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Millett, Paul C

Publications and source records attributed to Millett, Paul C.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Molecular dynamics simulations of intergranular fracture in UO2 with nine empirical interatomic potentials

The intergranular fracture behavior of UO2 was studied using molecular dynamics simulations with a bicrystal model. The anisotropic fracture behavior due to the different grain boundary characters was investigated with the View the MathML source symmetrical tilt S5 and the View the MathML source symmetrical tilt S3 ({1 1 1} twin) grain boundaries. Nine interatomic potentials, seven rigid-ion plus two core–shell ones, were utilized to elucidate possible potential dependence. Initiating from a notch, crack propagation along grain boundaries was observed for most potentials. The S3 boundary was found to be more prone to fracture than the S5 one, indicated by a lower energy release rate associated with the former. However, some potential dependence was identified on the existence of transient plastic deformation at crack tips, and the results were discussed regarding the relevant material properties including the excess energies of metastable phases and the critical energy release rate for intergranular fracture. In general, local plasticity at crack tips was observed in fracture simulations with potentials that predict low excess energies for metastable phases and high critical energy release rates for intergranular fracture.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Demonstrating the Temperature Gradient Impact on Grain Growth in UO2 Using the Phase Field Method

Grain boundaries (GBs) are driven to migrate up a temperature gradient. In this work, we use a phase field (PF) model to investigate the impact of temperature gradients on normal grain growth. GB motion in 2D UO2 polycrystals is predicted under increasing temperature gradients. We find that the temperature gradient does not significantly impact the average grain growth behavior, because the curvature driving force is dominant. However, it does cause significant local migration of the individual grains. In addition, the change in the GB mobility due to the temperature gradient results in larger grains in the hot portion of the polycrystal.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Intergranular fracture in UO2: derivation of traction-separation law from atomistic simulations

In this study, the intergranular fracture behavior of UO2 was studied by molecular dynamics simulations using the Basak potential. In addition, the constitutive traction-separation law was derived from atomistic data using the cohesive-zone model. In the simulations a bicrystal model with the (100) symmetric tilt E5 grain boundaries was utilized. Uniaxial tension along the grain boundary normal was applied to simulate Mode-I fracture. The fracture was observed to propagate along the grain boundary by micro-pore nucleation and coalescence, giving an overall intergranular fracture behavior. Phase transformations from the Fluorite to the Rutile and Scrutinyite phases were identified at the propagating crack tips. These new phases are metastable and they transformed back to the Fluorite phase at the wake of crack tips as the local stress concentration was relieved by complete cracking. Such transient behavior observed at atomistic scale was found to substantially increase the energy release rate for fracture. Insertion of Xe gas into the initial notch showed minor effect on the overall fracture behavior.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

A Quantitative Comparison between C0 and C1 elements for Solving the Cahn-Hilliard Equation

The Cahn–Hilliard (CH) equation is a time-dependent fourth-order partial differential equation (PDE). When solving the CH equation via the finite element method (FEM), the domain is discretized by C1-continuous basis functions or the equation is split into a pair of second-order PDEs, and discretized via C0-continuous basis functions. In the current work, a quantitative comparison between C1 Hermite and C0 Lagrange elements is carried out using a continuous Galerkin FEM formulation. The different discretizations are evaluated using the method of manufactured solutions solved with Newton’s method and Jacobian-Free Newton Krylov. It is found that the use of linear Lagrange elements provides the fastest computation time for a given number of elements, while the use of cubic Hermite elements provides the lowest error. The results offer a set of benchmarks to consider when choosing basis functions to solve the CH equation. In addition, an example of microstructure evolution demonstrates the different types of elements for a traditional phase-field model.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Guidance to Design Grain Boundary Mobility Experiments with Molecular Dynamics and Phase-Field Modeling

Quantitative phase-field modeling can play an important role in designing experiments to measure the grain boundary (GB) mobility. In this work, molecular dynamics (MD) simulation is employed to determine the GB mobility using Cu bicrystals. Two grain configurations are considered: a shrinking circular grain and a half loop grain. The results obtained from the half loop configuration approaches asymptotically to that obtained from the circular configuration with increasing half loop width. We then verify the phase- field model by directly comparing to the MD simulation results, obtaining excellent agreement. Next, this phase-field model is used to predict the behavior in a common experimental setup that utilizes a half loop grain configuration in a bicrystal to measure the GB mobility. With a 3D simulation, we identify the two critical times within the experiments to reach an accurate value of the GB mobility. We use a series of 2D simulations to investigate the impact of the notch angle on these two critical times and we identify an angle of 60? as an optimal value. We also show that if the notch does not have a sharp tip, it may immobilize the GB migration indefinitely.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Effects of Intergranular Gas Bubbles on Thermal Conductivity

Model microstructures obtained from phase-field simulations are used to study the effective heat transfer across bicrys- tals with stationary grain boundary bubble populations. We find that the grain boundary coverage, irrespective of the intergranular bubble radii, is the most relevant parameter to the thermal resistance, which we use to derive effec- tive Kapitza resistances that are dependent on the grain boundary coverage and Kaptiza resistance of the intact grain boundary. We propose a model to predict thermal conductivity as a function of porosity, grain-size, Kaptiza resistance of the intact grain boundary, and grain boundary bubble coverage.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Crack tip plasticity in single crystal UO2: Atomistic simulations

The fracture behavior of single crystal uranium dioxide is studied using molecular dynamics simulations at room temperature. Initially, an elliptical notch is created on either {111} or {110} planes, and tensile loading is applied normal to the crack planes. For cracks on both planes, shielding of crack tips by plastic deformation is observed, and crack extension occurs for crack on {111} planes only. Two plastic processes, dislocation emission and phase transformation are identified at crack tips. The dislocations have a Burgers vector of ?110?/2, and glide on {100} planes. Two metastable phases, the so-called Rutile and Scrutinyite phases, are identified during the phase transformation, and their relative stability is confirmed by separate density- functional-theory calculations. Examination of stress concentration near crack tips reveals that dislocation emission is not an effective shielding mechanism. The formation of new phases may effectively shield the crack provided all phase interfaces formed near the crack tips are coherent, as in the case of cracks residing on {110} planes.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Deformation Twins in Nanocrystalline Body-Centered Cubic Mo as Predicted by Molecular Dynamics Simulations

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In (1 1 0) columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrest by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Molecular dynamics simulations of He bubble nucleation at grain boundaries

The nucleation behavior of He bubbles in nano-grained body-centered-cubic (BCC) Mo is simulated using molecular dynamics (MD) simulations with a bicrystal model, focusing on the effect of grain boundary (GB) structure. Three types of GBs, the (100) twist S29, the ?110? symmetrical tilt (tilt angle of 10.1?), and the (112) twin boundaries, are studied as representatives of random GB, low angle GB with misfit dislocations, and special sigma boundaries. With the same amount of He, more He clusters form in nano-grained Mo with smaller average size compared to that in bulk. The effects of the GB structure originate from the excess volume in GBs. Trapping by excess volume results in reduction in mobility of He atoms, which enhances the nucleation with higher density of bubbles, and impedes the growth of He bubbles by absorption of mobile He atoms. Furthermore, the distribution of excess volume in GBs determines the distribution of He clusters. The effect of GBs becomes less pronounced with increasing vacancy concentration in the matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2012

PHASE-FIELD SIMULATION OF INTERGRANULAR BUBBLE GROWTH AND PERCOLATION IN BICRYSTALS

We present three-dimensional phase-field simulations of the growth and coalescence of intergranular bubbles in bicrystal grain geometries. We investigate the dependency of bubble percolation on two factors: the initial bubble density and the bubble shape, which is governed by the ratio of the grain boundary energy over the surface energy. We find that variations of each of these factors can lead to large discrepancies in the bubble coalescence rate, and eventual percolation, which may partially explain this observed occurrence in experimental investigations. The results presented here do not account for concurrent gas production and bubble resolution due to irradiation, therefore this simulation study is most applicable to post-irradiation annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Grain Boundary Percolation Modeling of Fission Gas Release in Oxide Fuels

We present a new approach to fission gas release modeling in oxide fuels based on grain boundary network percolation. The method accounts for variability in the bubble growth and coalescence rates on individual grain boundaries, and the resulting effect on macroscopic fission gas release. Two-dimensional representa- tions of fuel pellet microstructures are considered, and the resulting gas release rates are compared with traditional two-stage Booth models, which do not account for long-range percolation on grain boundary net- works. The results show that the requirement of percolation of saturated grain boundaries can considerably reduce the total gas release rates, particularly when gas resolution is considered.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Mesoscale modeling of intergranular bubble percolation in nuclear fuels

Phase-field simulations are used to examine the variability of intergranular fission gas bubble growth and percolation on uranium dioxide grain boundaries on a mesoscopic length scale. Three key parameters are systematically varied in this study: the contact angle (or dihedral angle) defining the bubble shape, the initial bubble density on the grain boundary plane, and the ratio of the gas diffusivity on the grain boundary versus the grain interiors. The simulation results agree well with previous experimental data obtained for bubble densities and average bubble areas during coalescence events. Interestingly, the rate of percolation is found to be highly variable, with a large dependency on the contact angle and the initial bubble density and little-to-no dependency on the grain boundary gas diffusivity.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Phase-field modeling of temperature gradient driven pore migration coupling with thermal conduction

Pore migration in a temperature gradient (Soret effect) is investigated by a phase-field model coupled with a heat transfer calculation. Pore migration is observed towards the high temperature domain with velocities that agree with analytical solution. Due to the low thermal conductivity of the pores, the temperature gradient across individual pores is increased, which in turn, accelerates the pore migration. In particular, for pores filled with xenon and helium, the pore velocities are increased by a factor of 2.2 and 2.1, respectively. A quantitative equation is then derived to predict the influence of the low thermal conductivity of pores.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Atomistic study of grain boundary sink strength under prolonged electron irradiation

Grain boundaries (GBs) are sinks and sources of point defects, which are produced in large number under radiation damage. As grain size decreases, many of the point defects sink into GBs. It is unknown whether GBs will saturate as sinks of point defects, particularly when grain size is at the nanoscale. Using a combination of molecular dynamics (MD) simulations and rate theory for BCC Mo, the authors show that large-angle GBs do not saturate as sinks of point defects. The MD simulations serve to provide direct measurement of defect evolution, and the rate theory serves to test whether grain boundary sink strength is a constant and to extend the MD results to realistic defect production rates.

36 MATERIALS SCIENCE↗
DOE OSTI2012

PERCOLATION ON GRAIN BOUNDARY NETWORKS: APPLICATION TO FISSION GAS RELEASE IN NUCLEAR FUELS

The percolation behavior of grain boundary networks is characterized in two- and three-dimensional lattices with circular macroscale cross-sections that correspond to nuclear fuel elements. The percolation of gas bubbles on grain boundaries, and the subsequent percolation of grain boundary networks is the primary mechanism of fission gas release from nuclear fuels. Both radial cracks and radial gradients in grain boundary property distributions are correlated with the fraction of grain boundaries vented to the free surfaces. Our results show that cracks surprisingly do not significantly increase the percolation of uniform grain boundary networks. However, for networks with radial gradients in boundary properties, the cracks can considerably raise the vented grain boundary content.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

An Object-Oriented Finite Element Framework for Multiphysics Phase Field Simulations

The phase field approach is a powerful and popular method for modeling microstructure evolution. In this work, advanced numerical tools are used to create a phase field framework that facilitates rapid model development. This framework, called MARMOT, is based on Idaho National Laboratory's finite element Multiphysics Object-Oriented Simulation Environment. In MARMOT, the system of phase field partial differential equations (PDEs) are solved simultaneously with PDEs describing additional physics, such as solid mechanics and heat conduction, using the Jacobian-Free Newton Krylov Method. An object-oriented architecture is created by taking advantage of commonalities in phase fields models to facilitate development of new models with very little written code. In addition, MARMOT provides access to mesh and time step adaptivity, reducing the cost for performing simulations with large disparities in both spatial and temporal scales. In this work, phase separation simulations are used to show the numerical performance of MARMOT. Deformation-induced grain growth and void growth simulations are included to demonstrate the muliphysics capability.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Energetics and diffusional properties of He in BCC Mo: an empirical potential for molecular dynamics simulations

This paper presents an interatomic potential for modeling of He defects and bubbles in body-centered-cubic (BCC) Mo. We utilize three existing framework: the Finnis-Sinclair (FS) potential for Mo-Mo, the Effective-Medium-Theory (EMT) for He-Mo, and the Hartree-Fock-Dispersion (HFD) potential for He-He interactions. The energetics of He defects and the diffusivity of He interstitial givens by the present potential agree well with ab initio calculations and experimental measurements. Furthermore, in agreement with theoretical prediction, it is shown that the introduction of He gas suppresses the surface diffusivity of BCC Mo, which decays exponentially with increasing He pressure acting on the free surface. The decay constant, with is correlated with the characteristic interaction volume for He-Mo, is close to the atomic volume of BCC Mo. This suppression effect is important to understand the mobility of small gas bubbles.

36 MATERIALS SCIENCE↗