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 studied in-situ using diffractive, calorimetric, and magnetometric techniques to show that the progression from ε to τ in Mn₅₄Al₄₆ at <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 Mn₅₅Al₄₅ in 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 L1₀ binary materials as well as materials that undergo a disorder–order transformation followed by displacive shear.

基于锰的无稀土永磁材料在电动机和设备中的应用显示出巨大的前景。Mn-Al系亚稳态铁磁τ相的磁性能介于高性能Nd-Fe-B磁体和低性能铁氧体磁体之间。然而，τ 形成的混合位移-扩散途径（从母体 ε 相到中间 ε' 相）仍不完全清楚。使用衍射、量热和磁力技术对这种相变进程进行了原位研究，表明 Mn ₅₄ Al _{46中从 ε 到 τ 的进程}在 <450 °C 时，ε 会有序化为 ε'。对每个相进行了密度泛函理论计算，并证实了实验观察结果，即 ε 到 ε' 到 τ 路径在能量上是有利的。淬火 ε 在 350 °C 的等温退火表明，ε' 具有铁磁性，也与理论结果一致，具有至少 50 kA/m 的中等矫顽力。观察到 τ 相沿着先前的 ε 相晶界成核并生长到 ε' 相区域。在 τ 相和 ε 相之间观察到 ε' 的边界前沿。Kissinger 和 Flynn-Wall-Ozawa 方法均用于确定 ε' 和 τ 相变的活化能，两个相的活化能值均为 ∼140 kJ/mol。所以，ε' 的有序变换和 τ 的混合位移扩散变换被证明可以克服相同量级的能垒。两种活化能均小于之前在 Mn 上测量的 τ 相活化能₅₅ Al ₄₅在没有显着的 ε' 有序放热的情况下，为 350 °C 下的 ε 到 ε' 到 τ 途径提供了动力学优势。这项研究的结果深入了解了 L1 ₀二元材料以及经历无序-有序转变和位移剪切的材料的相变。