Concentration of point defects and site occupancy behavior in ternary NiAl alloys

2004 ◽  
Vol 365 (1-2) ◽  
pp. 85-89 ◽  
Author(s):  
Y.L Hao ◽  
R Yang ◽  
Y Song ◽  
Y.Y Cui ◽  
D Li ◽  
...  
Keyword(s):  
Point Defects ◽  
Site Occupancy ◽  
Occupancy Behavior

Atom location by channeling enhanced microanalysis study of site distributions of alloying elements in titanium aluminides

2000 ◽  
Vol 15 (11) ◽  
pp. 2475-2481 ◽  
Author(s):  
Y. L. Hao ◽  
R. Yang ◽  
Y. Y. Cui ◽  
D. Li
Keyword(s):  
Transition Metal ◽  
Quantitative Determination ◽  
Point Defects ◽  
Titanium Aluminides ◽  
Site Occupancy ◽  
The Other ◽  
Planar Channeling ◽  
Antisite Defects ◽  
Metal Aluminides

The original equation of Spence and Taftø for quantitative determination of site occupancy using atom location by channeling enhanced microanalysis method has been extended to take into account both delocalization effect and the influence of point defects (antisite atomic distribution and vacancies). The outcome of this treatment suggests that, for crystals free of antisite defects, the accuracy of site occupancy is influenced by delocalization effect but is independent of both thermal and structural vacancies. For crystals free of structural vacancies, the accuracy of site occupancy is influenced by both delocalization effect and antisite defects, but is independent of thermal vacancies. The delocalization effect was shown to vary with channeling strength at a given channeling condition. For a binary ordered phase in which at least one of the host elements exhibits weak delocalization effect (as is the case for many transition-metal aluminides), a tangent-line method for obtaining the value of k (a parameter necessary for the calculation of site occupancy) was proposed, allowing the determination of the delocalization correction factor for the other host element. Application of this method to estimating the delocalization effect of Al in Ti3Al and TiAl under axial and planar channeling conditions, respectively, was demonstrated.

scholarly journals Site occupancy of transition elements in C15 NbCr2 laves phase: A first-principles study

2017 ◽  
Vol 53 (1) ◽  
pp. 13-18 ◽  
Author(s):  
Q. Long ◽  
J. Wang ◽  
Y. Du ◽  
X. Nie ◽  
Z. Jin
Keyword(s):  
First Principles ◽  
Site Occupancy ◽  
Laves Phase ◽  
Site Preference ◽  
First Principles Calculations ◽  
Transition Elements ◽  
First Principles Study ◽  
Weak Site ◽  
Occupancy Behavior

Using first-principles calculations, site occupancy behaviors of transition elements in C15 NbCr2 Laves phase are systematically investigated. Elements Y, Sc, Zr, Hf, Cd, Ta, Ti and Ag prefer to occupy the Nb site, and elements Zn, Pt, Re, Tc, Ir, V, Os, Rh, Ru, Ni, Co, Mn, Fe and Cu favor to occupy the Cr site; whereas elements Mo, W, Pd and Au have weak site preference for Cr or Nb site. The present calculations agree well with the available experimental and previously calculated results. It was found that the site occupancy behavior of transition elements in NbCr2 is mainly affected by the radii of transition elements. The present calculations also propose the correlation between the site preference energy and radii of transition elements.

scholarly journals Site occupancy behavior of the binary μ phase

2021 ◽  
pp. 122704
Author(s):  
Xuezhi Wu ◽  
Wei Liu ◽  
Xiao-Gang Lu ◽  
Yueshan Jiang ◽  
Yanlin He
Keyword(s):  
Site Occupancy ◽  
Occupancy Behavior

High-resolution structure determination by ALCHEMI

1983 ◽  
Vol 41 ◽  
pp. 178-181 ◽  
Author(s):  
Peter G. Self ◽  
Peter R. Buseck
Keyword(s):  
Site Occupancy ◽  
Real Space ◽  
Unit Cell ◽  
Bragg Angle ◽  
Electron Current ◽  
High Resolution Structure ◽  
Beam Incident ◽  
Crystallographic Planes ◽  
Electron Beam Incident ◽  
Resolution Structure

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

Microstructural Evolution in Reduced TiO2

1981 ◽  
Vol 39 ◽  
pp. 74-75
Author(s):  
W. T. Donlon ◽  
S. Shinozaki ◽  
E. M. Logothetis ◽  
W. Kaizer
Keyword(s):  
Microstructural Evolution ◽  
Point Defects ◽  
Extended Defects ◽  
Small Deviations ◽  
Controlled Atmospheres ◽  
Limited Solubility ◽  
Thin Foils ◽  
Heat Treated ◽  
Porous Polycrystalline ◽  
Crystallographic Shear

Since point defects have a limited solubility in the rutile (TiO2) lattice, small deviations from stoichiometry are known to produce crystallographic shear (CS) planes which accomodate local variations in composition. The material used in this study was porous polycrystalline TiO2 (60% dense), in the form of 3mm. diameter disks, 1mm thick. Samples were mechanically polished, ion-milled by conventional techniques, and initially examined with the use of a Siemens EM102. The electron transparent thin foils were then heat-treated under controlled atmospheres of CO/CO2 and H2 and reexamined in the same manner.The “as-received” material contained mostly TiO2 grains (∼5μm diameter) which had no extended defects. Several grains however, aid exhibit a structure similar to micro-twinned grains observed in reduced rutile. Lattice fringe images (Fig. 1) of these grains reveal that the adjoining layers are not simply twin related variants of a single TinO2n-1 compound. Rather these layers (100 - 250 Å wide) are alternately comprised of stoichiometric TiO2 (rutile) and reduced TiO2 in the form of Ti8O15, with the Ti8O15 layers on either side of the TiO2 being twin related.

Simulated Dark-Field Electron Micrographs of Point Defects and Organometallics

1975 ◽  
Vol 33 ◽  
pp. 200-201
Author(s):  
William Krakow
Keyword(s):  
Electron Micrographs ◽  
Point Defects ◽  
Structure Determination ◽  
Atomic Structure ◽  
Image Interpretation ◽  
Dark Field ◽  
Field Electron ◽  
Dark Field Microscopy ◽  
Dark Field Electron

Tilted beam dark-field microscopy has been applied to atomic structure determination in perfect crystals, several synthesized molecules with heavy atcm markers and in the study of displaced atoms in crystals. Interpretation of this information in terms of atom positions and atom correlations is not straightforward. Therefore, calculated dark-field images can be an invaluable aid in image interpretation.

Defects in Crystals

1968 ◽  
Vol 26 ◽  
pp. 244-245
Author(s):  
Kenneth R. Lawless
Keyword(s):  
Electron Microscope ◽  
Point Defects ◽  
Surface Defects ◽  
Chemical Properties ◽  
Material Point ◽  
Crystal Defects ◽  
Defects In Crystals ◽  
Irradiation Effects ◽  
Normal Lattice ◽  
Line Defects

One of the most important applications of the electron microscope in recent years has been to the observation of defects in crystals. Replica techniques have been widely utilized for many years for the observation of surface defects, but more recently the most striking use of the electron microscope has been for the direct observation of internal defects in crystals, utilizing the transmission of electrons through thin samples.Defects in crystals may be classified basically as point defects, line defects, and planar defects, all of which play an important role in determining the physical or chemical properties of a material. Point defects are of two types, either vacancies where individual atoms are missing from lattice sites, or interstitials where an atom is situated in between normal lattice sites. The so-called point defects most commonly observed are actually aggregates of either vacancies or interstitials. Details of crystal defects of this type are considered in the special session on “Irradiation Effects in Materials” and will not be considered in detail in this session.

Zone-axis ALCHEMI for the rapid assessment of site occupancies in garnets

1986 ◽  
Vol 44 ◽  
pp. 706-707
Author(s):  
M.T. Otten ◽  
P.R. Buseck
Keyword(s):  
Single Crystals ◽  
Rapid Assessment ◽  
Site Occupancy ◽  
Zone Axis ◽  
Synthetic Compounds ◽  
X Ray ◽  
Large Group ◽  
Crystallographic Planes

ALCHEMI (Atom Location by CHannelling-Enhanced Microanalysis) is a TEM technique for determining site occupancies in single crystals. The method uses the channelling of incident electrons along specific crystallographic planes. This channelling results in enhanced x-ray emission from the atoms on those planes, thereby providing the required site-occupancy information. ALCHEMI has been applied with success to spinel, olivine and feldspar. For the garnets, which form a large group of important minerals and synthetic compounds, the channelling effect is weaker, and significant results are more difficult to obtain. It was found, however, that the channelling effect is pronounced for low-index zone-axis orientations, yielding a method for assessing site occupancies that is rapid and easy to perform.

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