Microstructural Transformations in Alumina Gels

1984 ◽  
Vol 32 ◽  
Author(s):  
F. W. Dynys ◽  
M. Ljungberg ◽  
J. W. Halloran
Keyword(s):  
Phase Transformations ◽  
Microstructural Development ◽  
Transition Alumina ◽  
Hydrous Oxides ◽  
Alumina Gels ◽  
Alpha Alumina

ABSTRACTMicrostructural development in alumina gels is dominated by a series of phase transformations between the hydrous oxides to transition alumina phases, between transition phases, and transformation to alpha-alumina. These microstructural transformations are illustrated in boehmite AlOOH and bayerite Al(OH)3 gels.

The use of thin Film Substrates to Study Enhanced Solid-State Phase Transformations

1993 ◽  
Vol 319 ◽  
Author(s):  
PAUL G. Kotula ◽  
Dwight D. Erickson ◽  
C. Barry Carter
Keyword(s):  
Thin Film ◽  
Phase Transformation ◽  
Solid State ◽  
Phase Transformations ◽  
Pulsed Laser ◽  
Alumina Layer ◽  
Transition Alumina ◽  
Transmission Electron ◽  
Film Substrate ◽  
Alpha Alumina

AbstractA thin-film substrate geometry is described for the study of enhanced or seeded solid-state phase transformations. As an example of this approach, thin films of hematite have been used as substrates for the study of the seeded phase transformation of a boehmite-derived transition-alumina to α-A12O3. The hematite films were grown on bulk (0001) α-A12O3 single crystal substrates by pulsed-laser ablation. A layer of a boehmite sol was then spin-coated onto the thin film. The assemblages were then heated to 950°C, or 1000°C in order to induce the phase transformation. Specimens were imaged in cross section by transmission electron microscopy. No transformation was observed for specimens heated to 950°C. In specimens heated to 1000°C, the transition alumina was found to transform to alpha-alumina, starting at the surface of the hematite film, via solid-state heteroepitaxy. In this case, islands, growing out from the hematite film into the transition alumina layer, were observed.

Characterization of metallurgical-grade aluminas and their precursors by 27Al NMR and XRD

2007 ◽  
Vol 85 (10) ◽  
pp. 889-897 ◽  
Author(s):  
Linus M Perander ◽  
Zoran D Zujovic ◽  
Tania Groutso ◽  
Margaret M Hyland ◽  
Mark E Smith ◽  
...  
Keyword(s):  
Solid State ◽  
High Field ◽  
Structural Complexity ◽  
Magic Angle Spinning ◽  
Magic Angle ◽  
Nmr Studies ◽  
Transition Alumina ◽  
Angle Spinning ◽  
Alpha Alumina

The structure of metallurgical- or smelter-grade aluminas (MGAs) is complex and poorly understood. Ultra-high-field solid-state 27Al NMR results on industrial as well as on laboratory-prepared aluminas are discussed in relation to XRD results. It is demonstrated that high-field NMR can effectively be used to quantify the proportion of the thermodynamically stable alpha-alumina phase in these materials. The results demonstrate that 27Al NMR is a vital adjunct to XRD methods to quantify the transition alumina phases that invariably dominate the MGAs. The nature of the disorder in these materials, determined by 27Al NMR, is also compared with literature data, such as XANES and EXAFS studies, on typical laboratory-prepared materials. The utility of 27Al NMR studies to provide new insight into the structural complexity of metallurgical aluminas is shown.Key words: solid-state magic-angle-spinning NMR, metallurgical-grade alumina, transition alumina, gamma-alumina, coordination number.

A combined TEM/APFIM approach to the study of phase transformations: Phase identification in the Fe-Be system

1988 ◽  
Vol 46 ◽  
pp. 780-781
Author(s):  
M.G. Burke ◽  
M.K. Miller
Keyword(s):  
Phase Transformations ◽  
Microstructural Analysis ◽  
Analytical Electron Microscopy ◽  
Thin Foil ◽  
Atom Probe ◽  
Microstructural Development ◽  
Phase Identification ◽  
Probe Field ◽  
Isothermal Ageing ◽  
Microscopy Techniques

Phase transformation investigations rely on the identification and characterization of the microstructure in order to understand the formation, development, and relative stability of the constituent phases. Although transmission and associated analytical electron microscopy techniques have made substantial contributions by providing structural and chemical data necessary for the detailed microstructural analysis, the direct atomic structure and chemistry are not readily discernable. By combining TEM techniques with atom probe field-ion microscopy (APFIM), it is possible to obtain a complete structural and chemical analysis of the constituent phases. In this paper, the microstructural development which occurs during ageing in an Fe-25 at. % Be alloy is presented to illustrate the complementary nature of the techniques and demonstrate the applicability of the combined TEM/APFIM approach in the study of phase transformations.An Fe-25 at. % Be alloy was solution annealed at 1100°C for 0.5 h and water-quenched prior to isothermal ageing at 650°C for 4 h. Thin foil specimens were examined in a Philips EM430T operated at 300 kV and in a JEOL 200CX operated at 200 kV. FIM needle specimens were electropolished and analyzed in the ORNL energy-compensated APFIM.

Phase Transformations on Flame Sprayed Alumina

1996 ◽  
Author(s):  
R.S. Lima ◽  
C.P. Bergmann
Keyword(s):  
Phase Transformations ◽  
Diffraction Pattern ◽  
Phase Changes ◽  
Stable Phase ◽  
Alumina Powder ◽  
Stable Form ◽  
X Ray Diffraction ◽  
X Ray ◽  
Sprayed Coatings ◽  
Alpha Alumina

Abstract Flame-sprayed coatings of alumina were produced by an oxyacetylene flame spray system in order to study the phase transformations that occurs on alumina during the spraying. The X-ray diffraction pattern of the alumina powder to be sprayed, showed the main presence of the stable phase alpha alumina and an impurity probably resulting from the process of purification of alumina. For as-sprayed coatings, phase changes occur. The X-ray diffraction pattern shows the presence of the stable form alpha alumina, but also the metastable form gamma alumina and amorphous alumina. True density measurements were done using a helium pycnometer, as an aid to observe the phase transformations. The density of the powder to be sprayed was 3.98 g/cm3 and the density of the as-sprayed coatings was 3.62 g/cm3. This change of density is linked to the phase transformation during spraying. Scanning electron microscopy (SEM) of the incident particles was made after 1 second of deposition onto glass substrates in order to observe the degree of melting of the incident particles, by analyzing their profiles (degree of flattening).

Phase Transformations and Microstructural Development in Tial-Based Alloys

1990 ◽  
Vol 213 ◽  
Author(s):  
P. A. Beaven ◽  
TH. Pfullmann ◽  
J. Rogalla ◽  
R. Wagner
Keyword(s):  
Heat Treatment ◽  
Phase Transformations ◽  
Thermomechanical Processing ◽  
Critical Role ◽  
Subsequent Heat Treatment ◽  
Microstructural Development

ABSTRACTThe main features of the phase transformations occurring in alloys based on Ti-48 at% Al are described in order to assess their roles in microstructural development during thermomechanical processing and subsequent heat treatment. It is shown that the presence of Al-rich interdendritic γ formed during solidification plays a critical role during these processes.

Effect of Cerium Oxide Additions on the Densification Behaviour of Alumina Gels

1985 ◽  
Vol 60 ◽  
Author(s):  
Ratnesh Dwivedi ◽  
Jane Adams
Keyword(s):  
Cerium Oxide ◽  
Theoretical Density ◽  
Sintering Behavior ◽  
Growth Morphology ◽  
Pure Alumina ◽  
Densification Behaviour ◽  
Alumina Gel ◽  
Identical Manner ◽  
Alumina Gels ◽  
Alpha Alumina

AbstractSintering behavior of gel-derived alumina containing 2.8 mol percent cerium oxide (Ce2 O3) was studied at 1400°C. The growth morphology of alpha alumina in cerium oxide-containing aluminas was found to be significantly different form that of pure gel-derived alumina. Alumina gel containing 2.8 mol % Ce2 O3 was found to densify to 81% of the theoretical density as compared to 65% for pure alumina gels. The powdered cerium oxide-containing gels, pressed into a pellet, could be sintered to 97% of the theoretical density at 1400°C as compared to 87% found for pure alumina gel powder processed in an identical manner.

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