature above 450 °C. • Bubble coalescence occurred when annealing temperature was higher than 500 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Nanoscale size dependence parameters on lattice thermal conductivity of Wurtzite GaN nanowires

Graphical abstract: Temperature dependence of calculated lattice thermal conductivity of Wurtzite GaN nanowires. Highlights: Black-Right-Pointing-Pointer A modified Callaway model is used to calculate lattice thermal conductivity of Wurtzite GaN nanowires. Black-Right-Pointing-Pointer A direct method is used to calculate phonon group velocity for these nanowires. Black-Right-Pointing-Pointer 3-Gruneisen parameter, surface roughness, and dislocations are successfully investigated. Black-Right-Pointing-Pointer Dislocation densities are decreases with the decrease of wires diameter. -- Abstract: A detailed calculation of lattice thermal conductivity of freestanding Wurtzite GaN nanowires with diameter ranging from 97 to 160 nm in the temperature range 2-300 K, was performed using a modified Callaway model. Both longitudinal and transverse modes are taken into account explicitly in the model. A method is used to calculate the Debye and phonon group velocities for different nanowire diameters from their related melting points. Effect of Gruneisen parameter, surface roughness, and dislocations as structure dependent parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves. It was observed that Gruneisen parameter will decrease with decreasing nanowire diameters. Scattering of phonons is assumed to be by nanowire boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. At high temperatures and for dislocation densities greater than 10{sup 14} m{sup -2} the lattice thermal conductivity would be limited by dislocation density, but for dislocation densities less than 10{sup 14} m{sup -2}, lattice thermal conductivity would be independent of that.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Large Scale DD Simulation Results for Crystal Plasticity Parameters in Fe-Cr And Fe-Ni Systems

The development of viable nuclear energy source depends on ensuring structural materials integrity. Structural materials in nuclear reactors will operate in harsh radiation conditions coupled with high level hydrogen and helium production, as well as formation of high density of point defects and defect clusters, and thus will experience severe degradation of mechanical properties. Therefore, the main objective of this work is to develop a capability that predicts aging behavior and in-service lifetime of nuclear reactor components and, thus provide an instrumental tool for tailoring materials design and development for application in future nuclear reactor technologies. Towards this end goal, the long term effort is to develop a physically based multiscale modeling hierarchy, validated and verified, to address outstanding questions regarding the effects of irradiation on materials microstructure and mechanical properties during extended service in the fission and fusion environments. The focus of the current investigation is on modern steels for use in nuclear reactors including high strength ferritic-martensitic steels (Fe-Cr-Ni alloys). The effort is to develop a predicative capability for the influence of irradiation on mechanical behavior. Irradiation hardening is related to structural information crossing different length scales, such as composition, dislocation, and crystal orientation distribution. To predict effective hardening, the influence factors along different length scales should be considered. Therefore, a hierarchical upscaling methodology is implemented in this work in which relevant information is passed between models at three scales, namely, from molecular dynamics to dislocation dynamics to dislocation-based crystal plasticity. The molecular dynamics (MD) was used to predict the dislocation mobility in body centered cubic (bcc) Fe and its Ni and Cr alloys. The results are then passed on to dislocation dynamics to predict the critical resolved shear stress (CRSS) from the evolution of local dislocation and defects. In this report the focus is on the results obtained from large scale dislocation dynamics simulations. The effect of defect density, materials structure was investigated, and evolution laws are obtained. These results will form the bases for the development of evolution and hardening laws for a dislocation-based crystal plasticity framework. The hierarchical upscaling method being developed in this project can provide a guidance tool to evaluate performance of structural materials for next-generation nuclear reactors. Combined with other tools developed in the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the models developed will have more impact in improving the reliability of current reactors and affordability of new reactors.

36 MATERIALS SCIENCE↗