Quantifying the Early Stages of Crystallization in Co-Based Magnetic Amorphous Nano-Composite Alloys

Magnetic Amorphous Nano-Composites (MANCs) comprise nanoscale crystallites embedded in an intergranular amorphous matrix and are being developed for use in inductors and transformers for medium frequency power conversion and recent interest for energy efficient, high frequency motor applications. They are produced by annealing of amorphous precursors that are formed by melt spinning, deposition processes, or other rapid solidification techniques. The crystallites are largely composed of ferromagnetic Co-Fe encased within an amorphous stabilized matrix containing such elements as Si and B. Early transition metals (such as Nb and Ta) are also added to reduce the crystallite growth upon exposure to high temperatures. Recent work now includes additions of Mn (up to 6 at.% balanced by a reduction in Co), which results in unprecedented changes in magnetic permeability. In this work, we report a series of in situ annealing, ex situ annealing, and atom probe tomography measurements to reveal the early stages of crystallization. We use electron diffraction to measure the short range and medium range ordering at various annealing times and correlate that to atom probe tomography outcomes of the chemical partitioning. With increasing Mn content, the crystallinity of the transformation occurs at earlier stages and is explained by a change in the Co:B ratio. Through this combined quantification, we reveal the sequential underpinning structure and chemical evolution that accompanies the time-temperature-transformation behavior.