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Abstract
This thesis is devoted to consideration of one of the simplest possible types of nucleation realistically possible in crystalline solids, namely the homogeneous nucleation of one fcc phase within another, where the two phases have the same spatial orientations and differ only in composition, and sometimes also in lattice parameter. The first four chapters are devoted to collection and further development of the theoretical apparatus needed for these studies. The fifth chapter reports what appears to be the first sound solution to the very difficult problem of making a valid comparison between homogeneous nucleation theory and experiment in any condensed phase system. Three dilute Cu-rich Cu-Co alloys, whose thermodynamics, crystallography and diffusivities have been well characterized by previous investigators, were chosen as the experimental vehicle for this comparison. Alloy compositions were selected after being shown theoretically to transform at nucleation and growth rates convenient for experimental study. The agreement achieved between theoretical and experimental nucleation kinetics is very good and provides the first quantitative support for the essential correctness of homogeneous nucleation theory. In the sixth chapter, the discrete lattice plane approach to constructing critical nuclei for classical nucleation is shown to accounts successfully for the shapes of faceted GP zones and the temperature-dependence thereof in Al-Ag alloys, when some modifications in the regular solution thermodynamics (otherwise used in this thesis) are introduced in order to approximate better the thermodynamics of the fcc Al-Ag solid solution. In the last chapter, both classical theory and discrete lattice point non-classical theory with strain energy included are employed to develop a basis for explaining the origin of GP zones on Al-Ag and Al-Cu alloys. It is proposed that GP zones in Al-Ag alloys will normally form by homogeneous nucleation and growth whereas those in Al-Cu alloys may be able to form through, or with the assistance of, spinodal decomposition.
