Daniel Schwen

Idaho National Laboratory | INL · Department of Computational Mechanics and Materials

Additive manufacturing (AM) or rapid prototyping has become a crucial tool for reducing both cost and time while increasing efficiency in qualifying structural materials for reactor use. In this study, directed energy deposition (DED) was used to develop an as-built 316 L stainless steel sample with three regions of increasing Hf-dopant to study th...

Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here...

Compositionally-graded austenitic 316L stainless steel (SS) samples with five different Hafnium (Hf) concentrations (up to 1 wt.%) were additively manufactured by directed energy deposition and then were irradiated using 5 MeV Fe 2 + ions to 50 displacements per atom (dpa) at 500, 550, and 600 °C, respectively. A rastering beam was used to ensure h...

Engineering alloys generally exhibit multi-phase microstructures. For simulating their microstructure evolution during solid-state phase transformation, CALPHAD-guided multi-phase-field models coupled with micro-mechanics have proven to be a reliable simulation tool. Nevertheless, their efficiency and accuracy still depend on the homogenization sch...

Engineering alloys generally exhibit multi-phase microstructures. For simulating their microstructure evolution during solid-state phase transformation, CALPHAD-guided multi-phase-field models coupled with micro-mechanics have proven to be a reliable simulation tool. Nevertheless, their efficiency and accuracy still depend on the homogenization sch...

This paper presents an efficient and quantitative phase-field model for elastically heterogeneous alloys that ensures the two mechanical compatibilities$\unicode{x2014}$static and kinematic, in conjunction with chemical equilibrium within the interfacial region. Our model contrasts with existing phase-field models that either violate static compati...

This report summarizes efforts performed during Fiscal Year 2022 to develop capabilities for modeling structural component degradation in support of the U.S. Department of Energy's Nuclear Energy Advanced Modeling and Simulation Program. These efforts were centered around development of capabilities for the Grizzly code. Efforts focused both on fou...

A phase-field model to simulate the formation of both void and gas bubble superlattices is derived from a grand potential functional, assuming 1D diffusion of self-interstitial atoms. The model is capable of accounting for superlattice formation by either a nucleation and growth or spinodal decomposition mechanism; in this work, we focus on the nuc...

The last 2 years have been a period of unprecedented growth for the MOOSE community and the software itself. The number of monthly visitors to the website has grown from just over 3,000 to now averaging 5,000. In addition, over 1,800 pull requests have been merged since the beginning of 2020, and the new discussions forum has averaged 600 unique vi...

Understanding the impact of microstructure on corrosion rates can aid the development of corrosion-resistant alloys for molten salt reactors. In this work, we develop an electrochemical phase-field model for capturing the microstructure-dependent corrosion of structural alloys by molten salts. As a demonstration problem, we apply this model to capt...

This paper presents an efficient and quantitative phase-field model for elastically heterogeneous alloys that ensures the two mechanical compatibilities—static and kinematic, in conjunction with chemical equilibrium within the interfacial region. Our model contrasts with existing phase-field models that either violate static compatibility or interf...

Multimetallic layered composites (MMLCs) have shown an excellent potential for application under extreme environments, e.g., accident-tolerant fuel cladding, because of their low oxidation tendency and high corrosion resistance. Interfacial phases or complexions in nanocrystalline materials accelerate the annihilation of defects and enhance the rad...

Irradiation of crystalline materials modifies their microchemistry and microstructure. This includes solute segregation toward defect sinks such as grain boundaries (GBs), a phenomenon commonly known as radiation-induced segregation (RIS). Unlike in coarse-grained alloys where GBs are nearly static, RIS is usually accompanied and affected by either...

This study demonstrates the feasibility of using compositionally gradient specimens, fabricated by laser additive manufacturing (AM) and post-AM thermo-mechanical treatment, to accelerate alloy synthesis, radiation experiment, and the assessment of irradiation properties in light water reactor environments. The effects of minor Hf doping in austeni...

The phase-field method has been established as a de facto standard for simulating the microstructural evolution of materials. In quantitative modeling the assessment and compilation of thermodynamic/kinetic data is largely dominated by the CALPHAD approach, which has produced a large set of experimentally and computationally generated Gibbs free en...

The simulation of radiation effects in materials broadly falls into two categories. In the limit of short time and length scales lies the modeling of primary radiation damage, such as point defect creation, energy deposition, and ballistic mixing. This is followed by the modeling at longer time scales of thermally activated microstructure evolution...

This article presents a statistical approach for atomistic calculations of vacancy formation energy, which is expected to exhibit a probability distribution in concentrated solid-solution alloys, due to the variation in atomic environment. Demonstrated using a random FeCrNi ternary alloy, a general formulation is given for applications in random, c...

The phase-field method has been established as a de facto standard for simulating the microstructural evolution of materials. In quantitative modeling the assessment and compilation of thermodynamic/kinetic data is largely dominated by the CALPHAD approach, which has produced a large set of experimentally and computationally generated Gibbs free en...

The ability to identify features within finite element simulations and track them over time is necessary for understanding and quantifying complex behaviors as disparate as turbulent vortices in a flow field to microstructure evolution. We extend our previous research on feature identification in parallel unstructured meshes with the novel ability...

BISON is a nuclear fuel performance application built using the Multiphysics Object-Oriented Simulation Environment (MOOSE) finite element library. One of its major goals is to have a great amount of flexibility in how it is used, including in the types of fuel it can analyze, the geometry of the fuel being modeled, the modeling approach employed,...

Efficient solution via Newton’s method of nonlinear systems of equations requires an accurate representation of the Jacobian, corresponding to the derivatives of the component residual equations with respect to the degrees of freedom. In practice these systems of equations often arise from spatial discretization of partial differential equations us...

We use molecular dynamics (MD) to study radiation-induced mixing through multi-metallic layered composites' interfaces for nuclear reactor applications. Here, we consider the Incoloy-Ni and the Inconel-Ni system with four different compositions of Inconel. We investigate the irradiated structure of these composites at different temperatures by perf...

A monolithic fuel design based on a U-Mo alloy has been selected as the fuel type for conversion of the U. S. High-Performance Research Reactors. A critical phenomenon of interest regarding U-Mo monolithic fuel is the large amount of swelling that takes place during operation, particularly at high fission densities. The accurate prediction of fuel...

The current work proposes a novel approach for the implementation of periodic representative volume element (RVE) based multiscale modeling using the finite element method. The approach is based on a rigorous mechanics foundation that implements appropriate boundary conditions for the RVE analysis and simplifies the homogenization technique. Stress...

One of the most intriguing phenomena under radiation is the self-organization of defects, such as the void superlattices, which have been observed in a list of bcc and fcc metals and alloys when the irradiation conditions fall into certain windows defined by temperature and dose rate. A superlattice features a lattice parameter and a crystal struct...

We argue that radiation damage induced degradation of thermal conductivity does not set a lower limit on fuel grain sizes for the low enriched uranium fuel design of the Transient Reactor Test Facility (TREAT). Earlier work reports that smaller grains cause a larger degradation of thermal conductivity than larger grains constraining the smallest fe...

A phase-field model was developed to simulate the high burn-up structure formation and evolution in UO2. The model takes into account the interfacial energies of grain boundaries and bubble surfaces, the strain energy associated with dislocations, and the chemical energy of gas atoms. This enables the model to simulate the formation and growth of s...

Scientific communities struggle with the challenge of effectively and efficiently sharing content and data. An online portal provides a valuable space for scientific communities to discuss challenges and collate scientific results. Examples of such portals include the Micromagnetic Modeling Group (μMAG) [1], the Interatomic Potentials Repository (I...

Despite the extensive utilization of uranium dioxide (UO2) as a fuel in commercial nuclear reactors, there is only minimal information regarding the fundamental nature of radiation damage at high temperatures, such as those experienced by the fuel under operation. In this work, molecular dynamics simulations have been performed to determine the thr...

Ordering and self-organization are critical in determining the dynamics of reaction-diffusion systems. Here we show a unique pattern formation mechanism, dictated by the coupling of thermodynamic instability and kinetic anisotropy. Intrinsically different from the physical origin of Turing instability and patterning, the ordered patterns we obtaine...

A phase-field model of fission gas bubble evolution was developed and applied to gain an improved understanding of the microstructure-level processes leading to fission gas release from nuclear fuel, and to inform engineering-scale fission gas release models. The phase-field model accounts for multiple fuel grains and fission gas bubbles and tracks...

Conserved order parameter (Gas atom concentration). Cahn-Hilliard equation The non-conserved order parameters (bubble phase and grains). Allen-Cahn equations Nucleation of bubbles and grains is modeled directly using fluctuation terms and no priori assumptions are required. The free energy of the system is composed of the bulk thermodynamic part an...

Phase Field Model • U-7Mo and other nuclear fuels develop a unique microstructure under irradiation usually known as the High Burn-up Structure (HBS) • In this HBS, the as-fabricated microstructure transforms into a much finer one with a grain size that is orders of magnitude less than the initial grain size. • The increase in free energy due to th...

The restart of the Transient Reactor Test facility (TREAT) will once again provide the capability for rapid transient testing of fuel concepts. Under the auspices of the U.S. Department of Energy’s Office of Material Management and Minimization, research is underway to assess the feasibility of converting the current high-enriched uranium (HEU) fue...

Grand-potential-based phase-field model for multiple phases, grains, and chemical components is derived from a grand-potential functional. Due to the grand-potential formulation, the chemical energy does not contribute to the interfacial energy between phases, simplifying parametrization and decoupling interface thickness from interfacial energy, w...

The influence of anisotropic properties of powder particles on microstructural evolution during solid-state sintering processes is analyzed. Two types of anisotropy studied in the current work are direction-dependent interface diffusion anisotropy, and grain orientation dependent grain boundary energy anisotropy. A phase field modeling approach is...

During primary damage, the fraction of produced solute interstitials in the total interstitials can be either higher or lower than the solute concentration in an alloy, depending on the solute type. To understand which alloy property governs the over- or under-production behaviors of solute interstitials, molecular dynamics simulations are conducte...

Nanoscale metallic foams display mechanical properties which make them attractive for a variety of technological applications. We report simulated nanoindentation tests for a model polycrystalline nanoporous gold structure with 11 nm mean filament diameter and 35 nm average grain size, comparable to foams produced by dealloying. Hardness, plasticit...

Self-organized patterns, realized in non-equilibrium processes, have been widely observed in physics and chemistry. As a powerful tool to create far-from-equilibrium environments, irradiation produces a variety of types of defects, which can self-organize through physical interactions and chemical reactions. Such a process becomes complicated espec...

Nano-structured superlattices may have novel physical properties and irradiation is a powerful mean to drive their self-organization. However, the formation mechanism of superlattice under irradiation is still open for debate. Here we use atomic kinetic Monte Carlo simulations in conjunction with a theoretical analysis to understand and predict the...

In this work, we present a multiphysics phase field model for capturing microstructural evolution during solid-state sintering processes. The model incorporates modifications of phase field equations to include rigid-body motion, elastic deformation, and heat conduction. The model correctly predicts consolidation of powder particles during sinterin...

Molecular dynamics simulations are conducted to study the effects of alloying elements on the primary damage behaviors in three Fe-based ferritic alloy systems: (1) a Fe-Cr system in which the heat of mixing changes its sign with the Cr concentration; (2) a Fe-Cu system that has a positive heat of mixing; and (3) an ideal but artificial Fe-Cr syste...

Phase-field modeling is a microstructure-level simulation technique often used in the Integrated Computational Materials Engineering (ICME) approach to materials design. To perform quantitatively accurate phase-field simulations for this application, potential sources of error in model parameters such as interfacial energy must be fully understood....

This work presents a multi-physics, multi-scale approach to modeling the Transient Test Reactor (TREAT) currently prepared for restart at the Idaho National Laboratory. TREAT fuel is made up of microscopic fuel grains (r ≈ 20μm) dispersed in a graphite matrix. The novelty of this work is in coupling a binary collision Monte-Carlo (BCMC) model to th...

We present a novel phase-field model development capability in the open source MOOSE finite element framework. This facility is based on the 'modular free energy' approach in which the phase-field equations are implemented in a general form that is logically separated from model-specific data such as the thermodynamic free energy density and mobili...

Download from here before December 18, 2016: http://authors.elsevier.com/a/1Tz1m4r9SU3pu6 A multiphase field model is developed to study the effects of metastable ζ and γ hydrides on the nucleation and growth of the stable δ hydrides in α zirconium matrix. The model incorporates all the possible phases using the Gibbs free energies of formation fo...

Sintering is a processing technique which compacts powder materials into solids leading to microstructural evolution with reduced surface area and improved material density. Understanding the densification mechanism and grain growth kinetics during the powder compaction process during sintering is of immense importance in order to evaluate usabilit...

This paper addresses the role of misfit dislocations in the nucleation and growth of nanoscale He bubbles at interfaces. In a recent work, we studied the nanoscale effects on the capillarity equation and on equilibrium conditions. We proposed an expression for surface energy and for the equation of state, EOS, for He in bubbles, which have a size d...

We investigate the modifications to the Young–Laplace capillarity equation needed to describe nanoscale gas bubbles embedded in metals, scale at which the finite width of the interface region cannot be neglected. We focus in particular on the case of He in Fe. Using both, the concept of Tolman’s length that provides a curvature dependence for the i...

By exploiting Monte Carlo methodology and molecular dynamics, we computationally simulate the spinodal decomposition of iron– chromium binary alloys and analyze the relationship between the increase of yield strength induced by the phase separation phenomenon, and statistical parameters of the atomistic configuration. We successfully model the expe...

Renewed interest in fast nuclear reactors is creating a need for better understanding of fission gas bubble behavior in non-oxide fuels to support very long fuel lifetimes. Collisions between fission fragments and their subsequent cascades can knock fission gas atoms out of bubbles and back into the fuel lattice. We showed that these collisions can...

Helium (He) presents 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 bubbl...

A usual way to estimate the amount of gas in a bubble inside a metal is to assume thermodynamic equilibrium, i.e., the gas pressure P equals the capillarity force 2γ/R, with γ the surface energy of the host material and R the bubble radius; under this condition there is no driving force for vacancies to be emitted or absorbed by the bubble. In cont...

Compositional patterning in two-phase immiscible alloys during severe plastic deformation at elevated temperatures has been investigated. Kinetic Monte Carlo computer simulations were used to test the proposed idea that patterning derives from a dynamic competition between homogenization by forced chemical mixing and phase separation by thermally a...

a b s t r a c t We present a calculation of the critical sizes and nucleation rates for the nucleation of a 0 precipitates in an FeCr alloy. Our work combines the calculation of the FeCr free energy surface using molecular dynamics simulations with recently published data [1] for the interfacial free energies between the a and a 0 phases in FeCr to...

The fundamental processes of shear-induced chemical mixing in heterogeneous Cu-based alloy systems have been studied by molecular dynamics computer simulations. These simulations reveal that two very disparate mechanisms operate depending on whether or not the two phases are coherent. For the coherent systems, mixing occurs as dislocations transfer...

Forced chemical mixing in nanostructured Ag60Cu40 eutectic alloys during severe plastic deformation by high-pressure torsion (HPT) was quantitatively studied using x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. Nearly complete chemical homogenization of the original lamellar structure with a wavelength o...

The formation of surface hillocks in diamond-like carbon is studied experimentally and by means of large-scale molecular dynamics simulations with 5 Multiplication-Sign 10{sup 6} atoms combined with a thermal spike model. The irradiation experiments with swift heavy ions cover a large electronic stopping range between {approx}12 and 72 keV/nm. Both...

Shear-induced mixing in heterogeneous Cu alloy systems was investigated by molecular dynamics simulation. Each system, which comprised a single spherical particle within a Cu matrix, was subjected to cyclical shearing events to high strains at 100 K. The particles investigated were Cu, Ag, Ni, Fe, Nb, and V. fcc particles were observed to undergo “...

Vertical p-type Si nanowires (NWs) "in-situ" doped during growth or "ex-situ" by B ion implantation are investigated regarding their acceptor activation. Due to the much higher surface to volume ratio of the NW in comparison to bulk material the surface effect plays an important role in determining the doping behaviour. Dopant segregation and fixed...

The stability of model nanostructured Cu90Mo10 and Cu90W10 alloys during irradiation with 1.8 MeV Kr+ at very high temperatures was investigated. Significant coarsening occurs only above ∼0.6Tm in Cu90Mo10 and ∼0.8Tm in Cu90W10 (Tm is the melting point of Cu). Below these temperatures, nanoprecipitates nucleate and grow to a saturation diameter of...

We introduce a first passage based Monte Carlo [1] code to investigate the population evolution of Xe fission gas bubbles in UO2 fuels. Growth laws are obtained for homogeneous and heterogeneous re-solution models for a wide range of gas and bubble diffusivities. Under certain irradiation conditions, bubble populations find dynamic steady states. H...

Sputtering and the re-solution of Xe fission gas bubbles in UO2 due to electronic energy deposition of fission fragments is investigated using molecular dynamic (MD) simulations. First, a two-temperature model coupling the electronic (e-) and phonon (p-) systems is employed to determine the temperature profile along the tracks of fission fragments....