Current understanding of the evolution of the structure and defect generation in radiation damage cascades in structural materials - specifically Fe and Fe-based alloys - is largely based on molecular dynamics (MD) simulations using classical potentials. There are two major reasons for this: firstly, there are no experimental techniques that are capable of measuring defect evolution on the appropriate time scale (femto-seconds to pico-seconds and beyond); secondly, the systems sizes required (tens of thousands to millions and larger) for realistic simulations of radiation damage cascade evolution are beyond those accessible by standard ab initio electronic structure methods. In this presentation I shall discuss progress towards the development of ab initio methods that treat spin and ion dynamics on an equal footing that are capable of addressing system sizes used in classical MD. I will show results for Fe that address the importance of the disruption of the magnetic state of Fe during displacement cascades that are based on large scale (~10,000 atom) models and order-N electronic structure methods. I will also show how these approaches can be extended to even larger system sizes (>1,000,000-atoms) using a novel “representative-atom” course graining approach. The work described in this presentation was performed in collaboration with Aurelian Rusanu, Khorgolkhuu Odbahdrahk, Donald M. Nicholson, Yury Osetskiy, Roger Stoller (ORNL), and Yang Wang, (Pittsburgh Supercomputer Center, Carnegie-Mellon University), within the “Center for Defect Physics in Structural Materials” which is a Department of Energy, Office of Science, Energy Frontier Research Center (EFRC).

In this study, we performed light ion (400 keV Ne++) irradiations of A2Ti2O7 pyrochlore oxide compounds, where A = (Lu, Er, Y), at cryogenic temperature (~77 K). In each compound, we measured significant volume swelling (~5-8%) at fairly low displacement damage doses (less than 0.5 displacements per atom, or dpa). Our studies (using X-ray diffraction) also revealed that in the ion irradiated pyrochlore samples, cation antisite formation correlates directly with lattice swelling. For instance, in Lu2Ti2O7 (LTO), our measurements indicate that the volume increase per antisite defect pair is approximately 12 Å3. We also found that swelling is most pronounced in compounds with the largest A cations (Y and Er in these experiments, where both Y and Er have ionic radii of about 1 Å in 8-fold coordination). For instance, in LTO, we measured 2.6% volume swelling at a dose of only 0.07 dpa. The swelling in Y2Ti2O7 (YTO) appears to be slightly higher yet, ~3.0% volume swelling at 0.07 dpa, perhaps suggesting a larger ion radius effect for Y3+ in YTO versus Lu3+ in LTO. This is even more apparent in Er2Ti2O7 (ETO), where we measured 3.9% volume swelling at 0.07 dpa. The Er3+ cation is about 2.8% larger than the Lu3+ cation, which might explain the enhanced swelling in ETO compared to LTO in the presence of cation antisite defects. That is to say, if we compare A3+ cations Er3+ and Lu3+, a larger size disparity exists between the larger Er3+ cation and the small Ti4+ cation (the latter is only 0.6 Å in radius in 6-fold coordination), as compared to Lu3+ and Ti4+. It should be noted that this is difficult to quantify because the relative swelling also depends on the retained antisite defect concentration at any given dose. In the examples cited above, the measured antisite concentrations at 0.07 dpa in LTO and ETO are 22% and 15%, respectively. This suggests that there is significantly more swelling in ETO per antisite defect, compared to LTO. The pronounced irradiation-induced swelling of titanate pyrochlores will be discussed in terms of cation antisites and other defects that might cause swelling, namely cation and anion Frenkel defects. These discussions will include computational results based on first-principles calculations and temperature accelerated dynamics (TAD) simulations. These results are evaluated in the context of the role of lattice disorder on the radiation tolerance (or conversely, the radiation susceptibility) of complex oxide ceramics.

The fundamentals of the framework and the details of each component of self-evolving atomistic kinetic Monte Carlo (SEAKMC) will be presented. Applications of SEAKMC for annealing simulations of radiation damage will be demonstrated through two selected examples: cascade annealing of primary damage and the long-term evolution of a specific configuration of fifty vacancies. For the cascade annealing, a comparative study using SEAKMC and object kinetic Monte Carlo (OKMC) will be shown. For the evolution of vacancies, the results are compared with autonomous basin climbing (ABC), kinetic activation-relaxation technique (kART), and molecular dynamics (MD) simulations. It is found that SEAKMC possess the atomic fidelity that is similar to the MD but on a much longer time scale. The difference between SEAKMC and other methods will be elaborated. In addition, the unique predictive capabilities and the limitations of SEAKMC as well as its potential applications to a wide range of problems will be discussed. This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99.”

The diffusion of He atom in the grain boundaries of BCC Mo is studied using molecular dynamics simulations. Three grain boundaries, the {100} twist Σ29 (referred to as the twist), the <110> symmetrical tilt (tilt angle of 10.1; the tilt), and the {112} twin boundaries (the twin), are investigated to elucidate the effect of the grain boundary structure. The results show that due to the trapping, the introduction of grain boundaries reduces the effective diffusivity of He atom in reference to that of He interstitial in single-crystal BCC Mo, with increased diffusion barriers. For the twist and the tilt grain boundaries which present strong trapping to He, the dimensionality of diffusion is also changed from that in the perfect crystal, e.g., two-dimensional diffusion in the twist, and one-dimensional diffusion along the dislocation cores in the tilt grain boundaries. The change in diffusion dimensionality is found to enhance the preferable He bubble nucleation in the grain boundaries, with random nuclei distribution in the twist, and parallel distribution along the dislocation cores in the tilt grain boundaries.

The study of microstructure evolution in materials under extreme conditions is becoming possible thanks to incredible advances in experimental techniques, theory and computer simulation. Atomistic simulations are approaching the length and time scale of some experiments, and I will discuss plasticity in porous materials under compression as an example. In particular, nano-scale plasticity and failure of porous metals under extreme conditions are not well understood. In addition to porosity arising from mechanical failure at high strain rates, porous structures also develop due to radiation damage. Therefore, understanding the role of porosity on mechanical behavior is important for the assessment and development of materials like metallic foams, and materials for new fission and fusion reactors, with improved mechanical properties. We carry out tension and compression molecular dynamics simulations of Tantalum, as a model b.c.c. metal, with a collection of nanovoids. The effects of high strain rate on the stress-strain curves and on dislocation activity are examined. During loading we find massive total dislocation densities, comparable to recent experimental findings. We also estimate a much lower density of mobile dislocations, due to the formation of junctions. We compare homogeneous loading of our samples with non-equilibrium shock loading-release simulations. In both cases a significant fraction of dislocations survive unloading, unlike what happens in f.c.c. metals, and future experiments might be able to study similarly recovered samples and find clues to their plastic behavior during loading. Comparison to mechanical properties of materials with extremely high porosity (nanofoams) will also be discussed.

Irradiation creep is an important long-term macroscopic degradation phenomenon in nuclear structural materials that involves dislocation interactions with irradiation induced obstacles. The yield strength of dislocation-obstacle interactions in the context of irradiation creep exhibit complex relations, for example an inverse relation to strain rate at long time scales. To quantitatively assess the dislocation-vacancy cluster interaction in bcc Fe at the time scale beyond conventional atomistic simulations, we employ a newly developed approach, the Autonomous Basin Climbing (ABC) method (Kushima et al., JCP, 130, (2009)). The energy landscape and transition pathways in this interaction are determined by the ABC method. The directly simulated strain rates at the atomic level span a wide range from 108 s-1 down to 103 s-1, which is far beyond reach to traditional molecular dynamics (MD). We examine the relation between the critical resolved shear stress (CRSS) and the strain rate, and demonstrate the origin of the inverse behavior between CRSS and strain rate below 105 s-1. This behavior arises because of the competition of two driving forces: strain rate and thermal activation. At low strain rate, the obstacle has enough time to nucleate to a stable structure and thus has strong interaction with the dislocation. At high strain rate, however, the vacancy cluster is split into parts because of less time available for nucleation. The split vacancy cluster has larger surface area attached to the dislocation and yields a higher CRSS as well. Therefore the interaction leads to a “V” shape relation between the CRSS and strain rate (that is, decreasing at low strain rate and increasing at high strain rate) with the minimum at 105s-1. The interactions at high strain rates are directly benchmarked against MD simulations, and the results are well consistent with each other. This work shows that even a unit process can induce an inverse behavior, which complements the previous macroscopic models.

GaN devices are revolutionizing technologies from solid state lighting and displays to high power, RF electronics due to many advantageous material properties. A lesser known but potentially high impact property of GaN-based materials is the significant resistance to radiation-induced defect degradation, a feature that is important for space and military applications. To date, there have been only sparse studies on radiation-induced defects in GaN. This paper reports on the introduction of deep states within the GaN bandgap due to proton irradiation as a function of fluence, followed by trap characterizations via deep level optical spectroscopy and deep level transient spectroscopy (DLOS/DLTS) to probe traps throughout the entire GaN bandgap. Measurements were made on n-type GaN:Si (n~3×10^16cm^-3) layers grown on c-plane GaN templates by ammonia-based MBE. Schottky diodes were fabricated for CV-based measurements. Proton irradiation was carried out at 1.8MeV with fluencies being varied from 3×10^11cm^-2 to 1×10^13cm^-2 in multiple steps. Prior to radiation, DLTS and DLOS revealed states at EC-0.25eV, 0.62eV, 0.72eV, 1.3eV, 2.5eV and 3.3eV, all of which were observed in earlier studies of epitaxial GaN. The total concentrations for DLTS traps (0.25eV, 0.62eV, 0.72eV) and DLOS traps (1.3eV, 2.5eV and 3.3eV) were both low 10^15cm^-2. The impact of proton irradiation is dramatic. DLTS reveals the evolution of traps as a function of proton dose through an increase in the concentrations of each pre-radiation trap, and the introduction of two additional traps at EC-0.13eV and 0.16eV. Slight increases in DLTS trap concentrations were observed until a dose of 3×10^12cm^-2, after which large increases occurred, with a final concentration up to 7×10^15cm^-3 at the largest dose. Since radiation was done under vacuum, the energies of the two new states at EC-0.13eV and 0.16eV are suggestive of a nitrogen vacancy-related source, but more work is needed to verify this. Unlike the upper gap states, the DLOS-detected deeper levels (EC-1.3eV, 2.5eV, 3.3eV) were present in all samples under all fluences. These states were previously associated with interstitial carbon, gallium vacancies and CN substitutional acceptors, respectively, in n-GaN. Similar to the DLTS traps, each DLOS trap showed a threshold behavior at a flux of 3×10^12cm^-2, and the total trap concentration reached ~1×10^16cm^-3 after the largest dose. The removal of free carriers was strongly evident in these samples via a monotonic dependence on proton flux, reaching 1×10^16cm^-3 removed carriers at the highest flux. DLOS in conjunction with lighted CV measurements revealed the three traps at EC-1.3eV, 2.5eV and 3.3eV dictate carrier removal. This likely explains the often observed channel carrier removal in irradiated HEMTs and will direct growth optimization toward addressing these sources. Complete discussion of trap introduction kinetics and source identification will be made at the conference.

Radiation defects in oxides are complex and remain poorly understood. Here, we use a combination of high-resolution transmission electron microscopy and high-energy ion channeling to study ZnO crystals bombarded at room temperature with heavy ions (500 keV Xe). Results reveal an intriguing damage evolution, proceeding via the formation of a near-surface band of cavities. With further irradiation, a layered structure is formed, with alternating near-stoichiometric and Zn-rich layers. This explains the origin of the anomalous intermediate peak and step in ion channeling spectra, which have remained a puzzle for over a decade. Our results clearly show that the free surface has a profound impact on the radiation damage buildup in ZnO up to about 200 nm from the surface for the conditions used here. This changes our current understanding of radiation damage buildup in this material system. To explain the damage evolution observed, we propose a damage buildup scenario involving vacancy clustering, loss of O from the surface, and peculiarities of point defect transport through a Zn-rich defect band toward the surface. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

The development of quantitative models for microstructural evolution of materials under ex-situ and in-situ ion beam irradiation, as well as for interpreting electron microscope observations of real-time Brownian dynamics of defects and dislocations under irradiation, remains an outstanding problem. Addressing this problem is necessary for the interpretation of results of ion beam irradiation tests and for the development of micromechanical, and other microscopic, methods for the examination of irradiated materials, into tools for the accelerated qualification and development of materials for fusion and fission power generation technologies. Electron microscope observations of microstructures formed under ion irradiation show that microstructural evolution is driven by a complex interplay between generation of defects in collision cascades, their mobility and elimination at free surfaces, and interaction between the defects. Interactions between mobile defects are likely responsible for the formation of ordered structures dominating the observed microstructure in the limit of high (~1 dpa) irradiation dose and in the limit of high density of defects characteristic of ion irradiation experiments. Analysis performed in this work shows that by mapping a many-body diffusion equation for interacting defects onto a set of single-particle Langevin equations, it is possible to formulate a numerically efficient algorithm for modelling microstructural evolution. The algorithm does not rely on a mean-field approximation often adopted in a rate theory treatment of microstructure, and enables following the evolution of large ensembles of interacting defects in real time. Generalizing the Foreman-Eshelby analytical formula for the energy of elastic interaction between dislocation loops to the case of arbitrary non-collinear orientations of the loop Burgers vectors, and including elastic interactions in the Langevin dynamics simulations, we observe the spontaneous formation of stable ordered collinear Burgers vector loop configurations similar to those observed in in-situ ion irradiation experiments. This highlights the role played by the density of interacting defects in determining the pathways of microstructural evolution under irradiation. This work, part-funded by the European Communities under the contract of Association between EURATOM and CCFE, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was also part-funded by the RCUK Energy Programme under grant EP/I501045.

Cr based systems, which show complex structural-magnetic phase diagrams, play a major role in metallurgical technology. In this study, we address, by means of Density Functionnal Theory (DFT), the influence of magnetism on structural and energetical properties of defects in these systems. We point out that, in the magnetic ground state of Cr with the presence of spin density waves (SDW), the formation and migration of vacancies and non-magnetic impurities may be strongly anisotropic, that is, confined around the low-spin nodal planes. At variance, the behaviour of mag,etic impurities such as Fe may be rather isotropic [1]. We also show the impact of magnetism on the energetic of Cr surfaces and Fe/Cr interfaces, by determining the stability of various magnetic structures in the presence of surfaces and interfaces [2]. We show that the most stable structure of the spin-density wave is mainly dictated by a subtle balance between bulk and surface/interface influences, and strongly dependent on the surface/interface orientation. Regarding the Cr surfaces, we confirm the role of magnetism to lower the surface energy of Cr(100) with respect to Cr(110) as suggested experimentally. Concerning the Fe/Cr interfaces, magnetic frustrations are identified to be responsible for a higher formation energy of Fe/Cr(110) compared to that of Fe/Cr(100). This unusual anisotropy of interface energies is clearly different from the corresponding interfaces between Cr and a nonmagnetic element. Two ways are suggested to relax partially the magnetic frustrations at the (110) interface and to lower its formation energy. Noncollinear magnetic configurations can be developed where local moments of Fe and Cr atoms are perpendicular to each other. Also, the presence of SDWs with the low-moment nodal sites at Fe/Cr(110) offer another way to relax the magnetic frustrations and lower the interfacial energy. [1] R. Soulairol, Chu-Chun Fu and C. Barreteau, Phys. Rev. B, 83, 214103 (2011) [2] R. Soulairol, Chu-Chun Fu and C. Barreteau, Phys. Rev. B, 84, 155402 (2011)

We show that the shape of the kinetic energy distribution in radiation-induced thermal spikes may be described using a dimensionless number, (volume)^(2/3)/(surface area), known as compactness. The disorder produced in thermal spikes in Ni3Al is proportional to compactness because the thermal spike cooling rate, which determines the time available for thermal disordering, is inversely proportional to compactness. On the other hand, Frenkel pair production is inversely correlated to compactness because longer thermal spike lifetimes enhance vacancy-interstitial recombination.

In the irradiation damage process of steels, there exists an incubation period, a transient stage before steady-state void swelling. Understanding point defect behaviors in the period is important because it determines the service lifetime of steel components in nuclear systems. Though there have been a lot of theoretical analyses on the period, only a few experimental studies have been performed for the detection of vacancies and their clusters. It is due to the fact that transmission electron microscopy has been employed for most of these studies. Point defects and their small clusters in the incubation period are below the microscope&’s resolution limits and therefore are impossible to detect. Positron annihilation spectroscopy has made it possible to detect small vacant sites, which are below the resolution limit of transmission electron microscopy. In this study, defect structures of austenitic stainless steels and ferritic stainless steels in the period were investigated by positron annihilation lifetime measurement. Austenitic stainless steels and ferritic stainless steels are both important nuclear materials. The former has been used as nuclear reactor core materials primarily because they are highly corrosion resistant. The latter is expected for structural materials in fusion and fast breeder reactors. It is well known that the start of void swelling strongly depends on alloying elements. Therefore, we compared defect structures in commercial alloys and model alloys during the incubation period of void swelling. Alloys used were as follows. Austenitic stainless steels: commercial alloys (SUS316 SS, SUS316L SS, SUS304 SS, Ti added modified 316SS), model alloys (Fe-16.13Cr-16.96Ni, Fe-15.39Cr-15.92Ni-2.68Mo-1.89Mn, Fe-15.27Cr-15.8N-2.66Mo-1.88Mn-0.53Si, Fe-15.27Cr-15.8N-2.66Mo-1.88Mn-0.53Si-0.24Ti). Ferritic stainless steels: commercial alloys (F82H), model alloys (Fe-8Cr, Fe-8Cr-2W, Fe-8Cr-2W-0.1C, Fe-8Cr-2W-0.2V-0.04Ta, Fe-8Cr-2W-0.2V-0.04Ta-0.1C). They were irradiated with neutrons and electrons. Even neutron irradiation at 363 K and 573 K to a dose of 0.2 dpa, there existed an effect of alloying elements in austenitic stainless steels. At 363 K irradiation, micro-voids were detected only in Fe-Cr-Ni. At 573 K irradiation, voids were formed in all model alloys, though the concentration depended on alloying elements. In commercial alloys, precipitates and/or stacking fault tetrahedra were formed instead of vacancy clusters, which prevented void swelling. In ferritic stainless steels, the effect of carbon was clear. Neutron irradiated alloys with no carbon, vacancy clusters (micro-voids) were formed. While in alloys with carbon, metal carbides formed during tempering prevented the growth of vacancy clusters.

Irradiation drives a material far from its equilibrium state by introducing high densities of defects, whose dynamics lead to composition changes, phase transformations and microstructure evolution. These processes ultimately lead to irreversible changes in material properties of the components employed in nuclear reactors. This presentation focuses on irradiation-driven thermodynamic instabilities in alloys, which include phase changes and composition instabilities, with a special emphasis on the latter case. These instabilities are particularly important in investigating the behaviour of structural alloys and (alloy) fuels in a nuclear reactor environment and the response of alloy thin films to ion beams. Following a brief review of the related experimental and theoretical works, a detailed model of defect and species dynamics in a binary system under irradiation will be presented. This model consists of a set of reaction-diffusion equations governing the dynamics of vacancies, dumbbell-type interstitials and lattice atoms under irradiation; such dynamics includes the stochastic generation of defects by collision cascades as well as the defect and atomic reactions and diffusion. A noticeable feature of this model is that the atomic fluxes are derived based on the transitions of lattice defects, a treatment that is significantly different from the way thermal diffusion is classically handled in alloys. The set of reaction-diffusion equations are highly stiff, hence a stiffly-stable method, known as Gear method, has been used to numerically approximate the equations. For the Cu-Au alloy, a miscible binary system, the model results clearly demonstrate the formation of compositional patterns under high-temperature particle irradiation, with Fourier spectra that depend on the cascade characteristics, average composition and temperature. Both stability analysis and numerical simulation results will be presented. This work is supported by the U.S. Department of Energy as a part of a Computational Materials and Chemical Sciences Network project entitled "Computational Microstructure Science."

To support the R&D of high power spallation targets for neutron scatering sciences applications, structural materials such as ferritic/martensitic steels, austenitic steels, Ni-alloys, Al-alloys etc. have been irradiated high energy protons and spallation neutrons doses up to 20 dpa (in Fe) and 1800 appm He. High He contents produced by high energy protons cause significant changes in mechanical properties, which differ greatly from those induced by fission neutron irradiation in similar irradiation conditions. To understand the changes of mechanical properties, microstructure of the irradiated materials have been investigated in detail. Transmission electron microscopy (TEM) has been applied to obtain quantitative information of defect clusters, dislocation loops and helium bubbles. Position annihilation spectrometry (PAS) has been used to detect vacancy / helium clusters of sizes below the TEM resolution limit, typically 1 nm. Furthermore, atom probe tomography has also been utilized to analyze tiny precipitates (e.g. α) formed during irradiation. In this talk the results obtained from ferritic/martensitic steels and austenitic steel AISI 316L will be presented.

The thermal diffusion of defects as vacancies or interstitials is the main process which drives the material towards equilibrium after or in parallel to the damage production. During the irradiation the supersaturated defects can form clusters and one of the key questions in metals is the structure adopted by self-interstitial clusters. We propose in body centered cubic metals a new low-energy three dimensional periodic structure for these defect clusters that generalizes previous observations [1, 2]. The underlying crystal structure corresponds to the C15 Laves phase. Using density functional theory (DFT) we demonstrate that in iron these C15 aggregates with small size are highly stable compared to 2D loops and that they exhibit large antiferromagnetic moments. The systematic exploration of the energy landscape performed using an eigenvector following method, the Activation Relaxation Technique (ART) [3], and an empirical potential fitted to DFT defect data, confirms the exceptional stability of these clusters and shows how they can grow by capturing self-interstitials. Molecular Dynamics reveals that these C15 clusters can form directly in cascades and that they are highly immobile. This new morphology of self-interstitial clusters thus constitutes an important element to account for when predicting the microstructural evolution of iron base materials under irradiation. 1. M.-C. Marinica, F. Willaime, J.-P. Crocombette, Phys. Rev. Lett. 108, 025501 (2012). 2. D. J. Bacon, F. Gao, and Y. Osetsky, J. Nucl. Mater. 276, 1 (2000); D. Terentyev et al,. Phys. Rev. Lett. 14, 145503 (2008) 3. G.T. Barkema and N. Mousseau, Phys. Rev. Lett. 77, 4358 (1995); E. Cances et al., J. Chem. Phys. 130, 114711 (2009), M.-C Marinica et al., Phys. Rev. B 83, 094119 (2011).

Zr-2.5Nb samples removed after 7 years of service from a nuclear power reactor were investigated by traditional mechanical testing and whole pattern neutron diffraction line profile analysis of the irradiated and deformed materials. A significant increase in yield strength and subsequent strain softening are observed in the as-irradiated material. The line profile analysis allows the change in mechanical properties to be directly related to evolution of the microstructure. A four-fold increase in overall dislocation density accomplished entirely by increase in the Burgers vectors dislocations and profound change in the dislocation network arrangement are found to be created by the fast neutron irradiation. Comparison to microstructural evolution during plastic deformation shows a similar increase in dislocation density, but the increase is equally distributed amongst and type dislocation. Finally, plastic deformation of the previously irradiated material again increases the dislocation density significantly but, in contrast, does so through a ten-fold increase of the dislocation density relative to the as-irradiated material, while the Burgers vector density does not change. The different evolution of the and Burgers vector ratios in the unirradiated and irradiated Zr-2.5Nb during plastic deformation which can perhaps be explained with the strain localization effect previously reported in irradiated Zircaloy subjected to deformation.

Graphene, the one atomic sp2 -bonded planar carbon sheet, has inspired considerable research due to its unique thermal, mechanical, optical and electrical properties. So far, many applications based on graphene have been demonstrated, such as field effect transistors, sensors, quantum devices etc. However, in the fabrication of graphene devices, etching-induced defects which would affect the intrinsic properties of graphene are difficult to avoid completely due to the sensitive structure of graphene. Therefore, understanding the evolution of structural defects in graphene is necessary for fabricating graphene devices. In this work, atomic force microscopy and Raman spectroscopy have been used to investigate the evolution of the structural defects in the graphene obtained by mechanical exfoliation under different gas plasma. The defects were introduced intentionally into the graphene by adjusting the type of plasma gas (argon, oxygen, nitrogen) and plasma treating time. This enables us to study the detailed changes in the graphene during exposure to plasma. It is demonstrated that at the early stage of the process, when the graphene is non-defective, the rate of introducing defects was different for argon, oxygen, nitrogen plasma and the argon gas plasma was observed to be the fastest to introduce defects into graphene under the same plasma conditions (at fixed input power, flow rate) while the nitrogen gas plasma was the slowest. Interestingly, when the graphene has been introduced with the defects indicated by the presence of the D peak in Raman spectrum, the rates of further forming defects were nearly the same for these gas plasmas. We propose that the element number of the plasma gas is the key to cause defects at the early introduction of defects since the energy barrier is high. After the defects formed, the energy barrier would become lower which explains the later same rates of defects formation by these gas plasmas.

The nanocontact plastic behavior of single-crystalline Ta (100), Ta (110) and Ta (111) was studied as a function of temperature and indentation rate. Tantalum was selected as a representative body centered cubic (BCC) metal and revealed a unique deformation behavior. Experiments performed at room temperature exhibit a single pop-in event, while at 200°C a transition to multiple pop-ins was observed. These results are discussed in respect to the orientation, defect structures and temperature using both anisotropic finite element as well as MD simulations. The latter suggest that defect structures such as twins and stacking faults are active during plastic deformation and contribute to the observed behavior. Additionally, serrated flow is related to differences in the quasi-elastic reloading from changes in the defect mechanism due to the increase in testing temperature.

The propagation of an interfacial crack front in a disordered toughness landscape is considered. The nature of the propagation is numerically shown to crucially depend on both the variance and the spatial correlation of the toughness landscape. In case of small (relative) variance of the toughness or small values of the gradient in the direction of propagation, weak pinning conditions are observed: the front propagation is smooth and regular. In the opposite case strong pinning conditions are recovered: high velocity fluctuations and avalanche dynamics are encountered. The consequences of these two regimes on the effective toughness are discussed.

The core structure and Peierls stress of dislocations in TiN are studied using density functional theory (DFT) calculations, to elucidate possible slip systems in TiN bulk. We focus our study on the edge dislocations with Burgers vector a/2<110>, with slip planes of {001}, {1-10}, and {111}. Periodically repeating supercell containing a dipole of dislocations is used. The Peirels stress is calculated using the direct shearing method. The DFT results in TiN are compared to those in MgO, a pure ionic oxide, to understand the effect of complex bondings in TiN. In addition, the screw dislocations in TiN are also studied and the results will be discussed.

Silicon-based solar cells account for 87% of the world&’s photovoltaic market share. However, further adoption of this technology is still contingent on the reduction of crystal-growth defects that impede higher cell efficiencies. Herein, we focus on one of the most deleterious defects in multicrystalline silicon (mc-Si): Dislocations. These defects negatively impact the performance of solar cells and can lead to significant electrical conversion losses ^[1]. Successful techniques such as high-temperature annealing have been previously demonstrated to reduce dislocation density in mc-Si ^{[2, 3]}, therefore increasing the potential of improving cell performance. A better understanding of dislocation behavior may enable us to improve and optimize such annealing techniques, providing further dislocation density reduction (DDR). Crystalline features (e.g., crystalline orientation, crystalline size, or crystalline boundaries) are known as one of the dominating parameters that affect the dislocation motion in crystalline materials. However, their roles on dislocation motion and the corresponding DDR in mc-Si have not been studied in much detail. In this contribution, we present results from high-temperature annealed mc-Si and quasi-mono Si samples. A comparison of their crystalline features and the corresponding DDR is made 1) to elucidate the crystallographic orientation where DDR is more dominant and 2) to highlight the contribution of crystalline boundaries that are expected to act as effective dislocation sink sites. The understanding of the relation between the crystallographic characteristics and DDR can lead to a tailored in-situ process at the crystal-growth level to eradicate such defects, with a potential to increase efficiency at a marginal processing cost. _{[1] A. Bentzen et al., “Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing” Journal of Applied Physics99, 093509 (2006) [2] K. Hartman et al., “Dislocation density reduction in multicrystalline silicon solar cell material by high temperature annealing,” Applied Physics Letters93, 122108 (2008) [3] M.I. Bertoni et al., “Stress-enhanced dislocation density reduction in multicrystalline silicon,” Physica Status Solidi Rapid Research Letters 5, 28-30, (2010)}

Dislocation evolution in single-crystal tantalum compressed at pressures >50 GPa under hydrostatic and dynamic-pressure (i.e., high stain-rate and pressure) conditions using diamond anvil cell, gas-gun impact, and laser-shock techniques has been studied using transmission electron microscopy (TEM) technique. Results are presented in this talk to reveal that transitions of deformation mode from dislocation glide to shear transformations (i.e., twinning and omega transformation) occurred solely in tantalum compressed under dynamic conditions. It is proposed that the mode transitions take place in tantalum to accommodate insufficient dislocation flux resulting from the exhaustion of dislocation sources when dynamic-recovery reactions for dislocation annihilation and cell formation are largely suppressed when compressed under dynamic-pressure conditions. The exhaustion of dislocation sources occurs when the stress for dislocation multiplication exceeds the threshold stresses for shear transformations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

SISYPHUS (Stochastic iterations to strengthen yield of path hopping over upper states) is a combined Monte Carlo and Molecular Dynamics technique that allows one to achieve milliseconds and longer time-scales for several thousands of atoms. We have validated this technique through vacancy mediated diffusion in Iron and Tantalum metals at low temperatures. We then apply it to realistic strain-rate nanomechanics: strain-rate as low as 10/s while maintaining fully atomistic resolution for several thousands of atoms. We calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. It is found that the activation free energy depends significantly and non-linearly on the driving force (stress or strain) and temperature, leading to very high activation entropies for surface dislocation nucleation. We expect SISYPHUS to to be a useful tool to simulate a variety of defect nucleation and growth processes under realistic conditions.

The breakdown of Schmid&’s law in bcc metals has been known for a long time. It is generally believed that, at temperatures well below Tc, the flow stress and plastic deformation mechanisms in bcc metals are primarily controlled by the glide of ½<1 1 1> screw dislocations on {110} planes. Further, the observation of slip traces corresponding to slip systems with very low Schmid factor, often called anomalous slip, has been ascribed as a result of twinning-antitwinning asymmetry, the effect of stress components other than the shear stress in the slip direction or to dislocation-dislocation interactions. Since not all slip events give rise to slip traces on the crystal surface, slip trace analysis is often incomplete. Here the microcompression technique performed during in-situ Laue diffraction at the MicroXAS beamline of the Swiss Light Source is used to follow the sequence of activated slip systems in BCC single crystal pillars with diameters of 1-2 micron. Diffraction patterns are obtained in transmission with a 5-23 keV X-ray beam, with FWHM of 0.5-1 mu;m and complemented with Laue scans allowing the mapping of the spatial distribution of strain gradients in the deformed pillars, providing information on local crystallographic orientations and on the activated dislocation slip systems. Details of the method are described in [H. Van Swygenhoven, S. Van Petegem, JOM 62 (2010) 36]. Complementary slip trace analysis is carried out in SEM. Applied on bcc Mo alloy pillars in a [001] orientation obtained by directional solidification [J. Zimmermann et al. Acta Materialia in press], it demonstrated that slip occurs first on {112} slip planes having the highest resolved shear stress and not on the {110} planes as predicted from simulations on pure Mo. TEM 3D atom probe analysis revealed the presence of Al rich fcc precipitates in the bcc structure of directionally solidified Mo alloy, suggesting the precipitates being the reason for slip on {112} planes. Applied on 4 different single crystal bcc metals all having different values for the critical temperature (W, Mo, V and Nb) with the compression axis oriented in [155], [1 5 10], [154], [156] and [10 20 23] it allowed to determine the activated slip systems and their dependence on Tc, to evidence anomalous slip as a result of dislocation-dislocation interaction and to evidence crystal rotation according to resultant slip on (112) planes.

Silicon carbide is the archetypal polytypic material with more than 200 forms referenced. Among these polytypes, the cubic (3C-SiC) phase exhibits the most appealing properties for electronic applications. Despite this interest, the growth of high-quality 3C-SiC single-crystals is still hindered by its instability at the high temperatures (> 1900°C) that are usually required to grow SiC from the vapor phase. As a consequence, the grown crystals are of poor crystalline quality and contain large amounts of stacking faults (SFs) lying in the {111} planes and even inclusions of hexagonal polytypes (mainly 6H). The question of the relative stability of the different polytypes is a longstanding issue in the studies of SiC materials. Although it is clearly established that spatially correlated SFs are involved in the polytypic transformation, it remains unclear whether the occurrence of a particular polytype is due to thermodynamic reasons or whether it is related to kinetic effects. In this work we address the question of spatially correlated SFs in SiC single crystals undergoing the 3C-6H polytypic transformation. We make use of diffuse X-ray scattering (DXS) combined with Monte-Carlo simulations in order to investigate the spatial correlations in the SF arrangement at different points of the 3C-6H transformation path. We demonstrate that SF systematically group in the form of double (2SFs) and triple SFs (3SFs) whereas single (intrinsic) and multiple nSFs (with n>3) are nonexistent. Furthermore we show that, in the stacking sequence, each SF is followed by an exclusion zone with a width of at least three lattice spacings where no SF occur. These characteristics are rationalized using an Axial Next-Nearest Neighbor Ising (ANNNI) interaction model where we show that 2SFs and 3SFs are energetically favored. Using the same ANNNI approach we are able to conclude that the formation of the 6H polytype using spatially correlated SFs is driven by energetic considerations. Finally, the transformation kinetics as well as the activation energies of the transformation are deduced from the DXS data. These last results reveal that SF multiplication is ensured by double cross-slipping of dissociated dislocations.

Molecular composites such as plastic-bonded explosives or pharmaceutical tablets exhibit complex anisotropic deformation with implications for quality control and durability. Such composites are largely composed of micron-scale molecular crystals with some small amount of polymer binder. Bulk deformation and fracture mechanisms typically are influenced by crystal deformation and crystal-binder interfacial properties. Our investigations of the elastic-plastic responses of crystalline nitramine explosives and paracetamol by nanoindentation shed light on the elastic properties, plastic flow, and brittle failure characteristics that lead to, for example, poor tablet formation characteristics and localization of energy leading to initiation of detonation. The thermodynamic characteristics of several crystals and binder materials have been evaluated, and suggest that there is comparatively little influence on macroscopic response by comparison to the crystal properties themselves and especially formulation-derived chemical and microstructural effects. Formulation processes and additives further influence compaction behavior and microstructure-dependent fracture behavior. To begin to quantify these effects, we have performed several experiments focusing on the interface between the crystal constituents and matrix materials following simulated formulation procedures. Ellipsometry and neutron reflectometry were used to identify the interfacial structure of several molecular composites, with and without additives in the formulation. The difference in interfacial properties was also observed mechanically with nanoindentation. Synchrotron X-ray studies of molecular crystals, explosive binders and formulated composites revealed some intriguing possibilities in real-time observation of cracks, bubbles, delamination, and void collapse during high-speed loading events.

The mechanism of homogeneous dislocation nucleation in a defect free crystal under cylindrical nano-indentation has been studied by performing atomistic simulations. Previous work has shown that the nucleation process is governed by vanishing of energy associated with a single normal mode that exhibits a lengthscale that scales in an anomalous way with the geometrical loading parameters (indenter radius and film thickness).Here, we show that these scalings that were previously observed for a single particular crystallographic orientation in a two dimensional Lennard-Jones system are generic with respect to the lattice orientation.We also discuss our preliminary work on three dimensional (3D) models of Al.

We present a continuum formulation of dislocation dynamics using a vector representation of the dislocation density. This formulation gives rise to first-order dislocation kinetic equations in which the spatial derivatives are of curl-type. Upon numerical treatment, the curl-representation captures the dislocation multiplication process in a natural manner. The resulting kinetic equations are coupled with crystal mechanics by representing the plastic slip as eigenstrain. A staggered finite element scheme has been used to solve the dislocation kinetic equations and the elastic eigenstrain problems. Earlier investigations of the spatial and temporal statistics of the dislocation systems using discrete dislocation dynamics have made it possible to integrate into the above framework a stochastic component of the driving force for dislocation motion as well as dislocation processes such as cross slip and short-range reactions. The stochastic force is included to account for the density fluctuations. We demonstrate all of the above by presenting 3D solutions of the deformation of a copper crystal under multiple slip loading condition. This work was supported by the U.S. DOE Office of Basic Energy Sciences, Division of Materials Science & Engineering via contract # DE-FG02-08ER46494 at Florida State University.

In brittle polycrystalline materials, anisotropic shape changes - due to thermal expansion, composition changes, piezoelectricity, or other physical phenomena - can induce large misorientation stresses, often resulting in microfracture. While critical sizes for microfracture suppression can be evaluated with analytical models, predictions of failure probabilities must consider complex microstructures that are require numerical analysis. We have developed and implemented a stochastic model for such microstructure events , which we refer to as a Finite Element plus Monte Carlo (FE+MC) method, to predict microstructurally resolved failure probabilities in brittle polycrystalline materials. Our model considers randomly generated 2D polycrystalline virtual microstructures, to which elastic properties and orientation information can be assigned on a grain-by-grain basis. We perform a finite element analysis to calculate the resulting elastic stress distributions for a hypothetical defect-free microstructure and subsequently perform a Monte Carlo analysis by ‘distributing&’ flaws throughout the microstructure according to experimental flaw size distributions. Using superposition integral methods, we calculate the resulting stress-intensity factors for these flaws and extract failure probabilities by comparison to a fracture toughness (critical stress-intensity factor). We have validated our model for the case of uniaxial tension, for which we reproduce the expected Weibull distribution of failure probability. As a demonstration of the utility of this method in a more complex stress state, we consider electrochemical shock of polycrystalline LiXCoO2 electrodes. The predicted composition-dependent failure probabilities show good semi-quantitative agreement with in situ acoustic emission measurements.

The occurrence of jerky flow during deformation, which is also commonly referred to as Portevin-Le Chatelier (PLC) effect, has been investigated for nearly a century. Over the years, various theories have been proposed to account for this plastic instability, and now it is generally accepted that the nanoscale solute-dislocation interactions are responsible for the macroscopic serrations in stress-strain curve and the appearance of rapidly moving dislocation avalanches in the PLC bands. However, the underlying physics remains a matter of debate. In the current study, the high degree of depth sensitivity and the self-similarity that is maintained during instrumented indentation with a sharp indenter are fully exploited to improve our fundamental understanding of serrated flow behavior. Instrumented indentation tests were carried out on Ni-10Pd solid solutions from 30 °C to 450 °C with loading rates varying from 62.5 mN/s to 1000 mN/s. When the annealed Ni-10Pd was tested at temperatures above 350 °C, the load-depth curve was initially smooth and then became serrated after reaching a critical load. For the same loading rate, increasing the testing temperature from 350 °C to 450 °C resulted in an earlier onset of serrations with a lower load threshold. At a constant temperature (450 °C), increases in loading rates resulted in an earlier appearance of plastic instability with a higher load threshold. A modified cavity expansion model that accounts for the reconfiguration of dislocation substructures and the solute-dislocation interactions was developed and used to elucidate the influence of temperature, loading rate and pre-deformation on the occurrence of serrated flow behavior in Ni-10Pd during hot indentation.

Indentation of FCC single crystal is simulated using a three-dimensional multiscale plasticity framework. In addition to revealing the details of plastic deformation evolution under indentation loading, this work aims to compare induced local elastic strain and lattice rotation fields with X-ray measurements of the same fields in 3D. The simulation framework consists of coupling a dislocation dynamics library that explicitly simulates plasticity in the nanoscale regime with a finite element solver that applies the continuum mechanics laws in the continuum scale. The evolving dislocation microstructure during the course of simulation is extracted regularly and therefore used to calculate the induced local elastic strain and lattice rotation fields. The comparison between simulations and experiments involves computational coarse graining of the computed elastic distortion fields to match the pixel size of the X-ray data. Preliminary simulation results are presented along with the corresponding X-ray measurements, and the aspects of similarity between the predicted and measured fields are exposed. This work was supported by the U.S. DOE Office of Basic Energy Sciences, Division of Materials Science & Engineering.

High density misfit and threading dislocations have been a major issue for the growth of epitaxial wurtzite nitride semiconductor films such as (Al)GaN because of their deleterious electronic and optical effects. Therefore, evaluations of possible dislocation mechanisms leading to entanglements in such films are important for understanding the factors that lead to dislocation multiplication. This work compares non-coplanar dislocation junctions, a possible mechanism in dislocation multiplication, formed by two intersecting dislocations gliding on adjacent slip planes in hexagonal close-packed (hcp) crystals, a sub-lattice of wurtzite. We study similarities and differences in their evolutions during formation and destruction of dislocation junctions and compare their strengths under varying stress fields, elastic properties, dislocation lengths, and active slip systems. The transformations of originally sessile junctions into two passing glissile dislocations under the influence of an incrementally increasing applied stress field are studied via discrete dislocation dynamics (DDD) simulations and the strengths of junctions are quantitatively assessed by the yield surface plots, the loci of points that determine the resistance of junctions to de-stabilization. In light of the fact that common growth directions are [0 0 0 1] and <1 1 -2 2>, the active slip planes considered in this work are basal (0001), prismatic {1 0 -1 0}, and higher order δ-pyramidal planes {1 1 -2 2}. The calculated yield surfaces of junctions on the adjacent prismatic/basal and prismatic/δ-pyramidal {1 1-2 2} planes show a remarkable uniformity in shape across different material stiffness. However, the absolute magnitude of critical stresses needed to completely eliminate the junctions reduces with increasing material elasticity, for example 20 - 30 MPa for beryllium (Be) and 2 - 3 MPa for magnesium (Mg).

The force experienced by a segment of dislocation due to an applied stress can be accurately determined using the Peach-Koehler formula [1], yet its subsequent motion cannot. The manner in which it responds to this force is a question of fundamental importance for the dynamics of dislocations, and hence almost all mechanical properties of crystalline materials from tensile strength to fracture behaviour. However, it is only poorly understood from a theoretical standpoint. Computer simulations of dislocation dynamics generally treat the response as overdamped, and implement an “Aristotlean” velocity rule where the dislocation velocity is proportional to the applied force [2]. Experimental observations tell us that the dislocation response is very sensitive to temperature, strain rate, applied stress and the composition of the matrix in which the dislocation resides [3]. In this work we present recent progress in a new approach to understanding dislocation dynamics. Inspired by out-of-equilibrium statistical mechanics, the dislocation line is treated as a dynamically growing interface [4], driven through its environment by the applied force and thermal noise. Kink-like features emerge naturally from the underlying Frenkel-Kontorova model. The diffusive properties of the motion are obtained, and compared with the results of stochastic computer simulations. [1] M. Peach and J. S. Koehler, Phys. Rev. 80, 436-439 (1950) [2] V.V. Bulatov and W. Cai, Computer Simulations of Dislocations, OUP (2006) [3] See e.g. A. Giannattasio and S. G. Roberts, Philos. Mag. 87 (17) 2589 (2007) [4] See e.g. H. C. Fogedby, J. Phys.: Condens. Matter 14 1557 (2002) This work was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was also part-funded by the RCUK Energy Programme under grant EP/G003955.

In body-centered (bcc) crystals, screw dislocation motion is one of the rate-limiting process governing deformation. Screw dislocations are subjected to high intrinsic lattice friction as a consequence of their non-planar core structure. The most successful theoretical model describing this intrinsic behavior is the so-called Peierls-Nabarro (PN) model, which is formulated for two dimensions and in static conditions. The PN model has been shown to be insufficient to capture the breadth of behaviors in real materials. While 3D aspects of screw dislocation motion have received significant attention in the form of atomistic simulations, finite temperature effects are comparatively less understood. Here, we use thermodynamic integration via configurational averaging of atomistic simulations to provide a full thermodynamic extension of the PN model. In this fashion, we obtain free energy generalizations of the Peierls energy and stress and find that the coupling between the applied stress and the free energy barrier governs the finite temperature behavior of screw dislocations.

High densities of dislocations are one of the principle performance-limiting defects in multicrystalline silicon (mc-Si) solar cells. High temperature (>1200°C) annealing has been reported to reduce the dislocation density in ingot Si [1]. However, this high-temperature process may also cause significant impurity contamination, canceling out the positive effect of dislocation density reduction on cell performance. Recently, the authors have observed a significant dislocation density reduction at temperatures as low as 820°C after phosphorus gettering in mc-Si containing intermediate concentrations of certain metallic species [2]. It is hypothesized that a unidirectional flux of impurities in the presence of a gettering layer can drive dislocations in a preferential direction where they subsequently sink at grain boundaries or surfaces. Here, as a follow-up study, the correlation between dislocations and metallic impurities is further investigated. The dislocation density reduction after gettering at different temperatures (820, 920 and 1020°C) is compared to the reduction of interstitial iron during P diffusion gettering. Furthermore, according to the dislocation-impurity interaction force, an upper limit of the dislocation velocity as a function of the flux and velocity of various types of metallic impurities is estimated. [1] M. Bertoni et al., Solid State Phenomena Vols. 156-158 (2010) pp 11-18 [2] H.J. Choi et al., Proc. 38th IEEE Photovoltaic Specialists Conference, Austin, USA (2012)

Nematic Liquid Crystals (NLCs) under confinement have been of interest because of the richness in physical phenomenon as a consequence of frustration emanating from fixed orientation at curved bounding surfaces and their role in electro-optic devices. This application requires accurate characterization of the gradient elastic moduli of NLCs. In this work we develop and implement a characterization approach based on the defect-driven textural transformations that arise when a NLC is confined to a capillary. In the initial stage an unstable disclination defect of strength +1 nucleates in the axis of the capillary and quickly branches into two stable +1/2 disclination defects. The model includes: (1) the Kirchhoff branch balance equation which predicts the splitting of a +1 wedge disclination into two +1/2. (2) the curvature of the +1/2 disclination lines as a function of elastic properties. This model shows by increasing the ratio of tension strength to bending stiffness, the branch point angle increases, but the final defect distance decreases. (3) the aperture branching angle as a function of the elastic properties and the value of the curvature at the branch point. These three predictions form the basis for the evaluation of the elastic moduli on NLCs. The material property predictions are validated with experimental data obtained with the same capillary geometry. The key advantage of the implemented methodology is to use time-dependent textural transformations under micron-range capillary confinement to extract elastic parametric data needed to further develop NLCs application.

In recent years, femtosecond laser interaction with materials, especially with glassy materials, has attracted considerable interest, and been proved to be a promising method for the construction of three dimensional micro-optical components or devices inside transparent materials. Here, we report on protuberance structures on the surface and continuous micro-channels inside copper ions doped glasses induced by irradiation of 800 nm, 250 KHz femtosecond laser. Field emission scanning electron microscope and three dimensional measuring laser microscope images reveal that the induced structures are circular and linear protuberances and can be controlled from ten microns to hundreds microns in width, and one micron to ten microns in height. The protuberance structure is proposed to be formed as a consequence of the laser induced high temperature and pressure due to linear and non-linear absorption near the laser focal point, and low softening and melting temperature of the glass sample. Through the comparison of micro-Raman spectra acquired at the protuberance structure and glass surface, we find that the B coordination in borate glasses changed after femtosecond laser irradiation and the BO4 tetrahedron units increased at the copping of the protuberance structure. This is due to the difference in diffusion speed of various kinds of ions near the focal point where the temperature can be as high as 3000 K. Furthermore, micro-channels are formed inside the glass samples directly after femtosecond laser irradiation. Compared with conventional micro-channels inside glasses fabricated by laser irradiation and subsequent acid solution treatment, which are very short and always have a conical shape, our method can realize uniform and continuous micro-channels with a long distance. These observations are very important for understanding the femtosecond laser/glassy materials interaction process, and may also have applications in the fabrication of integrated optical devices, such as waveguides, micro lens and micro fluidic devices.

Several deformation mechanisms are available to accommodate the applied strain in tensile tests of polycrystalline face centered cubic metal nanowires. These include grain boundary sliding, dislocation deformation of grain interiors, fracture, slip localization,... We perform very large scale molecular dynamics simulations of the tensile deformation of polycrystalline Pt nanowires with and without notches and as a function of grain size to observe how these mechanisms compete in the failure of these structures. We observe that many of these mechanisms operate simultaneously and synergistically and that grain boundary triple lines are arguably the most important defects in dictating which mechanisms dominate. We further argue that in such cases, where so many mechanisms operate simultaneously, a more holistic approach is required to describe deformation and failure than is traditionally applied at larger length scales.

The thermal stability of nanocrystalline (nc) materials is an important issue for their technical application. Grain growth in polycrystalline materials occurs to reduce the energy contribution of the grain boundary areas to the total energy of the system. The high density of interfaces in nc materials provides a significant driving force for grain growth. Systematic studies of grain growth at room temperature and at elevated temperatures show that the thermal stability of nc materials can be improved either by addition of solutes or lattice defects [1]. However, most of these studies neglect the contribution of triple and quadruple junctions. This work is motivated by recent results showing that nc material produced by inert gas condensation could be stabilized against room temperature grain growth by severe plastic deformation [1]. Such material was inspected by high resolution TEM using an aberration corrected FEI Titan 80-300. A triple junction (Σ3:Σ3:Σ9) consisting of two intersecting Σ3 twin boundaries and a Σ9 grain boundary, which is connected to a quadruple junction (Σ3:Σ3:Σ3:Σ9) via the Σ9 grain boundary was studied. This configuration has most likely formed during deformation in which multiple twins merged. A comprehensive strain analysis of the triple junction using the geometric phase analysis (GPA) is presented and compared with a molecular dynamics (MD) simulation [2]. The strain field of the core of the triple junction unequivocally shows dislocation character with a tensile and a compressive part in a dipole arrangement opposite to each other. The intersecting boundaries result in a net translation of a Burgers vector corresponding to a lattice dislocation in an fcc metal (result from MD simulations) [2]. An analysis of the rigid-body rotation along the Σ9 grain boundary yields two rotational defects (disclinations) of opposite sign forming a dipole. This accounts for a short range strain field since back stresses of the opposite disclination are balancing it. The presence of such a disclination dipole embodied in a grain boundary is thought to act as a stabilizing element for nanostructured materials hindering grain growth. Further it appears to be an important by-product of materials exhibiting multiple twinning. Based on the observation that the core of the triple junction showed the characteristics of a dislocation strain field, its energy was estimated to be 1.7*10^-9 J/m using 1/2Gb². References: [1] Y Ivanisenko et al, Acta Materialia 57 (2009), p. 3391. [2] H Rösner et al, Acta Materialia 59 (2011), p. 7380. [3] Support by the Deutsche Forschungsgemeinschaft (FOR714) and the Karlsruhe Nano Micro Facility (KNMF) is gratefully acknowledged.

Plastic strain recovery has been observed in recent years in nanocrystalline materials in which permanent deformation subsides over time after unloading. It has been suggested from experiments that the inhomogeneous stress fields in the nanocrystalline material due to the large variations in its grain size distribution is responsible for this recovery mechanism. Numerical simulations show that local stresses trigger dislocation activity leading to plastic strain recovery. We will present large scale numerical simulations of plastic strain recovery by employing a phase field dislocations model coupled to a kinetic Monte Carlo algorithm to simulate plastic deformation during loading and unloading, and a creep model for stress relaxation after unloading. The model includes grain boundary sliding by incorporating phase field distributions localized in the grain boundary with energetics obtained from atomistic simulations. It also accounts for grain orientation by recourse to Eshelby&’s equivalent strain method to model inhomogeneity, and grain size distribution. Our results quantify the importance of these structural factors on plastic strain recovery.

We report the results of computational tensile and compressive tests for model bi-continuous nano-porous gold structures using atomistic simulations with empirical many-body potentials and molecular dynamics. We find foam compaction that results from pore collapse even under tensile deformation. We also find a surprising substantial tension-compression asymmetry and we explain the results in terms of the surface stress that sets the filament under compression providing a bias favoring yielding in compression. We provide an interpretation to this effect that predicts the tension-compression asymmetry to be significant as the ligament size falls below ~ 10 nm. As the ligament size grows, approaching the values tested in the experiments, the model recovers the symmetric behavior.

Historically, principles guiding the rational design of polycrystalline materials have focused on structures the scale of a single grain or smaller. The opportunity to engineer a microstructure at longer length scales as well depends fundamentally on the availability of suitable language and concepts to describe the statistical features of ensembles of grains in a microstructure. Our research is based on the construction of a combinatorial object, known as a swatch, that gives a complete description of the grain boundary network topology in a small region. By considering the frequencies of swatch types, or of local grain boundary configurations, the statistical features of the grain boundary network topology may be encoded as a discrete probability distribution. This allows us to calculate the topological entropy of a microstructure, or even a distance between microstructures that indicates the degree of their topological similarity or difference. The intention of this research is to provide the community with a rigorous means to measure, e.g., microstructure variability for a given processing condition, or the degree to which some processing route accurately produces a target microstructure. Prepared by LLNL under Contract DE-AC52-07NA27344.

The mechanical and thermodynamic properties of materials are crucially dependent on microstructure, in particular precipitates and interfaces. In this contribution we will report on some of our recent progress in modeling precipitation and interfaces in several metallic alloys. Using a recently developed hybrid Monte Carlo/molecular dynamics scheme based on the variance constrained semi-grandcanonical ensemble [1], we have studied the size and temperature dependence of the martensitic BCC-9R phase transition in Fe-Cu alloys. The critical size at which the transition occurs is strongly temperature dependent. More interestingly the phase transition exhibits first-order character at elevated temperature but terminates in a critical point at about 300 K. We argue that this feature should be of general character and also occur in other systems with unlike crystal structures. Explicit simulations of the interaction of screw dislocations with both BCC and 9R precipitates revealed an increase in the critical shear stress by approximately a factor of two across the transition. The variance constrained semi-grandcanonical ensemble also allows computing the derivative of the free energy across miscibility gaps. In this fashion it is possible to extract excess free energies associated with interface formation [2]. We have employed this approach to obtain the temperature and orientation dependence of interface free energies for both lattice models (representative for cluster expansions) and models with continuous coordinate degrees of freedom (e.g., empirical potentials). This approach takes into account both configurational and vibational degrees of freedom as well as lattice relaxations. It has already been applied to Ising models and Fe-Cr alloys. [1] Sadigh, Erhart et al., Phys. Rev. B 85, 184203 (2012) [2] Sadigh and Erhart, arXiv/cond-mat:1111.1880

In nanocrystalline alloys the fraction of atoms located in grain boundaries is significant in contrast to conventional coarse-grained materials. Additionally, large strains are inherent to their nanograins. These key differences affect near-equilibrium phase composition, which is crucial for the understanding of the nanocrystalline alloy behavior upon thermal activation. The impact of grain refinement on the solid solubility has been investigated theoretically as well as experimentally. A thermodynamic model based on the Vinet et al universal equation of state is developed describing the chemical equilibrium between the elastically strained solution phases [1]. Grain boundaries are approximated by a uniformly dilated lattice. To determine their excess volume molecular simulations and macroscopic measurements were performed. The grain boundary enrichment is calculated for several binary systems. The results show good agreement with a wide range of experimental data. The strain inside the nanograins depends primarily on the grain size as shown by X-ray diffraction. Using a simple model for the magnitude and the distribution of local strain variations of the solid solubility are predicted [2]. Weighted according to their respective molar fractions both contributions are used to calculate the global solubility as a function of grain size. Results in several model systems will be presented. At 500 K the calculated solubility of Ag in Cu with a grain size of 20 nm is increased at least ten-fold with respect to the coarse-grained Cu. Experimental evidence supports this conclusion. The spatial distribution of the solute is determined and the grain boundary segregation quantified using atom probe tomography. Since the majority of the increase in solubility is due to grain boundary enrichment, it scales with the inverse grain size. The applicability of the thermodynamic model to interstitial solid solutions will be demonstrated for Fe-C. [1] A. Kirchner and B. Kieback, Scr Mater 64 (2011) p. 406. [2] A. Kirchner and B. Kieback, J Nanomater (2012) in press.

Grain boundaries in face-centered-cubic (FCC) metals with low stacking fault energies often form broad and complex three-dimensional structures that are composed of arrays of stacking faults. A general question concerns how the pattern of such faults, and hence the structure of the boundary, is related to the orientational parameters, such as intergranular misorientation and boundary inclination, that describe the macroscopic geometry of the interface. Addressing such questions can provide insight concerning the underlying dislocation structure of grain boundaries. In this presentation, we analyze a thin (~1 nm) hexagonal-close-packed (HCP) intergranular layer at a 29° <110> tilt grain boundary in gold. Our analysis, which is based on HRTEM observations and atomistic calculations, shows that this boundary consists of a dense array of 60° (a/2)<110> crystal lattice dislocations that are distributed one to every two {111} planes. These dislocations dissociate into paired Shockley partial dislocations, creating a stacking fault on every other plane, thereby producing the hellip;ababhellip;, or HCP, stacking sequence. This distribution of dislocations is consistent with the measured intergranular misorientation, the interfacial inclination, and with the calculated rigid-body translation along the tilt axis. By establishing the interfacial dislocation arrangement, we also show how the HCP layer at the 29° boundary observed here is geometrically related to that found previously at the 80.6° Σ=43 <110> boundary as well as to stacking fault arrangements observed and calculated at other dissociated high-angle grain boundaries. The 29° boundary is also interesting because it forms vicinally to two low index planes, namely {111} and {131}. In response, we also discuss an alternative description of this interface employing a reference frame for which these planes are aligned and strained into coherency. This analysis, which can be generalized to other high angle grain boundaries, provides further insight concerning the boundary structure at the facet length scale by showing how a periodic array of interfacial disconnections relieves the coherency strain and dictates the final interfacial inclination. These results help establish how dislocation-based descriptions for grain boundary structures can be related between the high- and low-angle misorientation regimes.

The five-parameter grain boundary character distributions (GBCDs) of three polycrystalline metals with the fcc structure (Ni, Cu, and Au), processed to have similar microstructures, were measured and compared. The GBCDs of the metals are strongly correlated. Furthermore, they show no correlation with the grain boundary character distribution in bcc structured Mo. The results are also compared to an extensive set of grain boundary energies for Ni, Cu, and Au calculated by the embedded atom method. The results show a strong inverse correlation between the calculated energies and the observed populations. Using the computed energies, the most highly populated boundaries have populations that are consistent with the Boltzmann distribution. The results suggest that metals with isostructural crystal structures and similar microstructures have isomorphic grain boundary character distributions, allowing observations from one material to be extended to isostructural materials.

Five macroscopic parameters—boundary misorientation and plane orientation—describe the geometric structure of grain boundaries. Analogous to texture analysis, the ability to describe continuous functions of grain boundary parameters is essential in constructing statistical distribution functions and in formulating structure-property relationships in polycrystalline materials. The case of grain boundaries is complicated due to special relations that relate symmetrically equivalent geometric parameters. In this talk, a rigorous framework for developing continuous functions over a subset of the five-parameter space is presented, and referred to as the “high-symmetry single-axis grain boundary space”. This subset comprises all of the boundary-plane orientations with the misorientation axis constrained to lie along a single high-symmetry crystal direction. The single-axis grain boundary space is relevant to collections of grain boundaries in fiber-textured materials (thin films and severely extruded metals). Hyperspherical harmonics are used to construct continuous distribution functions for high-symmetry single-axis grain boundary parameters.

Crystal plasticity involves the motion of dislocations under stress. To date, atomistic simulations of this process have predicted Peierls stresses, the stress needed to overcome the crystal resistance in absence of thermal fluctuations, more than twice experimental values, a discrepancy best-known in body-centered cubic (b.c.c.) crystals. We show that a large contribution arises from the crystal zero-point vibrations, which increasingly accelerate dislocation motion for temperatures decreasing below half Debye temperature. Using Wigner&’s quantum transition state theory in atomistic models of crystals, we found a large decrease of the kink-pair formation enthalpy due to the quantization of the crystal vibrational modes. Consequently, the flow stress predicted by Orowan law is strongly reduced compared to its classical approximation and in much closer agreement with experiments. Other thermally-activated processes involving defects in crystals where quantum effects might play a role will be reviewed.

A phase field model has been developed to simulate the sintering behaviour of uranium dioxide and surrogate ceramics. The model takes into account different sintering mechanisms such as volume diffusion, grain boundary diffusion, surface diffusion and grain boundary migration. Direct relations between model parameters and material properties have been established through analytical means, which enables us to study the sintering and grain growth processes quantitatively with material specificity and over actual experimental time scales. The model takes into consideration the interaction between the pores and the grain boundaries, which is known to reduce the grain boundary migration in ceramics. The model successfully captures the different stages of sintering. Application of the model to uranium dioxide and ceria shows that the grain growth is controlled by the pore mobility. It also shows that the sintering rate is higher in ceria than in uranium dioxide for the same thermal conditions and initial microstructure. The effects of temperature and initial microstructure on the sintering and grain growth processes as predicted by the model agree well with experimental data. This research was supported as a part of the Energy Frontier Research Center for Materials Science of Nuclear Fuel funded by the U.S. Department of Energy, Office of Basic Energy Sciences under award number FWP 1356, through subcontract number 00122223 at Purdue University..

Understanding the mechanisms of oxygen defect clustering in uranium dioxide (UO2) is an important step towards modeling of microstructure evolution in this material under irradiation. In this work, Parallel Kinetic Monte Carlo (KMC) and thermodynamic analysis are used to investigate the dynamic clustering of oxygen defects in UO2 under hypostoichiometric, stoichiometric and hyperstoichiometric conditions, where the predominant defects are vacancies, Frenkel pairs and interstitials, respectively. A hierarchical approach is applied to describe the multiscale process from fast diffusion of vacancies to slow migration of interstitials and rare event of Frenkle pair generations. The results of this work such as the diffusion coefficients and cluster distribution will be compared to available experimental data. This work is supported by the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number FWP 1356.

Properties, such as non-planarity of 1/2[111] screw dislocation cores, are qualitatively the same in all BCC metals since they are principally of crystallographic nature. However, quantitatively structures and properties of crystal defects vary from material to material since they relate to the electronic structure and thus bonding that varies between different BCC metals. Comparative investigation of crystal defects in different transition metals is therefore important for understanding the underlying reasons for differences in their behavior. Although experiments provide valuable information, detailed structure of defects can only be examined through atomistic simulations with reliable interatomic potentials. In this talk, we present bond-order potentials (BOPs) constructed recently for a number of BCC transition metals (Mo, Ta, W, Nb, V, and Fe). The BOPs, a semi-empirical scheme based on the tight-binding model, are capable of capturing the essential aspects of mixed metallic and covalent bonding in transition metals. At the same time, the calculation can be performed in real-space, which is particularly suitable for modeling extended defects. Using BOPs, we investigate vacancies, self-interstitials, phonon spectra, as well as 1/2[111] screw dislocations that govern the mechanical behavior of BCC transition metals. In the latter case we are interested in the response of dislocation cores to different applied stresses, in particular stresses that may change the core but do not drive the dislocation motion. The goal is to reveal the reasons for the differences in the properties of different BCC transition metals, in particular in their mechanical behavior.

Calculations based on the Kohn-Sham Density Functional Theory (DFT) have been widely used to predict the band-gap levels of point defects in semiconductors. Several additional defect properties are needed in order to fully understand how defects influence the behavior of materials. We will discuss a set of ongoing efforts to predict defect properties using a general approach in which DFT at the atomistic scale is coupled with other models in order to bridge to longer length and time scales. For example, DFT results interpreted in light of a set of approximate bounds on the defect levels are combined with Franck-Condon theory in order to obtain activation energies for carrier-capture at defects. Likewise, kinetic Monte-Carlo simulations based on energies obtained from a cluster expansion fit to DFT results help us predict the effects of alloying on defect diffusion. Finally, rate equations and kinetic Monte-Carlo incorporating parameters obtained from DFT are used to investigate the effects of non-equilibrium carrier concentrations on defect diffusion and defect induced carrier recombination. The results of these calculations help us understand technologically important phenomena in solid-state lighting and the annealing of radiation damage. This work was supported, in part, by Sandia&’s Solid-State Lighting Science Energy Frontier Research Center, sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

The major objective of this research is to develop a unified physically-based representation of how the microstructure in crystalline materials, and how it affects the initiation and evolution of failure modes at different physical scales that occur due to a myriad of factors, such as dislocation-density interactions, grain morphology, grain heterogeneous microstructures, junction formation, and grain boundary misorientations and distributions. This dislocation density based multiple-slip crystal plasticity formulation is then coupled to specialized finite-element methods to predict the scale-dependent microstructural behavior, the evolving heterogeneous microstructure, and failure phenomena, such as shear-strain localization, and void coalescence. This methodology is then used to predict intergranular and transgranular failure and behavior for different high strain-rate failure scenarios

Nanoclustered ferritic alloys, or NFA's, consist of nanometer-size grains of metal oxides dispersed in ferritic steel. Our work to date has involved on-lattice simulation using an artificially dilated "nanocluster lattice" linked to a normal iron lattice. In this study, we expand our model of 12YWT and similar materials to include both on- and off-lattice simulation techniques. The off-lattice techniques use simple molecular statics relaxation combined with Metropolis sampling to produce simulations of off-lattice nucleation and growth phases of the nanoclusters. While significantly slower, such off-lattice techniques remove the assumptions of rigid intermolecular distances and imposed strain. They also provide a test of the on-lattice models, which show clustering behavior only for models with "nanocluster strain" (strain imposed by dilating the lattice inside nanoclusters but not outside it). The on-lattice models show appreciable levels of clustering, though the algorithm proves too inefficient to simulate cluster growth over large swaths of phase space (i.e., large numbers of Monte Carlo steps). The off-lattice simulations reveal a much higher tendency to form Ti[x]O[y] clusters than their on-lattice counterparts.

Point defects and their clusters are known to strongly affect material characteristics, like kinetics or mechanical behaviours. Knowledge of point defect properties is particularly important for nuclear materials because, under irradiation, a large amount of point defects is created and can evolve towards larger clusters. Ab initio calculations provide the most accurate way to obtain these properties. However, the long ranged elastic fields produced by the defects can make the calculated values strongly dependent on the simulation cell size. The precise computation of the formation and migration energies of the defects requires supercells in order to reduce the interaction between their periodic images. Supercells large enough to obtain converged values may be out of reach for ab initio calculations, especially in the case of self-interstitial and clusters. Here, we propose to couple ab initio calculations with the continuum theory of elasticity to model point defects and their clusters. In this approach, a point defect is represented by an elastic dipole which is deduced from the ab initio calculations. Elasticity theory is then used to model the elastic interaction between the defect and its periodic images. This allows us to remove this elastic contribution from our ab initio calculations so as to model an isolated defect. Our approach also predicts the variation with an applied stress of the defect energy landscape, without the need for a new ab initio calculation at each studied stress. The case of HCP zirconium is chosen to illustrate our purpose. Because of the above mentioned problem, the energetic landscape of the self-interstitial and the vacancy-clusters defects is still an unsolved question in this material. Our approach represents an original way to get converged values for the formation, migration, and binding energies with reasonable simulation cell sizes, thus shedding new light on the understanding of zirconium kinetic evolution under irradiation. We also examine with the same approach the influence of hydrogen impurity on vacancy-clusters formation. It allows us to understand how the presence of hydrogen can select some geometries of clusters, leading preferentially to the nucleation of basal or prismatic vacancy dislocation loops.

We have performed molecular dynamics simulations of defect processes and structural evolution following swift heavy ion irradiation in five pyrochlore compositions belonging to the Gd₂Ti_2-xZr_xO₇ system. We found that the irradiation effects ranged from the formation of disordered fluorite with minimal volume change to the development of an amorphous track with substantial volume expansion. The resistance to amorphization increased with the proportion of Zr (x). We also studied oxygen ion transport in these pyrochlores at five different temperatures and obtained the activation energies for anion diffusion as a function of composition. We observed significant changes in the cation anti-site disorder and anion migration energies with changes in the value of x. Our results indicate a strong link between fast oxygen ion conductivity and radiation tolerance. These findings will be discussed in light of prior molecular statics studies and experimental observations. This work is supported by the United States Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

A systematic inversion method to extract interatomic potentials across interfaces is presented based on a number-theoretical technique. Some concrete examples are presented in detail, such as the potentials between Ni atom and Mg ion and between Ni atom and oxygen ion are required for Ni/MgO interface. For further development and collaboration, the confusions caused by ill-posedness (in contrast, the well-posedness) are discussed by using a series of modified Möbius Inversion formulas.

Surface photovoltage (SPV) is a contactless probe monitoring illumination-mediated changes in the surface potential, primarily in semiconductors and insulators. Among many advantages SPV can offer are the abilities to detect surface states, to distinguish their charge sates and donor- vs. acceptor-like nature, and to determine conductivity type. Additional information, such as surface defect densities and their cross sections, can be deduced from the SPV transient measurements. However, when the studied materials have a complex microstructure, the transient SPV opens up new possibilities, as demonstrated in our work. We studied two different systems with nontrivial microstructure. The first - nano-porous Si layers synthesized on an n-type (100) Si and also permeated with nanoparticles of Ni and Co. The second - ZnO thin films grown on Si by atomic layer deposition at different temperatures. In these systems light-dark transient SPV was employed to monitor the dynamics of surface charge redistribution in response to illumination changes. A contact potential difference was measured by a Kelvin probe at room temperature using a white light source in a nitrogen gas environment. For all the studied samples we observed non-typical transient responses. For example, for the nano-porous Si samples sharp voltage spikes were detected in all the samples at both “light on” and “light off” events. Moreover, when transient SPV curves for all the studied samples are plotted on a log time scale, one can observe more nontrivial behavior - possible presence of more than one characteristic time. SPV transients resemble time evolution for the voltage of a capacitor in a RC-network with multiple time constants. We fitted the experimental curves employing this model to determine the time constants and other parameters involved. We argue that these different time scales reveal several distinct charge exchange mechanisms involving surface and interface defects. For example for the metal-permeated nano-porous Si specimens they may be associated with the complex microstructure giving rise to different mechanisms of charge recombination: at the top surface, in the pores, at the Si/metal contacts, as well as in the bulk of the Si wafer. On the other hand, in the ZnO/Si thin films the different channels of charge recombination could be at the ZnO surface, at the ZnO/Si buried interface, as well as inter-granular charge exchange.

Yttria-stabilized cubic zirconia (YSZ) is considered as a potential candidate of inert matrix fuel for actinide transmutation purposes. Therefore, numerous studies have been devoted to its behaviour upon ion irradiation. Recently, we demonstrated, through the use of RBS/C, XRD and TEM experiments that the damage build-up in YSZ irradiated with 4-MeV Au ions at RT occurs in three steps, and transitions from one step to the next one reveal microstructural transformations. Each step is thus characterized by specific features such as damage and strain levels and nature of radiation-defects (namely from point defects to network of dislocations). The present study aims at investigating, with the same experimental methodology based on the three above-mentioned techniques, the effect of the irradiation temperature on the response of YSZ. For this purpose, additional 4-MeV Au ion irradiations have been performed in a broad fluence range at both 500°C and 800°C. Surprisingly, it appears that a three-step disorder accumulation process still prevails above room temperature, and the corresponding microstructures are, as imaged by TEM, similar to those observed at RT. However, both the irradiation-induced disorder (RBS/C) and elastic-strain (XRD) levels are found to be lowered and the fluences at which microstructural transformations occur decreased. These results are tentatively explained by an enhanced defect-clustering rate that would allow lowering the required fluence for the defects to reach the critical size at which they transform into more stable ones.

Mono-nitrogen impurities may serve as charge-carrier traps in solar cells. WIth the use of silicon nitride passivation layers, there is a risk of formation of mono-nitrogen impurities. To address this, we study the diffusion of nitrogen impurities on a silicon lattice. Nitrogen usually exists as more stable pairs in silicon. Two mechanisms of nitrogen diffusion have been proposed in literature: (1) Diffusion of intact pairs or (2) a three-step process where a pair dissociates, the two nitrogen atoms diffuse and eventually recombine into pairs when they encounter other nitrogen atoms. Mechanism (2) is believed faster than mechanism (1) on a per-walker basis, but the large binding energy of a pair makes the pair more prevalent. In this work, we focus on mechanism (2). More specifically, we study analytically the escape probability for a nitrogen atom from its partner when they split up and the average distance traveled by the walker from dissociation until recombination. We also apply the method to the random walk of nitrogen atoms in phosphorous-doped silicon.

Li-ion batteries are appealing for a prolific variety of applications as they provide higher energy density when compared to other rechargeable batteries. The use of Li-metal as negative electrode improves the specific capacity but raises safety issues due to the reactions that take place with the gel-liquid electrolyte. Dendrites tend to grow establishing a preferred conduction channel leading to shortcuts. One of the possible ways to overcome this problem is to use solid electrolytes. A solid electrolyte material must have high ionic conductivity and be a good electronic insulator. Ionic conductivity occurs by means of ions hopping from site to site through a crystal structure; therefore it is necessary to have partial occupancy of energetically equivalent or near-equivalent sites. In favorable structures, the defects may be mobile, leading to high ionic conductivity. There are a small group of materials in which defect creation by doping is unnecessary since, in the parent stoichiometric crystal, there is already extensive disorder in the mobile ion sublattice above 0 K. An example of the latter solid electrolytes is the high temperature polymorph, α-AgI. The number of stoichiometric compounds that present technically useful order-disorder transitions is extremely limited; therefore most attempts for designing the solid electrolytes rely on chemical doping. We have analyzed the hopping movement, calculated activation barriers, established the role of the lattice, and calculated thermodynamic properties by means of first principles and phonon calculations at working temperatures. Moreover, we present experimental data for novel solid state ionic conductors for which we have determined that doping the material enhances the ionic conductivity in more than one order of magnitude from room temperature and up to ~230 °C.

The mono-quasicrystal stable is depend on the difference of different metal bond at surface. The quantum tunneling of potential well of different metal bond is key factor that the mono-quasicrystal can be prepared. The paper tries to discuss the relation between each quasicrystal and its edge bond difference which lead quantum tunneling effect. The process can be explained by my calculation.

During the last years carbon-based nanostructures (such as, fullerenes, carbon nanotubes and graphene) have been object of intense investigations. The great interest in these nanostructures can be attributed to their remarkable electrical and mechanical properties. Their inorganic equivalent structures do exist and are based on boron nitride (BN) structures. BN fullerenes, nanotubes and single layers have been already synthesized. Recently, the fracture patterns of single layer graphene [1] and multi-walled carbon nanotubes [2] under stress have been studied by theoretical and experimental methods. In this work we investigated the fracturing process of defective carbon and boron nitride nanotubes under similar stress conditions. We have carried out molecular dynamics simulations using the ReaxFF [3] force field. Our results show that the fracture patterns and von Mises stress profiles are completely different for carbon and BN tubes. The origin of these differences are explained in terms of bond order energies and chiralities. [1] Kim, K.; Artyukhov, V. I.; Regan, W.; Liu, Y.; Crommie, M. F.; Yakobson, B. I.; Zettl, A., Nano Letters 2012, 12, 293-297. [2] Kosynkin, D.; Higginbotham, A.; Sinitskii, A.; Lomeda, J.; Dimiev, A.; Price, B.; Tour, J. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature 2009, 458, 872-876. [3] Van Duin, A.; Dasgupta, S.; Lorant, F.; Goddard III, W., J. Phys. Chem A 2001, 105, 9396-9409.

Rare-earth manganese oxides (manganites) are of great scientific and technological importance. In particular, many hole-doped manganites in the perovskite structure show the famous colossal magnetoresistance effect. At the same time, a number of hexagonal manganites are currently under extensive investigations as interesting multiferroics. Due to the strong correlation effects and competing orders, self-organized microstructures are widely observed in these systems. Using a newly developed microwave impedance microscope, we are able to spatially resolve the glassy percolation network in (NdSr)MnO3 and (PrCaSr)MnO3 thin films through the metal-insulator transition. The ordering of the metallic phase along certain crystal axes provides compelling evidence that the substrate strain plays the dominant role. The same electrical imaging tool can also be applied to study the improper ferroelectrics ErMnO3 and YMnO3, showing that the domain walls and topological defects are more conducting than the bulk. Continuous research along the line is expected to shed some light on understanding the complexity of manganites, which will lead to practical applications of these fascinating materials.

Ever since the development of novel quantum computing algorithms, the promise of a general purpose quantum computer has attracted both scientists and engineers to the practical problem of realizing such a device. Superconducting qubits (SQs) represent a promising route to achieving a scalable quantum computer. However, the coupling between electro-dynamic qubits and (as yet largely unidentified) ambient parasitic noise sources has so far limited the functionality of current SQs by limiting coherence times of the quantum states below a practical threshold for measurement and manipulation. Further improvement can be enabled by a detailed understanding of the various noise sources afflicting SQs. Sapphire (α-Al2O3) is commonly used as a substrate for SQs and in this study we have investigated the magnetic stability of its surface by using first principles calculations. We have identified a plausible microscopic origin of magnetic noise arising from the substrate. In addition, the effects of both intrinsic and extrinsic defects on the quality of the substrate in SQ applications have been investigated. We will discuss the physical origin of this magnetic noise source as well as strategies for ameliorating its effects. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Starting from the atomic jump frequencies we use the Self Consistent Mean Field (SCMF) diffusion theory and Atomic Kinetic Monte Carlo (AKMC) to describe the alloy transport properties controlled by the vacancy and interstitial diffusion mechanisms. With the examples of Fe-Cr, Fe-C and Ni-Si model alloys, we show that diffusion properties are particularly sensitive to the variation of atom jump frequencies with the local atomic environment. We also insist on the importance and the difficulty of estimating physical point defect concentrations during diffusion and phase transformations in alloys. Due to its non-conservative nature, the point defect concentration is related to the microstructure formation. A mean field approach and related AKMC simulations yield a detailed picture of the complex kinetic coupling between vacancy and atoms occurring during a phase separation. A generalized atomic diffusion equation associated with non-uniform concentration fields is deduced.

The age-hardenable Al-Mg-Si-(Cu) (6xxx) alloys are widely used for industrial purposes due to properties like a high strength-to-weight ratio, good formability and resistance against corrosion. They display a large increase in hardness upon aging at elevated temperatures, which is attributed to the formation of a high density of nanosized precipitates emerging from supersaturated solid solution - particles that slow the overall dislocation movement in the host material. The ultimate goal is to obtain computational control over the microstructure whereby related physical properties can be estimated with sufficient precision. In principle this would allow for precise optimization of the alloy composition and the thermo-mechanical procedure for any product. In the mean time 'alloy and process design&’ used to tailor materials to the desired properties will have to rely on some limited set of the quantified parameters. Precipitate structure and morphology, coherency with the matrix and habit planes, as well as the precipitate sizes and numbers are parameters deciding the properties. For a number of years we have been focusing on solving precipitate crystal structures, as the different structures correlate strongly with strength. Atomic scale knowledge is a prerequisite for improved understanding of the physics governing the hardening process, especially so since it facilitates ab-initio calculations and modeling. Experimental tools used are advanced transmission electron microscopy (TEM) methods (quantitative nanobeam diffraction and high angle annular dark field scanning TEM) in order to determine the precipitate structures and interfaces of the metastable phases. This is combined with theoretical density functional studies, to determine the composition and give corrections and support to solve the structure [1]. Generally, experiments report at least one fully coherent precipitate-matrix interface for the metastable phase under investigation. Theoretically this may dictate a vanishing misfit between the cell dimensions of the bulk precipitate and the matrix along this given direction, in conjunction with the usual formation enthalpy minimization [2]. This approach upgrades the first-principles analysis from being a tool for structure verification to becoming a nearly independent tool for composition determination of metastable precipitates. Studies of precipitates in Al-Mg-Si alloys with small and higher additions of Cu will be presented. Also the effect of 10% pre-deformation on the precipitation microstructure will be discussed. [1] Hasting HS, Froslash;seth AG, Andersen SJ, Vissers R, Walmsley JC, Marioara CD, Danoix F, Lefebvre W, Holmestad R J. Appl. Phys. 106, 123527, 2009. [2] Torsaelig;ter M, Ehlers FJH, Marioara CD, Andersen SJ, Holmestad R Phil. Mag., DOI: 10.1080/14786435.2012.693214, 2012.

A material under sustained irradiation has non-equilibrium concentration of defects. At operating temperatures of nuclear reactors, dynamics of these defects result in microstructural evolution, phase transformations and local composition changes. In concentrated alloys, inverse Kirkendall effect (IKE) is found to be responsible for local composition changes: IKE is observed when defect fluxes preferentially drive certain atom species into or out of the local regions. In presence of defect sinks, radiation-induced segregation (RIS) of alloying species is observed in regions close to these sinks. Radiation-induced segregation near grain boundaries has been identified to be one of the reasons behind irradiation-assisted stress corrosion cracking, which increases the susceptibility of the material to inter-granular cracking. In addition, RIS has also been observed to affect evolution of void microstructures, change in sink strengths, etc. After a brief review of the literature, a model of defect and species dynamics in binary alloys under irradiation will be presented. This model consists of a set of reaction-diffusion equations with a stochastic, spatially-resolved, discrete defect generation term, representing cascade damage. An important feature of this model is that the boundaries have not been explicitly considered as defect sinks, which is significantly different from the way radiation-induced segregation has been studied in the literature. Instead, the role of boundaries as defect sinks has been ensured by defining defect-boundary interactions via a set of reaction boundary conditions. Defining defect-boundary interactions in this way makes it possible to capture the process of segregation as a consequence of boundary motion. The model is tested with Cu-Au alloy on a 2D slab geometry in which y-direction is taken to be periodic; material surface is free to move along x-direction. Gear's method has been used to integrate the system of reaction-diffusion equations in time. Degree of Cu enrichment and Au depletion has been observed to be spatially non-uniform in areas close to the boundaries. For a given dose rate, maximum degree of segregation has been observed at an intermediate temperature; it decreases when temperature is raised further. When the dose rate is decreased, a maximum degree of segregation is obtained by lowering the temperature, whereas when dose rate is increased, the maximum degree of segregation is found when temperature is raised. This work is supported by the U.S. Department of Energy as a part of a Computational Materials and Chemical Sciences Network project entitled "Computational Microstructure Science."

We describe a recently developed molecular dynamics approach for the simulation of the kinetic evolution of irradiation-induced point defects in predominantly ionic materials, such as MgO, UO2 and CeO2. By directly inserting Frenkel pairs into either or both sub-lattices of the crystal lattice and following their kinetic evolution, the approach aims essentially to capture the effects of electron irradiation. As in such experiments, the approach enables ‘selective sub-lattice irradiation&’, thus enabling deconvolution of the interplay between the distinct point-defect populations on the two sub-lattices in the overall lattice response. Our simulations reveal remarkably different cluster structures formed in different materials (e.g., neutral vs. charged, and two-dimensional loops vs. three-dimensional clusters). Analysis of the underlying cluster-formation mechanisms reveals in all cases a common, intricate partial self-healing response of the perfect crystal to the radiation-induced defects: namely, the lattice responds to point defects created during irradiation with the spontaneous creation of new point defects that lower the overall energy while neutralizing the cluster by screening their long-range Coulomb potential, thereby localizing the damage.

The Gibbs free energy of point defect formation is the key quantity for the derivation of defect concentrations. However, an accurate determination of defect formation energies over the entire relevant temperature window, i.e. from T = 0 K all the way up to the host melting temperature, is challenging both for experiment and theory. Experimentally, it is only possible to measure defect concentrations in a certain high temperature window due to infeasible long equilibration times at lower temperatures. The assessed high temperature range shows in good agreement an Arrhenius like dependence and therefore a linear extrapolation is performed to low temperatures, i.e., the defect entropy is assumed to be temperature independent. Theoretically, the challenge lies in the various entropic contributions that may become important at high temperatures such as electronic, harmonic and anharmonic excitations and that are computationally expensive. Consequently, in most theoretical studies on point defects only configurational entropy has been considered. Recent methodological advances provide now the opportunity to compute all these excitation mechanisms with high precision on a fully ab initio basis, making it possible to derive free energies for bulk systems and defects up to the melting temperature [1]. Applying these approaches to vacancies in Al and Cu we were able to derive vacancy formation energies over the entire temperature range. An analysis of these results shows that in particular the inclusion of anharmonic contributions gives rise to a hitherto not expected magnitude of non-Arrhenius effects. We show that the presence of these effects is not observable in conventional experiments measuring vacancy concentrations. Furthermore, we discuss the implications such effects have on enthalpies and entropies of formation. For example, it will be shown that non-Arrhenius effects change defect formation energies by several 0.1 eV. Even more stunning, the defect entropy is found to change by more than an order of magnitude, e.g., for the Al-vacancy from 0.2 to 2.3 kB. [1] B. Grabowski, L. Ismer, T. Hickel, and J. Neugebauer, Phys. Rev. B 79, 13416 (2009)

Ion implantation of single-crystal diamonds has garnered considerable interest in the last few decades due to the potential applications in quantum computing, photonics, and diamond devices. One particularly promising technique involves implantation of diamond crystals with light ions, resulting in a thin layer of buried lattice damage. This is followed by an annealing step, which sharpens the interface between the damage layer and the diamond lattice. Subsequent patterned implants and chemical etching allow for the fabrication of free-standing single-crystal diamond nanostructures. Clearly, if this technique is to be used for fabrication of photonic and optically-based quantum information devices, a detailed understanding of the effects of ion implantation and annealing on the opto-electronic properties of the diamond lattice is necessary. Coherent Acoustic Phonon (CAP) spectroscopy is an ultrafast optical method well suited to study these phenomena. CAP experiments are an advancement of standard ultrafast optical pump-probe techniques, and provide detailed depth-dependent information about the optical and electronic properties of materials in a non-destructive fashion. We will describe a systematic study of the modulation of CAP oscillation patterns in diamond at a variety of He+ implantation doses, and extract relationships for how the implantation modifies the complex index of refraction of the diamond lattice, as well as its first derivative with strain. Comparing these results with computer simulations of implantation damage profiles yields a calibration relationship between optical properties and induced damage. Comparison to cross-sectional high-resolution TEM shows that the spatial extent of the optical modification is much greater than the ion implantation-induced structural modification. These results will aid in the fabrication of photonic and quantum devices based on single-crystal diamonds.

The microstructural features, such as interfaces, GBs and dislocations, in irradiated materials have substantial effects on defect generation and annihilation. Tersoff and ReaxFF potentials are used to comparatively study defect process in SiC containing two-dimensional (2D) Si and C nano-dispersions (ND), and to investigate space charge layer effects on defect generation and annealing, as well as defect-interface interaction mechanisms. Specifically, molecular dynamics is employed to simulate defect generation in the vicinity of the interfaces between Si or C nano wire and SiC, and to determine cascade-interface interactions. The annealing using both potential models reveals that the damage within the Si nano-wire can be annealed out at 1.1 ns. There are several mechanisms for this striking phenomenon. Defects produced in Si nano wire dispersed SiC can be self annealed out at room temperature, where mobile vacancies provide an efficient mechanism for defect annihilation and inhibits interstitials escaping from cascade volume. In addition, space charge layers at the interfaces alternate the potential surfaces for interstitials and vacancies to migrate and promote defect annihilation at the interfaces, leading to the self healing of defects. The present simulations are compared to those in (NC) SiC, and shed light on the design of radiation-tolerant materials for applications in extreme environments.