Role of primary Zr-rich particles on microstructure and magnetic properties of 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

doi:10.1016/j.jmst.2020.04.018

Abstract

Abstract:

Trace Zr addition is essential for achieving desired cellular nanostructure and large coercivity in the pinning-controlled 2:17-type Sm-Co-Fe-Cu-Zr magnets that have served as the strongest high temperature permanent magnets for over 40 years. However, accompanying this is the formation of Zr-rich particles that may deteriorate the hard magnetic properties. Besides the formerly-reported 1:3R Zr-rich platelets, in this work, 1-2 μm sized Zr₆(Co, Fe)₂₃ (6:23) particles (Fm $\bar{3}$ m) and 100-200 nm sized 1:3R Zr-rich particles were also found based on combined structural identifications and element mapping analysis. Around such particles, the desired 1:5H cell wall precipitates that provide the strongest pinning force of magnetic domain wall motions are rare, forming the precipitate-free-zones (PFZs). The 1:5H-PFZs and the soft magnetism of both 6:23 and 1:3R Zr-rich particles act as local weak pinning points, which are unfavorable to retain the large magnetization in strong opposite fields and lead to poor squareness. As observed in a Sm₂₅Co_45.9Fe_19.5Cu_5.6Zr_4.0 (wt.%) magnet, the co-existence of such Zr-rich particles and the associated 1:5H-PFZs leads to a pretty low squareness factor of only 52.89 % given the large coercivity of 29.04 kOe. Our findings suggest that careful controlling the Zr content and avoiding its aggregation to form harmful 6:23 and 1:3R Zr-rich particles are essential for achieving high squareness as well as large energy product in the Sm-Co-Fe-Cu-Zr permanent magnets.

Key words: Permanent magnets, Sm-Co alloys, Microstructure, Precipitates

Tao Yuan, Xin Song, Xianglong Zhou, Wentao Jia, Munzali Musa, Jingdong Wang, Tianyu Ma. Role of primary Zr-rich particles on microstructure and magnetic properties of 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets[J]. J. Mater. Sci. Technol., 2020, 53: 73-81.

Figures/Tables 9

Fig. 1. (a) Room temperature initial magnetization curves and magnetization hysteresis loops for the thermally demagnetized magnets. (b) Second derivative curves of the initial magnetization curves, where the arrows indicate the pinning fields.

Table 1 Room temperature magnetic properties of the Fe-16.2 and Fe-19.5 magnets derived from the magnetization hysteresis loops.

Magnet	(BH)_max (MGOe)	B_r (kGs)	H_cj (kOe)	SF (%)	(BH)_max /(B_r²/4) (%)
Fe-16.2	28.2	11.05	34.32	64.45	92.38
Fe-19.5	26.6	11.10	29.04	52.89	86.29

Fig. 2. Back-scattered SEM image and elemental mapping images for the Fe-16.2 magnet. Solution-treated (a) and final (b) states, characterized using Hitachi-SU3500 and JEOL JSM-IT500HR SEM, respectively.

Fig. 3. (a) Bright field image for the Fe-16.2 final magnet taken along [001]2:17R zone axis. White circle indicates where the inset diffraction pattern was taken. (b)?(d) HR-TEM image and SAED patterns of the 6:23 Zr-rich particle (red circle region in (a)).

Fig. 4. (a) Bright field image for the Fe-16.2 magnet taken along [001]2:17R zone axis. (b) Enlarged view of the 1:5H precipitate free zone (PFZ) around the 6:23 Zr-rich particle.

Fig. 5. (a) Bright field image of the Fe-19.5 magnet taken along [100]2:17R zone axis, containing a microsized Zr-rich particle. (b) The corresponding SAED pattern. (c)-(f) STEM-EDS elemental mapping images. (g) Bright field image of the typical region free of 6:23 Zr-rich particle. (h) HR-TEM image around the 6:23 Zr-rich particle, the inset FFT pattern is for the 2:17R’ phase.

Fig. 6. TEM characterizations of the grain edges for the Fe-19.5 magnet. (a) and (b) Bright field image and SAED pattern taken by tilting grain 1 (G1) along [100]2:17R zone axis. (c) and (d) STEM image and Zr mapping image.

Fig. 7. TEM characterizations of the 1:3R Zr-rich nanoparticles at grain edge of the Fe-19.5 magnet. Bright field image, HR-TEM image, FFT patterns of the 1:3R Zr-rich nanoparticle and the 2:17R phase, characterized along the [100]2:17R (a?d) and [210]2:17R (e?h) zone axes, respectively.

Fig. 8. TEM characterizations of the solution-treated Fe-19.5 magnet. (a) HR-TEM image and (b) SAED pattern taken along [100]2:17R zone axis. (c)-(e) FFT patterns of the 1:3R Zr-rich nanoparticle, the 2:17R nano-variant and the region with stacking faults, respectively.

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