Microstructural design of soft magnetic CoFe - based alloys with targeted magnetic and electronic properties for transformer cores
- Author(s): Belcher, Calvin Harris
- Advisor(s): Apelian, Diran
- et al.
As the world’s need for electrical power grows, so too does the need for more efficient transformers and transformer cores to properly transmit and store power across large grids and distances. There is also an additional need for transformers that are more resilient to events such as electromagnetic pulse (EMP) and geomagnetic disturbances (GMD). The objective of this research is to use microstructural design approaches to develop CoFe – based soft magnetic materials for more resilient and efficient transformer cores. The studies presented here implement the use of non-equilibrium powder metallurgical processing techniques to design and fabricate microstructures necessary for efficient magnetic and electronic properties in transformer cores such as high saturation magnetic polarization and high electrical resistivity. The first study aimed to develop CoFe – P alloys with unstable nanocrystalline, solid solution microstructures and discusses the effect of a secondary intermetallic phase on the saturation magnetic polarization and electrical resistivity of the bulk alloy. Solid solution CoFe – P alloy powders were prepared by mechanically alloying metal powders; however, our results revealed the formation of a stable secondary phase occurred during spark plasma sintering (SPS) consolidation at relatively high temperatures. The magnetic properties of the precipitated intermetallic phase were found to be detrimental to the soft magnetic properties of the targeted CoFe – P alloy. In the second study, the columnar grain microstructures of additively manufactured CoFe are discussed with respect to thexii magnetocrystalline anisotropy of the B2 ordered BCC CoFe structure. Additive manufacturing holds potential as a key manufacturing process for brittle alloys such as CoFe and the microstructural control of additive manufacturing processes can enable the development of textured CoFe alloys for more efficient soft magnetic properties. Despite the columnar grains, the magnetic behavior of additively manufactured CoFe alloys was measured to be isotropic. The isotropic magnetic behavior was attributed to randomly oriented columnar grains, not textured, which were attributed to the 90° hatch rotation parameter used during the additive manufacturing process. The final study assesses the microstructural development of a CoFe – based soft magnetic composite with a continuous non-magnetic Al2O3 phase fully intersecting CoFe phase particles to produce a composite with high saturation magnetic polarization and high electrical resistivity. The CoFe – Al2O3 composite was developed by coating CoFe particles with Al2O3 and subsequently consolidating them using spark plasma sintering. Upon consolidation at relatively high temperature, the CoFe was found to diffuse into the Al2O3 coatings at the particle boundaries and was attributed to low resistivity in the composite. However, at lower consolidation temperature, diffusion of CoFe was not observed and a high electrical resistivity was achieved, while maintaining a high magnetic polarization. Together, these studies represent effective microstructural design approaches to the CoFe system of materials to develop more efficient and resilient soft magnetic transformer cores.