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Articles 91 - 104 of 104
Full-Text Articles in Physics
Coincidence Orientations Of Crystals In Tetragonal Systems, With Applications To Yba2cu3o7, Abha Singh, N. Chandrasekhar, Alexander H. King
Coincidence Orientations Of Crystals In Tetragonal Systems, With Applications To Yba2cu3o7, Abha Singh, N. Chandrasekhar, Alexander H. King
Alexander H. King
We have developed a method for the characterization of coincidence-site lattices (CSL's) in tetragonal or near-tetragonal orthorhombic structures, by suitable modifications to the method of Grimmer & Warrington [Acta Cryst. (1987), A43, 232-243]. We have applied our method to determine coincidence rotations and the associated information appropriate for forming constrained CSL's in the high-To superconductor YBazCu307-n. The unit cell is orthorhombic with lattice parameters a = 3.82, b = 3"89 and c = 11.67 A for the nominal composition. We present tables of coincidence rotation angles, .Z, CSL, DSCL and associated step vectors up to ,~ = 50. We find …
Nife Invar Alloys: Theoretical Insights Into The Underlying Mechanisms Responsible For Their Physical Properties, Duane D. Johnson, F. J. Pinski, J. B. Staunton, B. L. Gyorffy, G. M. Stocks
Nife Invar Alloys: Theoretical Insights Into The Underlying Mechanisms Responsible For Their Physical Properties, Duane D. Johnson, F. J. Pinski, J. B. Staunton, B. L. Gyorffy, G. M. Stocks
Duane D. Johnson
What are the "driving forces" responsible for various physical properties of alloys? What causes alloy systems to order chemically and/or magnetically? To answer these questions, we have been using a quantum mechanical (QM) method to calculate cohesive energies, magnetic properties, and thermodynamic phase instability of alloys. This scheme directly incorporates the inherent disorder of the high-temperature solid solution and in this sense goes beyond traditional "band theory." Although approximations have been made at various steps in the thermodynamical averaging, and in the QM treatment of the electrons, adjustable parameters have not been used. Recently, for FCC NiFe (INVAR) alloys, we …
Modified Broyden’S Method For Accelerating Convergence In Self-Consistent Calculations, Duane D. Johnson
Modified Broyden’S Method For Accelerating Convergence In Self-Consistent Calculations, Duane D. Johnson
Duane D. Johnson
A modification to Broyden’s method for obtaining stable and computationally efficient convergence in self-consistent calculations is developed and discussed. The method incorporates the advantages of two schemes proposed by Srivastava and by Vanderbilt and Louie without any increase in complexity. Its improvement over their methods is discussed. The present method is compared with two other widely used convergence methods, simple mixing and Anderson’s method, for the case of the disordered binary alloy Ni0.35Fe0.65 on the verge of a magnetic instability and is shown to be much improved in stability and rate of convergence.
Slip, Twinning, And Fracture At A Grain Boundary In The L12 Ordered Structure—Aς = 9 Tilt Boundary, Alexander H. King, M. H. Yoo
Slip, Twinning, And Fracture At A Grain Boundary In The L12 Ordered Structure—Aς = 9 Tilt Boundary, Alexander H. King, M. H. Yoo
Alexander H. King
The role of interaction between slip dislocations and a Σ = 9 tilt boundary in localized microplastic deformation, cleavage, or intergranular fracture in the Li2 ordered structure has been analyzed by using the anisotropic elasticity theory of dislocations and fracture. Screw superpartials cross slip easily at the boundary onto the (1$\overline 1$1) and the (001) planes at low and high temperatures, respectively. Transmission of primary slip dislocations onto the conjugate slip system occurs with a certain degree of difficulty, which is eased by localized disordering. When the transmission is impeded, cleavage fracture on the ($\overline 1$11) plane is predicted to …
Effects Of Chemical And Magnetic Disorder In Fe0.50mn0.50, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Effects Of Chemical And Magnetic Disorder In Fe0.50mn0.50, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Duane D. Johnson
We present the results of first‐principles calculations of the total energy and spin‐polarized electronic structure of disordered fcc Fe0.50Mn0.50. These self‐consistent calculations were performed using the Korringa–Kohn–Rostoker method and the coherent‐potential approximation (KKR‐CPA), using the local‐spin‐density approximation to treat exchange and correlation. We use a lattice constant of a=6.80 a.u.; information about stability can be inferred from calculated pressure and energy. The moments on the Fe and Mn sites are large but antiparallel to each other, resulting in a small net magnetization. A comparison of the electronic structure is made for fcc Fe, Mn, and FeMn in both the ferromagnetic …
Theoretical Study Of The Hyperfine Fields Of Ni And Fe In Fcc Ni,Fe, Alloys, H. Ebert, H. Winter, B. L. Gyorffy, Duane D. Johnson, F. J. Pinski
Theoretical Study Of The Hyperfine Fields Of Ni And Fe In Fcc Ni,Fe, Alloys, H. Ebert, H. Winter, B. L. Gyorffy, Duane D. Johnson, F. J. Pinski
Duane D. Johnson
Using charge self-consistent cluster Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band-structure calculations, the authors have studied the hyperfine fields on both Ni and Fe sites in the NixFe1-x alloys. To elucidate the influence of relativistic effects the hyperfine matrix elements have been calculated nonrelativistically as well as completely relativistically The dependence of the hyperfine fields on the environment of a given site has been studied by performing calculations for various atomic configurations around that site. The results of the work are compared with other theoretical data and with experiment.
Theoretical Study Of The Environmental Effects On The Hyperfine Fields Of Ni And Fe In Ni0.75fe0.25, H. Ebert, H. Winter, B. Gyorffy, Duane D. Johnson, F. J. Pinski
Theoretical Study Of The Environmental Effects On The Hyperfine Fields Of Ni And Fe In Ni0.75fe0.25, H. Ebert, H. Winter, B. Gyorffy, Duane D. Johnson, F. J. Pinski
Duane D. Johnson
The dependence of the hyperfine fields Bhf of Ni and Fe in Ni0.75Fe0.25 on the surrounding atomic configuration has been studied by performing charge selfconsistent 6 shell-cluster Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) bandstructure calculations. By replacing the CPA- scatterers in the various shells around the central atom by Ni - or Fe-atoms, respectively, it could be shown that the hyperfine fields vary linearly with the number of Fe-atoms within a given shell and that the changes of Bhf due to simultaneous changes of the atomic configurations of different shells are additive. The changes of the hyperfine fields upon ordering of …
The Further Geometry Of Grain Boundaries In Hexagonal Close-Packed Metals, Fu-Rong Chen, Alexander H. King
The Further Geometry Of Grain Boundaries In Hexagonal Close-Packed Metals, Fu-Rong Chen, Alexander H. King
Alexander H. King
A technique is given for finding partial DSC vectors appropriate to crystals with more than one atom per lattice site. The DSC lattice is made up of vectors that represent displacements of one crystal with respect to the other that leave the boundary structure shifted, but not complete. A new, rapid method for finding the step vectors associated with perfect DSC dislocations is described. Partial DSC vectors and step vectors for perfect DSC dislocations in hexagonal close-packed crystals are determined. The availability of reactions between lattice partial dislocations and grain boundaries in hexagonal close-packed crystals is also assessed.
Interaction Between Magnetic And Compositional Order In Ni‐Rich Ni C Fe1−C Alloys (Invited), J. B. Staunton, Duane D. Johnson, B. L. Gyorffy
Interaction Between Magnetic And Compositional Order In Ni‐Rich Ni C Fe1−C Alloys (Invited), J. B. Staunton, Duane D. Johnson, B. L. Gyorffy
Duane D. Johnson
We have developed a first‐principles electronic theory of concentration fluctuations in spin polarized binary alloys. It is a mean field theory of the state of compositional order and it is based on the local spin density (LSD) approximation for describing the electrons. The usual averages over the statistical mechanical ensemble are carried out with the aid of the self‐consistent Korringe–Kohn–Rostoker coherent‐potential approximation (SCF‐KKR‐CPA). To illustrate the main consequences of the theory we study the compositional short‐range order in the Ni c Fe1−c alloy system. We find that the ordering energy is almost entirely of magnetic origin.
Ferromagnetism Versus Antiferromagnetism In Face-Centered-Cubic Iron, F. J. Pinski, J. Staunton, B. L. Gyorffy, Duane D. Johnson, G. M. Stocks
Ferromagnetism Versus Antiferromagnetism In Face-Centered-Cubic Iron, F. J. Pinski, J. Staunton, B. L. Gyorffy, Duane D. Johnson, G. M. Stocks
Duane D. Johnson
Using a first-principles disordered-local-moment picture of itinerant-electron magnetism, we calculated the temperature and volume dependence of the magnetic moment and spin-spin correlations for fcc Fe in the paramagnetic state.
Density-Functional Theory For Random Alloys: Total Energy Within The Coherent-Potential Approximation, Duane D. Johnson, D. M. Nicholson, F. J. Pinski, B. L. Gyorffy, G. M. Stocks
Density-Functional Theory For Random Alloys: Total Energy Within The Coherent-Potential Approximation, Duane D. Johnson, D. M. Nicholson, F. J. Pinski, B. L. Gyorffy, G. M. Stocks
Duane D. Johnson
A density-functional-based theory is developed for calculation of the total energy and pressure of random substitutional alloys within the Korringa-Kohn-Rostoker coherent-potential approximation. The theory is used to calculate the concentration variation of the equilibrium lattice spacing of α-phase CucZn1−c alloys. We find, in agreement with experiment, that the variation is almost linear and that it deviates from Vegard's rule.
Fast Method For Calculating The Self-Consistent Electronic Structure Of Random Alloys, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Fast Method For Calculating The Self-Consistent Electronic Structure Of Random Alloys, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Duane D. Johnson
We describe a computationally efficient method for performing self-consistent-field calculations of the electronic structure of random solid-solution alloys within the multiple-scattering coherent-potential-approximation formalism (KKR-CPA). We utilize a cluster method for solving the KKR-CPA equations and integrate along a contour in the complex-energy plane to reconstruct the single-site electronic densities.
The Effects On Grain-Boundary Processes Of The Steps In The Boundary Plane Associated With The Cores Of Grain-Boundary Dislocations, Alexander H. King, D. A. Smith
The Effects On Grain-Boundary Processes Of The Steps In The Boundary Plane Associated With The Cores Of Grain-Boundary Dislocations, Alexander H. King, D. A. Smith
Alexander H. King
The contribution of the associated steps to the properties of grain-boundary dislocations are investigated. Two methods of step-height determination are given and it is shown that the step height, like the Burgers vector, must be conserved during the reactions of grain-boundary dislocations: this is not always automatic and additional coherent steps must take part in some reactions. The energies associated with steps have been estimated and compared with the elastic energies of grain-boundary dislocations; these comparisons suggest that (a) the reduction of elastic energy alone may not be a sufficient criterion for the progress of a reaction and (b) that …
Self‐Consistent Electronic Structure Of Disordered Fe0.65ni0.35, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Self‐Consistent Electronic Structure Of Disordered Fe0.65ni0.35, Duane D. Johnson, F. J. Pinski, G. M. Stocks
Duane D. Johnson
We present the results of the first a b i n i t i o calculation of the electronic structure of the disordered alloy Fe0.65Ni0.35. The calculation is based on the multiple‐scattering coherent‐potential approach (KKR‐CPA) and is fully self‐consistent and spin polarized. Magnetic effects are included within local‐spin‐density functional theory using the exchange‐correlation function of Vosko–Wilk–Nusair. The most striking feature of the calculation is that electrons of different spins experience different degrees of disorder. The minority spin electrons see a very large disorder, whereas the majority spin electrons see little disorder. Consequently, the minority spin density of states is smooth …