657 Matching Results

Search Results

Advanced search parameters have been applied.

The Mechanisms of Grain Boundaries - Slip Transmission, Migration, and Sliding

Description: During the last eight years, we have worked on the general problems associated with grain boundaries in metals with DOE support. This final report summarizes the work that has been performed. At the start of this work, we took a much more atomistic approach to grain boundaries. However, as we performed this research it became clear that such approaches had the drawbacks listed above, and that we were not proceeding toward the more general understanding of grain boundaries that we have hoped to achieve. We then moved toward more macroscopic based experiments that we could use to understand the structure and motion of grain boundaries. From these we were able to begin deducing some of the most important results of this work and to provide information that can be used by others to understand the role of grain boundaries in materials. We thus present this report in a topical way and provide the experimental and theoretical underpinning that is needed at each point as we go forward.
Date: March 2, 2005
Creator: Briant, Clyde L.
Partner: UNT Libraries Government Documents Department

Role of twinning in the optimization of the grain boundary character distribution

Description: The grain boundary character distribution (GBCD) is a microstructural property that describes the proportions of �special� and �random� boundaries as defined by the coincident site lattice model. Recently, there has been increased attention on determination of the GBCD and manipulation of the relative fractions in the microstructure through thermomechanical processing in order to improve material� s properties like corrosion and creep resistance. Most of the �optimization� treatments reported in the literature have been performed on fee materials with relatively low stacking fault energies and have resulted in microstructures with high fractions of {Sigma}3, {Sigma}9, and {Sigma}27 boundaries. It could be interpreted that annealing twins are solely required to improve the GBCD. However, in order to optimize the properties, it appears imperative that the formation of annealing twins disrupt the connectivity of the random boundary network, thus implying that {Sigma}3{sup n} reactions and resultant triple lines are critical. Experiments to modify the GBCD of oxygen-free electronic Cu and Inconel 600 through thermomechanical processing are presented and discussed in light of observations of the deformed and recrystallized microstructures.
Date: January 8, 1999
Creator: King, W E; Kumar, M & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Thermal modification of microstructures and grain boundaries in silicon carbide

Description: Polycrystalline SiC samples hot-pressed with aluminum, boron, and carbon sintering additions (ABC-SiC) were characterized using transmission electron microscopy. The study focused on the effects of high temperature treatment on microstructure.
Date: May 23, 2003
Creator: Zhang, Xiao Feng & De Jonghe, Lutgard C.
Partner: UNT Libraries Government Documents Department

Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO

Description: The understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to res olve the dislocation Core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.
Date: November 29, 1999
Creator: Kim, M.; Browning, N.D.; Pennyscook, S.J.; Sohlberg, K. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department

The breaking strain of neutron star crust

Description: Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of neutron star crust. With multimillion ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Due to the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gTavitational wave radiation could limit the spin periods of some stars and might be detectable in large scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in Magnetar Giant and Micro Flares.
Date: January 1, 2009
Creator: Kadau, Kai & Horowitz, C J
Partner: UNT Libraries Government Documents Department

Atomistic Simulations of Grain Boundary Pinning in CuFe Alloys

Description: The authors apply a hybrid Monte Carlo-molecular dynamics code to the study of grain boundary motion upon annealing of pure Cu and Cu with low concentrations of Fe. The hybrid simulations account for segregation and precipitation of the low solubility Fe, together with curvature driven grain boundary motion. Grain boundaries in two different systems, a {Sigma}7+U-shaped half-loop grain and a nanocrystalline sample, were found to be pinned in the presence of Fe concentrations exceeding 3%.
Date: May 22, 2005
Creator: Zepeda-Ruiz, L A; Gilmer, G H; Sadigh, B; Caro, J A & Oppelstrup, T
Partner: UNT Libraries Government Documents Department

Deformation by grain boundary sliding and slip creep versus diffusional creep

Description: A review is presented of the debates between the present authors and other investigators regarding the possible role of diffusional creep in the plastic flow of polycrystalline metals at low stresses. These debates are recorded in eleven papers over the past seventeen years. ln these papers it has been shown that the creep rates of materials in the so-called �diffusional creep region� are almost always higher than those predicted by the diffusional creep theory. Additionally, the predictions of grain size effects and stress exponents from diffusional creep theory are often not found in the experimental data. Finally, denuded zones have been universally considered to be direct evidence for diffusional creep; but, those reported in the literature are shown to be found only under conditions where a high stress exponent is observed. Also, the locations of the denuded zones do not match those predicted. Alternative mechanisms are described in which diffusion-controlled dislocation creep and/or grain boundary sliding are the dominant deformation processes in low-stress creep. It is proposed that denuded zones are formed by stress-directed grain boundary migration with the precipitates dissolving in the moving grain boundaries. The above observations have led us to the conclusion that grain boundary sliding and slip creep are in fact the principal mechanisms for observations of plastic flow in the so-called �diffusional creep regions. �
Date: November 4, 1998
Creator: Ruano, O. A.; Sherby, O. D. & Wadsworth, J.
Partner: UNT Libraries Government Documents Department

Transmission Electron Microscopy Characterization of Nanocrystalline Copper

Description: The microstructure and grain boundary structure of nanocrystalline Cu powders and a compact prepared by the inert-gas condensation technique have been characterized by transmission electron microscopy. The as-prepared particles are round in shape and have no distinct surface facets. Annealing twins (coherent {Sigma}3 boundaries) have been observed in the as-prepared Cu particles as well as in the compact. Pores are commonly found at grain boundaries, triple grain junctions and some in the interior of grains in the compact. In addition to twin boundaries, a number of special grain boundaries have been observed. These special grain boundaries have low-index interface planes, and sometimes have misorientation angles close to coincidence site lattice (CSL) orientations.
Date: November 1, 1999
Creator: Kung, H.; Sanders, P.G. & Weertman, J.R.
Partner: UNT Libraries Government Documents Department

Investigating the Atomic Scale Superconducting Properties of Grain Boundaries in High-T(Sub c) Superconductors

Description: Presented at Fourteenth International Congress on Electron Microscopy Cancun, Mexico, August 31-September 4, 1998, and published in Proceedings Over ten years after the discovery of high-TC superconductors, their widespread application into viable device structures is still limited by the deleterious effect of grain boundaries. One of the main difficulties associated with understanding this effect is that transport measurements are usually performed on the micron scale. However, the critical parameter for superconductivity, the coherence length, is only ~lnm. To understand grain boundaries on a fundamental level it is therefore necessary to investigate the properties on this atomic scale; a scale attainable only by electron microscopy [12]. As an example of the observed properties of grain boundaries in YB~C~07d (YBCO), the V(I) curves recorded across a 24o boundary for several magnetic fields are shown m figure 1, To explain these properties, a model where the grain boundary is composed of equally sized and spaced dislocation cores separated by a very small fraction of much stronger links has been developed (figure 1). These strong links may carry either the depairing current, the JC of the grains or another Josephson current (a depairing current seems unlikely in view of the field dependence of the experimental data). The simulated behavior obtained for this model, where the fraction of strong links is x=O.005 and JC is the observed J=(B) of the grains, exhibits qualitatively similar behavior to the experimental data (figure 1). However, the fit is not perfect, suggesting that the strong links are more likely to be regions of grain boundary with a higher Josephson current, rather than links with the JC(B) of the grains. Using electron microscopy we can look for the origin of these stronger coupled regions at the grain boundary. Figure 2 shows a Z-contrast image of a similar high-angle [001] tilt ...
Date: August 31, 1998
Creator: Browning, N.D.; Buban, J.P.; Christen, D.K.; Nellist, P.D.; Norton, D.P.; Pennycook, S.J. et al.
Partner: UNT Libraries Government Documents Department

Atomistic studies of grain boundaries and heterophase interfaces in alloys and compounds. Final report, July 1987-August 1998

Description: The overarching goal of the research supported by this grant was investigation of the structure and properties of interfaces in multicomponent systems by atomistic modeling. Initially, the research was devoted to studies of segregation to grain boundaries in binary disordered alloys. The next step was then studies of the structure and properties of grain boundaries in ordered compounds, specifically Ni3Al and NiAl, and grain boundary segregation in these compounds in the case of off-stoichiometry. Finally, the structure of Nb/sapphire interfaces, in particular the core configurations of the misfit dislocations, was studied.
Date: August 1, 1998
Creator: Vitek, Vaclav
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

Study of grain boundary segregation using the Auger electron emission technique. Annual technical progress report, III, January 1, 1973--December 31, 1973

Description: In Al/sub 2/O/sub 3/, it was found that additives segregated to grain boundaries. The role of additives on the densification of Al/sub 2/O/sub 3/ and MgO was measured. Segregation of S in Ni, P in W, and Bi in Cu was measured. Work continued on the effect of third element additions for controlling deterioration of properties and on intergranular corrosion in stainless steels. (DLC)
Date: January 1, 1973
Creator: Stein, D.F. & Heldt, L.A.
Partner: UNT Libraries Government Documents Department

Faceting of high angle grain boundaries in the coincidence lattice

Description: Faceting of high angle grain boundaries in SIGMA = 3 and SIGMA = 5 coincidence lattices of the fcc structure was studied using thinfilm bicrystal specimens of controlled geometry. A number of relatively high index boundaries in the coincidence lattices (containing relatively low planar densities of coincidence sites) was found to break up into low energy facets corresponding to low index planes of the coincidence lattices (containing high densities of coincidence lattice sites). These results are consistent with a general expectation that the grain boundary energy decreases as the planar density of coincidence sites increases, i.e., the two-dimensional periodicity of the boundary becomes shorter, and long-ranged distortions are reduced. (auth)
Date: November 16, 1973
Creator: Wagner, W.R.; Tan, T.Y. & Balluffi, R.W.
Partner: UNT Libraries Government Documents Department


Description: Grain boundaries in silicon-based ceramics have been characterized by high resolution electron microscopy including the technique of lattice fringe imaging, and this work is illustrated with examples from both hot-pressed silicon nitrides (MgO and Y{sub 2}O{sub 3} fluxed) and a magnesium-sialon (Mg{sub 1.86} Si{sub 1.67} Al{sub 2.47} O{sub 3.19} N{sub 3.81}). Room temperature observations of the glassy phase are consistent with it being only a partially wetting phase, indicating that it cannot form a continuous film. The atomic configuration of the grain boundaries in both materials is presented together with lattice fringe observations of segregation at grain boundaries in the magnesium-sialon.
Date: December 1, 1977
Creator: Clarke, David R. & Thomas, G.
Partner: UNT Libraries Government Documents Department

In Situ Spectroscopic Observation of Activation and Transformation of Tantalum Suboxides

Description: Using ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS), we were able to observe the process of oxidation of tantalum with different morphological parameters. Being able to trace surface evolution during oxidation, we evaluated activation energy of oxidation under the influence of strain and grain boundaries. It was found that the metal oxidized through three different stages and there was a transition stage where the phase transformation from suboxides to the equilibrium state of pentoxide. The applied stress and surface defects reduced the activation energy oxidation.
Date: December 16, 2009
Creator: Wang, Ke; Liu, Zhi; Cruz, Tirma Herranz; Salmeron, Miquel & Liang, Hong
Partner: UNT Libraries Government Documents Department

Influence of point defects on grain boundary motion.

Description: This work addresses the influence of point defects, in particular vacancies, on the motion of grain boundaries. If there is a non-equilibrium concentration of point defects in the vicinity of an interface, such as due to displacement cascades in a radiation environment, motion of the interface to sweep up the defects will lower the energy and provide a driving force for interface motion. Molecular dynamics simulations are employed to examine the process for the case of excess vacancy concentrations in the vicinity of two grain boundaries. It is observed that the efficacy of the presence of the point defects in inducing boundary motion depends on the balance of the mobility of the defects with the mobility of the interfaces. In addition, the extent to which grain boundaries are ideal sinks for vacancies is evaluated by considering the energy of boundaries before and after vacancy absorption.
Date: September 1, 2010
Creator: Foiles, Stephen Martin
Partner: UNT Libraries Government Documents Department

First principles determination of dislocation properties.

Description: This report details the work accomplished on first principles determination of dislocation properties. It contains an introduction and three chapters detailing three major accomplishments. First, we have used first principle calculations to determine the shear strength of an aluminum twin boundary. We find it to be remarkably small ({approx}17 mJ/m{sup 2}). This unexpected result is explained and will likely pertain for many other grain boundaries. Second, we have proven that the conventional explanation for finite grain boundary facets is wrong for a particular aluminum grain boundary. Instead of finite facets being stabilized by grain boundary stress, we find them to originate from kinetic effects. Finally we report on a new application of the Frenkel-Kontorova model to understand reconstructions of (100) type surfaces. In addition to the commonly accepted formation of rectangular dislocation arrays, we find numerous other possible solutions to the model including hexagonal reconstructions and a clock-rotated structure.
Date: December 1, 2003
Creator: Hamilton, John C.
Partner: UNT Libraries Government Documents Department

In-situ Fracture Studies and Modeling of the Toughening Mechanism Present in Wrought LCAC, TZM, and ODS Molybdenum Flat Products

Description: In-situ testing, ultrasonic C-scans, and metallography were used to show that a crack-divider delamination form of thin-sheet toughening occurs in wrought Low Carbon Arc Cast (LCAC) unalloyed molybdenum, Oxide Dispersion Strengthened (ODS) molybdenum, and TZM molybdenum at temperatures {ge} the Ductile to Brittle Transition Temperature (DBTT). Cracking along boundaries relieves mechanical constraint to free ligaments that may plastically stretch to produce toughening. Anisotropy in fracture toughness with lower values in the short-transverse direction is shown to produce the crack divider delaminations at the crack tip in the LT and TL orientations. The delamination zone increases with increasing stress-intensity to sizes significantly larger than the plastic zone, which leads to large increases in fracture toughness by the thin sheet toughening mechanism. Fracture in ODS Mo-alloys proceeds mainly along grain boundaries to produce small ligaments that exhibit ductility for both LT and TL orientations resulting in a lower DBTT and higher toughness values at lower temperatures than observed in LCAC and TZM. A combination of grain boundary fracture and cleavage is prevalent in LCAC molybdenum and TZM. The predominance for microcracking along grain boundaries to leave fine, ductile ligaments in ODS molybdenum can be attributed to a fine-grained microstructure with {approx} 1-2 {micro}m thickness of sheet-like grains. The presence of mixed grain boundary fracture and cleavage in LCAC and TZM can be attributed to a microstructure with a larger thickness of sheet-like grains (4-15 {micro}m).
Date: July 1, 2007
Creator: Cockerman, B. V. and Chan, K. S.
Partner: UNT Libraries Government Documents Department