Demand for high-performance permanent magnets is increasing rapidly for applications such as wind turbine generators and motors in both electric and hybrid cars. This market is served by rare earth (RE) magnets based on neodymium-iron-boron and samarium-cobalt. RE magnets are not without issues; they can chip, suffer thermal shock, and can suffer grain boundary corrosion. However, their biggest problems are: price volatility; that China largely controls the RE metals market; and that the extraction of RE metals creates severe environmental degradation. The compound MnAl has been of interest as a permanent magnet since the early 1960s. It has a theoretical energy product (a measure of its magnetic strength) between that of traditional AlNiCo magnets and RE magnets with a value comparable to that of bonded neodymium-iron-boron magnets. Further, it does not suffer from the issues associated with RE magnets, and potentially has the lowest cost per unit energy product of any permanent magnet. The magnetic properties of MnAl are affected by the numerous types of defects present, but how these defects affect the magnetic properties and under what conditions these defects occur is unknown. In this project, we will determine both the conditions under which defects and second phases form during processing of MnAl and how these control the magnetic properties. The expectation is that superior magnetic properties can be obtained by carefully manipulating the defect structure. While the work focuses on MnAl, understanding the relationship between the occurrence of defects, their production and their relationship to the magnetic properties in MnAl will be useful for understanding similar behavior in the related compounds MnBi, MnGa, NiFe, PtFe and CoPt, all of which are of interest as hard magnets. The project includes the training of both a Ph.D. student and several undergraduates in state-of-the-art techniques.
Tau-MnAl is a metastable phase that transforms from the high temperature epsilon phase, during which anti-phase boundaries (APBs), twins, stacking faults and dislocations are created. Depending on the processing conditions, the equilibrium beta and gamma 2 phases can also form. The fundamental difficulty with improving the magnetic properties of tau-MnAl is that there is no clear understanding on how they depend on the defect structure. The grain size can also influence the magnetic properties either directly or by affecting the beta and gamma 2 phases arrangement and defect formation. Our aim is to understand both the conditions under which APBs, twins, stacking faults, dislocations and second phases form during processing of tau-MnAl and how these control the magnetic properties. The local chemistry will also be explored at high resolution using atom probe tomography via collaboration with Dr. Baptiste Gault, Max-Planck-Institut fur Eisenforschung, Germany. Our working hypothesis is that we need a strong, c-axis alignment and a low density of APBs, twins and stacking faults (which locally disorder the material) for a high saturation magnetization, while a low density of APBs, twins and stacking faults but a high dislocation density are required for a high coercivity. It is thought that a fine distribution of beta and gamma 2 phases will also contribute to a high coercivity through magnetic domain wall pinning. While the work focuses on MnAl, understanding the relationship between the occurrence of defects, their production and their relationship to the magnetic properties in MnAl will be useful for understanding similar behavior in the related compounds MnBi, MnGa, NiFe, PtFe and CoPt, all of which are of interest as hard magnets. The project will include training of both a Ph.D. student and several undergraduates in state-of-the-art techniques.
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.
