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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

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

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

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

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

Grain boundary dislocations in plane matching grain boundaries

Description: Plane matching (PM) boundaries are defined as boundaries in which a single stack of planes of indices (hkl) in each crystal adjoining the boundary is either parallel, or nearly parallel, and where no other significant lattice matching exists. A search was made (by transmission electron microscopy) for possible intrinsic secondary grain boundary dislocation (GBD) arrays which might be present in such boundaries possessing a variety of deviations from the exact PM condition. (002), (220) and (420) PM boundaries of controlled geometry were prepared in gold thin film bicrystal specimens. Wide ranges of twist and tilt deviations from the exact PM orientation were introduced, and the orientation of the boundary plane was also varied for cases of fixed crystal misorientation. Arrays consisting of parallel GBDs were found in the (002) and (220) boundaries but not in the (420) boundaries. The Burgers vector of the GBDs was parallel to (hkl) with a magnitude given by the (hkl) interplanar spacing. Arrays were found in boundaries with twist deviations as large as 4/sup 0/ and tilt deviations as large as 14/sup 0/ and with a wide range of grain boundary plane orientations. The results suggest that discrete GBDs should be present in a variety of (002), (111) and (220) PM boundaries over a considerable range of misorientation which in certain cases may be as large as 20/sup 0/. Such boundaries should therefore appear rather frequently in general polycrystalline materials in agreement with the estimates of Warrington and Boon. It is pointed out that the physical basis for the stability of such GBD structures is not understood at present.
Date: August 1, 1979
Creator: Schindler, R.; Clemans, J.E. & Balluffi, R.W.
Partner: UNT Libraries Government Documents Department

Growth of second phase particles in a copper--beryllium alloy. Final technical report

Description: Growth of second phase particles from a solid solution of copper-beryllium was studied to determine this alloy's suitability for acoustic emission testing. Optical and Scanning Electron microscopes were used to study the microstructure. Micro and macro hardness tests were also performed. A hardness curve for aging at 550/sup 0/F was determined. Microscopic examination revealed the presence of large inclusions which make this alloy unsuitable for the acoustic tests envisioned.
Date: June 2, 1977
Creator: Bunch, R.; Wells, R. & Mukherjee, A.K.
Partner: UNT Libraries Government Documents Department

Influence of the interface on the crystallization of amorphous Ge in Pb/Ge multilayers

Description: The amorphous to crystalline phase transition of Ge in contact with Pb has been extensively investigated in Pb/Ge multilayers. We find that the Ge crystallizes at reduced temperatures in the range of 100-200/degree/C depending on the thickness of the Ge and the Pb. Simultaneously the texture of the Pb improves and the layered structure completely disappears. The basis for the reduction of the crystallization temperature is the enhanced diffusion kinetics at the interface, caused by the adjacent Pb layer. 20 refs., 4 figs.
Date: January 1, 1988
Creator: Sevenhans, W.; Locquet, J.P.; Bruynseraede, Y.; Homma, H. & Schuller, I.K.
Partner: UNT Libraries Government Documents Department

Recovery of deformed and hydrogen-charge palladium

Description: Positron lifetime and Doppler-broadening studies made at 300 K have been used to investigate the interaction between interstitial hydrogen and lattice defects in deformed Pd. Specimens were charged with hydrogen at 300 K to levels up to 0.1%. The presence of hydrogen was found to have no effect on the recovery curves of Pd upon annealing to 400/sup 0/C. By 400/sup 0/C the values for both lifetime and Doppler-broadening for both cold worked and cold worked plus hydrogen were below the values obtained for annealed pure Pd. This can be interpreted as gaseous-impurity-trapped vacancies being present after the 1200/sup 0/C anneal, but being swept away by the dislocation microstructure recovery between 200 to 400/sup 0/C.
Date: January 1, 1982
Creator: Snead, C.L. Jr.; Lynn, K.G. & Lynch, J.F.
Partner: UNT Libraries Government Documents Department

A novel phase transition in alloys

Description: The intermediate-phase of the intermetallic alloy CsPb is shown to be a plastic crystal characterized by jump reorientations of Cs{sub 4}Pb{sub 4} structural units. The wave vector variation of elastic and quasielastic intensities is well reproduced by a simple model if independent structural units jumping between the four orientations observed in the crystal at room temperature. This represents the first observation of a plastic crystal phase in a metal alloy. 17 refs., 6 figs.
Date: September 1, 1991
Creator: Saboungi, M.L. & Price, D.L.
Partner: UNT Libraries Government Documents Department

Group representation of bicrystal invariant translations with an application to the topology of secondary grain boundary dislocations

Description: All DSC-Lattice translations of one lattice with respect to the second lattice of a bicrystal are described as a group. It is shown that the matrix representation of this group can be used to solve topological problems connected with secondary grain boundary dislocations (SGBDs) such as finding the step in the boundary associated with the SGBD. We have formulated this problem by establishing the step vector S associated with the Burgers vector b of the SGBD in a cubic bicrystal. The problem is then solved in 2D, and the way to generalize to 3D is indicated.
Date: September 1, 1980
Creator: Brokman, A.
Partner: UNT Libraries Government Documents Department

Electron paramagnetic resonance at dislocations in germanium

Description: The first observation of the paramagnetic resonance of electrons at dislocations in germanium single crystals is reported. Under subband gap optical excitation, two sets of lines are detected: four lines about the <111> axes with g/sub perpendicular to/ = 0.34 and g/sub parallel/ = 1.94, and 24 lines with g/sub perpendicular to/ = 0.73 and g/sub parallel/ = 1.89 about <111> axes with the six-fold 1.2/sup 0/ distortion. This represents the first measurement of the disortion angle of a dislocation dangling bond. The possibility that the distortion results from a Peierls transition along the dislocation line is discussed.
Date: June 1, 1982
Creator: Pakulis, E.J.
Partner: UNT Libraries Government Documents Department

Internal magnetic fields in hcp-iron

Description: The magnetic behavior of hcp-Fe (epsilon iron) has been investigated by the Moessbauer Effect over a wide range of temperature T, pressure P, and applied magnetic field H/sub 0/. The internal field H/sub i/ of the induced moment is given by H/sub i/ = 0.20 +- .01 H/sub 0/ and is independent of T and P. Enhanced paramagnetism is suggested as the most likely origin for a contribution of this magnitude to H/sub i/.
Date: January 1, 1982
Creator: Taylor, R.D.; Cort, G. & Willis, J.O.
Partner: UNT Libraries Government Documents Department

Glass forming ranges of amorphous alloys

Description: Two main methods of synthesis of amorphous metallic alloys are based on the rapid solidification of molten alloys and on isothermal solid state reactions between pure metals. We discuss recent calculations for the glass forming range for both techniques and we compare the predicted homogeneity ranges of the amorphous phase with experiments. 10 refs., 2 figs.
Date: January 1, 1988
Creator: Schwarz, R.B.
Partner: UNT Libraries Government Documents Department