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Quarterly progress report for Q2 FY06 for Complex Transient Events in Materials Studied Using Ultrafast Electron Probes and Terascale Simulation (FWP SCW0289)

Description: In this quarter (Q2 FY06), the DTEM underwent a substantial reconfiguration of its laser systems. The cathode laser system was changed to provide greater numbers of electrons per pulse by lengthening the time duration of the pulse to 30 ns. The greater number of electrons per pulse has allowed us to acquire high quality pulsed images and diffraction patterns. The spatial resolution in the single pulsed image has been measured at better than 20 nm. The diffraction patterns are now more comparable to conventional electron microscope operation. Examples are found in the body of the report. We summarize important achievements in the following list: (1) Instrument performance and design improvements--(A) The laser system was changed for the cathode photoemission system (75 ns at 1053 nm wavelength converted to 30ns at 211 nm wavelength) to give longer electron pulses at the same current to yield more electrons per pulse. (B) New specimen drive laser constructed. (C) New computer monitored and controlled alignment systems installed for both laser systems to facilitate laser alignment through a user friendly computer interface. (2) Experimental Progress--(A) The spatial resolution of pulsed images was tested by imaging a cross-section of multilayer thin foils with 30 nm and 20 nm periods. Single pulse images were observed to have spatial resolution better than 20 nm. This combination of 20 nm spatial and 30 ns temporal resolution is thought to be highest combined spatial and temporal measurement ever made. (B) The quality of single pulse electron diffraction patterns have been improved to the point where differentiating the HCP from BCC patterns in Ti is substantially easier. The spatial coherence of the electron illumination on the specimen was improved to give much smaller diffraction spots in the pattern.
Date: March 29, 2006
Creator: Campbell, G. H.
Partner: UNT Libraries Government Documents Department

Atomic structure of the {sigma}5 (210)/[001] symmetric tilt grain boundary in yttrium aluminum garnet

Description: The {Sigma}5(210)/[100] symmetric tilt grain boundary in YAG was produced by UHV diffusion bonding precisely oriented single crystals. The boundary has been characterized by HREM along two different directions, parallel and perpendicular to the tilt axis. Models of the atomic structure of the boundary were formed following the Coincident Site Lattice scheme. The resulting models are equivalent to twins formed at the atomic scale. The high resolution images show no rigid crystal translations away from the perfect mirror reflection relation. Comparison of the simulated images using the atomic model as input with the experimental images identifies the plane of mirror symmetry. The atomic model is shown to be in good agreement with the experimental images when viewed parallel to tilt axis, but disagrees with the images perpendicular to tilt axis. Agreement between simulated and experimental images can be improved by changing the composition of the grain boundary with respect to the bulk. To reach a more certain conclusion on the structure of the grain boundary will require additional theoretical calculations.
Date: June 24, 1996
Creator: Campbell, G.H. & King, W.E.
Partner: UNT Libraries Government Documents Department

Experimental validation for atomistic simulations of the deformation of tantalum

Description: The transition metals exist in several crystal structures due to the influence of the d-bands on bonding. The central transition metals are stabilized in the body-centered-cubic (BCC) structure due to the approximately half filled d-bands. The d-bands have an inherent structure which imparts a directional dependence to the interatomic interactions [1]. These electronic effects, through their influence on the core structure of dislocations, cause the unusual mechanical properties observed in these metals [2]. The purpose of this project was the validation of the predictive abilities of the newly developed MGPT potentials in simulating the defect structures of BCC metals. The validation of these potentials will allow them to be applied with confidence in the simulation of materials behaviors under conditions that are not easily accessible to experimental confirmation.
Date: January 31, 2000
Creator: Campbell, G H
Partner: UNT Libraries Government Documents Department

Fragmentation in Biaxial Tension

Description: We have carried out an experiment that places a ductile stainless steel in a state of biaxial tension at a high rate of strain. The loading of the ductile metal spherical cap is performed by the detonation of a high explosive layer with a conforming geometry to expand the metal radially outwards. Simulations of the loading and expansion of the metal predict strain rates that compare well with experimental observations. A high percentage of the HE loaded material was recovered through a soft capture process and characterization of the recovered fragments provided high quality data, including uniform strain prior to failure and fragment size. These data were used with a modified fragmentation model to determine a fragmentation energy.
Date: June 13, 2006
Creator: Campbell, G H; Archbold, G C; Hurricane, O A & Miller, P L
Partner: UNT Libraries Government Documents Department

Transitions of Dislocation Glide to Twinning and Shear Transformation in Shock-Deformed Tantalum

Description: Recent TEM studies of deformation substructures developed in tantalum and tantalum-tungsten alloys shock-deformed at a peak pressure {approx}45 GPa have revealed the occurrence of shock-induced phase transformation [i.e., {alpha} (bcc) {yields} {omega} (hexagonal) transition] in addition to shock-induced deformation twinning. The volume fraction of twin and {omega} domains increases with increasing content of tungsten. A controversy arises since tantalum exhibits no clear equilibrium solid-state phase transformation under hydrostatic pressures up to 174 GPa. It is known that phase stability of a material system under different temperatures and pressures is determined by system free energy. That is, a structural phase that has the lowest free energy will be stable. For pressure-induced phase transformation under hydrostatic-pressure conditions, tantalum may undergo phase transition when the free energy of a competing phase {omega} becomes smaller than that of the parent phase {alpha} above a critical pressure (P{sub eq}), i.e., the equilibrium {alpha} {yields} {omega} transition occurs when the pressure increases above P{sub eq}. However, it is also known that material shocked under dynamic pressure can lead to a considerable increase in temperature, and the higher the applied pressure the higher the overheat temperature. This means a higher pressure is required to achieve an equivalent volume (or density) in dynamic-pressure conditions than in hydrostatic-pressure conditions. Accordingly, P{sub eq} for {alpha} {yields} {omega} transition is anticipated to increase under dynamic-pressure conditions as a result of the temperature effect. Although no clear equilibrium transition pressure under hydrostatic-pressure conditions is reported for tantalum, it is reasonable to assume that Peq under dynamic-pressure conditions will be considerably higher than that under hydrostatic-pressure conditions if there is a pressure-induced {alpha} {yields} {omega} transition in tantalum. The observation of {alpha} {yields} {omega} transition in shock-compressed tantalum and tantalum-tungsten alloys at {approx}45 GPa in fact reveals the occurrence of a non-equilibrium phase ...
Date: October 19, 2009
Creator: Hsiung, L L; Campbell, G H & McNaney, J M
Partner: UNT Libraries Government Documents Department

Nanosecond Time Resolved Electron Diffraction Studies of the (Alpha) to (Beta) Transition in Pure Ti Thin Films using the Dynamic Transmission Electron Microscope (DTEM)

Description: The transient events of the {alpha}-{beta} martensitic transformation in nanocrystalline Ti films were explored via single shot electron diffraction patterns with 1.5 ns temporal resolution. The diffraction patterns were acquired with a newly constructed dynamic transmission electron microscope (DTEM), which combines nanosecond pulsed laser systems and pump-probe techniques with a conventional TEM. With the DTEM, the transient events of fundamental material processes, that are far too fast to be studied by conventional bulk techniques, can be captured in the form of electron diffraction patterns or images with nanosecond temporal resolution. The transient phenomena of the martensitic transformations in nanocrystalline Ti is ideally suited for study in the DTEM, with their rapid nucleation, characteristic interface velocities {approx}1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of {approx}10{sup 10} K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long, intense electron pulse. Diffraction patterns show an almost complete transition to the {beta} phase within 500 ns. Postmortem analysis (after the sample is allowed to cool) shows a reversion to the {alpha} phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a ''massive'' short-range diffusion transformation mechanism.
Date: December 9, 2005
Creator: LaGrange, T; Campbell, G H; Colvin, J D; Reed, B & King, W E
Partner: UNT Libraries Government Documents Department

Rapid Laser Induced Crystallization of Amorphous NiTi Films Observed by Nanosecond Dynamic Transmission Electron Microscopy (DTEM)

Description: The crystallization processes of the as-deposited, amorphous NiTi thin films have been studied in detail using techniques such as differential scanning calorimetry and, in-situ TEM. The kinetic data have been analyzed in terms of Johnson-Mehl-Avrami-Kolomogrov (JMAK) semi-empirical formula. The kinetic parameters determined from this analysis have been useful in defining process control parameters for tailoring microstructural features and shape memory properties. Due to the commercial push to shrink thin film-based devices, unique processing techniques have been developed using laser-based annealing to spatially control the microstructure evolution down to sub-micron levels. Nanosecond, pulse laser annealing is particularly attractive since it limits the amount of peripheral heating and unwanted microstructural changes to underlying or surrounding material. However, crystallization under pulsed laser irradiation can differ significantly from conventional thermal annealing, e.g., slow heating in a furnace. This is especially true for amorphous NiTi materials and relevant for shape memory thin film based microelectromechanical systems (MEMS) applications. There is little to no data on the crystallization kinetics of NiTi under pulsed laser irradiation, primarily due to the high crystallization rates intrinsic to high temperature annealing and the spatial and temporal resolution limits of standard techniques. However, with the high time and spatial resolution capabilities of the dynamic transmission electron microscope (DTEM) constructed at Lawrence Livermore National Laboratory, the rapid nucleation events occurring from pulsed laser irradiation can be directly observed and nucleation rates can be quantified. This paper briefly explains the DTEM approach and how it used to investigate the pulsed laser induced crystallization processes in NiTi and to determine kinetic parameters.
Date: March 1, 2010
Creator: LaGrange, T; Campbell, G H; Browning, N D; Reed, B W & Grummon, D S
Partner: UNT Libraries Government Documents Department

A high resolution electron microscopy study of the {Sigma}11 (113)/[110] symmetric tilt grain boundary in pure Al and Al-1.5 wt% Cu

Description: Identical bicrystals of pure Al and Al-1.5 wt.% Cu were prepared by diffusion bonding in ultra-high vacuum. The boundary chosen was the {Sigma}11 (113)/[1{bar 1}0] symmetric tilt grain boundary. Characterization of the atomic structure of and segregation to the boundary was performed by high resolution transmission electron microscopy along the common [110]. The stable sites for Cu segregation were probed using electronic structure calculations. Boundary atomic structures were simulated using embedded atom potentials. The Cu was found to segregate to this boundary by occupying sites removed from the mirror plane boundary by one (113) plane. This prediction was compared to experimental micrographs through high resolution image simulation. The experimental results are consistent with the predictions of the theoretical calculations.
Date: March 1996
Creator: Campbell, G. H.; King, W. E.; Wien, W. L. & Foiles, S. M.
Partner: UNT Libraries Government Documents Department

Directly Imaging Fast Reaction Fronts

Description: Direct observation of fast intermetallic phase formation in Reactive Multilayer Foils (RMLFs) has been achieved. Snap-shots of the reaction appear to show development of mass-thickness contrast of the unmixed Al and Ni layers and an intermetallic phase. Electron imaging of these RMLF reaction fronts have never been attained in the past. The reaction front travels at {approx}10 meters per second as the nanoscale layers mix in an exothermic chain reaction, thus making traditional in situ electron microscopy {approx}10{sup 5} times too slow to produce such an image. The DTEM capability to produce several million electrons within nanoseconds for single-pulse imaging made this experiment possible. Additionally, the sample drive laser ensures reliable experiment initiation and repeatability. In no other way could such a high velocity event be captured at this magnification. RMLF reaction fronts continue to be analyzed via diffraction for complete phase evolution with respect to time. High quality diffraction patterns enable quantitative phase information to be obtained for future comparison to simulation.
Date: February 21, 2007
Creator: Kim, J S; LaGrange, T B; Reed, B W; Campbell, G H & Browning, N D
Partner: UNT Libraries Government Documents Department

Solving the Accelerator-Condenser Coupling Problem in a Nanosecond Dynamic Transmission Electron Microscope

Description: We describe a modification to a transmission electron microscope (TEM) that allows it to briefly (using a pulsed-laser-driven photocathode) operate at currents in excess of 10 mA while keeping the effects of condenser lens aberrations to a minimum. This modification allows real-space imaging of material microstructure with a resolution of order 10 nm over regions several {micro}m across with an exposure time of 15 ns. This is more than 6 orders of magnitude faster than typical video-rate TEM imaging. The key is the addition of a weak magnetic lens to couple the large-diameter high-current beam exiting the accelerator into the acceptance aperture of a conventional TEM condenser lens system. We show that the performance of the system is essentially consistent with models derived from ray tracing and finite element simulations. The instrument can also be operated as a conventional TEM by using the electron gun in a thermionic mode. The modification enables very high electron current densities in {micro}m-sized areas and could also be used in a non-pulsed system for high-throughput imaging and analytical TEM.
Date: December 29, 2009
Creator: Reed, B W; LaGrange, T; Shuttlesworth, R M; Gibson, D J; Campbell, G H & Browning, N D
Partner: UNT Libraries Government Documents Department

Time Resolved Annular Dark Field Imaging of Catalyst Nanoparticles

Description: Dynamic transmission electron microscopy (DTEM) has the potential to provide insight into nanoparticle catalyst dynamics by obtaining direct images with high spatial and temporal resolution. To date, the limited signal to noise ratios attainable for dispersed nanoparticle samples have made such studies difficult to perform at the highest resolution. These limitations have been overcome by the fabrication of an annular objective lens aperture that permits images to be obtained with a 5 fold increase in the signal to background ratio. This annular dark field imaging mode is shown here to vastly improve the contrast attainable in 15ns pulsed electron images and allows particles as small as 30nm in diameter to be observed.
Date: August 4, 2009
Creator: Masiel, D J; Reed, B W; LaGrange, T B; Campbell, G H; Guo, T & Browning, N D
Partner: UNT Libraries Government Documents Department

Quantitative high resolution electron microscopy of grain boundaries

Description: The {Sigma}11 (113)/[1{bar 1}0] symmetric tilt grain boundary has been characterized by high resolution transmission electron microscopy. The method by which the images are prepared for analysis is described. The statistics of the image data have been found to follow a normal distribution. The electron-optical imaging parameters used to acquire the image have been determined by nonlinear least-square image simulation optimization within the perfect crystal region of the micrograph. A similar image simulation optimization procedure is used to determine the atom positions which provide the best match between the experimental image and the image simulation.
Date: December 12, 1996
Creator: Campbell, G.H., King, W.E., Cohen, D., Carter, C.B.
Partner: UNT Libraries Government Documents Department

Fundamental Mechanisms Driving the Amorphous to Crystalline Phase Transformation

Description: Phase transformations are ubiquitous, fundamental phenomena that lie at the heart of many structural, optical and electronic properties in condensed matter physics and materials science. Many transformations, especially those occurring under extreme conditions such as rapid changes in the thermodynamic state, are controlled by poorly understood processes involving the nucleation and quenching of metastable phases. Typically these processes occur on time and length scales invisible to most experimental techniques ({micro}s and faster, nm and smaller), so our understanding of the dynamics tends to be very limited and indirect, often relying on simulations combined with experimental study of the ''time infinity'' end state. Experimental techniques that can directly probe phase transformations on their proper time and length scales are therefore key to providing fundamental insights into the whole area of transformation physics and materials science. LLNL possesses a unique dynamic transmission electron microscope (DTEM) capable of taking images and diffraction patterns of laser-driven material processes with resolution measured in nanometers and nanoseconds. The DTEM has previously used time-resolved diffraction patterns to quantitatively study phase transformations that are orders of magnitude too fast for conventional in situ TEM. More recently the microscope has demonstrated the ability to directly image a reaction front moving at {approx}13 nm/ns and the nucleation of a new phase behind that front. Certain compound semiconductor phase change materials, such as Ge{sub 2}Sb{sub 2}Te{sub 5} (GST), Sb{sub 2}Te and GeSb, exhibit a technologically important series of transformations on scales that fall neatly into the performance specifications of the DTEM. If a small portion of such material is heated above its melting point and then rapidly cooled, it quenches into an amorphous state. Heating again with a less intense pulse leads to recrystallization into a vacancy-stabilized metastable rock salt structure. Each transformation takes {approx}10-100 ns, and the cycle can be ...
Date: January 4, 2011
Creator: Reed, B W; Browning, N D; Santala, M K; LaGrange, T; Gilmer, G H; Masiel, D J et al.
Partner: UNT Libraries Government Documents Department

Electronic Effects on Grain Boundary Structure in BCC Metals

Description: The dominant factor in determining the atomic structure of grain boundaries is the crystal structure of the material, e.g. FCC vs. BCC. However, for a given crystal structure, the structure of grain boundaries can be influenced by electronic effects, i.e. by the element comprising the crystal. Understanding and modeling the influence of electronic structure on defect structures is a key ingredient for successful atomistic simulations of materials with more complicated crystal structures than FCC. We have found that grain boundary structure is a critical test for interatomic potentials. To that end, we have fabricated the identical {Sigma}5 (3l0)/[001] symmetric tilt grain boundary in three different BCC metals (Nb, MO, and Ta) by diffusion bonding precisely oriented single crystals. The structure of these boundaries have been determined by high resolution transmission electron microscopy. The boundaries have been found to have different atomic structures. The structures of these boundaries have been modeled with atomistic simulations using interatomic potentials incorporating angularly dependent interactions, such as those developed within Model Generalized Pseudopotential Theory. The differing structures of these boundaries can be understood in terms of the strength of the angular dependence of the interatomic interaction. We report here the results for Ta.
Date: November 15, 1999
Creator: Campbell, G.H.; King, W.E.; Belak, J.A.; Moriarty, J.A. & Foiles, S.M.
Partner: UNT Libraries Government Documents Department

Fatigue crack nucleation in metallic materials

Description: The process of fatigue crack nucleation in metallic materials is reviewed placing emphasis in results derived for pure FCC metals with wavy slip behavior. The relationship between Persistent Slip Bands (PSB`s) and crack initiation will be examined for both single crystals and polycrystals, including the conditions for inter- and transgranular crack nucleation and their connection to type of loading, crystallography and slip geometry. The latter has been found to be an important parameter in the nucleation of intergranular cracks in polycrystals subjected to high strain fatigue, whereby primary slip bands with long slip lengths impinging on a grain boundary produce intergranular crack nucleation under the right conditions. Recent results related to intergranular crack nucleation in copper bicrystals and crack nucleation in Cu/Sapphire interfaces indicate that this mechanism controls crack nucleation in those simpler systems as well. Furthermore, it is found that under multiple slip conditions the crack nucleation location is controlled by the presence of local single slip conditions and long slip lengths for a particular Burgers vector that does not have to be in the primary slip system.
Date: April 1, 1999
Creator: Peralta, P.; Laird, C.; Ramamurty, U.; Suresh, S.; Campbell, G.H.; King, W.E. et al.
Partner: UNT Libraries Government Documents Department

Mechanism of ductile rupture in the Al/sapphire system elucidated using x-ray tomographic microscopy

Description: The fracture of a thin metal foil constrained between alumina or sapphire blocks has been studied by a number of investigators. The systems that have been investigated include Al, Au, Nb, and Cu. Except for Al/Al{sub 2}O{sub 3} interfaces, these systems exhibit a common fracture mechanism: pores form at the metal/ceramic interface several foil thicknesses ahead of the crack which, under increasing load, grow and link with the initial crack. This mechanism leaves metal o none side of the fracture surface and clean ceramic on the other. This has not been the observation in Al/Al{sub 2}O{sub 3} bonds where at appropriate thicknesses of Al, the fracture appears to proceed as a ductile rupture through the metal. This paper addresses the question of why the fracture of the Al/Al{sub 2}O{sub 3} system appears to be different from other systems by probing the fracture mechanism using X-ray tomographic microscopy (XTM). The authors have experimentally duplicated the simplified geometry of the micromechanics models and subjected the specimens to a well defined stress state in bending. The bend tests were interrupted and XTM was performed to reveal the mechanism of crack extension.
Date: December 18, 1995
Creator: King, W.E.; Campbell, G.H.; Haupt, D.L.; Kinney, J.H.; Riddle, R.A. & Wien, W.L.
Partner: UNT Libraries Government Documents Department

Orientation imaging microscopy investigation of the compression deformation of a [011] ta single crystal

Description: High-purity tantalum single crystal cylinders oriented with [110] parallel to the cylinder axis were deformed 10, 20, and 30 percent in compression. The samples were subsequently sectioned for characterization using Orientation Imaging Microscopy (O&I) along two orthogonal sectioning planes: one in the plane containing [001] and [110] (longitudinal) and the other in the plane containing [1{anti 1}0] and[110] (transverse). To examine local lattice rotations, the Euler angles relative to a reference angle at the section center were decomposed to their in-plane and out-of-plane components. The in-plane and out-of-plane misorientation maps for all compression tests reveal inhomogeneous deformation everywhere and particularly large lattice rotations in the comers of the longitudinal section. Of particular interest are the observed alternating orientation changes. This suggests the existence of networks of dislocations with net alternating sign that are required to accommodate the observed rotations. Rotation maps from the transverse section are distinctly different in appearance from those in the longitudinal plane. However, the rotation maps confirm that the rotations observed above were about the [1{anti 1}0] axis. Alternating orientation changes are also observed on this section. Results will be directly compared with crystal rotations predicted using finite element methods and reviewed in light of the LLNL Multiscale Materials Modeling Program.
Date: January 8, 1999
Creator: Adams, B L; Campbell, G H; King, W E; Lassila, D H; Stolken, J S; Sun, S et al.
Partner: UNT Libraries Government Documents Department