Presentation Information
[O12-4]Discovery of Stable Tetragonal Phase in Mn-Al-Cu Ternary System
*Tomohito Maki1, Naoki Hashimoto2, Xiao Xu2, Toshihiro Omori2, Ryosuke Kainuma2 (1. Proterial, Ltd. (Japan), 2. Tohoku Univ. (Japan))
Keywords:
Mn-Al,stable phase,tetragonal structure,phase diagram
Rare-earth permanent magnets, such as Nd-Fe-B magnets, are used in motors for automobiles, railways, home appliances, and industrial applications due to their excellent magnetic properties. As the global market for permanent magnets is expected to expand, there are resource and price escalation risks associated with rare-earth elements. Mn-Al based permanent magnets are known as permanent magnets that do not use rare-earth elements. The main phase, τ-MnAl, is a metastable phase with a tetragonal structure and tends to decompose into non-ferromagnetic β-Mn and γ2-Mn5Al8 phases. The addition of carbon improves its stability, but the τ-phase remains metastable and transforms into non-ferromagnetic phases upon heat treatment, reducing its magnetic properties. In this study, we found a stable phase that has a tetragonal structure in specific regions of the Mn-Al-Cu ternary system.
To clarify the compositional region of the stable τ-phase and its phase equilibria with surrounding phases, we experimentally determined the phase diagram at various temperatures. Mn-Al binary and Mn-Al-Cu ternary alloys were prepared by induction melting and heat-treated at temperatures from 500 to 1000°C for 3 to 40 days and then water-quenched. At 700°C, the τ-phase was found to exist stably, although its existence region is extremely narrow. The composition of the τ-phase determined by an Electron Probe Micro Analyzer (EPMA) was Mn50.5Al47.0Cu2.5 (at%). At 600°C, the region of τ-phase expanded significantly while shifting to Mn-poor region with less than 50 at% Mn, compared to the phase diagram at 700°C. At 500°C, the τ-phase extended to a region with even less Mn content compared to the case at 600°C. The τ-phase was not observed at temperatures from 800 to 1000°C.
To evaluate the magnetic properties of the stable τ-phase, a Mn-Al-Cu ingot was prepared by induction melting and heat-treated at 600°C for 7 days. X-ray diffraction (XRD) measurement showed only diffraction peaks derived from the tetragonal structure, confirming that the τ-phase was obtained as a single phase. The alloy composition determined by Inductively Coupled Plasma (ICP) emission spectrometry was Mn49.1Al48.4Cu2.5 (at%), which has a lower Mn concentration compared to the conventional Mn-Al magnet. Mn-Al-Cu alloys of about 1.5 mm in size were prepared, and the magnetization curves were measured under an applied magnetic field of 0 to 7.2 MA/m. The magnetization curve at room temperature exhibited ferromagnetic behavior. Assuming that the density of the τ-MnAlCu phase is the same as that of the τ-MnAlC phase (5.1 x 103 kg/m³), the saturation magnetization Js was 0.81 T and the anisotropy field Ha was 3.4 MA/m, which were found to be comparable to those of the τ-MnAlC phase[1, 2].
The appearance of stable τ-phase in the Mn-Al-Cu system is expected effective to avoid the thermal decomposition, thus suppresses the deterioration of magnetic properties. The results of this study may lead to the development of cost-effective permanent magnets composed only of elements with low resource risks.
[1] L. Pareti, F. Bolzoni, F. Leccabue, and A. E. Ermakov, J. Appl. Phys. 59, 3824 (1986).
[2] J. Z. Wei, Z. G. Song, Y. B. Yang, S. Q. Liu, H. L. Du, J. Z. Han, D. Zhou, C. S. Wang, Y. C. Yang, A. Franz, D. Többens, and J. B. Yang, AIP Advances 4, 127113 (2014).
To clarify the compositional region of the stable τ-phase and its phase equilibria with surrounding phases, we experimentally determined the phase diagram at various temperatures. Mn-Al binary and Mn-Al-Cu ternary alloys were prepared by induction melting and heat-treated at temperatures from 500 to 1000°C for 3 to 40 days and then water-quenched. At 700°C, the τ-phase was found to exist stably, although its existence region is extremely narrow. The composition of the τ-phase determined by an Electron Probe Micro Analyzer (EPMA) was Mn50.5Al47.0Cu2.5 (at%). At 600°C, the region of τ-phase expanded significantly while shifting to Mn-poor region with less than 50 at% Mn, compared to the phase diagram at 700°C. At 500°C, the τ-phase extended to a region with even less Mn content compared to the case at 600°C. The τ-phase was not observed at temperatures from 800 to 1000°C.
To evaluate the magnetic properties of the stable τ-phase, a Mn-Al-Cu ingot was prepared by induction melting and heat-treated at 600°C for 7 days. X-ray diffraction (XRD) measurement showed only diffraction peaks derived from the tetragonal structure, confirming that the τ-phase was obtained as a single phase. The alloy composition determined by Inductively Coupled Plasma (ICP) emission spectrometry was Mn49.1Al48.4Cu2.5 (at%), which has a lower Mn concentration compared to the conventional Mn-Al magnet. Mn-Al-Cu alloys of about 1.5 mm in size were prepared, and the magnetization curves were measured under an applied magnetic field of 0 to 7.2 MA/m. The magnetization curve at room temperature exhibited ferromagnetic behavior. Assuming that the density of the τ-MnAlCu phase is the same as that of the τ-MnAlC phase (5.1 x 103 kg/m³), the saturation magnetization Js was 0.81 T and the anisotropy field Ha was 3.4 MA/m, which were found to be comparable to those of the τ-MnAlC phase[1, 2].
The appearance of stable τ-phase in the Mn-Al-Cu system is expected effective to avoid the thermal decomposition, thus suppresses the deterioration of magnetic properties. The results of this study may lead to the development of cost-effective permanent magnets composed only of elements with low resource risks.
[1] L. Pareti, F. Bolzoni, F. Leccabue, and A. E. Ermakov, J. Appl. Phys. 59, 3824 (1986).
[2] J. Z. Wei, Z. G. Song, Y. B. Yang, S. Q. Liu, H. L. Du, J. Z. Han, D. Zhou, C. S. Wang, Y. C. Yang, A. Franz, D. Többens, and J. B. Yang, AIP Advances 4, 127113 (2014).