The formation and growth of an amorphous phase or other metastable phases during interdiffusion reactions in metals has been the subject of much study during the past decade [1-4]. The thermodynamic and kinetic factors which control phase selection and growth during the evolution of a binary diffusion couple are of particular interest. Amorphous phase formation in binary A/B diffusion couples has been shown to be favored in systems satisfying several themodynamic and kinetic criteria. First, the free energy of formation of the amorphous phase from the reactants must be negative. This provides the thermodynamic driving force to form the amorphous phase. In general, this condition is best satisfied in cases where the crystalline metals A and B have a negative heat of mixing in the amorphous phase [1,2,4]. On the other hand, the formation of simple crystalline solid solutions of the parent metals should be relatively unfavorable. Thus amorphous phase formation occurs generally in systems for which crystalline terminal solid solutions are restricted. In such binary systems, it is generally true that other crystalline intermetallic phases exist which have lower free energies than the amorphous phase and thus a larger thermodynamic driving force for growth during interdiffusion. As such, additional kinetic constraints must be present which suppress the nucleation and/or growth of the competing crystalline intermetallic phases [2,4].