Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C, B, and rare earth elements
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- Liu, Z.W., Chen, C., Zheng, Z.G. et al. J Mater Sci (2012) 47: 2333. doi:10.1007/s10853-011-6049-8
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MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn55Al45 based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L10 (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L10 τ-phase and improved the hard magnetic properties. The Curie temperature TC of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on TC. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially JS. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L10 hard magnetic τ phase.
Hybrid cars and electric vehicles require increasing quantities of rare earth magnets, e.g., Dy-substituted Nd2Fe14B, SmCo5, and Sm2Fe17N, because of their excellent magnetic properties. However, rare earth (RE) resources have become a major issue in the international community. Developing high performance permanent magnets without RE elements is therefore an urgent need. MnAl alloys are attractive candidates to replace rare earth permanent magnets because of their superior mechanical strength, excellent machinability, and reasonable magnetic properties. Importantly, these hard magnets are relatively low cost because they do not contain rare earth or other elements in limited supply such as Co and Ni. The hard magnetic properties of MnAl alloys derive from the formation of a L10 intermetallic phase (tetragonal τ-MnAl) characterized by strong, uniaxial magnetocrystalline anisotropy (106 J/m3) with an easy c-axis [1–4]. Unfortunately, the τ phase is metastable, forming from a quenched-in high-temperature hexagonal (ε) phase by annealing at ~550 °C . The parent hexagonal disordered ε-phase is antiferromagnetic (TN = 97 K) . It has been found  that addition of small amounts of carbon stabilizes the τ phase and prevents the decomposition of the alloy into the stable but nonmagnetic γ (Al8Mn5) and β (Mn) phases. The stabilized τ-phase can also be obtained by slow cooling (<8 °C/min) from the temperature interval, where the ε-phase is stable (from above 800 °C) [1, 7].
The magnetic properties of Mn–Al–C alloys are dependent on the composition and microstructure, which is in turn strongly influenced by τ-phase formation . Some previous investigations showed that an alloy with 1.7 at.% C has the best hard magnetic properties since the uniformly dispersed fine Mn3AlC phase in the τ phase can pin the domain walls and improve coercivity [5, 8–10]. The best magnetic properties with maximum energy product (BH)max up to 64.4 kJ/m3 have been reported in an anisotropic Mn–Al–C alloy obtained by high-temperature extrusion [11, 12].
Despite the above progress, MnAl-based alloys have not been fully studied and explored, possibly because of the existence of RE magnets (such as NdFeB and SmCo) with very good magnetic properties. There has not been much work on MnAl alloys except those focused mainly on MnAl thin films . The previous literature also reveals inconsistent. Zeng et al.  investigated the MnAl base alloys produced by mechanical milling and subsequent annealing. A high coercivity of ~380 kA/m was obtained for nanostructured Mn54Al46 annealed at 400°C. They also obtained Curie temperatures TC from 346 to 382 °C for Mn–Al binary alloys and 329 to 338 °C for MnAlC ternary alloys with 1.7 at.% C . However, Fazakas et al.  investigated the Mn54Al44C2 alloy synthesized by melt spinning followed by annealing and found very different results. The TC of this alloy was only 247 °C and the coercivity was less than 103 kA/m, which is much lower than the values reported by Zeng et al. . Except for this study, synthesis of MnAl-based hard magnetic alloys by rapid quenching has received almost no attention. The melt spinning is a feasible technique to prepare magnetic materials, such as NdFeB and SmCo alloys. This technique is useful in formation of nanostructures and is scalable for mass production as well as for fundamental investigations of phase transitions. Hence, melt spinning was used to prepare MnAl-based alloys in this study. The MnAl magnets with elemental additions other than C have been rarely studied. Hence, in this study, the phase transformations and the effect of heat treatment on MnAl(C) alloys prepared by melt spinning are investigated. RE elements (Pr and Dy) having 4f magnetism were used as dopants to improve the magnetic properties. In addition, substitution of C by B, another group IIA element, was also examined to elucidate its effects on the structure and properties.
MnAl alloy ingots with various compositions, including Mn55−xAl45Cx (x = 0, 1, 1.7 and 2), Mn53.5Al45B1.5, and Mn52.3Al45C1.7RE1 (RE = Pr or Dy) were produced by argon arc melting. Special care has been paid to melting the alloys because of the large differences in the melting point and saturated vapor pressure of the raw materials (Mn, Al, and C). The as-prepared ingots were used to prepare ribbon samples by a single-roller melt spinning technique under protective atmosphere (argon) at a wheel speed of 40 m/s. The evolution of the microstructure of the as-spun samples was examined by X-ray diffraction (Philips, Cu Kα radiation). Differential scanning calorimetery (Perkin-Elmer TGA7) was employed to study the structural transformations in the temperature range of 300–1000 °C with a temperature ramp rate of 20 °C/min. The as-spun ribbons were annealed at 500–650 °C for 10 min under argon atmosphere. The magnetic properties at various temperatures were characterized by a physical properties measurement system (PPMS, Quantum Design Co.) equiped with a vibrating sample magnetometer (VSM), using an applied field of 5 T.
Results and discussion
The magnetic properties of melt spun Mn–Al based alloys are much lower than that reported for MnAlC magnets prepared by hot extrusion of annealed gas-atomized powders ; processing has an important effect on the microstructure and properties. Zeng et al.  also suggested that the magnetic hysteresis behavior of Mn–Al–C is extremely sensitive to the microstructure and defects introduced during the formation of the τ-phase within the high-temperature ε-phase. High hard magnetic properties for commercial products produced by hot extrusion are a result of high anisotropy, grain size reduction and carbide precipitations. The coercivities of our samples are also lower compared to those obtained by Zeng  from ball milled MnAlC samples. However, as indicated earlier, the magnetic properties obtained in this work are very close to these obtained by Fazakas et al.  using the same preparation method of melt spinning. The difference in the properties between melt spun and ball milled alloys could result from these two distinct preparation methods, which lead to different grain structures and defects. It is not easy to achieve nanostructures in MnAl alloys by melt spinning because of the low glass formability of this alloy. We believe that the magnetic properties can be improved by reducing grain size and inducing anisotropy. The results thus indicated that an appropriate processing technique and treatment process has to be developed for making full use of this low cost permanent magnet.
As-quenched MnAl alloys with C doping or C and RE additions exhibits a single phase structure, consisting of the hcp ε phase. After annealing at suitable temperatures, ε can transform to the metastable ferromagnetic τ phase. The effects of composition and heat treatment on the phase transition and hard magnetic properties have been investigated. Addition of C is beneficial to the formation of the τ phase and, thus to the hard magnetic properties. C content also has a significant effect on the TC of τ phase. 2% C addition reduced the TC from 346 to 258 °C. The Mn53.3Al45C1.7 ribbon after annealing at 650 °C for 10 min has the best combined magnetic properties. Doping of rare earth elements Pr can slightly improve the hard magnetic properties, but Dy does not have a positive effect. B does not stabilize the hard magnetic phase, the unstable ε phase transform to intermediate phases during annealing. Our results indicated that the properties of MnAl-based alloys depend strongly on the processing method.
This study is partly supported by the Natural Science Foundation of China (Grant Nos. 50874050 and 51174094) and the Fundamental Research Funds for the Central Universities, SCUT (Grant No. 2009ZZ0025). ZWL also thank the School of Materials Science and Engineering at the Nanyang Technological University for a visiting professorship.