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Stoller, Roger E

Publications and source records attributed to Stoller, Roger E.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Primary Radiation Damage Formation in Metals in State-of-the-art Report on Structural Materials Modelling

The processes that give rise to changes in the microstructure and the physical and mechanical properties of materials exposed to energetic particles are initiated by essentially elastic collisions between atoms in what has been called an atomic displacement cascade. The formation and evolution of this primary radiation damage mechanism are described to provide an overview of how stable defects are formed by displacement cascades, as well as the nature and morphology of the defects themselves. The impact of the primary variables cascade energy and irradiation temperature are discussed, along with a range of secondary factors that can influence damage formation.Radiation-induced changes in microstructure and mechanical properties in structural materials are the result of a complex set of physical processes initiated by the collision between an energetic particle (neutron or ion) and an atom in the lattice. This primary damage event is called an atomic displacement cascade. The simplest description of a displacement cascade is to view it as a series of many billiard-ball-like elastic collisions among the atoms in the material. This chapter describes the formation and evolution of this primary radiation damage mechanism to provide an overview of how stable defects are formed by displacement cascades, as well as the nature and morphology of the defects themselves. The impact of the relevant variables such as cascade energy and irradiation temperature is discussed, and defect formation in different materials is compared.

atomic displacement cascades↗
DOE OSTI2015

Effect of helium on irradiation creep behavior of B-doped F82H irradiated in HFIR

The diameter of pressurized tubes of F82H and B-doped F82H irradiated up to similar to 6 dpa have been measured by a non-contacting laser profilometer. The irradiation creep strains of F82H irradiated at 573 and 673K were almost linearly dependent on the effective stress level for stresses below 260 MPa and 170 MPa, respectively. The creep strain of (BN)-B-10-F82H was similar to that of F82H IEA at each effective stress level except 294 MPa at 573K irradiation. For 673K irradiation, the creep strain of some (BN)-B-10-F82H tubes was larger than that of F82H tubes. However, the generation of similar to 300 appm He did not cause a large difference in the irradiation creep behavior at 6 dpa.

Ando, M.↗
DOE OSTI2015

Displacement Cascade Damage Production in Metals

Radiation-induced changes in microstructure and mechanical properties in structural materials are the result of a complex set of physical processes initiated by the collision between an energetic particle (neutron or ion) and an atom in the lattice. This primary damage event is called an atomic displacement cascade. The simplest description of a displacement cascade is to view it as a series of many billiard-ball-like elastic collisions among the atoms in the material. This chapter describes the formation and evolution of this primary radiation damage mechanism to provide an overview of how stable defects are formed by displacement cascades, as well as the nature and morphology of the defects themselves. The impact of the relevant variables such as cascade energy and irradiation temperature is discussed, and defect formation in different materials is compared.

defect creation↗
DOE OSTI2014

Breakthrough in Understanding Radiation Growth of Zirconiu

The efforts of many scientists for more than a half of a century have resulted in a substantial understanding of the response of Zr-based materials to irradiation. However, the models of radiation growth proposed to date have not played a decisive role in creating radiation-resistant materials and cannot predict strain rates at high irradiation doses. The main reason for this is the common assumption that, regardless of the incident particle mass and energy, the primary damage consists of single vacancies and self-interstitial atoms (SIAs), both diffusing three-dimensionally. Thus, the models ignore the distinguishing features of the damage production in displacement cascades during fastparticle, e.g., neutron, irradiation; namely, the intra-cascade clustering of vacancies and SIAs and one-dimensional diffusion of SIA clusters. Over the last twenty years or so, the production bias model (PBM) has been developed, which accounts for these features and explains many observations in cubic crystals. The cascades in hcp crystals are found to be similar to those in cubic crystals; hence one can expect that the PBM will provide a realistic framework for the hcp metals as well. It is shown in this paper that it reproduces all the growth stages observed in annealed materials under neutron irradiation, such as the high strain rate at low, strain saturation at intermediate, and breakaway growth at relatively high doses. It accounts for the striking observations of negative strains in prismatic directions and co-existence of vacancy- and interstitial-type prismatic loops, which have never been explained before. It reveals the role of cold work in the radiation growth behavior and the reasons for the alignment of basal vacancy-type loops along the basal planes. The critical parameters determining the high-dose behavior are revealed and the maximum growth rate is estimated.

Golubov, Stanislav I↗
DOE OSTI2013

Irradiation response in weldment and HIP joint of reduced activation ferritic/martensitic steel, F82H

This work investigates irradiation response in the joints of F82H employed for a fusion breeding blanket. The joints, which were prepared using welding and diffusion welding, were irradiated up to 6 dpa in the High Flux Isotope Reactor at the Oak Ridge National Laboratory. Post-irradiation tests revealed hardening in weldment (WM) and base metal (BM) greater than 300 MPa. However, the heat affected zones (HAZ) exhibit about half that of WM and BM. Therefore, neutron irradiation decreased the strength of the HAZ, leaving it in danger of local deformation in this region. Further the hardening in WM made with an electron beam was larger than that in WM made with tungsten inert gas welding. However the mechanical properties of the diffusion-welded joint were very similar to those of BM even after the irradiation.

Hirose, Takanori↗
DOE OSTI2013

Cascade Defect Evolution Processes: Comparison of Atomistic Methods

Determining the defect evolution beyond the molecular dynamics (MD) time scale is critical in bridging the gap between atomistic simulations and experiments. The recently developed self-evolving atomistic kinetic Monte Carlo (SEAKMC) method provides new opportunities to simulate long-term defect evolution with MD-like fidelity. In this study, SEAKMC is applied to investigate the cascade defect evolution in bcc iron. First, the evolution of a vacancy rich region is simulated and compared with results obtained using autonomous basin climbing (ABC) +KMC and kinetic activation-relaxation technique (kART) simulations. Previously, it is found the results from kART are orders of magnitude faster than ABC+KMC. The results obtained from SEAKMC are similar to kART but the time predicted is about one order of magnitude faster than kART. The fidelity of SEAKMC is confirmed by statistically relevant MD simulations at multiple higher temperatures, which proves that the saddle point sampling is close to complete in SEAKMC. The second is the irradiation-induced formation of C15 Laves phase nano-size defect clusters. In contrast to previous studies, which claim the defects can grow by capturing self-interstitials, we found these highly stable clusters can transform to <111> glissile configuration on a much longer time scale. Finally, cascade-annealing simulations using SEAKMC is compared with traditional object KMC (OKMC) method. SEAKMC predicts substantially fewer surviving defects compared with OKMC. The possible origin of this difference is discussed and a possible way to improve the accuracy of OKMC based on SEAKMC results is outlined. These studies demonstrate the atomistic fidelity of SEAKMC in comparison with other on-the-fly KMC methods and provide new information on long-term defect evolution in iron.

Xu, Haixuan↗
DOE OSTI2013

On the Use of SRIM for Computing Radiation Damage Exposure

The SRIM (formerly TRIM) Monte Carlo simulation code is widely used to compute a number of parameters relevant to ion beam implantation and ion beam processing of materials. It also has the capability to compute a common radiation damage exposure unit known as atomic displacements per atom (dpa). Since dpa is a standard measure of primary radiation damage production, most researchers who employ ion beams as a tool for inducing radiation damage in materials use SRIM to determine the dpa associated with their irradiations. The use of SRIM for this purpose has been evaluated and comparisons have been made with an internationally-recognized standard definition of dpa, as well as more detailed atomistic simulations of atomic displacement cascades. Differences between the standard and SRIM-based dpa are discussed and recommendations for future usage of SRIM in radiation damage studies are made. In particular, it is recommended that when direct comparisons between ion and neutron data are intended, the Kinchin-Pease option of SRIM should be selected.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Recent Progress of R&D Activities on Reduced Activation Ferritic/Martensitic Steels

Several types of reduced activation ferritic/martensitic (RAFM) steel have been developed over the past 30 years in China, Europe, India, Japan, Russia and the USA for application in ITER TBM and future fusion DEMO and power reactors. The progress has been particularly important during the past few years with evaluation of mechanical porperties of these steels before and after irradiation and in contact with different cooling media. This paper presents recent RAFM steel results obtained in ITER partner countries in relation with different TBM and DEMO options

Huang, Q.↗
DOE OSTI2013

Molecular Dynamics Simulation of Cascade-Induced Ballistic Helium Resolutioning from Bubbles in Iron

Molecular dynamics simulations have been used to assess the ability of atomic displacement cascades to eject helium from small bubbles in iron. This study of the ballistic resolutioning mechanism employed a recently-developed Fe-He interatomic potential in concert with an iron potential developed by Ackland and co-workers. The primary variables examined were: irradiation temperature (100 and 600K), cascade energy (5 and 20 keV), bubble radius (0.5 and 1.0 nm), and He-to-vacancy ratio in the bubble (0.25, 0.5 and 1.0). Systematic trends were observed for each of these variables. For example, ballistic resolutioning leads to a greater number of helium atoms being displaced from larger bubbles and from bubbles that have a higher He/vacancy ratio (bubble pressure). He resolutioning was reduced at 600K relative to 100K, and for 20 keV cascades relative to 5 keV cascades. Overall, the results indicate a modest level of He removal by ballistic resolutioning. The results can be used to provide guidance in selection of a resolution parameter that can be employed in cluster dynamics models to predict the bubble size distribution that evolves under irradiation.

displacement cascades↗
DOE OSTI2013

Self-interstitial configurations in hcp Zr: a first principles analysis

Analysis of microstructure evolution in irradiated Zr and Zr alloys based on the modern radiation damage theory suggests some important features of self-interstitial atoms (SIAs). Alignment of vacancy loops and voids along basal planes requires anisotropic interstitial transport with a dominant contribution along the basal plane. Under neutron irradiation this can be explained by one-dimensional mobility of SIA clusters but experiments under electron irradiation indicate unambiguously that even the single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the data obtained by ab initio calculations within the last decade reported stable SIA configurations that should provide essentially three-dimensional diffusion. To clarify this issue, an extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that simulation cell size, type of pseudopotential, the exchange-correlation functional, and the c/a ratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes. The results provide a confirmation of basic mechanisms for microstructural evolution under irradiation.

Samolyuk, German D↗
DOE OSTI2013

The influence of transition metal solutes on dislocation core structure and values of Peierls stress and barrier in tungsten.

Several transition metals were examined to evaluate their potential for improving the ductility of tungsten. We investigate the dislocation core structure, Peierls stress and barrier of $1/2\langle111\rangle$ screw dislocations in binary tungsten-transition metal alloys (W$_{1-x}$TM$_{x}$) based on first principles electronic structure calculations. The periodic quadrupole approach was applied to model the structure of $1/2\langle111\rangle$ dislocation. The alloying with transition metals was modeled using the virtual crystal approximation. In order to verify the applicability of this approach, the equilibrium lattice parameter and elastic constants were calculated for tungsten alloyed with the set of transition metals. Reasonable agreement was obtained between results using the virtual crystal approximation and those using both a conventional super-cell approach and existing experimental data. Increasing the concentration of a transition metal from the VIIIA group leads to reduction of the $C^\prime$ elastic constant and increase of elastic anisotropy A=$C_{44}/C^\prime$. It was demonstrated that alloying W with a group VIIIA transition metal changes the structure of the dislocation core from symmetric to asymmetric, similar to results obtained for W$_{1-x}$Re$_{x}$ alloys in the earlier work of Romaner {\it et al} (Phys. Rev. Lett. 104, 195503 (2010)). Following the core symmetry change, the values of the Peierls stress and barrier are reduced. This combination of two effects could lead to increased ductility in a tungsten-based alloy\comments. Our results demonstrate that similar effects could be achieved with any of the transition metals from the VIIIA group.

Samolyuk, German D↗
DOE OSTI2013

Coarse graining approach to First principles modeling of structural materials

Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an e cient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the rst principles. We used the Locally Self-consistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screw-dislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force eld. The local moments in the neighborhood of the defect cores are calculated with rst-principles based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs signi cantly and makes large-scale structures amenable to rst principles study. Work is sponsored by the USDoE, O ce of Basic Energy Sciences, Center for Defect Physics, an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the O ce of Science of the USDoE under Contract No. DE-AC05-00OR22725.

coarse grained↗