scholarly journals Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

2021 ◽  
Vol 77 (5) ◽  
Author(s):  
Zhiping Luo
Keyword(s):  
Reaction Mechanism ◽  
Tetragonal Structure ◽  
Collapse Mechanism ◽  
Intermediate Structure ◽  
Orientation Relationships ◽  
Transformation Mechanism ◽  
Initial Stage ◽  
Transformation Mechanisms ◽  
Lattice Distortions ◽  
Diffraction Patterns

γ-Alumina is a used material, while its precise crystal structure and transformation mechanism derived from boehmite have remained unclear in the literature for decades. In this work, quantitative electron microscopy has been applied to study the crystalline structure of γ-alumina and its transformation mechanism from boehmite. Based on Rietveld refinement of electron diffraction patterns, a new tetragonal structure model, with a space group of I41/amd (No. 141), was proposed for the γ-alumina phase, with Al cations on 4a, 8c, 8d and 16g sites and O anions on the 16h site, which could provide better fits than current models. During the boehmite to γ-alumina transformation induced by e-beam irradiation, when the boehmite layers were oriented along the edge-on direction, a shrinkage caused by dehydration was directly observed. Two kinds of boehmite to γ-alumina transformation mechanisms, namely collapse and reaction mechanisms, were elucidated crystallographically in detail with new insights through an intermediate structure, and the reaction mechanism was demonstrated to produce much reduced changes in dimensions and volume, compared with the collapse mechanism. The experimental observations supported the reaction mechanism, which occurred through partial occupation of the dehydrated space by diffusion in the initial stage of the transformation, without the formation of voids that only appeared after the initial stage. Filling tetrahedral interstices of the intermediate structure with Al cations in different ways yields tetragonal or cubic γ-alumina structures, and the tetragonal structure is energetically favorable because of smaller lattice distortions required, compared with the cubic structure. The crystallographic orientation relationships of γ-alumina with the parent boehmite phase deduced from the proposed mechanisms are consistent with the experimental observations.

Moire Fringes on Precipitates in Quenched and Aged Beryllium

1968 ◽  
Vol 26 ◽  
pp. 426-427
Author(s):  
F. J. Fraikor ◽  
A. W. Brewer
Keyword(s):  
Ternary Phase ◽  
Volume Fraction ◽  
Age Hardening ◽  
Orientation Relationships ◽  
Solute Content ◽  
Moiré Fringes ◽  
Moire Patterns ◽  
Total Volume Fraction ◽  
Diffraction Patterns ◽  
Moiré Patterns

A number of investigators have examined moire patterns on precipitate particles in various age-hardening alloys. For example, Phillips has analyzed moire fringes at cobalt precipitates in copper and Von Heimendahl has reported on moire fringes in the system Al-Au. Recently, we have observed moire patterns on impurity precipitates in beryllium quenched in brine from 1000°C and aged at various temperatures in the range of 500-800°C. This heat treatment of beryllium rolled from vacuum cast ingots produces the precipitation of both an fee ternary phase, AlFeBe4, and an hcp binary phase, FeBe11. However, unlike a typical age-hardening alloy, the solute content of this material is low (less than 1000 ppm of Fe and 600 ppm of Al) and hence the total volume fraction of precipitates is small. Therefore there is some difficulty in distinguishing the precipitates and their orientation relationships with the beryllium matrix since the weak precipitate spots generally do not appear on the diffraction patterns.

Direct imaging of order-disorder and antiphase domain structures in Ti3Al intermetallic compound

1990 ◽  
Vol 48 (4) ◽  
pp. 480-481
Author(s):  
H. Q. Ye ◽  
T.S. Xie ◽  
D. Li
Keyword(s):  
Intermetallic Compound ◽  
Volume Fraction ◽  
Fundamental Problem ◽  
Antiphase Domain ◽  
Atomic Scale ◽  
Orientation Relationships ◽  
Domain Structures ◽  
Α Phase ◽  
Diffraction Patterns ◽  
Two Phases

The Ti3Al intermetallic compound has long been recognized as potentially useful structural materials. It offers attractive strength to weight and elastic modulus to weight ratios. Recent work has established that the addition of Nb to Ti3Al ductilized this compound. In this work the fundamental problem of this alloy, i.e. order-disorder and antiphase domain structures was investigated at the atomic scale.The Ti3Al+10at%Nb alloys used in this study were treated at 1060°C and then aged at 700°C for 2 hours. The specimens suitable for TEM were prepared by standard jet electrolytic-polishing technique. A JEM-200CX electron microscope with an interpretable resolution of about 0.25 nm was used for HREM.The [100] and [001] projections of the α2 phase were shown in Fig.l.The alloy obtained consist of at least two phases-α2(Ti3Al) and β0 structures. Moreover, a disorder α phase with small volume fraction was also observed. Fig.2 gives [100] and [001] diffraction patterns of the α2 phase. Since lattice parameters of the ordered α2 (a=0.579, c=0.466 nm) and disorder α phase (a0=0.294≈a/2, c0=0.468 nm) are almost the same, their diffraction patterns are difficult to be distinguished when they are overlapped with epitaxial orientation relationships.

Crystallization of Amorphous Sputtered NiTi Shape-Memory Alloy Films

1995 ◽  
Vol 398 ◽  
Author(s):  
F.F. Gong ◽  
H.M. Shen ◽  
Y.N. Wang
Keyword(s):  
Shape Memory ◽  
Annealing Temperature ◽  
Optimum Shape ◽  
Niti Shape Memory Alloy ◽  
X Ray Diffraction ◽  
Initial Stage ◽  
Average Grain Size ◽  
Optimum Heat ◽  
Diffraction Patterns ◽  
Optimum Heat Treatment

ABSTRACTThe crystallization of amorphous sputtered NiTi films was investigated for selected heat treatments. From x-ray diffraction patterns, when the films were annealed below the crystallization temperature, the intensity of the broad maximum centered at 2θ = 43.5° increased with increasing the annealing temperature and time. When the films were annealed at 550°C for 0.5hr, parent B2 phase and Ni4Ti3 precipitates appeared. For annealing temperature above 700°C, the films showed embrittlement and volatilization. Therefore the optimum heat treatment for the optimum shape memory effect is found. The average grain size increased slowly in the initial stage of annealing but remained almost unchanged when the films were annealed for more than 1hr. This is because the grain boundary grooving caused by the titanium at a certain temperature volatilization seriously hinder the grain growth.

Defining the hematite topotaxial crystal growth in magnetite–hematite phase transformation

2020 ◽  
Vol 53 (4) ◽  
pp. 896-903
Author(s):  
Flávia Braga de Oliveira ◽  
Gilberto Álvares da Silva ◽  
Leonardo Martins Graça
Keyword(s):  
Electron Backscatter Diffraction ◽  
Inverse Pole Figure ◽  
Orientation Relationships ◽  
Transformation Matrices ◽  
Hematite Phase ◽  
Pole Figures ◽  
Magnetite Crystal ◽  
Diffraction Patterns ◽  
Backscatter Diffraction ◽  
Specific Orientation

Magnetite and hematite iron oxides are minerals of great economic and scientific importance. The oxidation of magnetite to hematite is characterized as a topotaxial reaction in which the crystallographic orientations of the hematite crystals are determined by the orientation of the magnetite crystals. Thus, the transformation between these minerals is described by specific orientation relationships, called topotaxial relationships. This study presents electron-backscatter diffraction analyses conducted on natural octahedral crystals of magnetite partially transformed into hematite. Inverse pole figure maps and pole figures were used to establish the topotaxial relationships between these phases. Transformation matrices were also applied to Euler angles to assess the diffraction patterns obtained and confirm the identified relationships. A new orientation condition resulting from the magnetite–hematite transformation was characterized, defined by the parallelism between the octahedral planes {111} of magnetite and rhombohedral planes \{10\bar {1}1\} of hematite. Moreover, there was a coincidence between one of the octahedral planes of magnetite and the basal {0001} plane of hematite, and between dodecahedral planes {110} of magnetite and prismatic planes \{11\bar {2}0\} of hematite. All these three orientation conditions are necessary and define a growth model for hematite crystals from a magnetite crystal. A new topotaxial relationship is also proposed: (111)Mag || (0001)Hem and (\bar {1}\bar {1}1)_{\rm Mag} || (10\bar {1}1)_{\rm Hem}.

Phase Transformation Crystallography of Lath Martensite

2006 ◽  
Vol 979 ◽  
Author(s):  
Xiao Ma ◽  
R.C. Pond
Keyword(s):  
Phase Transformation ◽  
Habit Plane ◽  
Lath Martensite ◽  
Martensitic Transformations ◽  
Phenomenological Theory ◽  
Orientation Relationships ◽  
Transformation Mechanism ◽  
Ferrous Alloys ◽  
Model Phase ◽  
Good Agreement

AbstractOur current understanding of martensitic transformations has been based on the Phenomenological Theory of Martensite Crystallography developed in the 1950s. Recently, a Topological Model of martensitic transformations has been presented wherein the habit plane is a semi-coherent structure, and the transformation mechanism is shown explicitly to be diffusionless. This approach is used here to model phase transformation crystallography of lath martensite in ferrous alloys. A range of network geometries is predicted corresponding to orientation relationships varying from Nishiyama-Wasserman to Kurdjumov-Sachs. Experimental observations from the literature of the dislocation and disconnection arrays, habit plane and orientation relationship are in good agreement with the model.

Solvothermal route to Bi3Se4 nanorods at low temperature

2001 ◽  
Vol 16 (12) ◽  
pp. 3361-3365 ◽  
Author(s):  
Yuan-fang Liu ◽  
Jing-hui Zeng ◽  
Wei-xin Zhang ◽  
Wei-chao Yu ◽  
Yi-tai Qian ◽  
...  
Keyword(s):  
Electron Microscope ◽  
Reaction Mechanism ◽  
Low Temperature ◽  
Hydrazine Hydrate ◽  
Photoelectron Spectra ◽  
Elemental Selenium ◽  
X Ray ◽  
Transmission Electron ◽  
Microscope Images ◽  
Diffraction Patterns

Nanorods Bi3Se4 were synthesized directly through the reaction between BiCl3 and elemental selenium in an autoclave with hydrazine hydrate as solvent at 165 °C for 10 h. X-ray powder diffraction patterns, x-ray photoelectron spectra, and transmission electron microscope images show that the products are well-crystallized hexagonal Bi3Se4 nanorods. The solvent hydrazine hydrate played an important role in formation and growth of Bi3Se4 nanorods. The possible reaction mechanism was proposed.

Mechanism of the morphotropic transformation between the rutile and corundum structural types

2003 ◽  
Vol 59 (4) ◽  
pp. 456-462 ◽  
Author(s):  
H. Katzke ◽  
R. Schlögl
Keyword(s):  
Simple Model ◽  
Phase Segregation ◽  
Transformation Process ◽  
Transformation Mechanism ◽  
One Dimensional ◽  
X Ray ◽  
Structural Types ◽  
Crystallographic Shear ◽  
Diffraction Patterns ◽  
Shear Planes

The rutile/corundum structural transformation which is based on crystallographic shear is discussed in terms of a one-dimensional disorder model. The transformation process is described by a simple model based on the structural relationship between the rutile-type and corundum-type phases. The model is able to handle randomly spaced crystallographic shear planes, the so-called Wadsley defects, as well as clustered CS planes. Calculations hsow that simply modifying the probability parameters of the model can lead to phase segregation. X-ray powder diffraction patterns are calculated for the proposed transformation mechanism as a function of the stoichiometry x in MO2−x in order to show the influence of such defects on the intensities and linewidths of the Bragg reflections.

scholarly journals Микроструктура и фазовый состав сплава дисилицидов железа и хрома

2022 ◽  
Vol 56 (2) ◽  
pp. 187
Author(s):  
Е.И. Суворова ◽  
Ф.Ю. Соломкин ◽  
Н.А. Архарова ◽  
Н.В. Шаренкова ◽  
Г.Н. Исаченко
Keyword(s):  
Hexagonal Structure ◽  
Tetragonal Structure ◽  
Orientation Relationships ◽  
Spontaneous Crystallization ◽  
Crystal Lattices ◽  
Transmission Electron ◽  
Orthorhombic Modification ◽  
Scanning Transmission ◽  
Before And After ◽  
Iron Monosilicide

The phase composition, microstructure, and interphase interfaces of the disordered CrSi2-FeSi2 solid solution obtained by spontaneous crystallization (before and after annealing) have been investigated by scanning, transmission electron microscopy, electron diffraction, and X-ray energy dispersive spectrometry. The as-grown samples contained the phases of CrSi2 with the P6422 hexagonal structure and FeSi2 with the P4/mmm tetragonal structure. Annealing of the samples led to the phase transformation of tetragonal FeSi2 into the orthorhombic modification Cmca. Precipitates of cubic iron monosilicide FeSi with space group P213, nano-precipitates of Si and silicon silicide Cr5Si3 with a tetragonal structure I4/mcm were observed inside the FeSi2 grains. Impurities of interstitial Cr atoms with a concentration up to 2.0 at% are found in iron (di)silicides grains in all samples. The structure of the CrSi2 phase remains unchanged after annealing; the concentration of impurity iron atoms is about 0.7 at%. Orientation relationships between the crystal lattices of the phases are established and strains due to the mismatch of the crystal lattices are determined.

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