Enabling L10 Ordering in Bulk FeNi Alloys and an Alternative for Nd2Fe14B- Based Permanent Magnets

NON-TECHNICAL SUMMARY NdFeB based magnets are in increased demand for use in wind mills, DC motors in electric vehicles and numerous other key technologies. The high cost of rare-earth (RE) elements and heavy reliance on imports of Nd and other rare-earth elements make the replacement of NdFeB magnets with low-cost magnets made of more abundant elements an urgent economic and national security need. A specific type of ordered arrangement of Ni and Fe atoms in a specific FeNi alloy provides powerful magnetic properties comparable to that of NdFeB magnets. While an FeNi alloy with such an ordered arrangement of Fe and Ni atoms has been observed in meteorites, efforts to make such an alloy on the earth has so far been unsuccessful. This is because such an arrangement is stable only below 300 degree Centigrade, but at these temperatures, the atomic jumps needed to achieve the ordered crystals will require millions of years. This project investigates (i) introduction of extensive amount of crystal defects in FeNi single crystals using a unique approach and (ii) how they can be used to increase the atomic jump rates and obtain the desired structure in a practical time frame. This work also sheds light on the operating mechanism in meteorites to form this ordered arrangement in the FeNi alloy. Strategic need to minimize the current reliance on RE imports underscores the impact of the proposed research. The project trains several graduate and undergraduate students, enhances course content in several courses, and improves research facilities. Outreach efforts are made to high school students, female and under-represented student groups, and the broader community. TECHNICAL SUMMARY There is an increasing demand for NdFeB based magnets for use in wind mills, DC motors in electric vehicles and numerous other key technologies. The high cost of rare-earth elements and heavy reliance on the imports of Nd and other rare-earth elements make the replacement of NdFeB magnets with low-cost magnets made of more abundant elements an urgent economic and national security need. Equiatomic FeNi alloy phase with L10 ordered crystal structure provides powerful permanent magnet properties comparable to NdFeB magnets. While this phase has been observed in neutron irradiated FeNi single crystals and asteroids, it has defied terrestrial synthesis in bulk form due to its low critical ordering temperature of around 320 degree centigrade and consequently low diffusion kinetics. This project overcomes the barrier for L10 long-range ordering in FeNi alloy by enhancing diffusion kinetics at temperatures below the critical ordering temperature. This is achieved by increasing the dislocation density and nonequilibrium vacancy concentration through extreme deformation and by alloying additions that enhance diffusion kinetics. FeNi single crystals are grown using the vertical Bridgman crystal growth technique. Dislocation and other defect densities are characterized using x-ray diffraction and scanning transmission electron microscopy. Short- and long-range order are examined using diffuse scattering peaks and superlattice peaks in the x-ray diffraction patterns. Ordering and local atomic environments are also examined using extended x-ray fine spectrum (EXAFS). Vibrating sample magnetometry is used to assess the magnetic properties. The work also elucidates the operating mechanism in meteorites to form this ordered phase in the FeNi alloy. Strategic need to minimize the current reliance on rare-earth element or rare-earth magnet imports underscores the impact of the proposed research. The project trains several graduate and undergraduate students, enhances course content in several courses, and improves university research facilities. Outreach efforts are made to high school students, female and under-represented student groups, and to the broader community. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.