(3N-Lab Workshop on Permanent Magnets, Tsukuba, March 24th, 2017.)
Microstructure and coercivity of Sm2(Co,Fe,Cu,Zr)17 type sintered magnets

H. Sepehri-Amin1, J. Thielsch2, T. Ohkubo1, J. Fischbacher3, T. Schrefl3, O. Gutfleisch4, and K. Hono1

1 National Institute for Materials Science, Tsukuba 305-0047, Japan
2 IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, D-01171 Dresden, Germany
3 Center for Integrated Sensor Systems, Danube University Krems, Austria
4 Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany

Abstract:

Fig. 1. (a) and (b) STEM-HAADF image and (c)
and (d) 3DAP atom maps of Sm and Cu and
calculated composition profile obtained from
the Sm2Co17-type magnets quenched and slowly
cooled down from aging temperature of 850℃ [3].
  Excellent hard magnetic properties of Sm2Co17-type sintered magnets at elevated temperature make them the only choice for the applications above 300℃. However, the coercivity of Sm2Co17-type permanents magnets strongly depends on the cooling rate from aging temperature of ~850℃; No coercivity for rapidly quenched sample while the coercivity is enhanced to ~2.0 T after slow cooling [1]. This has been correlated to the Cu content and its distribution in the cell boundary phase [2]. Questions raise here; does just small increase of Cu in the cell boundary phase substantially enhances coercivity or are there other microstructural features influencing the coercivity of Sm2Co17-type permanent magnets? In this work, we have revisited the microstructure of Sm2Co17-type sintered magnets with different coercivity levels and discussed the coercivity mechanism by employing finite element micromagnetic simulations to answer to these open questions [3].
  Commercial Sm(Co0.784Fe0.100Cu0.088Zr0.028)7.19 sintered magnets with two different heat treatment conditions, one quenched rapidly and the other slowly cooled from 850℃ were studied. The sample slowly cooled down from 850℃ showed the high coercivity of 2.6 T, while coercivity of the quenched sample was only 0.14 T. Figure 1 (a) and (b) show high resolution STEM-HAADF images obtained from the quenched and slowly cooled samples respectively. Sm2Co17 matrix phase, SmCo5 cell boundary phase, and Z-phase are observed in the microstructure. Unlike the quenched sample, SmCo5/Sm2Co17 interface is sharp and smooth in the slowly cooled sample. Figure 1 (c) and (d) show 3D atom maps of Sm and Cu and their composition profiles obtained from the two different cell boundaries of the quenched and slowly cooled samples. Enrichment of 8.6 at. % of Cu and 7.7 at. % Fe was found in the cell boundary of the quenched sample while the SmCo5 cell boundary phase of slowly cooled down sample contains 15.4 at. % of Cu and 3.0 at. % Fe. In addition, the distribution of Cu broader than that of Sm was found in the cell boundary of the quenched sample. Micromagnetic simulations showed that the enrichment of Fe in the cell boundary and the broad distribution of Cu results in a smaller gradient of K1 through the 2:17/1:5 interface, which decreases the pinning strength of the cell boundary phase. This explains the low coercivity in the quenched sample.
[1] D. Goll, et al. Appl. Phys. Letters 76 (2000) 1054-1056.
[2] X. Y. Xiong, et al. Acta Mater. 52 (2004) 737-748.
[3] H. Sepehri-Amin, et al. Acta Mater. 126 (2017) 1-10.

Related Links:
Page top