Microstructural Engineering and Phase Transformations Study of α″-Fe16N2 for Rare-Earth-Free Permanent Magnets

Abstract

Permanent magnets are one of the most important building blocks of motors, generators, sensors, hard disks and more. High-energy product permanent magnets contain a significant amount of rare-earth elements. The extraction process of rare-earths are expensive and energy intensive, and also hazardous for human health and the environment. Moreover, supply chain issues of the rare-earths added instability in the permanent magnet market. Additionally, we are observing an increased demand for permanent magnets in past couple of years because of the increase in vehicle electrification, and increasing demand for renewable energy sources. In these regards, there is a necessity to produce a rare-earth-free permanent magnet to reduce the detrimental effect on human health and the environment and have a stable supply chain. A promising rare-earth-free permanent magnet should possess a high remanent magnetic flux density (Br), a large coercivity (Hc), and consequently, a large energy product ((BH)max). α″-Fe16N2 has been emerging as one of the promising candidates because of its large magnetocrystalline anisotropy of 1.8x107 erg/cm3 and high saturation flux density of 2.6-2.9 T and the abundant availability of iron and nitrogen on the earth. In this dissertation, a method was demonstrated of fabricating α″-Fe16N2 ribbons with an optimized microstructure to obtain high coercivity. We developed a coercivity model based on the microstructure analysis and refined the microstructure using a different alloy system. We have designed a new alloy based on B and Cu doped into the Fe to obtain the suitable microstructure for high coercivity, hence high-energy product. The alloy design and development have been analyzed using a scanning electron microscope, and x-ray diffraction. The solid-state phase transformations are the key to forming the thermodynamically metastable martensitic FeN phases, such as α′-Fe8N and α″-Fe16N2. In this dissertation, we demonstrated nitrogen diffusion kinetics in the Cu and B doped Fe and found that the N diffusion coefficient is two magnitudes lower in the FeCuB matrix than the undoped nanocrystalline Fe. We also found the activation energy for N diffusion in FeCuB matrix as of 76 kJ/mol. The activation energy is important to understand the iron nitride phases in the Cu and B doped Fe. The martensitic phase transformation of FeN was also studied and optimized to obtain the α″-Fe16N2 phase. The change in microstructure due to solid-state phase transformations was also analyzed to understand the N and other alloying element’s behavior on the Fe matrix and their relation to magnetic properties. Finally, we also demonstrated a newly developed soft magnet with C doping in the FeN compound, Minnealloy, with an ultra-high saturation magnetization of 2.8±0.15 T, which is 27% higher than pure iron, and almost five times higher than Ferrite, the most used soft magnetic material. High saturation flux density will be helpful in reducing the machine size and weight.