Equal Channel Angular Extrusion (ECAE) Processing of Tau-MnAl Magnets

对于电动和混合动力汽车中的风力涡轮机发电机和电动机等应用，对高性能永磁体的需求正在迅速增加。该市场由基于钕铁硼和钐钴的稀土（RE）磁体服务。稀土磁体并非没有问题;它们可能会碎裂，遭受热冲击，并可能遭受晶界腐蚀。然而，他们最大的问题是：价格波动;中国在很大程度上控制了稀土金属市场;稀土金属的提取造成了严重的环境退化。自20世纪60年代初以来，化合物MnAl作为永磁体一直受到关注。它具有介于传统AlNiCo磁体和RE磁体之间的理论能量积（其磁强度的量度），其值与粘结钕铁硼磁体的值相当。此外，它不会受到与RE磁体相关的问题的困扰，并且潜在地具有任何永磁体的每单位能量产品的最低成本。MnAl的磁性能受到存在的多种类型的缺陷的影响，但是这些缺陷如何影响磁性能以及这些缺陷在什么条件下发生尚不清楚。在本项目中，我们将确定在MnAl加工过程中形成缺陷和第二相的条件以及这些缺陷和第二相如何控制磁性能。期望通过仔细地操纵缺陷结构可以获得上级磁性能。虽然这项工作的重点是MnAl，但了解缺陷的发生，它们的产生及其与MnAl中磁性能的关系之间的关系将有助于了解相关化合物MnBi，MnGa，NiFe，PtFe和CoPt中的类似行为，所有这些都是令人感兴趣的硬磁体。该项目包括培养一名博士和一名研究生。Tau-MnAl是一种亚稳相，它是从高温下的Al 2 O3相转变而来的，在此过程中会产生反相界（APB）、孪晶、堆垛层错和位错。取决于加工条件，还可以形成平衡β和γ 2相。提高τ-MnAl磁性能的根本困难在于，对它们如何依赖于缺陷结构没有清楚的理解。晶粒尺寸也可以直接或通过影响β和γ 2相排列和缺陷形成来影响磁性。我们的目的是了解在加工过程中形成的APB，孪晶，堆垛层错，位错和第二相的条件以及这些如何控制磁性能。还将与德国马克斯-普朗克研究所的Baptiste Gault博士合作，利用原子探针断层摄影术，以高分辨率探索当地的化学性质。我们的工作假设是，对于高饱和磁化强度，我们需要强的c轴对准和低密度的APB、孪晶和堆垛层错（其局部地使材料无序），而对于高饱和磁化强度，需要低密度的APB、孪晶和堆垛层错，但是需要高位错密度。据认为，β和γ 2相的精细分布也将通过磁畴壁钉扎而有助于高矫顽力。虽然这项工作的重点是MnAl，但了解缺陷的发生，它们的产生及其与MnAl中磁性能的关系之间的关系将有助于了解相关化合物MnBi，MnGa，NiFe，PtFe和CoPt中的类似行为，所有这些都是令人感兴趣的硬磁体。该项目将包括培训一名博士和一名研究生。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.