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Development of local shear bands and orientation gradients in fcc polycrystals

Description: A finite element formulation which derives constitutive response from crystal plasticity theory is used to examine localized deformation in fcc polycrystals. The polycrystals are simple, idealized arrangements of grains. Localized deformations within individual grains lead to the development of domains that are separated by boundaries of high misorientation. Shear banding is seen to occur on a microscopic scale of grain dimensions. The important consequences of these simulations are that the predicted local inhomogeneities are meeting various requirements which make them possible nucleation sites for recrystallization.
Date: May 1, 1995
Creator: Beaudoin, A. J., Jr.; Mecking, H. & Kocks, U. F.
Partner: UNT Libraries Government Documents Department

Bicrystals with strain gradient effects

Description: Boundary between two perfectly bonded single crystals plays an important role in determining the deformation of the bicrystals. This work addresses the role of the grain boundary by considering the elevated hardening of a slip system due to a slip gradient. The slip gradients are associated with geometrically necessary dislocations and their effects become pronounced when a representative length scale of the deformation field is comparable to the dominant microstructural length scale of a material. A new rate-dependent crystal plasticity theory is presented and has been implemented within the finite element method framework. A planar bicrystal under uniform in-plane loading is studied using the new crystal theory. The strain is found to be continuous but nonuniform within a boundary layer around the interface. The lattice rotation is also nonuniform within the boundary layer. The width of the layer is determined by the misorientation of the grains, the hardening of slip systems, and most importantly by the characteristic material length scales. The overall yield strength of the bicrystal is also obtained. A significant grain-size dependence of the yield strength, the Hall- Petch effect is predicted.
Date: January 9, 1997
Creator: Shu, J.Y.
Partner: UNT Libraries Government Documents Department

Relationship between nanocrystalline and amorphous microstructures by molecular dynamics simulation

Description: A recent molecular dynamics simulation method for growth of fully dense nanocrystalline materials crystallized from melt was used with the Stillinger-Weber three-body potential to synthesize nanocrystalline Si with a grain size up to 75{Angstrom}. Structures of the highly constrained grain boundaries (GBs), triple lines, and point grain junctions were found to be highly disordered and similar to the structure of amorphous Si. These and earlier results for fcc metals suggest that a nanocrystalline microstructure may be viewed as a two-phase system, namely an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular, glue-like phase. Analysis of the structures of bicrystalline GBs in the same materials reveals the presence of an amorphous intergranular equilibrium phase only in the high-energy but not the low-energy GBs, suggesting that only high-energy boundaries are present in nanocrystalline microstructures.
Date: August 1, 1996
Creator: Keblinski, P.; Phillpot, S.R.; Wolf, D. & Gleiter, H.
Partner: UNT Libraries Government Documents Department

Growth of Oriented C11(b) MoSi(2) Bicrystals Using a Modified Czochralski Technique

Description: Oriented bicrystals of pure C11<sub>b</sub> MoSi<sub>2</sub> have been grown in a tri-arc furnace using the Czochralski technique. Two single crystal seeds were used to initiate the growth. Each seed had the orientation intended for one of the grains of the bicrystals, which resulted in a 60&deg; twist boundary on the (110) plane. Seeds were attached to a water-cooled seed rod, which was pulled at 120 mm/h with the seed rod rotating at 45 rpm. The water- cooled copper hearth was counter-rotated at 160 rpm. Asymmetric growth ridges associated with each seed crystal were observed during growth and confirmed the existence of a bicrystal. It was also found that careful alignment of the seeds was needed to keep the grain boundary from growing out of the boule. The resulting boundary was characterized by imaging and crystallographic techniques in a scanning electron microscope. The boundary was found to be fairly sharp and the misorientation between the grains remained within 2&deg; from the disorientation between the seeds.
Date: June 2, 1999
Creator: Chu, F.; Garrett, J.D.; McClellan, K.J.; J.R., Michael; Mitchell, T.E. & Peralta, P.
Partner: UNT Libraries Government Documents Department

Practical superconductor development for electrical power applications - quarterly report for the period ending December 31, 1998.

Description: This is a multiyear experimental research program focused on improving relevant material properties of high-T{sub c} superconductors and on development of fabrication methods that can be transferred to industry for production of commercial conductors. The development of teaming relationships through agreements with industrial partners is a key element of this program. Recent results on Y-123 bicrystals and grain boundaries, flux dissipation, novel sheathed Bi-2223 tapes, AC losses in multifilament Bi-2223 conductors, and oxygen diffusion in Bi-2223 are discussed.
Date: January 29, 1999
Partner: UNT Libraries Government Documents Department

Xe precipitates at grain boundaries in Al under 1 MeV electron irradiation.

Description: Crystalline nanoprecipitates of Xe have been produced by ion implantation into mazed bicrystalline Al at 300 K, in which the matrix grain boundaries are mainly 90 deg tilt boundaries. Within Al grains, Xe nanocrystals are fee, isotactic with the Al and cuboctohedral in shape with {l_brace}111{r_brace} and {l_brace}100{r_brace} facets. With an off-axial imaging technique, the nanocrystals were structure imaged against a relatively featureless matrix background. In contrast to metal precipitates in Al, such as Pb, Xe precipitates straddling a matrix grain boundary are bicrystals as small as approximately 2 nm in diameter. Larger Xe precipitates tend to avoid boundaries which are inclined away from asymmetrical orientation and which thus have a significant twist component. Under the 1 MeV electron irradiation employed for HREM observation, small Xe nanocrystals near a grain boundary may migrate to the boundary and coalesce with other Xe precipitates. The structural observations are rationalized on a simple geometrical interpretation.
Date: October 23, 1998
Creator: Allen, C. W.
Partner: UNT Libraries Government Documents Department

Local and bulk melting of Cu at grain boundaries

Description: We investigate gain boundary (GB) melting using molecular dynamics simulations on face-centered-cubic Cu bicrystals with symmetric {l_angle}110{r_angle} tilt grain boundaries. Two representative types of GBs are explored: {Sigma} = 11/(113)/50.48{sup o} (low GB energy) and {Sigma} = 27/(552)/148.41{sup o} (high GB energy). The temperature and temporal evolutions of the Cu bicrystals under stepped heating are characterized in terms of order parameters and diffusion coefficients, as ell as the nucleation and growth of melt. Within the GB region, continuous local melting precedes discontinuous bulk melting, while continuous solid state disordering may precede local melting. Premelting may occur for local melting but not for bulk melting. For {Sigma} = 11/(113)/50.48{sup o}, premelting of the GB region is negligible, and local melting occurs near the thermodynamic melting temperature. The GB region as a whole is superheated by about 13% before its bulk melting. In the case of {Sigma} = 27/(552)/148.41, considerable premelting is observed for local melting, while the bulk melting occurs with negligible superheating. The exact melting behavior of a general GB depends on the GB energy, but is likely bracketed within these two cases.
Date: January 1, 2008
Creator: Luo, Shengnian; Han, Li - Bo; An, Qi; Fu, Rong - Shan & Zheng, Lianqing
Partner: UNT Libraries Government Documents Department

Hierarchical Characterization of Deformation Heterogeneities inBCC crystals

Description: Deformation behavior of body-centered cubic (BCC) metals is being investigated by white beam x-ray microdiffraction to characterize the dislocation structure that results from uniaxial compression experiments. The measurements were performed on molybdenum single crystals and a tantalum bicrystal as part of a hierarchical characterization effort. Results show heterogeneities in the deformed structure and misorientation maps consistent with results obtained from Orientation Imaging Microscopy (OIM). Additionally, the technique allows for the determination of the active glide systems as well as of the dislocation densities in function of the position in the sample.
Date: January 1, 2005
Creator: Magid, Karen R.; Lilleodden, Erica T.; Tamura, Nobumichi; Florando, Jeff; Lassila, Dave; Barabash, Rozaliya I. et al.
Partner: UNT Libraries Government Documents Department

Grain boundary transport properties in YBa{sub 2}Cu{sub 3}O{sub x} coated conductors.

Description: Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa{sub 2}Cu{sub 3}O{sub x} show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley. A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.
Date: November 2, 2000
Creator: Berghuis, P.; Miller, D. J.; Kim, D. H.; Gray, K. E.; Feenstra, R. & Christen, D. K.
Partner: UNT Libraries Government Documents Department

Size Effect in Cleavage Cracking in Polycrystalline Thin Films

Description: The reliability of polycrystalline thin films is essential to assuring safe performance of micro/nano-electromechanical systems. Usually, they are of through-thickness grain structures and are brittle at working temperatures, and therefore their fracture properties are dominated by the resistances offered by grain boundaries to cleavage cracking [1,2]. As a cleavage crack front propagates across a high-angle grain boundary, it would first penetrate across a number of break-through points [3], and the persistent grain boundary areas would then be separated through shear fracture or ligament bending [4,5]. It is, therefore, envisioned that as the film thickness is smaller than the characteristic distance between the break-through points, which is often in the range of 0.5-5 microns, the crack front transmission can be significantly confined by the film surfaces, leading to an either beneficial or detrimental size effect. That is, the fracture toughness of the polycrystalline thin film is not a material constant; rather, it highly depends on the film thickness. Since this important phenomenon has not received the necessary attention, we propose to carry out a systematic study on fracture resistances of bicrystal silicon films. The film thickness will range from 1 to 1000 microns, and the crystallographic orientations across the grain boundaries will be controlled precisely so that the size effect and the geometrical factors can be analyzed separately. The study will starts with thick films. Once the crack front transmission process is relatively well understood, it will be extended to thin films. This project will shed light on crack-boundary interactions in confining microenvironments, which has both great scientific interest and immense technological importance to the development of fine-structured devices.
Date: July 27, 2007
Creator: Qiao, Yu
Partner: UNT Libraries Government Documents Department

Direct determination of grain boundary atomic structure in SrTiO{sub 3}

Description: In the electroceramic SrTiO{sub 3} the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for tilt boundaries in SrTiO{sub 3} bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imagine to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO{sub 3}.
Date: December 1, 1994
Creator: McGibbon, M.M.; Browning, N.D.; McGibbon, A.J.; Chisholm, M.F. & Pennycook, S.J.
Partner: UNT Libraries Government Documents Department


Description: Bulk Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi2212) bicrystals containing a single high quality [001] twist grain boundary junction were prepared in order to investigate the orbital symmetry of the superconducting order parameter in highly anisotropic Bi-based high temperature superconductors. The misorientation angles of the bicrystals ranged from 0 to 180{degree}. The microstructure in the vicinity of the junction was characterized using high-resolution, nano-probe analytical microscopy. The authors found that some high angle twist junctions were able to carry a critical current density similar to their constituent single crystals. These results cannot be interpreted in terms of a pure d{sub x{sup 2}-y{sup 2}}-wave order parameter for superconducting Bi2212.
Date: July 18, 1998
Partner: UNT Libraries Government Documents Department

Multilength-scale modeling: Crystal-plasticity models in implicit finite element codes

Description: A multidisciplinary team of researchers from Chemistry & Materials Science and Mechanical Engineering have recently completed the first year of an LDRD Director`s Initiative. This initiative involves the development of physics-based multilength-scale models to optimize casting and deformation processing of polycrystalline materials. The purpose of this initiative is to develop models to simulate the material`s mechanical response on the meso length-scale. The meso length-scale refers to the realm where the material microstructure (e. g., grain size and shape, crystallite orientation, etc.) is resolved but dislocations are homogenized. The recent development of two experimental technologies has provided a unique capability to validate material models on the mesolength-scale. The first of these technologies is the ultrahigh-vacuum diffusion bonding machine. This machine has been demonstrated to build bicrystals with high-precision planar grain boundaries joining two precisely oriented crystallites. The second technology is Orientation Imaging Microscopy. This technology has been demonstrated to precisely determine the local lattice orientation in a microstructure. Hence, this provides a powerful tool to characterize local lattice rotations following specified levels of deformation. The coupling of these two technologies provides a tool for detailed study of a material`s mechanical response due to the presence of an interface or grain boundary. 6 refs., 6 figs.
Date: March 1, 1996
Creator: Sam, D.D. & King, W.E.
Partner: UNT Libraries Government Documents Department

Angular dependence of the Josephson critical current across c-axis twist junctions of layered superconductors.

Description: The authors studied theoretically the c-axis Josephson critical current for bicrystals of high temperature superconductors twisted an angle {phi}{sub 0} about the c-axis with respect to each other. They used the effective Lawrence-Doniach models appropriate for the cases of pure s-wave or d{sub x{sup 2}-y{sup 2}}-wave order parameters, and of a dominant d{sub x{sup 2}-y{sup 2}}-wave order parameter combined with a subdominant one of either s-wave or d{sub xy}-wave form, as a function of the temperature and {phi}{sub 0}. The results demonstrate that this new phase-sensitive experiment can serve as a very useful test of order parameter symmetry. In particular, the recent zero-field experiments in the vicinity of T{sub c} by li et al. on c-axis twist junctions of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} are very difficult to explain with a dominant d{sub x{sup 2}-y{sup 2}}-wave order parameter.
Date: July 31, 1998
Creator: Klemm, R. A.
Partner: UNT Libraries Government Documents Department

The structure and faceting behavior of tilt grain boundaries in aluminum

Description: This work describes a number of experimental observations on the structure and behavior of {Sigma}99 and other near-90{degree} <110> tilt boundaries in bicrystals of aluminum. The continuous bicrystal structure employed in these studies is based on the symmetry properties inherent in heteroepitaxial growth. A thin film grown in this geometry consists of intertwined grains surrounding each other but with only two grain orientations.
Date: November 1, 1990
Creator: Dahmen, U.
Partner: UNT Libraries Government Documents Department

X-Ray Microdiffraction Characterization of DeformationHeterogeneities in BCC Crystals

Description: The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (mu XRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot ''streaking'', which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that mu XRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures.
Date: November 22, 2004
Creator: Magid, K.R.; Lilleodden, E.T.; Tamura, N.; Florando, J.N.; Lassila, D.H.; LeBlanc, M.M. et al.
Partner: UNT Libraries Government Documents Department

LLE review, Volume 77. Quarterly report, October--December 1998

Description: This volume of the LLE Review, covering the period October--December 1998, includes two articles addressing issues applicable to direct-drive ICF on the National Ignition Facility (NIF): laser-plasma interactions and laser-irradiation uniformity. Additional highlights of the research presented in this issue are: (1) P.B. Radha and S. Skupsky present a novel charged-particle diagnostic that performs simultaneous {rho}R measurements of the fuel, shell, and ablator regions of a compressed ICF target, consisting of an inner DT fuel region, a plastic (CH) shell, and an ablator (CD), by measuring the knock-on deuteron spectrum. (2) F. Dahmani, S. Burns, J. Lambropoulos, S. Papernov, and A. Schmid report results from stress-inhibited laser-driven crack propagation and stress-delayed damage-initiation experiments in fused silica at 351 nm. Research is underway presently to determine the ramifications of these findings for large-aperture systems, such as OMEGA. (3) V. Goncharov presents an analytic theory of the ablative Richtmyer-Meshkov instability, which shows that the main stabilizing mechanism of the ablation-front perturbations is the dynamic overpressure of the blowoff plasma with respect to the target material. The perturbation evolution during the shock transit time is studied to determine the initial conditions for the Rayleigh-Taylor phase of the instability and to analyze the level of laser imprint on ICF direct-drive targets. (4) J.M. Larkin, W.R. Donaldson, T.H. Foster, and R.S. Knox examine the triplet state of rose bengal, a dye used in photodynamic therapy, that is produced by 1,064-nm excitation of T{sub 1}. (5) R. Adam, M. Currie, R. Sobolewski, O. Harnack, and M. Darula report measurements of the picosecond photoresponse of a current-biased YBCO microbridge coupled to a bicrystal YBCO Josephson junction.
Date: December 31, 1998
Creator: Regan, S.P.
Partner: UNT Libraries Government Documents Department

Boundary Migration in Rutile

Description: TiO{sub 2} is a vital material in several technologies including, photocatalysis, gas sensing, biomaterials and optical coatings. Among the several crystal structures of this oxide, rutile has the highest density and microhardness, the highest index of refraction and the highest temperature stability. The processing of dense polycrystalline materials often includes the addition of a liquid-forming phase at higher temperatures. This technique is known as liquid-phase sintering and has been studied extensively. Rutile boundaries containing an amorphous phase have been used to study boundary migration and grain-boundary grooving. Visible-light (VLM), scanning electron (SEM) and transmission electron microscopy (TEM) in addition to electron-backscatter diffraction (EBSD) and a focused-ion beam (FIB) tool were used to characterize boundary migration in rutile. EBSD analysis was carried out on a Philips XL30 FEG SEM equipped with a DigiView 1612 high-resolution, high-speed CCD camera. A 2.5 cm sample-to-camera distance was used and {approx}70{sup o} sample tilt. A Philips CM30 operated at 300 kV was used for TEM characterization and an FEI DB235 was used for FIB work. Pulsed-laser deposition (PLD) has been used to deposit thin films ({approx}100 nm thick) of silica glass on single-crystals of rutile. The film/substrate assembly is then fabricated into bicrystals of known boundary-plane orientation by hot pressing. Bicrystals were fabricated with boundary planes of nominal surface orientation of (001) and (110). After diffusion bonding a surface perpendicular to the interface is cut and polished. Bicrystals are then heat treated in air at 1650 C for varying lengths of time. Figure 1 is a VLM image of a rutile bicrystal which as been heat treated for 4 hours. During this heat treatment migration of the boundary initiates at parallel grooves contained in the crystal on the right-hand side. EBSD analysis shows that this parallel set of grooves is due to the presence of ...
Date: August 1, 2003
Creator: Gilliss, S.R.; Ravishankar, N.; Farrer, J.K. & Carter, C.B.
Partner: UNT Libraries Government Documents Department

Grain boundary structure and solute segregation in titanium-doped sapphire bicrystals

Description: Solute segregation to ceramic grain boundaries governs material processing and microstructure evolution, and can strongly influence material properties critical to engineering performance. Understanding the evolution and implications of grain boundary chemistry is a vital component in the greater effort to engineer ceramics with controlled microstructures. This study examines solute segregation to engineered grain boundaries in titanium-doped sapphire (Al2O3) bicrystals, and explores relationships between grain boundary structure and chemistry at the nanometer scale using spectroscopic and imaging techniques in the transmission electron microscope (TEM). Results demonstrate dramatic changes in solute segregation stemming from small fluctuations in grain boundary plane and structure. Titanium and silicon solute species exhibit strong tendencies to segregate to non-basal and basal grain boundary planes, respectively. Evidence suggests that grain boundary faceting occurs in low-angle twis t boundaries to accommodate nonequilibrium solute segregation related to slow specimen cooling rates, while faceting of tilt grain boundaries often occurs to expose special planes of the coincidence site lattice (CSL). Moreover, quantitative analysis of grain boundary chemistry indicates preferential segregation of charged defects to grain boundary dislocations. These results offer direct proof that static dislocations in ionic materials can assume a net charge, and emphasize the importance of interactions between charged point, line, and planar defects in ionic materials. Efforts to understand grain boundary chemistry in terms of space charge theory, elastic misfit and nonequilibrium segregation are discussed for the Al2O3 system.
Date: May 17, 2002
Creator: Taylor, Seth T.
Partner: UNT Libraries Government Documents Department

Atomic relaxation modes in grain boundaries

Description: High-resolution electron microscopy (HREM) in combination with computer simulations of the fully relaxed atomic structures and energies of symmetric and asymmetric grain boundaries (GBs) in Au <110> tilt bicrystals has been applied to a study of atomic relaxation modes. These investigations indicate that: (1) Atomic relaxations are typically dominated by short-range interactions and the tendency of the solid to assume a local atomic environment similar to the bulk. (2) Misfit localizations are likely within a structural unit whenever the GB unit cell is large. (3) When there is a length mismatch along the GB the atomic relaxations can assume quasiperiodic character, generating densely spaced regions of structural disorder, akin to misfit dislocations. (4) Atomic relaxations can take the form of stacking disorder.
Date: January 1, 1992
Creator: Merkle, K.L. & Wolf, D.
Partner: UNT Libraries Government Documents Department

Pre-precipitation phenomena at grain boundaries

Description: A high spatial resolution study of the behavior of Zn solute in Al bicrystals has been conducted using x-ray energy-dispersive spectroscopy (EDS) in a TEM/STEM instrument. Specimens subjected to short annealing cycles are found to exhibit a periodic distribution of Zn along the grain boundary plane prior to the appearance of any evidence suggesting precipitation has occurred. The periodicity of the segregation event bears no exact relationship with the transition lattice models of boundary structure but its behavior as a function of misorientation angle does agree with the expected intervals of atomic relaxation in the O-lattice. High resolution images and convergent beam microdiffraction are used to confirm the structural characteristics of these solute-rich regions.
Date: March 1, 1982
Creator: Briceno-Valero, J. & Gronsky, R.
Partner: UNT Libraries Government Documents Department