MICROSTRUCTURE AND MAGNETIC AFTEREFFECT IN BULK-HARDENED RARE EARTH-COBALT-COPPER BASE PERMANENT MAGNET ALLOYS

HAL Id: jpa-00214011

https://hal.archives-ouvertes.fr/jpa-00214011

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MICROSTRUCTURE AND MAGNETIC AFTEREFFECT IN BULK-HARDENED RARE EARTH-COBALT-COPPER BASE PERMANENT

MAGNET ALLOYS

A. Ray, H. Mildrum, K. Strnat, R. Harmer

To cite this version:

A. Ray, H. Mildrum, K. Strnat, R. Harmer. MICROSTRUCTURE AND MAGNETIC AFTEREFFECT IN BULK-HARDENED RARE EARTH-COBALT-COPPER BASE PERMA- NENT MAGNET ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-554-C1-555.

�10.1051/jphyscol:19711188�. �jpa-00214011�

JOURNAL DE PHYSIQUE Colloque C I, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - ⁵⁵⁴

MICRO STRUCTURE AND MAGNETIC AFTEREFFECT IN BULK-HARDENED RARE EARTH-COBALT-COPPER

BASE PERMANENT MAGNET ALLOYS (*) A. E. RAY, H. MILDRUM, K. STRNAT and R. HARMER

University of Dayton, Ohio, U. S. A.

RbumB. - Dans le compose SmCo3, sCuFeo,s, Ie durcissement magnktique dO a la precipitation du cuivre a Ctk etudi6. Les propriktes magnetiques sont likes B la structure microscopique et a la dimension des particules. L'aftereffect magnbtique est reporte pour differents btats metallurgiques du compose.

Abstract. - The magnetic hardening of SmC03.~CuFeo.

by a precipitation heat treatment has been studied, and the magnetic properties are related to microstructure and the particle size. Observations of the magnetic aftereffect in C ~ C O ~ . ~ C U F ~ ~ . ~ in different metallurgical states are reported.

I. Introduction. - In the alloys of the basic types SmCo,, CeCo, and MMCo, (MM = Ce-rich mischmetal), substantial permanent magnet properties can be developed in the massive state by partial sub- stitution of the Co by Cu or Cu + Fe andanappropriate heat treatment [I, 2, 31. In practice the most desirable condition for these alloys is a ferromagnetic

RCO, >>

matrix (R = rare earth) containing an intragranular precipitate of one or more R-Cu-based phases. Preci- pitate particles of the proper size and nature can effec- tively impede domain wall motion and thus cause coercive forces [4, 51 which can reach values of many thousand Oersted in these alloys of extremely high crystal anisotropy.

A pronounced magnetic aftereffect has been obser- ved in these alloys [4, 51, first by J. J. Becker. After magnetization reversal has been initiated, the induction continues to change strongly even at constant field strength for periods of several minutes.

11. Precipitation Hardening of SmCo, . ,CuFe,. , .

- The coercivities of massive samples of the substi- tuted RCo5 alloys are strongly dependent on the heat treatment. On the other hand, the coercivities of fine-grained powders are not significantly affected by the thermal processes.

An arc melted alloy with the nominal composition SrnCo,.,CuFe,., was homogenized at 1 100 OC for 3 hours, then rapidly quenched t o room temperature.

A very large grained, apparently single-phase micro- structure was developed by this process. Specimens of the homogenized alloy were then heated to 374O, 4190, 4730, 525O, or 576 OC for 4 h. Each specimen was then crushed and screened, various particle size fractions separated, and hysteresis loops measured on magnetically oriented samples of these. Selected results for ,Hc are shown in Figure 1.

The coarsest fraction, representing the oriented massive material, shows a pronounced rise of MHc above 4000C, followed by a decrease. We attribute this to precipitation of a second phase which can most effectively impede domain-wall motion after a 473 OC anneal. Since we could not observe discrete

(*) This work was supported in part ^by the ^U. S. Air Force Materials Laboratory, Wright Patterson Air Force Base, Ohio under Contract No F33615-69-C1172.

0 1

1

0 100 200 300 400 500 600

TEMPERATURE OF HEAT TREATMENT

FIG. 1. - SmCo3. ~CuFeo. Dependence of the intrinsic coer- cive force of oriented powders upon temperature of last heat

treatment and particle size.

precipitate particles with either optical or scanning electron microscopes, we suspect that the precipitate may still be coherent with the matrix. Discrete, orien- ted particles were observed, however, after heating the alloy to 525 OC and 576 OC (Fig. 2) above the temperature of maximum ,H,.

Finer powders of the alloy show increased coercive force, and for < 20 pm grains the effect of the heat treatment is no longer noticeable. For the finest frac- tions of any thermal history, ,H, depends on grain size as it does for pure SmCo,. Here, each grain contains only a few or no precipitate particles ; not enough t o cause wall motion coercivity. Note that the density of precipitate particles in the overaged alloy of Figure 2 is about 1 per 120 pm2. Their density a t maximum Hc is probably the same.

The precipitate particles in the overaged alloy are about 2 pm thick and 5 pm long. Electron micro- probe scans show the particles to be richer in Sm and Cu and poorer in Co and Fe than the surrounding

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711188

MICROSTRUCTURE AND MAGNETIC AFTEREFFECT IN BULK-HARDENED EARTH-COBALT-COPPER C 1 - 555

FIG. 2. - SmCo3.sCuFeo.s homogenized 1 100°C for 3 h, quenched, then reheated at 576 OC for 4 h. Scanning electron

micrograph, absorbed electron image.

matrix. Similar results have been reported for a heavily overaged Ce-Co-Cu alloy [6].

111. Aftereffect observations. - The magnetic after- effect will be discussed using demagnetization curves of samples pressed from oriented 74-105 pm particles of CeCo, ,,CuFe,. , (packing density 40- 50 %, tin binder). Figure 3 is for nearly optimal heat

FIG. 3. - Magnetic aftereffect of precipitation heat treated C ~ C O ~ . ~ C U F ~ ~ . ~ . Curve (1) traced at 1 kG/s traversal rate ; (2) several 3-minute waiting periods ; (3) steady-state curve ;

(4) B vs. H corresponding to (I).

treatment. The aftereffect is negligible in the upper part where the sample is still almost saturated. In the steep part of the curve, however, B-H drops as much as 1.5 kG, or 50 % of B,, in three minutes. The steady state curve 3 is thus appreciably different from

the quick traversal curve 1, and ,HC is lower. Because of the low packing fraction of our sample, the B vs.

H curve is almost unaffected. But for a massive sample of this alloy having twice the remanence, ,HC, (BH),,, and, especially, the dynamic behavior would be adver- sely influenced by the aftereffect.

Figure 4 is for the same alloy in the cc homogenized

state. Here, the aftereffect shows up near the rema- nence point, and the B vs. H curve is strongly displaced by it. In magnets made from cc pure

CeCo,, the aftereffect is barely noticeable, as is true of SmCo,

Ce Co,, Cu, Fe,, Ann 24HRS. AT llOODC SLOWLY COOLED Ann IHR AT I I O O ~ C WATER QUENCHED

0

FIG. 4. - Magnetic aftereffect of CeCo 3. sCuFeo.

homoge- nized and quenched. Curves (1) to (4) as in figure 3 ; (5) B vs. H curve with waiting periods ; (6) steady-state B vs. H curve.

and MMCo,. That it is present at all is presumably due to impurity particles such as oxides which also cause some wall coercivity in the massive state. The strong aftereffect of Figure 4 together with the sizeable ,H, value indicates that the quenched alloy contains a submicroscopic c( precipitate ^B which can interact with Bloch walls. Micrographs of the overaged state show precipitate in the grain boundaries and uniformly throughout the grains, but clear (Cu-depleted) regions near the boundaries. We think that unimpeded wall motion through the latter corresponds to the hori- zontal portion of the B-H curve (Fig. 3, top). Along the steep sides of the hysteresis loop, walls have to penetrate the interior of the grains where the uniform precipitate causes a fairly constant wall coercivity.

The aftereffect may correspond to a continued creep- ing of wall sections through regions of locally reduced H,, , under the influence of demagnetizing fields.

IV. Acknowledgement. - We are grateful for the experimental assistance of Mr. David Walsh. The Th. Goldschmidt A.-G. and the Ronson Corporation have kindly provided rare earth materials.

References

[I] NESBITT E A.), J . Applied Physics, 1968, 40, 1259. [5] MILDRUM (H.), RAY (A. E.) and STRNAT (K.), Proc.

[Z] TAWARA &.) and SENNO (H,), Japan. J. Applied Physics, 8th Rare Earth Res. Conf., Reno, Nevada, 1970,

1968, 7, 966. Vol. 1, 21.

f31 s r ~ N . 4 ~ (K.), RAY (A. E.) and HERGET (C.), this issue. [6] SENNO (H.) and TAWARA (Y.), Japan. J. Applied [4] STRNAT (K.), ZEEE Trans. Magnetics, 1970, MAG-6, Physics, 1969, 8, 118.

182.