58 Matching Results

Search Results

Advanced search parameters have been applied.

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

Preview of Behavior of Grain Boundaries in Creep of Aluminum Bicrystals

Description: Note presenting testing of the gliding of one metal crystal with respect to another, along their mutual grain boundary, during high-temperature creep tests conducted with bicrystals of pure aluminum. Stresses of from 1 to 100 psi and temperatures from 200 to 650 degrees Celsius were employed. Results regarding grain boundary glide rate, influence of orientation difference between grains, influence of temperature, influence of applied stress, structure of the operated grain boundary, and structural changes within grains are provided.
Date: July 1952
Creator: Rhines, F. N. & Cochardt, A. W.
Partner: UNT Libraries Government Documents Department

Grain-Boundary Behavior in Creep of Aluminum Bicrystals

Description: Note presenting a study of the gliding of one metal crystal with respect to another parallel to their mutual grain boundary in pure aluminum bicrystals during isothermal creep at a range of temperatures and static stresses. Results regarding induction period, cyclic motion, influence of time on rate of grain-boundary displacement, influence of temperature on rate of grain-boundary displacement, influence of stress on rate of grain-boundary displacement, influence of relative crystal orientations on rate of grain-boundary displacement, and microstructural observations are provided.
Date: December 1955
Creator: Rhines, F. N.; Bond, W. E. & Kissel, M. A.
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

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 1990
Creator: Dahmen, U.
Partner: UNT Libraries Government Documents Department

Atomic-resolution characterization of interface structure and chemistry in the STEM

Description: Combination of Z-contrast imaging and EELS (electron energy loss spectroscopy) allows the local structure and chemistry of interfaces to be determined on the atomic scale. In this paper, these two complementary techniques are used to analyze the structure and chemistry of a nominally 25 degree [100] symmetric tilt boundary in an electroceramic SrTiO{sub 3} bicrystal.
Date: March 1, 1994
Creator: Browning, N. D.; McGibbon, M. M.; McGibbon, A. J.; Chisholm, M. F.; Pennycook, S. J.; Ravikumar, V. et al.
Partner: UNT Libraries Government Documents Department

Simulations of the dislocation array at Ge/Si interfaces

Description: When Ge is grown epitaxially on Si(001), the 4% mismatch between the lattice parameters of Ge and Si can produce a regular two-dimensional grid of (a/2) [1,{plus_minus}1,0] edge dislocations at the interface, a checkerboard with a spacing of {approximately} 100 {Angstrom}. We have performed classical molecular dynamical simulations of this checkboard in large microcrystals. Results show the expected 5-fold plus 7-fold ring structure at the cores of the individual dislocations, and a new closed symmetric structure of 18 atoms at their intersections. Tetrahedral coordination is everywhere retained, with relatively small changes in the bond lengths of less than 10 and in the bond angles of less than 25%. The energetics and dislocation offset of the system are explored for the Stillinger-Weber and Tersoff potentials.
Date: December 1, 1993
Creator: Kaplan, T.; Chisholm, M. F. & Mostoller, M.
Partner: UNT Libraries Government Documents Department

Image simulations of Ge twin boundaries

Description: General high-angle tilt grain boundaries may be described by an arrangement of repeating structural units. Some grain-boundary defects may also be modeled by the incorporation of structural units of related boundary structures into the boundary. The simulation of these structures requires the use of prohibitively large unit cells. The possibility of modeling these boundaries by the superposition of image simulations of the individual structural units is investigated.
Date: December 31, 1992
Creator: McKernan, S. & Carter, C. B.
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

Boron and hydrogen in Ni{sub 3}Al: Part 2, Mechanical testing of bicrystals

Description: To provide a sensitive measurement of the effect of boron segregation on the strength and ductility of Ni{sub 3}Al grain boundaries, bicrystal tensile tests were performed on small specimens of boron doped Ni{sub 76}Al{sub 24} cut from extremely large-grained boules. Five specimens with the same ``random`` or low-symmetry grain boundary (disorientations measured by means of backscattered Kikuchi patterns) and two specimens with a second random grain boundary were tested in quenched and slow-cooled conditions. Duplicate tests performed in a low (7 ppM) water-vapor environment showed that the fracture mode and the stress and strain at fracture are altered by environmental embrittlement at individual, partially strengthened grain boundaries.
Date: January 4, 1994
Creator: Johnson, P. E.; Gourdin, W.; Gonis, A.; Kioussis, N. & Vaudin, M.
Partner: UNT Libraries Government Documents Department

Interface science of controlled metal/metal and metal/ceramic interfaces prepared using ultrahigh vacuum diffusion bonding

Description: We have designed, constructed, and are operating a capability for production of controlled homophase and heterophase interfaces: an ultrahigh vacuum diffusion bonding machine. This machine is based on a previous design which is operating at the Max Planck Institut fuer Metallforschung, Institut fuer Werkstoffwissenschaft, Stuttgart, FRG. In this method, flat-polished single or polycrystals of materials with controlled surfaced topography can be heat treated up to 1500C in ultrahigh vacuum. Surfaces of annealed samples can be sputter cleaned and characterized prior to bonding. Samples can then be precisely aligned crystallographically to obtain desired grain boundary misorientations. Material couples can then be bonded at temperatures up to 1500C and pressures up to 10 MPa. Results are presented from initial work on Mo grain boundaries and Cu/Al{sub 2}A{sub 3} interfaces.
Date: April 1, 1993
Creator: King, W. E.; Campbell, G. H.; Coombs, A. W.; Johnson, G. W.; Kelly, B. E.; Reitz, T. C. et al.
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