Structure of Grain Boundaries in L12 Alloys at Finite Temperatures: Effects of Deviations from Stoichiometry

1991 ◽  
Vol 238 ◽  
Author(s):  
M. Yan ◽  
V. Vitek ◽  
G. J. Ackland
Keyword(s):  
Monte Carlo ◽  
Temperature Dependence ◽  
Grain Boundaries ◽  
Low Temperatures ◽  
Boundary Region ◽  
Many Body ◽  
Atomic Structures ◽  
Intergranular Brittleness ◽  
Significant Chemical ◽  
Very High

ABSTRACTAtomic structures of grain boundaries in Ni3Al and Cu3Au has been studied using a Monte Carlo relaxation technique and many body central force potentials to describe interatomic forces. The purpose of this work is to investigate the temperature dependence of the compositional order in the boundary region for both stoichiometric and non-stoichiometric alloys. In the former case the boundaries in Ni3Al remain ordered up to very high temperatures while boundaries in Ni3Al disorder at relatively low temperatures. In the non-stoichiometric case, the surplus of nickel in the bulk leads to segregation to grain boundaries and to a significant chemical disordering in the boundary region. The surplus of aluminum also leads to segregation but without disordering in the boundary region. Using these results we discuss the intrinsic brittleness of grain boundaries in Ni3Al as well as the effect of stoichiometry on the intergranular brittleness.

scholarly journals Effect of Ordering Energy on Grain Boundary Structure in L12 Alloys

1988 ◽  
Vol 133 ◽  
Author(s):  
G. J. Ackland ◽  
V. Vitek
Keyword(s):  
Grain Boundaries ◽  
Tight Binding ◽  
Structural Features ◽  
Boundary Structure ◽  
Many Body ◽  
Atomic Structures ◽  
Empirical Potentials ◽  
Grain Boundary Structure ◽  
Moment Approximation ◽  
Ordered Alloys

ABSTRACTAtomic structures of grain boundaries in two L12 ordered alloys with very different ordering energies have been calculated. The interatomic forces are represented by many-body empirical potentials based on the second moment approximation to the tight-binding density of states. Grain boundaries in strongly ordered alloys show very little relaxation. This results in the presence of columns of cavities in the boundaries. On the other hand boundaries in weakly ordered alloys, and also in pure f.c.c. materials, show extensive relaxations leading to much more homogeneous structures. We suggest that these structural features may be responsible for the intrinsic brittleness of grain boundaries in compounds such as Ni3Al in contrast with boundaries in pure f.c.c. metals or ordered alloys such as Cu3Au which are only embrittled by segregation.

Theoretical analysis of tilt grain boundaries in GaN at the atomic scale

2002 ◽  
Vol 743 ◽  
Author(s):  
Jun Chen ◽  
Pierre Ruterana ◽  
Gérard Nouet
Keyword(s):  
Grain Boundaries ◽  
Rotation Angle ◽  
The Other ◽  
Atomic Scale ◽  
Threading Dislocations ◽  
High Angle ◽  
Structural Units ◽  
Atomic Structures ◽  
Very High ◽  
Weber Potential

ABSTRACTEpitaxial layers of GaN contain a very high density of threading dislocations. In the first stage of growth they may form low and high angle grain boundaries. Energetic calculations of <0001> tilt grain boundaries have been performed with the Stillinger-Weber potential modified to take into account the wrong bonds Ga-Ga and N-N. The variation of the energy has been calculated as a function of the rotation angle. Two minima exist with special atomic structures based on a limited number of structural units. They are used to describe the other misorientations in terms of dislocation cores.

Monte Carlo Algorithms for Moments of Transition Arrays

1988 ◽  
Vol 102 ◽  
pp. 79-81
Author(s):  
A. Goldberg ◽  
S.D. Bloom
Keyword(s):  
Monte Carlo ◽  
State Vector ◽  
Basis Vector ◽  
Collective State ◽  
Dispersion Characteristics ◽  
Dipole Strength ◽  
The Third ◽  
Monte Carlo Algorithms ◽  
Third Moment ◽  
Very High

AbstractClosed expressions for the first, second, and (in some cases) the third moment of atomic transition arrays now exist. Recently a method has been developed for getting to very high moments (up to the 12th and beyond) in cases where a “collective” state-vector (i.e. a state-vector containing the entire electric dipole strength) can be created from each eigenstate in the parent configuration. Both of these approaches give exact results. Herein we describe astatistical(or Monte Carlo) approach which requires onlyonerepresentative state-vector |RV&gt; for the entire parent manifold to get estimates of transition moments of high order. The representation is achieved through the random amplitudes associated with each basis vector making up |RV&gt;. This also gives rise to the dispersion characterizing the method, which has been applied to a system (in the M shell) with≈250,000 lines where we have calculated up to the 5th moment. It turns out that the dispersion in the moments decreases with the size of the manifold, making its application to very big systems statistically advantageous. A discussion of the method and these dispersion characteristics will be presented.

The Effect of Specimen Thickness and Probe Size on Aem Analysis of Cr-Depletion Profiles in Sensitized Stainless Steel

1982 ◽  
Vol 40 ◽  
pp. 518-519
Author(s):  
E. L. Hall
Keyword(s):  
Grain Boundaries ◽  
Boundary Region ◽  
Chromium Concentration ◽  
Foil Thickness ◽  
Specimen Thickness ◽  
Probe Size ◽  
Analytical Electron ◽  
Analytical Electron Microscope ◽  
Sensitization Process ◽  
Compositional Gradients

Sensitization in stainless steels is caused by the formation of chromium-rich M23C6 carbides at grain boundaries, which depletes the adjacent matrix and boundary region of chromium, and hence leads to rapid intergranular attack. To fully understand the sensitization process, and to test the accuracy of theories proposed to model this process, it is necessary to obtain very accurate measurements of the chromium concentration at grain boundaries in sensitized specimens. Quantitative X-ray spectroscopy in the analytical electron microscope (AEM) enables the chromium concentration profile across these boundaries to be studied directly; however, it has been shown that a strong effect of foil thickness and electron probe size may be present in the analysis of rapidly-changing compositional gradients. The goal of this work is to examine these effects.

Finding the Right Problems: Some HREM Applications to Interfaces

1984 ◽  
Vol 42 ◽  
pp. 376-379
Author(s):  
D. Cherns
Keyword(s):  
Grain Boundaries ◽  
High Resolution Electron Microscopy ◽  
Boundary Structure ◽  
Atomic Structures ◽  
Grain Boundary Structure ◽  
Resolution Electron ◽  
Point To Point ◽  
The Right ◽  
New Generation ◽  
Better Than

The use of high resolution electron microscopy (HREM) to determine the atomic structure of grain boundaries and interfaces is a topic of great current interest. Grain boundary structure has been considered for many years as central to an understanding of the mechanical and transport properties of materials. Some more recent attention has focussed on the atomic structures of metalsemiconductor interfaces which are believed to control electrical properties of contacts. The atomic structures of interfaces in semiconductor or metal multilayers is an area of growing interest for understanding the unusual electrical or mechanical properties which these new materials possess. However, although the point-to-point resolutions of currently available HREMs, ∼2-3Å, appear sufficient to solve many of these problems, few atomic models of grain boundaries and interfaces have been derived. Moreover, with a new generation of 300-400kV instruments promising resolutions in the 1.6-2.0 Å range, and resolutions better than 1.5Å expected from specialist instruments, it is an appropriate time to consider the usefulness of HREM for interface studies.

TEMPERATURE DEPENDENCE BEHAVIOR OF ELECTRICAL RESISTIVITY IN NOBLE METALS AT LOW TEMPERATURES

2014 ◽  
Vol 5 (3) ◽  
pp. 982-992 ◽  
Author(s):  
M AL-Jalali
Keyword(s):  
Experimental Data ◽  
Electrical Resistivity ◽  
Temperature Dependence ◽  
Concentration Dependence ◽  
Low Temperatures ◽  
Noble Metals ◽  
Phonon Scattering ◽  
Theoretical Models ◽  
Residual Resistivity ◽  
Electron Phonon Scattering

Resistivity temperature – dependence and residual resistivity concentration-dependence in pure noble metals(Cu, Ag, Au) have been studied at low temperatures. Dominations of electron – dislocation and impurity, electron-electron, and electron-phonon scattering were analyzed, contribution of these mechanisms to resistivity were discussed, taking into consideration existing theoretical models and available experimental data, where some new results and ideas were investigated.

THE ANXIETY DISORDER IN HIGH SCHOOL STUDENT

2017 ◽  
Vol 4 (23) ◽  
Author(s):  
Jagdish Rathod
Keyword(s):  
High School ◽  
High School Students ◽  
Anxiety Disorder ◽  
High School Student ◽  
Growth And Development ◽  
Social Problems ◽  
Educational Development ◽  
Many Body ◽  
School Students ◽  
Very High

Stress situations such as study tight deadlines or important social obligations. Of fen makes nervous or fearful. In high school students they facing educational and social problems. This period is already growth and development period at this time so many body changes occurs in the students body. Individuals with anxiety disorder experience excessive anxiety. Fear as worry, causing them either to anode situations. The result of survey on the basis of anxiety disorder in high school students. Is very high in ahmednagar tarakpur in some area. They need special treatments for educational development.

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