The phase transformation behavior of Mn-Al rare-earth-free permanent magnets
The phase transformation behavior of Mn-Al rare-earth-free permanent magnets
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DOI：
10.1016/j.jmmm.2023.171331
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发表时间：
2023-09
影响因子：
2.7
通讯作者：
Thomas Keller;Dylan Barbagallo;Natalya Sheremetyeva;Tushar Kanti Gosh;Katherine S. Shanks;G. Hautier;Ian Baker
中科院分区：
文献类型：
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作者：
Thomas Keller;Dylan Barbagallo;Natalya Sheremetyeva;Tushar Kanti Gosh;Katherine S. Shanks;G. Hautier;Ian Baker
锰基无稀土永磁材料在电机和器件中具有广阔的应用前景。Mn-Al系亚稳态铁磁τ相具有介于高性能NdFeB磁体和低性能铁氧体磁体之间的磁性能。然而，τ形成的混合置换-扩散途径，从母体ε相到中间ε‘相，仍然不完全清楚。用衍射法、量热法和磁强计对这一相变级数进行了现场研究，结果表明，在Mn54Al46at<；450°C中，从ε到τ的相变级数涉及ε到ε‘的有序化。对各个阶段进行了密度泛函理论计算，证实了实验观察到的ε-ε‘-τ途径是有利的。350C等温热处理表明，ε‘是铁磁性的，具有中等的矫顽力，至少为50kA/m。观察到ε相沿先前的τ相晶界形核并生长到ε’相区。在ε相和τ相之间观察到ε‘的边界锋面。用基辛格方法和Flynn-Wall-Ozawa方法计算了ε‘和τ相变的活化能，得到两个相变的活化能为140kJ/∼。因此，对ε‘的有序变换和对τ的混合位移-扩散变换克服了相同量级的能垒。在没有明显τ‘有序放热的情况下，这两个激活能都小于之前在Mn55Al45上测得的ε相激活能，这为350℃下ε-ε’-τ途径提供了动力学有利条件。
Rare-earth-free permanent magnet materials based on Mn show great promise for applications in electric motors and devices. The metastable ferromagnetic τ phase of the Mn-Al system has magnetic properties between those of the high-performance Nd-Fe-B magnets and the lower-performance ferrite magnets. However, the hybrid displacive-diffusional pathway of τ formation, from the parent ε phase through the intermediary ε’ phase, is still not fully understood. This phase transformation progression was studiedin-situusing diffractive, calorimetric, and magnetometric techniques to show that the progression from ε to τ in Mn54Al46at <450 °C involves the ordering of ε into ε’. Density functional theory calculations were performed on each phase and confirmed the experimental observation that the ε to ε’ to τ pathway is energetically favorable. Isothermal annealing of quenched-in ε at 350 °C demonstrated that ε’ is ferromagnetic, also in agreement with theoretical results, with a moderate coercivity of at least 50 kA/m. The τ phase was observed to nucleate along the prior ε phase grain boundaries and grow into the ε’ phase regions. A boundary front of ε’ was observed between the τ and ε phases. Both Kissinger and Flynn-Wall-Ozawa methods were used to determine the activation energies for the ε’ and τ phase transformations with values of ∼140 kJ/mol obtained for both phases. Therefore, the ordering transformation to ε’ and the hybrid displacive-diffusional transformation to τ were shown to overcome the same magnitude energy barrier. Both activation energies were less than previous τ phase activation energies measured on Mn55Al45in the absence of a significant ε’ ordering exotherm, providing a kinetic benefit to the ε to ε’ to τ pathway at 350 °C. The results of this study give insight into the phase transformation of L10binary materials as well as materials that undergo a disorder–order transformation followed by displacive shear.