Thermodynamics and Kinetics of Nanostructure Formation in Soft-Magnetic Nanocrystalline Alloys (Overview)

Thermodynamic and kinetic conditions for nanocrystallization of Fe-based amorphous precursors into a duplex structure of α-Fe nanocrystals within a residual amorphous matrix are considered in view of recent literature. The effect of copper addition on nanograin size is explained in terms of the CuFe metastable phase diagram and the nature of the various heteroatomic interactions. Addition of larger atoms such as Nb and Zr that are rejected together with boron atoms at the α-Fe nanocrystallization interface is found to generate diffusion double-layers with sharp concentration gradients. The diffusion double-layers explain measured differences in Curie temperatures Tc of the remaining grain boundary amorphous phase and bulk glasses of the same composition. They also successfully predict the occurence of a broad maximum in Tc of the remaining amorphous phase during nanocrystallization annealing. The concentration gradients have an additional thermodynamic stabilization effect on the amorphous interlayers by reducing the driving force for the formation of intermetallics.