High-rate plastic deformation is the subject of increasing experimental activity. High power laser platforms such as the National Ignition Facility offer the possibility to study plasticity at extremely high rates in shock waves and, importantly, in non-shock ramp-compression waves. Here we describe the development of the theory of the kinetics of governing the relaxation of a 1D elastically compressed state to a 3D state due to plastic flow, and explain how the results of molecular dynamics (MD) simulations inform the analytic theory. This theory is compared with experiment, the MD simulations and other plasticity models. In the MD simulations we focus on the high-rate deformation of multi-million atom (nanoscale) tantalum systems at pressures up to a few Mbar. We also report an analytic theory of the rate of plastic relaxation associate with dislocation flow and compare it to the MD simulations. We compare both with laser-drive Rayleigh-Taylor flow experiments that allow inference of material strength and in-situ x-ray diffraction experiments that measure lattice compression and the associated timescales. We show that theoretical calculations of the plastic relaxation time are consistent with the experimental bounds. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

A suite of plate-impact experiments have been conducted to determine the dominant factors in dynamic damage evolution in uniaxial-strain, tensile (spall) experiments. The first group of experiments addresses the effect of microstructure by using copper samples with varying grain sizes while maintaining similar loading conditions (peak compressive stress ~1.6 GPa). In a second set, the density of grain boundaries in copper samples and the compressive stress (~1.6 GPa) were held constant while the tensile loading characteristics were tailored by controlling the geometry of flyers and targets. For similar loading conditions, the damage fields were observed to depend on the grain boundary density: void growth and coalescence were observed to dominate the damage behavior in samples with either small (30um) or large grained specimens (200um); whereas within intermediate grain sized specimens (60 and 100um), most of the damage was restricted to individual voids. For the second portion of the study the characteristics of the damage fields were observed to strongly depend on the characteristics of the tensile pulse. In this case, an increasingly large plastic dissipation, in the form of grain misorientation, and more advanced stages of damage growth and coalescence were observed in the samples deformed at lower tensile stress rates.

This co-design project aims at addressing the challenges presented by the advent of exascale computing resources by forging a qualitatively new predictive-science capability that exploits evolving high-performance computer architectures for applications in materials science by simultaneously evolving the four corners of science, methods, software and hardware. Our test application is a multiscale code based on the Heterogeneous Multiscale Method (HMM) applied to shock compression of materials. Multiscale applications are particularly adapted to heterogeneous computing because the different scales studied in materials science can be linked to the different levels in the computer architecture. In this presentation, we will introduce the concept of co-design as well as our approach, before giving a brief overview of our application with HMM and presenting our first results on the compression of copper samples. Comparisons to full Molecular Dynamics simulations and other HMM simulations on traditional computer architectures will be included.

The mechanical behavior and microstructural characteristics of a high-performance construction steel, ASTM A913, under blast loading conditions at elevated temperatures is a topic of interest for blast resistant infrastructure. The resulting high deformation rates, greater than 10s^-1, have the propensity to create localized shear in metals. This localization of shear forces, occurring in such short times, can create adiabatic shear bands (ASBs). Practically, ASBs are unfavorable as they are more brittle than the surrounding matrix, and hence are sites for crack initiation and propagation. Drop-weight and Split Hopkinson Pressure Bar experiments generate strain rates sufficient to create ASBs and are carried out in conjunction with ASTM fire profiles. The resulting microstructural changes and differences in relative hardness of ASB regions compared to the matrix are measured with optical microscopy and nano-indentation techniques. During subsequent heating curves (as in prolonged fire events), it is possible that ASBs may become “annealed.”

Previous molecular dynamics (MD) simulations have revealed that the preferred nucleation sites for partial dislocations at grain boundaries (GB) are related to the local atomic interface structure which already indicates the crucial role of the GB structure at high stresses. Moreover, shock experiments discovered different post-mortem defect structures for low-energy and high-energy grain boundaries. In the present study, MD simulations are conducted to understand the structural origin of the differences in GB response under shock compression. We present shock MD simulations in copper bicrystals with misoriented crystals, which corresponds to recent shock experiments of a columnar polycrystal. The defect evolution in the MD studies is compared with the experimental post-mortem TEM results, and the different GB responses are discussed. In particular we will address the structural origin of superior failure resistance of twin GBs in a polycrystalline interface network. More general, we will discuss the current understanding of the role of interfaces in the framework of stress-driven interface motion and its implication for shock loading conditions, where the driving forces for GB motions are orders of magnitude higher compared to the conventional GB mobility regime at elevated temperatures and long time scales.

Hydrocarbon polymers, foams and nanocomposites are increasingly being subjected to extreme environments. Molecular scale modeling of these materials offers insight into failure mechanisms and complex response. Classical molecular dynamics (MD) simulations of the principal shock Hugoniot will be presented for two hydrocarbon polymers, polyethylene (PE) and poly(4-methyl-1-pentene) (PMP). We studied two reactive and two non-reactive classical MD interaction potentials. We show that the ReaxFF of van Duin et al. has the best agreement with experiment. We compare these results with recent DFT calculations and experiments conducted at Sandia National Labs. DFT results were in excellent agreement with experiment unto 100s of GPa, while the best MD results showed agreement up to approximately 50 GPa. We extend these results to include low-density polymer foams using nonequilibrium MD techniques. We find good quantitative agreement with both experiment and hydrocode simulations. Further, we have measured local temperatures to investigate the formation of hot spots and polymer dissociation near foam voids. Nanoparticle-doped polymer and foam results will also be presented and compared to recent experimental results. 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 expanding cloud of explosively disseminated materials, sepecifically powers and liquids, exhibits a uniform ‘spiky&’texture arising from the prolific and regular jetting. In this work, we investigate the hydrodynamic behavior of those jets consisting of wet powders with viscous binders, including the onset and development of jetting as well as the jet size distribution. Although the break-up of exploding wet powders with varying binder content can be characterized by the Mott-Linfoot distribution, the mean jet size gets significantly reduced with the increasing liquid content. Opposed to the Rayleigh-Taylor (RT) instability accounting for the ‘billowing&’ fireballs from standard condensed-phase explosives, which cannot explain the break-up features of wet powders, we carry out stability analyses based on the plane strain condition to predicte the onset of the breakup and the size of jets. An instability criterion is proposed and developed based on the opposing forces of stabilizing inertial pressures and destabilizing viscous resistance which decreases with the liquid content.

The dynamic behavior of SiC ceramics under severe plane shock loading is investigated using multi-million-atom molecular-dynamics simulations. We reveal the interplay between shock-induced compression, structural phase transformation (SPT) and plastic deformation. The calculated shock Hugoniot for a wide range of particle velocity (U_p) from 0.1 km/s to 4 km/s agree well with experiments. The dynamic response of single crystalline models indicate no induced plasticity or SPT for Up < 2 km/s. The generated elastic wave travels above the longitudinal sound velocity (V_L) at ~V_L+0.7*U_p km/s in agreement with experiments. For intermediate particle velocity, 2 km/s > U_p > 3 km/s, the generated shock wave splits into an elastic precursor and a zinc blend-to-rocksalt SPT wave. That is induced by the increase in shock pressure to over 90 GPa and results in increase of density to ~4 g/cm³. For U_p > 3 km/s a single overdriven transformation shock wave is generated. We compare these results with those of plane shock loading on nanostructured SiC models.

The design of explosives for the destruction of biological agents requires detailed knowledge about the temperature distribution inside the post-detonation fireball. Optical pyrometry and spectral line fitting provide temperature data from near the surface of the fireball. Accessing information about the conditions inside the fireball requires new approaches. We recently reported on the initial development and testing of Eu-doped Y2O3 nanoparticles that were seeded into the fireball. The initial nanoparticles are amorphous in nature and consist of carbonate precursors. Exposure to the heat inside the fireball changes the amorphous structure of the material. The nanoparticles are collected at the end of the explosion test and subsequently analyzed. The optical signatures from the dopant ions provide information about the amorphous structure of the material. This information is compared with reference samples which have been heated under controlled conditions in a pyroprobe heater. We have refined the synthesis conditions of the nanoparticles and eliminated several uncertainties in determining the reference data. New calibration curves and their uncertainties for the reference samples were completed. A new set of explosion tests was conducted at the NSWC Indian Head. The optical signatures of these new test samples were measured and their characteristic spectral peak width and peak position determined. Comparison with the reference data yields the temperature that the samples were exposed to. We compare these optically determined temperatures with temperatures measured by thermocouples during the explosion. The Eu:Y2O3 samples which rely on changes in the amorphous structure are suitable for temperatures up to near the crystallization temperature. Structural changes due to temperatures beyond this crystallization point are difficult to quantify. As such, we are also utilizing Eu-doped ZrO2 nanoparticles with the goal to measure temperatures beyond the crystallization point of these nanoparticles. When ZrO2 crystallizes, it forms three different structural phases, monoclinic, tetragonal, and cubic. The ratio between the tetragonal and the monoclinic phases can provide information about the temperature that the samples were exposed to.

This work investigates the shock response of composite pellets, which utilize a low melting (Bi) and a high melting (W) material. Samples were mixed using low-energy ball milling, followed by uni-axial pressing with and without heating. A shock wave was driven into the samples using a high energy laser drive, and the shocked samples were collected for post-mortem analysis. On the laser drive side, we observe several hundred micrometer deep craters, which presumably form as Bi is shock-melted, and material is unloaded as tensile stresses develop from the release wave interactions. The spall surface showed various behaviors, from no damage to large spall regions, depending on the composition of the sample. We find that the depth of the crater (i.e. the melting depth) is primarily governed by the composition and sample porosity. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-561454

Phononic materials, due to their unusual acoustic dispersion properties, are potentially interesting platforms for mitigation and manipulation of shock waves. It is currently not understood, however, to what extent the presence of band gaps in phononic structures influences the propagation of a non-linear superposition of acoustic waves that is a shock front. We seek to investigate the interactions by the in situ observation of shock waves incident on open-cell phononic structures. Utilizing cylindrically focused pulsed laser excitation, we are able to generate a propagating plane-wave shock within an absorbing fluid medium, which is then targeted onto periodic or quasiperiodic microstructures fabricated by multiphoton lithography (MPL) direct write. The shock may be imaged by a separate probe pulse, allowing for direct visualization of the interaction (transmission/reflection) with picosecond snapshots. Alternatively, a streak camera may be employed to track the average amplitude of the shock wave through the duration of the interaction event, albeit with reduced detail. Further, the use of MPL allows for the quick adaptation of fabrication parameters to rapidly test many different pattern arrangements and characteristic sizes. In this manner, we are able to determine the nature and degree of phononic manipulation of shock waves by a given structure. In addition, we compare these results to finite element simulations of linear acoustic pulse interactions to elucidate the degree to which new behaviors occur that are specific to non-linear excitation.

The critical shortage of abundant, affordable, and clean energy calls upon novel materials that are superior to any known material existing today with extreme properties for energy production, storage, conversion, and transfer. The extreme pressure-temperature (P-T) environments present a vast, unexplored, fertile ground to search for transformative materials and phenomena. The Center for Energy Research in Extreme Environments (EFree) established in 2009 has set a mission on accelerated discoveries of novel materials and phenomena in the extreme environments and recoveries of favorable properties for energy applications, and made advances in the following thrusts. 1. Novel Chemical Energy. 2. Novel Superconducting, Electronics, and Magnetic Materials. 3. Novel Nanophase, Mesoporous, Amorphous, and Structural Energy Materials. To recover the novel high-pressure materials metastably at ambient pressure far away from equilibrium is central to the EFree approach. Mounting examples reveal that the combination of high pressures and low temperatures not only brings matter, but also sustains matter, very far away from equilibrium. In these studies, pressure provides a powerful means for continuously tuning the free energy of the system; x-ray photons excite systems into highly metastable states, and low temperature and chemical tuning prevent the system from reaching equilibrium. By varying these parameters, characterizing the dynamically compressed or stressed and electronically excited materials with time-resolved probes, and combining these efforts with first-principles calculations, transition mechanisms and energy landscapes can be revealed. and Structural Energy Materials Under Extreme P-T. To recover the novel high-pressure materials

Due to the light weight, the abundance and the environment friendliness of Magnesium, Mg-based hydrides stand as promising candidates for solid state hydrogen storage. In the perspective of developping reversible compounds that show fast sorption kinetics and mild operation temperature, research has been directed towards development of new materials based on the combination of Mg and Transition Metal (TM) elements. Actually, reactive ball milling and high pressure anvil cell have been used to synthesize ternary Mg-TM-H phases in spite of the Mg-TM immiscibility and absence of binary metal compound in the binary Mg-TM system. Among the Mg-based ternary compounds, the Mg6~7TMHx (TM = Ti, V, Ta, Hf, Zr and Nb) phases are of particular interest not only because of their competitive properties for hydrogen storage (Mg7TiHx contains 6 wt.% of H2 released from 605 K [2]), but also from a more fundamental point of view. These compounds have only been synthesized under high pressure (HP) high temperature conditions (6-8 GPa, 873 - 1073 K), and can be described as a superstructure of the HP cubic polymorph of MgH2. The presence of hydrogen and partial substitution (1/8 Mg atoms) are responsible for the stabilization, at ambient conditions, of the HP cubic polymorph of MgH2. Several theoretical calculations and experiments are carried on in order to manage to stabilize the metal lattice after H2 has been released and improve the reversibility of these compounds. In this work, structural characterizations were performed by in-situ X-ray angle dispersive diffraction by use of a laboratory set-up. A monochromatic (lambda=0.56Å) micro-focused X-rays source was illuminating a specific cell assembly [3] fit on a Paris-Edinburgh press. The MgH2:TiH2 system was investigated in the pressure range 0- 10 GPa and in the temperature range 300- 1400 K. Results concerning MgH2, TiH2 and 7 MgH2: TiH2 compositions will be presented. References [1] D. Kyoi, T. Sato, E. Rönnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Comp. 375 (2004) 253. [2] D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 372, 213-217(2004) [3] G. Morard, M. Mezouar, N.Rey, R. Poloni, A. Merlen, S. Le Floch, P. Toulemonde, S. Pascarelli, A. San-Miguel, C. Sanloup And G. Fiquet; High Press. Res. 27 (2007) 223.

We present ab initio calculation of the phase diagram and equation of state of Ta in a wide region of volume-temperature phase space, with volumes 9-180 ų/atom, and up to 20,000K, and 7Mbars. All calculations are based on first principles, using thermodynamic integration and modelling, and a single experimental input of the melting point at atmospheric pressure. We cover solid, fluid, and gas single and two-component phases. In the high density region we construct the melting curve; in the low density region we present the first ab initio calculation and analysis of the gas-vapor critical point and its surrounding. Our results show excellent agreement with existing experimental data, and are compared against previous theoretical predictions.

In-situ high temperature and high pressure Raman and Brillouin spectroscopy have been used to study the structure and elastic properties of silica-rich glasses quenched from temperatures near the glass transition under pressures up to 4 GPa. In-situ high temperature light scattering experiments were carried out by using a newly developed emulated platelet light scattering geometry in a Linkam TS1500 optical furnace (maximum temperature of 1500C). In-situ high pressure experiments were carried out in membrane-driven diamond anvil cell. A solid-medium piston-cylinder apparatus was used in the pressure-quenching experiments to prepare densified glasses. Pressure-quenched glasses have permanently “densified” characteristics, with density, elastic moduli and hardness increase dramatically. With the increase of the quenching pressure, the anomalous behaviors (positive temperature derivative and negative pressure derivative of elastic moduli) in silica-rich glasses gradually diminish. Detailed studies of the relaxation of densified glasses as a function of time and annealing temperature will be presented as well. Our studies show that after densified glasses relax back to the undensified state based on the elastic properties and the major characteristic bands in Raman spectra, yet the D1 line (~492 cm-1, generally associated with the symmetric oxygen breathing vibrations in 4-member rings) seems to take much longer time to recover.

The natural calcium phosphate apatites are important minerals to geochemistry and biology [1,2]. Rare earth silicates with the apatite structure have been widely investigated due to their potential applications as catalyst [3], ionic exchangers [4], oxygen ion conductors and luminescent materials [5]. As a new class oxygen ion conductor, lanthanide silicate apatite shows even higher oxygen conductivity at moderate temperature than that of the stabilized zirconia [6]. Most of the rare earth silicate apatites have a structure isotropical with calcium phosphate, which crystallize in a space group of P63/m. It has a very open structure, where all the SiO4 tetrahedra are separated and stack along the c-axis. There are oxygen channels in the apatite structure paralleling the c-axis and previous research indicated that the interstitial oxygen ions in these channels play an important role for the interpretation of the high oxygen conductivity for these kind of materials, though the high oxygen ion conduction mechanism generated by the cation vacancy has not yet been fully understood. The lanthanide silicates RE9.33Si6O26 (RE: La, Nd, Ce, Eu, Sm, Gd), which has the hexagonal apatite structure (P63/m), were investigated at high pressure. A subtle phase transition was observed at pressure from sim;13.3 GPa to ~25 GPa by in situ synchrotron x-ray diffraction and confirmed by infrared absorption and Raman scattering measurements. The high-pressure phase has a structure similar to that of the initial hexagonal apatite structure, but the symmetry is reduced to P63 through the displacement of one oxygen site and tilting of the SiO4 tetrahedra. Interestingly, the high-pressure phase of La-Si-O system has an abnormally lower compressibility, which is caused by the change in symmetry that allows the tilting of the SiO4 tetrahedra, and the bulk modulus of the high-pressure phase is only half that of the apatite structure. The structural behavior of these apatite-type silicates was also studied by quantum mechanical calculations. References 1M.J. Kohn, T.E. Cerling, Rev. Mineral Geochem. 48, 455 (2002). 2G.H. McClellan, J.R. Lehr, Am. Miner. 54, (1374) 1969. 3T. Wakabayashi, S. Kato, Y. Nakahara, M. Ogasawara, S. Nakata, Catal. Today 164, (575) 2011. 4L. Leon-Reina, E.R. Losila, M. Martinez-Lara, S. Bruque, M.A.G. Aranda, J. Mater. Chem. 14, (1142) 2004. 5S. Ferdov, R.A. Sa Ferreira, Z. Lin, Mater. Chem. 18, (5958) 2006. 6 S. Nakayama, M. Sakamoto, J. Eur. Ceram. Soc. 18, 1413 (1998).

Ammonia borane is considered to be a promising hydrogen storage material due to its high hydrogen density and mild hydrogen desorption conditions. We conducted in situ Raman spectra and Infrared (IR) measurement on ammonia borane loaded in diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 degree Celsius under the pressure ~0.7 GPa. Raman spectra show the hydrogen was desorbed within 5 minutes at 140 degree Celsius. The hydrogen was sealed in the DAC well and cooled down near to room temperature. After applying higher pressure up to 17 GPa shows interactions between the products and no rehydrogenation was detected.

Tungsten is the primary material choice as the plasma-facing material for the divertor region in the International Thermonuclear Experimental Reactor (ITER). [1] ITER is designed to produce 500MW of nuclear fusion power sustained for up to 1000 seconds. Although these strong plasma-facing fusion material properties are well suited to the application, blistering and embrittlement of tungsten due to high exposure to hydrogen and helium irradiation is a serious materials challenge for the viability of these materials in the ITER plasma. [2] In the last few years, it was shown [3] that other microstructural modifications such as nanostructure formation can also occur on the surface of tungsten under helium irradiation. Suppression of point defect accumulation by annihilating the freely migrating defects (interstitial and vacancy) to defect sinks such as grain boundaries is believed to lead to higher helium fluence threshold values of bubble formation and nanoscale structure formation in tungsten. Increasing the grain boundary area requires the formation of ultrafine or nanocrystalline tungsten materials. Moreover, in-situ irradiation-transmission electron microscopy (TEM) experiments are crucial tests to address the importance of grain boundaries and the nanoscale grains in mitigating the irradiation damage due to helium irradiation. In this work, in-situ irradiation-TEM experiments were performed in the University of Huddersfield on ultrafine tungsten materials (< 500nm) prepared by severe plastic deformation techniques. The in-situ work was performed at low energies (2-8 KeV) and different temperatures (up to 950 oC). Bubble formation and distribution on different grains and grain boundaries as well as defect generation and migration are illustrated. Correlation between the in-situ irradiation-TEM results and the morphological changes observed from ex-situ irradiation-scanning electron microscopy (SEM) work performed on similar samples, is illustrated. Comparison with other in-situ irradiation-TEM works performed on commercial tungsten samples is also discussed. [1] Lipschultz B, et al. Nucl. Fusion 47 (2007) 1189-1205. [2] Zinkle SJ, Ghoniem NM. Fus. Eng. Design, 51-52 (2000) 55. [3] Shin Kajita, Wataru Sakaguchi, Noriyasu Ohno, Naoaki Yoshida, Tsubasa Saeki, Nucl. Fusion 49 (2009) 095005

Recrystallization and grain growth are typically considered to be thermally-activated Arrhenius diffusive processes. However, recent observations and simulations suggest that mechanically-induced grain growth of nanocrystalline metals may occur independent of temperature, or even be encouraged at low temperatures. Zhang, Weertman, and Eastman (APL, 2005) observed indentation-induced grain growth in nanocrystalline Cu both at room temperature and at 77 K. At the lower temperature, they observed a stronger propensity for abnormal grain growth, resulting in a small number of very large grains. The possibility of low-temperature mechanically-induced grain growth is also supported by molecular statics simulations which suggest the possibility of grain growth even at 0 K (Sansoz and Dupont, APL, 2006). Our own molecular dynamics simulations of a large survey of different grain boundaries suggest that a significant fraction of these boundaries behave anti-thermally: their mobility actually increases with decreasing temperature. A custom-built liquid helium (4 K) Vickers indenter permits ex-situ study of deformation induced grain-growth while a liquid nitrogen (77 K) in-situ TEM straining stage permits direct observation of grain growth at cryogenic temperature. Grain growth has been observed at 4 K, 77 K, and 293 K in both nanocrystalline and nanotwinned Cu. The nanotwinned Cu showed the highest propensity for mechanical grain growth at all temperatures, which was surprising given that coherent sigma-3 boundaries possess zero mobility. The role of impurity content, crystallographic texture, and boundary character will be discussed. 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.

Extreme strain processing has advanced to the stage that nanoscale bi-metallic composites can be fabricated in bulk. It is found that under severe plastic deformation, bi-metal interfaces uniformly self-organize into a certain structural type, characterized as ordered and containing regular arrays of misfit dislocations and steps. We demonstrate that these atomic and meso-scale features can control slip and twinning activities by dictating where and which dislocation transmission and nucleation events are possible. Based on these results, we hypothesize that interface structures that are stable under extreme strains must orient themselves such that interface/dislocation interactions are optimized. To prove this concept, we integrate characterization across scales, atomic-scale modeling, dislocation theory, and crystal plasticity.

The formation of cleared channels in deformed irradiated materials has important consequences for determining the macroscale mechanical properties as well as the susceptibility to stress corrosion cracking. To understand the source of dislocations that generate these channels, how the dislocations interact with and annihilate irradiation produced defects, and how these dislocations interact with grain boundaries straining experiments have been performed in-situ in the transmission electron microscope as a function of temperature. This talk will show for stainless steels with varying composition that freshly generated dislocations create the channels, the progress of these dislocations through the channels is erratic and intermittent, the channel width reflects the width of source and complex cross-slip reactions, and dislocation pileups form in the grain interior due to the strength of the defect field. The interaction of dislocations with grain boundaries and the criteria controlling the slip transfer process across a grain boundary will be discussed. In addition, the application of electron tomography to defect characterization will be introduced.

Vacancy accumulation in a 4-degrees (110) bcc Fe and a 2-degrees (111) fcc Cu twist boundaries has been studied. These interfaces are characterized by different sets of screw dislocations, two sets of a_0/2<111> and one set of a_0/2<100> in Fe and three sets of a_0/6<112> in Cu. We observe that vacancies agglomerate preferentially at the misfit dislocation intersections, where their formation energy is lower. In bcc the dislocation structure remains stable, but in fcc the interface rearrange itself increasing the stacking fault area. To perform this study a kinetic Monte Carlo algorithm coupled with the molecular dynamics code LAMMPS has been developed. Atomic positions are relaxed at every step after an event takes place to account for long-range strain fields. The events considered in this work are vacancy migration hops. The rates are calculated via harmonic transition state theory with the energy at the saddle point obtained either by a linear approximation considering the relaxed energy of the initial and final configurations or the nudged-elastic band method depending on the vacancy position in the sample.

The effect of additional defects on the process of dislocation emission from grain boundaries in fcc materials is studied using atomistic simulation. We report fully three dimensional atomistic molecular dynamics studies of the mechanical response of identical samples with and without a large concentration of vacancies and with and without intergranular H impurities. The presence of vacancies aids the dislocation emission process. The presence of H also influenced the processes of dislocation emission from the grain boundaries, as well as the various mechanisms of grain boundary accommodation of the strain. H also affected the processes of nucleation and propagation of intergranular cracks.

Through in situ Cu ion irradiation at room temperature in a transmission electron microscope (TEM), we investigated the evolution of defect clusters at Σ3 {112} incoherent twin boundary in Cu and at Cu-Nb interfaces. During irradiation, the density of defect clusters was observed to increase with irradiation dose and then saturate. The saturation defect concentration was lower for Cu-Nb interface as compared to the twin boundary in Cu. Post in situ irradiation, high resolution TEM was used to explore the defects at the interface in detail and a model was proposed to explain the difference in the two interfaces in terms of sink efficiency. This research is sponsored by DOE, Office of Basic Energy Sciences.

Interfaces are important in structural materials such as Oxide Dispersion Strengthened (ODS) nanocpmposite steels as they act as sinks for radiation-induced vacancies and interstitials. However, they are difficult to access due to the geometric complexity of the internal interfaces. Fundamental mechanisms of radiation-induced defect evolution and annihilation at interfaces are being investigated using model nanocomposite systems with multilayer geometry. Large, regularly spaced interfaces in nanocomposite model (Cu/Nb) systems make them ideal to explore the physics of defect interactions at interfaces. In the present studies, synchrotron techniques; Extended X-Ray Absorption Fine Structure (EXAFS), X-Ray Reflectivity (XRR) and Grazing Incidence Wide Angle X-Ray Scattering (GI-WAXS) are used to study interface structure in Cu/Nb nanocomposite systems. EXAFS studies will provide information on local structure of Cu/Nb nanocomposites with elemental sensitivity (both Cu and Nb edge), the radial distribution of atoms around the selected central site, including bond lengths, coordination numbers, and thermal disorder. Novel approach of using atomic positions and path lengths computed from Molecular Dynamics (MD) simulations to build models for EXAFS data analysis will be discussed.While XRR and GIWAXS studies are used to study electron density profile and multilayer geometry, and structure of the Cu/Nb nanocomposites respectively. In addition the internal microstructure of Cu/Nb nanocomposites investigated by Transmission Electron Microscopy will be reported. Changes induced in the structure of Cu/Nb nanocomposites after He-ion irradiation will be studied.

In-situ transmission electron microscopy (TEM) is a powerful technique to study variation processes in local structures within solids in response to various stimuli under extreme environments. We have utilized high-voltage electron microscopy (HVEM) and ion-accelerator combined TEM, to uncover elementary processes of microstructural evolution in crystalline materials, upon irradiation with energetic particles, such as electrons and ions. In this talk, I will present recent results of our studies on the dynamic behaviors of radiation-produced defects, such as directly visible point-defect clusters and even "invisible" single point defects, in metals and semiconductors (e.g. [1]-[4]). [1] Arakawa, K. et al., “Changes in the Burgers Vector of Perfect Dislocation Loops without Contact with the External Dislocations,” Phys. Rev. Lett., 96 (2006) 125506. [2] Arakawa, K. et al., “Observation of the One-Dimensional Diffusion of Nanometer-Sized Dislocation Loops,” Science, 318 (2007) 956. [3] Arakawa, K., Amino, T., and Mori, H., “Direct Observation of the Coalescence Process between Nanoscale Dislocation Loops with Different Burgers Vectors,” Acta Mater., 59 (2011) 141. [4] Amino, T., Arakawa, K., and Mori, H., “Activation Energy for Long-Range Migration of Self-Interstitial Atoms in Tungsten Obtained by Direct Measurement of Radiation-Induced Point-Defect Clusters,” Philos. Mag. Lett., 91 (2011) 86.

Porous materials with high surface-to-volume ratio have the potential to be extremely radiation resistant. In nanoscale foams, ligaments form an open sponge-like 3D structure with large number of surfaces that provides the perfect sinks for defects. Recently, a model has been proposed [1] that defines a window of radiation endurance based on the combined effect of a length scale and a time scale which depend on the irradiation condition: the length scale determined by the relation between ligament size and collision cascade sizes and the time scale given by the relation between defects diffusion to ligaments surfaces and the time between collision cascades. A window of radiation resistance can be defined when ligaments are sufficiently small so that defect migration to the ligament surface happens faster than the time between cascades and at the same time sufficiently large not to be destroyed by cascade induced melting. To help understand defect evolution in nanoporous materials under irradiation, an experimental/computational campaign has been launched aimed at finding the boundaries of the tolerance window. In this work we report on temperature, dose and dose-rate radiation response of nanoporous gold (np-Au). Foam microstructure was characterized using transmission electron microscopy (TEM) before and after ion irradiation. Our results show significant changes, such as the formation of stacking fault tetrahedra (SFT) observed in irradiation at room temperature. To guide the analysis of these observations, Molecular Dynamics (MD) simulations were performed on Au ligaments exploring different diameters, primary knock-on atom&’s (PKAs) energies, impact locations and orientations. The possible mechanisms to explain relevant features of radiation damage in nanoscale Au foams are discussed. [1] E. M. Bringa, J. D. Monk, A. Caro, A. Misra, L. Zepeda-Ruiz, M. Duchaineau,F. Abraham, M. Nastasi, S. T. Picraux, Y. Q. Wang, D. Farkas, “Are Nanoporous Materials Radiation Resistant?”, to appear in Nano Letters July (2012). dx.doi.org/10.1021/nl201383.

Ultrafine grain structures of materials processed by severe plastic deformation methods have been widely investigated during the past 15 years. Beside grain refinement and the resulting higher yield stress, it is now well established that the atomic mobility might be dramatically increased during severe plastic deformation. This feature could give rise to specific phenomenon like precipitate dissolution, formation of supersaturated solid solution or grain boundary segregations. Both the underlying physical mechanisms of deformation induced atomic transport and the influence on the grain size refinement mechanisms and on the final properties (especially strength, and thermal stability) are not fully understood yet. This presentation reports about recent experimental data collected by atom probe tomography and high resolution transmission electron microscopy, bringing new information in the field. A special emphasis will be first given on the concomitant precipitate decomposition and grain boundary segregation of solute elements in steels and aluminium alloys. The second part of the talk will be devoted to a nanostructured metallic glass-aluminium composite. It will be shown how the solid state amorphization of the crystalline aluminium phase takes place during SPD through intense atomic scale mixing. This phenomena leads to the formation of a unique amorphous structure with a nanometer scale modulated composition.

Copper-based high strength and high electrical conductivity nanocomposite wires reinforced by Nb nanotubes are prepared by severe plastic deformation, applied with an Accumulative Drawing and Bundling process (ADB), for the windings of high pulsed magnets. The ADB process leads to a multi-scale Cu matrix containing up to N=85⁴ (52.2 10⁶) continuous parallel Nb tubes with diameter down to few tens nanometers. After heavy strain, The Nb nanotubes exhibit a homogeneous microstructure with grain size below 100 nm. The Cu matrix presents a multi-scale microstructure with multi-modal grain size distribution from the micrometer to the nanometer range. The use of complementary characterization techniques at the microscopic and macroscopic level (in-situ tensile tests in the TEM, nanoindentation, in-situ tensile tests under high energy synchrotron beam) shed light on the role of the multi-scale nature of the microstructure in the recorded extreme properties.

A series of plate impact experiments were conducted to investigate the effect of dynamic loading rate on the uniaxial dynamic tensile response of commercially pure 1050 aluminum. The loading rate (kinetic effect) was varied by altering the shock-wave shape, while the total defect density loaded in dynamic tension was held constant (spatial effect). The maximum tensile stress magnitude was held constant for all experiments in order to solely examine the effects of dynamic loading rate. Samples were soft recovered and analyzed via Electron Backscatter Diffraction (EBSD) to correlate damage to microstructural features. An optical velocimetry (VISAR) trace from the free surface was utilized to correlate the effects of damage growth rate observed through EBSD to changes in free surface velocity pull back rate. As the dynamic tensile evolution rate was increased a transition from a slow damage mechanism of individual void growth to a fast damage mechanism of void nucleation is observed. As the rate is increased to even higher levels the damage mechanisms are overdriven and very little damage is observed. The free-surface velocity histories correlated well with damage observed in the microstructure. Additionally, the effect of dynamic loading rate on dislocation generation and plastic flow in the microstructure will be discussed.

The dynamic deformation behavior of bcc metals such as tantalum has long been attracting considerable research attention. Conventional dynamic tests, however, were limited to strain rates of approximately 10^3/s, and the efforts in understanding plasticity and dynamic behavior of bcc metals have focused mostly on the role of dislocations. In this work, we study dynamic behavior of Tantalum by utilizing two recently developed techniques, i.e., miniaturized Desktop Kolsky Bar and Dynamic Shear Compression experiments, which allow dynamic testing under compressive and shear dominant modes respectively at strain rates of ~10^4/s, one order higher than conventional methods. Experiments were carried out at both ambient and cryogenic temperatures. Extensive deformation twinning has been identified at high rates and low temperatures, and a critical twinning threshold has been derived from abundant compression and shear experiments and compared to values previously speculated from single-crystal experiments. The evolution of twinning structures, from nucleation, growth, propagation and interaction with dislocation slip has been studied in details by using transmission electron microscopy. The stress-strain behaviors and a sudden up-turn of the flow stress with strain rate approaching 10^4/s are discussed in terms of the microstructure evolution, which may or may not involve deformation twinning. We also discuss appropriate constitutive models for describing the material&’s behavior.

We report on a first-principles study of an unusual structural rotation of bulk single-crystal Al under uniaxial compressive strains. Density functional theory based calculations were used to explore this behavior, which involved collective shuffling of atoms. It was found that under strains either along the <11-2> or the <111> direction, beyond a critical stress of about 13 GPa, the Al crystal can rotate through collective shear in the Shockley partial direction (i.e., <11-2> ) on the {111} planes, in an attempt to relieve internal stresses. This phenomenon reveals a possible mechanism leading to the onset of homogeneous dislocation nucleation in Al under high uniaxial compressions.

The oxide dispersion strengthened (ODS) alloys are good candidate for the future nuclear plants because of their excellent high temperature creep properties [1-2]. However, under these radiation conditions, not only ballistic damage is generated, but also gas accumulation, helium in particular. To gain insight about the ODS resistance, our first step was to study the matrix of ODS alloys: pure α-Fe and FeCr model alloys. In this work, an ultra high purity Fe and Fe- ( 5 ,10 ) wt % Cr alloys were irradiated at 500°C within the JANNuS (Joint Accelerators for Nano-science and Nuclear Simulation) facility [3]. Some irradiations were performed in in-situ mode up to 1 dpa to follow the nucleation and the evolution of the radiation damage while others were performed in ex-situ mode to gain insight about the damage formation at higher doses (up to 100 dpa). They were realized under mono-beam (Fe only) and dual-beam (Fe + He) conditions to dissociate the ballistic damage from the gas implantation. The radiation-induced dislocation loops, cavities and helium bubbles were characterized by transmission electron microscopy (TEM). The evolution of Cr was analysed by means of scanning TEM/energy dispersive X-ray spectrometry (STEM/EDX) and atom probe tomography (APT). We will present in this paper results concerning the effect of He on the loop mobility, the heterogeneous nucleation of bubbles on the dislocation loops [4], the effect of Cr on the radiation-induced swelling and the evidence of a radiation-induced segregation (RIS) on a dislocation loop. A part of these experimental data was then used to validate a rate theory (RT) numerical code developed in our laboratory. This work is realized within the European Fusion Development Agreement (EFDA) program. [1] E.A. Little and D.A. Stow, J. Nucl. Mater. 87 (1979) 25-39. [2] F.A. Garner, M.B. Toloczko and B.H. Sencer, J. Nucl. Mater. 276 (2000) 123-142. [3] JANNUS platform web site: jannus.in2p3.fr. [4] D. Brimbal, B. Décamps, A. Barbu, E. Meslin and J. Henry, J. Nucl. Mater. 418 (2011) 313-315.

The Born-Oppenheimer approximation is the keystone for molecular dynamics simulations of radiation damage processes; however, actual materials response involves nonadiabatic energy exchange between nuclei and electrons. In this work, time dependent density functional theory is used to calculate the electronic excitations produced by energetic protons in Al. We study the influence of these electronic excitations on the interatomic forces and find that they differ substantially from the adiabatic case, quantitatively revealing a nontrivial connection between electronic and nuclear stopping that is absent in the adiabatic case. These results provide quantitative evaluation of effects in the early stages of radiation damage cascades that have so far been considered within empirical formulations. PRL 108, 213201 (2012)

NF616 (ASTM code: P92 9Cr nominal alloy composition) and HCM12A (ASTM code: T122, 12Cr nominal alloy composition) are third Generation ferritic-martensitic (F-M) steels originally developed for non- nuclear applications in power generation industry as boiler and turbine materials. Both alloys exhibit improvements in creep rupture strength and maximum operation temperatures compared to the previous generations of F-M steels, which makes them potential candidates for in-core applications in the Sodium-Cooled Fast Reactor and fusion reactor concepts. In this study, an in situ TEM investigation of the microstructure evolution in NF616 and HCM12A alloys under 1 MeV Kr ion irradiation has been performed. NF616 and HCM12A were irradiated to 10 dpa between 20 K and 773 K at Argonne IVEM Facility. The evolution in the irradiation induced microstructure was followed by at various doses systematically recording micrographs using conditions appropriate for imaging defect clusters and recording diffraction patterns. The alloys exhibited similar behavior under irradiation. Below an initial onset dose observed at all temperatures no defect accumulation was visible. Above this dose defects appeared to be created in cascade events (becoming visible from one frame to the next). The onset dose for defect accumulation shifted to higher doses with temperature. In both alloy at temperatures between 20 K and 573 K, the density of these defect clusters density increased until reaching saturation at 5-6 dpa. Defects were continuously created and eliminated throughout the irradiation; their average size was constant throughout the irradiation in this temperature range (sim;3-4 nm). Dynamic observations highlighted the restricted mobility of defects and very limited interaction of defects with each other and with the initial ferritic- martensitic microstructure. Because of this, the defects exhibited what appeared to be ion beam- induced sudden jumps and jerks below 573 K. On the other hand, defects showed increased thermal mobility at 673 K, which resulted in the formation of larger defects by growth and coalescence. These results are discussed in comparison to cluster dynamics modelling and to other irradiation damage results in the literature.

Ferritic-martensitic (F-M) alloys are attractive candidates for structural components in sodium-cooled fast reactors, and stainless steels are widely used as core structural materials in light water reactors. To reach the high doses expected in both applications, high dose rate self-ion irradiation is used. In this study, F-M alloys HT9, HCM12A, T91 and a model 9Cr alloy were irradiated at 400 or 500°C to doses of 30 to 500 dpa using 5 MeV Fe++ ions at 0.001 dpa/s. Stainless steel alloys 304 and 316 were irradiated with Fe++ or Ni++ to doses of up to 200 dpa. Samples were prepared using FIB for TEM to characterize dislocation microstructure, voids, and RIS, and for APT to characterize RIP. Results show that [100] type dislocation loops are dominant at all doses but at 500 dpa, loops are substantially larger and the density is lower than at lower doses. At all doses, Cr, Si and Ni enrich at grain boundaries. Ni/Si/Mn-rich, Cu-rich and Cr-rich precipitates nucleate in T91 and HCM12A at 7 or 30 dpa. At a very high dose of 500 dpa, significant coarsening of Ni/Si/Mn-rich and Cu-rich precipitates occurs and a high density of radiation-induced Cr-rich carbides was observed in HCM12A. The evolution of RIS and precipitation at high dose and high temperature and their potential effect on the alloy mechanical properties will be presented and discussed.

In the past years, several new materials and coatings have been developed to extend the lifetime of industrial and medical products. To investigate these materials, wear measurements using radioactive tracers is an accepted and approved technique and widely used for several applications. These measurements are typically performed using the method of ion beam activation. A suitable proton beam, produced in a cyclotron with kinetic ion energies of 10-50 MeV, hits a specific part of the sample and activates it. However, this method has some disadvantages. First of all, one needs a high current and high energy proton beam. Only a few materials, mainly metal alloys can withstand this bombardment without showing any changes of the physical properties of the sample. Additionally, these high energy protons show a very high penetration depth, thus a large number of unspecified radioactive isotopes are spread over hundreds of micrometer even millimeter. A more precise method is the implantation of radioactive tracers in a defined depth. 7Be pointed out to be a very good tracer because of its small half-life of about 53 days and its only emitted low energy gamma-ray of 478keV. In addition, light elements show only small straggling during implantation. Thus, the implantation depth can be chosen with an accuracy of only a few tens of nanometers depending on the impact energy. This method is already in use e.g. for wear measurements in automotive industry [1][2] by using a tandem accelerator with ion energies of a few MeV. However, to investigate coatings or layers with a thickness of a few tens of nanometer, this energy is still to high. At the RUBION of the Ruhr-Universitaet Bochum, we developed an implanter which is able to produce a low energy radioactive 7Be ion beam with only a few keV. This system consists mainly of an electron cyclotron resonance ion source (ECRIS) put on high potential followed by a 90° analyzing magnet for mass (over charge) separation and an implantation chamber. The talk will give the first results of a new low energy 7Be implantation setup at the Ruhr-Universitaet Bochum and discusses advantages and disadvantages of those ion beams.

Materials under intense ion irradiation are subjected to microstructural changes due to energy deposition that leads to atomic displacements, formation of extended defects, electronic excitation and ionization, and heat generation. In addition, the implanted species incorporates into the material structure and gas bubbles may form in case of high-dose He⁺ ion implantation. As a result, the material property could be modified dramatically. Material resistance or susceptibility to ion irradiation is associated with its microstructural stability. It is important to study and understand the fundamental processes involved during ion irradiation. In-situ observation of microstructural evolution under He⁺ ion irradiation provides an access to the processes and generates data to improve the scientific understanding of the processes. This knowledge is expected to have a significant impact on various important applications, including rational designs of structural materials for fusion reactors, cladding materials for nuclear fuels, nuclear waste forms, and nuclear radiation monitoring systems. Previous studies of microstructural changes under ion irradiation have been performed primarily using ion accelerators or implanters, microscopes and spectrometers. The currently available in-situ capability for studying ion-irradiation-induced microstructural changes is based on a transmission electron microscope (TEM) coupled with an ion accelerator. Recent technological advances lead to emergence of helium ion microscope (HIM). Compared to conventional secondary electron microscope (SEM), HIM has demonstrated an enhanced imaging capability with a greater surface sensitivity, higher spatial resolution, larger depth of field and sharper Z contrasts. While imaged using secondary electrons or backscattered ions, the material is simultaneously under He⁺ ion irradiation. This unique feature provides an in-situ method for examining microstructures under He⁺ ion irradiation at a sub-nanometer resolution within the beam dwelling time, typically on the order of tens to hundreds of microseconds per pixel. In-situ HIM studies include particle aggregation, grain growth or shrinkage, volume expansion or compaction, cracking, buckling, formation of voids and helium bubbles, material decomposition, phase segregation and phase transformation. The HIM data is complementary to that from a combined TEM and accelerator system. As a demonstration, this presentation will show a number of HIM movies for microstructural evolution in various ceramic materials. Structural effects from thermal annealing only and electron irradiation through SEM, where nuclear processes are avoided, will be also shown.

Due to increasing technologic interests for ceramics in applications with radiative environment (nuclear or space), studies of material behaviour during swift heavy ion irradiation are highly topical to understand their stability under intense electronic excitations. High density of electronic excitation entails in crystalline materials important structural variations with various phase transition kinetics which depend on material characteristics (compositions, electrical properties, crystallographic structures, etc). To facilitate the study of transition kinetics as a function of ion fluence, at the beamline IRRSUD of GANIL (swift heavy ion accelerator, Caen, France) a new X-ray diffractometer (called “ALIX”) has been recently set up. This equipment allows us to perform for the first time in-situ X-ray diffraction simultaneous to irradiation. On the IRRSUD beam line the energy range is from 0.3 to 1 MeV/u implying a mean ion depth penetration in solid matter around few micrometers. Then X-Ray diffraction measurements under grazing incidence are set up to allow the analysis of the topmost part of the sample. Also simultaneous irradiation and diffraction is made easier by the energy discrimination between the diffracted X-rays and X-rays from ion-target interaction.[ref.1] This equipment is open to the scientific community. In this communication, results about successive and simultaneous irradiation-diffraction experiments will be presented to highlight the effects of electronic excitation on structural properties of polycrystals. Three different oxide materials have been chosen in order to interpret the damage build-up induced by ion irradiation, they are MgO, SrTiO3 and Nd2Zr2O7 chosen for their potential application as matrices for immobilization and/or transmutation of nuclear wastes. During experiments using ions with a relatively high electronic energy loss Se around 20 keV/nm, different sensitivities to electronic excitation are observed. By interpreting the transition kinetics, we have observed three cases for these materials: no transition, direct transition as well as complex multiple impact transition to the amorphous state. The defect overlapping models will be discussed to interpret the observed transition kinetics. In order to support the purposed models, additional transmission electron microscopy measurements have been used to deliver track characteristics like diameters and microstructures. ref.1: C. Grygiel et al, Rev. Sci. Instrum. Volume 83, Issue 1, 013902 (2012)

Ceramic textile composites are being developed specifically targeting several operating challenges encountered in hypersonic flight. Extreme service conditions of ultrahigh temperatures from 1200°C to nearly 1600°C in combination with service loads and hostile environmental atmospheres are well beyond the realm of current commercial structural materials. Using novel strategies in materials selection and coatings along with integral 3-D architectural design, woven ceramic-matrix composites (CMC&’s), such as SiC_f-SiC_m materials, make the development of such ultrahigh-temperature structures a feasible proposition. Lifetime prediction and damage assessment for such complex architectures presents a formidable challenge though, as the collection of reliable physical and engineering mechanical data, not to mention the characterization of damage in 3-D, at temperatures of 1200-1600°C, is so difficult. To this end, we have developed a facility to perform such characterization using synchrotron x-ray micro-tomography capable of subjecting samples to tensile loads at temperatures of 2300°C, which has proven to be a tool of choice for non-destructively evaluating the component in three-dimensions at spatial resolutions around a micron. We report several phenomena governing failure that occur over time in a model SiC/SiC fiber-matrix composite at 1600°C ranging from cracking within individual fibers to the fracture of entire fiber bundles/tows at these extreme temperature conditions system. This new ability to image complex 3-D materials undergoing failure under combined extreme physical conditions, for the first time opens new possibilities for evaluating ceramic textile materials in real time. Indeed, the technique can be extended in studying not just textile composites but a myriad of new structural materials.

Particle irradiation appears to be an effective method for manipulating properties of individual carbon nanotubes (CNTs). This potential, however, remains unexplored for macroscopic assemblies of cross-linked CNTs. Here, we study structural and electrical properties of ultralow-density cross-linked CNT-based nanofoams exposed to ion irradiation at room temperature over a wide range of ion masses and fluences. For all irradiation conditions studied, the electrical resistance of nanofoams initially increases with a rate that scales with the number of ballistically generated displacements. This process is attributed to the buildup of defects in graphitic nanoligaments. Irradiation with Ne and heavier ions leads to a decrease in the electrical resistance at large fluences, which is attributed to radiation-induced foam densification. In addition, heavy-ion bombardment causes amorphization of CNTs and smoothing of ligament surfaces. These results demonstrate that ion bombardment can be used for tailoring density, ligament morphology, and electrical properties of CNT-based foams. They also show an overall excellent radiation stability of CNT nanofoams compared to the case of more extensively studied nanoporous silica. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Understanding the behavior of carbon materials at extreme conditions is of fundamental interest and of significant importance for technological applications. To date, reliable experimental data is still scarce as experiments are difficult. However, the past decade has shown that atomistic numerical simulation provides an reliable alternative.[1] In this contribution we present accurate results of atomistic simulations of the transformation of carbon structures at conditions up to T=4000K and P=20GPa, focusing on two aspects: 1) nucleation of liquid carbon and 2) graphitization of single wall carbon nanotubes (SWCNTs). We employed a state-of-the-art bond-order potential (LCBOB)[2] and advanced numerical techniques[3] to ensure that our simulations yield accurate predictions. 1) The melting temperatures of diamond has been established with reasonable accuracy. However, for the reverse process, nucleation of diamond from liquid carbon, no experimental data is available. We performed accurate numerical simulations of this process and found the nucleation rate, at constant supersaturation, to be strongly dependent on the pressure. We demonstrate that the pressure dependence originates from a change in the local structure of the supercooled liquid.[4] This result may have important novel insights on the structure carbon-rich planet interiors. 2) In a second study we addressed pressure-induced graphitization of SWCNTs at extreme conditions. Our numerical simulations of (5,5) and (10,10) SWCNTs show that at temperatures below 2000~K small nanotubes coalesce before forming graphitic structures, whereas larger (10,10) nanotubes graphitize via a collapse mechanism. At temperatures above 2000~K for both the small and larger nanotubes the coalescence appears to dominate the graphitization mechanism. We observed that the resulting graphitic structures often contains sp3-interlinking defects between sheets. This may explain the formation of nano-diamond forms at pressure below the graphite-diamond coexistence line. In conclusion, our numerical simulations have provided novel insight in structural transformation of carbon at extreme conditions. They provide a reference for interpreting experimental data, and will serve as a starting point for exploring, by numerical simulation, the dynamical aspects of the structural transformations among liquid carbon, diamond, graphite, and carbon nanostructures. References: [1] A.A. Correa, S.A. Bonev and G. Galli, PNAS 103, 1204 (2006). L.M. Ghiringhelli, J.H. Los, E.J. Meijer, A. Fasolino, and D. Frenkel, Phys. Rev. Lett. 94, 145701 (2005). [2] H. Los, L.M. Ghiringhelli, E.J. Meijer, A. Fasolino, Phys. Rev. B. 72, 214102 (2005). [3] D. Frenkel and B. Smit, Understanding Molecular Simulation, Academic Press (2001). [4] L.M. Ghiringhelli, C. Valeriani, E. J. Meijer, and D. Frenkel, Phys. Rev. Lett. 99, 055702 (2007). [5] F. Colonna, A. Fasolino, and E.J. Meijer, submitted for publication.

Nanostructured carbon materials, including single-wall carbon nanotubes (SWCNTs), are attractive for nanoelectronic devices such as FETs, interconnects, advanced wires and cables, transparent conductive electrodes, and solar cells. The availability of electronic-type-separated SWCNTs from large-scale separation via density gradient ultracentrifugation (DGU) has substantiated the use of SWCNTs in many of these applications, where the unique electronic properties of metallic (M-SWCNTs) and semiconducting (S-SWCNTs) SWCNTs are critical to achieving the necessary functionality (e.g., drain current on/off ratio, high open circuit voltage, high conductivity, high mobility, and high gain), thus making them competitive with existing alternatives. The potential use of these materials in space systems necessitates a basic understanding of material survivability in harsh radiation environments prior to deployment. Using commercially available electronic-type-separated SWCNTs, thin-films of approximately 80 - 100 nm thickness were prepared by vacuum filtration and transferred onto quartz substrates. The resulting samples were irradiated with 150 keV B+ and 150 keV P+ to fluences ranging from 1E12 to 1E15 ions/cm2, and the structural and electrical properties of electronic-type-separated SWCNT thin-films were investigated after each irradiation. Raman spectroscopy results indicate that the ratio of the Raman D to G' band peak intensities (D/G') is a more sensitive indicator of SWCNT structural modification induced by ion irradiation by one order of magnitude compared to the ratio of the Raman D to G band peak intensities (D/G). The increase in sheet resistance (Rs) of the thin-films follows a similar trend as the D/G' ratio, suggesting that the radiation induced variation in bulk electrical transport for both electronic-types is equal and related to localized defect generation. The characterization results for the various samples are compared based on the displacement damage dose (DDD) imparted to the sample, which is material and damage source independent. Therefore, it is possible to extend the analysis to include data from irradiation of transferred CVD-graphene films on SiO2/Si substrates using 35 keV C+ ions, and compare the observed changes at equivalent levels of ion irradiation-induced damage to that observed in the SWCNT thin-film samples. Ultimately, a model is developed for the prediction of the radiation response of nanostructured carbon materials based on the DDD for any incident ion with low-energy recoil spectra. The model is also related to the defect concentration, and subsequently the effective defect-to-defect length, and yields a maximum defect concentration (minimum defect-to-defect length) above which the bulk electrical transport properties in SWCNT thin-films and large graphene-based electronic devices rapidly degrade when exposed to harsh environments.

Polymer composites containing conducting fillers have been extensively investigated for various applications such as heating elements, electromagnetic interference (EMI) shielding, electronic packaging, radar absorption, reinforcement structure and high charge storage capacitor. Consequently, high aspect ratio fillers such as carbon nanotubes (CNTs) which favor reinforcement and electrical properties/percolation at lower volume fraction are desirable. In heating unit applications, due to Ohmic joule heating of nanotube composite, electrical energy can be converted heat energy easily and quantitatively. In addition, more conducting with low heat capacity composite is ideal for practical heating applications. Though several researchers have examined on the electrical behavior of CNT device or composite under heating or cooling conditions for sensor applications, but only few works have been studied on its Ohmic joule heating behavior and thermal degradation. Here, we report observation of hardening and thermal degradation of polymer base carbon nanotube composites through electrical joule heating. In addition, we investigate how the resistance of nanotube composite behaves under electric heating from room temperature to temperatures up to 250°C. The fabrication of highly conducting carbon nanotubes (CNTs) / polydimethylsiloxane (PDMS) composites are presented with the aim of heating unit applications. We have seen that, in same temperature, material properties of nanotube composites, e.g., thermal stability and mechanical properties, undergo electrical joule heating are quietly different from composites go through convection heating. For example, nanotube composites after electric heat-aging at 200°C have suffered severe thermal degradation which are easily fragile while nanotube composite after convection heat-aging showing enhanced mechanical properties possibly due to further chemical changes in the polymer (cross-linking). Various analyses, i.e., SEM (Scanning Electron Microscopy) characterization, Raman spectroscopy, tensile testing and TGA (The thermogavimetric analyzer), were conducted to interpret the behavior of hardening and thermal degradation of nano-composites. Consequently, through isothermal mode of TGA, effect of Ohmic joule heating at 200°C is corresponding to effect of convection heating at 350°C. The above observations are expected to suggest useful design guide for nanotube composite fabrications as a heating elements.

Ceramic carbides materials such as SiC, due to their refractory nature and their low neutron absorption are believed to be promising candidates for high temperature nuclear or aerospace applications. However, SiC brittleness has limited its structural application. For counteracting this phenomenon, a reduction of grain size (below 100 nm) accompanied by a high final density (also for fission products retention) seem to be the solutions for improving mechanical properties. Sintering additives are usually employed, generally boron-based or yttrium and aluminum oxides, in order to achieve high densities and fine grain sizes. The presence of additives permits to improve the mechanical properties, although they present drawbacks under irradiation. In order to avoid these problems, no sintering additives were used in this study. The counterpart of the expected mechanical properties improvement by reducing the grain size is the drastic decrease of the thermal conductivity due to the phonon scattering at the GB. With the aim of reducing this effect, multiwall carbon nanotubes (MWCNTs) were introduced into the SiC matrix in order to improve thermal conductivity. The MWCNTs are characterized by having a strong toughness which should also help to enhance the mechanical properties as reviewed by several authors. For this study β-SiC nanopowders with a mean particle size of 20 nm produced by laser pyrolysis were employed. MWCNTs with lengths between 400 and 680 µm were synthesized by aerosol assisted catalytic CVD. De-agglomeration of the powder has been performed in an aqueous medium under magnetic stirring and ball-milling. Subsequently, green bodies were prepared by slip-casting of slurries. In this context, non-conventional densification routes such as SPS allowed to achieve high final densities avoiding an exaggerated grain growth. Finally, samples were subjected to mechanical characterization (hardness, toughness) with the aim of correlating the final microstructures to the mechanical behavior.

High temperature, gas-cooled reactors are among the Generation IV nuclear reactor concepts. Zirconium carbide is a candidate material for use in the fuel, in which it would be subjected to high radiation doses. However, the effects of radiation on ZrC, especially at high doses, are not yet well understood. The goal of this study is to gain an increased understanding of the irradiation response of ZrC by using ion irradiation. In-situ ion irradiations were performed at the IVEM-Tandem facility in order to observe damage as it developed. The IVEM is a transmission electron microscope modified to allow simultaneous imaging and ion irradiation. Samples of ZrC_0.9 were irradiated at a range of temperatures from 20 K to 1073 K. Irradiations were performed with 1 MeV double-charged Kr ions. During irradiation, the same area in the sample was examined using appropriate imaging conditions to reveal the development of displacement damage, and diffraction patterns were systematically taken during irradiation to fully characterize the irradiation response. The highest doses achieved ranged from 2 to 10 dpa. No evidence of amorphization was observed even after 10 dpa at 20 K. Nor was there any evidence of void formation recorded after 5 dpa at 1073 K. The microstructure development showed an initial damage structure consisting of a low density of black-dot type damage, the density of which increased with dose. After the initial stage, at low temperatures, damage continued to accumulate as black-dot damage, with increasing density. At 673 K and above, small loops were formed exhibiting double-arc contrast, aligned principally in two directions. Also with increasing temperature, considerable defect cluster mobility was observed during the experiment. This mobility resulted in the formation of larger dislocation loops and of depleted zones near the foil edge, both of which increased with temperature. The diffraction patterns showed new ring intensity during irradiation at all temperatures. The rings were indexed and belong to a fcc type lattice, but the phase is unknown. At temperatures above 673 K to 1073 K, the diffraction patterns showed streaking from {111} to {220} ZrC matrix diffraction spots. These results will be discussed in detail and compared to computer simulations done as another task in this project.

Research on ion-solid interactions often focuses on predicting and mitigating detrimental effects on materials from particle irradiation, for instance, in ion-implantation doping of electronic devices, nuclear reactors or space applications. In this work, we show that swift heavy ion (SHI) irradiation can have beneficial recovering effects on the damaged structure of irradiated materials. For this purpose, we adopted an integrated approach that combined experimental characterizations, namely with Rutherford backscattering spectrometry and channeling (RBS/C) and transmission electron microscopy (TEM) techniques, and molecular dynamics (MD) simulations. In particular, we investigated the microstructure change of damaged SiC crystals subjected to irradiation with highly ionizing 0.87-GeV Pb particles (with an electronic energy loss of Se~33 keV/nm). The influence of the initial damage state, i.e. partial amorphization vs. full amorphization has been scrutinized. Results show that SHI irradiation induces defect recovery of the damage structure in both cases. This finding is deduced from the clear decrease of the disorder level measured by RBS/C and is supported by TEM images. A dramatic effect of the initial damage state is clearly put forward: (i) recrystallization takes place at the buried amorphous/crystalline interface in the case of crystals that were completely amorphized, while (ii) this damage recovery occurs over the entire damaged thickness for partially amorphous crystals (i.e crystals where amorphization just initiated at the damage peak). MD calculations implemented to simulate the thermal spike phenomenon associated to the tremendous electronic energy deposited during Pb ion irradiation do succeed in confirming experimental findings.

We have used a thermal spike model to study the microstructural evolution of BaTiO₃ subjected to swift heavy ion irradiation and compared our findings to corresponding experimental observations. We first developed a new empirical potential for BaTiO₃ that provides good agreement with the experimental melting temperature, lattice constants, and elastic constants. We used this potential to perform molecular dynamics simulation of the response of BaTiO₃ to the extreme condition of swift heavy irradiation. The simulations reproduce damage structures observed in this material under irradiation with 1.5 GeV 238U⁺ ions to various ion fluences at room temperature at the Grand Accélérateur National d'Ions Lourds facility in France. The damage consists of an amorphous core surrounded by defect domains, such as distorted structures and polycrystalline domains. The simulations shed light on the stages in microstructural evolution of ceramics in extreme environments. This work is supported by the United States Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfers their energy mainly to the electronic system (over 10 keV/nm) with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate and quartz are relevant optical materials with anysotropic structure. The amorphous phase is certainly isotropic. In addition, the refractive index in amorphous phase exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density different than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. In the case of lithium niobate, for certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion-generated amorphous nano-tracks for a variety of ion energies and doses. The surface deformation (hillcocks and swelling) has been compared with experimental data and molecular dynamic simulations in quartz. A very remarkable feature for X cut-samples of lithium niobate is that the maximum shear stress appears on preferential planes that form +/-45° with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Since the inception of nuclear power, issues involving nuclear safety and reliability have posed serious practical, financial, security, health and safety problems, due to the heightened pressures, temperatures, and levels of corrosion. Next generation reactor environments though promise to even surpass conventional nuclear power output and efficiency by increasing the operating temperatures, pressures, and neutron flux over conventional nuclear technology [1]. Concurrently, we have set strict criteria required for safe use of next generation fast reactor and fusion technology [2]. Meeting both heightened operating conditions and strict safety criteria, several materials issues are being investigated at the micron to sub-Ångström scale from the minor actinide containing oxide fuel sources to damage resistant steels. When deposited at increasingly smaller thicknesses, thin oxide layers show metastable structures, examples include SrTiO₃ (STO), YSZ deposited on CeO₂ [3]. Recent developments within the nuclear materials community hypothesize multilayered materials can address concerns regarding nucleation, growth, and the migration of voids or inert gas bubbles and fission fragments at higher temperatures and radiation environments [4]. A recent example study was performed by Misra et al. where conventional TEM was used to show metallic multilayer composites with smaller layer thicknesses contained a large number of defects at the interfaces that offered improved mechanical strength and resistance to both radiation induced swelling and embrittlement. In detail, researchers found that, with reduced layer thickness and after vacuum annealing, no growth of helium bubbles occurred that could have lead to swelling of the film [5]. The fundamental interfacial chemistry associated with this mechanism [6, 7] was not shown and the only conclusion was that smaller layer thickness leads to increased radiation damage performance. In this area of research, there therefore exists a need to characterize candidate radiation resistant material systems for both their interfacial structure and chemistry. Applying the latest experimental capabilities of aberration corrected transmission electron microscopy, we have chosen to study the evolution of interfacial structure and chemistry at oxide interfaces with the effects of ion radiation. In particular, we will be showcasing the analytical expertise and knowledge necessary [8] to understand the following oxide/oxide interfaces under irradiation: STO/CeO₂ and YSZ/CeO₂. Following experimental results, theoretical predictions will be used to investigate the resolved interfacial chemistry and structure, inclusive of mechanical [9] and corrosive properties. [1] K. Ehrlich, Philos. Trans. R. Soc. London, Ser. A 357, 595 1999. [2] E. E. Bloom, S. J. Zinkle, and F. W. Wiffen, J. Nucl. Mater. 329-333, 12 2004. [3] C. M. Yang, S. Azad, V. Shutthanandan, D. E. McCready, C. H. F. Peden, L. Saraf, S. Thevuthasan “Microstructure of ZrO2-CeO2 hetero-multi-layer films grown on YSZ substrate”, Acta Materialia. 53, 1921-1929 (2005). [4] M. Natasi, Q. M. Wei, Y. Q. Wang, A. Misra, “Nucleation and growth of bubbles in He ion-implanted V/Ag multilayers”, Philosophical Magazine 91, 553-573 (2011). [5] A. Misra, M. J. Demkowicz, X. Zhang, R. G. Hoagland, “The radiation damage tolerence of ultra-high strength Nanolayered composites”, JOM, 62-65 (2007). [6] K. Yu-Zhang, J. D. Embury, K. Han, A. Misra, “Transmission electron microscopy investigation of the atomic structure of interfaces in nanoscale Cu-Nb multilayers”, Philosophical Magazine 88, 2559-2567 (2008). [7] Q. Wei, A. Misra, “Transmission electron microscopy study of the microstructure and crystallographic orientation relationships in V/Ag multilayers”, Acta Materialia 58, 4871-4882 (2010). [8] J. A. Aguiar, H. Yang, M. C. Sarahan, N. D. Browning, “Interfacial Atomic Structure and Chemistry at Ceria Grain Boundaries”, Microsc. Microanal.17 (Suppl 2) 1248 (2011). [9] P. Hosemann, E. Stergar, L. Peng, Y. Dai, S.A. Maloy, M.A. Pouchon, K. Shiba, D. Hamaguchi, H. Leitner, J. Nucl. Mater. (2010) doi: 10.1016/ j.jnucmat.2010.12.200 [10] This work was supported by Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.

The primary damage induced within a uranium dioxide matrix subjected to a flux of energetic ions was investigated by classical molecular dynamics. UO2 was modeled using the set of empirical potentials based on a rigid ion model [1]. Displacement cascades were initiated by accelerating a uranium primary knock-on atom to a kinetic energy up to 100 keV. Cascades were also purposely overlapped within the same simulation box so as to study the response of the material to increasing damage levels. During cascade overlap sequences, the growth of nanometric voids was observed [2]. Complementary experimental studies have been performed by conducing in-situ experiments on the JANNuS platform of Orsay. Transmission electron microscopy observations confirmed that nano-void formation occurs at ambient in UO2 thin foils irradiated with energetic heavy ions. However these voids appeared to be unstable since they are not observed at higher temperatures (600°C). The presence of a little soluble or insoluble element which is likely to segregate inside the formed nano-voids and to stabilize them has also been studied. The obtained results, in conjunction with several other observations taken from the literature of ion implanted [3] or neutron irradiated uranium dioxide, suggest a radiation damage controlled heterogeneous mechanism for insoluble fission product segregation in UO2. The mechanisms which are responsible of void formation under irradiation at the atomic scale have been identified from the coupling between dedicated experimental observations and calculation results. The involved phenomena may occur in other ceramics as suggested by many observations. Obtained results can also be correlated to recent works which suggest that void formation may also occur under an electronic energy loss regime [4]. [1] N. D. Morelon, D. Ghaleb, J. M. Delaye, L. Van Brützel, Phil. Mag. 83 (2003) 1533. [2] G. Martin, P. Garcia, C. Sabathier, L. Van Brutzel, B. Dorado, F. Garrido, S. Maillard, PLA 374 (2010) 3038. [3] C. Sabathier, L. Vincent, P. Garcia, F. Garrido, G. Carlot, L. Thome, P. Martin, C. Valot, Nucl. Instr. and Meth. B 266 (2008) 3027. [4] T. Sonoda, M. Kinoshita, N. Ishikawa, M. Sataka, A. Iwase, K. Yasunaga, Nucl. Instr. and Meth. B 268 (2010) 3277-3281.

Actinides mixed dioxides are currently used in PWR nuclear reactors and are considered as reference fuels for several Gen III and Gen IV concepts. Moreover, they could act as matrices for the recycling of minor actinides, either directly in the core or in fertile blankets. In these conditions, the consequences of the incorporation of trivalent elements, such as americium or curium, in the fluorite-type structure of MO₂ should be carefully assessed when dealing with key-steps of the nuclear fuel cycle, such as reprocessing. This study was then first focused on the dissolution of (An^IV,Ln)O₂ and (An^IV,An^IV)O₂ samples (An^IV = Th, U ; Ln = La-Yb) as model compounds for future mixed oxides fuels. Since the influence of conventional parameters such as temperature or acidity was mainly described in the literature, a particular attention was paid to structural (chemical composition and homogeneity, crystal structurehellip;) and microstructural (crystallization state, density, pore size and distribution, hellip;) parameters. On the one hand, the incorporation of trivalent lanthanide elements in both CeO₂ and ThO₂ matrices was found to drastically increase their dissolution rate, in relation with the formation of oxygen vacancies weakening the crystal structure. For example, the normalized dissolution rate of Ce_1-xLn_xO_2-x/2 solid solutions was increased by 4 orders of magnitude when shifting from x = 0.1 to x = 0.5. Another significant effect of composition was found to arise from the cationic homogeneity of the solid solutions considered. Such effect was particularly evidenced for Th_1-xU_xO₂ samples, whose dissolution is often promoted by redox reactions involving U(IV). In these conditions, the kinetics of alteration was generally slowed down when improving the distribution of cations at the microscale, namely by using wet chemistry routes of preparation. On the other hand, microstructural parameters were generally found to be significant only for samples of high chemical durability, i.e. ThO₂, CeO₂, and derivative solid solutions. Nevertheless, they must be taken into account when studying the evolution of all the solid/solution interfaces. Indeed, ESEM observations performed in operando during the dissolution allowed imaging the preferential zones of release for several solids which can be located either at grain boundaries, triple junctions or through the formation of intragranular corrosion pits. It also allowed evaluating the strong evolution of the reactive surface during the dissolution of the ceramics.

The next generation of power generation facilities will require novel materials that can sustain extreme environments such as the harsh conditions in a fusion reactor. Conventional material development using for example neutron reactor irradiation experiments is an expensive and time consuming process. To improve this situation, we developed a miniaturized experimental approach that employs proton irradiation of nanoscale copper samples in combination with quantitative nanomechanical testing in situ in a transmission electron microscope. This allows to measure the full stress-strain behavior and at the same time to observe the underlying deformation mechanisms [1]. We find that for the investigated material condition there is a transition in deformation mechanism, from size-dependent material behavior governed by the activation of single armed spiral sources for dimensions below ~400 nm to a size independent bulk-like yield strength governed by the interaction between irradiation defects and moving dislocations for larger samples. The value of the transition length scale depends on the exact material and material condition, but it&’s occurrence should be a universal feature. Thus, this new approach allows to conveniently measure bulk mechanical properties and at same time to determine the underlying strengthening mechanisms in irradiated materials. Therefore, this method should significantly aid the development of novel materials to be used in future fusion or fission power plants. [1] Kiener D, Hosemann P, Maloy SA, Minor AM. Nat. Mater. 2011;10:608.

The underlying mechanisms that control the mechanical response of brittle materials to dynamic compression are not well understood. Nevertheless, indirect probes and mechanisms extrapolated from static experiments suggest rich materials behavior. However, it has not been possible to distinguish shock-induced mechanical phenomena such as plasticity, twinning, phase transformations, fracture, or intergranular kinematics from one another since in these materials post-experiment microstructural analyses and many standard characterization techniques are extremely difficult. The absence of reliable data in these areas has precluded the development of predictive models. Here, we present our efforts to obtain the first time-resolved, in situ observations of the mechanical response of brittle materials to shock compression. The IMPULSE (IMPact system for ULtrafast Synchrotron Experiments) platform under development by us at the Advanced Photon Source is a transformative new tool that enables the simultaneous capture of multi-frame diffraction patterns, phase contrast imaging, and velocimetry. These probes are being used to elucidate the mechanisms of the dynamic response along with their heterogeneity, anisotropies, interdependencies, and the role of microstructure. Initial results for organic molecular crystals and materials of interest to ballistics applications, such as glasses and ceramics, will be presented.

For tailoring superior hydrogen storage materials, it is important to understand their atomistic reaction mechanisms. By in-situ Transmission Electron Microscopy (TEM), we can observe nanostructural change of the materials under controlled temperatures and gaseous atmosphere. So far, many types of in-situ Environmental Cells (EC) for TEM have been developed and applied [1-3]. We have developed the EC for High Voltage Electron Microscope (HVEM) with an acceleration voltage of 1250 kV, aiming to high-resolution images at a relatively high pressure (asymp;0.1 MPa). We carried out an in-situ observation in hydrogenation process of particles Pd and Mg. The samples were kept in vacuum, and then in atmosphere of H2 at 0.05 MPa at RT. With gas introduction, hydrogenation and the growth of hydride around the particles were confirmed from Diffraction Pattern and Bright Field image. It can be expected that we obtain more atomic-scale information about various gas-solid reaction as well as hydrogen storage materials.

Iron sulfide scales form during the degradation of steels in the presence of H2S, notably in the extreme environments that exist in oil and gas fields. This research aims to provide a quantitative, atomic-scale description of the growth mechanisms and barrier properties of iron sulfide corrosion films in aggressive environments. Although not the primary Fe-S phase encountered under so called “sour” acidic environments, pyrite (FeS2) is used as a model system in this study to identify the unique reactivity of surface vacancies towards hydrogen sulfide and other inorganic molecules. The dissociative chemisorption of H2S is a necessary process for the growth of iron sulfide scales and also liberates hydrogen, which can dissolve in the metal and cause embrittlement. Here we study anion vacancy sites on (100) growth faces of high purity FeS2 made by chemical vapour transport, using a combination of x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). We quantitatively assess the formation of sulfur vacancy sites on the surface and correlate the electronic structure of defective pyrite with its chemical reactivity towards H2S and H2O. The experimental results obtained are interpreted by ab-initio simulations performed at our laboratory.

The oxidation kinetics, at elevated temperature in air ambient, of titanium layer sputter deposited on 2 micron CVD diamond grown on silicon substrate has been studied. The titanium layer was protected by refractory metal alloy barrier layer. The sample was annealed in air at 500C over a long period of time. The rate of oxidation of the titanium layer was studied with Rutherford Backscattering Spectroscopy (RBS) data. We also report the effect of sputtering the barrier layer in Argon-Nitrogen gas mixture (5% nitrogen by flow rate) on the barrier characteristics. The overall goal is to apply the effective barrier layer on specialized, wide band gap semiconductor based devices that will be operated at high temperatures. Surface morphology of the layer and its effect on barrier layer effectiveness was studied as well.

Currently, ceramics are being used under increasingly demanding environments. These materials have to exhibit phase stability and resist chemical attack during service. This research involves the study of the high-temperature stability of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3 2SiO2) deposited by chemical vapor deposition (CVD), the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion.

Ohmic microswitches are of great interest in a wide variety of applications ranging from cell phones to aerospace and defense. These devices have unique and significant performance advantages over solid state switch alternatives; however, their market penetration has been slow due to high cost. Making reliable electrical contacts in the microswitch is the bottleneck inhibiting affordable mass production of these devices. Mechanically, the switch surfaces must withstand 100 billion repetitive contact cycles without failure. Each cycle generates stresses equal to the hardness of the interface material. Electrically, the resistance must at remain the 1-2 ohm level. The challenge in meeting this requirement is to prevent hydrocarbon adsorption on the contacting surfaces. Too much deposit prevents electrical transmission, while too little leads to adhesion and wear. A contact material is needed that can maintain this delicate balance in the harsh environment created by high repetitive stress. The conducting oxide RuO2 is of interest because it adsorbs adventitious hydrocarbons to a lesser degree than more commonly used materials, and because due to its 20 GPa hardness it is less prone to adhesion and wear. Here we measure the reliability of RuO2 and Pt-coated microswitches in clean and contaminated environments with N2 and N2:O2 background gases. We demonstrate that RuO2 material performs very poorly in contaminated N2, but very well in contaminated N2:O2. A simple mechanochemical model is proposed that is consistent with the results. Furthermore, we introduce trace contaminant in a controlled manner to characterize device performance as a function of hydrocarbon concentration and thereby establish the threshold levels to maximize device lifetime. This information provides insight into the materials and gaseous environment required to survive the extremely harsh cycling conditions to which ohmic microswitch interfaces are exposed.

Online monitoring of reducing gases in oxygen free condition is of interest for several applications including the control of fuel cells operation. The detection of high concentrations of hydrogen, moisture, carbon dioxide, methane and carbon monoxide is a challenging task for gas sensors operating at high temperatures under such extreme conditions due to the degradation or/and contamination of sensing materials such as metal oxides (SnO2), nitrides (GaN) an metals (Pd). We report here on a novel concept that is employed to protect and stabilize gas sensors operating under harsh environments. The feasibility of our approach is demonstrated for SiBCN/SnO2 and Si3N4/GaN chemiresistors employed for the detection of hydrogen and carbon monoxide. The microporous amorphous SiBCN layers - obtained via polymer pyrolysis route - were studied as a protecting coating for semiconducting metal-oxide gas sensors. The synthesis of amorphous SiBCN-ceramics has been realized through pyrolysis of poly(organoborosilazanes) in argon, so-called polymer pyrolysis route. The top SiBCN microporous membrane offers a protection to the sensing layers based on SnO2 providing therefore an opportunity for CO /H2 sensing with SnO2 in extremely harsh conditions. Unprotected SnO2 sensors have been reduced to metallic tin after sensor test at 550 C in 1 vol.% H2/N2, while SiBCN membrane coated sensor remain stable under high hydrogen concentrations. Hence, a protection of the SnO2 sensing layer in harsh reducing conditions was achieved. GaN based sensors are known to be capable of operation in harsh environmental conditions. The large bandgap of GaN allow high temperature operation with chemical stability and mechanical robustness. However, GaN is typically contaminated with oxygen impurities that lead to the degradation of GaN performance under reducing environments. We studied the effect of microporous Si3N4 membranes on the performance of GaN sensors. Hydrogen selective Si3N4 ceramic coatings were prepared by dry ammonia pyrolysis of commercially available polysilazane. The uncoated GaN showed very high sensor signals towards CO in oxygen-free background whereas the microporous Si3N4 membrane coated and ammonia treated sensors showed almost no signal to CO at 530 C. The H2 sensor signals were still quite remarkable for Si3N4-coated GaN sensor confirming high permeance of H2 through Si3N4 membrane compared to CO permeance due to size-selective transport of gases through the Si3N4 ceramic layers. The results presented here demonstrate the potential of amorphous microporous ceramic coatings in the enhancement of the performance and stability of gas sensors operating under harsh environments.

Surface acoustic wave (SAW) devices are ideal candidates for gas sensors due to their small size, low cost of production and high sensitivity [1]. Increasing restrictions on pollution and emissions create the necessity for sensors that can operate in the harsh environments found in vehicle exhaust systems and industrial production [2]. Gallium nitride (GaN) is a robust, chemically inert piezoelectric semiconductor, making it an attractive material for SAW devices designed to detect and monitor gases in harsh environments [3]. In this work, SAW devices designed to operate at the 5th, 7th, 9th and 11th harmonics are fabricated on GaN thin films and their frequency response is measured. Devices are thermally cycled up to 1000 °C and exposed to gases typical of combustion engine vehicle exhaust. Devices are then re-measured and compared. SAW devices fabricated in this work have measured operating frequencies up to and above 2 GHz, with Q-factors up to and higher than 2000, depending on the harmonic mode. SAW devices on GaN showed good chemical stability and had negligible measured changes in frequency response when annealed up to 600 °C. [1] D. S. Ballantine Jr. et al., "Acoustic Wave Sensors: Theory, Design, & Physico-Chemical Applications", 1st ed., (Academic Press, 1996). [2] C.K. Ho, A. Robinson, D. R. Miller and M. J. Davis, Sensors, (5) (2005) 4-37. [3] S.J. Pearton et al., J. Phys.: Condens. Matter, (16) (2004)961-994.

The mechanical behavior of a range of ion-implanted amorphous Ge thin films has been studied at extreme pressures using nanoindentation. Early work on a-Ge in extreme pressure environments reported that a phase transformation was the energetically favorable deformation pathway [1]. However, details of the formation of high-pressure phases and the effect of film thickness on the phase transformation remained unclear. This current study highlights the importance of the confinement of the amorphous Ge layers between the indenter tip and the underlying crystalline Ge substrate in the phase transformation pathway. Following a discontinuity (‘pop-in-event&’) on loading the nanoindentation data fall into two distinct data sets. In one case, the end-phase is predominately polycrystalline diamond-cubic Ge, and in the other set an end-phase matrix composed of a relatively unstable mixture of the high-pressure crystalline phases of Ge is formed. If the material under load is, by and large, confined under the indentation tip an unstable mixture of the high-pressure Ge end-phases can routinely be formed. This mixture is composed principally of the r8 and bc-8 Ge phases, which we observe to be unstable and to further transform to hexagonal Ge following full unloading. However if the metallic phase is able to escape the constraint of the indentation tip, the end-phase is predominately diamond-cubic Ge - a transition reminiscent of the phenomena of so-called ‘explosive crystallization&’ [2]. 1. O. Shimomura, S. Minomura, N. Sakai, K. Asaumi, K. Tamura, J. Fukushima, and H. Endo, Phi-los. Mag. 29, 547 (1974). 2. T. Takamori, R. Messier, and R. Roy, Appl. Phys. Lett. 20, 201 (1972).

Amorphous silicon (a-Si) can be regarded as a model system for a covalently bonded, disordered tetrahedral network. However, its deformation behaviour under pressure, both in a diamond anvil cell or by the use of a pointed diamond tip (i.e. nanoindentation) is not well understood. When pure ion-implanted a-Si is relaxed by thermal annealing, it is close to an ideal continuous random network and undergoes a phase transition to a soft metallic phase with β-Sn structure at a pressure of ~11 GPa induced by nanoindentation. Other forms of a-Si appear to deform under pressure in different ways, however. For example, films of a-Si prepared by deposition techniques do not readily deform via a phase transition under indentation testing and instead preferably deform via plastic flow. Furthermore, when such deposited films are probed in diamond anvil cells various transformation pathways are reported with either crystallization into the β-Sn structure or transformation to a high-density amorphous phase. It is therefore of interest to examine such deformation behaviour as well as the stability of the a-Si under pressure using controlled preparation of different forms of a-Si. In this study the deformation (and transformation) processes under pressure were investigated for several forms of a-Si prepared using various methods that result in a variety of nanostructures. Ion-implantation generates a pure, voidless and (after thermal annealing) ‘ideal&’ form of a-Si. In contrast, magnetron-sputtering or growth via molecular beam epitaxy results in materials with nanostructure ranging from larger voids (in the order of 10 nm) to materials with thin, but long voids (1 nm in diameter). Such a-Si films were studied by nanoindentation followed by ex-situ characterization in a transmission electron microscope and by in-situ X-ray diffraction in a diamond-anvil cell using synchrotron radiation. Although the inclusion of larger scale voids causes the a-Si to deform via plastic flow compared to phase transformation for the ideal a-Si, both these forms have the same hardness as determined by nanoindentation. In contrast, when the nanostructure consisted of thin, but long nanopores, a ~10% higher indentation load had to be applied to deform the film plastically. This is somewhat surprising as the lower mass-density of such a nanoporous film may have been expected to lower the indentation load necessary for deformation. The result was confirmed by high-pressure application in a diamond-anvil cell as the onset of the phase transition to the β-Sn structure was significantly inhibited. Thus, it appears that the size of the nanostructured features in a-Si can play a significant role in controlling the pressure at which plastic deformation (or phase transformation) occurs. This insight may assist in better understanding the mechanical stability of amorphous materials.

This paper presents terahertz circuit structures implemented by multi-wafer level bonding of bulk-micromachined silicon wafers. The proposed terahertz interaction circuit experienced very high temperature and pressure during exerting specified beam radiations, but showed good signal responses in spite of those circumstances. The silicon-based MEMS techniques enabled us to fabricate this novel radiation circuit structure which can avoid mechanical distortions under harsh environments. A metal-to-metal eutectic bonding scheme was optimized for attaching four wafers precisely and parylene material was adopted to be a role as an etch stop during silicon penetration process without any charging effect and poor step coverage on entire wafer. ‘Silicon sidewall smoothing&’ process was performed to diminish scallops for avoiding return loss (S11) distortions resulting from the change of actual electrical length of the circuit lines. The optimized bonding metals and process combinations were experimented to increase the reliability including bonding strength and the surface treatment with argon gas plasma was successfully implemented. The after-fabrication yield of the circuits achieved 80 % and the S11 characteristics of the fabricated circuits identically showed similar values to each other, agreeing well with the HFSS simulation results. The measured S11 response was -30 dB level at 98 GHz. These results implicated that this bulk-micromachined silicon structure utilizing four wafers bonding can be applicable to the terahertz source circuit radiating electromagnetic waves.

Confined micro-explosion induced by a tightly focused femtosecond (fs-) laser pulses inside a transparent solid opens new strategy strategies for synthesis of new materials and the study of Warm Dense Matter (WDM) at the laboratory bench-top. In this presentation we extend the experiments of confined microexplosion into the domain of opaque materials. Fs-laser pulses can deposit a volume energy density up to several MJ/cm3 in a sub-micron volume. This creates highly non-equilibrium, hot, dense and short-lived plasmas with conditions favorable for arrangement of atoms into unusual material phases. New phases formed at pressures above 100 GPa (1 Mbar) and temperatures above 104 K can be synthesized using this approach. The essential novelty of the method is a record-fast quenching of material at ~1015 K/s so that the new material phase remains confined in a strongly localized region - ‘frozen&’ inside a bulk crystal, and can be analyzed after the ‘micro-explosion&’. Silicon is an interesting material to study both from a technological and fundamental standpoint using this method as at least twelve crystalline phases of silicon are currently known to exist at extreme pressures and temperatures. Si demonstrates twelve different crystalline structures with increased pressure and temperature up to 250 GPa. The presented experiments have a potential to extend the pressure range into unexplored area of TPa. We present here the results of structural characterization in silicon crystal exposed to strong shock wave induced by fs-laser micro-explosion in confined geometry. The conditions of confinement were realized by focusing fs-pulses on a Si surface buried under a 10-µm thick SiO2-layer formed by oxidation of a Si-wafer. 170-fs laser pulses with the energy up to 2.5 µJ were tightly focused to the intensity well above the threshold ~1012 W/cm2 for optical breakdown and plasma formation. The shock-wave modified areas of Si crystal were cut-out from a bulk using a focused-ion beam and characterized with scanning and transmission electron microscopy, Raman microspectroscopy and material characterization via nanoindentation. A void surrounded by a shock-wave-modified Si was observed under the surface in the region where the fs-laser was focused. The Si surrounding the void appears to have undergone a transition to an amorphous state. The results demonstrate that confined micro-explosion opens up new perspectives for studies of WDM at the laboratory tabletop, and unfolds new routes for formation of super-dense and super-hard materials.

When intense femtosecond laser pulses are incident on a semiconductor material, it has generally been assumed that ultrafast phase transitions follow a non-thermal pathway due to destabilization of the lattice. In this talk, results using molecular-dynamics simulation with an empirical potential whose parameters were determined by fitting to finite-temperature density-functional theory calculations will be presented. In the case of melting, the results are consistent with a discontinuous (first order) phase transition occurring at a suppressed melting temperature. In the case of laser ablation, the phase-explosion mechanism is relevant even at very high levels of electronic excitation. In fact, phase explosion is driven by fast expansion of the material that occurs when a high density of electron-hole pairs is present. Finally, potential applications of this simulation approach to other problems in extreme environments are explored, including radiation damage in solids relevant for nuclear materials.

Low chromium alloy 600 (15 wt%) and higher chromium alloy 690 (30 wt%) are being utilized in critical pressurized water reactors (PWR) components with alloy 600 being replaced in favor of the more corrosion and stress-corrosion resistant alloy 690. It is commonly accepted that this improved behavior is a result of a protective chromia layer formed on the alloy 690 surfaces and crack walls. Surface condition also plays an essential role in the degradation resistance of these alloys and many plant components are put into service with highly deformed, ground surfaces. Direct comparisons have been made in the current research between alloy 600 and alloy 690 materials in two different surface finishes (colloidal silica or 1200 grit SiC). Coupons of both alloys were exposed to 360°C PWR primary water conditions. Metallographic cross-sections were prepared and examined using high resolution, low kV, low angle backscatter scanning electron microscopy (SEM), and subsequent regions of interest were prepared by focused ion beam (FIB) milling for both transmission electron microscopy (TEM) and three dimensional atom probe (APT) investigations. Intergranular attack (IGA) occurred rapidly for the alloy 600 in both surface conditions. Grain boundaries intersecting the polished surface allowed for immediate IGA, while for the ground surface, the IGA was delayed first propagating through the nanocrystalline re-crystallized layer at the surface before reaching the bulk grain boundaries. In stark contrast, the corrosion microstructures on the alloy 690 surfaces showed little to no IGA. At the intersection of grain boundaries with the polished surface, Cr-rich oxidation occurred and produced extensive intergranular Cr depletion below the surface. The nanocrystalline microstructure at the alloy 690 ground surface served as a path for quick diffusion of Cr to the surface to form a thin, protective chromia layer. APT was also used to confirm the level of Cr diffusion ahead of leading IGA in these samples. These results will be explored to postulate how bulk Cr concentration and surface microstructure affect grain boundary corrosion and susceptibility to intergranular stress-corrosion cracking.

The cracking susceptibility of nickel base super alloy X-750 to stress corrosion cracking in boiling water reactor (BWR) environment has been investigated. Crack growth rates were determined in normal and hydrogenated water chemistry at 288°C. Post test analyses including SEM, TEM and APT were performed in order to investigate the cracking mechanisms. The role of grain boundaries morphology (character, presence of carbide), and gamma prime phases will be discussed. The characterization of crack tips will be presented.

The fracture behaviour of individual grain boundaries has been studied in order to understand the mechanisms controlling stress corrosion cracking in nuclear reactors. In particular, the role of oxidation in facilitating crack initiation and propagation has been reviewed. Nickel alloys from pressurized water reactors (PWRs) have been tested in simulated primary water conditions to induce grain boundary oxidation. Microcantilevers containing an oxidized grain boundary plane have been prepared and tested and the results compared with a finite element model. The brittle nature of the oxide was demonstrated and the required stress to fracture measured.

Localized corrosion and stress-corrosion cracking in high-temperature water environments are critical issues impacting the reliable operation of many industrial systems, including power generation systems. Despite decades of research, the mechanistic basis of the materials degradation in these environments is still unclear. In this work, we employ transmission electron microscopy (TEM) and atom-probe tomography (APT) to study the grain boundary oxidation and corrosion behavior of model binary Ni-base alloys exposed to simulated pressurized water reactor (PWR) primary water environments at 360 °C. Combining the analytical power of TEM and APT, we have identified striking differences in the oxidizing behavior of Ni-5Cr and Ni-5Al binary alloys. Ni-5Cr exhibits continuous intergranular oxidation (primarily Cr2O3) that is accompanied by severe depletion of Cr from the grain boundary for distances of several microns beyond the oxidation front. In contrast, Ni-5Al exhibits discontinuous intergranular oxidation (primarily NiAl2O4) that is accompanied by some void formation without detectable long-range Al depletion along the grain boundary. Intriguingly, neither system exhibits the formation of MO-type oxides, which are the dominant intergranular oxidation products in similarly corroded commercial Ni-16Cr-7Fe alloy 600. The implications of these observations in the context of internal oxidation mechanisms in high-temperature water environments will be discussed.

Brittle fracture is often controlled by grain boundary behavior. Until now measuring the fracture properties of single grain boundaries has required macroscopic bi-crystals. We have developed techniques to measure the fracture toughness of selected grain boundaries using micro-cantilevers (typically asymp;5mu;m wide and 25mu;m long), made by focused ion-beam machining, followed by loading using a nanoindenter. The grain boundary plane, misorientation and local chemistry have been characterized using FIB, TEM-EDX and EBSD. At room temperature the fracture behavior of bismuth-doped-copper and tungsten alloys have been investigated. In bismuth doped copper the results show a variation of fracture toughness between 1-7MPam^0.5 which is in good agreement with the literature values. There is no dependence on crystallography, either in fracture plane of the boundaries or mis-orientation is seen. Previous work on Cu-Bi bi-crystals has focused on “special” low sigma boundaries and this is the first time a large number of special and non special boundaries have been tested. While micro-fracture testing has been applied to a wide range of materials including; metals, semi-conductors and nano-crystalline materials, these tests have all been performed at room temperature. For many applications it important to be able to test fracture properties of materials over the range of temperatures at which they operate. Using a high temperature nanoindenter, with vacuum and controlled atmosphere capabilities and an AFM like scanning stage, the fracture behavior of single crystal silicon and tungsten alloys have been investigated up to 750°C. Silicon was chosen as it has well documented brittle to ductile transition on the macro-scale and has previously been used a calibration material for micro-fracture testing. Tungsten alloys have been chosen as they are a candidate material for plasma facing components in fusion power reactors, experiencing a large range of temperatures (from 5K to 3000K). While grain boundary fracture has been seen to control brittle fracture behavior in these alloys on the macro-scale, the effect of temperature on single boundaries has not been studied. Thus high temperature micro-mechanical fracture tests are an important tool in understanding and developing these alloys. Cantilevers were tested at loading rates of 5µN/s at temperatures ranging from 23°C to 750°C. Single crystal silicon micro-cantilevers show a brittle-to-ductile transition temperature of asymp;400°C which is in good agreement with literature values for bulk tests. Ultra high purity tungsten and tungsten 5wt% tantalum micro-cantilevers have also been tested. While pure tungsten is found to be ductile by 600°C the W-5wt%Ta is brittle across the full temperature range. EBSD and TEM-EDX are now being used to understand how the crystallography and local chemistry influence the grain boundary behavior.

Chromia-forming alloys are mainly used in high temperature industry, the a-Cr2O3 protective oxide scales formed at high temperature improving the corrosion resistance of the metal. However, it is usually mentioned that the compressive stress state generated in the oxide films during oxidation, or even increased on cooling due to the differential between thermal expansion coefficient of the substrate and the film, may finally lead to spalling by the buckling phenomenon. Thus, to increase the durability of materials it is crucial to better understand such buckling and spalling phenomena in thermal oxide films in order to limit or even avoid it. This is particularly true for chromia-forming alloy which is one of the main protective thermal oxide films which develop on structural material, e.g., stainless steel used at relatively high temperature. To investigate the eventual relation between the stress level in the oxide film and delamination appearance, previous studies have already been done mainly in alumina scales and to a lower extend in chromia scales. From a mechanistic point of view, it is crucial to study the competition between different stress relaxation mechanisms which may lead or not to damage situation, e.g., by delamination processes. Also, it is necessary to determine if a univocal correlation exists between stress magnitude and damage appearance, or if some specific other physical parameters have also to be taken into account. The aim of this communication is to underline the application of both powerful X-ray diffraction and Raman spectroscopy techniques to access the in plane stress in the oxide layer at a macroscopic and local scale. In particular, stress mapping associated with the observed damages have been measured in our laboratory and at synchrotron facilities.

The sulfur-iodine thermo-chemical cycle (S-I cycle) is one of the promising nuclear hydrogen production methods combined with a high temperature gas-cooled reactor. During the operation of S-I cycle, heat exchanger materials will experience extremely aggressive environments caused by sulfuric acid (H2SO4) and hydrogen iodide (HI), which are the working fluid of the system. To provide better corrosion resistance in such environments, aluminum-rich zone was formed on the surface of Alloy 617 using physical vapor deposition followed by either electron beam re-melting or diffusion treatment. Coupon type specimens were exposed for 100 h in flowing H2SO4 or HI at 850oC. Argon gas was used as a carrier gas. Several high temperature austenitic alloys including Alloy 800HT and Hastelloy C-22 were also tested for comparison purpose. After 100 h exposure, the surface treated Alloy 617 showed negligible weight change in both environments. However, scanning electron microscope observation showed porous surface oxide and internal oxidation for electron beam treated specimens. On the other hand, the diffusion treated specimens showed better corrosion resistance in a sulfuric acid environment, such that the thickness of surface oxide was less than a half of electron beam treated specimens. Especially, in a hydrogen iodide environment, the damage on the diffusion treated specimen was considerably less significant probably due to the protection by very thin aluminum-rich oxide on the surface.

The corrosion of structural materials ranks among the most complex dynamical processes in materials science. It involves surface and interface reactions, electronic and ionic transport and the occurrence of a variety of phase transformations, all driven by extreme chemical and mechanical driving forces. The ability to predict the occurrence and mechanisms of corrosion requires detailed knowledge about fundamental electronic, thermodynamic, kinetic and mechanical properties of the various phases participating in the corrosion reactions. First-principles statistical mechanical methods are now capable of predicting a wide variety of thermodynamic and kinetic properties as well as the couplings between chemistry and mechanics that determine the rate and mechanism of degradation of structural materials. In this talk, we will describe how corrosion of Zr in aqueous environments can be modeled from first principles.

Understanding the mechanism and kinetics of self-limiting metal oxidation is among the most pertinent topics in the physics and chemistry of surfaces and thin films. Classic Cabrera-Mott theory stipulates that the limited growth of oxide films results from electron tunneling from the metal through the oxide film to adsorbed oxygen leading to the formation of oxygen anions on the oxide surface. This leads to an electric field across the oxide film that assists ion migration and makes low-temperature oxide film growth possible. A central issue for the engineering of metal/oxide interfaces is understanding the formation of the self-generated electric field and ion transport through the oxide film. In this work we present experimental confirmation of the existence of the built-in electric field, based on X-ray photoelectron spectroscopy measurements of aluminum oxidation, and show that the magnitude of the electric field is tunable by controlling the surface coverage of adsorbed oxygen. The oxygen anion coverage exhibits a Langmuir isotherm behavior with changes in temperature and oxygen pressure. We demonstrate that by changing the oxidation conditions (temperature and oxygen gas pressure), the self-limiting growth behavior of an oxide film can be manipulated thereby allowing control over the passivation properties of the metal surface. These observations also illustrate the more general feasibility of tuning an interfacial reaction via self-adaptation to its environment.

Degradation of steels in harsh, H₂S-containing environments (called sour corrosion) leads to formation of iron sulfide corrosion scales on the exposed surface. A quantitative description of the barrier properties and surface reactivity of these corrosion films is necessary for predicting corrosion lifetimes for products used in the petroleum industry, where such harsh environments are normally encountered. In this study, we use density functional theory (DFT) modeling and nudged elastic band (NEB) methods to assess the kinetics of key reactions that govern the formation and growth of iron sulfide corrosion films. For this purpose, we calculate adsorption energies for H₂S chemisorption and activation barriers for dissociation of H₂S molecules atop a model iron sulfide surface of (100) pyrite, as well as the diffusion of charged point defects in pyrite. Anion vacancies on the surface are found to show stronger adsorption and easier dissociation of H₂S compared to defect-free adsorption sites. A reactivity model based on the electronic structure of the defect-free and defective pyrite surfaces is developed to explain the enhancement of reaction rates at surface sites with anion vacancies. The corrosion behavior of the surface in high [H₂S] conditions is analyzed by assessing the inter-adsorbate interactions on surfaces with high H₂S coverage. Together with experimental characterization of pyrite crystals using x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), this study provides a mechanistic and quantitative description of the kinetics of the initial stages of sour corrosion and a measure of the corrosion protectiveness of the iron sulfide film.

The use of heavy liquid metals (HLM), namely Pb or Pb-based alloys, in energy-related applications, is under consideration, due to their favourable thermal and neutronic properties. However, concerns are related to their compatibility with structural steels in terms of corrosion and mechanical resistance. To protect the steels from direct contact with HLM, the formation of an oxide layer is aimed at. Thermally grown protective oxide scales on steel surface, exposed to HLMs, can be achieved in operando by dissolving a suitable level of oxygen into the HLM. Structural materials, envisaged to be used in HLM environment, are austenitic steels (AS), ferritic-martensitic steels (FM) and ODS steels. Experiments have shown that, below 500°C, the AS - 15-15Ti steel and the FM - T91 steel are protected by stable, relatively thin FeCr-based oxide scales. However, above 500°C, AS steels suffer severe dissolution attack, while FM steels form thick, fast growing oxide scales, which reduce heat transfer from the fuel pins and may break off, eventually blocking the coolant channels. The improvement of corrosion resistance has been attempted by alloying the steels with strong oxide-forming elements (e.g. Al). Aluminum has shown its potential to protect steel surfaces in contact with HLM (containing small amounts of oxygen) forming slowly growing Al-rich oxide scale. However, the development of a thin, continuous, dense, stable, adherent Al2O3 layer, providing an effective corrosion barrier in HLM, requires a minimal Al-level, which unfortunately affects negatively the mechanical properties of the steel. Therefore a surface alloying process, using intense pulsed electron beam (GESA-process), was developed and optimized in KIT. The procedure consists in two steps: (i) coating the steel surface with an Al-containing alloy layer and (ii) melting the deposited layer and the steel surface to form an homogeneous, well bonded coating. In order to find the suitable composition for the formation of Al2O3 protective scale, oxide maps, for the oxidation of pure Fe-Cr-Al bulk alloys in oxygen-containing HLM, were drawn using grazing incidence synchrotron X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy and thermo-gravimetric measurements. It was found that an Al content of at least 8 wt.% guarantees the formation of a thin Al2O3 scale on coatings containing also 12-16 wt.% Cr. Based on these results, FeCrAl-based layers were deposited on flat and tube specimens (T91) and then exposed to HLM. The corrosion experiments showed the good protective behavior of Al scales (<0.001 mm thickness) in HLM with 10minus;6 wt.% oxygen up to 650°C and for exposure times up to 10,000 h. Creep test in HLM, at 550°C, show strain, 2nd creep rates and creep-to-rupture times comparable with those of non-coated T91, tested in air. The Al surface alloying by GESA process improves the fretting resistance up to a factor of 5 between 450°C and 550°C, compared with non-coated T91.