Nanostructured metals are often sought for the perceived benefits of high strength (at similar densities) in many impact applications. We focus on the behavior of these materials at the extreme conditions of high pressure and high strain rate developed in these applications, approaching this from both mechanics and materials perspectives.
We begin by examining the strengthening mechanisms within nanostructured metals deformed at high strain rates. While grain boundary related mechanisms are often considered, the related effects of twin boundaries and slip-twin interactions can play a major role. We examine these features by considering ultra-fine-grain aluminum and aluminum alloys. We also consider a conventional grain size high-strength aluminum alloy where the high strength arises from control of specific features at the nanoscale, demonstrating that much of such alloy development is in fact a form of nanostructure control. We explore the reasons for the high strength of this alloy by considering the contributions of various strengthening mechanisms to the total strength of the material, using quantitative TEM together with models for each mechanism. Finally, we consider the failure mechanisms that may develop in nanomaterials, and discuss the tradeoffs involved in such materials.

The improvement of materials or the development of new ones often requires the design of new advanced processing techniques. Resistance Sintering (RS) - also called Spark Plasma Sintering (SPS) - has demonstrated its potential for improving the rapidity and efficiency of the sintering stage for the powder metallurgy technique. Thanks to higher heating rates and lower sintering temperatures than what is used for hot pressing, a limitation of the grain growth phenomenon during the sintering stage can be achieved. Thus, this kind of “flash” process becomes a pertinent choice for (i) the processing of bulk nanostructured parts and (ii) the fast sintering of fairly dense products. However, full densification of the materials still remains difficult to reach notably on account of the limited applied load (<100MPa). It is expected that a change in the pressure nature and/or pressure value could result in an additional increase of the potential of these “flash” processing methods.
In this context, in addition to the conventional quasi-static uniaxial loading, a dynamic loading, based on the use of Split Hopkinson Pressure Bar (SHPB) technique, has been implemented leading to a new process: the Dynamic Compaction Resistance Sintering (DCRS). The high deformation rates (10^2 - 10^4 /s) and high stress level imposed to the specimen (asymp;GPa) available with this kind of dynamic loading are here used in combination with the quasi-static loading (5 to 100MPa) to achieve tailored properties.
After a presentation of the new DCRS process, the potential of this technique is discussed through the analysis of the sintering behavior of a copper/oxide powder and the subsequent properties of the sintered products. The influence of the addition of one to multiple dynamic loadings at different sintering temperatures is evaluated through the analysis of mechanical properties. These properties are correlated, via micromechanical models, with the microstructure observations in order to take into account the oxide particle reinforcement, the grain size as well as the density of structural defects induced by the dynamic loading

We have investigated the high strain rate mechanical properties of microstructure engineered commercial purity titanium with multiple structural length scales. The materials were fabricated by consolidating pre-mixed nanocrystalline powders produced by cryo-milling under different conditions. Microstructural examinations via transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD) revealed the multiple structural length scales within the various materials. Both quasi-static (strain rate ~0.001/ s) and high strain rate (strain rate ~1000/ s) uniaxial compressive loading conditions were used to evaluate the mechanical properties of the microstructure engineered titanium. High strain rate compression was performed using a Kolsky bar (or Split-Hopkinson Pressure Bar) system. Our results show that under high strain rate compression, most of the samples exhibit highly localized adiabatic shear localization even though strong strain hardening is present in these samples. The high strain rate behavior is discussed using a mechanistic model for adiabatic shear localization that takes into account strain hardening, strain rate hardening and thermal softening.

Nanostructured metals are very attractive for structural applications because of their outstanding mechanical properties compared to those of their corresponding bulk materials: ultra-high strength. However, these materials typically have poor creep behavior and high strain rate sensitivity [1, 2]. Previous work has systematically investigated the mechanical properties such as the Young&’s modulus, yield strength, creep behavior and strain rate sensitivity of nanocrystalline Nickel (nc-Ni) LIGA components using micro-tensile, load relaxation, strain rate jump tests and micro-pillar compression.
Since its introduction by Uchic nearly a decade ago [3], micro-compression testing has become a preferred technique for investigating size-dependent plasticity. During this period, it has been adapted for testing a variety of test geometries (cantilever bending, tensile, double-cantilever beam fracture, etc.) and extended to elevated temperatures and variable strain rates [4]. However, the achievable strain rates have usually been limited to the near-quasistatic regime: <10^-2 s^-1.
In this study, we examine nc-Ni produced by electrodeposition with mean grain sizes within a range of 15 nm to 120 nm using high-speed micro-compression and extend this range by 4 orders of magnitude to ~10^2 s^-1 in order to examine the deformation mechanisms of small volumes of nanostructured material at high strain rates.
References
[1] M.A. Meyers et al, Prog. in Mat. Sc. 51 (2006) 427-556
[2] N. Wang et al, Mat. Sc. and Eng. A237 (1997) 150-158
[3] M.D. Uchic, D.M. Dimiduk, J.N. Florando, W.D. Nix, Science, 305 (2004) 986-989.
[4 J.M. Wheeler, J. Michler, Review of Scientific Instruments, 84 (2013) 064303.

Pearlitic steel wires exhibit tensile strengths above 7 GPa after severe plastic deformation. the deformation refines the lamellar structure and leads to cementite decomposition. The correlations between deformation and cementite decomposition in pearlite are not understood. In the present work, a local electrode atom probe and TEM were jointly used to characterize the nanostructure evolution of pearlitic steel, cold-drawn up to a true strain of 6. The carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon-dislocation interaction.

There is much interest in the recent years in the nanoscale metallic multilayered materials due to their unusual mechanical properties such as very high flow strength and stable plastic flow to large strains as well as extraordinary radiation damage tolerance. In an effort to shed light on fundamental mechanisms underlying these unusual performances, successive uniaxial compression experiments (total strains = 1%, 2%, 10% and 20%) on nanoscale Cu/Nb multilayer pillars using ex situ synchrotron-based X-ray microdiffraction technique were conducted. We found significant X-ray peak broadening in both Cu and Nb layers initially (up to strains of about 4%) which was then followed by the saturation of the X-ray ring width broadening (up to large strains of 20%). This has been further confirmed using quantitative analysis involving peak width measurements of different crystal planes of the metals (ΔK vs. K) to further eliminate the effects of crystal size and instrumentation broadening. This observation indicates that the interfaces in the nanolayered Cu/Nb are very stable and effective in trapping and annihilating dislocation content during mechanical deformation, and explains why the Cu/Nb nanolayers can be deformed to large plastic strains without any onset of plastic instabilities leading to their demonstrated abilities to survive both extreme radiation as well as mechanical environments.

Mg alloys are among the lightest alloys but their strengths are usually low. Here we report a new mechanism to make them ultrastrong and moderately ductile. Stacking faults with nanoscale spacing were introduced into a Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr (wt.%) alloy by conventional hot rolling, which produced a yield strength of ~575 MPa, an ultimate strength of ~600 MPa, and a uniform elongation of ~ 5.2%. Low stacking fault energy played an essential role in producing a high density of stacking faults which impeded dislocation slip and promoted dislocation accumulation. These findings provide guidance for development of Mg alloys with superior mechanical properties.

Nanostructured metals are a group of materials whose compositional grain size is smaller than several hundreds of nanometers. These nanocrystals usually exhibit unique mechanical properties compared with their coarse-grained counterparts. However, the creep deformation of them are incredibly enhanced at elevated temperature and high stress because of the inevitable grain growth behavior. Therefore, the applications of those excellent structural materials are quite limited at extreme conditions. As a result, a systematic investigation of the creep behaviors of nanostructured metals is not only motivated by the scientific curiosity, but also makes sense for the engineering purpose to expand the application environment and service life time of such novel materials. In this study, extensive atomistic modellings are applied to understand the creep mechanisms of nanostructured metals at extreme conditions. We target at building a bridge linking the phenomenological deformation modes and the physics underlying them. The following points are reached from our simulations with atomic details.
1. With increasing applied stress, the deformation mechanisms change from diffusive mechanisms, etc., grain boundary diffusion and its accommodated grain boundary sliding and migration, to displacive deformation modes, which is dislocation nucleation from grain boundary. The conclusions are supported by atomistic details, along with reasonable physical parameters, such as stress exponent, grain size exponent, and activation enthalpy[1,2].
2. For the first time we explain why transition of deformation modes happens with varying external stress and temperature conditions. It is based on a physical model with Arrhenius-like constitutive equations for creep[1].
3. Strong entropic effect is found in the creep deformation of nanostructured metals. The relationship between activation entropy and activation enthalpy lies well in an empirical law described by Meyer-Neldel compensation rule[3].
All those findings provide deeper physical understanding on the deformation of nanostructured metals at extreme conditions. Based on our results, sound physical modes are potentially built to describe the novel deformation modes. These fundamental studies benefit providing strategies to stabilize nanostructured metals at extreme conditions towards widening the applications of those promising structural materials.
References:
[1] Y. J. Wang, A. Ishii, and S. Ogata, “Transition of Creep Mechanism in Nanocrystalline Metals” Phys. Rev. B 84 (2011) 224102.
[2] Y. J. Wang, A. Ishii, and S. Ogata, “Grain Size Dependence of Creep in Nanocrystalline Copper by Molecular Dynamics” Mater. Trans. 53 (2012) 156-160.
[3] Y. J. Wang, A. Ishii, and S. Ogata, “Entropic Effect on Creep in Nanocrystalline Metals” Acta Mater. 61 (2013) 3866.

I will give some highlights of recent TEM experiments with in situ ion irradiation of nanostructured metals. These experiments, by users of the IVEM-Tandem Facility at Argonne National Laboratory. feature metal samples prepared with unusual nano-microstructure designed to resist or accommodate extreme conditions of high levels of irradiation at a range of temperatures up to 800 °C. Examples of such samples will include nano-grained, nano-twinned, and precipitate enhanced structures. Dynamic image data taken under irradiation at temperature will illustrate the interactions of irradiation defects with these nanostructures in real time. In addition to these fundamental interactions, the formation of a nano-grained material due to high levels of irradiation (approaching 100 dpa), which may reach a steady state, will be described.

Stacking fault tetrahedra are detrimental defects in neutron or proton irradiated structural metals with face-centered-cubic structures. Their removal is very challenging and typically requires annealing at very high temperatures, incorporation of interstitials or interaction with mobile dislocations. We present an alternative solution to remove stacking fault tetrahedra discovered during room-temperature, in situ Kr ion irradiation of epitaxial nanotwinned Ag with an average twin spacing of ~ 8 nm. A large number of stacking fault tetrahedra are removed during their interactions with abundant coherent twin boundaries. Consequently the density of stacking fault tetrahedra in irradiated nanotwinned Ag is much lower than that in its bulk counterpart. Two fundamental interaction mechanisms are identified, and compared to predictions by molecular dynamics simulations. In situ studies also reveal a new phenomenon: radiation induced frequent migration of coherent and incoherent twin boundaries. Such twin boundary migration is closely correlated to the absorption of radiation generated dislocation loops. Potential migration mechanisms are discussed.

Recent attempts to develop radiation resistant alloys have focused on creating high densities of unbiased sinks for point defects. Interfaces have been widely recognized as stable, unbiased sinks for point defects produced under irradiation, however a number of computational works have shown that the efficiency of these sinks will depend on the detailed structure of the interface. Little experimental work has been performed, to date, to directly characterize and quantify the sink strength. In this study we grow single crystalline Cu-Ag films, bounded by different coating layers, such as Nb and Ni, and use high-sensitivity in situ resistivity measurement to monitor irradiation-enhanced Ag precipitation from the initial stages of the irradiation (under short pulses) through the final stage (under prolonged irradiation). Samples with a free surface are also studied for comparison. It is found that the efficiency of the interfaces plays a significant role on the precipitation kinetics.
During prolonged irradiation the precipitates grow and coarsen, and the sample becomes suitable for XRD/TEM/STEM investigations. By measuring the size of the precipitates as a function of their distance from an interface, the relative strengths of internal sinks, such as the precipitates themselves, and the interfaces are deduced.

Many theoretical predictions have suggested that the confined length scales and increased interface density of various nanostructured materials may result in desired thermal, mechanical, and radiation properties. In this presentation, we will highlight a unique transmission electron microscope (TEM) that has been developed at Sandia National Laboratories. This in-situ ion irradiation TEM is capable of observing in real time with nanometer resolution ion irradiation with a range of high energy (1 MeV to 14 MeV) ion beams using various ion species (H, He, Si, Ni, Cu, Fe W, and Au), performing 4D tomographic reconstructions as a function of dose, heating the sample during ion beam exposure, performing microfluidic flow and mixing, heating at pressures as high as 1 atm, observing ion beam induced luminescence within the TEM, ion implanting of He at energies up to 10 keV, and performing quantitative small scale mechanical property testing during ion beam exposure. Each of these capabilities will be briefly highlighted with a nanomaterial application in the associated extreme environment; Focusing on an in-depth example of the application of concurrent heavy ion irradiation and He implantation into a high-purity nanostructured tungsten sample produced by severe plastic deformation.
This work was partially supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. 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.

Tungsten has been identified as the leading solid material for fusion plasma facing component applications because it has many desirable properties, such as high melting temperature, good thermal conductivity, low sputtering and erosion yield, high strength, low thermal expansion, and high resistance to swelling. Plasma facing components in advanced fusion devices will be exposed to an extremely hostile environment that includes severe and variable heat loads, damage from neutron bombardment, and surface modification due to impingement by energetic particles. A fusion-relevant 14 MeV neutron source is not available to conduct irradiation experiments so it is essential to develop quantitative models of W property evolution as a function of neutron flux and fluence, temperature, and concentration of gaseous transmutation products such as He and H.
Multi-scale models are being developed to simulate neutron-induced damage in bulk W. Molecular dynamics simulations are being performed to study the effects of primary-knock-on atom (PKA) energy, temperature, and He content on displacement cascades in W. For pure W, simulations show that for PKA energies < 30 keV sub-cascades do not form, and the defect survival efficiency is small due to a small damage volume. The number of surviving Frenkel pairs displays sub-linear dependency on PKA energy. For PKA energies > 30 keV interconnecting sub-cascades are observed, and strong clustering of like defects is found, which gives rise to a super-linear dependency on PKA energy. Initial work on the affect of He on cascade damage indicates that at low-temperatures, He atoms are more effective than self-interstitial atoms in finding vacancies. However, the effectiveness of He filling with increasing temperature is influenced by the interplay between increased He mobility to find a vacancy versus the decreased stability of a He-vacancy complex.
A lattice-based object kinetic Monte Carlo code is being developed to perform kinetic simulations of microstructure evolution (kSOME). This code is designed to account for the migration, emission, transformation and recombination of all types of intrinsic point defects and their complexes. In addition, methods for treating in detail the interactions of these point defects with sinks such as dislocations, grain boundaries and free surfaces are included. The code is being employed to study the long-time annealing of displacement cascades in W for comparison with experimental measurements of visible features of the irradiated microstructure. The database of displacement cascades described above provides a source term for kSOME simulations. Other needed input data, such as activation energies for migration and dissociation of defects, and their capture radii are obtained from atomic-level calculations. The evolution of radiation damage is being investigated as a function of time, temperature, dose and dose-rate. We report our latest results from these simulations.

We report on an experimental and simulation campaign aimed at exploring the radiation and mechanical response of nanoporous gold (np-Au) foams.
In previous work, we studied the response of np-Au foams under irradiation at room temperature and different dose-rates [E. Fu, et al. APL 101 (2012) 191607]. Our experimental findings show that 400 keV Ne++ ion irradiation for a total dose of 1 dpa leads to the formation of Stacking Fault Tetrahedra (SFTs) at high and intermediate dose-rate, while no SFTs are formed at low dose-rate. An atomic-view of the process was previously reported based on Molecular Dynamics simulations (MD). It was observed that vacancy migration distance and nanofoams ligament size play a key role in explaining the dose-rate dependent defect accumulation.

In this work, we go a step further and make use of nanoindentation techniques to investigate the changes in np-Au foams mechanical properties. We study their deformation behavior under compression before and after ion irradiation at room temperature. We discuss our experimental findings in view of recent MD simulations of ligament deformation behavior under compression [L. Zepeda-Ruiz, et al., APL (2013) in press]. Simulations predict that nanoscale foams soften under irradiation in very good agreement with our hardness nanoindentation measurements.

FePd based alloys have attracted tremendous interest during the past 30 years - initially due to their Invar as well as magnetic properties,
and, more recently, owing to their magnetic shape memory (MSM) behavior with theoretical strains up to approximately 5%. When processing
Fe-Pd ferromagnetic shape memory thin films, selection of the desired phases and their transformation temperatures constitutes one of the
largest challenges from an application point of view. In the present contribution we demonstrate - using a combination of experiments and
molecular dynamics (MD) computer simulations - that irradiation with 1.8 MeV Kr ions is the method of choice to achieve this goal.
Single crystalline Fe7Pd3 thin films that are grown with molecular beam epitaxy on MgO (001) substrates and subsequently irradiated
with ions reveal a phase transformation along the whole phase transformation path ranging from fcc austenite to bcc martensite. While for
10^14 ions/cm2 a fcc-fct phase transformation is observed, increasing the fluence to 5x10^14 ions/cm2 and 5x10^15 ions/cm2 leads to a phase
transformation to the bcc phase. Pole figure measurements reveal an orientation relationship for the fcc-bcc phase transformation according
to Nishiyama/Wassermann. The results are interpreted in terms of generalized stresses due to (i) point defects and (ii) deviations from
the equilibrium short-range order. Generalization of these concepts to other types of external forcings, such as severe plastic deformation, are
also discussed.
[1] A. Arabi-Hashemi and S. G. Mayr, Phys. Rev. Lett. 109 (2012), 195704

Recent developments within the nuclear materials community have lead current researchers to hypothesize that composite materials can address concerns regarding nucleation, growth, migration, and fission product evolution at higher temperatures and radiation environments. Advanced microscopy allows for unprecedented insight into the physical mechanisms responsible for the improved performance of these materials. To extend our understanding of radiation tolerance, and connect macroscale properties with potential sub-Ångstrom changes in electronic and atomic structure, the use of aberration corrected energy filtered microscopy is a necessary tool.
Studying ceramic interfaces offers opportunities to understand inherently complex phenomena in connection with material performance and behavior. Tailoring ceramic interfaces considers a variant of structural and chemical properties. Differences in crystal structure are selected to regulate changes in interfacial structure, misfit dislocation spacing, and the concomitant role of vacancy-interstitial energetics. In the later, the termination layer at the interface can play a major factor in determining the interfacial energetics. The same energetics are responsible for the microstructural evolution at interfaces and the concomitant role on radiation tolerance. The loss of crystallinity at interfaces leads to both changes in atomic coordination and swelling that can cause mechanical failure. Comparing the interfacial structure, chemistry, electrostatics, and response before and after irradiation can thereby lead to control of radiation tolerance and tailoring the onset of intergranularity.
In this work, we examine the evolution of electronic structure in connection with the formation of intergranular films following light ion irradiation at structured interfaces. Combining controlled epitaxial growth of thin films and ion beam radiation, we have performed aberration corrected transmission electron microscopy coupled with spectral imaging on a series of irradiated CeO2-STO oxide interfaces to address both simultaneous changes in atomic structure and chemistry. We determine changes in the interfacial termination layer are the structural pinning sites for the onset of intergranularity. Based on these experimental results, first-principles based calculations investigate the resolved interfacial chemistry and structure, including effects on mechanical and functional properties. The results provide critical insights into heterogeneous interfaces, radiation damage, and how composite materials can be tailored for radiation tolerance.
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 Basic Energy Sciences under Award Number 2008LANL1026 and ORNL&’s Shared Research Equipment (SHaRE) User Facility, sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences.

We have determined the energetics of defect formation and migration in M_nAX_n-1 phases with M = Ti, A = Si or Al, X = C and n = 3 using density functional theory calculations. In the Ti₃SiC₂ structure the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular conguration (C-Si = 1.889 #8491;) and to two apical Ti atoms (C-Ti = 2.057#8491;). This carbon-metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti-Si, 17.3 eV for Ti-C, and 6.1 eV for Si-C. Broadly similar behaviour was found for Frenkel and antisite defect energies the Ti₃AlC₂ structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti3AlC2 is dierent and the energy is lower compared with the Ti₃SiC2 structure. This eect, together with the observation of a stable C interstitial defect coordinated by 3 silicon species and 2 titanium species in Ti₃SiC₂, will have important implications for radiation damage response in these materials.

The European Space Agency (ESA), in collaboration with the Japan Aerospace Exploration Agency (JAXA), is preparing a mission to the planet Mercury. This inner solar system planetary mission, named BepiColombo, is the 5th cornerstones of ESA&’s long-term science programme. The high heat loads and high levels of radiation of external surfaces require materials to be used in unchartered environments. A challenging mission orbit with high heat loads coming from the 700 °K hot sub solar point of Mercury is a specific challenge.
External materials are expected to reach up to about 500°C while irradiated with a UV/VUV intensity up to 11 times higher than in earth orbit. Various facilities are available at the Materials Space Evaluation and Radiation Effects Section at ESTEC. Following earlier work synergistic effects like UV, particle and temperature are degradation factors that need to be considered.
Due to the harsh environment only high performance materials can be expected to resist the space environmental loads. In some cases newly processed materials were developed by modification of the micro and nanostructure. Examples of those will be shown as well as their response measured by various in-situ as well as ex-situ measurements like UV-VIS-NIR-FIR measurements and surface analysis like XPS. In addition, the effect of radiation induced contamination effects on the top surface has become a critical area to be investigated which has been shown to influence functional properties of materials. Again the changes are analysed by thermo-optical characterisation techniques, Raman as well as surface analytical chemistry.
As one example results will be shown on a nano-structured ceramic material exposed with and without contamination sources which were exposed up to 500C and up to a duration of 29 000 esh (equivalent sun hrs). Also results of the full lifetime simulation of this coating will be shown.

the addition of solute atoms to nano-structured metals can have a profound influence on their thermal stability and mechanical properties. This can result from the thermodynamic stabilization of grain boundaries (when referenced to a dilute solution) due to solute segregation, or from a significant reduction of grain boundary mobility due to solute drag.
We have been using a combination of density functional theory calculations, molecular dynamics simulations and a meso-scale defect model to generate and test new analytic relations that illustrate how the combined properties of solvent and solute atoms influence grain boundary stability and mobility. This talk will focus on three aspects of this work. The first is a perturbation approach that allows rapid calculation of solute substitution energies for embedded-atom method potentials without having to carry out simulations that relax system strains. This approach speeds up the efficiency of Monte Carlo simulations that can be used to establish solute atom distributions in complex microstructures. The second aspect to be discussed focuses on the influence of grain boundary segregate density on grain boundary motion at high driving forces. In the final part of the talk our attempts to combine Monte Carlo and molecular dynamics simulations to model the influence of solute atoms on grain boundary mobility at driving forces that better mimic experiment will be presented.
This work was supported by a grant from the Office of Naval Research.

Recent advances in surface engineering has led to a new set of thermal barrier coatings based on refractory metals. Refractory metals, such as tungsten and its alloys, are primary candidate materials for coatings of structural components in extreme environments. In this talk we present the first set of micro-mechanical experiments to characterize the microstructure, mechanical properties, and damage modes of a new set of micro-patterned tungsten dendritic coatings developed by ULTRAMET. We show that these coatings demonstrate ultrahigh strength beyond that of nanocrystalline tungsten.

The automotive industry is aggressively pursuing use of advanced high-strength steels (AHSS) in automobile body structures. Among these are nano-precipitate strengthened steels (NPSS). The NPSS achieves high strength while maintain high ductility by utilizing nano-sized precipitates in a single-phase matrix. A critical concern of the NPSS for auto body applications is the thermal stability of these nano-precipitates—they must possess sufficient resistance to particle growth and coarsening in a number of body assembly steps such as welding during automotive body construction. Preliminary test results (hardness and TEM) suggest that the dissolution of the nano-precipitates cause the significant degradation of the NPSS weld.
We have performed SANS experiments to scan a 2mm diameter aperture along welded NPSS sample in 1mm steps. Asymmetry (elongation) of the scattering in the base NPSS material disappears in the weld. The asymmetry of the scattering away from the weld indicates that the nanoparticles or regions of different scattering length density are oriented, characteristic of elongated particles that lay in the plane of the plate material; the loss of anisotropy in the weld with a similar intensity at Q=0 indicates that the nanoparticles are not dissolving but are losing orientation, elongation, or both while maintaining their volume and density.

Nano-scale metallic multilayers exhibit superior mechanical properties and a resistance to harsh environments due to the nature of their interfaces. Incoherent interfaces are generally stronger, acting as barriers to slip transmission, and are dislocation and radiation-induced defect sinks. These interfaces, specifically between to immiscible metals like Cu and Nb, have also shown resistance to thermal degradation, making them even more attractive as a hard coating material. Coherent interfaces, however, show more ductility from the continuous slip system across the boundary but tend to be less resistant to thermal degradation due to the similarity of their crystal structures. Molecular dynamic simulations of Cu-Nb bilayers indicate decreasing temperature dependence as the layer thickness decreases when the layer thicknesses are on the order of tens of nanometers. A new trilayer system, with a mixture of incoherent and coherent interfaces, is investigated to determine the thermal stability of mixed interfaces using elevated temperature nano-indentation. Results show reduced temperature sensitivity as the individual layer thickness decreases with the hardness of the 30 layer dropping 35% where as the 5 nm sample only drops 15% across the same temperature range. By choosing the layer thickness and material selection of the multilayer films, mechanical properties can be specifically tailored, making them useful as wear resistant coatings.

Functionally graded and hierarchical composite coatings have continued to be a topic of interest as materials that can function under extreme operating conditions. These materials are needed to meet the increasing demands of the defense, automotive, and aerospace industries. While many methods have been proposed and developed for manufacturing these composites, most are either very process intensive, limited in size resolution, or result in cracked and porous materials. Electrochemical deposition of metal-matrix-ceramic composites provides a unique and practical means of manufacturing functionally graded and hierarchical structures that can avoid the manufacturing difficulties that hinder competing techniques. Here we present novel nickel matrix - alumina particle reinforced composites using a rotating disk electrode (RDE). The volume fraction of particles is controlled by the rotation rate of the RDE, so higher order composite structures can be formed by simply varying the rotation rate during deposition. We demonstrate a variety a microstructures including uniform, graded, and layered composite materials. Micro-tensile tests were performed to characterize the mechanical behavior of the uniform and layered composite materials under ambient conditions and high temperatures, via joule heating of the sample. The hierarchical composites showed unique strength and ductility. This manufacturing technique provides a practical means of generating materials more suitable for extreme environmental conditions.

Because of their high volume fraction of interfaces, nanocrystalline metals have a tendency to coarsen, especially under the high temperatures required for, e.g., powder consolidation or other shape-forming processing operations. Certain alloying conditions can help bypass such instability by providing a solute segregated grain boundary configuration that is energetically preferable compared to any bulk states, rendering these alloys stable against both grain growth and bulk phase separation. In this talk we describe our recent modeling and experimental work on nanocrystalline tungsten, for which a high-temperature sintering step is a processing requirement. Guided by Monte Carlo simulations, we identify a minority addition of Ti as a stabilizer, and experimentally show that with this addition a nanocrystalline grain structure can be retained at the sintering temperature of 1100°C for at least one week. Scanning transmission electron microscopy and atom probe tomography results show an inhomogeneous solute distribution after high temperature exposure, in line with the expectations of our Monte Carlo simulations.

Coherent {111} twin boundaries (CTBs) have much lower energy than typical grain boundaries, and hence show improved thermal stability. However, {112} incoherent twin boundaries (ITBs) have energies similar to low- and high-angle grain boundaries, making them less stable, and more importantly, critical to the thermal stability of nanotwinned metals. Epitaxial nanotwinned Ag films with (110) and (111) texture were deposited with CTBs angled to the growth direction in (110) films and parallel to substrate in (111) films. After annealing at temperatures up to 800 °C (homologous temperature of 0.87), we found twins were removed from (110) films, but remained with remarkably fine sub-100 nm spacing in (111) films. The influence of orientation on thermal stability of nanotwins is discussed. Furthermore, from the range of microstructures created, we formulate a strengthening model that considers the contributions of both CTBs and ITBs to hardness.

There are many proposed powder routes to obtain nanocrystalline alloys, all of which require rapid low-temperature consolidation. Unfortunately, accelerated sintering methods, such as liquid phase sintering and activated sintering, do not adapt simply to nanocrystalline systems. Here we explore a new enhanced sintering mechanism that is specifically useful to nanostructured alloy powders. The method employs super-saturation and phase separation to achieve rapid densification of nanocrystalline alloys. The approach is demonstrated here using nanocrystalline tungsten powder prepared by high-energy ball milling. We quantitatively analyze the sintering kinetics using the master sintering curve approach, and analyze the sintered microstructures by microscopy and elemental mapping. Generality of this approach to other binary metal systems is also discussed.

Mechanical characterization of thin films at elevated temperature is extremely important for the reliable operation of a micro/nano-scale device. However, due to the difficulty of testing nano, micro-scale free-standing thin films at elevated temperatures, this behavior is relatively unknown. Here we introduce our in-situ SEM (scanning electron microscope) mechanical tester which enables tensile testing in vacuum while observing the microstructural change. The displacement resolution of the tester is 10 nm with 250 µm stroke and the load resolution is 9.7 µN which is further adjustable by changing the spring of the tester.
A silicon-based micomachined heater is fabricated for mechanical testing at elevated temperatures inside the SEM. Each micro-heater consists of a tungsten heating element which also serves as a resistance thermometer. A current passed through the heating element heats the heater and the free standing metal thin film. Potential drop between the voltage probes is monitored during current flow and the temperature change is determined from a four-point thermistor resistance measurement that has been calibrated to temperature.
Tensile test were performed on submicron-thick Cu and Au films at various temperatures up to 430°C. Stress-strain curves show a significant decrease in yield strength and initial loading and unloading slope for samples tested at higher temperature, which we attribute to grain boundary sliding with lattice diffusion and dislocation climb.

Transition metal nitrides have remarkable properties that make them suitable in a wide range of applications, including wear resistant coatings, diffusion barriers, and decorative surfaces. Previous studies of these nitrides have focused mainly on single-to-nano-crystalline structures, and very little is known for their amorphous state. Low substrate temperatures in combination with high deposition rates and ion-bombardment-induced recoil mixing during PVD processing are favorable for formation of amorphous thin films, as reported for metals, oxides, and carbides. We propose amorphous multicomponent transition metal nitrides as a new class of durable materials that could add to the range of possible applications, due to the homogeneous structure and mixed character of bonding.
We have recently shown that addition of Al and Si to TiN (e.g. Ti0.26Al0.46Si0.28N1.17) in cathodic arc evaporation promotes renucleation and x-ray amorphous structure [1]. The amorphous films are thermally stable up to 900 °C and exhibit age hardening up to 1100 °C with an increase in hardness from 19.4 GPa to 31.6 GPa.
In this study, we turn focus on the Ti-B-Si-N system and the role of B instead of Al. (TiB2)1-xSixN (0le;xle;1) thin films were grown on Si(001) substrates by reactive DC magnetron sputtering from TiB2 and Si targets in a 20%-N2/Ar atmosphere. Compositional analysis of the films was performed by elastic recoil detection analysis, and the structural information was gained by X-ray diffraction, and analytical transmission electron microscopy. The mechanical properties of the films were characterized by nanoindentation.
We show that by adding both B and Si, two well-known grain refiners to TiN, creating a multicomponent system, the as-deposited films assume an x-ray diffraction amorphous state - regardless of composition and growth temperature. For (TiB2)1-xSixN with xge;0.33, the films are also electron diffraction amorphous. The thermal stability and crystallization behavior is studied using in-situ annealing experiments and small-angle x-ray scattering experiments. The mechanical properties are characterized using in-situ annealing in combination with nanoindentation. In addition, we will present a comparison with Ti-B-Si-N films grown by industrial scale reactive arc evaporation as well as with industrial scale high power impulse magnetron sputtering.
[1] H. Fager, J.M. Andersson, J. Lu, M.P. Johansson Jöesaar, M. Odén, and L. Hultman (2013), submitted.

Ion beam synthesis (IBS) and ion beam processing (IBP) have proved to be suitable methods to obtain materials based on NPs and to tune their properties. However, a general understanding of the behavior of nanostructures under irradiation is still lacking. This is due to the fact that ion-matter interaction is a complicated process. It can drive the nanocomposite material into novel experimental configurations that are far off-equilibrium. Thus, one of the challenges in this field is the developing of new experimental routes permitting to obtain valuable information on the behavior of the ion-driven NPs. In this regards, one longstanding and intriguing problem is the possibility to use ion-irradiation to inverse the thermodynamic stability of an ensemble of NPs. For example, under a thermodynamically-driven process, larger particles grow at the expense of the smaller ones (Ostwald ripening-OR). On the other hand, under irradiation, this evolution can be reversed, i.e. the smaller NPs become more stable than the larger ones. This processed is called inverse Ostwald ripening (IOR).
Here, we first rapidly depict the history of IOR phenomenon from the original paper of Nelson (1970) to the analytical model developed by Heinig. After that, we show that insights into the evolution of NPs under ion irradiation can be obtained if a model system is used, i.e. metallic NPs are first chemically synthesized and then sandwiched between two silica layers.
This model system is used with a twofold objective: i) to study the behavior of NPs when irradiation conditions are changed, ii) to experimentally check the Heinig model.
We show that the Heinig model correctly describes the IOR process. Moreover, our model system is used to give an experimental estimation of all the parameters contained into this model, i.e. the evolution of the capillarity length with temperature, the diffusivity under irradiation and the steady-state concentration for both planar and curved surfaces. Conversely, we show that the temperature threshold for regime transformation is not well defined and that the passage from OR-to-IOR regime is not a direct process but at least two new intermediate regimes must be introduced.

It is generally accepted that one-dimensional carbon nanostructures (such as carbon nanofibers and carbon nanotubes) are improving the mechanical properties (Young modulus) of polymeric matrices, enhancing the temperature stability of these matrices and imparting new electrical features (depending on the concentration, size, and nature of the filler). However, the behavior of polymer-based nanocomposites to ionizing radiation was not investigated in detail. A thorough analysis of the effect of gamma irradiation on thermal stability of isotactic polypropylene-vapor grown carbon nanofibers, as derived from thermogravimmetric analysis in air and in inert atmosphere (nitrogen) is reported.
Polymer-based nanocomposites were obtained by dispersing vapor grown carbon nanofibers within isotactic polypropylene. The Marlex HLN-120-01 isotactic polypropylene from Philips Sumika Polypropylene Company, used in this work, has a density 0.906 g/cm³ and a melt flow rate at 230 ^oC of 12 g/10 min. The nanofiller of choice was vapor grown carbon nanofibers with diameters ranging between 60 and 100 nm and lengths between 30,000 and 100,000 nm, supplied by Pyrograf Products (PR-24AG). The nanofibers were purified and disentangled in dichloromethane and deionized water followed by vacuum filtering and drying at 110 ^o for 24 h. Nanocomposites containing various amounts of filler, ranging from 0 to 20 % wt. filler have been prepared by extrusion at 180 ^oC for 9 minutes at 65 rpm, followed by a 5 minutes mixing at 90 rpm and 180 ^oC. The as obtained samples have been hot pressed into sheets with a thickness of about 0.6 mm at about 180 ^oC and by a force of 1000 N for 100 s. All measured samples had the same size (1cmx1cm), shape, and surface. Samples of nanocomposites have been subjected to gamma irradiation by a ⁶⁰Co source, in air, at room temperature. The integral doses were 0, 9, 19, and 28 kGy. The dose rate was 1 kGy/h.
Experimental data suggested a decrease of the thermal stability of the nanocomposites due to gamma irradiation. For a better understanding of the oxygen role, thermogravimmetric measurements have been performed both in air and nitrogen. Such a result may have important consequences on the applications of such nanocomposites in aerospace, with emphasize on polymer-carbon nanostructures composites used on satellites orbiting in Low Earth and Geostationary Orbits.

Graphene nanoplatelets (GNPs) are novel nanofillers possessing attractive characteristics, including robust compatibility with most polymers, high absolute strength, and cost-effectiveness.
In this study, GNPs were used to reinforce epoxy/ carbon fiber composite (CFRP) laminates to enhance their mechanical properties under environment aging effects. The mechanical properties of epoxy/carbon fiber composite laminates, such as ultimate tensile strength, flexural properties and interlaminar shear strength (ILSS) were tested at two temperatures under high-moisture condition: high temperature (85 degrees Celsius/ 85%RH) and room temperature (25degrees Celsius/ 85%RH).
Consequently, The overall mechanical properties of GNPs/CFRP composites laminates were greatly enhanced with adding GNPs under environment aging effects. The ultimate tensile strength, flexural strength and interlaminar shear strength (ILSS) of CFRP with a 19%, 13% and 17% improvement for 0.25 wt% GNPs loading under 85 degrees Celsius / 85%RH, respectively. Moreover, the flexural modules of CFRP laminate with 0.75 wt% GNPs-added showed a 13% increased compared with neat CFRP laminates under 85 degrees Celsius/ 85%RH.

Metals containing nanoscale growth twins have received much attention recently as a potential alternative to nanocrystalline metals as a high strength material. Nanotwinned (nt) Cu, in particular, has been shown to achieve strengths comparable to nanocrystalline Cu, while maintaining good amounts of ductility, in addition to being relatively mechanically and thermally stable. To date, the vast majority of experimental work has focused only on nanotwinned copper, and much of the assumptions on the potential of growth-twinned metals have been based on this. In order to develop an in-depth understanding about the contributions of twins to the mechanical performance of metals, it is necessary to expand the study to various systems. The current research involves the synthesis, deformation, and microstructural characterization of two additional systems: nt-Ag and nt-CuAl alloy. The synthesis of these materials by magnetron sputtering will be discussed. In addition, microstructural changes and overall deformation modes in deformed areas of the various nt metals will be presented.

Carbon nanotube (CNT) is chemically stable and electrically conductive material. One of the applications of CNT is filler into insulating materials for decreasing its electrical resistance. In this study, dispersion fluid of polytetrafluoroethylene (PTFE) was mixed with CNT dispersion to form CNT/PTFE composite film. Although PTFE is electrically insulating material, this composite film shows electrical conduction. The application of this composite film is anticorrosion coating to bipolar plate (electrode) of fuel cell, because it has chemical stability in addition to the electrical conductivity.
The CNT/ PTFE composite film was formed from dispersion fluids of the CNT and the PTFE. CNT dispersion was made from multi-wall type CNT. Cellulose derivatives were added into water to disperse the CNT. Water based commercial PTFE dispersion was used in this study. The dispersion fluids of the CNT and the PTFE were mixed and stirred by applying the ultrasonic wave. The CNT/PTFE resin dispersion was applied to stainless steel bipolar plate at the thickness 50 micro m. The bipolar plates were at 350 oC for 20min.
Pure PTFE showed the low conductivity below measuring limit. The CNT/ PTFE composite film of 25% CNT showed high conductivity of 20S/cm. The conductivity increased up to 30S/cm with an increase in the CNT concentration up to 75%. This result indicates that the CNTs form the electrical network in the PTFE film and modify the film into electrically conductive material.
The CNT/ PTFE composite film was coated on the bipolar plates (electrodes) of a fuel cell. The fuel cell using the bare stainless steel (SS) bipolar plates showed maximum output power of 1.5W. The coating of the CNT/ PTFE composite film on bipolar plates increased the maximum output power up to 3.9W, which is higher than that of bare SS bipolar plates by 2.6 times.
The electrochemical measurements was perform to evaluate the performance of the CNT/ PTFE composite film as a anticorrosion carting of the bipolar plates (electrodes). Polarization curve were measured in 1M H2SO4 from corrosion potential to 1500mV vs. Ag/AgCl for the SS bipolar plate coated with the CNT/PTFE composite film. The Ag/AgCl electrode was used as the reference.
The bare SS bipolar plate was 8x10-5A/cm2 in the passive current at the potential of 800mV vs. Ag/AgCl. The passive current was decreased to 2.8 x10-5A/cm2 with the coating of the CNT/ PTFE composite film. The decrease in the passive current suggests that the CNT/PTFE composite film protects the SS bipolar plates from the surface corrosion during the operation of the fuel cell.

Wind energy is one of the most useful power sources which transform it to electric energy due to its low costs, no pollution and high efficiency. In the wind energy system, composite materials are widely applied for wind turbine blades to generate power, seismic retrofitting and repair of concrete structures. Compared to metal blades, the wind turbine blade fabricated by polymer composite is one of the key components in the wind energy system due to its low density and high durability. In addition, fiber reinforced plastics (FRP) emerged as an attractive potential to substitute the metallic materials in many weight-critical applications such as aerospace, automotive and various industrial applications due to their low densities and excellent mechanical properties high specific strength and specific modulus). Based on these advantages mentioned above, FRP is suitable materials for the application to wind turbine blade. Graphene oxide (GO) possesses outstanding mechanical and thermal properties for using in polymer composites due to its unique structure and properties. GO with oxygen-contained groups shows the hydrophilicity may disperse well in the polymer matrix, thus enhancing the comparability. Comparing with the carbon fiber reinforced epoxy (CF/Epoxy), GO-CF/Epoxy with the addition of 0.75 phr GO shows 17 % and 13 % increase in flexural and tensile strength, respectively. GO-CF/Epoxy composite demonstrates 41 % improvement in the coefficient of thermal expansion. The use of GO as nanofiller in CF/Epoxy
shows a great potential application to wind turbine blade.

Porous inorganic nanowire aerogels with particular properties have tremendous applications. However, creating inorganic nanowire aerogels has remained an outstanding challenge. Here, we present a facile methodology to enable ultralight and highly porous inorganic nanowire aerogels production from in-situ nanowire gels composed of interconnected inorganic nanowires obtained by hydrothermal synthesis without supporting materials. The in-situ hydrogel formation is based on the self-assembly of one-dimensional (1D) nanowires into a cross-linking network during growth of nanowires from the precursor suspension. The resultant nanowire aerogels exhibit high porosity, high surface areas, extremely low densities and superelasticity and the superior properties are demonstrated as oil/solvent absorbents, which are tens of times higher than those of conventional ones. This work suggests the inorganic nanowire aerogels a widespread potential for applications in industry as well as topics regarding environment, energy, and thermoelectric devices research.

Solute segregation in grain boundaries (GB) could enhance the thermal stability of nanocrystalline metals due to the reduction of grain boundary energy and thus the capillary driving force for grain growth. In the present work, the effects of substitutiaonal grain-boundary segregation of Ni, Cr, Ta and Zr on bcc-Fe Σ3 (111)<110> tilted symmetry grain boundary are investigated energetically by first-principles calculation. It was found that the Σ3 grain boundary energy of bcc-Fe could be reduced obviously by Zr, moderately by Ta and not significantly by Ni and Cr. Zr and Ta was predicted to enhance the thermal stability of nanocrystalline Iron, which is well agreed with experiments [1-2]. Furthermore, the mechanism of this alloying segregation enhanced GB stabilization phenomena was studied from the both views of bonding electron charge densities and atomic strain energy.

1. Darling, K.A., et al., Grain-size stabilization in nanocrystalline FeZr alloys. Scripta Materialia, 2008. 59(5): p. 530-533.
2. Darling, K.A., et al., Stabilized nanocrystalline iron-based alloys: Guiding efforts in alloy selection. Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 2011. 528(13-14): p. 4365-4371.

In order to improve combustion efficiency, micro-particles in fuel generally have been pulverized by being penetrated through small holes in pulverization disk with extremely high pressure and cavitation effect. The disk is made of a single crystal diamond synthesized by the high pressure and high temperature method and high cost as well. To generalize this pulverization method widely, its high material cost must be reduced.
As a result, we pulverized water using some ceramic disks with low cost; silicon carbide, silicon nitride, boron carbide, and partially stabilized zirconia (PSZ) at a pressure of 200 MPa for 5 hours. The disks were first damaged by the bombardment of particles and finally etched much faster than the single crystal diamond disk. PSZ indicated the good pulverization performance and the application possibility as the disk among these samples because it has relatively higher toughness. However, PSZ disk was not sufficient enough to use since it showed the cavitation wear at the surface caused by the low hardness of PSZ. Then, we deposited nanocrystalline diamond (NCD) films that have great hardness and surface roughness on PSZ disk to protect the surface from damage by the cavitation effect. The NCD films on PSZ were grown in hot filament chemical vapor deposition reactor from gas mixtures containing CH4 (10 sccm), H2 (190 sccm), and N2. N2 gas was added to this mixture at concentrations from 0 to 10 sccm. In 0 sccm of N2 gas flow rate, large exfoliation and cracks were showed because of poor adhesion strength. However, the nitrogen-doped NCD films improved the exfoliation more remarkably than that of the non-doped NCD film, and also decreased the progress of film cracking. In X-ray photoelectron spectroscopy analysis, nitrogen concentration linearly increased in the films with increasing N2 gas flow rate, and the dissolved oxygen from PSZ matrix, which helps to etch the interface between the crystal growth surface and PSZ, was also contained in the films with higher N2 gas flow rate without etching the interface. The adhesion strength between film and PSZ was evaluated by nitrogen doping. Then, nitrogen-doped NCD films exhibited a significantly higher adhesion on mechanical load than non-doped NCD film.
In this report, we suggest to substitute a single crystalline diamond with PSZ by coating nitrogen-doped NCD film, which can protect surface from the bombardment of particles and the cavitation wear. By this novel technique, it is concluded that total cost is lowered, and this pulverization technique is spread especially in the environmental fields.

We developed a new highly porous polyimide (PI)-silica composite with good flexibility, good mechanical strength and resistance to high temperature. The composite was prepared by a new process including (1) phase separation of polyimide precursor (polyamic acid), solvent, and silicon alkoxide mixture induced by high pressure CO2 (40°C, 20 MPa), (2) formation of silicate by sol-gel reaction, and (3) supercritical CO2 extraction of the solvent. SEM observation revealed that the composite had a bimodal porous structure including both micro- and nano-sized pores. Silica nanoparticles (<100 nm in diamiter) were highly dispersed in the PI matrix. The densities of the resulting composites were in the range of 0.26 - 0.36 g/cm3, which is lower than the popular porous PI had been prepared by conventional physical foaming technique (0.77 - 1.45 g/cm3) and/or water vapor induced phase separation technique (0.36 - 0.41 g/cm3). Relative dielectric constant of the material were lower than 1.5 at 1 MHz. The porous PI-silica composite sheets were flexible enough to be folded without cracking, and had Young&’s modulus of 0.36 - 0.80 GPa. The onset decomposition temperatures were around 600°C. Notably, the silica composite had higher Young&’s modulus and onset decomposition temperature than the porous PI of similar density. The porous PI-silica composite is highly promising as a flexible thermal insulation material for high temperature, and a thermal resistant low-k material.

Radiation induced segregation at grain boundaries is known to be a key factor in irradiated assisted stress corrosion cracking in bulk Fe- and Ni- based alloys. In order to develop insights into advanced alloys or processing routes that are more radiation tolerant, a systematic study has been completed to determine the fundamental response of model Ni-Cr alloys under high temperature irradiation as function of both grain size and grain boundary character distribution. Two model austenitic fcc alloys: Ni-5Cr and Ni-18Cr were thermomechanically processed to induce a wide range of critical grain boundaries including a large length fraction of both coherent and incoherent twin grain boundaries. In addition to the thermomechanically processed bulk samples, free standing films of nanocrystalline Ni-5Cr and Ni-18Cr were deposited using physical vapor deposition. All samples were irradiated with heavy ions (Ni ⁴⁺ ) and protons at 500°C. Following the irradiation, a grain boundary specific FIB-EBSD preparation method was used to investigate the interaction of irradiation induced defects, defect denuded zones, and solute segregation with specific grain boundaries by STEM-EDS and correlated GB site-specific atom probe tomography. Results will be presented showing the variation in radiation induced segregation with the full grain boundary character distribution as function of Cr concentration (Ni-5Cr vs. Ni-18Cr). The results indicate anisotropic solute segregation behavior as function of the inclination angle for the Σ3 twin grain boundary. In addition, the results show that not all high angle or low angle GBs respond equally under irradiation which has significant implications for ultrafine grained materials where there is a high density of grain boundary area.

Recent studies have described how surface modifications affect the mechanical properties and electrical conductivities of carbon nanotubes (CNTs) and their distribution in epoxy resin. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and creep behaviors of the composites thus formed is investigated, and the strength of the material is tested at different environments.
The test results also indicate that mechanical strength increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it. An SEM image of the fracture surface reveals debonding and the pulling out of longitudinal fibers because of poor interfacial bonding between fiber and matrix, which reduce overall strength.
Moreover, the creep behaviors of carbon fiber (CF) /epoxy resin thermosetting composites and CNTs/CF/ epoxy resin composites were tested and analyzed at different stresses, orientations of fiber, temperatures and humidities. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.
Additionally, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55 celsius degree. Possible room temperature creep mechanisms have been proposed and discussed. With increasing number of creep tests, the creep strain decreased due to strain hardening which occurred during creep. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle theta; between the orientation of fiber and the direction of the applied stress.
Furthermore, the mechanical strengths and creep strain of CF/epoxy resin composites and CNTs/CF epoxy resin composites performed post-curing and aging pretreatments at a constant temperature for different days and at different temperatures for one day prior to creep testing were also investigated.
Finally, the Findley power law was used to fit the curves for creep strain in the CF/epoxy resin composites and MWCNTs/CF/epoxy resin composites under various test conditions. And Larson-Miller equation can be adopted to precisely predict the low-temperature-long-term creep behavior by the high-temperature-short-term creep behavior.

Design of radiation tolerant materials has been an active research subject for decades. Recent studies show that layer interface in certain metallic materials may lead to enhanced radiation tolerance. Here we present studies on ion irradiation tolerance in nanolayered TiN/AlN and TiN/MgO films with different individual layer thickness (5-50 nm) prepared by pulsed laser deposition. We examined the evolution of microstructure and mechanical properties of He ion irradiated multilayers. The suppression of amorphization in AlN and the mitigation of defect clustering in MgO are observed in the two multilayer systems. Layer interface also mitigates radiation induced softening and hardening respectively in TiN/AlN and TiN/MgO multilayer films. A clear size-dependent enhancement of radiation tolerance is observed in the TiN/AlN multilayer films. Our studies suggest that nanolayer interfaces could act as effective sinks for irradiation induced defect clusters in nitride-based ceramic nanocomposites, and shed light on selection and design of radiation tolerant ceramic materials with unique interfaces.

The degree of resistance to structural degradation of metals in corrosive environments depends, to a large extent, on the mechanical and chemical stability of the passive film that forms on the surface. A mechanistic description of the passive film breakdown process is needed to both accurately quantify the protectiveness of these films and systematically design better barrier layers. Here, we investigate, using Density Functional Theory (DFT) calculations, the mechanical stability of surface films composed of the layered iron-sulfide, mackinawite (Fe1+xS) formed in industrially-important extreme ‘sour&’ corrosion environments commonly encountered in the petroleum industry [1]. Mackinawite crystals have low mechanical strength because weak van der Waals forces are mainly responsible for inter-laminar cohesion [2]. Our results showed that the toughness of mackinawite films decreases by over 75% upon the introduction of hydrogen interstitials, which are natural by-products of the cathodic corrosion reaction. Through a quantification of formation energies of different native and hydrogen-related defect configurations and activation barriers for defect migration, we propose a mechanism involving interplay between atomic hydrogen interstitials and native point defects leading to accumulation of hydrogen molecules in inter-laminar spaces inside the mackinawite crystal. By acting as internal catalytic sites for H2 defect generation, the native defect sites (cation vacancies) convert mechanically benign atomic hydrogen interstitials into deleterious H2 molecules that result in a marked reduction in the fracture toughness of the mackinawite crystal. Our mechanistic explanation provides insight into inter-defect interactions and degradation of layered chalcogenides that are currently being investigated for applications ranging from photocatalysis to energy storage [3, 4].
References
1. Azevedo, C.R.F., Failure analysis of a crude oil pipeline. Engineering Failure Analysis, 2007. 14(6): p. 978-994.
2. Jeong, H.Y., J.H. Lee, and K.F. Hayes, Characterization of synthetic nanocrystalline mackinawite: Crystal structure, particle size, and specific surface area. Geochimica et Cosmochimica Acta, 2008. 72(2): p. 493-505.
3. Zhou, W.J., et al., Synthesis of Few-Layer MoS2 Nanosheet-Coated TiO2 Nanobelt Heterostructures for Enhanced Photocatalytic Activities. Small, 2013. 9(1): p. 140-147.
4. Chang, K. and W.X. Chen, In situ synthesis of MoS2/graphene nanosheet composites with extraordinarily high electrochemical performance for lithium ion batteries. Chemical Communications, 2011. 47(14): p. 4252-4254.

Under pressure in a diamond anvil cell (DAC), a number of high pressure (metallic) phases of both Si and Ge are possible, and, on pressure release further metastable phases that can be semi-metals or semiconducting can form. However, the end phase material powders are not easy to extract from a DAC for property measurements and to exploit any unique properties. However, under nanoindentation it is also possible to induce phase transformations immediately under the indenter tip in zones that have dimensions from several micrometers to below 100 nm. For example under indentation, diamond cubic and amorphous Si and Ge can transform to the first metallic phase (β-Sn) at around 10-12 GPa pressure. On unloading this phase transforms to a series of metastable phases on unloading some of which are stable at room temperature.
In this paper we first outline our recent measurements that illustrate the various phases of both Si and Ge that can be obtained after full pressure release. For Si several phases are possible including the bc8 and r8 phases and on annealing the hexagonal diamond (hd) and an entirely new phase (Si-XIII). In Ge, the previous literature constitutes a very confusing picture for possible end phases with bc8, st12, hd and even dc Ge reported from both DAC and indentation experiments. We have considerably resolved this issue in a series of recent results. Our measurements show for the first time that the r8 phase is the dominant end phase after pressurization via indentation. However, if the indentation conditions significantly depart from hydrostatic behaviour, the st12 phase can result. In this presentation we focus on the detail of the transformation pathways that lead to r8 Ge and it subsequent instability under annealing where it transforms via bc8 to hd Ge. We use Raman, XRD and TEM for phase determination and TEM for showing the microstructure.

Single crystals of the uranyl peroxide nanocluster, U24Py12, were analyzed by in situ Raman spectroscopy, synchrotron X-ray diffraction, and small angle X-Ray scattering in diamond anvil cells, as well as electrospray ionization mass spectrometry. The U24Py12 cluster, [(UO2)24(O2)24(HP2O7)6(H2P2O7)6]30-, is composed of edge-sharing uranyl hexagonal bipyramids with peroxide, hydroxide, and pyrophosphate as equatorial coordinating ligands. At pH~12, clusters of U24Py12 form crystals with the approximate chemical formula Na8[(UO2)24(O2)24(P2O7)12]. At ambient pressures, U24Py12 is tetragonal (P42/mnm: a = 22.746(2) Å, c = 30.426(4) Å). U24Py12 single crystals are transparent yellow, and can grow up to 1 mm in diameter; single crystals less than 200 microns in diameter were used. Pressures ranged from ambient to 50 GPa. Two symmetric stretch modes of the uranyl ion are evident in the Raman spectra with wavenumbers of 810 and 830 cm-1; they are clearly observed at pressures up to 5.4 GPa and shift toward higher wavenumbers with increase of pressure. Between 5-15 GPa, the uranyl ion vibrational modes overlapdue to stress in the crystal. At pressures higher than 15 GPa, the uranyl ion vibrational mode broadens further and shifts to higher wavenumbers, which persist up to 50 GPa (highest measured pressure). After pressure quenching, Raman spectra exhibit one vibrational mode at 840 cm-1. High pressure X-ray diffraction measurements were completed at Argonne National Laboratory to quantify structural unit cell changes over the pressure ranges investigated. U24Py12 undergoes a phase transition to higher symmetry at approximately 5 GPa, and eventually partially amorphizes at 17 GPa, irreversibly. Unlike other nanoclusters previously investigated, such as U60, the U24Py12 cluster shows phase transitions that are at least partially reversible, indicating the overall strength of the structure and cluster orientation.

Multiscale dislocation dynamics plasticity (MDDP) is used to investigate the deformation mechanisms of copper single crystal subjected to high strain rate shock and monotonic loading. The simulations are carried out at strain rates ranging between 1.0×10^4/s and 1.0×10^10/s where mechanisms of homogeneous and heterogeneous nucleation of dislocation are implemented. For strain rates below 10^7/s, heterogeneous nucleation of dislocation sources dominates the induced plasticity, while homogeneous nucleation of small dislocation loops acts as the main mechanism of plastic flow for strain rates greater than 10^7/s. Detailed analysis of strain rate sensitivity in shock-less monotonic loading shows the following features: a) small strain rate hardening up to 10^5/s b) large strain rate hardening for strain rate below 10^7/s, and c)stress saturation due to homogeneous nucleation for strain rate beyond 10^7/s. In shock loading simulations on the other hand, the yielding stress exhibits a power law dependency on strain rate. Additionally, the homogeneous nucleation simulations show that the stresses exhibit an elastic overshoot followed by rapid relaxation. The relaxation process leads to the transformation of the 1D state of strain of shock loading to a 3D state. Based on MDDP results, we develop models for dislocation density evolution, saturated dislocation density, and stress relaxation time at different pressures. Moreover, an extension of high strain rate Orowan equation that accounts for homogeneous nucleation is derived.

The discovery of graphene and methods to isolate it raised the prospect of a new class of nanoelectronic devices based on its extraordinary physical and electrical properties. Soon enough it was observed that its electronic properties are strongly sensitive to the environment and also by molecules assembled on its surface. In this context, studies regarding the interaction between hydrogen (H2) and graphene devices have become particularly important because of the possibility of using graphene as a hydrogen storage material and a new type of gas sensor devices. Here, the electronic properties of monolayer graphene have been investigated (in situ) under the exposure of molecular hydrogen using the gate voltage-dependent conductivity as a function of temperature. The field-effect transistor mobility of graphene is highly sensitive to H2 exposure, showing an asymmetrical effect in the hole and electron mobilities. What is fully reversible and happens in all temperatures analyzed. Moreover, charging effects are dependent of temperature and also changes in minimum of conductivity is observed. Analysis on all these aspects suggests that the scattering mechanism that causes the asymmetric behavior is due to H2 acting as a short-range scattering center as well as being intercalated between graphene and the SiO2 substrate. Further analysis to explain the influence of hydrogen on the electron and hole mobilities will be presented.

Clathrate gas hydrates have long fascinated researchers and have been the focus of much in depth study over the past fifty years. Gas hydrates impact societal problems related to conventional and non-conventional energy production and storage in addition to global climate change. Most laboratory studies on hydrate formation and agglomeration phenomena have been carried out in high-pressure batch reactors. Thermal resistances and mass transport limitations introduce challenges when hydrate formation and dissociation rates are of similar time scales. However, the study of clathrate hydrates in microscale laminar flow offers that opportunity to add new knowledge on hydrate science. Our high-pressure microfluidic synthesis and dissociation of methane (sI) and propane (sII) hydrates, on demand and in a matter of minutes, shows the potential to bridge the knowledge gap between the laboratory study of hydrates and the production systems that encounter them. In the present work, we will elucidate the rational behind the high-pressure microfluidic synthesis of hydrates and the recent advances we have made in the field.

Nanocrystalline (nc) metals possess superior mechanical strength in comparison to their bulk counterparts. However they typically have poor thermal stability at elevated temperatures. Here we present a strategy to enhance the thermal stability of nanocrystalline metals. Co-Cu-Ag tri-phase immiscible nanocomposites (TPIN) are fabricated via magnetron sputtering. All co-sputtered films are composed of nanocrystalline grains. Thermal stability and mechanical properties of several TPIN are studied and compared to nanocrystalline binary Cu50Ag50 and monolithic Cu. After annealing at 973 K, 92% of the eutectic temperature of Cu and Ag, the grain size of Co34Cu33Ag33 TPIN remains ~115 nm, whereas that of Cu50Ag50 film increases to over 700 nm. Meanwhile mechanical tests show that hardness of Co34Cu33Ag33 TPIN is much higher than that of Cu50Ag50 film. The remarkable enhancement of thermal stability of nanocrystalline metals is largely achieved via introduction of a third immiscible element (phase) to the binary alloys, forming immiscible tri-phase nanostructures with abundant triple junctions.

Lead chalcogenide NCs, designated as PbS, PbSe, and PbTe in this review, have recently been the subject of extensive research owing to their unique behavior. Notably, they possess relatively narrow band gaps and large Bohr exciton radii to ensure strong quantum-confinement effects, thereby making them suited to the investigation of the properties of a size-quantized system.1 These features make them ideal candidates for photovoltaic and thermoelectric applications. However, all of these applications require the integration of lead chalcogenide NCs into scalable and robust device structures, and there are issued to be addressed such as the stability of these lead chalcogenide nanoparticles, and how to improve the electron transfer between them in the presence of organic ligands.
I will present how to use the unique in-situ synchrotron high-pressure small angel x-ray scattering (SAXS) and wide angel x-ray scattering (WAXS) technique to investigate the structural stability and assembly behavior of lead chalcogendie nanoparticles.2-4 It will include: (1) the first report of reversal Hall-Petch effect of PbTe nanoparticles, (2) the pressure-controlled switches between amorphization and crystallization of PbTe nanoparticles, (3) the size-dependent phase transitions of PbSe nanoparticles under high pressure, (4) the relationship of pressure-induced interparticle spacing reduction versus the enhanced coupling effect in PbSe nanoparticle supercrystals.
References:
(1) Quan, Z.; Valentin-Bromberg, L.; Loc, W. S.; Fang, J. Chemistry - An Asian Journal 2011, 6, 1109.
(2) Quan, Z.; Fang, J. Nano Today 2010, 5, 390.
(3) Quan, Z.; Luo, Z.; Loc, W. S.; Zhang, J.; Wang, Y.; Yang, K.; Porter, N.; Lin, J.; Wang, H.; Fang, J. J. Am. Chem. Soc. 2011, 133, 17590.
(4) Quan, Z.; Siu Loc, W.; Lin, C.; Luo, Z.; Yang, K.; Wang, Y.; Wang, H.; Wang, Z.; Fang, J. Nano Lett. 2012, 12, 4409.

The characterization of micro- or nanoscale materials under a combination of time, temperature, and load requires management of system stability as thermal drift can be of a similar magnitude to the desired measurement. However, it has been shown that the influence of load frame thermal drift can be minimized from material creep measurement using dynamic (force modulation) methods. These tests can then be performed either in situ, with an electron microscope, or ex situ for thousands of seconds under both load and temperature. This enables activation energy and creep mechanism analysis for a variety of materials under direct observation. Here, the properties of tetragonal indium, BCC tungsten, FCC iridium single crystals are explored as a function of time, temperature, and load under direct observation in a scanning electron microscope. This is then combined with additional techniques, including; two-point-probe electrical testing, for rapid Joule heating, and EBSD for determination of crystal deformation.

Ultrathin oxide films on metal supports represent a unique combination of materials systems with unprecedented properties and potential applications ranging from heterogeneous catalysis to electronic devices. Particularly, the oxidation of NiAl alloys has received extensive interest for its ability to form a well-ordered Al2O3 film. Using scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM), we studied the ultrathin oxide film growth during the initial oxidation of NiAl(100) at elevated temperatures. It is shown that the growth of the oxide film proceeds as thin stripes with a stripe width less than 1 nm and stripe spacing of about 3 Å. Increasing oxygen exposure results in both the lengthening and widening of these stripes. In-situ LEEM observations show that the oxide stripes prefer to nucleate at surface areas with steps and can growth continuously across surface steps. While the stripes grow in both the length and width directions, the growth rate along the length direction is much larger than that in the width direction. It is also shown that the oxide stripes start to shrink at 925°C when the oxygen exposure is stopped. For a single stripe, it shrinks in the reverse direction of its growth direction. The microscopic processes governing the growth and decomposition of the oxide stripes will be discussed.

Ice (solid water) is not only the most familiar crystal for us on the earth, but also material which exists universally in the outer space. Since ice takes on a variety of polymorphic structures depending on ambient pressures and temperatures, ice on the outer space is expected to show a variety of exotic structures and properties due to its extreme conditions (low temperature, low pressure, and under high energy cosmic ray irradiation). Moreover the ice structures on the extreme conditions have attracted significant research interest because that is expected to show the behavior and properties of water on the outer space. Cryogenic transmission electron microscopy (cryo-TEM) is expected as a powerful method for discovery of novel crystal structure of ice on the outer space because that inner column of a transmission electron microscope on cryo-TEM observation provides suitable low temperature (~95 K) and low pressure (~10-5 Pa). Moreover, since electron beam irradiation during the TEM observation is considered to mimic the effect of cosmic ray irradiation onto the crystalline ice, the TEM observation is also expected to be able to discover the behaviors of ice on the outer space.
In this study, we observed crystalline structure and phase transition of ice thin film under electron beam irradiation by cryo-TEM operating at several accelerating voltages (25-2000 kV) to clarify the phase of ice on the outer space under cosmic ray irradiation [1, 2]. The crystal structure of ice thin film was identified to Ic phase [3] by transmission electron diffractometry. Under high energy electron beam irradiation, the phase transition of ice thin film from Ic phase to XI phase [3] was observed. We note that although the phase transition of ordinary ice to XI phase has been assumed to require an extremely long time (>104 year), the phase transition occurred on the time-scale of a laboratory experiment. The results may suggest that electron beam irradiation induces the phase transition and that the phase transition to ice XI readily occurs in outer space. We will discuss how electron beam irradiation onto ice thin film affects the phase transition, and prospective unique properties of ice in the outer space, based on the TEM observations.
References
[1] K. Kobayashi, H. Yasuda, Chem. Phys. Lett., 547, 9 (2013).
[2] K. Kobayashi, H. Yasuda, Physica B, 411, 88 (2013).
[3] V. F. Petrenko, R. W. Whitworth, Physics of Ice, Oxford Univ. Press, Oxford, 1999.

We use molecular dynamics and phase field modeling to demonstrate that Cu-Nb multilayer nanocomposites with individual layer thickness above 2-4nm remain morphologically stable when subjected to 100keV collision cascades, characteristic of neutron or heavy ion irradiation. Initial defect generation during collision cascades is simulated by molecular dynamics with an embedded-atom method (EAM) potential fitted to the experimental Cu-Nb phase diagram. Microstructure evolution after collision cascades is modeled via the phase field method. The probability of morphological instability increases rapidly as layer thicknesses decrease below 2nm. These results suggest that multilayer composites may remain morphologically stable under irradiation extremes, provided their layer thicknesses are above 2-4nm.
This material is based upon work supported as part of the 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.

Significant microstructural damage, in the form of defect clusters, typically occurs in metals subjected to heavy ion irradiation. High angle grain boundaries (GBs) and layer interfaces have long been postulated as sinks for defect clusters, such as dislocation loops. Here we provide direct evidence, via in situ Kr ion irradiation under a transmission electron microscope, that high angle GBs in nanocrystalline (NC) Ni can effectively absorb radiation induced dislocation loops, and thus significantly reduce the density and size of dislocation loops in Ni. Another approach to enhance radiation tolerance of Ni is to design multilayers with appropriate interfaces. Ag/Ni multilayers with individual layer thickness of 5 and 50 nm were subjected to in situ Kr ion irradiation at room temperature to 1 displacements-per-atom. Direct observations of frequent loop absorption by layer interfaces suggest that these interfaces are efficient defect sinks.

In general, materials with a large fraction of buried interfaces are expected to have a superior radiation resistance. However, grain refinement to the nanometer regime, which increases the volume fraction of grain boundaries (GBs), can result in both increased and decreased tolerance to radiation damage, depending on the material type and details of the microstructure. In addition to increasing the density of interfacial sinks, grain refinement can lead to other effects such as the change in the distribution of GB types or in the density of dislocations. In this talk, we will use the example of SiC to demonstrate that radiation tolerance of ceramics also depends on the details of the energy landscape for defect recombination. The energy landscape effects can be strongly coupled to the effects of the grain refinement. Furthermore, our experimental studies reveal that those nanocrystalline (nc) SiC samples that exhibit a superior radiation resistance are also characterized by a high density of stacking faults (SFs). A possible role of SFs in defects evolution has been discovered through our first principle calculations. We will also present our results on the effects of GB stresses on the sink strength, which quantifies the ability of GBs to annihilate defects. For polycrystalline materials it is typically assumed that random high-energy GBs act as perfect planar sinks and provide the upper limit for rate of defect annihilation. Here, using a combination of first principle calculations, mesoscale simulations, and the linear elasticity theory we discover that sink strength of a GB with low misorientation angle can in fact exceed that of a random high-angle GB. This effect is particularly pronounced in nc materials and it is contrary to the conventional wisdom that GBs with small misorientation angles are poor defect sinks due to their low dislocation densities. The unusually high sink strength of low-angle GBs arises from the presence of GB stress fields. The results are qualitatively the same for nc metals and nc ceramics, which will be demonstrated on the examples of Cu and SiC. Our discovery provides an opportunity to enhance radiation resistance of nc materials through GB engineering.

We capture a new grain growth mechanism in nanocrystalline CeO2 via classical molecular dynamics (MD) simulations. Grain growth is generally known to occur by two mechanisms, i.e., grain rotation and movement of curvature-driven grain boundaries. We show, in addition to these two mechanisms, a new mechanism where grain growth takes place by ‘grain disorder&’. In this mechanism, a nanocrystalline grain first completely disorders and then is consumed by the growth of a neighboring grain. We further show that experimentally-observed grain growth in nanocrystalline fluorite-structured oxides under irradiation at 160 K, i.e., well below the thermal grain growth temperatures, also occurs via the disorder mechanism.
This work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The computer simulations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.

The kinetic/thermodynamic stabilization recent results of grain growth in nanomaterials-based metals, alloys and compounds are generalized and discussed in detail [1]. The attention is taken to the using possibility of the regular solution approximation to estimate thermodynamic properties of nanomaterials taking into account both inner regions of nanograins and their grain boundaries. This situation is considered by the example of alloys-based iron, cupper, tungsten, nickel, etc. As applied to ferritic and ferritic/martensitic steels the thermal stability in radiation conditions is also discussed. The nanomaterials improved radiation stability is connected with their great quantity of interfaces (in the form of grain boundaries, triple junctions, and quaternary those), which can function as the sinks for the radiation defects [2]. In addition to the removal facilitation of radiation defects, there are other phenomena in the nanomaterials irradiation such as the nanostructure amorphization and recrystallization acceleration. Moreover, in the latter case, it should be borne in mind the possibility of abnormal grain growth. Some little-explored and unexplored problems are pointed.
References
1. R.A. Andrievski. Thermal stability of nanostructures. Submitted to J. Mater. Sci.
2. R.A. Andrievski. Behavior of radiation defects in nanomaterials. Rev. Adv. Mater. Sci.
v. 29, 54 (2011).

An overview of the irradiation, thermal and plastic flow stability of metallic nanocomposites will be presented. It is shown that low-energy interphase boundaries with special crystallographic orientation relationships produced either by physical vapor deposition or severe plastic deformation processing can exhibit morphological stability at elevated temperatures. Nanocomposites (model systems such as copper-niobium) containing these special interfaces exhibit high strengths, high deformability, and irradiation stability. The interfaces in these composites can act as effective traps for helium leading to stable storage of helium at interfaces and also as sinks for radiation-induced point defects resulting in suppression of void swelling. The integration of atomistic modeling and theory with experiments is shown to be a key factor in the bottom-up design of nanocomposites for tailored response at extreme environments.
This research is sponsored by DOE, Office of Science, Energy Frontier Research Center program.

Though a single carbon nanotube (CNT) exposes superior thermal conductivity, a carbon buckypaper made of randomly networked carbon nanotubes has much lower thermal conductivity due to difficult in phonon transport from one tube to another. We found that ion bombardment can introduce inter-tube carbon displacements as stable point contacts to assist phonon transport. The ion irradiation simultaneously introduces defects within the tube, thus the thermal conductivity of a buckypaper is determined by a competing effect between inter-tube-displacement-mediated phonon transport and intra-tube defect-caused phonon scattering. The competition leads to first increasing, then decreasing thermal conductivity as a function of ion fluences, with a peak enhancement by 550% observed. This competing effect was confirmed by molecule dynamics simulations. The thermal conductivity can be further enhanced by post irradiation annealing to remove defects within the tubes, but without removal of inter-tube displacements since they are more thermally stable.

Ion beam irradiation of nanotubes (NTs) has been investigated as a scalable technique to alter atomic structure, and the electronic, mechanical and acoustic properties [1 and references therein]. Structural modification under ion irradiation is also important when assessing suitability of nanomaterials for radiation hard environments.
We report on an in situ transmission electron microscope (TEM) ion irradiation experiment examining the stability of single-walled NTs conducted at the IVEM-Tandem facility at Argonne National Laboratory. The TEM was operated at a voltage of 100 keV and images and video-rate data were collected. Single-walled carbon NTs (SWCNTs) and single-walled boron nitride NTs (SWBNNTs) were irradiated with 30 keV Kr⁺ ions (1.25 e10 ions/cm²/sec) to doses of up to 1e15 ions/cm² at room temperature. All regions of both specimens displayed ion-beam disordering and eventual amorphization, at roughly the same dose (5e14 ions/cm²). This suggests that the accumulation of defects was the major mechanism for structural change, and that the ion beam interactions were similar for both materials.
Diameter shrinkage was also observed for both MWCNTs and SWBNNTs, and at high ion dose, this was responsible for the eventual breakage of the NTs. The irradiation of certain areas was observed using video-rate image capture. In a particular SWBNNT and at low ion dose, a catastrophic ion scission event (< 33 msec) was seen, suggestive of a collision cascade [2]. Multi-wavelength ex situ Raman measurements were also carried out on these samples at various stages of the irradiation and we correlate these results with the TEM images to obtain a cross-section for defect production due to interaction with ions of this energy and species.
References
[1] A. V. Krasheninnikov and F. Banhart. Nature Materials, 6, 723, (2007)
[2] A.C.Y. Liu, R. Arenal, G. Montagnac, accepted Carbon available online 10.1016/j.carbon.2013.05.062 (2013).
[3] We thank P. Baldo and E. Ryan for their assistance in the in-situ ion irradiating TEM measurements. Raman spectroscopy measurements have been carried out at the ENS-Lyon (France). We acknowledge G. Montagnac for his assistance in these experiments.

The potential applications of nano-scale materials in space applications require the knowledge of the behavior of such materials under irradiation, as their properties may degrade due to defects created by high energy particles coming from the open space. At the same time, numerous experiments (see [1,2] for an overview) on the bombardment of 2D materials with energetic particles indicate that the conventional theory of defect production in bulk materials not always works in the systems with reduced dimensionality or it requires considerable modifications. In this talk, our latest theoretical results on the response of graphene [3-6] and inorganic 2D materials like BN [7] and dichalcogenides (MoS2, etc.) [8] to electron and ion irradiation will be presented, combined with the experimental results obtained in collaboration with several groups [5,6,8-11]. The production of point defects will be discussed, followed by the analysis of their agglomeration and evolution in the 2D systems. The role of electronic excitations in defect production will be touched upon, and the effects on the mechanical and electronic properties will be addressed as well. Finally, the analogies in defect morphologies in 2D materials with the same symmetry (e.g., graphene, bilayer silica, BN and MoS2) will be discussed.
1. A.V. Krasheninnikov and F. Banhart, Nature Materials, 6 (2007) 723.
2. A. V. Krasheninnikov, K. Nordlund, JAP 107 (2010) 071301.
3. O. Lehtinen et al., PRB 81 (2010) 153401.
4. S. Standop et al., Nano Letters 13 (2013) 1948.
5. J. Kotakoski, et al., PRL 106 (2011) 105505.
6. J.C. Meyer et al., PRL 108 (2012) 196102.
7. N. Berseneva, et al., PRL 107 (2011) 035501.
8. H.-P. Komsa, et al., PRL 109 (2012) 035503.
9. R. Nair, et al., Nat. Phys. 8 (2012) 199.
10. M. Kalbac et al., Adv. Mater 25 (2013) 1004.
11. R. Nair, et al., Nat. Comm. 4 (2013) 2010.

Short-pulse laser irradiation of a metal target can create conditions for generation of non-equilibrium phases and unusual microstructure in the surface region of the irradiated target. The shallow melt depths produced by the short pulse laser irradiation and the high thermal conductivity of metals can result in very high cooling rates, strong undercooling and rapid resolidification. In this work, the melting and resolidification processes occurring under conditions of extreme heating and cooling rates are investigated in large-scale atomistic simulations performed with a computational model that combines the classical molecular dynamic method with a continuum description of the laser excitation of conduction band electrons, electron-phonon coupling and electron heat conduction. The kinetics of the resolidification process and the microstructure of the surface region are found to be defined by a competition between the epitaxial regrowth of the substrate and nucleation of crystallites within the undercooled melted region. The dependence of the final microstructure of the surface region on the irradiation conditions is discussed based on the results of the atomistic simulations. The results of the simulations are related to experimental observations. The implications of the computational predictions for practical applications, as well as for the theoretical description and continuum-level modeling of the laser-induced processes are discussed.

High-fluence ion implantation followed by subsequent annealing was used to synthesize epitaxial Pb nanoparticles in Si(100) wafers and in single crystalline Al and Cu films (grown by molecular beam epitaxy), i.e. three matrices in which Pb has a limited solubility. By varying the implantation fluence and temperature, as well as the annealing temperature and duration, the Pb nanoparticle size (5-20 nm) and size distribution can be tuned. The size, crystal structure, epitaxial orientation and elastic strain of the nanoparticles were investigated by Rutherford backscattering and channeling spectrometry, x-ray diffraction and small angle scattering, and transmission electron microscopy. Different growth regimes point to different scaling behaviors of the average particle radius with the implantation parameters, as will be explained in terms of the classical nucleation theory. In turn, tuning the particle size, in combination with the selection of the matrix, allows to tune the particle lattice parameters.
The melting and solidification of these Pb nanoparticles was studied by real time x-ray diffraction, revealing large superheating and supercooling effects. These thermal hysteresis effects, which drastically depend on the size of the nanoparticles, are due to the epitaxial alignment, as well as to different levels of lattice mismatch between the Pb particles and the matrices, leading to a different pressure on the Pb nanoprecipitates. Further analysis of the lattice constant of the Pb nanocrystals gives unambiguous evidence that the elastic strain related pressure effect can be used to modify the melting behavior of the embedded Pb nanoparticles, both for increasing or decreasing the melting point.
We investigated the superconducting properties of Pb nanoparticles (with a diameter varying between 8 and 20 nm) in an Al matrix, as this is an interesting system consisting of two superconductors with different critical parameters. The critical fields and critical currents of these hybrid superconductors were characterized extensively to establish the normal/superconducting phase boundary. The Al/Pb nanocomposites display a single sharp superconducting transition with a critical temperature Tc, which increases linearly with the Pb/Al volume ratio. The experimental data are in agreement with the theoretical predictions of the proximity effect in the Cooper limit for strongly coupled superconductors, taking into account a slightly reduced electron-phonon coupling constant. This good agreement between theory and experiment illustrates that the quality of the interface (in particular the electronic transparency) between the Pb nanoparticles and the Al matrix is excellent, which we attribute to the ion beam synthesis process.
In this presentation, we will discuss the intimate interplay between the structural properties of the Pb nano particles, e.g. the particle size, the matrix and the interface quality, and their thermal and superconducting response.

TiO2 ceramics with a grain size ranging from 150nm to 5mu;m were processed at various sintering temperatures, atmospheres and dopants. Dielectric properties; such as dielectric constant, dielectric loss, polarization-electric field relationship and dielectric breakdown strength were measured using samples with acceptor and donor type dopants. Correlation between microstructural development and dielectric properties was interpreted through analysis of the results obtained by impedance spectroscopy measurements. It was found that nanostructured TiO2 ceramics exhibit higher insulation resistance, lower dissipation factor (<0.02% and extremely high voltage breakdown (>300 V/mu;m) as compared to coarse grained materials (<100 V/mu;m). Dielectric properties of TiO2 ceramics were significantly affected by dopants, sintering atmosphere and cooling rate during sintering. The role of grain boundaries and defect chemistry in nanostructured titanium oxide ceramics will be discussed with respect to possible mechanisms of voltage breakdown and high energy density capacitor applications.

Ke Han, Jun Lu, Robert Walsh, Vince Toplosky, Rongmei Niu, Xiaowei Zuo and Liping Deng
National High Magnetic Field Laboratory
Recently, increasing material innovation has created a potential for unprecedented advances in DC and pulsed magnet to achieve ever high record fields. These record high magnetic fields are generated by applying very high density of electrical current through conductors. High field magnets which operate in the range 5-100 T require extremely high strength conductors in order to resist Lorentz forces. In addition, high field magnets require conductors to have very low electrical resistivity in order to minimize the temperature rise due to Joule heating. The design of a high field coil requires not only high strength conductors, but also high strength structure materials, and insulators. Most of the necessary properties are only achievable in nanostructured composites. Various fabrication routes have been developed and used to fabricate nanostructured materials for high field magnets. Fabricated key nanostructured materials possess unique combinations of mechanical and physical properties in order to meet the special requirements of high field magnets. Because certain high field magnets are operated at cryogenic temperatures, cryogenic temperature properties are also important.
The unique microstructure of these magnet materials with face-centered-cubic (fcc) matrix provides us with unprecedented properties. A number of Cu matrix conductors have strength levels close to steels and they have sufficient ductility because the size of their grains reaches nanometer scales. However, their electrical resistance is also increased because of their refined microstructure. The application of these materials requires understanding of the mechanisms of strengthening and of electron transport which are operative in materials with nano-scale microstructures. For instance, both the increase of strength and electrical resistivity can be attributed to same microstructure parameter changes, such as the distance between the interfaces, the dissolution of strengthening component, and density of the defects. In addition to mechanical strength and electrical resistivity, we also need to study the fatigue endurance of those composites at cryogenic temperatures because most of the high field magnets are operated in cyclic modes at very low temperatures.
The other type fcc matrix materials are superalloys. This paper mainly summarizes our studies of cryogenic properties of these alloys with nanostructured components. The refined structure in this alloy provides a strength level up to 2500 MPa at cryogenic temperatures.
Acknowledgement: This work was supported by the US NSF (Cooperative Agreement No.DMR-0084173), the US DOE and the State of Florida. We thank Dr Tyler for editing.

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 neutrons and high energy synchrotron beam) shed light on the role of the multi-scale nature of the microstructure in the recorded extreme mechanical properties [Acta Mat, 57 (2009), p3157; Acta Mat, 58 (2010), p6504; Adv. Eng. Mat., 14-11 (2012), p998].
In addition, combined implantation/irradiation experiments (dual beam with 10keV He and 4 MeV Au) have been performed in-situ in the TEM on these Cu/Nb nanocomposite metals: while the coarse Cu channels exhibit a very rapid degradation with the formation of large bubbles at grain boundaries and inside Cu grains, the Cu/Nb nanocomposite regions exhibit much higher resistance to defect formation. In particular, nanometer-size bubbles first form at Cu-Nb interfaces while the Nb nanostructures interior remains bubble-free up to even larger dose.

Future applications require advanced materials that can maintain their properties at extreme conditions such as high strains, high strain rates, high temperatures and radiation. Nanostructured metals represent a promising material class that can satisfy these applications. In particular, nanolayered bimetallics have exhibited outstanding strength, thermal stability and radiation resistance. To date, there is a lack of knowledge on how to control the properties of nanolayered bimetallics during fabrication.
Using an accumulative roll bonding (ARB) technique, we fabricated meter-sized Cu-Nb layered composites with nano-sized individual layer thicknesses. After a strain of 10.5, with layer thicknesses reduced from 2 mm to 58 nm, the sample was then cut into two pieces. One was continuously rolled along the original rolling direction until the layer thickness reached 10 nm (longitudinally rolled, ARB-LR), while the other was rotated 90o and rolled in the transverse direction until layer thickness became 20 nm (cross-rolled, ARB-CR). Neutron diffraction and transmission electron microscopy analysis reveal that these two samples are comprised of totally different interface features. In ARB-LR, the dominant interface is faceted {112}<111>Cu//{112}<110>Nb, while the prevailing interface in ARB-CR can be characterized as a non-faceted flat {110}<111>Cu//{001}<110>Nb. In addition, both samples show high hardnesses of ~4.0 GPa. All the samples are stable under annealing for one hour at a relatively high temperature of 500 °C, which is 0.46Tm of Cu (Tm: melting temperature). The most exciting result is that unlike the ARB-LR sample, the ARB-CR sample retained its the ultra-high strength after an annealing temperature of 600 °C for one hour. We rationalize the observed simultaneous high strength and thermal stability in terms of atomic interface structures and boundary densities.
In conclusion, we report tunable interfaces and high thermal stability in nanolameller composites fabricated via severe plastic deformation. These results give insight into designing advanced materials by controlling interfaces via science-guided mechanical processing.

To bring high pressure and high temperature (HPHT) reservoirs on-stream, there is an urgent need to develop advanced alloys, where higher strength allows design of critical wall thicknesses necessary to counter burst and collapse of tubular(s) deployed downhole. However, the presence of sour gases coupled with HPHT, poses significant engineering challenges associated with corrosion, environmental cracking (EC) and hydrogen embrittlement (H2E), especially of high strength alloys. In this area nano-materials can potentially offer some reliefs in abetting reduce susceptibility to EC and H2E while providing necessary strength. Our talk first addresses the technology gaps in alloy requirements for deep-water and sour HPHT. We will then reflect on the potentials of harnessing bulk nanocrystalline materials and coating in oil and gas environments. As research in progress, experiments delving into environmental cracking susceptibility of nanocrystalline materials and “surface nanocrystallized” stainless steels in contrast to their coarse grain (CG) commercial counterparts will be discussed. To assess susceptibility of nanocrystalline materials in contrast to its coarse grained counterpart (CG) to EC and H2E, tests (NACE TM0177 - Method C and NACE TM0198-2004 - SSRT) in air and sour environment (with and without cathodic polarization - CP) were performed. A CP of 1100 mV below open circuit potential (OCP) of the alloys in selected sour environment was applied during quasi static tensile tests. In line with recent publications that nano-materials perform better than their CG counterparts, our results affirm that grain refinement and grain boundary engineering abets corrosion performance and EC / H2E susceptibility.

Copper foils with different microstructures were produced by magnetron sputtering in order to evaluate the effect of nanotwins on corrosion behavior. The Cu samples had the following microstructures: a.) coarse grained b.) fully nanotwinned columnar grains c.) partially nanowtinned columnar grains d.) columnar grains without nanotwins. These foils were tested in 3.5% NaCl solution (pH ~ 8.0) by linear polarization, potentiodynamic polarization, and immersion corrosion techniques. It was found that fully nanotwinned copper has a higher polarization resistance (R_p) and a lower corrosion current density (I_corr) than non-nanotwinned, partially nanotwinned, and coarse grained copper foils. The passive layers after the polarization and immersion tests were examined by X-ray diffraction, EDS, and SEM. It was determined that fully nanotwinned specimens did not show pits, while the other specimens generally showed pits. These observations highlight the potential of nanotwins for corrosion protection.

The rise of global energy demands, coupled with the decline in easily accessible oil and gas reserves, has encouraged investigations into new methods for enhanced oil recovery (EOR). Conventional EOR uses thermal and chemical methods (e.g., steam stimulation, polymer flooding) to mobilize residual oil remaining in a reservoir after initial production. Recently, nanotechnology has gained recognition in the petroleum industry for its potential to support EOR in applications including remote sensing and imaging for reservoir exploration and management. Here, we describe the development of unique core-shell nanoparticle carriers with a burst-release mechanism for delivery of sensors or chemicals to the reservoir for EOR. To achieve burst-release delivery, the shell material remains impervious to severe reservoir environments for a defined time period prior to the burst release. For example, the shell material must resist degradation from reservoir environments such as high heat (e.g., 75-150° C) and high pressure (e.g., 3000-15000 psi). Moreover, the shell exclusively degrades in response to a specific chemical trigger, ultimately enabling burst delivery of sensors or chemicals into the reservoir.
Here, we used epoxy polymers as the shell material, owing to their versatility and exceptional material properties. We characterized a variety of epoxy polymer systems before and after exposure to simulated reservoir conditions under high pressure, high salinity, and high temperature (e.g., >5000 psi, >100°C). Using dynamic mechanical analysis, we evaluated the glass transition temperatures and storage moduli. Results show nearly identical mechanical properties after exposure to simulated reservoir conditions, suggesting that the epoxy polymers readily tolerate these harsh environments. Importantly, we show the ability to incorporate labile disulfide crosslinks into the epoxy networks that degrade exclusively in response to thiol compound trigger. For example, an epoxy system containing the disulfide component, 4-aminophenyl disulfide, remains stable after exposure to high temperatures and high salinity, but readily degrades in the presence of 2-mercaptoethanol. The effects of differing parameters on the degradation kinetics of the disulfide-containing epoxy systems were evaluated, demonstrating the importance of disulfide percentage within the epoxy polymer. We further simulated the dimensions of core-shell particles using thin film experiments. Bi-layer thin films were prepared, where the first layer comprising a model sensor (e.g., rhodamine-B fluorophore) was overlaid with a second layer comprising a disulfide epoxy polymer thin film. The degradation kinetics of the epoxy thin film was evaluated by immersing this bilayer system into a thiol solution at high temperature and monitoring fluorophore release. Overall, we show the usefulness of this robust shell chemistry as a component of core-shell particles for controlled delivery in EOR applications.

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 and Eu-doped ZrO2 nanoparticles that were seeded into the fireball of an explosion to measure its temperature. In order to cover a larger temperature range and to be able to determine temperature and time from the kinetics, we are currently testing various core/shell nanoparticles. The samples currently being evaluated include: (i) Er,Yb:ZrO2/Eu:Y2O3; (ii) Tb:ZrO2/Eu:Y2O3; (iii) Tb:ZrO2/Tm:Y2O3 + Eu:ZrO2; (iv) Yb2O3/Er:Y2O3; (v) Yb2O3/Er:Y2O3 + Eu:ZrO2; and Tb:ZrO2/Tm:Y2O3/Eu:ZrO2.
Several approaches are being pursued: (i) amorphous to crystalline transition, (ii) diffusion, and (iii) built-in reference. In the first approach, amorphous core/shell nanoparticles, similar to the individual nanoparticle sensor materials, are used and their transition to a crystalline state is monitored via luminescent dopants. This approach relies on nucleation and growth of the nanoparticles. Depending on the specific optical signature to be detected, changes in the polymorphic nature of some of the materials (ZrO2) are required. In the second approach, we are evaluating the diffusion of elements such as Yb from Yb2O3 into Er:Y2O3 and detect such changes via upconversion luminescence. In the third approach, we are using fully crystallized cores for intensity calibration standards and monitor the luminescence intensities of the shells.
After heating the samples by various means in the laboratory, they are evaluated using optical spectroscopy. Also, some samples have been exposed to explosion tests at the NSWC - IH Division. 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 temperature the material has been exposed to.

Iron is the basic component of most technological alloys and steels used as structural materials in the nuclear industry of today, and is foreseen to continue playing this role in future reactors. How iron and iron alloys behave under fast neutron irradiation is critical for the nuclear community. Modelling can help explain observations and provide predictions to interpolate or extrapolate phenomena over length- and time scales not accessible to experiments. Here will be presented the current state-of-the-art of ab initio modelling of radiation damage in iron. The insights that come from using quantum mechanical methods will be discussed, as well as the limits of these approaches. Much of the current understanding of radiation damage characteristics in iron is based on simulations using classical semi-empirical model potentials. The importance of maintaining the quantum mechanical description of the physics as far as possible, in order to obtain correct results, will here be exemplified by recent applications, such as ab initio molecular dynamics simulations of displacement cascades.

Precipitates of Cr at grain boundaries (Σ3 <110> {112} as an example) in α-Fe have been studied using molecular dynamics with a two-band embedded atomic model potential. The accumulation and segregation of Cr atoms and the evolution of the GB configurations depend on local vacancy concentration, Cr concentration and temperature. At the early stage, the strong attraction of Cr with the GB core provides a pathway for Cr atoms to quickly accumulate within the GB core. It is of interest to find that high vacancy concentrations enhance the long-distance migration of Cr, thus leading to the formation of the large Cr precipitates (α' phases) and voids near the GB core. The surfaces of voids are found to be largely depleted by Cr, which is strongly dependent on vacancy concentration. With the increase of Cr concentration, the size and number of Cr precipitates increase, and the strong segregation of Cr at the GB is observed. Also, the number and size of Cr precipitates significantly increase with increasing the temperatures from 300 K to 1000 K. The accumulation and segregation of Cr atoms at the GB lead to significant deformation of the GB structure and the formation of several GB steps, causing the displacement and broadening of the GB interface.

Interfaces and grain boundaries that act as sinks for radiation-induced defects offer a promising avenue for designing radiation tolerant materials. Determining how interfaces interact with those defects is key for optimizing advanced materials, especially oxides that have inherent complexity due to electrostatic effects and are often susceptible to amorphization. Structure and stoichiometry of the interfaces are one of the factors governing radiation tolerance in nanostructured oxides. For the semi-coherent SrTiO3/MgO interface, experiments suggest that the orientation is dependent on the deposition temperature as the SrTiO3 (110) films are favored at lower temperatures whereas (100) films are favored at higher temperatures. Moreover, recent experiments show that upon irradiation, (110) oriented SrTiO3 undergoes amorphization at very low doses at which the (100) grains remain crystalline. Understanding this behavior and how it relates to the atomic structure of the interface is crucial for developing design principles for new materials. We use atomistic simulations to investigate the structure of experimentally observed orientations of the SrTiO3/MgO interface and the possible implications on the aforementioned behavior. Both SrO and TiO2 terminated interfaces have been considered. As expected, the periodic interface misfit dislocations primarily relax the lattice mismatch between SrTiO3 and MgO. The structure of these misfit dislocations depends on the orientation between the two materials. For certain orientations, multiple stackings are possible leading to different types of coherent regions of the interface. We systematically study the dependence of defect interaction with the interface as a function of atomic structure, in particular how that interaction depends on the local structure of the interface, including the misfit dislocations, their intersections, and the different coherent regions. We discuss the implications of these interactions for the evolution of radiation-induced defects near this interface in light of the experimental observations. The behavior predicted for the present oxide/oxide interfaces will be contrasted with the metal/metal interfaces to identify the similarities and differences between the two types of materials systems.
This material is based upon work supported as part of the 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.

The presence of helium is inescapable in the structural materials of a fusion reactor, including reduced activation ferritic/martensitic steels. It is well known that the nucleation, growth and diffusion of He bubbles will significantly affect the global evolution of microstructures and consequently degrade the mechanical properties of materials. However, the nucleation and diffusion of He bubbles are rare events within the achievable time scale of atomic-level simulations, and also, the potential energy surfaces of their migration paths are generally rough, with numerous local minima, where the existing accelerated dynamics approaches may fail to determine minimum energy pathways for important kinetic processes. A new approach, the so-called self-consistent accelerated molecular dynamics is being developed to simulate rare events along rough potential energy surfaces. We have applied this method to simulate the migration of small stable He-V clusters in Fe, as well as the growth of these clusters into larger He bubbles, for time scales on the order of ~102 s. It is of interest to note that the migration mechanism for HenV clusters is governed by the creation and migration of self-interstitial atoms (SIAs), as observed in previous MD simulations at high temperatures. When two small He-V clusters migrate into a critical separation distance, they spontaneously grow to a larger cluster by a mechanism involving Ostwald ripening and SIA emission. The implications of the present results to mesoscale simulations of He behavior and microstructural evolution are discussed.

Self-interstitial atoms are only found in materials far from equilibrium because of their high formation energies (~4eV). Therefore, interstitial loops are a principal signature of radiation damage created by neutrons or ions and are central to radiation-induced property changes in iron-based structural materials applied in current and advanced nuclear energy systems. Unlike other bcc metals, two types of interstitial loops, ½ <111> and <100>, are formed in bcc iron and its alloys. However, the mechanism by which <100> interstitial dislocation loops are formed has remained undetermined since they were first observed more than fifty years ago. In this talk, we describe our atomistic simulations that have provided the first direct observation of <100> loop formation. The process was initially observed using our self-evolving atomistic kinetic Monte Carlo (SEAKMC) method, and subsequently confirmed using molecular dynamics simulations. Formation of <100> loops involves a distinctly atomistic interaction between two ½ <111> loops, and does not follow the conventional assumption of dislocation theory, which is Burgers vector conservation between the reactants and the product. The process observed is different from all previously proposed mechanisms. Thus, our observations might provide a direct link between experiments and simulations and new insights into defect formation that may provide a basis to increase the radiation resistance of these strategic materials.
This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCS99.”

Designing truly radiation tolerant alloys requires a fundamental understanding of how radiation damage affects the microstructure of a material. In particular, it is important to understand how interstitials and vacancies created by cascade events migrate to sinks such as grain boundaries and dislocations, and the mechanisms by which these defects are absorbed at the boundary. For this purpose, free-standing ultrafine grained (10 nm to 1 mu;m) iron films were generated at Los Alamos National Laboratory using physical vapor deposition and irradiated in-situ using the Hitachi H-9000NAR TEM at Argonne National Laboratory&’s IVEM Tandem facility using 1 MeV Kr²⁺ ions. The in-situ irradiation experiments have shown that defect clusters and dislocation loops contribute significantly to defect transport to grain boundaries as they glide to and annihilate at grain boundaries, enhancing the formation of a denuded zone. In the current work, the behavior of defect clusters and dislocation loops observed during the in-situ irradiation experiments has been correlated with molecular dynamics simulations to understand the fundamental mechanisms of defect cluster absorption at grain boundaries and the role of grain boundary character. These results are particularly important in light of the role of the glissile interstitial defect clusters on the formation of the irradiated microstructure in iron (i.e. the formation of large dislocation loops by coalescence), and will inform the optimization of fine-grained nanostructured ferritic alloys for advanced nuclear reactors.

Irradiation assisted stress corrosion cracking is driven by a combination of stress, a susceptible microstructure, an aggressive environment, and irradiation. While the origin of this SCC mode remains poorly understood, emergence of localized deformation as a key process in IASCC has resulted in considerable experimental and simulation studies. Irradiation induces localized deformation at high temperature by confining dislocation movement to narrow channels. It is the intersection of these dislocation channels with grain boundaries at which cracks form and eventually lead to failure. Experiment and simulation have shown that the normal stress at these locations can exceed the applied stress by a factor of 2-3 and that cracking correlates well with high normal stress. Shear stresses in the channel can add an additional component to the normal stress at the channel-boundary intersection. While the exact role of localized deformation in stress corrosion cracking is not fully understood, it is known that the degree of localized deformation correlates well with SCC susceptibility. Further, both experiments and simulations indicate that cracks nucleate preferentially at high angle grain boundaries that are perpendicular to the loading direction, and are not oriented for easy deformation under the applied load. We have performed a detailed analysis of the elastic stress and plastic strain tensors at dislocation channel - grain boundary intersections and correlated these values to IG cracking frequency. This paper will review recent progress in understanding the behavior of localized deformation and the impact on stress corrosion cracking, and present new findings on the micromechanics of dislocation channel - grain boundary interactions and their impact on IASCC.

The first motivation in this work is to improve our knowledge about the mechanisms able to drive nano-precipitate growth in a given substrate by ion assisted irradiation using an experimental approach derived from those developed by Kögler [1], Sakuma [2], Lindner [3] or Zhu [4]. Our final goals are to produce homogeneous dispersion of nano-precipitates in a model substrate and then to acquire new data about the radiation tolerance of these as-synthesized composite materials.
Single and dual beam irradiations were carried out using the accelerators of the Jannus platform at CEA Saclay in order to produce nano-precipitate dispersions in suitably chosen model substrates. In order to improve the quality and/or the orientation relationship of/between the ion beam-synthesized nanoprecipitate in/and substrates, the irradiations were carried out at room- and high-temperature. Three different systems have been investigated:
i) Carbon or carbon + silicon beam irradiation of silicon single crystals ;
ii) Carbon beam irradiation of yttria stabilized zirconia single crystals ;
iii) Carbon + titanium beam irradiation of iron single and polycrystals.
We first present the experimental configurations that have been optimized for the different irradiations and then describe the characterization tools used to analyze the evolution of both the composition and the microstructure of the investigated systems. Five techniques were coupled for this purpose: nuclear reaction analysis (NRA), Rutherford backscattering spectrometry in channeling geometry (RBS-c), X-ray diffraction, Raman micro-spectrometry and high resolution transmission electron microscopy (HR-TEM). Then, some results related to each system (Si/C+ + Si+), (YSZ/C+) and (Fe/C+ + Ti+) are detailed and discussed. In silicon, the formation of 4 nm SiC nano-precipitates is evidenced. In YSZ, the new microstructure growing during irradiation is assumed to correspond to ZrOxCy. In iron, the formation of TiC is expected due to its low exothermic enthalpy of formation.
[1] R. Kögler et al., Appl Phys A76(2003)827.
[2] D. Sakuma et al., J. Nucl. Mater. 329-333(2004)392.
[3] J.K.L. Lindner et al., Mater. Sci. Engin. C26(2006)857.
[4] S. Zhu et al., Appl. Phys. Lett. 80(2002)4327.

Oxide Dispersion Strengthened (ODS) alloys based on a Fe-Cr ferritic or ferritic-martensitic matrix (chromium content is about 9~ 18%), constitute a good candidate for structural components in a fusion reactor where they will be submitted to high operating temperatures (T>550°C) and high neutron irradiation damage (200 dpa). ODS steels are made through mechanical alloying process and their good properties depend on the composition, and size and of the homogeneous dispersion of nanometric oxide (very often made of Y-Ti- O) particles. But the exact composition of these nanometric oxides and their formation mechanism are not yet clarified. The excess vacancy defects introduced in ODS fabrication process, by mechanical alloying, have been considered to be one of the key points for oxide composition and formation mechanism. By combining the techniques of positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry (SIMS), we aimhope to obtain information about the interaction between vacancy defects and impurities such as Y, Ti, and O in Fe and Fe-Cr alloy.
For these first studies, we have implanted yttrium in high purity Fe samples (99.99%). Different implantation conditions were chosen. Some samples were implanted by 1.2MeV yttrium at room temperature (300K) and low temperature (77K) at doses from 0.176 to 5.28 dpa and other were implanted by 800keV yttrium. The vacancy defects themselves without influence of impurities in pure Fe were also studied for comparison. PAS is used to characterize the possible V-Y complexes or precipitates in pure Fe and SIMS is used to determine the implanted yttrium profile.
Vacancy clusters were detected at damage dose of 0.176 dpa in yttrium implanted samples. But with the dose increaseing of doses (to 3.52 and 5.28 dpa), the detected vacancy cluster concentration is reduced in the region where Y is implanted indicating that Y atoms are involved in the processes that lead to the defect distribution change which is detected. Two possible assumptions have been proposed: a) the formation of Y-V or Yn-Vm complexes due to a favorable binding energy at least for Y-V pairs. b) Y precipitates can be formed due to the Y implanted concentration which is higher than its solubility and their positron affinity which makes possible trapping. After-implantation annealing is still under investigation.

Oxide Dispersion Strengthened (ODS) steels consist of a ferritic matrix with dispersed nanometer-size oxide particles. Compared to conventional steels these materials exhibit two remarkable properties that are not fully understood yet: (i) Stability: Up to rather high temperatures the number and size of the oxygen-rich nanoparticles do not change significantly. (ii) Tolerance: The nanoclusters act as sinks for transmutation helium, vacancies and self-interstitials. The first property is the reason for the improved creep strength at high temperature, whereas the second property is related to the radiation resistance of these materials. Therefore, ODS steels are promising candidates for applications as structural materials in extreme environments, i.e. at high temperature and intense particle irradiation, such as in advanced nuclear fission and fusion reactors.
The detailed structure and composition of the nanoclusters containing Y, Ti, O, along with other minor alloying and impurity elements, is still under discussion. In this work simulated annealing (SA) based on the Metropolis Monte Carlo method on a rigid lattice is used, in order to determine the structure of the oxide nanoclusters with the lowest formation energy. The ferritic matrix of ODS steels is modeled by bcc-iron. Since cluster sizes up to a few nm are of interest, first-principle methods cannot be used throughout since they are computationally too expensive. However, extensive first-principle calculations on the structure and energetics of point defects and small clusters must be performed in order to obtain the parameters describing the atomic interactions in the rigid lattice used for SA. In this work not only parameters for pair interactions but also for triple interactions are determined. SA is performed for various clusters and the results are compared with available experimental and theoretical data from literature. The binding energies of the oxide nanoclusters obtained in this work can be used as input parameters of coarse-grained method such as object kinetic Monte Carlo simulations and rate theory that are often used to consider the evolution of a system of nanoclusters.

Nanostructured ferritic alloys (NFAs) have shown great promise as fuel claddings, first wall and structural components for advanced fission and fusion reactor systems due to their capability of withstanding high neutron fluence, high temperatures and pressures, and high concentrations of helium as a result of (n,α) transmutation reactions. As helium diffuses through a matrix, it preferentially nucleates along grain boundaries and within voids creating pressurized helium bubbles and consequently deformation in the surrounding lattice. Large concentrations of helium can potentially lead to helium embrittlement and stress corrosion cracking, both having catastrophic consequences in reactor environments. High number densities of complex oxide nanoclusters in NFAs such as, Y_2 Ti_2 O_7, Y_2 O_3 and Y_2 TiO_5, have been shown to act as trapping sites for helium atoms, preventing the nucleation and growth of helium bubbles at preferential nucleation sites such as grain boundaries and voids. Understanding the energetic mechanisms responsible for the trapping of helium bubbles at nanocluster surfaces is essential to furthering the understanding and development of materials suitable to be used in the harsh environment of advanced fission and fusion reactor applications.
We present results from density functional theory calculations on the energetic interactions of helium atoms with the complex oxide nanoclusters known to exist in nanostructured ferritic alloys. Results on the energetic stability of helium occupying a variety of lattice sites, the migration barriers of helium in the nanoclusters, and the stability of He - Vacancy clusters will be presented. The results of these simulations will serve to further the understanding of the energetic interactions occurring between transmutation product helium and complex oxide formations resulting in helium trapping at the surface of nanoclusters. On a broader scale, this research can shed light on prevention of helium embrittlement and stress corrosion cracking for materials in environments with high temperatures, pressures and high concentrations of helium.

We report in situ transmission electron microscopy (TEM) observations of agglomerated 3 mol% yttria-stabilized zirconia (3YSZ) during electric field assisted sintering (EFAS). EFAS has the potential of increasing consolidation rates, reducing sintering temperatures and holding time and, hence, minimizing the grain growth. Resulting nanogranular microstructures often reveal improved macroscopic properties, such increased toughness, better corrosion or oxidation resistance, optical transmission, and altered electrical properties.
To separate the effects of temperature and electrical field strength on green bodies, we have employed the Protochips Aduro TEM sample holder. Direct TEM observations while exposing particle agglomerates to elevated temperatures and/or electrical fields provides new information on morphological and structural changes, formation of necks and pore closure during sintering. The size of growing necks as a function of temperature was determined along with the corresponding dihedral angles. To our knowledge, this is the first real-time TEM observation that was used to generate consolidation curves for 3YSZ agglomerates (area shrinkage as a function of temperature). The projected area of the agglomerates initially increased slightly as a result of thermal expansion, and then decreased due to consolidation. Without applying electric field, the onset of the consolidation is around 1,000°C, i.e. consistent with previous work by Cologna et al.1. Thus, results validate that in situ TEM is feasible to address questions about the governed mechanisms during EFAS and flash sintering. Experiments to systematically study the effect of the applied electric field on the consolidation of 3YSZ nanoparticles are underway and preliminary results will be presented.
1. Cologna Marco, Rashkova Boriana, Raj Rishi, Flash sintering of nanograin zirconia in <5 s at 850 C, J. Am. Ceram. Soc., 2012.