No description available.

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No description available.

The precipitation and crystallization of aluminum (oxy)hydroxides from chloride solutions by neutralization appears to be quite complex and depends on several parameters, namely, temperature, the OH/Al molar ratio, aging and solution composition. The precipitation of aluminum from AlC13 solutions was found to be complete at substoichiometric OH/Al ratios (typically ~ 2.7), and resulted in the production of an amorphous material that transformed to crystalline phases upon aging at OH/Al ≥3. The predominant phases precipitated from 0.5M AlCl3 solutions at 22, 60 and 95° C at an OH/Al ratio of 3 after 24 hours of aging were pseudoboehmite, bayerite and boehmite, respectively. Increasing the OH/Al ratio and aging time at both 22°C and 60°C promoted crystallization of bayerite over poorly crystalline boehmite. At 95°C, regardless of aging time or degree of neutralization, boehmite was consistently obtained. Increasing the AlCl3 concentration to 2.0M proved to suppress the crystallization of bayerite at 22°C and 60°C, and favoured the crystallization of pseudoboehmite instead. Between the bayerite and pseudoboehmite products, the former exhibited the best settling behaviour (30% settled slurry density after 1 week), while settling did not occur at all for the latter. / Precipitates produced from mixed AlCl3-NaCl solutions exhibited an increasing tendency to form pseudoboehmite over bayerite with increasing sodium chloride concentration. Upon neutralizing mixed AlCl3-MgCl 2 systems, the final solution pH decreased significantly with increasing magnesium chloride concentration. Furthermore, increasing concentrations of MgCl2 promoted the precipitation of poorly crystalline mixed magnesium-aluminum-hydroxide hydrates, identified as Mg6Al2(OH)18·4.5H 2O by XRD. Increases in temperature (up to 125°C), aging time (48 hours) and OH/Al ratio (3.45) were unsuccessful in improving the crystallinity or transformation of these precipitates.

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Thesis (doctoral)--Lund University, 1996. / Added t.p. with thesis statement inserted.

Thesis (doctoral)--Lund University, 1996. / Added t.p. with thesis statement inserted.

The formation of grain boundary precipitates of the high temperature β phase from the supersaturated ɤ phase has been examined in Ag-5.64 wt.% aluminum alloys at 688°C . Large grained samples were used and the boundary misorientations were determined by X-ray diffraction. At low angle boundaries only primary sideplates formed while above a misorientation of 17° lenticular precipitates were dominant. Precipitate growth was studied on individual grain boundaries using a statistical technique. The lengthening and thickening rates were independent of the grain boundary misorientation indicating that grain boundary diffusion was not significant under these conditions. The precipitates grew with constant shape, with both the length and thickness increasing parabolically with time. By approximating the shape of the precipitate to that of an oblate spheroid growing with constant shape, an equivalent diffusion coefficient was calculated. The value obtained was in good agreement with measurements obtained from diffusion couples. The nature of the quenched β phase was also examined using optical and electron microscopy. The βphase transformed rapidly on cooling, forming a massive [symbol omitted] product or an acicular martensite at higher quenching rates. The structures were very similar to those reported for the Cu-Ga and Cu-Al systems. Many grain boundary precipitates showed unequal growth into the two matrix grains. Measurements of the matrix habit plane suggested that a possible orientation relationship existed between the precipitate and that grain into which no development occurred. In this case the precipitate nucleated in one grain but grew into the opposite grain. Precipitates which developed equally into both grains exhibited no apparent habit relationship with either grain. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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