The Formation of Monocrystalline Alnico Magnets by SecondaryRecrystalization MethodsE. Steinort, E. R. Cronk, S. J. Garvin, and H. Tiderman Citation: Journal of Applied Physics 33, 1310 (1962); doi: 10.1063/1.1728707 View online: http://dx.doi.org/10.1063/1.1728707 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/33/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Some Aspects of Precipitation and Magnetic Properties of Alnico Alloys J. Appl. Phys. 40, 1308 (1969); 10.1063/1.1657644 Magnetic Properties of Alnico Alloy Phases and Temperature Instability of Permanent Magnets J. Appl. Phys. 40, 1307 (1969); 10.1063/1.1657643 Long Term Magnetic Stability of Alnico and Barium Ferrite Magnets J. Appl. Phys. 31, S82 (1960); 10.1063/1.1984613 “Interaction Anisotropy” Model of the Structure of Alnico Magnet Alloys J. Appl. Phys. 31, S78 (1960); 10.1063/1.1984611 Influence of Various Heat Exposures on Alnico V. Magnets J. Appl. Phys. 30, S115 (1959); 10.1063/1.2185848

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JOURNAL OF APPLIED PHYSICS SUPPLEMENT TO VOL. 33, NO.3 MARCH, 196?

The Formation of Monocrystalline Alnico Magnets by Secondary Recrystalization M.ethods

E. SJ'EINORJ'

Cenlro-Magneti Permanenti, S. P. A., Mum!, Italy

AND

E. R. CRONK, S. J. GARVIN, AND H. TIDERMAN

Thomas &> Skinner Inc., Indianapolis, Indiana

A brief review of the progress of the" Alnico Art" is given, trac­ing early attempts at increasing energy yield by changes in com­position to present day techniques of promoting polycrystaIIine growth parallel to the magnetic axis. Energy product values have steadily risen to the present commercially available level of 7.5 (MGO). Since research has apparently reached a temporary im­passe in further improvement in polycrystalline Alnico magnets, except by an uneconomic fabrication from a much larger initial casting or expensive foundry mold techniques an, attempt to take advantage of the 12.0 mega gauss-oe energy product available from single crystals of Alnico is an obvious solution. Two known methods of producing single crystals sufficiently large to be of practical value are described, and the obvious economic and pro­duction problems discussed. New work on a third method is de­scribed in which normal foundry techniques are used, but the formation of a single monocrystal of the entire magnet is promoted

T HIS paper is based on the original work of Mr. Eberhard Steinort of Centro-Magneti Perman­

enti, Milan, Italy, and attempts to outline the basic concepts of his invention.

The materials presently known under the generic title of Alnico had their origin in the period 1931-1933 through studies by T. Mishima, K. Honda, and H. Matsumoto of the iron-nickel-aluminum, and iron­nickel-cobalt-titanium systems. Progress in developing magnets with progressively higher energy products was initially confined to changes in composition until heat­treating techniques to develop magnetic anisotropy were introduced in 1938. Even though very little atten­tion was given to crystalline texture, energy product values of 5.0 MGO were common during World War 2.

Since the energy product values in directions perpen­dicular to the preferred magnetic axis fell as low as 0.6 MGO, it was felt for some time that the ultimate had been attained. However, R. E. Bernius and D. Ebling independently demonstrated that the properties could be further improved by obtaining preferred crystal orientation parallel to the desired magnetic axis. Thus, properties close to 8.0 MGO have been reported for castings with nearly 100% preferred crystal orientation.

by secondary recrystallization. It is shown that if a sufficiently large grain edge strain can be induced in a polycrystalline aggre­gate, reSUlting in secondary recrystallization, formation of a single large crystal is possible. It is also shown that if a sufficiently large internal stress can be introduced by both mechanical and thermal means, the required grain-edge shift can be accomplished. The additions of normally prohibitive amounts of either carbon, nitrogen, manganese, or other "gamma-phase" promoting elements will serve to provide the necessary mechanical strain, and methods of controlling their effect on the magnetic results are shown. The thermal stresses are accomplished by a simple maintainance of 6O-80°C gradient across the magnet poles. This process has yielded single crystals approaching lib in size whose energy prod­uct is 11.0 megagauss-oe. The practical nature of this process is discussed, and possible mass production techniques outlined.

literature describes two methods of growing mono­crystals of considerable size from a melt. The Bridgman method uses a crucible of a shape designed to provide a nucleus of crystallization with a chosen crystallographic orientation for the slow growth of a single crystal. This method can be applied to Alnico but can hardly be con­sidered a practical manufacturing technique. The Czochralski method is based on dipping a previously prepared, properly oriented seed crystal into the melt and then lifting it slowly from the molten bath. This technique also can be used with Alnico type alloys but has, obviously, severe limitations for production pur-poses.

The literature also abounds with examples of recrys­tallization and growth induced in malleable metals such as pure iron. Here the polycrystalline metal is subjected to mechanical strains which induce critical internal directional stresses. By suitable heat treatment of the strained metal, the internal stresses are relieved through movements of the grain edges reSUlting in the phenom­enon commonly described as recrystallization and growth. The factors that appear to influence the growth of large grains are:

(1) Internal mechanical strains causing Jarge energy level differentials in the grain structure.

(2) A critical heat treating temperature. (3) The presence of a marked thermal gradient dur­

ing the early stages of heat treatment.

It is clear now that a further step can be taken to improve the magnetic properties of an Alnico casting. This involves replacing the polycrystalline structure of the cast mass with a single crystal whose (100) axis corresponds to the preferred axis of magnetization. Measurements on individual large crystals cut from a While mechanical straining of the Alnico alloys is larger polycrystalline mass indicate that energy prod- impractical, it seemed to us that other means might be ucts over 9.0 MGO are attainable. available for establishing the conditions necessary to

Our studies have been concerned with a practical promote the process of secondary recrystallization. The means of making monocrystalline Alnico magnets. The attainment of the critical heat treatment temperature

1310

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FORMATIO:-\ OF MONOCRYSTALLI:-\E ALNICO MAGNETS 1311

pre~ellt~ no great difficulty and critical ~tresses can be induced metallurgically by the close proximity of two distinct crystalline textures with different lattice con­stants as well as by the effects of the differing thermal coefficients of expansion of the two phases. If a suitable thermal gradient can be induced as part of the anneal­ing process, such a polycrystalline mass could be con­vert ed to a single crystal whose preferred (100) axis is parallel to the desired magnetic axis.

Thus, prompted by the random observation of the secondary recrystallization phenomenon in an Alnico casting, we have sought to establish the most favorable conditions for the formation of a monocrystal from a polycrystalline casting. The study sought to examine:

(a) The phases of the Alnico alloy system and their effect on grain growth.

(b) The most suitable composition for secondary reeryst all iza t ion.

(c) The most suitable temperature range, time, and thermal gradient for recrystallization.

A. THE PHASES OF THE ALNICO ALLOY SYSTEM

Tn the diagram of Fig. 1, the vertical dotted line represents an alloy of the Alnico 5 family. This alloy has a pure alpha-structure both above 1200°C as well as in the range 900-930°C. Since it is known that for good magnetic properties, gamma-phase precipitation must be avoided, heat treatment between 930° and 1180°C cannot be used unless this region is traversed very quickly. The deleterious effects of the presence of gamma phase on the magnetic properties are so great that a 7% content of gamma phase will lower energy values by about 25%. Preferred magnetic orientation is the result of the directional precipitation below 900°C of submicroscopic particles of alpha prime, a second alpha phase. Since the presence of even minor quantities of gamma-phase precipitates obstructs the proper alpha­prime formation and hence obstructs magnetic orienta­tion, it is reasonable to conclude that these gamma precipitates can also cause major lattice distortions with

1100°(- --

1000"C

J.I£LT + 0< I I

- - - - - -----CURIE TE'*'f:RA.TvRE

aod'C- ---- . --- ------

FIG. 1.

(a)

(b)

(e)

FIG. 2.

associated large internal mechanical strains, sufficient to induce large-crystal growth under suitable conditions.

It might be mentioned here that it may appear that we are proposing something of an absurdity since we first state that only magnets of pure alpha + alpha­prime phase content will give high magnetic properties, and then we propose extensive gamma-phase precipita­tion. However, it is part of the process to use the pres­ence of the gamma phase to induce mechanical strains, and then when recrystallization into a monocrystal has occurred, to suppress the gamma phase by proper heat treatment.

According to the phase diagram in Fig. 1, the gamma phase should dissolve above 1200°C and the structure should revert to pure alpha phase. However, a certain time interval is required for this solution process, and it

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1312 STEINORT, CRONK, GARVIN, AND TIDERMAN

FlG.3.

is therefore possible to raise the magnet to the crys­tallization temperature range of 1260° to 1310°C before the dissolution of the gamma phase has gone to com­pletion. Thus, the two requirements for crystalline growth, i.e.,

(a) heat treatment at the critical recrystallization temperature and

(b) the presence of internal strains in the crystal lattice can be fultilled.

In order to prevent gamma-phase dissolution before the recrystallization temperature is reached, it is neces­sary to either raise the temperature at an extremely fast rate, or provide increased stability of the gamma phase above 1200°C. It was 'decided that a combination of both methods would provide the best possible com­promise.

B. MOST SUITABLE COMPOSITION FOR SECONDARY RECRYSTALLIZATION

Certain elements, which may be referred to as gamma­phase precipitants, are known to \viden the gamma-

I4(XX)

12000

10000

8000

2000

-H 900 800 700 600 500 400 300 200 100 COERClVIT'i' - OERSTEDS

1-1(.=840

FIG. 4.

phase loop in the iron-phase diagram. These include carbon, nitrogen, manganese, ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, and gold.

Such elements have heretofore been avoided in Alnico alloys, but we have found that small percentages added to the alloy will accelerate the gamma-phase precipita­tion at 930° to 1150°C, and increase the stability of this phase above 1200°C during the transition to the recrys­tallization temperature. We have also determined that once recrystallization to a monocrystal has occurred, it is possible to bring all of this gamma-phase precipitate into solution, and by proper cooling, traverse the pre­cipitation temperature region at such a rate as to pre­vent further gamma-phase development.

C. THE MOST SUITABLE TEMPERATURE RANGE, TIME, AND TEMPERATURE GRADIENT CON­

DITIONS FOR RECRYSTALLIZATION

As described, the cast alloy must first be heat treated at about lOOO°C for periods of 30-60 min to promote the gamma-phase precipitation needed for straining of

FIG. 5.

the crystal lattices. Figs. 2(a), 2(b), and 2(c) show the progressive development of this precipitate. Once this precipitate has been formed, the temperature must then be rapidly raised to the critical recrystallization tem­perature range of 1250° to 1310°C. The gamma phase proceeds to dissolve at temperatures above 1200°C but the presence of gamma-phase stabilizing elements and the establishment of a thermal gradient of approxi­mately 20°Cjcm length from the base to the top of the casting, combined with a sufficiently rapid temperature rise, serves to promote monocrystal formation. The structure is free from all gamma phase in the 1250-13100 C temperature range, but the alpha-gamma equilibrium zone must then be traversed by rapid blast cooling to avoid gamma-phase precipitation during the cooling cycle.

In order to arrive at the best possible magnetic prop­erties in a monocrystal, it is essential that the (100) axis

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FOR MAT ION 0 F M 0 N 0 CRY S TAL LIN E A L N leo MAG NET S 1313

of the crystal structure be parallel to the desired mag­netic axis. This may be done by providing a properly oriented crystal nucleus at a preferred surface of the magnet body, and causing crystal growth to proceed from this nucleus. Since even unchilled normal sand castings have crystals lying close to the surface that are all oriented perpendicular to the surface, they can be used as the crystal nucleus. Figure 3 shows four steps in the growth of a monocrystal from a polycrystalline aggregate.

It has been found by experiment that if a temperature gradient in the order of 20°Cj cm of magnet length is maintained in the direction of the preferred magnetic axis, the monocrystal will grow from the surface nucle­ation bed through the entire magnet. Such monocrys­talline growth absorbs and converts to the monocrys­talline structure not only the properly oriented crystals, but the transversely and randomly oriented crystals as well, until the entire casting becomes substantially a single crystal.

CONCLUSIONS

We have described a process which is capable of pro­ducing monocrystalline Alnico magnets wtih superior magnetic properties.

Experiments with an alloy containing 24% cobalt, 14% nickel, 8% aluminum, 3% copper, the balance being iron, contaminated with 0.08% carbon or 0.35% manganese, following the procedure outlined, and then proceeding by conventional heat-treating methods to develop a preferred magnetic orientation parallel to the (100) axis of the monocrystalline casting, resulted in monocrystals of the proper orientation. Figure 4 shows graphically magnetic properties that have been obtained on such magnet castings and Fig. 5 shows photographi­cally the internal appearance of a monocrystal.

While such properties have also been obtained by other laboratory methods, we feel that this procedure of exploiting the phenomenon of monocrystal formation from conventional polycrystalline castings offers the greatest scope for practical application.

JOURNAL OF APPLIED PHYSICS SUPPLEMENT TO VOL. 33. NO.3 MARCH, 1962

The Preisach Diagram and Interaction Fields for Assemblies of y-Fe203 Particles

G. BATE

Development Laboratories, Data Systems Division, International Business Machines Corporation, Poughkeepsie, New York

The Preisach diagram calculated for an assembly of 'Y-Fe203 particles at a packing density of 20% by volume has been used to obtain a distribution function of the particle remanence-coercivities in the absence of interaction fields. This function, which has a peak at 275 oe, is then compared with the function obtained by differentiating the remanence hysteresis loop, i.e., with the distribution function in the presence of inter­action fields. It is found that the latter function is broader than the former; this can be qualitatively ex­plained in terms of a two-particle model for the interacting particles. The magnitude of the interaction fields can be estimated from the Preisach diagram and is found to have a maximum value of roughly 300 oe.

INTRODUCTION

T HE Preisach diagram for assemblies of particles of 'Y-Fe20a in magnetic recording tape has been dis­

cussed recently by Daniel and Levinel and by Wood­ward and Della Torre.2 In a previous paper,a the author has described an experiment which establishes the statistical stability of the diagram and shows that it can be used to predict accurately the results of other rema­nence experiments. In this paper, the Preisach diagram is used to find the distribution of remanence-coercivities pH r of the particles in the absence of interaction fields. This distribution is then compared with that measured in the presence of interaction fields and, finally, the magnitude of these fields is discussed. The samples were of tape containing oxide particles (length,,-,0.5-11l, axial

1 E. D. Daniel and I. Levine, J. Acoust. Soc. Am. 32, 1 (1960). 2 J. G. Woodward and E. Della Torre, J. App\. Phys. 31, 56

(1960). 3 G. Bate, J. App\. Phys. (to be published).

ratio,,-,7: 1) at a packing density of 20% by volume; previous work4 ,5 indicates that these particles should be single domains. The method of calculating the diagram has been described previously3 and as we are concerned here with changes of remanence we need consider only that part of the diagram for which a>o>b, where a, b, are, respectively, the positive and negative remanence­coercivities of a particle.

RESULTS

Figure 1 shows part of the Preisach diagram calcu­lated from experimental results as described in reference 3 for a nonoriented sample; 86% of the particles whose representative points lie in this quadrant have both positive and negative switching fields less than 585 oe. By summing the numbers over either the vertical or the horizontal strips or by differentiating the. descending

4 A. H. Morrish and S. P. Yu, J. App\. Phys. 26, 1049 (1955). 5 A. H. Morrish and L. A. K. Watt, Phys. Rev. 105, 1476 (1957).

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