STRUCTURE AND PROPERTIES OF A RAPIDLY SOLIDIFIED Mn-Al-C MAGNET

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STRUCTURE AND PROPERTIES OF A RAPIDLY SOLIDIFIED Mn-Al-C MAGNET

J. Gau

To cite this version:

J. Gau. STRUCTURE AND PROPERTIES OF A RAPIDLY SOLIDIFIED Mn-Al-C MAGNET.

Journal de Physique Colloques, 1985, 46 (C6), pp.C6-259-C6-262. �10.1051/jphyscol:1985645�. �jpa-

00224900�

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Colloque Ce, supplément au n°9, Tome >i(>, septembre 19â5 page Cé-259

STRUCTURE AND PROPERTIES OF A RAPIDLY S O L I D I F I E D M n - A l - C MAGNET

J . S . Gau

Research Laboratories, Eastman Kodak Company, Rochester, N.Y. 14650, U.S.A.

Résumé - Une phase nouvelle ordonnée, groupe ponctuel de symétrie 6 mm, a été identifiée dans des rubans de Mn-Al-C hypertrempés. Les poudres de Mn-Al-C obtenues par recuit des rubans à 500°C présentent un moment magné-

tique de 56 uem/g et un champ coercitif de 1640 Oe. Les aimants obtenus par frittage à chaud sous pression ou non, présentent des produits (BH) maximum inférieurs à 1 MGOe, valeurs attribuées à l'effet des pores et à l'absence de texture.

A b s t r a c t - A new o r d e r e d p h a s e w i t h t h e 6 mm p o i n t g r o u p was i d e n t i f i e d i n Mn-Al-C r i b b o n s p r o c e s s e d b y r a p i d s o l i d i f i c a t i o n . The Mn-Al-C p o w d e r s made from RSP r i b b o n s a f t e r a n n e a l i n g a t 500°C h a v e a m a g n e t i c moment of 56 emu/g and a c o e r c i v e f o r c e of 1640 Oe. The maximum e n e r g y p r o d u c t s (BH) o f s i n t e r e d m a g n e t s w i t h and w i t h o u t h o t p r e s s i n g a r e l e s s t h a n 1 MGOe, w h i c h a r e a t t r i b u t e d t o l a r g e d e m a g n e t i z a t i o n a c t i o n by t h e p o r e s and t o l a c k of t e x t u r e d e v e l o p m e n t s .

I - INTRODUCTION

The permanent magnet Mn-Al-C owes its characteristics to the ordered tetragonal x phase / l / , which is obtained from the high-temperature disordered E phase by a two-step transformation sequence either by controlled cooling or by quenching and annealing / 2 / .

In this study, a rapid solidification process (RSP) using a melt-spinning technique was used to investigate other possible new phases that might show unique properties. This process also provides a new route to prepare metal powders, and indeed, RSP crystalline alloys have gained new importance for magnetic material applications / 3 / . Structural characterization of the RSP Mn-Al-C ribbons and the magnetic properties of Mn-Al-C powders and sintered magnets are assessed.

II - EXPERIMENTAL METHODS

The rapid solidification processing of the Mn-Al-C alloy was done in an experimental melt-spinning apparatus consisting of a copper wheel rotating at 5000 rpm. The alloy ingot with a nominal composition of 69.7 wt % Mn, 29.8 wt % Al, and 0.5 wt % C was contained in a quartz tube and melted by RF inducting heating. The entire apparatus was enclosed in a He-filled chamber.

The RSP ribbons were first crushed by use of pestle and mortar, and the powders were contained in a BN (boron nitride) crucible and sintered in a multistation hot-press furnace. The powders were sintered at 1000°C for various times with or without pressing.

Structural characterization was done by scanning electron micro- scopy, x-ray diffraction, and transmission electron microscopy coupled with convergent-beam electron diffraction to determine a new struc- ture.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985645

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The magnetic properties of the powders and sintered magnets were measured with a PAR model 155 VSM (vibrating sample magnetometer), which has a maximum driving field of 5 kG.

I11 - ^{RESULTS AND} DISCUSSION

(1) Structural Characterization

The as-cast ribbon fragments are shown in Figure l-(a).

Rapid liquid quenching gave ribbons an average of 20 um thick and 2-4 mm long. Figure l-(b) shows the dendritic morphology of these cast ribbons.

Debye-Scherrer X-ray diffraction studies show that the crystal 9as a hexagonal 4isordered phase with lattice parameters of a = 2.697 A and c = 4.396 A. This could mislead one to interpret the phase in RSP ribbons as the high-temperature disordered

phase /3,4/.

Convergent-beam electron diffraction (CBED) studies show that it is, in fact, a new ordered phase.

Figure 2 shows various high-symmetry zone axis microdiffraction patterns where the existence of superlattice reflections is noted.

Figure 3 shows the high'-symmetry [0001] zone axis CBED pattern; both the whole pattern and the central disk have 6 mm symmetry. This observation limits the diffraction group to either 6 mm or 6 mm/R. To distinguish between these two, it is necessary to displace the condenser aperture to the Bragg angle, that is, halfway between the central disk and the diffracted disk, and the specific position of the dark field shows only m (mirror) symmetry, as shown in Figure 3-(b), instead of 2 mm. Therefore, the diffraction group is identified as 6 mm. Finally, referring to Table 3 of reference /5/, the krystal point group is 6 mm, i.e., no inversion center.

The RSP ribbons are not magnetic. Magnetism is strongly related to chemical bonding /6/, which is reflected in the crystal structure.

The brittleness of the as-cast RSP ribbons is also attributed to the resultant ordered phase, which crystallizes in a low-symmetry crystal structure and does not have enough slip system to permit general plastic deformation.

When the ribbons are annealed at 400-600°C, the metastable hexagonal ordered phase changes to the ferromagnetic

phase. Twins, in addition to grain boundaries, are the typical structural features shown by TEM. However, some randomly distributed precipitates were seen in the bright- and dark-f ield images. With the corresponding ring pattern of the precipitates in the selected-area diffraction (SAD) pattern, the precipitates were shown to have the fcc structure of the perovskite-type Mn3A1C precipitates.

(2) Properties of Powders

Mn-A1-C powders obtained by crushing the brittle ribbon fragments with a pestle and mortar were annealed at about 500°C for 2 h and then water quenched to room temperature. The resultant magnetic properties were: magnetic moment, 56 emu/g (at 5 kOe field); coer- civity, 1640 Oe. The coercivity of magnetic recording material generally categorized as hard magnetic material /7/ can be neither too high to be driven by the recording head nor too low to be due to instability; meanwhile, a level of remanent magnetization with a magnetic moment of at least 80 emu/g must be carefully optimized with respect to the coercivity, the coating thickness, and the recording density. Magnetic Mn-A1-C powders can be competitive for magnetic recording medium applications by use of alloy chemistry to enhance magnetization and by process modification to alter coercivity through the control of the size, shape and distribution of powders.

(3) Structure and Properties of Sintered Magnets

Manganese oxides are formed on the surfaces of sintered

Fig. 1 - ^SEM micrographs: (a) as-cast ribbon fragment; (b) dendrite morphology.

Fig. 2 -- Various high-symmetry zone Fig. 3 - ^A CBED pattern of the axis electron microdiffraction [0001] zone axis (a) 6

symmetry patterns of the RSP ribbon. for both the central disk and the whole pattern (b) with the [l0101 reflection revealing m (mirror) symmetry.

(b) Hot pressinp

Ff g. 4 - (a) Demagnetization curves of sintered magnets without and w ~ t h hot pressing. (b) Hysteresis loops of a hot-pressed magnet with uniaxial pressing direction parallel and perpendicular to the

induction field as indicated.

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magnets because of the reaction between the residual oxygen in the sintering atmosphere and atomic Mn, which has a very high vapor pressure. No significant microstructural differences except grain size were observed between magnets sintered with and without hot pressing. The structural characteristics of the sintered magnet are less complicated than those in the extruded state /8/. The demagneti- zation curves of the sintered magnets are given in Figure 4-(a). The maximum energy products are less than 1 MGOe. The magnetic properties of sintered magnets are inferior to those of cast magnets intended as permanent magnets. One of the main reasons is that no texture has ever been developed. Figure 4-(b) shows the hysteresis loops of a hot-pressed magnet with a uniaxial pressure of 6000 psi measured with the magnet parallel and perpendicular to the induction field. A magnetically uniaxial material with crystalline anisotropy would have unequal values of coercivity for the two directions, should texture be present. The hot pressing offers a slight increase in coercivity and induction by contributing plastic flow for higher densification and less pore density. Furthermore, a uniaxial magnet with randomly oriented crystals should have a retentivity of 0.5. However, the ratio of remanance to saturation is less than 0.5, as shown in Figure 4-(b). The low ratios suggest that there is significant internal demagnetization, probably atpores.

IV - CONCLUSION

A new hexagonal ordered phase was observed in rapidly solidified Mn-A1-C ribbons. The 6 mm point group of the new structure was determinedby convergent-beam electron diffraction.

The magnetic properties of powders obtained by crushing the RSP ribbons and annealed at 500°C were evaluated; these powders have potential for magnetic recording media applications.

The magnetic properties of sintered magnets were inferior to those of cast ones. Even with a uniaxial pressure of 6000 psi, there was no development of texture.

ACKNOWLEDGMENT

The author gratefully acknowledges Professor G. Thomas for his guid- ance and Pgofessor W. L. Johnson of Cal. Tech. for conducting RSP experiments. The alloy ingot was kindly supplied by the Matsushita Electric Industrial Company of Japan. Thanks are also extended to Drs. R. Mishra and M. Sarikaya for helpful discussions. This work was done when the author was affiliated with Department of Materials Science and Engineering, University of California, Berkeley, and supported by the U. S. Department of Energy.

REFERENCES

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