HAL Id: jpa-00214010
https://hal.archives-ouvertes.fr/jpa-00214010
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.
INVESTIGATIONS OF PERMANENT MAGNET ALLOYS BASED ON COBALT AND CERIUM
MISCHMETAL
K. Strnat, A. Ray, C. Herget
To cite this version:
K. Strnat, A. Ray, C. Herget. INVESTIGATIONS OF PERMANENT MAGNET ALLOYS BASED
ON COBALT AND CERIUM MISCHMETAL. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-
552-C1-553. �10.1051/jphyscol:19711187�. �jpa-00214010�
JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 552
INVESTIGATIONS OF PERMANENT MAGNET ALLOYS BASED ON COBALT AND CERIUM MISCHMETAL
K. STRNAT and A. E. RAY University of Dayton, Ohio, U. S. A.
and C. HERGET,
Th. Goldschmidt A.-G., Essen, Germany
Rhumb. - L'auteur est arrivk a produire de grands champs coercitifs compris entre 4 000 et 5 000 Oe avec des poudres d'alliage MMCo5, oh MM est un mischmktal riche en ckrium. Ces rksultats sont obtenus aprb broyage, traite- ment B l'acide et zinguage par diffusion. On a etudik le durcissement obtenu par prkcipitation dans M M C O ~ - ~ C U ~ . On a obtenu a ce jour les valeurs M H ~
=4 320 Oe et MdMP
=0,98 pour x
=2. Ainsi MMCo5 et MMCo3Cu2 sont des matCriaux interessants pour fabriquer des aimants permanents relativement Cconomiques du type RCo5.
Abstract. - The intrinsic coercivity of powders of MMCo5 (MM = cerium mischmetal) could be raised to values between 4 000 and 5 000 Oe by ballmilling, an acid treatment and zinc diffusion coawfing, alternatively. The precipitation hardening of M M C O ~ - ~ C U Z was studied and optimized. M H ~ = 4 320 Oe and Mr[Mp
=0.98 were achieved for x
=2. MMCos and MMCo3Cuz are thus attractive choices for relatively inexpensive permanent magnet materials of the RCOS type.
I. Introduction. - Of all the rare earth-cobalt alloys which have utility for permanent magnets [I], those in which the rare earth (R) component is the conventional cerium-rich mischmetal (MM)
(I)are the economically most attractive at this time. However, the properties of MMCo, powders prepared by nor- mal grinding techniques are not good enough.
The intrinsic coercive force of particles that can be well oriented appeared limited to MHc = 2 500 Oe[2,3].
Higher values, to > 4 000 Oe, could be achieved only by means that also largely destroyed the nepded particle anisotropy [4]. This paper is a report about successful attempts t o apply to MMCo, various methods of improving the coercivity which had pre- viously been tried on powders of other related subs- tances.
11. Grinding in Liquid Nitrogen. - Since the H,- limitations are attributed to plastic deformation of the particles, grinding in a more brittle state at low temperatures had been suggested and tried before with indications of improvement [I, 41. We ground in an attritor mill [3, 51, with balls and powder immer- sed in liquid nitrogen (LN), and measured hysteresis loops on aligned, epoxy-bonded, dilute samples.
Figure 1 shows the results of grinding two different ingots in LN and for comparison in iso-octane at room temperature. Coercivities up to 4 550 Oe with a loop squareness of M J M , = 0.78 were obtained. Such fine powders were pyrophoric in air at room tempera- ture after the LN had evaporated.
111. Particle Surface Treatments. - Becker [6] des- cribed methods of improving the coercivity of YCo5 and SmCo, particles, produced by normal grinding, by removing their strongly deformed, oxidized and irregularly shaped surface layer in an acid bath, o r magnetically deactivating it with zinc. We have beneficially applied both methods first to PrCo, [5]
and now to MMCo,. Figure 2a, b shows the impro-
.f
POWDER IGNITED OCTANE0 3 0 6 0 90 120 150
GRINDING TIME (MINUTES1
FIG. 1. - Coercivity of MMCos powders made by attritor grinding in liquid nitrogen and in iso-octane at room temperature
(Alloy A induction melted, B arcmelted),
FIG. 2. - Improvement of M H ~ by surface treatment of par- ticles made by mortar grinding : Curve (a) before, (b) after immersion in acid mixture. Curve (c) before, (d) after zinc
diffusion treatment.
(1)
MM
=Ronson Ceralloy 100 X. Analysis : 54.5 weight vements on mOrtar-ground powders that
% Ce, 26 La, 13 Nd, 5 Pr, remainder mostly other rare earths. were soaked for 8 minutes in a solution of 1 part
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711187
INVESTIGATIONS OF PERMANENT MAGNET ALLOYS BASED ON COBALT C 1 - 553 each of nitric, orthophosphoric and sulfuric acids,
5 parts acetic acid and 32 parts water. This raises ,HC for the < 20 pm fraction by a factor 4 to 4 000 Oe, with a remanence ratio of 0.9. Figure 2c, d shows the effects of heating powders to 4000C for one hour with 5 % Zn dust added. The optimum in this case is ,HC = 4 800 Oe and Mr/Mp = 0.85 for 10-20 pm particles. In both cases, the highest coercivities pos- sible by ballmilling were far exceeded - even with these rather coarse powders - while the alignability of the particles in a field is maintained.
IV. Precipitation Hardening of MM(Co, Cu), alloys. - SmCo,, CeCo, and solid solutions of these can be magnetically hardened in bulk, form by substituting copper or Cu f Fe for a part of the cobalt [7, 81 but an earlier attempt t o do this with MMCo, failed [3]. We investigated a series of alloys of the type MMCo,-,Cu,, varying composition and heat treatment. The alloys were produced by arcmelting.
Hysteresis loops were measured on coarse (74- 105 pm), magnetically aligned particles, which act essentially like grain-oriented, massive material.
Figure 3 summarizes coercive-force data for all
alloys in various states of heat treatment. The opti- mum properties, ,H, = 4 320 Oe with excellent loop squareness (Mr/Mp = 0.98), were obtained for an alloy of x = 2, homogenized at 1 1000C, cooled moderately slowly in a quartz bulb in stagnant air and then held four hours at 500 OC. This alloy has a saturation 4 nM, = 5 350 G . Energy products up to 7 MG . Oe should thus be possible for massive pieces.
Figure 4 shows the dependence of ,Hc on the heat treatment for two of the compositions. The final heat treatment does little to improve the properties of the slow-cooled alloy, but it can nearly completely restore the coercivity after lowering the latter drasti- cally by solution annealing and quenching.
01
I I 1 I I I0 2 0 0 400 600
TEMPERATURE OFHEAT TREATMENT (OC) FIG. 4. - Dependence of ixHc on temperature of the last heat treatment : (a) and (c) 4 hours at temperature after slow cooling from 1 100 OC. (b) and ( d ) 4 hours at temperatfre after quench-
ing from 1 100 OC.
I
