Spatially Tuned Microstructure and Properties in Soft Magnetic Nanocrystalline Alloy Via Transverse-Oriented Induction Heating

A novel processing technique involving transverse induction heating is demonstrated to produce microstructure variation in soft magnetic alloys over macroscopic lengths, enabling enhanced “permeability engineering” at component level with previously unattainable spatial/temporal control of thermal processing. In electric motors, local control of grain size in bulk crystalline alloys may overcome tradeoffs between low coercivity and high strength via spatially selective annealing. For amorphous/nanocrystalline alloys, degree of crystallization, grain size, and phase identity can be tailored resulting in modification of magnetic and mechanical properties for electric motors, inductors, and transformers. Here we develop analytical models of transverse coil fields and compare with results obtained using finite element modeling. We apply modeling and experimental processing to amorphous Co-based soft magnetic ribbon. By demonstrating the concept, multiple crystallization events provide easy indicators of a highly inhomogeneous temperature profile, and ribbon geometry demonstrates an important role due to eddy current concentration at ribbon edges.