Σ5 Symmetrical Tilt Research Articles

Atomic structure of grain boundaries in UO2 Bicrystals: A coupled high resolution transmission electron Microscopy/Atomistic simulation approach

In this work, high resolution transmission electron microscopy and numerical simulation at the atomic scale were coupled, for the first time to our knowledge, to study the atomic structure of two Σ5 symmetric tilt grain boundaries in uranium dioxide (UO2) bicrystals. We show that whereas the Σ5 (021)/[100] is fully symmetric, with the grain boundary plane forming a mirror symmetry plane, a rigid body translation exists between the crystals in the Σ5 (031)/[100]. We particularly believe that the excellent agreement between experimentally observed and simulated structures highlights the relevance of the coupled approach. Such result emphasize that the use of CRG and Yakub empirical potentials including a Morse contribution can be relied upon to model the behavior of UO2.

Stable and metastable structures and their energetics of asymmetric tilt grain boundaries in MgO: a simulated annealing approach

A simulated annealing (SA) method based on molecular dynamics is employed to reveal atomic structures of asymmetric tilt grain boundaries (ATGBs) in MgO. Σ5 and Σ13 ATGBs with the [001] tilt axis are systematically investigated. The ATGBs after SA simulations dissociate into saw-toothed nanofacets composed of multiple structural units. These nanofacets are lower in GB energy than those obtained from a γ-surface method with structural optimization, demonstrating the importance of SA-based methods for obtaining low-energy structures of ATGBs. For most of the Σ5 ATGBs, the nanofacets consist of only structural units of Σ5 symmetric tilt GBs (STGBs). For the Σ13 ATGBs studied, their nanofacets do not consist of only Σ13 STGBs but always contain non-Σ13 structural units, which probably results from a large difference between the excess volume of Σ13(510) and Σ13(320) STGBs. It is also found that ATGBs have a larger number of metastable structures whose GB energies are close to the lowest energy structure than STGBs, due to the fact that ATGB nanofacets are more tolerant of variation in facet junction, structural units and their arrangement. Consequently, the lowest energy structures have low probabilities of being formed than metastable structures.

Diffusion in the bulk and in a symmetric tilt grain boundary of the Fe[sbnd]Cr system

Classical and quantum calculations were performed in characterizing the interstitials transport parameters in the bulk of a diluted FeCr alloy and, in the Σ5 symmetric tilt grain boundary (GB) structure. For the bulk, in the same line of ideas as the multi-frequency model, we obtain the relevant transport parameters, namely: i) the involved jump frequencies in the associated multi-frequency model, ii) the full set of phenomenological Onsager coefficients, iii) the interstitial wind parameter, as well as, iv) the tracer self- and solute diffusion coefficients. Our calculations include the effects of entropy and ferromagnetic transition in the activation energy. We compare present calculations with available experimental data. As is expected, we have found in the bulk that Fe and Cr diffuse mainly via a vacancy mechanism, while diffusion by interstitials is several orders of magnitudes slower. For the GB, our calculations are in accordance with experimental data and show that diffusion is almost two orders of magnitude higher than in the bulk. Also, contrary to the bulk Cr diffuses slower than Fe in the GB. Present results show that both, interstitial and vacancy mechanisms, are important in studying diffusion in the GB.

The Formation and Evolution of Defects in Nanocrystalline Fe During Indentation: The Role of Twins in Pop‐Ins

Nanoindentation is a most common experimental tool used for obtaining information on the mechanical behavior of materials. This is done by qualitatively relating the occurrence of pop‐ins (in the load–displacement plots) to microstructural changes such as dislocation formation, fracture of surface oxides, or slip transmission. The present study takes a first approach in directly verifying the micro‐plasticity processes that give rise to such pop‐ins by performing molecular dynamics indentation simulations in BCC Fe‐nanocrystals with a Σ5 symmetric tilt boundary. The simulations allow to track the material behavior throughout the indentation process, and illustrate that each pop‐in is related to twin formation, twin growth, de‐twinning, dislocation nucleation and glide, or dislocation–grain boundary interactions. For the particular Σ5 boundary considered, it is found that the pop‐ins are most closely associated with twin formation. Although pop‐ins have been related to dislocation nucleation, a direct correlation between twinning and pop‐ins has not been shown before. Adding C segregants to the Fe sample, reduced the formation of twins after initial yielding, and allowed for dislocation activity to become the more dominant deformation mechanism.

Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO2

Ionic transport at grain boundaries in oxides dictates a number of important phenomena, from ionic conductivity to sintering to creep. For nuclear fuels, it also influences fission gas bubble nucleation and growth. Here, using a combination of atomistic calculations and object kinetic Monte Carlo (okMC) simulations, we examine the kinetic pathways associated with uranium vacancies at two model grain boundaries in UO2. The barriers for vacancy motion were calculated using the nudged elastic band method at all uranium sites at each grain boundary and were used as the basis of the okMC simulations. For both boundaries considered – a simple tilt and a simple twist boundary – the mobility of uranium vacancies is significantly higher than in the bulk. For the tilt boundary, there is clearly preferred migration along the tilt axis as opposed to in the perpendicular direction while, for the twist boundary, migration is essentially isotropic within the boundary plane. These results show that cation defect mobility in fluorite-structured materials is enhanced at certain types of grain boundaries and is dependent on the boundary structure with the tilt boundary exhibiting higher rates of migration than the twist boundary.

Role of electronic effects on the incorporation of Cr at a Σ5 grain boundary in UO2

Due to the important role of microstructure on nuclear fuel performance, it is desirable to obtain improved control of fuel microstructure. For example, Cr2O3 has been used as a grain growth promoter during the sintering of UO2; however, the influence of Cr incorporation on the bonding within the grain boundary is not well understood. Here, we quantify the energetics associated with Cr incorporation and segregation in a model UO2 system with a Σ5 symmetric tilt grain boundary using density functional theory calculations. The results indicate that Cr prefers to reside in a U substitutional site and segregate to the grain boundary. The Bader charge analysis and the charge density difference analyses confirm that Cr forms bonds with neighboring O atoms that weaken the ionic nature of adjacent U–O bonds, especially when it occupies U substitutional site at the grain boundary. The implications of these findings for the usage of Cr2O3 as a growth promoter are discussed.

The role of charge and ionic radius on fission product segregation to a model UO2 grain boundary

The segregation energies of a range of fission products in UO2 to a Σ5 symmetric tilt grain boundary have been calculated using empirical potentials and their dependency on site, charge, and ionic radius has been determined. Density functional theory calculations provide information about the detailed bonding environment around the segregates. While most of the fission products prefer to reside in sites with large free volume, there are some that form strong bonds with neighboring oxygen ions, and thus prefer sites with high oxygen coordination. This result provides insight into nuclear fuel design to enhance control of fission product retention.

First-principles study of the hydrogen absorption at Σ5 symmetrical tilt grain boundary in B2-TiFe alloy

We present a density functional theory (DFT) study of the hydrogen-metal interaction in the B2-TiFe alloy with Σ5(310) symmetrical tilt grain boundary (GB) and (310) free surface (FS). The influence of hydrogen on the electronic properties of alloy with GB and FS is analyzed for different hydrogen sorption sites. The hydrogen absorption/adsorption, binding and segregation energies are calculated at GB and FS. Our calculations reveal that H segregates more strongly to the surface than to the GB that results in decrease in the Griffith work, i.e., H makes the fracture of the GB easier.

Substitutional Impurity Segregation to the Σ 5 (310)/[001] Stgb in Cu Doped Aluminum and Ag Doped Copper

Abstract The phenomenon of segregation is of long standing scientific interest and has been studied extensively, both theoretically as well as experimentally. For our investigations we have chosen the Σ5 symmetric tilt grain boundaries (STGB) in two face-centered cubic (FCC) metals, aluminum and copper. Both metals were doped with only 1 at% of the impurity species (Cu and Ag). One of our major goals in this study was to investigate the size effect on segregation of an impurity to distinct sites at the grain boundary. Therefore we have selected the Ag as an impurity in Cu and Cu as an impurity in Al. The latter one is of special interest for applications like interconnects in microcircuits, where one of the major controlling factors of electromigration is expected to be the diffusion or segregation of Cu atoms at Al grain boundaries. The model grain boundaries have been fabricated with ultra-high vacuum diffusion bonding of single crystals which bonds the bicrystals under highly controlled environmental conditions.

A study of dynamic embrittlement in bicrystals of Cu-7%Sn

In a kinetic model [1] for the phenomenon of dynamic embrittlement, the cracking rate is predicted to be proportional to the diffusivity of the embrittling species along the grain boundary. To test this model, bicrystals of Cu-Sn with Σ5 symmetrical tilt boundaries in which tin is the embrittling element are being used. The diffusivities parallel and perpendicular to the tilt axis are expected to be different, therefore, the crack growth rates in these two directions should vary in the same ratio as the diffusivities. Measurements of the crack-growth rate along the [100] direction in the Cu-Sn alloy bicrystal at 265 °C showed that the cracking occurred by decohesion along the grain boundary with almost no observable plasticity at a maximum crack growth rate of approximately 10 −6 m s −1. A model for steady-state crack growth has recently been developed and is discussed here.

Study of Dynamic Embrittlement in Alloy Bicrystals

ABSTRACTIn a kinetic model [1] for the phenomenon of dynamic embrittlement, the cracking rate is predicted to be proportional to the diffusivity of the embrittling species along the grain boundary. To test this model, bicrystals of Cu-Sn and Fe-Si with Σ5 symmetrical tilt boundaries are used in which tin and sulfur, respectively, are the embrittling elements. The diffusivities parallel and perpendicular to the tilt axis are expected to be different, therefore the crack growth rates in these two directions should vary in the same ratio as the diffusivities.Preliminary measurements of crack growth rate along the [100] direction in the Cu-Sn alloy bicrystal are presented. The cracking occurred by decohesion along the grain boundary with almost no observable plasticity. The steady state crack growth was found to be approximately 10∼6 m/sec.