The dissolution mechanism and kinetics of a number of strontium master alloys in liquid aluminum and A356 alloys have been investigated. The dissolution behaviour of the strontium alloys was found to show marked differences depending on strontium content. Dilute strontium alloys containing less than 63 weight percent Sr were observed to exhibit simple dissolution in both melts. The dissolution rate and the recoveries of these alloys were found to increase with increasing melt temperature. Over the temperature interval of 675-775$ sp circ$C dissolution was determined to be mass-transfer controlled with associated activation energies of 10-20 kcal/mole. Experimental rate constants for dissolution showed good agreement with a mass transfer correlation expressed as k = (0.67(Gr.Sc)$ sp{1/4}$ + 0.58(h/r)) (D/h). The dissolution of high strontium master alloys $(>$63 wt. pct. Sr), unlike the dilute alloys, was accompanied by the formation of various intermetallics. Additions at low melt temperatures yielded the exothermic formation of those intermetallics that have the lowest Sr content as seen in the relevant phase diagram, i.e., SrAl$ sb4$ in liquid Al and SrAl$ sb2$Si$ sb2$ in liquid A356. Due to low reaction rates at these low melt temperatures, these intermetallics formed as dispersed particles that could easily dissolve in the melt yielding high recoveries. At high melt temperatures, the associated chemical reactions yielded, as products, the higher Sr intermetallics which formed with little or no exothermicity. These intermetallics were observed to be scarcely soluble in the melt resulting in low strontium recoveries. The dissolution times of these alloys were found to show good agreement with calculated values based on a two-stage dissolution model comprising an initial exothermic reaction period and a subsequent free dissolution period. The strontium master alloys were classified in two groups; the high Sr alloys that are efficient at low temperatures of 675-7

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Thèse (M.Eng.) -- Université du Québec à Chicoutimi, 2002. / Document électronique également accessible en format PDF. CaQCU

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The product of interaction between Al and TiO2 at elevated temperature has a wide range of applications in refractory, structural and electronics industries (refractory tiles, tank armor, fuel cells, and microelectronic devices). This research attempts to understand the extent of interaction between Al and TiO2 when the reactant surfaces are in contact at elevated temperature and normal atmospheric pressure. The interfacial region between the reactant compounds is examined using analytical techniques; and the formation of TiAl as the interfacial compound is described. The thermodynamics of the Al – Ti – O system is explained as it relates to the particular conditions for the Al – TiO2 reaction research. Thermodynamic principles have been used to demonstrate that the formation of TiAl is favored instead of other TixAly compounds for the set of conditions outlined in this thesis. A study of the mechanism of interactions in the interfacial region can help towards being able to determine the reaction kinetics that lead to the control of microstructure and thus an improvement in the material performance. An appropriate model that describes the formation of TiAl at the interface is described in this study. The formation of TiAl at the interface is a result of the reduction reaction between TiO2 and Al. The O released during the reduction of TiO2 has been investigated and demonstrated to partly remain dissolved in TiAl at the interfacial region. Some O reacts with Al as well to form crystalline Al2O3 in the interfacial layer. / Ph. D.