ECAE processing of t-MnAl magnets

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 Nd2Fe14B and Sm2Co17. 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. L10-structured t-MnAl has been of interest as a permanent magnet since the early 1960s. It has a  theoretical energy product, (BH)max, between that of AlNiCo magnets and RE magnets with a value (12 MGOe) comparable to that of bonded Nd2Fe14B magnets. Further, it does  not suffer from the issues associated with RE magnets, and potentially has the lowest cost per MGOe of any permanent magnet. The enigma is that the theoretical (BH)max has never been achieved: mechanically-milled particulates can show high coercivity (HC) but low saturation magnetization (MS) while warm-extruded material can show high MS but low HC. t-MnAlis a metastable phase that transforms from the high temperature ε phase, during which anti-phase boundaries (APBs), twins, stacking faults and dislocations are created. Depending on the processing conditions, the equilibrium β and γ2phases can also form. The fundamental difficulty with improving the magnetic properties of t-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 β and γ2 arrangement and defect formation. 

We are using equal channel angular extrusion (ECAE) to process t-MnAl billets over a range of temperatures. Multiple passes will be performed utilizing Routes A (extruded billet is fed back into the  ECAE jig in the same orientation) and BC(extruded billet is rotated 90o clockwise about its axis between passes). We will determine the texture of the extruded billets, and the density and arrangement of APBs, twins, stacking faults, dislocations and second phases after each pass, and relate these to the magnetic properties. The local chemistry will be explored at high resolution using atom probe tomography via collaboration with Dr. Baptiste Gault, Max-Planck-Institut fur Eisenforschung, Germany. To extend the strain range and, hence, defect densities studied we will also explore using severe plastic deformation through a collaboration with Prof. Gheoghe Gurau, University of Galati, Romania. 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 MS, while a low density of APBs, twins and stacking faults but a  high dislocation density are required for a high HC. It is thought that a fine distribution of β and γ2 phases will also contribute to a high HC through magnetic domain wall pinning.

Funded by the National Science Foundation.

Faculty contact: Ian Baker