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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

Computer simulation of boundary effects on bubble growth in metals due to He.

Description: Atomistic simulation methods were used to investigate and identify the relevant physical mechanisms necessary to describe the growth of helium gas bubbles within a metal lattice. Specifically, molecular dynamics simulations were performed to examine the material defects that originate from growing spherical He bubbles in a palladium crystal. These simulations consist of a model system containing bubbles within a metal and near a free surface. The simulation code employed was ParaDyn using the Embedded Atom Method to model the constitutive properties of Pd atoms in a FCC lattice. The results of these simulations are compared with previously run calculations of He bubbles in a bulk lattice [l]. These simulations show the influence of the free surface on defect creation and evolution. Features compared include the formation of inter-bubble dislocations, bubble pressure and swelling as functions of He to metal (He/M) concentration.
Date: March 1, 2003
Creator: Zimmerman, Jonathan A.
Partner: UNT Libraries Government Documents Department

Numerical Modeling of the Stability of Face-Centered Cubic Metals with High Vacancy Concentration

Description: The objective of this research is to assess the possibility of forming an atomically porous structure in a low-density metal, e.g., Al with vacancies up to 0.20/lattice site; and to examine the effects of hydrogen and vacancy concentration on the stability of an atomically porous structure that has been experimentally produced in nickel. The approach involves numerical modeling using the Embedded-Atom Method (EAM). High vacancy concentrations cause the Al lattice to disorder at 300K. In contrast, Ni retains the face-centered-cubic structure at 300K for vacancy concentrations up to 0.15 Vac/lattice site. Unexpectedly, the lattice with 0.15 Vac/lattice site is more stable than the lattice with 0.10 or 0.20 Vac/lattice site. The Ni systems with 0.10 and 0.15 Vac/lattice site exhibit domains consisting of uniform lattice rotations. The Ni lattice with 0.15 Vac/lattice site is more stable with an initial distribution of random vacancies compared to ordered vacancies. The equilibrium lattice structures of Ni a d Al containing vacancies and H are less ordered to structures with vacancies only at 300K.
Date: December 1, 1998
Creator: Somerday, Brian P. & Baskes, M. I.
Partner: UNT Libraries Government Documents Department

Grain boundary dissociation by the emission of stacking faults

Description: A range of <110> symmetric tilt grain boundaries (GBs) are investigated in several fcc metals with simulations and high resolution electron microscopy. Boundaries with tilt angles between 50.5{degree} and 109.5{degree} dissociate into two boundaries 0.6 to 1.1 nm apart. The dissociation takes place by the emission of stacking faults from one boundary that are terminated by Shockley partials at a second boundary. This is a general mode of GB relaxation for low stacking fault energy metals. The reasons for the occurrence of this relaxation mode are discussed using the theory of GB dislocations.
Date: May 1995
Creator: Rittner, J. D.; Seidman, D. N. & Merkle, K. L.
Partner: UNT Libraries Government Documents Department


Description: Microstructural characterization of plutonium by electron backscattered diffraction (EBSD) has been previously elusive primarily because of the extreme toxicity and surface oxidation rates associated with plutonium metal. In this work, initial electron backscattered diffraction pattern (EBSP) observations of a plutonium-gallium (Pu-Ga) alloy were made. Samples were prepared using standard metallographic and electropolishing techniques that were performed inside gloveboxes and/or an open front hood to prevent spread of radioactive contamination. A scanning Auger microprobe (SAM), equipped with an ion-gun, was used to characterize and remove surface chemical impurities (in particular carbon (C) and oxygen (O)) and a specially designed vacuum transfer device was used to minimize oxidation during the sample transfer from the SAM to the scanning electron microscope (SEM). EBSD patterns of the {delta}-phase (face-centered-cubic) were captured and the experimental techniques and parameters used to perform EBSD characterization are described in detail.
Date: June 1, 2001
Creator: BOEHLERT, C. J.
Partner: UNT Libraries Government Documents Department

Coupled finite element-Monte Carlo simulation of microstructure and texture evolution during thermomechanical processing

Description: A novel simulation technique for predicting the microstructure and texture evolution during thermomechanical processing is presented. The technique involves coupling a finite element microstructural deformation model based on crystal plasticity with a Monte Carlo simulation of recovery and recrystallization. The finite element model captures the stored energy and the crystallographic orientation distributions in the deformed microstructure. The Monte Carlo simulation captures the microstructural evolution associated with recovery and recrystallization. A unique feature of the Monte Carlo simulation is that it treats recrystallization as a heterogeneous subgrain growth process, thus providing the natural link between nucleation and growth phenomena, and quantifying the role of recovery in these phenomena. Different nucleation mechanisms based on heterogeneous subgrain growth as well as strain induced boundary migration are automatically included in the recrystallization simulation. The simulations are shown to account for the extent of prior deformation on the microstructure and kinetics of recrystallization during subsequent annealing. The simulations also capture the influence of the presence of cube orientations in the initial microstructure, and the operation of non-octahedral slip during deformation of fcc polycrystals, on the recrystallization texture.
Date: November 1, 1998
Creator: Radhakrishnan, B.; Sarma, G. & Zacharia, T.
Partner: UNT Libraries Government Documents Department

Atom-by-Atom and Concerted Hopping of Adatom Pairs on an Open Metal Surface

Description: Atom-by-atom and concerted hopping of ad-dimers on the open (100) surface of fcc metals are studied by means of density-functional calculations. The adatom interaction is relatively short-ranged, and beyond next-nearest neighbors ad-dimers are effectively dissociated. Diffusion takes place by a simple shearing process, favored because it maximizes adatom coordination at the transition state This holds for Al, Au, and Rh, and is likely a general result because geometrical arguments dominate over details of the electronic structure.
Date: August 25, 1999
Creator: Bogicevic, A.; Ovesson, S.; Lundqvist, B.I. & Jennison, D.R.
Partner: UNT Libraries Government Documents Department

Investigation of Copper Segregation to the S5(310)/[001] Symmetric Tilt Grain Boundary (STGB) in Aluminum

Description: The {Sigma}5 (310)/[001] symmetric tilt grain boundary (STGB) in the face centered cubic (FCC) metal aluminum with 1at% copper has been studied. The model grain boundary has been fabricated by ultra-high vacuum diffusion bonding of alloy single crystals. The segregation of the copper has been encouraged by annealing the sample after bonding at 200 C. TEM samples of this FCC-material were prepared with a new low voltage ion mill under very low angles. The atomic structure of the {Sigma}5 (310)/[001] STGB for this system was modeled with electronic structure calculations. These theoretical calculations of this interface structure indicate that the Cu atoms segregate to distinct sites at the interface. High resolution electron microscopy (HRTEM) and analytical electron microscopy including electron energy spectroscopic imaging and X-ray energy dispersive spectrometry have been used to explore the segregation to the grain boundary. The HRTEM images and the analytical measurements were performed using different kinds of microscopes, including a Philips CM300 FEG equipped with an imaging energy filter. The amount of the segregated species at the interface was quantified in a preliminary way. To determine the atomic positions of the segregated atoms at the interface, HRTEM coupled with image simulation and a first attempt of a holographic reconstruction from a through-focal series have been used.
Date: November 15, 1999
Creator: Plitzko, J.M.; Campbell, G.H.; King, W.E. & Foiles, S.M.
Partner: UNT Libraries Government Documents Department

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.; Chen, H. & Anderson, J. P.
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

Point defects in metals

Description: A review is presented of the knowledge of point defects in pure metals. Data are tabulated for vacancies and divacancies in fcc metals, and for defects in bcc metals. Production of Frenkel defects by irradiation is also discussed; data are tabulated for both fcc and bcc metals. Questions that need answering are posed. (16 references). (DLC)
Date: January 1, 1973
Creator: Koehler, J.S.
Partner: UNT Libraries Government Documents Department

Transmission electron microscopy study of cascade collapse in copper during in-situ ion-irradiation at elevated temperatures.

Description: The basic mechanisms driving the collapse of point defects produced in collision cascades are investigated by transmission electron microscope (TEM) characterization of defect microstructure produced in fcc-Cu irradiated with low-fluences of heavy (100 keV Kr) ions at elevated temperature (23--600 C). Areal defect yields are determined from direct TEM observation of the total defect production integrated over the duration of the in-situ ion-irradiation. They are unequivocally demonstrated to decrease with increasing lattice temperature. This decrease in defect yield indicates a proportional decrease in the probability of collapse of cascade regions into defects of size where visible contrast is produced in a TEM.
Date: January 29, 1998
Creator: Daulton, T. L.; Kirk, M. A. & Rehn, L. E.
Partner: UNT Libraries Government Documents Department

Phonon frequencies and elastic constants of cubic Pu from electronic structure theory

Description: The phonon frequencies and elastic constants of plutonium are calculated using a model for the electronic structure that treats the valance electrons as a pseudopotential and the f-electrons in tight-binding theory. An effective interaction between ions is presented with electron screening treated in the Thomas-Fermi approximation and the f-electrons contributing bonding and repulsive terms to the potential. The phonon frequencies and elastic constants are calculated using the face-centered cubic lattice structure for both the {alpha}-, and {delta}-phases of Pu. The electronic structure predicts the qualitative behavior of the elastic constants and the transverse branches of the phonon dispersion curves in agreement with experimental values of the elastic constants for B-phase Pu.
Date: November 1, 1996
Creator: Straub, G.K.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of nanophase face-centered-cubic titanium

Description: Unalloyed Ti is sputter deposited in the formation of two trilayer films. Each layer within the combinations of Ni-Ti-Ni and Ti-Ni-Ti is less than 0.1 {mu}m thick. High resolution imaging and electron diffraction results are presented for the microstructural characterization of both trilayer films. Nanophase fcc Ti is initially found in crystalline layers grown on Ni whereas hcp Ti is found in layers grown without a Ni epilayer. Conditions are further examined under which the fcc to hcp transition occurs for Ti deposited on Ni.
Date: February 1, 1995
Creator: Jankowski, A.F. & Wall, M.A.
Partner: UNT Libraries Government Documents Department

Diffusion and dissociation of two-dimensional islands on FCC metal (100) surfaces

Description: The mobility and the stability of small two-dimensional islands on a substrate are basic issues of surface science and thin-film growth. In this article, the authors present the main results from a series of theoretical studies of island diffusion and dissociation on several fcc metal (100) surfaces, with and without the effects of hydrogen as surface impurities. They found that a collective atomic process, shearing of a dimer belonging to a compact island, in many cases provides the most effective pathway for island diffusion. Consideration of this novel atomic process leads to the possibility of observing a new set of critical island sizes in dynamical island growth or coarsening. When H is introduced into the Ni system, the mobility of adatoms and islands of all sizes are enhanced. This conclusion suggests that H will function as an anti-surfactant in Ni(100) homoepitaxy.
Date: September 1, 1996
Creator: Zhang, Z. Y.; Shi, Z. P. & Haug, K.
Partner: UNT Libraries Government Documents Department

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

Atomic structure of [110] tilt grain boundaries in FCC materials

Description: High-resolution electron microscopy (HREM) has been used to study the atomic-scale structure and localized relaxations at grain boundaries (GBs) in Au, Al, and MgO. The [110] tilt GBs play an important role in polycrystalline fcc metals since among all of the possible GB geometries this series of misorientations as a whole contains the lowest energies, including among others the two lowest energy GBs, the (111) and (113) twins. Therefore, studies of the atomic-scale structure of [110] tilt GBs in fcc metals and systematic investigations of their dependence on misorientation and GB plane is of considerable importance to materials science. [110] tilt GBs in ceramic oxides of the fcc structure are also of considerable interest, since in this misorientation range polar GBs exist, i.e. GBs in which crystallographic planes that are made up of complete layers of cations or anions can join to form a GB.
Date: April 1, 1997
Creator: Merkle, K.L. & Thompson, L.J.
Partner: UNT Libraries Government Documents Department

A computer simulation of an extended dislocation barrier in a FCC crystal

Description: A computer simulation has been performed to examine the formation and breakdown of various extended dislocation barriers in a crystal of atomistically simulated aluminum. Under an appropriate stress tensor an extended mixed dislocation was found to decompose by one of its partial dislocations dissociating into a 1/6[110] type stairrod dislocation and a glissile partial on an intersecting glide plane. The mechanism is shown to provide an efficient way to generate stacking faults, can be relevant to twin formation, and provides a stable barrier to dislocation motion.
Date: November 1, 1995
Creator: Kurtz, R.J.; Hoagland, R.G. & Hirth, J.P.
Partner: UNT Libraries Government Documents Department

Fully relativistic surface green function and its application to surface spectroscopies

Description: A fully relativistic layer-KKR formalism was developed and implemented for calculating the single-particle Green function in atomic layers parallel to crystalline surfaces magnetic and non magnetic materials: The method was applied to the calculation surface spectroscopies, such as low energy electron diffraction (LEED), angle-resolve ultraviolet photo emission spectroscopy (UPS), and photoelectron scattering. Numeric tests were performed for non magnetic actinide surfaces and magnetic Fe surface Theoretical angle-resolved UPS spectra are presented for uranium monolayers on Pt(111) and for f.c.c. u(lll) surfaces. We find that u island formation can take place if a peak in the UPS spectra appears just before the Fermi energy immediately as u is deposited on P and we suggest an experimental procedure for testing this prediction. An intensity map photo excited electrons from the 2p{sub 3/2} core states of Fe(110) surface is also show Sizable magnetic anisotropy is found due to the interference between exchange and spin-orbit interaction, which is suitable for studying a possible surface-induced magnetism actinide adlayers.
Date: June 30, 1993
Creator: Tamura, E.
Partner: UNT Libraries Government Documents Department

Atomic-level studies of cluster diffusion on metal surfaces

Description: Direct observations of atomic motion with the field ion microscope (FIM) are providing detailed information on the mechanisms and energetics by which small clusters migrate across metal surfaces. An important result to emerge from these studies is that the activation energies of surface diffusion for small clusters on fcc(100) surfaces are strongly correlated with their shape. For Rh clusters on Rh(100) this correlation leads to an oscillatory behavior in the cluster mobility as a function of cluster size. For Pt on Rh(100) the activation energy is constant as clusters increase in size from three to five atoms and is also correlated with shape. The atomic-level mechanism involved in cluster diffusion on fcc(100) surfaces is inferred from a comparison of the measured activation energies to previous theoretical calculations.
Date: December 1, 1995
Creator: Kellogg, G.L.
Partner: UNT Libraries Government Documents Department

Observation of an elementary cuboctahedron of Xe nanocrystal in an Al matrix.

Description: When a noble gas element such as Xe is implanted in an fcc metal matrix such as Al at room temperature, a fine dispersion of precipitates forms. The precipitates are elementary fcc crystals up to diameters of several nanometers (for Xe in Al, 8-10 rim), above which they are non-crystalline. The precipitates exhibit a cube-on-cube orientation relation with the matrices and have lattice parameters which are much larger than those of the matrices (a{sub Xe} = 1.5a{sub Al}). Thus the interphase interfaces are incommensurate though the lattices are isotactic. The precipitates assume the shape of matrix cavities; for an Al matrix, at equilibrium this is a cuboctahedron, a {r_brace}111{l_brace} octahedron truncated at the corners on {l_brace}100{r_brace}. Fig. 1 is a sketch of a dispersion of such cuboctahedra, viewed approximately along a {l_angle}110{r_angle}. For this study specimens were prepared in the HVEM-Tandem Facility at Argonne National Laboratory by implanting 35 keV Xe to a dose of 4x10{sup 19} m{sup {minus}2} into well-annealed 5N Al discs which had been thinned by jet electropolishing. The range of the implant is approximately 25 nm. Specimens were examined at high resolution in the JEOL ARM-1000 high voltage electron microscope (HVEM) at the High Resolution Beam Station of the National Research Institute for Metals (NRIM), Tsukuba, Japan. The HVEM was operated at 1 MeV with a LaB{sub 6} electron source. A series of studies of electron irradiation effects in this material have been conducted, which have revealed a number of irradiation-induced phenomena including migration within the matrix, changes in shape, faulting, melting, crystallization and coalescence of Xe precipitates. In this presentation, the structure of the smallest possible cuboctahedral Xe nanocrystal will be discussed and its apparently random migration under the influence of the electron irradiation will be demonstrated.
Date: March 10, 1999
Creator: Allen, C. W.; Birtcher, R. C.; Donnelly, S. E.; Furuya, K.; Ryan, E. A. & Song, M.
Partner: UNT Libraries Government Documents Department

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

Reconstruction of a high-angle twist grain boundary by grand-canonical simulated quenching

Description: A formalism for obtaining the zero-temperature structure of mono- component solids by simulated quenching in the grand-canonical ensemble is outlined. The structure of a high-angle twist grain boundary on the (110) plane of an fcc metal is investigated. The lowest energy structure is found to be a twinned structure with a density approximately 20% higher than the structure obtained from canonical-ensemble energy-minimization.
Date: November 1, 1991
Creator: Phillpot, S. R. & Rickman, J. M.
Partner: UNT Libraries Government Documents Department