860 Matching Results

Search Results

Advanced search parameters have been applied.

Mechanism for diffusion induced grain boundary migration

Description: Grain boundaries are found to migrate under certain conditions when solute atoms are diffused along them. This phenomenon, termed diffusion induced grain boundary migration (DIGM), has now been found in six systems. The observed phenomenon and empirical data are used to discard certain concepts for the driving force and the mechanism. A mechanism is proposed in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.
Date: August 1, 1980
Creator: Balluffi, R.W. & Cahn, J.W.
Partner: UNT Libraries Government Documents Department

Atomistic simulations of surface relaxations in Ni, Al, and their ordered alloys

Description: We have performed a series of simulations to examine the atomistic nature of surface relaxations in pure metals and ordered alloys. The surface relaxations (..delta..d/sub n,n+1/) are shown to be oscillatory and to decay rapidly into the bulk. The period and form of the oscillation may be determined by simple geometrical arguments. The oscillation wavelength is always of order an atomic diameter. In pure metals, the surface layer of atoms always displaces inward. However, in the ordered alloys the larger atom may displace outward. On planes composed of more than one atom types, rippling occurs.
Date: January 1, 1986
Creator: Chen, S.P.; Voter, A.F. & Srolovitz, D.J.
Partner: UNT Libraries Government Documents Department

Volume dependence of computed grain boundary energy

Description: Over the past five years there have been numerous studies of grain boundary structure using the method of computer molecular statics which assume pairwise central potentials for the interatomic interaction. Emphasis is usually placed on relative grain boundary energies but these may be inaccurate due to various, but related, approximations and constraints implicity imposed on the calculation-namely central forces, finite model size, fixed border conditions and volume dependent contributions to the energy of the system. It is the purpose of this work to clarify how these particular properties of the model can affect the computed grain boundary energy and demonstrate instances in which the quoted energy has strictly been inaccurate. The implication of these results, especially on how they affect the method of relaxation and the resulting grain boundary structure is discussed.
Date: August 1, 1980
Creator: Bristowe, P.D. & Brokman, A.
Partner: UNT Libraries Government Documents Department

Theoretical studies of the crystal structure of rare earths and actinides at zero temperature

Description: Accurate total energy electronic structure calculations have recently been carried out over a range of volumes for selected rare-earth and actinide elements in crystal structures, experimentally observed in these elements. Correct zero temperature crystal structures are obtained, and calculated equilibrium properties are in reasonable agreement with experiment. Results of these calculations indicate that the interactions underlying crystal structure stability are similar in itinerant f-electron metals and transition metals. The stable crystal structure at a particular volume is determined by a balance between one-electron bandwidths and band fillings and the electrostatic energy of the crystal lattice. Broad bands favor high-symmetry, close-packed structures while narrow bands favor low-symmetry, open structures; allowing for expansion as well as contraction, both transition and actinide elements can be stabilized in both low- and high-symmetry crystal structures.
Date: January 1, 1993
Creator: Wills, J.M. (Los Alamos National Lab., NM (United States)) & Eriksson, O. (Uppsala Univ. (Sweden). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

Relaxation effects in shock-induced transitions in bismuth

Description: The indication given by the baratol experiments is that as the shock in the mixed region progresses and decays, the lead state does not change along a stable Hugoniot but appears to change from a sort of metastable Hugoniot, in accordance with Romain's data, to a stable Hugoniot reminiscent of the earlier results. If this is so, then the Bi I-III transition is exhibiting a relaxation effect similar to that seen in the Sb I-III transition. The transition detected by Romain could then be interpreted as the II-III transition seen in the metastable region. The few experiments outlined here merely hint at an explanation. Additional precision work in the stress range 6-10 GPa, some with thin samples, is needed to resolve this behavior.
Date: January 1, 1981
Creator: Neal, T.R.
Partner: UNT Libraries Government Documents Department

Carbon in high-purity germanium

Description: Using /sup 14/C-spiked pyrolytic graphite-coated quartz crucibles for the growth of nine ultra-pure germanium single crystals, we have determined the carbon content and distribution in these crystals. Using autoradiography, we observe a rapidly decreasing carbon cluster concentration in successively grown crystals. Nuclear radiation detectors made from the crystals measure the betas from the internally decaying /sup 14/C nuclei with close to 100% efficiency. An average value for the total carbon concentration (/sup 14/C + /sup 12/C) is approx. 2 x 10/sup 14/ cm/sup -3/, a value substantially larger than expected from earlier metallurgical studies. Contrary to the most recent measurement, we find the shape of the beta spectrum to agree very well with the statistical shape predicted for allowed transitions.
Date: October 1, 1981
Creator: Haller, E.E.; Hansen, W.L.; Luke, P.; McMurray, R. & Jarrett, B.
Partner: UNT Libraries Government Documents Department

Role of dislocation theory in the design of engineering materials

Description: The science of materials development has progressed to a stage in which it is possible to compose a recipe for new materials. The recipe has three steps: given a desirable set of properties and material constraints, one identifies a composition and microstructure to achieve them; given a desirable composition and microstructure, one identifies a processing sequence to achieve them; given a trial alloy, one conducts analytical testing to identify its shortcomings and overcomes them. In effecting each stage of this recipe, it is usually necessary to be aware of and understand the role of the dislocations which determine material properties, define material microstructure, and control its evolution. The role of dislocations is discussed. The text contains examples of particular alloy development efforts, and suggestions for research in dislocation theory which might contribute to the solution of significant problems in materials development.
Date: June 1, 1980
Creator: Morris, J.W. Jr.
Partner: UNT Libraries Government Documents Department

Equation of state of beryllium

Description: A new, wide-range equation of state (EOS) has been constructed for Be. The composite theoretical model incorporates ionization equilibrium and condensed-matter and multiphase physics. It also satisfies all thermodynamic equilibrium constraints. The theoretical EOS has been compared with all available high-pressure and high-temperature Be data, and satisfactory agreement is generally achieved. The most interesting feature is the theoretical prediction of melting at just below 220 GPa (2 Mb), indicating an extremely wide pressure range for solid Be. A striking feature is the appearance of shell-structure effects in physical-process paths: 2 large loops appear on the principal Hugoniot and the behavior of release isentropes from rho = rho/sub 0/ is significantly affected.
Date: September 15, 1977
Creator: Graboske, H. & Wong, L.
Partner: UNT Libraries Government Documents Department

Representation of orientation distributions

Description: This paper illustrates the principles presented with a particular experimental texture: from the surface layer of a copper polycrystal cold-rolled to 60% reduction in thickness. Four incomplete pole figures (200, 220, 222, and 113) were determined by x-ray diffraction in reflection geometry. The measured pole figures nearly exhibited orthorhombic symmetry (as expected), which was then strictly enforced by averaging the four quadrants of the pole figure. The orientation distribution function was obtained using the expansion in spherical harmonics (with only even-order coefficients up to l = 18).
Date: January 1, 1985
Creator: Wenk, H.R. & Kocks, U.F.
Partner: UNT Libraries Government Documents Department

Theory of alloy phases

Description: Various non-thermodynamic approaches to understanding and predicting phase diagrams are explored from the viewpoint of solid-state physics. The review is intended to indicate the scope of activity and some of the progress which has been made. (GHT)
Date: January 1, 1977
Creator: Watson, R.E.; Ehrenreich, H. & Bennett, L.H.
Partner: UNT Libraries Government Documents Department

Crystalline electric fields in mixed valent systems

Description: The inelastic neutron studies of rare-earth-based mixed valent systems have all shown remarkably similar results: a broad quasielastic line with half width on the order of 10 MeV. This width exhibits a strong temperature dependence in those systems which undergo a valence transition and is only weakly temperature dependent in those systems which show no transition. A surprising result was the absence of crystalline electric field (CEF) excitations. Recent measurements on the alloy Ce/sub .9-x/La/sub x/Th/sub .1/ have revealed the existence of CEF excitations. For x = 0, the valence transition is strongly first order and occurs near T/sub 0/ approx. 150 K. The inelastic spectra exhibit the typical broad quasielastic scattering. As x increases, T/sub 0/ decreases due to internal pressure effects, and a well-defined, but broad, excitation appears near E = 15 MeV. This is interpreted as a CEF excitation between the GAMMA/sub 7/ and GAMMA/sub 8/ levels of the Ce/sup 3/+ ion. For x = 0.40, the valence transition is almost completely suppressed and the excitation becomes even sharper.
Date: January 1, 1980
Creator: Shapiro, S.M.
Partner: UNT Libraries Government Documents Department

Fluorescence line-narrowing studies of rare earths in disordered solids

Description: This dissertation is made up of two experimental studies dealing with apparently diverse topics within the subject of rare earths (RE) in solids. The first study, described in Part II, concerns the vibrations of a disordered host material about an optically active rare-earth ion as manifested by vibrationally-assisted-electronic, or vibronic transitions. Part III of the dissertation describes an investigation of the influence of site anisotropy on the purely electronic, laser transition of Nd/sup 3 +/ in glass. These two studies are bound together by the common experimental technique of laser-induced fluorescence line narrowing (FLN). By exciting fluorescence with monochromatic light of well-characterized polarization, one may select and observe the response of a single subset of the optically active ions and obtain information that is usually masked by the inhomogeneous nature of disordered solids.
Date: August 10, 1982
Creator: Hall, D.W.
Partner: UNT Libraries Government Documents Department

Computer simulation study of the structure of vacancies in grain boundaries

Description: The structure of vacancies in grain boundaries has been investigated by computer molecular statics employing pairwise potentials. In order to gain an impression of the vacancy structures which may occur generally, a number of variables was investigated including: metal type, boundary type, degree of lattice coincidence and choice of boundary site. In all cases the vacancies remained as distinguishable point defects in the relatively irregular boundary structures. However, it was found that the vacancy often induced relatively large atomic displacements in the core of the boundary. These displacements often occurred only in the direct vicinity of the vacancy, but in certain cases they were widely distributed in the boundary, sometimes at surprisingly large distances.
Date: January 1, 1981
Creator: Brokman, A.; Bristowe, P.D. & Balluffi, R.W.
Partner: UNT Libraries Government Documents Department

Structure of grain boundaries in iron

Description: The present research has been carried out as part of the Steel Research Group's program to improve the properties of ultra-high- strength steels. It is an investigation of the mechanism of cohesion at grain boundaries as a step toward the design of alloy steel with improved resistance against boundary decohesion under stress corrosion conditions. This research involves a theoretical study of grain boundaries in iron alloys, examining the mechanism of cohesion as affected by structure including imperfections, by the average alloy composition and by segregation at the boundaries. This paper summarizes what we have done to date in obtaining approximate grain boundary structures. We have tested the approximate interatomic potential for iron which we have obtained by using it to compute the interplanar spacings at the surface and then comparing the computed results with experimental observations. Results are discussed. 51 refs., 9 figs., 3 tabs.
Date: January 1, 1988
Creator: Harrison, R.J.; Spaepen, F.; Voter, A.F. & Chen, Shao-Ping
Partner: UNT Libraries Government Documents Department

Crystallographic considerations of the delta in equilibrium. cap alpha. displacive transformation in plutonium alloys

Description: Determination of invariant-plane strain crystallographic solutions for martensitic transformation between the FCC delta and monoclinic ..cap alpha.. phases in plutonium alloys, using three possible lattice correspondences and 53 possible lattice-invariant shear systems, identifies the most probable delta-..cap alpha.. lattice correspondence. The operative lattice-invariant shear systems are predicted by comparison of both shape strain magnitudes and computed interfacial energies. For delta ..-->.. ..cap alpha.. transformation twinning on (001) (100)/sub ..cap alpha../ is favored, giving a (.817, .538, .208)/sub delta/ habit and a (.947, .269, .174)/sub delta/ shape strain of magnitude m/sub 1/ = .324. The ..cap alpha.. ..-->.. delta transformation favors slip on (111) (101)/sub delta/, giving a (.255, .844, .471)/sub ..cap alpha../ habit and (.822, .466, .355)/sub ..cap alpha../ shape strain of magnitude m/sub 1/ = .417.
Date: June 10, 1986
Creator: Adler, P.H. & Olson, G.B.
Partner: UNT Libraries Government Documents Department

Determination of vacancy mechanism for grain boundary self-diffusion by computer simulation

Description: It is currently well established that the fast self-diffusion which occurs along grain boundaries (GBs) in metals must occur by a point defect exchange mechanism. For example, it is known that rapid GB diffusion can transport a net current of atoms along GBs during both sintering and diffusional creep, and that the two species in a binary substitutional alloy diffuse at different rates in GBs. However, it has not been possible to establish firmly whether the defect mechanism involves the exchange of atoms with vacancy or interstitial point defects. It has been suspected that the vacancy exchange mechanism must apply but it has been difficult to prove this hypothesis because of a lack of detailed information at the atomistic level. The results are presented of an effort to establish the GB self-diffusion mechanism in a bcc iron ..sigma.. = 5 (36.9/sup 0/) (001) (310) tilt boundary using the combined methods of computer molecular statics and molecular dynamics simulation.
Date: June 1, 1981
Creator: Balluffi, R. W.; Kwok, T.; Bristowe, P. D.; Brokman, A.; Ho, P. S. & Yip, S.
Partner: UNT Libraries Government Documents Department

Atomistic studies of grain boundaries with segregated impurities. Progress report, July 1, 1980-June 30, 1981

Description: The aim of this research project is to carry out atomistic studies of grain boundaries with impurities. The emphasis of the investigation is on the effect of segregation upon the structure of grain boundaries and their properties. Since the necessary precursor of any atomistic study is a description of interatomic interactions an empirical scheme for the construction of interatomic potentials was first developed. In this scheme which was summarized in a previous report, the interatomic interactions are described by pair potentials together with a volume dependent term which represents a major contribution to the overall energy of the studied system. A new relaxation method had to be developed that includes the volume dependent term into the calculations which are then carried out at constant pressure rather than at constant volume. Furthermore, this method also inlcudes the possibility of dealing with spatially varying concentration of impurities. It is described in more detail in the report.
Date: February 1, 1981
Creator: Vitek, V.
Partner: UNT Libraries Government Documents Department

Oxygen diffusion in vanadium-based alloys

Description: The experimental study of transport and equilibrium properties of oxygen in vanadium-based alloys was made by EMF measurements on solid electrolytic cells over the temperature range of 873 to 1423/sup 0/K. The oxygen diffusion in vanadium was not significantly modified by small additions of Ti, Cr, Ni, Nb and Ta. The increase in the activation energy for oxygen diffusion in the V-based alloys containing Cr, Ni, Nb and Ta probably reflects the effect of these substitutional solutes on the activity coefficient of oxygen. The oxygen activity was increased by the addition of 1 at % of Cr, Ni and Nb, and decreased by the addition of Ti and Ta. However, the effects in the alloys containing Nb and Ta are very small.
Date: January 1, 1981
Creator: de Avillez, R.R.
Partner: UNT Libraries Government Documents Department

Ferroelectric domain structure of Pb(Zr /sub 52/Ti /sub 48/)O/sub 3/

Description: Ferroelectric domains are twins that are formed when PZT undergoes a phase transformation from a non-ferroelectric cubic phase to a ferroelectric tetragonal phase upon cooling below approx. 375/sup 0/C. The tetragonal phase is spontaneously polarized in the direction of c-axis, making each twin a ferroelectric domain.
Date: August 1, 1980
Creator: Goo, E.K. & Mishra, R.K.
Partner: UNT Libraries Government Documents Department

Electrochemical properties of small clusters of metal atoms and their role in the surface-enhanced Raman scattering

Description: Starting with equations for the shift of the reversible redox potential of small metal particles with size, the electrochemical properties of these particles are discussed. Approximate equations are given for the relationship between the particle size and the surface charge, the potential of zero charge, the surface potential, work function and quantities related to this function. The influence of these properties on redox reactions, electrosorption and chemi-sorption are discussed. The results are used to explain experimental observation in connection with the surface enhanced Raman effect.
Date: June 1, 1981
Creator: Plieth, W.J.
Partner: UNT Libraries Government Documents Department