This presentation will examine two emerging strategies to develop high-performance materials with simultaneous high radiation resistance, high strength, good toughness and corrosion resistance, and moderate fabrication cost. A face-centered cubic 27%Fe-27%Mn-28%Ni-18%Cr high entropy alloy (HEA) was utilized in exploratory studies to compare HEA irradiation behavior with conventional Fe-Cr-Ni and Fe-Cr-Mn alloys. Significant differences in irradiation behavior compared to conventional alloys (in particular solute segregation to grain boundaries and defect microstructures) were observed following 5 MeV Ni ion irradiation (1-10 dpa at room temperature and 500°C). We will also describe results from an investigation of the effects of 5 MeV Ni ion irradiation at room temperature and elevated temperature (200°C) to 0.1 and 1 dpa on the BAM-11 bulk metallic glass (BMG) with a composition of 52.5%Zr-17.9%Cu-14.6%Ni-10%Al-5%Ti (Tg~390°C). Characterization of the HEA and BMG samples utilized nanoindentation mechanical properties testing, x-ray diffraction, and electron microscopy. Although the two model alloys examined in this scoping study did not possess complete radiation resistance, several aspects of their observed response to irradiation suggest the potential for developing alternative HEA and BMG materials that could exhibit favorable stability to irradiation.

Near-term initiation of new commercial deployment of advanced nuclear power reactors requires further development, testing and qualification on new structural materials with improved properties, specifically with respect to radiation, corrosion, thermal and other degradation processes. The International Atomic Energy Agency (IAEA) launched several coordinated research activities in this very important topic. The projects are focused primarily on the stability or instability of nano-dispersoids under irradiation, as observed in several studies. An alternate approach to explore the radiation performance of oxide dispersoid-strengthened (ODS) alloys to high dpa levels is to use charged particle irradiation at greatly accelerated dpa rates to simulate neutron damage. Such approach has been used successfully to study many aspects of void swelling, phase stability, and irradiation creep in other alloy systems [1]. However, some aspects of accelerated charged particle irradiation that produce behaviour atypical of neutrons at lower dpa rates. Because ODS alloys are rather complex in microstructure and very little data at high exposure are available, it is necessary to first demonstrate that modelling can bridge the gap between neutron and charged particle irradiation for a much simpler system before moving to more complex alloys. The research activities contributes to knowledge-base on the radiation-induced response of the ODS material in the following areas: (a) Radiation-induced swelling of ODS materials, (b) Radiation stability of oxide nanoparticles and nanoclusters (or nanofeatures) against swift-ion irradiation, (c) Re-crystallisation effects on the radiation resistance of ODS materials - by testing the radiation tolerance of ODS materials after extrusion and after re-crystallisation to gain insights into the effect of grain size on the radiation tolerance of ODS materials, (d) The effect of temperature on radiation damage development, (e) The effect of He on radiation damage development, (f) Synergistic effects of ballistic damage and simultaneous He and H accumulation on damage formation. The ion irradiations are carried out at different facilities in order to study the phenomena listed above, including: CEA/Saclay Triple-beam ion irradiation Jannus facility (France), ANSTO ANTARES accelerator, and STAR accelerator; Plasma Inert Ion Implantation (Australia); JINR IC-100 cyclotron for applied research (Russia); ITEP heavy ion RFQ HIP-1 (Russia) and Kyoto University DuET Accelerator (Japan). The in-situ TEM and post-irradiation experiments (PIE), static and dynamic nano-hardness measurements, micro-cantilever tests, compression test on micro-pillars, and APT analysis are scheduled in period 2013/2014.
References:
[1] P. Hähner, A.Zeman (ed.), Proceedings of the 2nd IAEA-EC Topical Meeting on Development of New Structural Materials for Advanced Fission and Fusion Reactor Systems, Ispra, Italy, 16-20 April 2012, J.Nucl.Mat.442 (2013) 408-538.

We present in-situ experiment with swift heavy ions at the GANIL facility. One of the applications of these ions is the simulation of materials under real-world conditions, such as radiative environments found in nuclear or space applications. Indeed, swift heavy ion irradiation leads to the deposition of high energy density with electronic excitations, known to entail modifications depending on material characteristics (compositions, electrical properties, crystallographic structures, etc).
Since few years, many efforts are under way to allow in-situ experiments directly at large scale infrastructures to provide time-saving and avoid reproducibility problems. For the study of induced structural phase transitions and kinetics, a X-ray diffractometer (“ALIX”) has been set up at the low-energy IRRSUD beamline of the GANIL facility [1]. This equipment allows to perform in-situ X-ray diffraction measurements simultaneous to irradiation. A special configuration, grazing incidence X-ray diffraction, is required to probe the upper irradiated part of sample due to the energy range of IRRSUD, i.e. 0.3-1 MeV/A, which implies a mean projected range in solid matter around only a few micrometers.
In this communication, key parameters will be presented about the GANIL facility and the capability of the ALIX setup to perform simultaneous irradiation - diffraction by using energy discrimination between X-rays from diffraction and from ion-target interaction. To illustrate its potential, results of sequential or simultaneous irradiation - diffraction will be detailed to show radiation effects on the structural properties of ceramics. Different standard oxide materials are chosen to highlight and to interpret damage build-up induced by ion irradiation, which are MgO, SrTiO3, ZrO2 and Al2O3, which were studied for their potential application as inert matrices for the immobilization of radioactive wastes. During experiments using ions with a relatively high electronic energy loss Se around 20 keV/nm, different sensitivities to electronic excitations are observed. By interpreting the transition characteristics, various cases are described for these materials, ranging from no transition, crystal-to-crystal transition to amorphization through different overlapping-track processes. Evolutions of structural parameters and damage accumulation models will be discussed to interpret the observed transition kinetics. To support the XRD results, transmission electron microscopy studies will be presented.
[1]: C. Grygiel et al., Rev. Sci. Instrum. Volume 83, Issue 1, 013902 (2012)

The interaction of ions with solids results in energy loss to both atomic nuclei and electrons. At low energies, nuclear energy loss dominates, leading to damage production via ballistic processes. At high energies typical of fission products and swift heavy ions, electronic energy loss dominates, leading to intense local ionization. At intermediate ion energies, nuclear and electronic energy losses are of similar magnitude and can lead to additive, competitive or even synergistic processes that affect damage production, defect recovery and microstructure evolution. This energy regime includes energies of primary knock-on atoms created by fission and fusion neutrons, energies of ions used to investigate neutron damage in materials, and ion energies used to modify or generate novel defects and structures in materials to tailor properties or create unique functionalities. We have integrated experimental and computational approaches to investigate the separate and combined effects of nuclear and electronic energy loss on the response of ceramics to ion irradiation over a range of energies. Experimentally, ion mass and energy are used to control the ratio of electronic to nuclear energy loss; whereas, large scale molecular dynamics simulations that include both ballistic collision processes and local heating, or inelastic thermal spike, from ionization via electron-phonon coupling are used to model these processes. Using these approaches, an additive effect of nuclear and electronic energy loss on damage production in amorphous silica and crystalline MgO is demonstrated over a wide range of energies. Similarly, the competitive effects of nuclear and electronic energy loss are confirmed for in situ transmission electron microscopy studies and in studies on ionization-induced recovery of pre-damaged states in SiC using ions and energies (MeV to tens of MeV) with high ratios of electronic to nuclear energy loss. A threshold in electronic energy loss for defect recovery in SiC has been determined. Finally, a synergistic effect of nuclear and electronic energy loss is observed in LiNbO₃. These results have significant implications for modeling the response of materials to extreme irradiation environments.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Tungsten is a prime candidate for building divertor (“exhaust”) components in fusion reactors. During its service life, the component would be subject to fast neutron displacement damage at high temperatures (plasma facing parts: 1200°C; structural parts: 500°C). Using self-ion implantation as an analogue of the primary knock-on atoms of fusion neutrons, the damage behaviour and microstructural evolution in tungsten (> 99.9999 wt%) were investigated as a function of temperature (R.T. → 800°C) and dose (le; 10¹⁸ W⁺m^-2; ~ 1.0 dpa) with in-situ 150 keV W⁺ ion irradiations on the IVEM-Tandem facility at Argonne National Laboratory. Dislocation loops with Burgers vectors of ½<111> and <100> coexisted, among which loops of interstitial nature dominated. In addition, spherical voids of size less than 2 nm were observed.
This work focuses on the thermal stability of the radiation damage induced by the 150 keV W⁺ ions. Thin foils of tungsten irradiated at R.T. up to 1.0 dpa were then annealed in-situ for up to 120 min at temperatures between 300°C and 800°C. Three major conclusions could be drawn: (1) radiation-induced SIA clusters were most stable as ½<111> dislocation loops and tended to configure into strings/rafts through elastic interactions, whilst vacancies resided in both spherical voids and ½<111> loops; (2) b = <100> is not a stable defect configuration in tungsten and the fraction of such loops decreased with increasing annealing temperature and/or time; (3) microstructural transformation during annealing was very sensitive to temperature, but less sensitive to annealing time. The majority of changes were completed within 15 min of annealing, and were associated with the annihilation of small (1-2 nm) dislocation loops. The origin of these trends are discussed by considering defect mobility with respect to the annealing stages in tungsten as well as the enthalpy of formation of defect configurations predicted by previous DFT calculations.

In situ measurements of irradiation effects are enormously valuable for systematic investigation of the effects of dose, dose rate and temperature, since variations in microstructure from one sample to another are avoided. This is particularly true for irradiation induced creep (IIC), yet such measurements are scarce owing to their difficulty. On the one hand, in situ neutron have become prohibitively expensive and time consuming, while on the other, ion irradiations are plagued by short penetration depth of ions in solids and the need for homogeneous damage. Typically high energy light ions in the MeV energy range or GeV heavy ions are employed to overcome this problems but they suffer from high electronic stopping which limits doses to less than 1 dpa /and/or simply do not reflect neutron damage. We have overcome some of these difficulties of in situ creep measurements by using bulge tests thin-film specimens or compression tests on micro-pillars in conjunction with MeV heavy ion on irradiations. We have employed the first method to study IIC in nanocrystalline Cu alloys, and the second to study IIC in amorphous Cu-Ti alloys. We have also modeled IIC in these materials using molecular dynamics simulations. We will first describe our methods in this presentation and then show that the creep response in nanocrystalline materials is directly related to that in amorphous alloys. In the limit that the grain size goes to zero, the creep response in the nanocrystal in fact approaches that of the glass. The model and preliminary results suggest that the magnitude of the creep response in these materials is nearly the same for all alloys and glasses
This work was supported by the U.S. DOE Office of Basic Energy Sciences under grant DEFG02-05ER46217.

In future generation nuclear systems cooled by Heavy Liquid Metals (HLMs), fuel cladding will be exposed to an extremely harsh environment, in which radiation dose will approach 150 displacements per atom (dpa) at a temperature of up to 800°C. In addition, corrosion of structural steels by HLMs stands as a major bottleneck. In this framework, Al2O3 coatings are being investigated for protecting steels [1].
Here, fully dense and compact, nanocrystalline/amorphous Al2O3 coatings are grown by Pulsed Laser Deposition. The mechanical properties of the coating are assessed with high accuracy and precision trough a novel opto-mechanical method, based on the combination of ellipsometry, Brillouin spectroscopy and nanoindentation [2], whereas the adhesive strength is evaluated by nanoscratch tests. The deposition process is tailored so as to obtain an advanced material with metal-like mechanical properties (E=195±9 GPa and nu;=0,29±0,02), strong interfacial bonding and outstanding wear resistance (ratio of hardness to elastic modulus H/E=0,049-0,091). Corrosion aspects are examined by short- (500 hours) and mid-term (2000 hours) exposure of samples to stagnant HLMs at 600°C. Post-test analysis reveals no signs of corrosion [1].
Concerning high dose radiation damage, the performance of the alloy substrate-ceramic coating system is studied by irradiation with 12 MeV Fe ions up to over 100 dpa at 600°C at the JANNUS platform of the CEA center of Saclay. The irradiation parameters (600°C, Fe self-ions, implantation depth beyond coating with an ENSP ratio around 100) are chosen in order to reach a compromise between the expected operating conditions in HLM-cooled fast reactors (ENSP around 4 in Al2O3) [3], and requirements for nanoindentation measurements. Post-test analysis is conducted by profilometry, SEM, TEM, nanoindentation and nanoscratch measurements. Results are compared to previous work on ion [3-5] and neutron [6,7] irradiation of crystalline Al2O3, standing to which the coating is expected to exhibit formation of dislocation loops of around 10-15 nm with densities similar to 1023 m-3 , along with macroscopic swelling of up to asymp;10% or more.
Finally, other coating materials and structures of interest, including cermets and graded barriers with an engineered composition, are proposed and discussed.
[1] F. García Ferré, M. Ormellese, F. Di Fonzo, M.G. Beghi. Corr Sci 77 (2013) 375
[2] F. García Ferré et al. Acta Mater 61 (2013) 2662
[3] S.J. Zinkle. J Nucl Mater 219 (1995) 113
[4] C.J. McHargue. Mater Sci & Eng A253 (1998) 94
[5] S.J. Zinkle, C. Kinoshita. J Nucl Mater 251 (1999) 200
[6] T. Yano, K. Ichikawa, M. Akiyoshi, Y. Tachi. J Nucl Mater 283 (2000) 947
[7] F.W. Clinard Jr. et al. J Nucl Mater 122 (1984) 1386

Reactor materials must withstand irradiation to extremely high doses while under stress at high temperature and in aggressive environments. Cladding and structural materials in fast reactors and fusion reactors will reach 200 dpa, and concepts such as the Traveling Wave Reactor could top 500 dpa. Test reactors have the capability of achieving only a few to perhaps 30 dpa per year, while ion irradiation has the potential to reach high dose levels in comparatively short amounts of time, at low cost and with minimal sample activation. The challenge is in determining how to conduct such high damage rate irradiations so as to replicate the reactor irradiated microstructure.
Ferritic-martensitic (F-M) alloys are attractive candidates for structural components of fusion reactors due to their excellent dimensional stability, thermal properties and low activation. Evolution of the irradiated microstructure (dislocation loops, voids, and radiation-induced precipitates (RIP)) was studied in 5 MeV Fe++ (self-ion)-irradiated T91, HT9 and HCM12A at high doses (>100 dpa) in the temperature range 400-500°C with and without He pre-implantation. Samples were prepared by the focused ion beam (FIB) lift-out method for dislocation microstructure, voids, precipitates, and grain boundary RIS characterization by transmission electron microscopy and atom probe tomography.
Results showed that the dislocation microstructure developed rapidly and was relatively stable with increasing dose. However, precipitates and voids continued to evolve to high dose. Ni/Si/Mn-rich, Cu-rich precipitates, Cr-rich precipitates and chromium carbides all continue to evolve up through 500 dpa and their behaviors depended sensitively on the alloy composition. Voids began to form over a dose range of 100-140 dpa (K-P quick calculation using SRIM) in samples pre-implanted with He. He is found to be very effective in nucleating voids and shortening the incubation for swelling.

Metallic glasses are emerging as a new category of materials with many promising features, e.g. high irradiation tolerance, high strength, etc. The structure-property relation in metallic glasses, however, is still a challenging and unsolved issue. We investigated the relation between the stability of a metallic glass system and its atomic structure. The stability is observed strongly related to the collective arrangement of atoms, rather than the properties of single atoms. Particularly, in an instantly quenched system, the “soft” atoms with the lowest 5% atomic shear modulus tend to cluster together and build up few soft regions; while in an annealed system, the soft atoms are more homogenously distributed and overall leads to the suppression of soft regions and stabilization of the system. We further studied the quantitative relation between the energy barriers and maximum atomic displacements in local activation processes. A quadratic shape is identified, indicating the activation processes are localized and elastic-like. By fitting the curvature of the quadratic shape, we are able to identify the effective size of a typical local activation, which can further serve as input parameter for other models at different levels. We also discussed the implications of our findings and the connections with previous studies.

In this study various nano-mechanical test techniques were used to study the length-scale dependence of the operative mechanisms of time-dependent plastic deformation of single crystal gold micro-spheres at both room and high temperature. Au micro-spheres of various diameters from 0.8 to 3 µm were fabricated on a (0001) sapphire surface using e-beam lithography and a sputter deposition technique followed by in-vacuum annealing. Nano-indentation technique with a flat-punch indenter was used to perform depth-controlled uniaxial micro-compression and constant-load creep test of 1800 second duration at different temperature. Finite element simulation of micro-compression test was also performed to analyse the stress state within the deformed micro-spheres. This allowed the experimentally obtained force vs. displacement data were converted to average stress vs. average strain data. Finally, the average activation energy and the average activation volume of the deformation process was calculated to assess the operative time-dependent deformation mechanism in the Au micro-spheres.

Grain boundaries and triple junctions in metals are high energy regions in the structure where voids, carbides, and other secondary phases nucleate and grow. This leads to decreased performance of the metal, especially when subjected to conditions of extreme temperature and irradiation. There has been significant research in this area over the past three decades or so with concentration on classification of grain boundaries into special and non-special types. Triple junctions have also been characterized in a similar manner having zero to three "special" boundaries that form the junction. Alloy 617 has been shown to have an extended creep rupture life is being considered for use in next generation nuclear power plants. Carbide and void distributions in this material have been analyzed as related to grain boundary and triple junction structure. It is demonstrated that characterization of these distributions from single section planes can yield significant information as to the performance of the metal.

Chalcogenide glasses are semiconductors with unique optical, electronic and structural properties, which are useful for various applications. These characteristics can be enhanced by the addition of silver using a process known as photodoping. Silver photodoping in chalcogenide glasses can be accomplished using a bandgap light to ionize the silver atoms, creating fast moving ions, which diffuse and bond with the chalcogenide glass structure. Due to the importance of these properties for many applications, silver diffusion has been researched for visible and sub bandgap light, while the diffusion mechanics are unknown for short wavelength such as gamma radiation. Short wavelengths have the capability of depositing a greater amount of energy into the material and thus can ionize a greater number of silver atoms, which should affect the diffusion mechanisms. In this study, we investigated the diffusion of silver in a two different compositions from Ge-Se and Ge-Te systems irradiated to five different doses. A special film structure was used, which has the capability to be analyzed using Energy Dispersive X-ray Spectroscopy (EDS) to determine the silver diffusion distance. These results have been modeled using Comsol multiphysics® software to create a timeline of the silver diffusion and determine an equation that models this behavior. This equation was then adapted to a different geometry corresponding to fabricated devices, which were also measured at discrete radiation doses. The measured and simulated results show a close correlation. The greatest change in measured device conductivity occurs at the same doses where the silver concentration crosses a preliminarily defined threshold. Benefits as a derivative of this work will show whether the behavior of silver ions under gamma radiation is comparable to photodoping using bandgap light.

Silica-based optical fiber and fiber-based sensor integration in radiative environments is more and more considered since these components and systems present unique advantages for data transport, plasma diagnostics and for temperature/strain distributed sensing in harsh environments [1]. However, ionizing radiations generate point defects inside the silica matrix leading to macroscopic changes in the optical fibers properties. Defects in amorphous silica are studied experimentally for almost fifty years (see for example [2] for E&’γ center in a-SiO2) but the attribution of absorption/photoluminescence bands to an atomic structure is still a major problematic in the domain [1,3]. Indeed experimental identification of defects is still complex and requires the combination of numerous experimental characterization means such as Electronic Paramagnetic Resonance (EPR) and absorption and photoluminescence spectroscopy. But each experimental means has its own intrinsic limitations. For example, EPR can only detect paramagnetic defects. Moreover, the wide variety of the studied radiative environments in terms of dose and dose rate, applications parameters (fiber type, operating wavelength,..) implies to multiply high costs radiation tests to estimate the vulnerabilities of various commercial or prototype fibers and if necessary to investigate innovative treatments to improve their hardness. That is why, today, the development of a multi-scale simulation procedure appears mandatory to overcome the future challenges [4]. In this paper, we will present the latest results we obtained through an approach coupling experimental and theoretical characterizations of defects in pure and doped (Germanium, Fluorine, Phosphorous) amorphous silica in a multi-scale scheme [5]. On the experimental side, obtained data such as the concentration of a type of defects, coming from Electronic Paramagnetic Resonance, absorption and photoluminescence spectroscopy will be discussed in function of irradiation dose and dopants. On the theoretical side, results coming from first principles calculations will be presented. Computations using Density Functional Theory (DFT) and Many Body Perturbation Theory (MBPT) to the DFT as the GW approximation and the resolution of the Bethe Salpeter Equation have been performed to characterize the structural, electronic and optical properties of defects in pure and doped amorphous silica (see [6] for results in Germanium doped amorphous silica).
[1] S. Girard et al., IEEE Trans. Nuc. Sci., 60(3), 2015 (2013).
[2] R.A. Weeks, J. Appl. Phys., 27, 1376 (1956).
[3] L. Skuja Journal of Non-Crystalline Solids 239, 16 (1998).
[4] J. L. Bourgade et al., Rev. Sci. Instrum., 79, 10F304 (2008).
[5] S. Girard et al., IEEE Trans. Nuc. Sci., 55(6), 3473 (2008); S. Girard et al., IEEE Trans. Nuc. Sci., 55(6), 3508 (2008).
[6] N. Richard et al., J. Phys.: Condens. Matter, 25, 335502 (2013).

Understanding radiation damage in materials is critical for both predicting performance as well as designing advanced materials for even more demanding applications. There are significant challenges in the development of advanced nuclear energy systems, many of which are related to the materials used. Thus, there is a critical role for fundamental materials science in understanding the interaction between radiation damage evolution and the structure of the material in question.
In this presentation, the response of polycrystalline SrTiO3, fabricated by the flash sintering method, to ion beam irradiation was investigated by means of transmission electron microscopy (TEM) and atomic scale modelling. The samples were implanted with 250 keV Ne ions to fluences between 1.11x1016 and 2.25x1016 ions/cm2 at room temperature and then observed using scanning transmission electron microscopy and high voltage TEM. We observe that this material has a large number of Ruddlesden-Popper (RP) faults, related to the non-stoichiometry of the material, in a random and sometimes complicated arrangement. These faults have a significant impact on the radiation damage evolution of the material. In particular, the faults amorphize more quickly than the surrounding SrTiO3 matrix. We examine the interaction of point defects with the RP faults using atomistic modelling and determine that both the thermodynamic and kinetic properties of defects are influenced significantly by the presence of the faults, providing insight into the experimental observations. We conclude that planar defects such as RP faults can have a significant influence on the radiation damage evolution of SrTiO3 and might be one avenue for controlling radiation tolerance in complex materials.

Some of the current fleet of nuclear power plants is poised to reach their end of life and will require an operating life time extension. Therefore, the main structural components, including the reactor pressure vessel (RPV), will be subject to higher neutron exposures than originally planned. These longer operating times raise serious concerns regarding our ability to predict the reliability of RPV steels at such high doses. In this study, several RPV steels were irradiated at high doses to study the degradation of the mechanical properties and related microstructural changes. It is well known that copper-enriched precipitates are key microstructural futures that are responsible for radiation hardening of RPV steels for high-copper welds. At high doses, (Ni-Mn)-enriched precipitates may start contribute to embrittlement process. In this study, the evolution of copper-, nickel-, manganese- and silicon-enriched precipitates is studied by means of small-angle neutron scattering (SANS) and compared to results of atom-probe tomography (APT) measurements. These techniques are used to measure number density, volume fraction, radius of precipitates, and the solute partitioning. Evolution of these microstructural features is compared to degradation of fracture toughness and hardening of these steels.

We have developed in-situ cryogenic nanomechanical testing system to study the
mechanical behavior of nano-sized materials at low temperatures. The cryogenic system
has been installed in the scanning electron microscope and has the capability to reduce
the sample temperature down to 130 K. The simultaneous cooling of both the sample and
the diamond tip ensures the reliable mechanical tests with the low thermal drift
comparable with that at room temperature. Here, nanopillars with the diameter of
400~1300 nm were fabricated from two body-centered-cubic metals, niobium and
tungsten, and uniaxial compression tests were done at 160 K to study the temperature dependent size effects on their mechanical behaviors. Stress-strain curves exhibit the
higher yield strengths and larger strain burst sizes at lower temperature. We discuss these
two differences in stress-strain curves based on the increase in the intrinsic lattice
resistance and the surface-induced multiplication of dislocations. Dislocation dynamics
simulation was used to understand the temperature effects on the surface-induced
multiplication and its relation to the strain burst size.

Fusion devices based on thermonuclear reaction are intensely studied nowadays, the most important one being realized in Cadarche, France. As problems under investigation are the material composition of the first wall of the reaction chamber, erosion, deposition and fuel retention processes of the mixed layers produced during device operation. The high energy fluxes usually found in the tokamak reactor(10-100 MW/m2) are simulated worldwide using ion and electron beams, hot plasmas and laser irradiation.
For this purpose we study here the behaviour of the 200-500 nm thin films of Be-W, Be-C and C-W mixed layers prepared by thermionic vacuum arc (TVA) method in interaction with single or multiple terawatt laser beam pulses, as well as under the interaction with the plasma produced by laser irradiation in a gaseous environment (air, deuterium, helium). A high power terawatt laser system (TEWALAS) having 20-30 x 10 - 15 s pulse duration, 400-450 mJ pulse energy, 10 Hz repetition rate was used for experiments. The coatings were characterized before and after laser irradiation by: Scanning electron microscopy (SEM), Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Part of structural and morphological modifications on the irradiated surfaces were not observed with longer (nanosecond) laser pulses. Formed structures were observed to depend on the number of pulses but also on the ambient gas.

The effect of dilute S addition (42---140 ppm) on the corrosion resistance of Zircaloy-4 (Zr-4) was investigated in lithiated water with 0.01 M LiOH at 360 C and 18.6 MPa by autoclave tests. The microstructures of the alloys, outer surface and fracture surface of the oxide film formed on the alloys were observed by TEM, EDS and SEM. The results show that the addition of dilute S makes the corrosion resistance of Zr-4 better, and it becomes more obvious with the increase of S content. The solid content of S in α-Zr is between 42 ppm and 140 ppm for the alloys in this study. The excess S was precipitated as Zr9S2 with tetragonal structure. Dilute S addition in Zr-4 can improve the corrosion resistance in lithiated water with 0.01 M LiOH at 360 C and 18.6 MPa by delaying the formation of micro-pores and cracks, and the evolution process of ZrO2 columnar grains to equiaxed grains.

As diesel particulate filters operate over a range of temperatures, mechanical properties for these porous materials need to be measured to design against failure at elevated temperature. Fracture toughness measurements of porous silicon carbide were performed at temperatures up to 900°C in laboratory air using the double torsion testing methodology. The double torsion tests were performed on rectangular plate specimens fabricated from walls of honeycomb aftertreatment devices. The fracture toughness values were found to be 0.42±0.05, 0.55±0.03, 0.90±0.18 and 0.72±0.20 MParadic;m at 20, 500, 800 and 900°C, respectively. At elevated temperatures, the SiC and free Si nodules oxidize to form SiO2. This SiO2 skin likely seals small cracks and may have a viscous nature at elevated temperature, which would increase the fracture toughness.

Developing inherently tough materials that are resistant to mechanical failure has long been a fundamental and challenging task in materials science and engineering. Nature has demonstrated the ability to combine organic and inorganic components in complex architectures in order achieve composite materials possessing incredible mechanical properties, such as toughness and stiffness. More often, these biomineralized materials display mechanical characteristics that are far superior to man-made synthetics. One such structure is the hammer-like dactyl club of the mantis shrimp. This crustacean utilizes its raptorial appendage to smash open heavily mineralized prey on which it feeds with incredible force and speed. Here, we investigate the structural features and mechanisms by which the dactyl club resists catastrophic failure from one of the most substantial impacts found in nature. Through a multi-scaled structural, mechanical, and simulated approach, we reveal distinct regions containing both oriented crystalline hydroxyapatite and amorphous calcium carbonate and phosphate phases. Moreover, we find a highly developed underlying organic matrix consisting of chitin fibers arranged in a Bouligand pattern. As a result, this composite material exhibits numerous mechanisms for defense against intense and repetitive impacts, making it an appealing model of study for the development of biologically-inspired high performance materials.

Extensive MD modeling of displacement cascades in iron formed under surface irradiation by 25 keV self-ions has revealed several damage mechanisms. It is important that the particular mechanism depends mainly on the interaction between a supersonic displacement front (shockwave) formed during the thermal spike stage of a displacement cascade. Depending on the depth of the thermal spike center relative to the surface, the three following mechanisms were observed and investigated:
1. If the supersonic displacement front decelerated below the speed of sound well before it reaches the surface, a usual bulk-like displacement cascade formed. This case results in a relatively low number of vacancy and self-interstitial defects in the form of point defects and small clusters which are mainly of interstitial type.
2. If the supersonic-sonic transition occurs just near the surface, atoms on the displacement front flow out to the surface creating a significant number of adatoms and a large vacancy cluster just below the surface.
3. If the supersonic front intersects the surface, it creates a deep crater and a large rim of ad-atoms. It is interesting that the total volume of the crater may reach thousands of atomic volumes demonstrating that this is a very powerful mechanism of surface damage creation. The undersurface damage in this case is reduced compared with a bulk cascade.

Many other details were observed and will be discussed such as the formation of specific surface features due to correlated glide of many hundreds of atoms to the surface and sputtering.

The anomalous temperature-independent phonon friction found in simulations of Brownian motion of nano-scale self-interstitial atom defects and dislocation loops, as well as in simulations of diffusion of kinks on screw dislocations, is a surprising phenomenon that appears at odds with the extensive studies of temperature-dependent phonon drag of dislocations. Analysis show that the anomalous thermal friction effect stems from the anharmonicity of interatomic interaction in the highly deformed core regions of nano-scale dislocation loops, where the treatment of strain fields of defects require going beyond elasticity. We also explore the effect of elastic interactions on microstructure formed in ion-irradiated metals, for example iron or tungsten, where nano-dislocation loops form spatially locally ordered structures. We show that such ordered dislocation loop structures evolve spontaneously as a result of Brownian motion of loops biased by angular-dependent long-range elastic interactions. Patterns of spatially ordered nano-dislocation loops form once the density of defects produced by irradiation exceeds a threshold value, highlighting the critical effect of dose rate on microstructural evolution.

Fe-Cr binary alloys are representative of the reduced activation ferritic/martensitic steels, which are envisaged as the most promising candidates for structural materials in the fast fission reactors and in the future fusion systems [3]. The behavior of chromium and its role on the microstructural evolution in high temperature radiative environments in these alloys is not well known. In our study, we have investigated the influence of chromium on the dislocation loop microstructure induced by self-ion irradiation in highly pure bcc Fe-Cr binaries (with 5,10 and 14wt.% Cr). To study this effect, we have performed two types of ion irradiations at 500 °C, at the JANNuS facility in France:
- Ex-situ, up to 45 dpa, to gain insight about damage formation at higher doses.
- In-situ, (TEM coupled to beam line), up to 0.33 dpa, to follow nucleation and evolution of radiation damage.
Irradiations were realized under mono-beam (Fe only) and dual-beam (Fe + He) conditions to dissociate the ballistic damage from gas implantation effects.
Post-irradiation characterisation was performed by conventional TEM, X-ray energy dispersive spectroscopy (EDS) in an analytical scanning TEM and atom probe tomography (APT).
We have observed that the evolution of the dislocation loop microstructure in Fe-Cr is significantly different from pure Fe. The presence chromium in Fe matrix affects the burger vector of these loops. Thus far, at 500 °C, only a<100> type loops are reported in pure Fe [1,2] upon ion irradiation. However, from our mono beam irradiations on Fe(10,14)%Cr, we have observed both a<100> and a/2<111> type loop populations. We hence conclude that chromium stabilizes the glissile a/2<111> type dislocation loops. This effect then induces a finer loop microstructure, with smaller average sizes but a higher number density.
Furthermore, TEM and analytical STEM/EDS revealed that the plane of these dislocation loops were enriched in chromium. The enrichment was seen irrespective of the presence of simultaneous He implantation. APT analysis on Fe10%Cr showed that the enriched zones were heterogeneously distributed on the habit plane of these loops. Excess chromium in these areas was quantified by both APT and EDS. We speculate that such an existence of chromium rich zones inside the dislocation loops, which is a defect free region in bcc alloys, can not be due to pure radiation induced segregation process.
[1] D. Brimbal, PhD. thesis, University of Paris 11, 2011
[2] M.L. Jenkins et. al, J. Nucl. Mater, 389,197-202, 2009
[3] R.L. Klueh and D.R. Harries, ISBN 0-8031-2090-7

While irradiation-assisted stress corrosion cracking is an important issue for life extension of current light water nuclear reactors, the basic mechanisms for crack initiation remain poorly understood. In this study we have ascertained how the presence of an irradiation hardened matrix influences the criteria for predicting slip transfer across grain boundaries. The importance of the local resolved shear stress increases with irradiation damage level as the dislocations must experience sufficient shear stress to propagate through the irradiation hardened matrix. This converts the rate limiting step from dislocation nucleation from the grain boundary to propagation away from it. If the grain boundary cannot transfer the slip, the grain boundary will adopt an alternate relief mechanism such as an out-of-plane displacement and crack nucleation. These processes may cause the oxide film to rupture, exposing the metal to the environment. Results from a combination of in situ TEM deformation experiments and FIB-TEM post mortem investigations that probe the response of grain boundaries to impinging dislocations in irradiated materials will be presented.

Self-interstitials rarely play a significant role in crystalline materials except under irradiation where they are produced at the same rate as vacancies. The structure and kinetics of interstitial-type defects are being revisited by ab initio electronic structure calculations in metals. Calculations on self-interstitials in hexagonal close packed metals (Ti, Zr and Hf) revealed new low energy configurations and do not call for the expected fast basal migration. In body centered cubic iron, ab initio calculations predict that self-interstitial clusters can adopt a 3D periodic morphology, corresponding to the C15 Laves phase structure. The formation energies of clusters with 5 to 8 interstitials are lower by approximately 4 eV than that of conventional two-dimensional loops. Using an empirical potential than quantitatively reproduces this key feature, we show that these clusters: can form in cascades, have a very low mobility, and have a lower formation energy than 2D loops up to sizes of about 40 self-interstitials.

The development at CEA of new ferritic/martensitic (F/M) ODS materials for the cladding for GENIV Sodium Fast Reactors and fusion reactor is a key issue [1]. F/M ODS exhibit very low dimensional changes (creep and void swelling) under irradiation up to high doses, higher than 150 dpa. They contain a very high density (>10²⁴m^-3) of Y-Ti-O nanosized particles, responsible of the excellent behavior at high temperature, and favoring the trapping of He in the case of fusion reactor. Nano-phases have been extensively studied by SANS, APT and TEM. However, their exact nature is not yet well understood. They appear to range from coherent solute enriched clusters with complex shell structures as found by APT, to near stoichiometric complex oxides, such as Y₂TiO₅ and Y₂Ti₂O₇, as found from SANS and TEM analyses. Nevertheless, for these nano-oxides, the main point is to remain stable under irradiation up to high fluence and high temperature.
Here, we focus on the results obtained at the MARS beamline, the French beamline dedicated to the analysis of active materials [2]. Two experimental techniques, XRD and XAFS at the Y-K edge, have been performed on a subassembly with the DY alloy as cladding material irradiated in the French Sodium Fast Reactor Phenix. DY is a Fe-13Cr-1.5Mo + 1TiO₂ + 0.5Y₂O₃ ODS, developed by SCK/CEN Mol and elaborated by DOUR Metal in the seventies [3]. The irradiation dose was between 0 to 81 dpa and temperatures went from 400 to 580°C along the fuel pin. Four locations along the fuel pin were investigated; with a reference for the non-irradiated state [4].
This presentation will concern the results obtained by XRD and XAFS with a special regard about the influence of the position of the samples in the neutron flux on the Y-Ti-O clusters composition and crystallographic phase. These results, combined with previous TEM analysis, will give a new insight into the atomic configuration on the Y-Ti-O nanoclusters [4].
[1] P. Dubuisson, et al., J. Nucl. Mater. 428, issues 1-3 (2012) 6-12.
[2] J-L Béchade, et al., J. Nucl. Mater. 428, issues 1-3 (2012) 183-191.
[3] A. De Bremaecker, et al., J. Nucl. Mater. 428 (2012) 13-30.
[4] I. Monnet, et al., J. Nucl. Mater. 335 (2004) 311-321.

Oxide dispersion strengthened (ODS) ferritic alloys are promising candidate as first-wall/blanket structural materials for fusion reactors due to lower swelling rate. Even though some materials are quite stable structurally under only neutron irradiation, simultaneous He implantation at a rate of 10~2000 appm, driven by high dose (up to 200 dpa) of energetic neutrons in the reactors, can significantly degrade the structural property (i.e. swelling resistance), it becomes indispensable to understand how the He accumulates in the materials, in particular how He transitions from nanoscale He-contained gas bubbles into voids upon reaching a critical size, leading to void swelling and further detrimental changes to mechanical properties. To attain fusion relevant conditions a thin layer of NiAl was deposited on top surface, producing and injecting energetic He into target substrate about 8 µm below the NiAl coating, when irradiated in the High-Flux Isotope Reactor (HFIR) via a two-step thermal neutron reaction sequence from Ni element. PM2000, one of commercial ODS alloys, was coated on one side with a 4 µm thick NiAl film and irradiated in HFIR to a neutron displacement damage dose of 21.2 dpa at 500°C. The dpa are primarily produced by fast neutrons, while the in situ He injection (ISHI) to 1230 appm derives from the thermal neutron two-step ⁵⁹Ni(n,α) reaction.
This presentation summarizes TEM analyses to compare two different irradiation conditions to PM2000, i.e., simultaneous He-injected/neutron-irradiated side vs. only neutron-irradiated side. TEM analyses of the He implanted region reveal a high density of small He bubbles (<2 nm) aligned predominantly within dislocation loops in the bcc matrix. The voids (5-10 nm) always formed only within large amorphous Y-Al-O ODS particles (15-30 nm). On the only neutron-irradiated side of the sample, no He bubbles could be detected, and voids were found on ~10% of the ODS particles. The He-implanted/neutron-irradiated side contains a dominant density of dislocation loops (<200>{200} and ½<111>{111}) and a low density of line dislocations, whereas the un-implanted side possesses predominantly line dislocations with a lower density of dislocation loops. In both regions, nanoscale 2^nd precipitates (fcc) formation exhibiting a same orientation relationship with the bcc matrix were confirmed by diffraction patterns of 9 different zones and dark-field TEM (DFTEM). Chemical analysis using energy dispersive x-ray spectroscopy (EDS), energy-filtered TEM (EFTEM) and atomic probe tomography (APT) indicates these phase is rich in Al and Ti. Furthermore, 3^rd Fe₃C formation is confirmed by diffraction analysis. Substantial Cr segregation around the dislocation loops is noted, concurrent with Fe depletion. Unlike Fe and Cr, Al and Ti are segregated in the same region. The amorphous Y-Al-O particles contain a high Al concentration (>20 at%) and Cr segregation/Fe-depletion at the periphery.

Helium (He) in structural metals precipitates into bubbles, causing severe damage. Using multiscale modeling, we find that at fcc-bcc interfaces, He is initially trapped in stable, sub-nanometer platelet-shaped clusters, not bubbles. This behavior occurs due to the spatial heterogeneity of the interface energy: He wets high energy, “heliophilic” regions while avoiding low energy, “heliophobic” ones. We confirmed this prediction with neutron reflectometry, which showed that interfacial He bubbles form only above a critical He concentration and provided evidence for the presence of stable He platelets below the critical He concentration. Our work paves the way for the design of composite structural materials with increased resistance to He-induced degradation by tailoring the types of interfaces they contain.
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.

Nuclear reactions generate insoluble helium (He), which forms nano-sized bubbles that can lead to swelling and embrittlement of irradiated materials. Structural materials for nuclear reactors must be designed to be immune against radiation damage and to operate in harsh thermomechanical environments. Ferritic steels and metallic glasses serve as useful candidates for such applications due to their high strength, corrosion resistance, and potential for improved ductility upon irradiation. Interface-containing materials are also useful candidates as interfaces between dissimilar metals, i.e. fcc and bcc metals, have been shown to exhibit enhanced radiation resistance.
We fabricated, irradiated, and mechanically tested 100 nm-diameter nano-tensile samples created via templated electron-beam lithography and electrodeposition. Materials systems studied include monolithic Ni- and Fe-based metallic glasses, single crystalline Fe, and metallic glass/Fe with a single interface. Samples were implanted with He to create a uniform He concentration throughout the gauge or to create a peak He concentration at the interface. He bubble size and diffusion were controlled by varying implantation temperature and subsequent annealing temperature, at values ranging from room temperature to 400°C. We present the results of in-situ uniaxial tensile experiments and TEM on these as-fabricated and helium-implanted samples. We discuss the effects of helium implantation and temperature on the microstructure and tensile properties of the studied materials.

The generation of point defects during the irradiation of austenitic stainless steel is known to result in irradiation assisted stress corrosion cracking of material, partly due to radiation induced segregation (RIS) of solute atoms in the grain boundaries, and due to the formation of defect and solute clusters in lattice. Grain boundaries are effective sinks of point defects generated in the lattice. The concept of nanostructuration aims at providing more grain boundary surface area and a shorter migration distance for the point defects, and thus faster annihilation of these defects. At the same time, this nanostructuration can reduce the grain boundary segregation by decreasing the number of point defects reaching the grain boundary per unit area of grain boundary. The present study evaluates the effect of nanostructuration on the inter and intragranular segregation of solute atoms under ion irradiation of ultra-fine grained 316L austenitic stainless steel samples, produced by high pressure torsion (HPT) at 400°C with 10 turns under a compressive load of 6GPa.
Atom probe tomography study reveals the major segregation of silicon atoms and enrichment of chromium and Molybdenum atoms at grain boundaries after the HPT experiment. This probable non equilibrium segregation of solute elements is found to be undisturbed with thermal aging of the deformed sample at 450°C for 5hrs under vacuum condition. Preliminary results suggest that, in the case of ion irradiated ultra-fine grained samples with Fe5+ ions at 450°C and a total dose of 5dpa, the level of grain boundary segregation is lesser than that of coarse grained irradiated sample. These results are further discussed based on the defect generation, migration and annihilation mechanisms.

The emergence of metal nanoparticles and other nanostructured materials in applications ranging from medical procedures to solar cells has raised concerns regarding their stability in different extreme environments. This presentation will highlight recent work undertaken at Sandia National Laboratories&’ Ion beam Laboratory to understand the stability of gold nanoparticles to various ion beam irradiation and implantation conditions. To achieve an understanding of the dynamics that occur during the bombardment of gold nanoparticles, this study utilized an in situ ion irradiation transmission electron microscope (I3TEM). This facility permits real time observation with sub-nanometer resolution of the nanoparticles during ion irradiation with a 6 MV Tandem accelerator, ion implantation with a 10 kV Colutron, or both concurrently. In the ion irradiation portion of this study, the particles were bombarded with either copper or gold ions at 3 MeV. The evolution of the supported nanoparticles during ion irradiation showed both thermal and sputtering effects, as evidenced by the formation of large agglomerates, as well as new satellite particles surrounding the original particles. To fully understand the extent of both of these effects, a series of electron tomography and in situ ion irradiation steps were used to produce a time separated series of 3D models during ion irradiation. In addition to heavy ion irradiation, the evolution of the particles was investigated during ion implantation of noble gasses The insight gained from these experiments will be used to further elucidate the governing mechanisms associated with the structural evolution of the gold nanoparticles, as a function of nanoparticle size and structure in addition to irradiation condition.
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.

For the shock compression of materials, both the elastic and plastic deformation of the materials usually happen and finish within nanoseconds. The superfast loading rate makes it quite suitable for the molecular dynamics (MD) simulation method to be employed to study the underlying physics of this phenomenon. However, the limited length scale of the state-of-art MD technique still presents an obstacle for shock simulation on atomic scale. A noteworthy issue is that a tensile wave will be readily reflected back from the back surface on shock loading of a finite size MD model, thus makes it difficult for the MD model to mimic shock compression of bulk samples in order to capture the true non-equilibrium processes within the model. Here we present a wave transparent boundary (WTB) scheme for MD simulation of shock compression of materials. The WTB is simply based on the assumption that, wave propagation is steady and stable on the local time and space scale, regardless of the linear or nonlinear nature of the wave. We have tested the WTB by MD shock simulation of Cu single crystal with rectangular shock loads as well as trapezoidal shock loads. By comparing with shock simulations without WTB employed, the WTB shows very good performance for the waves to propagate through the outward boundary in WTB. It can be seen that much reduced size of MD model on the shock load dimension can be used for the non-equilibrium MD shock simulation with the WTB to mimic shock compression of the bulk sample. We expect that the WTB can be used on MD shock simulation of a broad range of materials and various kinds of shock loads.

Fused silica (FS) optics are critical components for high peak-power laser systems such as the National Ignition Facility. These optics experience extraordinary irradiation environments: 8-10 J/cm² nominal operating fluence over optical surface areas of about 2500 cm² per optic, with laser contrast and modulation responsible for regions with even higher fluences. The initiation of laser-induced damage on FS surfaces is an important factor for determining NIF operating parameters, as damage to FS optics ultimately require optics to be refurbished or replaced. Current mitigation methods for reducing damage initiation focus on eliminating absorbing near-surface impurities (e.g. ceria, a common optics polishing compound), and near-surface electronic defects (e.g., silica fractures due to scratches).
While most studies link strongly absorbing particulates such as metals to damage, we will show that a wide variety of materials on a FS surface can initiate damage, even when these materials are optically transparent at the operating wavelength. We have surveyed several materials and morphologies and tested their propensity to initiate damage on FS surfaces when irradiated at 355 nm. Laser-matter interactions between individual particles less than 100 microns in size are coincident with laser-induced damage on FS surfaces, which occurs between 20-30 J/cm² for a wide range of solid compounds. Indirect evidence suggests that submicron particles behave similarly. Based on this more complete understanding of the possible identities and processes responsible for forming damage initiators, we have implemented processes to significantly reduce the damage density on fused silica.
LLNL-ABS-645241

Recent recoveries and analysis of meteorites, including Martian meteorites,containing carbon-rich inclusions have led to evidence of complex phase transformations resulting from impacts in the asteroidal belt or on Mars : understanding the processes at play during the shock process is important in view to infer the original structure and the history of the recovered samples. In particular the transitions that occur in carbon-rich regions can lead to the formation of ultra-hard carbon phases which resist to polishing by conventional diamond powders. The formation of nano and micrometer-sized diamonds from graphite has also been shown to occur close to the hardest mineral phases such as silicates.
In this study, we use large-scale molecular dynamics simulations to focus on the mechanisms at play during the graphite to diamond transition, under conditions close to meteoritic impacts. We model the meteorite sample with a copper matrix containing graphite inclusions, using a Sutton-Chen EAM potential and the LCBOPII potential for copper and carbon respectively.
Inclusions of various topologies and sizes are considered and we analyse the role of shock strength on the induced phase transformation properties. We discuss the graphite/diamond transition threshold and mechanism with respect to recent shock simulation results for monocrystalline graphite.
All the simulations were run with the STAMP MD code on the Curie supercomputer (PRACE project #2011040525).

The creation of highly non-equilibrium conditions in solids by irradiation with ultrashort laser pulses has been a topic of extensive research in recent years. Advances in experimental characterization techniques, such as ultrafast electron diffraction (UED), has enabled structural data to be obtained with femtosecond resolution. The measured intensities of the Bragg peaks reflect, not only the lattice temperature effects, but also the structural changes due to phase transitions such as melting. De-convoluting these effects from the integrated structural data is challenging, therefore a detailed description of the structural evolution is difficult to achieve.
We recently carried out a combined experimental/ modelling study of femto-second laser irradiated of thin gold films and obtained a detailed description of the atomistic dynamics of the photo-induced solid to liquid phase transition. We used UED to measure the time evolution of the Bragg peak intensities and a coupled two temperature - molecular dynamics (2T-MD) model to simulate the atomistic dynamics following laser irradiation. Crucially, the time and length scales of the experiment and modelling were the same, therefore we can compare the Bragg peak evolution obtained from the two methods directly. We obtained excellent agreement between the calculated and experimental Bragg peak intensities over the full experimental time scale for all fluences, which suggests that the modeled atomistic dynamics are a true representation of the structural dynamics of the laser irradiated gold films. We identified three distinct types of melting dynamics induced by the different experimental fluences. In the low fluence regime we found heterogeneous melting, where melting initiated at the free surfaces and the melt front propagated to the interior of the film. At the intermediate fluence we observed homogeneous melting in which molten seeds grew and coalesced in the interior of the film. For high fluence irradiation we identified non-thermally accelerated melting. In this case the reduced screening due to the redistribution of the conduction electrons had a significant effect on the interatomic interactions, which resulted in rapid expansion of the film. We will present the details of the experimental and modelling methods, as well as the calculated atomistic dynamics of the gold films in the three fluence regimes.

In this work the ablation characteristic of glass substrate was investigated during femtosecond laser machining. The glass substrate was irradiated for 800 fs at the wavelength of 1552 nm. Morphological changes of the ablated regions were characterized by means of a 3-dimensional laser scanning microscope. The experimental results show two different ablation regimes for crater depth and removal volume with varying the laser fluence. The first ablation regime shows a linear dependence of crater diameter on its depth. In the second ablation regime, the crater depth and removal volume undergo a rapid increase over the laser fluences greater than the threshold energy density of 8.2 J/cm2. The crater diameter and depth have a nonlinear relationship. The ablation phenomena in the first ablation regime can be characterized by the diffusion length of the conduction band electrons plus a femtosecond laser pulse instead of the optical absorption coefficient of dielectrics. The rapid increase of the crater depth and removal volume in the second regime can be explained that the diffusion length (140 µm) in the second regime is sufficiently longer than 1.3 µm in the first regime. This means that the diffusion length of the electrons over the threshold energy density is long enough to carry on carrier diffusion into the bulk. Each diffusion length of the conduction band electrons in two different ablation regimes was obtained in this work.

The ballistic formation and thermalization of collision cascades are believed to be reasonably well understood. In contrast, our current understanding of the evolution of defects after cascade thermalization is very limited despite the fact that such defect dynamic processes play the dominant role in the formation of stable radiation disorder in most nuclear materials. Here, we use a pulsed-ion-beam method to gain insight into defect interaction dynamics. We measure the effective time constant of defect interaction by studying the dependence of damage on the passive part of the beam duty cycle, while the effective defect diffusion length is estimated from the dependence of damage on the active part of beam cycle. A defect lifetime of about 5 ms and a characteristic defect diffusion length of about 30 nm are measured for single-crystal Si irradiated with 500 keV Ar ions at room temperature. We further study how these dynamic parameters depend on the average density of collision cascades and the maximum instantaneous defect generation rate. Finally, we discuss implications of these findings for the development of predictive models of radiation damage buildup in solids.

Very little is known about the nucleation and growth of solidification under extreme conditions. In fact, very little is known about the properties of liquids (e.g. viscosity) along the melt curve under extreme conditions. To gain some window into these phenomena, we have performed a series of large-scale non-equilibrium molecular dynamics simulations using Finnis-Sinclair potentials for Tantalum and Iron. Of the limited experimental data available, probably the most interesting experiments use electrostatic levitation to measure the relaxation of liquid droplets and infer viscosity at extreme temperature. Our simulations agree with the experiments at the melting point. However, the temperature dependence (activation energy) is significantly softer in the simulation, in contrast with our simulations on copper, which are in agreement with the experiments at all temperature. By performing the simulations in a constant heat ensemble, we are able to study the properties of a nucleant in the hot dense liquid and map out the Gibbs-Thomson curve and critical nucleation size along the melt curve. We also present results for the wetted layer on the surface of a solid nucleant in the hot dense liquid. The talk will conclude with a discussion of the Exascale Co-design Center for Materials in Extreme Environments (ExMatEx).
* Work performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Increased scaling of nonvolatile memory (NVM) with traditional charge-trapping flash devices poses many problems, including charge retention and state detection. Programmable Metallization Cell (PMC) memory has shown potential as a solution to these issues. While most research on NVM is focused towards commercial applications, there is still a need for such memory solutions for applications in aerospace or nuclear environments. In these environments, the technology can be exposed to a range of ionizing radiation (electrons, β-particles, x-rays, etc.) Thus, the development of radiation-hardened devices is necessary for increased advancements in these fields.
There is principle concern related to these devices because the preliminary materials studies related to the Ge-Se glass films show that electron beam, x-rays and other forms of ionizing radiation cause increased Ag diffusion in the chalcogenide matrix films between the two electrodes, which could affect their performance. However, the composition dependence of these effects, as well as the device technology related aspects, has not been studied in detail. Thus, further examination is required in order to gain knowledge about the result of ionizing radiation over the performance of such devices.
The present study is related to the investigation of the influence of electron-beam radiation over blanket Ge-Se films, with compositions containing 20, 30 and 40 at. % Ge with a laterally positioned Ag source in addition to PMC devices with a vertical structure, based on the same Ge-Se compositions. PMC devices were fabricated with one W electrode, one Ag electrode and an amorphous Ge-Se insulating material analogous to that of the blanket films. The Ag electrode also acts as a source of Ag ions for conductive bridge formation when the device is biased in the “write” state.
Raman spectroscopy revealed the structural changes occurring under e-beam irradiation. The Ag diffusion due to e-beam irradiated diffusion into the chalcogenide glass was evaluated through Energy Dispersion Spectroscopy (EDS) studies. The molecular analysis of the composition, occurring due to reaction of Ag with the chalcogenide glass was accomplished through X-ray Diffraction (XRD). Electrical device performance was established through current vs. voltage (I-V) measurements. Radiation stability of the PMC devices is examined through charge retention after radiation exposure.
The obtained data are discussed based on the structural and compositional analysis of the films, as well as the device structure. Conclusions are made based on the most stable material for the performance of the PMC devices under the studied influence, which accordingly to our data are the Ge40Se60 glasses.

The mechanical behavior of nanocrystalline (NC) metals has attracted widespread interest, though the majority of current efforts have focused on (nominally) pure metals. By comparison, the mechanisms of strengthening and deformation in NC alloys, especially those with high segregation propensity and strong chemical interactions, such as Al-O, are poorly understood. Herein, NC Al-O thin films are synthesized by means of confocal co-sputtering, which enables wide-range and quasi-independent control over impurity content and grain size. Detailed characterization combining transmission electron microscopy (TEM) with three-dimensional atom probe tomography identify the multiple chemical states of O in a complex composite-like microstructure, including nanosized α-Al₂O₃ precipitates, O-rich clusters segregated along grain boundaries, and O solute atoms. Individual contributions of these strengthening features to the mechanical properties of NC Al-O thin films as measured by instrumented nanoindentation are well delineated by an analytical model with microstructural inputs. Furthermore, the influence of O impurities on the deformation mechanisms of NC Al films is discussed based on micro-tensile testing and quantitative in situ TEM, with special attention paid to the stress-driven microstructural instability, e.g. grain growth and grain boundary migration. We propose a critical interfacial excess of O impurities required to stabilize the NC grain structure and hinder stress-driven GB migration, providing a new avenue for controlling NC deformation mechanisms apart from grain size alone.

Nanostructured Cu and Ag films with and without a high density of twin boundaries were indented at approximately 77 K with a load of 0.25-0.5 N. Utilizing precession-enhanced electron diffraction in the TEM, the crystallographic texture, grain size, and grain-to-grain mis-orientation were quantified. The nanotwinned films underwent grain growth in the pile-up region of the indent. In particular the <001> textured Cu film had marked increases in the sigma 3 and sigma 5 boundary fractions as compared regions that did not undergo indentation. The Cu film without twin boundaries experienced little to no statistically observed grain growth. The intrinsic twinned grain structure seems to facilitate the observed grain growth, either as a result of the increased mobility of the twin boundaries and/or a complex mechanically-induced detwinning mechanism.

We report quantitative mechanical characterization of silicon carbide (SiC) nanowires (NWs) via in-situ tensile tests inside scanning electron microscopy using a microelectromechanical system. The NWs are synthesized using the vapor-liquid-solid process with growth direction of <111>. They consist of periodic segments of pure 3C structures and highly defective structures along the NW length. The SiC NWs are found to deform linear elastically until brittle fracture. Their fracture origin is identified in segments of the pure 3C structures with inclined stacking faults, rather than the highly defective structures. The fracture strength increases as the NW diameter decreases, approaching the theoretical strength of 3C SiC.

The equation of state of carbon under extreme temperature and pressure conditions is important to understand to devise models of planets and their interiors, such as Neptune and Uranus, as well as extra-solar carbon planets. Moreover, a detailed knowledge of carbon properties under such conditions is critical for a better understanding of carbon-based materials for several important applications. Here we use a density functional tight binding model to study the free expansion and cooling of shock induced metallic liquid carbon. Three different shock states and three expansion rates (kx-1 = 50 ps, 100 ps, and 250 ps) are investigated. For all cases, graphite like sheets are formed upon cooling and equilibration. Expansion at lower shock conditions (900GPa, 7000 K) resulted in ~13% increase in graphite quantities formed compared to the expansion at high shock conditions (1700GPa, 30000 K).
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and was funded by Laboratory Directed Research and Development grant #12-ERD-052. LLNL-ABS-645492.

Focused ion beam (FIB) processes have been widely used for a variety of material modification purposes including cutting, milling, formation of trenches, and fabrication of through-thickness nanopores in free-standing thin films. These nanopores have a variety of interesting applications, including for measuring and sequencing of DNA. In the present work we carry out some of the first ever large-scale molecular dynamics simulations of focused ion beam machining of silicon, with realistic length scales, energies, and experimentally accessible fluxes. In considering nanopore formation in a free-standing film, we find that there is a threshold ion delivery rate above which the mechanism underlying nanopore formation changes. At lower fluxes, nanopores form by means of an erosion mechanism. The erosion rate is proportional to the ion flux, so the nanopore formation process is limited by the sputter rate of the target material. But at higher fluxes, nanopores form via a thermally dominated process, consistent with an explosive boiling mechanism. In this case, mass is rapidly rearranged via bubble growth and coalescence, much more quickly than would occur during sputtering. This remarkable process allows the system to rapidly overcome the energy barrier to nanopore formation in the freestanding film, and for systems in which the hole diameter is comparable to the film thickness, mass transport is influenced by a microscopic Marangoni flow. A complete understanding of this explosive boiling mechanism has the potential to greatly enhance FIB-based material processing.

Ion beam irradiation along channeling direction will lead to selective grain growth in crystalline materials. In contrast to normal grain growth (NGG) by heat treatment, where the movement of grain boundary is driven by grain boundary curvature, the driving force for ion beam induced selective grain growth (IBSGG) mainly come from the difference of volume free energy between channeling and randomly oriented grains. With less irradiation damage, channeling oriented grains are selected to grow at the expense of others and, consequently, leading to texturing. IBSGG and texturing has already been investigated in fcc metallic thin films, e.g. Au and Ag. In this study, we are going to present IBSGG and texturing in bcc W thin film with (110) fiber texture. Instead of only one activated channeling system observed in fcc metals, three different channeling systems (i.e. <111> and <100> axial channeling and (110) planar channeling) are simultaneously activated in bcc W film, which leads to the development of three in-plane textures. Due to the two fold crystallographic symmetry around the fiber axis of <110>, <100> axial channeling and (110) planar channeling can be suppressed by 180 degree rotation of the sample, around the fiber axis, during ion irradiation. In addition, irradiation temperature shows great effect on the IBSGG process. Irradiation at liquid nitrogen temperature leads to continuous growth of selected grains as function of ion fluence while irradiation at room temperature only slightly increases the grain size at low fluence and has no further obvious effect. Resistivity measurements and TEM characterization are carried out on samples irradiated at both temperatures to reveal the effect of irradiation temperature on the microstructure and defect development.

One of the most topical issues surrounding oxygen interstitials in UO2+x is the relative stability of point and cluster defects. Density functional theory (DFT), corrected for onsite Coulomb interactions (DFT + U) provides a reasonable description of oxygen interstitials, but has issues with the U dependence and multi minima. Using DFT with the HSE06 (Heyd-Scuseria-Ernzerhof) hybrid functional, we will present (i) the formation energy of point defect and Willis 2:2:2 cluster with hybrid DFT and compared with DFT + U, (ii) localized U5+/U6+ ions form and (iii) the position of the localized gap state. Our results may provide important information that the preference of stable structure can be explained in terms of the excess-hole distribution to achieve a localized solution during structural relaxation. This work is supported by the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number FWP 1356.

Nanoindentation technique is commonly used and accepted way to study of the materials&’ surface mechanical properties.
Many real life applications require the materials to work in the extreme atmospheric conditions such as elevated temperatures. To assure the component reliability it is imperative to test it in the real working conditions. This is particularly challenging for thin film protective coatings at small scales when performed at elevated temperatures. In the light of numerous challenges related to the instrumentation, nanomechanical characterization at high temperatures has been limited up to present.
Addressing such instrumentation gap, we have recently developed a unique method that allows for complex characterization of a material&’s surface at elevated temperatures. A newly-designed, radically-different, low-drift heating stage for precise control of temperature up to 600 oC, combined with an improved nanoscale dynamic mechanical testing capability and in-situ scanning probe microscopy-based imaging, resulted in drift-free nanomechanical properties measurements method. High system stability and unique engineering approach resulted in drift free measurements even over extended period of time giving a good base for creep measurements at the nanoscale.
Such experiments are particularly interesting for investigation of materials such as superalloys and bond coats which were designed and optimized to serve in extreme atmospheric conditions. Based on the example of an intermetallic PtNiAl bond coat, we will demonstrate how our recently developed methodology can be utilized for dynamic nanoscale mechanical characterization over extended period of time. We will demonstrate how the creep measurements could be utilized for further understanding of the fundamental materials properties, specifically activation energy calculation.

High entropy alloys (HEAs) have attracted extensive attention due to their remarkable combination of strength, ductility, thermal stability, and potential radiation resistance. However, little is known about why these alloys are stable in a single-phase solid solution or how to predict which combinations of elements will form a single phase HEA. Here, we present density functional theory calculations of the heat of formation of several potential HEAs, in an effort to assess the role of the entropy of mixing in the stability of these alloys. The compositions studied here include both single-phase and multi-phase alloys. The heats of formation show no significant differences, regardless of their single or multiple phase formation, and no trends that could explain the stability of the single phase materials. Moreover, all of the calculated heats of formation are positive. These findings indicate that the entropy of mixing is insufficient to explain the unique stability of these alloys, and highlights the need for new criteria to explain the formation of single-phase solid solutions. We also discuss the minimum energy structures of several FCC and BCC alloys as well as their relative phase stability.
Work Supported by Department of Energy - Basic Energy Sciences, Materials Sciences and Engineering Division and the Scientific User Facilities Division.

Ion implantation using a focused ion beam (FIB) allows for good control over concentration and distribution of ions in the target material. The downfall of doping using an ion beam is the occurrence of damage in the crystal, including amorphisation, due to bombardment of the target by the highly energetic ions. In our studies, a Ga-ion source FIB was used to dope SCF-grown Ge nanowires deposited on FIB-patterned Si3N4 membrane grids. These pre-patterned girds allow for high resolution imaging in the TEM by opening “windows” and for ease of navigation to find the same nanowire, in the same orientation, in both the TEM and SEM/FIB time and time again. These nanowires were imaged, before and after doping, in the TEM and annealed in-situ using a heating stage holder. Our experimental results have shown that the recrystallisation of a nanowire which has been almost fully amorphised along its entire length, to an extent where small crystallites remain but have lost orientation with respect to the rest of the nanowire due to the loss of rigidity, returns a highly defective crystalline structure. However, if the doping is limited to only a section of the nanowire (i.e. using focused ion beam capabilities only a small portion of the nanowire can be irradiated), annealing recovers a near perfect crystalline nanowire at temperatures as low as 390°C. When the NW is irradiated along the full length, but only small amount of amorphisation is observed as small surface pockets, the recovery proceeds again to a single crystalline nanowire with well-developed facets. Similar results have been observed for Ge fins, where a near perfect crystalline structure is recovered [1] due to the existence of a large single crystalline substrate. We have determined that the ease of recrystallisation is attributed to the large templating “seed” available to a partially irradiated NW. In search for alternative annealing procedures, a comparative study of crystal recovery observed by ex-situ TEM of rapid thermal and microwave annealed Ge nanowires will also be presented. These methods can supply better control over dopant diffusion and subsequent junction formation.
1. Duffy, R.; Shayesteh, M.; McCarthy, B.; Blake, A.; White, M.; Scully, J.; Yu, R.; Kelleher, A. M.; Schmidt, M.; Petkov, N.; Pelaz, L.; Marques, L. A., The curious case of thin-body Ge crystallization. Applied Physics Letters 2011, 99 (13).

In this work we continue our previous investigations of the severe plastic deformation (SPD) on the magnetic properties of 4-f elements, with special accent on magnetic anisotropy and magnetic transformations.
We report the magnetic properties of thin Nd, Sm and Gd ribbons obtained with the help of SPD technique. Severe plastic deformation procedures are very interesting for designing novel functional materials. Depending on the degree of deformation, magnetic, structural or thermodynamic properties could be varied in severely deformed materials, especially in thin ribbons of SPD-treated materials.
The interest in this matter is far from being purely academic. As it shown in [1] a significant depression of magnetic and thermodynamic properties occurs in severely deformed samples of Gd. The reason of such behavior is in a giant magnetic anisotropy induced by SPD. This unexpected phenomena drives to a new thermodynamic and magnetic properties of severely deformed Gd ribbons [1] which are inapplicable after the SPD-treatment for magnetocaloric applications without additional heat treatment procedure. The heat treatment regimes are directly connected with the degree of plastic deformation [2].
Nevertheless, new magnetic properties induced by SPD-treatment allow us to obtain novel magnetic materials (especially hard magnetic materials). That can be realized because of two different processes which occur during SPD-treatment: texturing polycrystalline materials with low-crystal symmetry during plastic deformation and induced magnetic anisotropy which cannot be explained by any of the known mechanisms and is possibly associated with occurrence of significant orbital moments on atoms near lattice defects [1].
Authors appreciate RFBR grant 12-07-00676-a for financing this work.
References
[1] S. V. Taskaev, M. D. Kuz`min, K. P. Skokov, D. Yu. Karpenkov, A. P. Pellenen, V. D. Buchelnikov and O. Gutfleisch, JMMM 331, 33 (2013).
[2] S. V. Taskaev, V. D. Buchelnikov, A. P. Pellenen, M. D. Kuz&’min, K. P. Skokov, D. Yu. Karpenkov, D. S. Bataev and O. Gutfleisch, J. Appl. Phys. 113, 17A933 (2013).

Here we reported that, under a creep condition of relatively intense stress, dispersive precipitation of titanium carbide nanoparticles could be promptly brought out within the austenite grain when the iron-nickel-based superalloy was fabricated through the specific production of the ingenious alloy design, applicable electro-slag remelting and multi-temperature-step solution treatment created in this study. Analytical electron microscopy showed that both the titanium carbide and coexistent M₂₃C₆ precipitates are cubo-octahedral in shape, and remain the cube-to-cube orientation relationship and coherent {111}/{100} interphase interface with the austenite. As a result, at 650°C the dispersion-strengthened alloy 800H exhibited a time-to-rupture that is increased by 1.8 times of magnitude relative to the commercial creep-resistant product of the same grade. This improvement in creep resistance can be attributed to the mechanism that the nanometer-scale intragranular titanium carbides and the submicron intergranular M₂₃C₆ act as pinning points for individual dislocations and grain boundaries, respectively, resulting in the raised Orowan strengthening and suppressed grain boundary sliding. The present alloy material also demonstrated the superior short-time tensile properties on both sides of high-temperature strength and ductility. Since the mentioned strengthening media are thermally stable and able to emerge readily prior to the formation of gamma-prime phase at the temperature range studied, the present work suggested an improved grade of creep-resistant alloy for applications in a relatively harsh environment that the creep lifetime may be shorter than the incubation period of the common strengthening phase for superalloys.

A defect disorder model of UO₂ founded on density functional theory results of defect energetics has been extended to investigate local off-stoichiometry near UO₂ surfaces. While bulk UO₂ crystals contain defects and electronic charge carrier densities that solely depend on the oxygen partial pressure and temperature, surfaces were found to significantly modify the defect equilibrium states. Analysis of local defect densities near flat surfaces in UO₂ showed that significant defect segregation occurs and that, under fixed thermo-chemical environment, local stoichiometry can change from hyper to hypo as a function of distance from the surface. A generalization of the theory to void surfaces has led to the discovery that voids in UO₂ must contain oxygen gas. This important finding implies that, as a major component of UO₂ , oxygen may have a bigger role to play in the irradiation response of the material than previously believed. It was also found that voids are surrounded by significant defect segregation regions at void sizes in the range few tens to few hundred nanometers. This discovery also implies that the average O:U ratio of irradiated UO₂ crystals containing ensembles of voids will be different from the unirradiated material under the same thermo-chemical conditions. The discovered electrochemical aspect of voids in UO₂ gave us new insight into how to construct microstructure evolution models. A number of chemical characterization experiments are now being designed to validate this prediction. This research was supported as a part of the Energy Frontier Research Center for Materials Science of Nuclear Fuel funded by the U.S. Department of Energy, Office of Basic Energy Sciences under award number FWP 1356, through subcontract number 00122223 at Purdue University.

Nuclear power plants, spent nuclear fuel storage pools, nuclear fuel processing plants, nuclear powered ships and terrorist threats all provide strong motivation for developing inexpensive technology to detect and monitor the spread of radioactive contamination over very large areas. Such technology would have been extremely beneficial during the recent Fukushima nuclear disaster in Japan, as it would have been during the infamous Chernobyl and Three Mile Island events. Similarly, the ability to monitor the widespread distribution of radioactive contamination during a possible terrorist attack with a dirty bomb, like the hypothetical events publicized following the 911 attack on the Twin Towers and Pentagon, all provide motivation for the development of such technology.
This motivation has led to the development of Lawrence Livermore National Laboratory&’s (LLNL&’s) patented paints and coatings that are capable of detecting widespread radiological agents in the environment (landscape, road signs, buildings, rooms, processing equipment, piping and pipelines, military vehicles, transportation containers, trucks, small unmanned aerial vehicles, balloons, and other relevant applications). This novel technology incorporates special radiation-sensitive pigments into an organic binder that can be applied as a paint or coating. These paints and coatings enable the detection of such radioactive sources and contaminants through scintillation of the inorganic or organic pigment, which is selected based upon the particle being detected (alpha, beta or gamma). Multifunctional paints can be formulated that enable the simultaneous detection of alpha, beta and gamma rays, with energy discrimination. Recent modifications of the formulation also enable neutron detection.

We present recent results on the effect of radiation on the structure and microstructure of nanocrystalline yttria stabilized zirconia (10SYZ). Dense samples of with grains at the nanoscale (25, 40, 100 nm) were prepared by spark plasma sintering and subjected to two studies: direct grain boundary energy measurement using differential scanning calorimetry, and ion irradiation to test overall structural radiation effects. After irradiation, evidences suggest the existence of an optimum gran size towards which all samples evolve to. The samples with small grain sizes (25 and 40 nm) showed growth while the larger (100 nm) showed a decrease in grain size as characterized by XRD and TEM. From the calorimetric studies we determined the total grain boundary energy of each sample and used the data to quantitatively hypothesize that the contribution from the point defect generation competes with the grain boundary energy, creating an energetic valley corresponding to the optimal grain size. The work suggested that doping the grain boundary to lower its energy can enable a control of the optimal size. We present preliminary data on lanthanum doped YSZ, showing a remarkable control of grain growth linked to the lowering of the grain boundary energy measured by calorimetry, suggesting La is a strong candidate for radiation studies.

We have used molecular dynamics simulations to study phase transformations induced by swift heavy ion irradiation in complex ceramics, such as BaTiO₃ and Gd₂Ti_xZr_2-xO₇ pyrochlore for x values from 0 to 2. We varied the energy deposited per unit length of the track from 5 keV/nm to 20 keV/nm. We characterized features of the ion track that are difficult to characterize experimentally, such as the volume expansion of the material in the ion track and changes in the coordination number inside the track. In BaTiO₃ and Gd₂Ti₂O₇, there is considerable volume swelling and amorphization in the track, while Gd₂Zr₂O₇ resists swelling and amorphization. The microstructural evolution in complex ceramics is controlled by efficient annihilation of cation and anion Frenkel pairs, and the energy cost of residual defects. These simulation results will be discussed in light of experimental characterization at comparable scales.

Swift heavy ion (GeV) irradiation-induced amorphization and temperature-induced recrystallization of isometric pyrochlore, Gd2Ti2O7 (Fd3m) are compared with the response of a monoclinic-layered perovskite-type structure, La2Ti2O7 (P21). Irradiation experiments were performed at the GSI Helmholtz Center with 181Ta ions of 2GeV kinetic energy, which is enough to create extreme environments within tracks by electronic excitation. Angle-dispersive synchrotron powder XRD measurements were completed at ambient conditions and after annealing to temperatures up to 850°C at the Cornell High Energy Synchrotron Source. Raman spectroscopy and transmission electron microscopy (TEM) were completed for analysis of ion-induced structural modifications as a function of fluence. Small angle X-ray scattering (SAXS) measurements were performed at the Australian Synchrotron in transmission mode on samples irradiated to a fluence of 5×1011 ions/cm2.
The amorphous fraction increased with increasing ion fluence as evidenced by the XRD patterns of both compositions, which showed a reduction in the intensity of the crystalline diffraction maxima concurrent with the growth in intensity of a diffuse scattering from amorphous domains. The cross-sections for the radiation-induced crystalline-to-amorphous transformation, as determined from the evolution of the integrated peak intensities as a function of fluence, reveal that La2Ti2O7 is more susceptible to amorphization. An extreme environment created within the tracks by heavy ions were evidenced by TEM, which confirmed complete amorphization within ion tracks (d ~10.6nm) for the perovskite-type material was evident; whereas, a concentric, core-shell morphology formed in the ion tracks of pyrochlore, with an outer shell of disordered crystalline pyrochlore (d ~9.1nm). The diameters of ion tracks from TEM were in good agreement with those from SAXS analysis. The radiation response of both titanate oxides can be understood in terms of differences in their structures and A-site cations. While the radiation resistance of pyrochlore A2B2O7 materials decreases as function of the cation radius ratio rA/rB, this study correlates this behavior with the stability field of monoclinic structures, where rLa/rTi > rGd/rTi.
Isochronal annealing resulted in complete recrystallization of La2Ti2O7 at 775°C and of Gd2Ti2O7 at 850°C after they had been fully amorphized. The annealing behavior agrees with the enhanced-kinetic damage recovery in La2Ti2O7 during irradiation with low-energy ions at elevated temperatures. The difference in the recrystallization behavior may be related to structural constraints, i.e., the recovery of low symmetry vs. a high symmetry structures. These experimental results suggest that the activation energies of thermal annealing from the ion damage are related to those of damage production; thus, there is a relation between the kinetics of radiation-induced defect accumulation and annealing.

A chemical decomposition and related phase transformation are observed in 2.2 GeV 197Au irradiated SnO2 nanopowder. X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the transformation from the tetragonal SnO2 structure (P42/mnm) to the tetragonal SnO structure (P4/nmm). Rietveld refinement of the XRD data determined the structure and proportion of these phases up to a fluence of 2.4x1013 ions/cm2. The initially intense diffraction maxima corresponding to SnO2 gradually decrease in intensity with an increase in fluence. At a fluence of 2x1012 ions/cm2, diffraction maxima corresponding to SnO become evident and increase in intensity as fluence increases. Both Raman and TEM analysis confirm the transformation to tetragonal SnO. These data are consistent with a multi-impact model, confirmed by TEM as no single tracks were observed. Previous swift heavy ion irradiations of SnO2 have led only to grain size effects, changes in crystallinity, and “hole” formation, making the current results important for determining the parameters that control the radiation response in SnO2 as well as determining the underlying mechanisms of swift heavy ion-induced phase transformations. The formation of SnO, apparent O loss from the transformation regions, and associated Sn reduction are discussed in terms of thermodynamic, kinetic, and thermal-spike model considerations.

Despite much research, the detailed atomistic structure and evolution of the defects produced under irradiation remains unknown in many cases. We discuss the structure of two systems which reveal interesting aspects of irradiation behavior. We first analyze the structure of dislocations and dislocation loops in UO₂, with particular focus on the segregation of the impurities to the dislocations core. It is found, that the driving force behind segregation is the dislocations strain field; thus a simple model predicting behavior of impurities is constructed. Second, we examine simulations of the formation of voids in UO₂ and their interactions with grain boundaries. Because voids affect the thermal conductivity of the fuel, we also perform the thermal transport simulations of the system with the voids and compare its results with available analytical models. It is found that small voids affect thermal transport stronger than predicted by the simple models. This work was supported by the DOE-NE Nuclear Energy University Program 10-2258 and by the DOE-NE Advanced Modeling and Simulation (NEAMS) Program, and FUELS: Integrated Performance and Safety Code (IPSC) Project.

The extraordinary nano-structure of metallic particles in light water reactor fuels points to possible high reactivity through increased surface area and a high concentration of high energy defect sites. We have analyzed the metallic epsilon particles from a high burn-up fuel from a boiling water reactor using transmission electron microscopy and have observed a much finer nanostructure in these particles than has been reported previously. The individual round particles that vary in size between ~20 and ~50 nm appear to consist of individual crystallites on the order of 2-3 nm in diameter. It is thought that in-reactor irradiation induced the formation of the nano-structure. The composition of these metallic phases is variable yet the structure of the material is consistent with the hexagonal close packed structure of epsilon-ruthenium. These findings suggest that unusual catalytic behavior of these materials might be expected.

Fusion energy has the potential to provide a sustainable, environmentally friendly and almost inexhaustible long-term energy source. One of the main obstacles on the path to commercial magnetic confinement fusion reactors is the availability of sufficiently resilient materials. Plasma facing components will be exposed to high operating temperatures (>1300 K), bombardment with energetic particles (up to 20 MWm-2) and intense irradiation with 14.1 MeV fusion neutrons up to total doses of tens of displacements per atom (dpa). Some of the harshest conditions will be experiences by the divertor, which functions as the exhaust for helium produced in the fusion reaction. Currently the main candidate materials for the divertor plates in ITER are tungsten and its alloys due to their high melting point, good resistance to sputtering, high thermal conductivity, low tritium retention rate, and stability under irradiation. During operation of the divertor, large amounts of helium are deposited near the material surface, whilst in the bulk helium is generated by transmutation. As such it is vital to understand the modifications that occur in tungsten due to the presence of helium.
We have used synchrotron X-ray micro-beam Laue diffraction to quantify the lattice swelling associated with helium-implantation in a tungsten, 1% rhenium alloy. Using density functional theory (DFT) calculations we estimate the volume of formation of different helium-associated defects in tungsten. By comparing simulations and experimental measurements we estimate that approximately one third of the implanted helium is located at interstitial sites, whilst the remainder is confined in substitutional helium defects. Based on this we predict the anticipated changes in elastic modulus associated with the retained helium.

The production and aggregation of radiation-induced point defects in uranium dioxide (UO2) is a critical step in microstructural evolution, and consequently can have significant effects on material properties such as thermal and mass transport and mechanical properties. In this work we started with molecular dynamics (MD) simulations to investigate the cascade-induced defect production. The defect cluster population, lattice expansion, and the degradation of thermal conductivity were studied. The lattice expansion agrees well with our proton-irradiation experiments and the degradation of thermal conductivity is consistent with the classical theoretical model. Next the kinetic properties of twelve types of defect clusters of high formation frequencies in cascades were investigated with temperature accelerated dynamics (TAD) simulations. We found that the migration mechanisms of these clusters are very complex and non-intuitive. Finally these defect cluster reaction and migration mechanisms were used as inputs for kinetic Monte Carlo (KMC) modeling to study the defect clustering effects on the oxygen diffusivity in UO2+x. The results show that defect clustering effects are significant even at small x. This work is supported by the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number FWP 1356.

Heavy ions travelling at relativistic velocities can create cylindrical damaged zones in the solids, which provides unique opportunity to explore nonequilibrium phase transition pathway under the extreme conditions within the ion tracks. Nanoscaled phase transition within the ion tracks in pyrochlore Gd₂Ti_xZr_2-xO₇ (x = 0 to 2) created by extremely high ionizing energy density of 30 MeV C₆₀ cluster and 1.5 GeV ²³⁸U ion have been investigated in the present study. Atomic-scaled transmission electron microscopy reveals a complicated ion track structure in Gd₂Ti₂O₇ and Gd₂TiZrO₇ with combination of amorphous core and shell of disordered, defect fluorite structure in the C60 tracks, consistent with that created by monoatomic swift heavy ions. However, due to the different velocity of projectiles, C₆₀ cluster created larger ion tracks in diameter since it deposited stronger energy density compared to that of U ions. Disordered fluorite structure with no amorphous phase is observed in the ion track of Gd₂Zr₂O₇ by C₆₀ cluster, highlighting the exceptional irradiation resistance of this Zr-end member pyrochlore. Inelastic thermal-spike calculations are employed to simulate the size of ion tracks and interpret the complex core-shell morphologies from combined effects of deposited energy density and different energy loss dE/dx.

Cracking during nanoindentation of brittle solids with sharp pyramidal indenters like the Vickers and Berkovich has been modeled and systematically examined using cohesive zone finite element simulations. Although the initial stages of crack initiation cannot be adequately captured by these techniques, the cracks that form eventually evolve into long-crack geometries that behave much like those observed in experiment, provided the crack length is approximately ten times greater than the cohesive zone size. Once this is achieved, the simulations accurately describe how cracks grow and change during loading and unloading and how the sequence of cracking events depends on material parameters like the elastic modulus to hardness ratio and the fracture toughness. The modeling provides important new insights into indentation cracking such as the realm of material behavior over which the classic Lawn, Evans, and Marshall method for measuring fracture toughness from indentation crack lengths can be expected to work as well as a predicted changeover in cracking geometry from radial-median dominated to Palmqvist surface cracking as the modulus-to-hardness ratio increases. Salient results are presented and compared to experimental behavior.
* Research sponsored by the National Science Foundation under contract CMMI-0926798.

The plasma-facing components of future fusion power plants must be designed to accommodate property changes from radiation damage and helium implantation. An understanding of these property changes is therefore extremely important. Ion irradiation and nanoindentation are common techniques to study these irradation effects, however until now mechanical testing has been limited to room temperature. This is a far cry from the temperatures at which these materials will be in service.
A MicroMaterials Nanotest has been fitted with a vacuum system (<10^-5 mbar) and independent tip and sample heating to 750°C. This allows the testing of oxidising materials, such as tungsten - the key plasma facing material - that cannot otherwise be tested at temperature. Typical thermal drift rates of <0.10 nm/s are achieved for >90% of the indents performed up to 450°C and rates of <0.15 nm/s are achieved for >90% of the indents performed up to 750°C. These are the lowest drift rates obtained from any high-temperature nanoindenter system.
Using this system, the mechanical properties of pure tungsten and tungsten implanted with helium ions have been explored over a range of temperatures and strain rates. This gives a quantitative assessment of the mechanical properties of the radiation-damaged material, and the effect of the presence helium. Testing at different strain rates gives an understanding of the damage structures present causing the change in mechanical properties.
The hardness of pure tungsten decreases strongly with increasing temperature, from ~6 GPa at 50°C to ~3 GPa at 250°C, after which it remains constant. Helium implantation to levels of 600 appm produces an increase in hardness of ~4 GPa at 50°C that decreases to ~2 GPa by 750°C. This decrease in hardening with temperature is thought to be due to the increased mobility of dislocations to bypass helium-vacancy clusters that cause the hardening effect. These results are important for the design of plasma-facing components for any future nuclear fusion device.

A variety of samples of ferritic and oxide dispersion strengthened (ODS or nanoferritic) steels were placed the ATR reactor as part of the ATR National Scientific User Facility -UC Santa Barbara irradiation. Samples were in reactor for two years achieving doses of 4- 6 dpa at temperatures of ~290°C. Samples were shipped to the hot cells in Wing 9 in the CMR facility at Los Alamos National Laboratory and then tested in tension. This talk will present results from room temperature and 300°C tension tests from this irradiation. The results will be compared to control materials and similar materials from previous irradiations.

Computational materials science has provided great insight into the response of materials under extreme conditions that are difficult to probe experimentally. For example, shock-induced plasticity and phase transformation processes in single-crystal and nanocrystalline metals have been widely studied via large-scale molecular dynamics simulations, and many of these predictions are beginning to be tested at advanced 4th generation light sources such as Argonne's Advanced Photon Source (APS) and SLAC's Linac Coherent Light Source (LCLS). I will give two examples from our work on the mechanical response of metals to shock loading: (i) copper and iron single crystals, probed via ultrafast in situ X-ray diffraction; and (ii) grain boundaries in copper, and deformation processes probed at an atomistic scale with post situ high-resolution TEM. I will then discuss outstanding challenges in modeling the response of materials to extreme mechanical and radiation environments, and our efforts to tackle these as part of the multi-institutional, multi-disciplinary Exascale Co-design Center for Materials in Extreme Environments (ExMatEx). As we look ahead from the current petascale (10¹⁵ operations per second) era towards the exascale (10¹⁸ operations per second) platforms expected to be deployed by the end of this decade, multiscale, or scale-bridging, techniques are particularly promising. ExMatEx is an effort to do this by initiating an early and extensive collaboration between computational materials scientists, computer scientists, and hardware manufacturers.

Ferritic/Martensitic steels are considered as a primary candidate of structural material of the first wall of future nuclear fusion devices. It is widely known that integrity of this material is influenced by the accumulation of He atoms. One of the worst scenarios is a dense He segregation at grain boundaries without bubble formations, where large part of grain boundary interfaces are covered by He atoms. Because He atoms weaken the atomic bond between surrounding Fe atoms, the He segregation causes grain boundary embrittlement. Although this is an extreme case, it is worthwhile to investigate how much the grain boundary strength is lost in the worst condition. In this talk, we numerically analyze the loss of grain boundary strength in α-Fe using the first principles and empirical potential calculations as a function of He concentration at various grain boundaries. We then show a common feature over different grain boundaries [1], which is a useful information for the macroscopic assesment of structural integrity of components in a fusion power plant.
Reference
[1] T. Suzudo et al., Modeling Simul. Mater. Sci. Eng. 21 (2013) in press.

Recent progress in both in situ and ex situ small-scale mechanical testing methods has greatly improved our understanding of mechanical size effects in volumes from a few nanometers to a few microns. Besides the important results related to the effect of size on the strength of individual nanostructures, the ability to systematically measure the mechanical properties of small volumes through mechanical probing allows us to test samples that cannot easily be processed in bulk form, such as a ion-irradiated materials or single crystals of specific compositions. This talk will describe our recent results from in situ TEM compression and tensile testing and the incorporation of new techniques such as dislocation tomography, digital image correlation and environmental testing at cryogenic temperatures.

The Stress Corrosion Cracking (SCC) has been detected on core shrouds and primary water re-circulation piping made of low carbon stainless steels in boiling water reactors (BWRs). The material hardening strongly affects SCC propagation behavior, and SCC growth rate increases with increasing hardness of austenitic stainless steels by cold work or neutron irradiation.
Research works have been proceeding with improving SCC resistance using chemical composition control of stainless steels. It was reported that high SFE materials showed better SCC resistance than the low SFE materials due to suppressing the material hardening by high SFE value. In this study, SCC growth rate tests were performed using 15% cold worked Types 316L and 310L stainless steels under simulated BWR water environment. The Type 310L stainless steel has high SFE value due to chemical composition control, and measured SCC growth rate of the Type 310L stainless steel is lower than ones of low SFE stainless steels.
On the other hand, the oxidation behavior is one of the most important factors for SCC on austenitic stainless steels as well as the material hardening behavior, and the influence of the chemical composition control for increasing SFE value on the oxidation behavior in BWR is still unclear. In this study, therefore, immersion tests using types 316L and 310L stainless steel specimens were also conducted under simulated BWR water environment, and then the surface oxide films on the specimens were analyzed with micro-Raman spectroscopy and glow discharge optical emission spectroscopy to contribute to clarification on oxidation behavior of the austenitic stainless steels. In the results of the tests and analyses, NiFe2O4 content of outer oxide layer on high SFE stainless steels were higher than that on low SFE stainless steels, and the inner oxide film on 310L stainless steel was detected high Cr content.
From the above results, SCC resistances and oxidation behaviors of high SFE austenitic stainless steels under simulated BWR water environment will be discussed in this paper.

Ceramic-matrix composites (CMC&’s) are being developed for extreme service conditions at ultrahigh temperatures from 1200°-1600°C at loads and hostile environments well beyond the realm of current structural materials. Using various strategies and coatings with integral 3-D architectural design, woven CMCs make the development of such ultrahigh-temperature structures feasible. Fracture assessment and lifetime prediction presents a formidable challenge though as reliable mechanical data and damage characterization must be achieved in 3-D at high temperatures. Using a newly developed synchrotron x-ray micro-tomography facility to perform such characterization under tensile loading at temperatures approaching 2000°C (at spatial resolutions <1 micron), we report here the contrasting damage modes in a woven SiC-fiber/SiC-matrix composite under tensile load at 27° and 1750°C in both un-notched and notched samples. This ability to image complex 3-D materials undergoing failure at extreme temperatures opens new possibilities for evaluating ceramic textile materials in real time.

Creation of localized heating in complex solids at a specific location and with a controlled intensity is of great importance in pyrotechnics and explosives&’ handling, as well as material processing and fabrication in general. Our predictive models of such events (e.g., impact initiation of explosions), however, are poor and our control of such events on a microscale with typical energy sources (such as laser, electric current or shockwaves) are either non-selectively destructive or limited to specific material compositions. On the other hand, ultrasound is a relatively weak delocalized energy, but one that can induce dramatic thermo-mechanical effect if localized in a small region. Examples include acoustic cavitation in liquids, ultrasonic welding with the help of energy guides in solids, and clinical hyperthermia by a focused ultrasonic beam. How ultrasound can induce intense localized heating at the interior of bulk solids, however, remains barely explored. In this work, we have generated well-controlled hot spots in polymeric composites using ultrasound and study their dynamics with a thermal imaging microscope. We observe that ultrasonic energy spontaneously and exclusively concentrates on delaminating interfaces in such composites, with local heating rate up to 30,000 K/s on sub-mm scale, while the rest of the material remains cool and undisturbed. Hot spot locations can be precisely controlled by manipulations of interfacial adhesion, and hot spot intensity can be controlled by ultrasonic energy input. Insights from this work provide interesting possibilities for a new degree of control over the thermal events as well as chemical reactions inside composite solid structures.

High-entropy alloys are a new class of compositionally complex (multi-element) alloys, often designed around equiatomic compositions. A subset of these alloys can exhibit extremes of behavior including substantial solid solubilities, large elongations to fracture, high ultimate tensile stresses, and a breakdown of the usual inverse relationship between strength and ductility. Here we present recent results of mechanical tests performed on a model high-entropy alloy. Of special interest is its behavior at extremes of temperature and loading rates. Strong temperature dependence of strength was observed at cryogenic temperatures with relatively weak strain rate dependence. Very high values of energy absorption (factors of two of more higher than those of austenitic steels and Ni-free TWIP steels) were measured under impact conditions coupled with high elongations to fracture of ~45-55% over eight decades of strain rate. These impressive results are due to the alloy&’s high work hardening rate, which is roughly independent of strain rate. Wherever possible, our temperature and strain rate dependencies are put in perspective by comparing to data in the literature on compositionally simple BCC and FCC metals. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. FO also received funding from the Alexander von Humboldt Foundation through a Feodor Lynen Research Fellowship.

Tungsten-Rhenium alloys are being considered as likely candidate materials for plasma facing components in fusion reactors which experience the most extreme environmental conditions. Bulk sample alloys have been produced of Pure W & W-1%Re, by the arc melting of high purity powders and tested to analyse the effects of alloying additions and radiation damage on the mechanical properties. Ion-implantation has been used to simulate the effects of transmuted helium damage and direct implantation from fusion plasma and the associated displacement damage. Implantations performed to a depth of ~3µm with nominal helium concentrations of 300appm & 3000appm were carried out at 300°C using singly charged ions at the National Ion Beam Centre, University of Surrey. Additionally, helium implantation and simultaneous self-ion and helium implantations were carried out using 2MeV He+ and 4MeV W9+ ions at 800°C using the JANNUS irradiation platform at CEA Saclay, France.
The nature of the small scale damage profile produced necessitates the use of micromechanical testing techniques to determine the mechanical properties of the damaged material. Nanoindentation has been used to obtain indentation moduli and hardness data from the implanted samples. Micro-Cantilever tests have also been performed on various samples to obtain both elastic properties and plastic/yield data. All implanted samples showed significant increases in indentation hardness after implantation with the 3000appm implanted samples showing increases of up to 107% (6.36GPa). Surface Acoustic Wave microscopy was used to measure the elastic modulus, which was found to have decreased by ~2% in samples implanted with 3000appm helium. Interestingly AFM measurements of the nanoindents in implanted samples showed a substantial change of the pileup structure, with a smaller, more confined plastic zone. Further microscopy is being performed to both quantify the modulus values and to understand the change in fundamental deformation mechanisms after irradiation associated with the change in pile up.

A new approach is proposed and demonstrated for fabricating interpenetrating ceramic-metal composites by infiltration of ceramic foams with molten metals/alloys. Ti2AlC foams with open-pore structures were prepared and used as the infiltration preforms. Aluminum alloy 6061 (AA6061) was infiltrated into the Ti2AlC foams utilizing spark plasma sintering technique, resulting in AA6061/Ti2AlC composites. The microstructure, composition, and distribution of phases in the AA6061/Ti2AlC composites were studied using a combination of X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The composites were tested under compression from room temperature to 600 oC, a temperature near the melting point of aluminum (660 oC), to study the effect of AA6061 inclusion on the nonlinear, hysteretic, elastic behavior of Ti2AlC. The results are compared with those of the pure components of the composites, i.e., solution-heat-treated AA6061 and Ti2AlC foam.

Using Al-TiN and Cu-TiN as model systems, we have explored the unit mechanisms that
enable unprecedented mechanical properties at nanometer length scales, such as yield strengths approaching the theoretical limit for perfect crystals, work hardening rates within a factor of two of the Young&’s modulus, plastic co-deformability of the ceramic and metal nanolayers and enhanced crack growth resistance. A variety of tests such as compression, indentation fracture, notched 3-point bend and in situ indentation in TEM were used to study strength, deformability and crack growth resistance. Density functional theory, dislocation dynamics, dislocation theory and fracture mechanics were used to interpret the emergence of the macro-scale properties in terms of the unit mechanisms. The role of the nanostructuring length scales and interfaces in maximizing the strength and toughness of metal-ceramic nanolaminates will be highlighted.
This research is sponsored by DOE, Office of Science, Office of Basic Energy Sciences.

The brittleness of glass is the most limiting factor in its widespread applications. It's commonly understood that water molecules play the fundamental role in slow crack propagation in glasses, but the mechanisms at the crack tip which lead to fracture of glasses is not fully known. Futhermore, glasses are integral parts of various systems involving (external or self-generated) irradiation (satellites, glass packages of radioactive waste...) which gradually alters the atomic arrangement at the microstructure level. It's crucial to understand how these microstructure modifications at the atomic and/or meso scale impact the failure properties observed at the macro-scale. Our study is focused on the impact of irradiation on mechanical properties of simplified borosilicate glasses. To simulate α- and β- decays specimens are irradiated by electrons and light/heavy ions (He2+/Au+ respectively). The subcritical stress corrosion fracture properties of glasses are studied using DCDC samples. This permits fracture growth in this regime and micro/nano hardness tests point out variations at nano/meso scale. These results will be presented and linked to structural modifications investigated by RAMAN, NMR and EPR analysis.

We present a novel continuum model of dislocation dynamics that predicts the main features of the crystal deformation at the mesoscale, including the evolution of the dislocation density distribution (dislocation pattern), slip patterns, internal stress and the stress-strain behavior under an arbitrary mechanical loading. The model is based in a set of kinetic equations of the curl type that govern the space and time evolution of the dislocation density in all slip systems. These equations are directly coupled via cross slip and short range reactions and indirectly through the long-range stress field of dislocations. The kinetic equations are coupled to crystal mechanics, stress equilibrium and deformation kinematics, through a staggered finite element scheme customized to capture the crystallographic nature of slip. We present results for the evolution of dislocation density and dislocation patterns, stress-strain behavior and hardening, and explain the role of cross slip and short range reactions in both pattering and hardening. We also present a probabilistic description of the internal elastic lattice rotation, stress and elastic strain fields and compare the statistics of the rotation analysis with discrete dislocation dynamics predictions and X-ray measurements. This work was supported by the U.S. DOE Office of Basic Energy Sciences, Division of Materials Science & Engineering via contract # DE-FG02-08ER46494 at Florida State University and by funding from the School of Nuclear Engineering at Purdue University. The work is performed in collaboration with Shengxu Xia (Purdue) and B.C. Larson (ORNL).

Light ion irradiation produces vacancies and interstitials that may migrate to grain boundaries (GBs), generally in unequal amounts. We use atomistic modeling to study the response of mixed tilt/twist Σ9 and Σ11 GBs and an asymmetric Σ3 GB in copper (Cu) to continuous vacancy fluxes. We find that GB ground states cannot be reached through annealing alone, but require removal of excess atoms from the GBs. GB energies vary periodically as function of number of vacancies created. Although GB ground states recur, they may exhibit distinct GB structures, some of which are faceted. We describe how GB properties, such as vacancy formation energies and volumes, depend on the number of vacancies created in the GB. Furthermore, we find that continuous vacancy fluxes to GBs drive GB migration and shearing.
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.

The formation of cleared channels in deformed irradiated materials has important consequences for determining the macroscale mechanical properties as well as the susceptibility to stress corrosion cracking. To understand the source of dislocations that generate these channels, how the dislocations interact with and annihilate irradiation produced defects, and how these dislocations interact with grain boundaries straining experiments have been performed in-situ in the transmission electron microscope as a function of temperature. This talk will show for stainless steels that the loop population depends on irradiation temperature, dislocations generated from grain boundaries create the channels, the progress of these dislocations through the defect field is erratic and intermittent, the channel width reflects the width of source and complex cross-slip reactions, dislocation pileups form in the grain interior due to the strength of the defect field, grain boundary pile-ups are difficult to form initially and the conditions for predicting slip transmission through a grain boundary will be introduced. In addition, the use of electron tomography to recover the three-dimensional nature of the damage will be demonstrated.

When a metal is subjected to a ‘weak&’ shock a longitudinal elastic wave propagates into the metal activating mechanisms of plastic relaxation such as dislocation motion and twinning in its wake. Traditional methods of dislocation dynamics treat the elastic fields of dislocations quasi-statically. That is to say the explicit time dependence of the elastic fields is ignored; the force acting on a dislocation segment is calculated by evaluating the static stress fields of other dislocation segments in positions they occupy at a given instant in time. We have shown that when this approach is applied to shock loading it violates causality: dislocations are activated ahead of the shock front. To avoid this nonsense it is essential to treat explicitly the time dependence of the elastic fields associated with the creation and movement of dislocations. Building on the pioneering work of Markenscoff and Clifton [1] we showed in [2] how this can be done in 2D-simulations of the plastic relaxation of elastic shocks by the creation and movement of edge dislocations: Dynamic Discrete Dislocation Plasticity (D3P).
In this talk I will outline what is involved in the fully time-dependent approach of D3P. I will also present recent unpublished results on the application of D3P to the decay of the elastic precursor in aluminum.
This research was carried out in the Centre for Doctoral Training in Theory and Simulation of Materials at Imperial College London, which is funded by the EPSRC under grant no. EP/G036888/1. B.G.-L. acknowledges the support of the Department of Universities and Education of the Basque Government.
1. “The nonuniformly moving edge dislocation”, X Markenscoff and R J Clifton, J. Mech. Phys.
Solids vol 29, 253-262 (1981).
2. “A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading”, B Gurrutxaga-Lerma, D S Balint, D Dini, D E Eakins, A P Sutton, Proc. R. Soc. A vol 469, 20130141 (2013).

Using a combination of scanning probe microscopy (SPM) and field ion microscopy (FIM) in ultra-high vacuum (UHV), indentation can be carried out with atomically-defined indenters at a length scale which is directly comparable with atomistic simulations. Starting with a spherical tungsten indenter of radius ~3-12 nm, we have investigated indentation response on Au(111) at the atomic scale. Scanning tunneling microscopy (STM), using the same tip, provides high resolution imaging of defect structures on the surface after indentation.
In this talk, we will discuss results of recent indentation experiments on Au(111) single crystals with an emphasis on phenomena which are unique to the atomic length scale (tips of radius < 10 nm and penetration depths < 2 nm). This length scale is intrinsically interesting because it is at these low penetration depths that plastic deformation is initiated in the substrate [1]. We find evidence for heterogeneous dislocation nucleation in these experiments, a feature which is suggested by molecular dynamics simulations with atomically rough W(111) tips [2].
Molecular dynamics simulations support numerous experimental observations including adhesion and tip wetting, material pileup, and indent morphology. Some behaviors are not well reproduced, however: Notably, the maximum adhesive pull-off force is significantly higher in simulation than in experiment, and varies in experiments as the tungsten tip wets with gold from the substrate [3]. We believe that the disparity in adhesive forces may point to the need to close not only the length scale, but also the time scale gap.
[1] W. Paul, D. Oliver, Y. Miyahara and P. H. Grütter, Phys. Rev. Lett. 110, 135506 (2013)
[2] D. Oliver, W. Paul, M. El Ouali, T. Hagedorn, Y. Miyahara, Y. Qi, and P. Grütter, Nanotechnology (in press for Nov 2013)
[3] W. Paul, D. Oliver, Y. Miyahara, and P. Grütter, Nanotechnology (in press for Oct 2013)

Tribology as the science of the interaction of surfaces in relative motion has many important technological applications. In typical tribological conditions, the actual loading conditions on the materials are often very severe in terms of stresses, strains and temperatures. Even the strongest materials can be heavily deformed or intermixed, phase transitions and amorphisation can occur and wear debris can be produced. Unfortunately, mechanistic details almost always depend on a multitude of parameters, such as material combinations, atmospheric conditions, or loading rates. This makes it hard to develop general mechanistic models. Nevertheless is it possible to identify frequent characteristics of tribological contacts, like intermixing or phase transformation in the contact area and the generation of lamellar folding processes producing debris reminiscent of flaky pastry. The elucidation of the mechanism underlying these patterns by atomistic simulations is the main goal of this talk. We demonstrate the emerging patterns during the shear deformation and surface machining process of nanocrystalline materials by molecular dynamics simulations. This involves intermixing processes, the interplay between dynamic recrystallization and grain growth, and material deformation phenomena processes which induce the disruption of the laminar surface deformation by fold formation. For the latter, we will show that the polycrystalline bulk structure induces anisotropic material flow behavior, depending on the crystallographic orientations of the grains. We demonstrate that folding can lead to layered structures that act as precursors to lamellar wear debris.

We report here on the creation of a multi-center semi-empirical local basis set approach (MSEL) for long time scale (e.g., > 1 ns) simulation of experiments under extreme thermodynamic conditions. Density Functional Theory (DFT) has been shown to accurately reproduce the high pressure-temperature chemistry, phase boundaries, and EOS of many materials. DFT simulations, though, scale poorly with computational effort and thus are generally limited to nanometer system sizes and picosecond time-scales. In contrast, chemical kinetic effects and phase changes observed in experiments can span up to nanosecond timescales and significantly longer length scales. MSEL holds promise as a high throughput simulation capability by providing orders of magnitude increase in computational efficiency while retaining most of the accuracy of Kohn-Sham DFT. Our approach is based on density functional tight binding with self-consistent charges, with improved determination of both quantum mechanical electronic states and empirical functions. We show that MSEL interaction potentials can be created by expanding on density functional tight binding in two different ways: (1) an extended atomic basis set is used in order to maintain transferability between different material phases and conditions, (e.g., up to d-orbital interactions for carbon); (2) the many-site DFT Hamiltonian is approximated as a sum of two and three-center terms, and is mapped onto a combination of radial grids and empirical functions for efficient calculation. Here, we develop MSEL models for carbon at pressures up to 2000 GPa and temperatures up to 20,000 K. Our new interaction potential for carbon yields accurate material properties for diamond, graphite, the BC8 phase, and simple cubic carbon, as well as for the shock Hugoniot of diamond compressed up to the conducting liquid. Our results provide a straightforward method by which the MSEL approach can be made to provide equation of state and long-time scale chemical kinetic data at a similar accuracy to standard quantum codes. Our methods could be extended to any number of materials related to geology and materials science, including iron oxides, SiO₂, and hydrocarbon systems.

The development of metallic alloys is arguably one of the oldest of sciences, dating back at least 3,000 years. It is therefore very surprising when a new class of metallic alloys is discovered. High Entropy Alloys (HEA) represent such a class, a class that is receiving a great deal of attention due to their unusual combinations of strength, ductility, thermal stability, corrosion and wear resistance that make them candidates for technological applications. The term “high entropy alloys” typically refers to alloys that are comprised of 5, 6, 7 or even more elements, at or near equi-atomic composition, that form simple solid solution alloys on simple underlying lattices such as FCC and BCC. The appellation “High Entropy Alloys” refers to an early conjecture that these unusual systems were stabilized as solid solutions by the high entropy of mixing associated with the large number of components. In this presentation, I will present a simple model that provides answers to a number of fundamental questions posed by the very existence of these alloys, not the least of which are: what are the driving mechanisms for their unexpected stability, which combinations of elements can give rise to HEAs, and how does the number of possible alloys depend on the number of constituents species? The approach is based on ab initio “high through-put” calculations of the enthalpy of formation of simple binary phases. In particular we have developed a 31-element “enthalpy matrix” that allows one to discriminate between combinations of elements that form single-phase solid solution HEAs and similar combinations that do not. Despite the increasing entropy, our model predicts that the number of potential single-phase multicomponent alloys that can form decreases with an increasing number of components. Interestingly, out of more than two million possible 7-component alloys considered, fewer than twenty single-phase alloys are likely.
This work was supported by the Materials Sciences and Engineering Division of DOE-BES and the “Center for Defect Physics of Structural Materials”, a Department of Energy, “Energy Frontier Research Center”.

When germanium (Ge) is compressed it can undergo a phase transformation at a pressure of around 10 GPa to a soft metallic (beta-Sn) phase. This transformation can be achieved both in a diamond anvil cell (DAC) and under a diamond indentation tip. According to the literature, on depressurisation there appear to be many possible phase transformation pathways with many end phases reported following complete pressure release. As a result there is considerable controversy in the literature with up to six (meta)stable end phases reported such as: the body centred cubic (bc8) phase, simple tetragonal (st12), an hexagonal diamond (hd) phase, the lowest free energy diamond cubic phase (dc), amorphous Ge (a-Ge) and even traces of the rhombohedral (r8) phase. In this study we set about resolving the transformation pathways and correlating them with the loading/unloading conditions including the role of hydrostatic and sheer stress.
The main experiments conducted were nano-indentation using both spherical and Berkovich indenters (of various sizes) under different conditions. We used a-Ge as the starting material to avoid the tendency for a crystalline starting phase (dc-Ge) to deform under indentation loading via twinning and dislocation slip. We also compared our indentation results with behaviour in a DAC under loading conditions involving both predominantly hydrostatic pressure and those where significant shear was introduced. Analysis of end phases was via Raman spectroscopy and electron microscopy and X-ray diffraction. In addition, for phase assignment of Raman peaks we have compared our experimental data with Raman frequencies of selected phases calculated from first principles density functional perturbation theory.
The indentation results show that, if the soft (beta-Sn) phase can be confined entirely under a spherical indenter (under near-hydrostatic conditions), the end phase on unloading is predominantly r8 which is highly unstable and readily transforms at room temperature to the more stable hd-Ge phase. On the other hand, if the (beta-Sn) phase forms at the edges of the indenter contact and in high shear regions, we observe a predominantly dc-Ge phase with traces of st12-Ge. Our indentation results have been compared with DAC results under hydrostatic and sheer conditions and the transformation pathways obtained are in agreement with the above behaviour. We will discuss possible models to explain such behaviour.