Search Results

Advanced search parameters have been applied.
open access

Lattices

Description: Because lattice theory is so vast, the primary purpose of this paper will be to present some of the general properties of lattices, exhibit examples of lattices, and discuss the properties of distributive and modular lattices.
Date: August 1966
Creator: Rintala, Richard Arne
Partner: UNT Libraries
open access

Countable Additivity, Exhaustivity, and the Structure of Certain Banach Lattices

Description: The notion of uniform countable additivity or uniform absolute continuity is present implicitly in the Lebesgue Dominated Convergence Theorem and explicitly in the Vitali-Hahn-Saks and Nikodym Theorems, respectively. V. M. Dubrovsky studied the connection between uniform countable additivity and uniform absolute continuity in a series of papers, and Bartle, Dunford, and Schwartz established a close relationship between uniform countable additivity in ca(Σ) and operator theory for the classical continuous function spaces C(K). Numerous authors have worked extensively on extending and generalizing the theorems of the preceding authors. Specifically, we mention Bilyeu and Lewis as well as Brooks and Drewnowski, whose efforts molded the direction and focus of this paper. This paper is a study of the techniques used by Bell, Bilyeu, and Lewis in their paper on uniform exhaustivity and Banach lattices to present a Banach lattice version of two important and powerful results in measure theory by Brooks and Drewnowski. In showing that the notions of exhaustivity and continuity take on familiar forms in certain Banach lattices of measures they show that these important measure theory results follow as corollaries of the generalized Banach lattice versions. This work uses their template to generalize results established by Bator, Bilyeu, and Lewis.
Date: August 1999
Creator: Huff, Cheryl Rae
Partner: UNT Libraries
open access

Shock compression and quasielastic release in tantalum

Description: Previous studies of quasielastic release in shock-loaded FCC metals have shown a strong influence of the defect state on the leading edge, or first observable arrival, of release wave, due to large density of pinned dislocation segments behind the shock front, their relatively large pinning separation, and a very short response time as determined by drag coefficient in shock-compressed state. This effect is entirely equivalent to problems associated with elastic moduli determination using ultrasonic methods. This is particularly true for FCC metals, which have an especially low Peierls stress, or inherent lattice resistance, that has little influence in pinning dislocation segments and inhibiting anelastic deformation. BCC metals, on the other hand, have a large Peierls stress that essentially holds dislocation segments in place at low net applied shear stresses and thus allows fully elastic deformation to occur in the complete absence of anelastic behavior. Shock-compression and release experiments have been performed on tantalum (BCC), with the observation that the leading release disturbance is indeed elastic. This conclusion is established by examination of experimental VISAR records taken at the tantalum/sapphire (window) interface in a symmetric-impact experiment which subjects the sample to a peak longitudinal stress of approximately 7.3 GPa, in comparison with characteristic code calculations.
Date: January 1, 1993
Creator: Johnson, J. N.; Hixson, R. S.; Tonks, D. L. & Gray, G. T. III
Partner: UNT Libraries Government Documents Department
open access

Theoretical studies of zirconia and defects in zirconia. Final report

Description: Supported by this grant the author has performed total energy electronic structure calculations for cubic, tetragonal, and monoclinic zirconia. The results of these calculations agree with the observed ordering of structures in the phase diagram. He has developed model potentials based on the total energy results. Molecular dynamics calculations using these model potentials give a good description of the phase transitions in and the thermal properties of zirconia.
Date: November 1, 1995
Creator: Jansen, H.J.F.
Partner: UNT Libraries Government Documents Department
open access

First-principles study of the contractive reconstruction of gold and silver monolayers on gold, silver and aluminum

Description: Using first-principles calculations in conjunction with modeling techniques, the author has investigated the structures of Au and Ag monolayers on a number of metal surfaces. Au(100) has a c(26 {times} 68) surface unit cell and the reconstruction has been interpreted as the top layer transforming to a contracted hexagonal-close-packed layer, superimposed on the square lattice of the underlying substrate atoms. Similar reconstructions have been observed on the 5d fcc metals Ir and Pt, but not in the 4d Rh, Pd, and Ag. The author studied the energetics of a monolayer of Au and Ag using first-principles calculations. The author found that it is energetically favorable for both Au and Ag to transform from a square to hexagonal arrangement and to contract to a higher surface density, but Au gains substantially more energy than Ag. This is true both for a monolayer in isolation as well as on top of a jellium surface. The author also calculated the mismatch energy (energy loss when the top layer loses registry with the substrate) for Au and Ag, and found that Ag has a slightly higher mismatch energy. The first-principles results thus offer a strong indication that Au(100) can reconstruct but Ag will not. The reconstruction is further studied with a 2 dimensional Frenkel-Kontorowa model, with parameters extracted from the total energy calculations. The author found that it is indeed energetically favorable for the top layer of Au(100), but not for Ag, to transform to a hexagonal-close-packed structure and contract. 85 refs., 34 figs., 8 tabs.
Date: November 16, 1990
Creator: Takeuchi, Noboru.
Partner: UNT Libraries Government Documents Department
open access

Experimental studies of cascade phenomena in metals

Description: We review recent ion-irradiation experiments which have been performed to investigate the collapse of displacement cascades to dislocation loops in a range of metals and alloys. Many of the results including the dependencies of the collapse probabilities on irradiation temperature, and ion dose, energy and mass, can be explained within the framework of a thermal spike/cascade melting model which has been suggested by computer molecular dynamics simulations. Other aspects, such as the dependence of collapse propabilities on the crystal structure and the effects of alloying and impurities, are less well understood.
Date: June 1, 1992
Creator: Jenkins, M. L.; Kirk, M. A. & Phythian, W. J.
Partner: UNT Libraries Government Documents Department
open access

The crystal and molecular structure of azatranes: Azavanadatran (Z=t-Bu), monoazasilatrane (Z=H), azalithatrane (Z=Clo*4*), azaphosphatrane (Z=Me), azagermatrane (Z=t-Bu) and Azaalumatran (Z=nothing)

Description: The crystal and molecular structures of azatranes have been extensively studied for a variety of M atoms. The crystal and molecular structures of six similar cage compounds: (t-Bu)NV(MeNCH{sub 2}CH{sub 2}){sub 3}N, HSi(OCH{sub 2}CH{sub 2}){sub 2}(HNCH{sub 2}CH{sub 2})N, O{sub 4}ClLi(HNCH{sub 2}SH{sub 2}){sub 3}N, MeP(Me{sub 3}NCH{sub 2}CH{sub 2}){sub 3}N, t-BuGe(HNCH{sub 2}CH{sub 2}){sub 3}N, and Al(Me{sub 3}SiNCH{sub 2}CH{sub 2}){sub 3}N were determined by the use of three-dimensional, single crystal x-ray diffraction.
Date: April 23, 1996
Creator: Wang, T.
Partner: UNT Libraries Government Documents Department
open access

Experimental studies of cascade phenomena in metals

Description: We review recent ion-irradiation experiments which have been performed to investigate the collapse of displacement cascades to dislocation loops in a range of metals and alloys. Many of the results including the dependencies of the collapse probabilities on irradiation temperature, and ion dose, energy and mass, can be explained within the framework of a thermal spike/cascade melting model which has been suggested by computer molecular dynamics simulations. Other aspects, such as the dependence of collapse propabilities on the crystal structure and the effects of alloying and impurities, are less well understood.
Date: June 1, 1992
Creator: Jenkins, M. L.; Kirk, M. A. & Phythian, W. J.
Partner: UNT Libraries Government Documents Department
open access

Atomic defects and diffusion in metals

Description: The tracer self-diffusion data for fcc and refractory bcc metals are briefly reviewed with respect to (i) the available monovacancy formation and migration properties and (ii) the high-temperature diffusion enhancement above that expected for mass transport via atomic exchange with monovacancies. While the atomic-defect mechanism for low-temperature self-diffusion can be reliably attributed to monovacancies, the mechanisms responsible for high-temperature mass transport are not so easily defined at this time; both divacancies and interstitials must be seriously considered. Possibilities for improving our understanding in this area are discussed. 68 references, 7 figures.
Date: November 1, 1981
Creator: Siegel, R. W.
Partner: UNT Libraries Government Documents Department
open access

Dipole interactions and trapping effects of positive muons in fcc and bcc metals

Description: In this paper the results of calculations for a single muon trapped at various lattice sites in an external magnetic field are presented. As the direction of the magnetic field is changed, the results show a large variation of the local field experienced by a muon at the octahedral (o-sites) and tetrahedral (t-sites) symmetry lattice interstitial sites. These results show that a careful measurement of the ..mu../sup +/ depolarization rate for various external magnetic field orientations would indicate a preferential position for the trapped particle. At this time analysis of the experimental data does not present a clear interpretation for the determination of the ..mu../sup +/ site. The calculational procedure is outlined and the results, including an estimate of the effects of lattice distortion on the ..mu../sup +/ depolarization rate when the muon is localized are presented.
Date: January 1, 1976
Creator: Straub, G K & Parkin, D M
Partner: UNT Libraries Government Documents Department
open access

High energy spin waves in BCC iron

Description: We have studied the dispersion relation of the spin wave excitations in bcc iron by neutron inelastic scattering at the spallation neutron source ISIS. Magnetic intensity was followed up to 550meV along the (100) direction. The general form of the dispersion curve is in qualitative agreement with that calculated from a spin-polarised band model, and in particular we have confirmed the prediction of propagating modes extending above 300meV. 7 refs., 3 figs.
Date: January 1, 1990
Creator: Perring, T.G. (Oxford Univ. (UK). Clarendon Lab.); Boothroyd, A.T.; Paul, D.McK. (Warwick Univ., Coventry (UK). Dept. of Physics); Taylor, A.D.; Osborn, R. (Rutherford Appleton Lab., Chilton (UK)); Newport, R.J. (Kent Univ., Canterbury (UK). Dept. of Physics) et al.
Partner: UNT Libraries Government Documents Department
open access

Phase stability of fcc- and hcp-based intermetallics: The Ti-Al and Cd-Mg systems

Description: In this paper we summarize results of first-principles phase stability studies of fcc- and hcp-based Ti-Al alloys and of the hcp-based Cd-Mg system. In particular, heats of formation for ordered alloy compounds are calculated with the linear muffin tin orbital method; effective cluster interactions are determined from the results of these calculations and are used to derive thermodynamic properties and composition-temperature phase diagrams.
Date: June 1, 1992
Creator: Asta, M.; McCormack, R. (California Univ., Berkeley, CA (United States). Dept. of Materials Science and Mineral Engineering); van Schilfgaarde, M. (SRI International, Menlo Park, CA (United States)); Ceder, G. (Massachussetts Inst. of Tech., Cambridge, MA (United States). Dept. of Materials Science) & de Fontaine, D. (California Univ., Berkeley, CA (United States). Dept. of Materials Science and Mi
Partner: UNT Libraries Government Documents Department
open access

Interdiffusion in the Ni-Cr-Co-Mo system at 1300/sup 0/C

Description: Interdiffusion was investigated with solid-solid diffusion couples in the ..cap alpha.. (fcc) region of the quaternary Ni-Cr-Co-Mo system at 1300/sup 0/C for the determination of diffusion paths and diffusional interactions among the components. The concentration profiles for a given couple exhibited a common cross-over composition, Y/sub c/, which reflected the relative depths of diffusion in the terminal alloys. Interdiffusion fluxes were calculated directly from the concentration profiles and the quaternary interdiffusion coefficients were calculated at selected compositions. Ni and Co exhibited up-hill diffusion against their individual concentration gradients in a direction opposite to the interdiffusion of Cr. Quaternary diffusion paths were presented as a set of partial diffusion paths on the basis of relative concentration variables.
Date: January 1, 1985
Creator: Heaney, J.A. III & Dayananda, M.A.
Partner: UNT Libraries Government Documents Department
open access

Diffuse scattering studies as a tool for characterizing the local order structure and for obtaining pairwise interaction energies

Description: Diffuse scattering is a mature method for characterizing the local order structure of alloy systems. An extension of such structural investigations, for alloys at equilibrium, allows one to obtain pairwise interaction energies. Having experimental pairwise interaction energies for the various coordination shells offers one the potential for more realistic Monte Carlo modelling of alloy systems as they relax toward equilibrium. The diffuse scattering method and the recovery of the interaction energies are reviewed and some preliminary results are used to demonstrate the kinetic {Iota}sing modeling technique.
Date: January 1, 1992
Creator: Epperson, J.E. (Argonne National Lab., IL (United States)); Chen, H. & Anderson, J.P. (Illinois Univ., Urbana, IL (United States). Dept. of Materials Science and Engineering)
Partner: UNT Libraries Government Documents Department
open access

Investigation of irradiation strengthening of body centered cubic metals and their alloys. Final report

Description: The purpose of the proposed research was to determine how neutron damage affects the low temperature deformation mechanism or mechanisms in body centered cubic metals. The aims of this research can be stated more specifically as follows: the determination of the barrier to dislocation motion introduced by neutron irradiation damage; determine the mechanism by which the dislocation overcomes this barrier; and determine the density of the neutron produced barriers which are overcome by thermally activated motion of the dislocation.
Date: January 1, 1978
Creator: Arsenault, R.J.
Partner: UNT Libraries Government Documents Department
open access

Segregation induced embrittlement of grain interfaces

Description: Conditions are discussed under which an interface, possibly containing an adsorbed species, is capable of sustaining an atomistically sharp cleavage crack, rather than having any such crack blunt out via dislocation nucleation. Two models for ductile versus brittle interface response are discussed. One of these is applied to model grain boundaries in face centered cubic materials, and is used to compare predictions with experimental observations on the embrittlement of Cu polycrystals by the addition of dilute concentrations of Bi. The general results are in qualitative agreement, although the model predicts behavior that is, in general, more ductile than that observed experimentally. Several possible reasons for this excess ductility are discussed. An appendix presents an analysis of the work of reversible separation of an interface containing a segregated species. This is intended to clarify some issues recently raised in the literature on interfacial embrittlement, especially concerning the distinctions that must be drawn between expressions for cohesive energy reductions for interfacial separation at constant chemical potential, ..mu.., versus separation with constant concentration, GAMMA, of the adsorbate species.
Date: May 1, 1977
Creator: Mason, D D
Partner: UNT Libraries Government Documents Department
open access

Production of freely-migrating defects during irradiation

Description: During irradiation at elevated temperatures, vacancy and interstitial defects that escape can produce several different types of microstructural changes. Hence the production rate of freely-migrating defects must be known as a function of irradiating particle species and energy before quantitative correlations can be made between microstructural changes. Our fundamental knowledge of freely-migrating defect production has increased substantially in recent years. Critical experimental findings that led to the improved understanding are reviewed in this paper. A strong similarity is found for the dependence of freely-migrating defect production on primary recoil energy as measured in a variety of metals and alloys by different authors. The efficiency for producing freely-migrating defects decreases much more strongly with increasing primary recoil energy than does the efficiency for creating stable defects at liquid helium temperatures. The stronger decrease can be understood in terms of additional intracascade recombination that results from the nonrandom distribution of defects existing in the primary damage state for high primary recoil energies. Although the existing data base is limited to fcc materials, the strong similarity in the reported investigations suggests that the same dependence of freely-migrating defect production on primary recoil energy may be characteristic of a wide variety of other alloy systems as well. 52 refs., 4 figs.
Date: September 1, 1986
Creator: Rehn, L. E. & Okamoto, P. R.
Partner: UNT Libraries Government Documents Department
open access

Shock compression and quasielastic release in tantalum

Description: Previous studies of quasielastic release in shock-loaded FCC metals have shown a strong influence of the defect state on the leading edge, or first observable arrival, of release wave, due to large density of pinned dislocation segments behind the shock front, their relatively large pinning separation, and a very short response time as determined by drag coefficient in shock-compressed state. This effect is entirely equivalent to problems associated with elastic moduli determination using ultrasonic methods. This is particularly true for FCC metals, which have an especially low Peierls stress, or inherent lattice resistance, that has little influence in pinning dislocation segments and inhibiting anelastic deformation. BCC metals, on the other hand, have a large Peierls stress that essentially holds dislocation segments in place at low net applied shear stresses and thus allows fully elastic deformation to occur in the complete absence of anelastic behavior. Shock-compression and release experiments have been performed on tantalum (BCC), with the observation that the leading release disturbance is indeed elastic. This conclusion is established by examination of experimental VISAR records taken at the tantalum/sapphire (window) interface in a symmetric-impact experiment which subjects the sample to a peak longitudinal stress of approximately 7.3 GPa, in comparison with characteristic code calculations.
Date: June 1, 1993
Creator: Johnson, J. N.; Hixson, R. S.; Tonks, D. L. & Gray, G. T. III
Partner: UNT Libraries Government Documents Department
open access

3D dislocation dynamics: stress-strain behavior and hardening mechanisms in FCC and BCC metals

Description: A dislocation dynamics (DD) model for plastic deformation, connecting the macroscopic mechanical properties to basic physical laws governing dislocation mobility and related interaction mechanisms, has been under development. In this model there is a set of critical reactions that determine the overall results of the simulations, such as the stress-strain curve. These reactions are, annihilation, formation of jogs, junctions, and dipoles, and cross-slip. In this paper we discuss these reactions and the manner in which they influence the simulated stress- strain behavior in fcc and bcc metals. In particular, we examine the formation (zipping) and strength of dipoles and junctions, and effect of jogs, using the dislocation dynamics model. We show that the strengths (unzipping) of these reactions for various configurations can be determined by direct evaluation of the elastic interactions. Next, we investigate the phenomenon of hardening in metals subjected to cascade damage dislocations. The microstructure investigated consists of small dislocation loops decorating the mobile dislocations. Preliminary results reveal that these loops act as hardening agents, trapping the dislocations and resulting in increased hardening.
Date: February 19, 1999
Creator: Hirth, J P; Rhee, M; Zhib, H M & de la Rubia, T D
Partner: UNT Libraries Government Documents Department
open access

The structures of bare and deuterated Co{sub 19}

Description: The structures of bare Co{sub 19} and deuterated Co{sub 19}D{sub m} clusters are examined by the chemical probe method, and earlier assignments of bare Co{sub 19} as an fcc octahedron are reconsidered. New experimental measurements of the reactivity of Co{sub 19} with ammonia, nitrogen, and deuterium are presented, and together with earlier measurements of the reactivity with water suggest that bare Co{sub 19} has an hcp structure (D{sub 3h} symmetry). The adsorption of deuterium on Co{sub 19} is found to proceed in steps, leading to successive saturation levels at Co{sub 19}D{sub 4}, Co{sub 19}D{sub 14}, and Co{sub 19}D{sub 18}. Using binding rules derived from earlier studies of larger cobalt and nickel clusters, possible D-atom binding sites on Co{sub 19}D{sub m} (both fcc and hcp) are proposed.
Date: July 1, 1997
Creator: Parks, E.K. & Riley, S.J.
Partner: UNT Libraries Government Documents Department
open access

The crystal structure of neptunium metal

Description: From abstract: "In this paper the number of displaced atoms in equilibrium in a thin cyclotron-bombarded target is estimated as a function of the bombarding particle energy and target temperature."
Date: January 4, 1952
Creator: Zachariasen, William H.
Partner: UNT Libraries Government Documents Department
Back to Top of Screen