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

电动和混合动力汽车中的风力发电机和电机等应用对高性能永磁体的需求正在迅速增长。这一市场是由基于钕-铁-硼和钐-钴的稀土(RE)磁体服务的。稀土磁铁并非没有问题；它们可能会出现碎屑，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着稀土金属市场；稀土金属的开采造成了严重的环境退化。自20世纪60年代初以来，化合物MnAl一直是人们感兴趣的永久磁铁。它有一个介于传统铝镍钴磁体和稀土磁体之间的理论能量积(磁强度的量度)，其值与粘结的钕-铁-硼磁体相当。此外，它不会受到与稀土磁体相关的问题的困扰，而且可能是所有永磁体中单位能量产品成本最低的。MnAl的磁性受到多种缺陷的影响，但这些缺陷是如何影响磁性的，以及这些缺陷是在什么条件下发生的，目前尚不清楚。在这个项目中，我们将确定在加工过程中缺陷和第二相形成的条件以及这些条件如何控制MnAl的磁性。人们的期望是，通过仔细地操纵缺陷结构，可以获得优异的磁性。虽然这项工作的重点是MnAl，但了解缺陷的发生、产生及其与MnAl磁性之间的关系将有助于理解相关化合物MnBi、MNGA、NiFe、PTFE和CoPT的相似行为，所有这些化合物都是令人感兴趣的硬磁体。该项目包括对一名博士生和几名本科生进行最先进技术的培训。Tau-MnAl是一种从高温epsilon相转变而来的亚稳相，在此期间会产生反相界(APB)、孪晶、层错和位错。根据加工条件的不同，还可以形成平衡的β相和伽马2相。提高Tau-MnAl的磁性的根本困难在于对它们与缺陷结构的关系没有清楚的认识。晶粒度也可以直接或通过影响β相和伽马2相的排列和缺陷的形成来影响磁性能。我们的目的是了解Tau-MnAl在加工过程中形成APBS、孪晶、层错、位错和第二相的条件以及这些条件如何控制磁性能。通过与德国马普研究所的Baptiste Gault博士合作，还将使用原子探测器断层扫描以高分辨率探索当地的化学物质。我们的工作假设是，要获得高饱和磁化强度，需要强的c轴取向和低密度的APB、孪晶和层错(它们局部无序)，而要获得高的矫直力，需要低密度的APB、孪晶和层错，但需要高的位错密度。人们认为，β相和伽马2相的细小分布也有助于通过磁畴壁钉扎获得高的矫直力。虽然这项工作的重点是MnAl，但了解缺陷的发生、产生及其与MnAl磁性之间的关系将有助于理解相关化合物MnBi、MNGA、NiFe、PTFE和CoPT的相似行为，所有这些化合物都是令人感兴趣的硬磁体。该项目将包括对一名博士生和几名本科生进行最先进的技术培训。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.