recent developments in high-energy alnico alloys
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Recent Developments in HighEnergy Alnico AlloysEdward R. Cronk Citation: Journal of Applied Physics 37, 1097 (1966); doi: 10.1063/1.1708350 View online: http://dx.doi.org/10.1063/1.1708350 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/37/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Development of high-energy neutron imaging for use in NDE applications AIP Conf. Proc. 497, 693 (1999); 10.1063/1.1302076 Recent Developments in Computing, Processor and Software Research for HighEnergy Physics Phys. Today 38, 77 (1985); 10.1063/1.2814701 The Development of High-Energy Accelerators Am. J. Phys. 35, 897 (1967); 10.1119/1.1974288 Developments in highenergy physics Phys. Today 7, 14 (1954); 10.1063/1.3061658 Recent advances in highenergy physics Phys. Today 6, 14 (1953); 10.1063/1.3061226
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JOURNAL OF APPLIED PHYSICS VOLUME 37, NUMBER 3 1 MARCH 1966
Recent Developments in High-Energy Alnico Alloys
EDWARD R. CRONK
Thomas &- Skinner, Inc., Indianapolis, Indiana
A brief review of the "state of the art" with emphasis on developments of the past several years is given. It is shown that with but two major exceptions, all significant advancements in energy product enhancement have been achieved by special foundry techniques to promote crystal anisotropy, and not by alloy composition variations. The exceptions are found in the advent of the family of high-coercive-force alloys using increased amounts of cobalt and titanium known today as Alnico 8, and in secondary recrystallizing techniques on special forms of Alnico 5 to produce a single large crystai from a polycrystalline aggregate. The latter technique was described in an earlier paper at this conference [E. Steinort el al., J. Appl. Phys. Suppl. 33, 1310 (1962)]' Work on producing crystalline anisotropy in a modified form of Alnico 8 has produced energy products as high as 10.5 MG·Oe and magnets are commercially available as Alnico 9 with energy products averaging 9.0 MG·Oe. Further work is described which shows that it is possible to apply the techniques of secondary recrystallization to a columnar aggregate of Alnico 9 and achieve further energy product enhancement to 12.5 MG·Oe for a large single crystal.
A T the time of the introduction of the Honda and ft Mishima work in 1933 in the form of the ternary iron-nickel-aluminum alloy, a new family of magnetic materials for permanent magnet use was introduced. These alloys have not changed greatly in their metallurgical concept in the intervening years. Progress in developing better permanent magnet Alnico materials was confined almost entirely to variations in chemistry until the advent of the magnetically anisotropic alloys in the late 1930's.I,2 Commercial exploitation of this material was delayed by the advent of World War II and it was not until the late 1940's that significant work was started on further improvement of magnetic properties. It is significant to note that during this entire period very little emphasis was placed on controlling the crystalline structure of the Alnico castings, but it was known that pieces cut from casting surfaces that exhibited improved crystalline anisotropy also showed improved magnetic properties parallel to the crystal axis.
During the later 1940's, Bernius and Ebeling3 demonstrated that the magnetic properties of the Alnico 5 alloy could be considerably enhanced for a complete casting, if a high degree of crystalline anisotropy was introduced by special foundry techniques where a single flat wall of the casting mold contained a cold surface with high thermal conductivity and capacity. During the early years of the application of crystal growth techniques, the corp.mon generic form of Alnico 5, containing approximately 8.0%, AI, 24.0% Co, 14.0% Ni, 2.5% Cu, balance Fe, was used almost exclusively and demonstrated energy products approaching 6.5 MG·Oe for a combination columnar crystal and equiaxed polycrystalline aggregate. This material was marketed as Alnico 5 DG. Since the technique involved not only pouring the molten Alnico against a cold chill plate but also against cold sand-mold wall surfaces, crystal growth tended to develop from all these sur-
1 J. F. Dillinger and R. M. Bowrth, Physics 6, 279 (1935). 2 D. A. Oliver and J. W. Shedden, Nature 142, 209 (1937). 3 D. G. Ebeling, U.S. Patent 2 295 082.
faces and 100% crystalline anisotropy could not be achieved.
When small Alnico magnets were cut from the chilled end of such a casting, which then would exhibit close to 100% crystal orientation, it was found that energy products approaching 8.0 MG·Oe were technically possible. This prospect then led researchers to concentrate more effort on developing techniques which would assure castings with such a crystal structure directly from foundry procedures.
Almost all United States and European programs concentrated on the use of exothermic type molds, which presented only one cold surface to the molten poured mass, and thus columnar crystal growth from this chill plane could assure a high degree of crystalline anisotropy.
Mold materials are generally made up of sand, binders, aluminum powder, and suitable oxidizers. The material is handled very similar to baked sand molds, but must be ignited prior to pouring to produce the necessary mold wall temperatures that will prevent casting chilling on any but the desired surface. The chill plates can be massive iron or copper slabs, or water cooling can be used for large volume production. The material produced by this method has been known in England as "Columax," and in the United States as "Columax" and Alnico V-7.
Recently, considerable work has been done in Japan and Europe4 on special zone-melting techniques which produce a high degree of crystalline anisotropy in a polycrystalline elongated rod. However, since this process is quite expensive, it has not achieved commercial significance, but does illustrate that more than one avenue of research has been used to achieve comparable results.
In the early 1960's, Eberhart Steinort demonstrated5- S
4 N. Makino, Cobalt 17, 3 (1962). 5 E. Steinort et al., J. AppJ. Phys. SuppJ. 33, 1310 (1962). 6 E. Steinort, U.S. Patent 3 085 036. 7 W. Wright and R. Ogden, Cobalt 24,140 (1964). 8 E. Steinort, Paper presented at the Arbeitsgemeinschaft
Ferromagnetismus Meeting, "Discussionstagung Grundlagen und Werkstoffe der Dauermagnete," Karlsruhe, April 1962.
1097
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1098 EDWARD R. CRONK
..
10.0
9.0
8.C
~ IO 2
g 6.0
~ 5.0
~ 4.0 (!)
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~ 1933
1965-1939-
'" 0 ~ u :z: ii: It)
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1935 1939
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1949 1956 1962 1965 YEAR
FIG. 1. Chronology of Alnico energy product improvement.
that by a secondary recrystallization method, a monocrystalline mass could be converted from an existing polycrystalline aggregate of Alnico 5. This process is still in the development state but has achieved energy products approaching 10.0 MG· Oe.
Until approximately 1962, all of the work on Alnico concerned with the production of energy products in the 6 to 10 MG·Oe region centered on the common alloy known as Alnico 5 and its variations. However, with the introduction into the American market in the early 1960's of a new family of alloys now known as Alnico 8, a new field was open. This material exhibits a lower remanent induction, but has a coercive force approximately 2t times, and an energy product equal to Alnico 5 with an equiaxial crystal structure.
Figure 1 illustrates the progress of energy product development from 1933 through 1962. It shows that, while steady advancements have been achieved by the use of columnar crystal structure material, a point of diminishing returns seems to have been reached. Figure 2 illustrates that the "squaring factor" has reached such a high value that the usefulness of the
------
1000 900 800 600 600 500 400 300 200 100 0 OEMAGETIZING FORCE - H- OERSTEDS
FIG. 2. Demagnetization characteristics of varying crystalline forms of Alnico 5.
Alnico 5 composition is somewhat limited, in a maximum energy product form, since it requires a high degree of load permeance match to any associated structure. As the curve indicates, a shift from an optimum shear-line slope of 16.0 to 20.0 for the monocrystal material introduces a 12% loss in available energy product. This compares with losses of 6.0% and 3.5% for Columax and Alnico 5 for proportionate shifts in shear-line slope. Further increases in energy product are virtually impossible above the 10.0-MG·Oe region since the remanence tends to be limited at approximately 14 kG by the inherent composition of the alloy. With limitations on maximum coercive force in the 700 to 900 Oe region for Alnico 5, no amount of "squaring" can achieve significant energy product improvement.
Since researchers have not been able to develop an Alnico alloy that will exhibit a higher remanence, their
--:--
2.0 1.8 1.4 1.2 1.0 .8 .6 .4 .2 o DEMAGNETIZING FORCE -H-KOE
FIG. 3. Demagnetization characteristics of Alnico 8, 9, and V-7.
attention has now turned in the direction of increasing coercive force, since it had been demonstrated by the introduction of the isothermally treated, high-cobalt, high-titanium materials known as Alnico 8, that the coercive force does not have any such sharp upper limit or indeed, if it does, it is far beyond the present technology.
Since Alnico 8 has a high titanium content, which is necessary to enhance the coercive force of the material but, at the same time, introduces a very marked grainrefining effect, it was originally quite difficult to foresee how columnar crystal growth could be achieved by the best possible techniques developed for the Alnico 5 family. Early experiments indicated that the Alnico 8 alloy, containing approximately 34.0% Co, 7.0% AI, 15.0% Ni, 4.0% Cu, 5.0% Ti, balance Fe, when poured into exothermic molds would not exhibit any great amount of crystal growth and for some time there was considerable doubt in the Alnico industry that any practical results could be achieved. About 1962, it was
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R E C E N T DE VEL 0 P MEN T SIN HI G H - ENE R G Y A L N I C 0 ALL 0 Y S 1099
discovered by several independent researchers, both in England and the United States, that the addition of sulfur9- U to Alnico 8 produced sharp increases in columnar crystal growth tendencies and, at the same time, did not seem to produce any deleterious effects upon the magnetic properties. Since that time, further work on changing the alloy composition slightly to improve the coercive force without further reduction of the remanent induction, resulted in the introduction into the American market in early 1964 of a material known as Alnico 9. At the present time, Alnico 9 appears to be in a "pilot production" stage in the industry. Needless to say a very high consumer interest has been shown in this material because of its outstanding properties and potential for design problems solu-
FIG. 4. View of columnar crystal development in typical Alnico 9 casting.
tion. Figure 3 shows a typical energy product curve for Alnico 9 as compared to Alnico 8 and Alnico V-7 (Columax) as it is commercially produced today. Alnico 9 offers the designer a high energy product material which does not have too high a "squaring factor" and, at the same time, offers far greater promise of improvement than alloys based on the Alnico 5 composition. One of the important things to note about Alnico 9 is an increase in the remanent induction of approximately 25% over Alnico 8 as opposed to a 10% increase for Alnico V -7 when compared to Alnico 5. It does seem feasible that still further improvement is possible, and research efforts in this direction are yielding results indicating that a remanence approaching 12000 G will be soon available. lO •n
Figure 4 is a photograph of a typical piece of Alnico 9
9 M. McCaig, J. App!. Phys. Suppl. 35, 958 (1964). 10 J. Harrison, British Patent 987, 636. 11 J. E. Gould, Cobalt 23,82 (1964).
FIG. 5. View of mono crystal section of Alnico 5 and Alnico 9 alloys.
which has been produced with a columnar crystal growth 2 in. in length with a cross section 3X3 in. This crystal growth has been readily attainable with certain size castings and it appears, owing to the very high operating coercive force of the material, that longer grain growth will not be required. We have found that, while the phenomena of grain growth due to the additions of sulfur are not fully explained, the addition of approximately 0.45% columbium (niobium) further enhances grain growth and also enlarges the grain cross section.lO,ll This phenomenon is fully understood in the steel industry and exhibits the same general characteristics when used in Alnico 5 or other alloys. It also increases the coercive force approximately 50 to 1000e and it is recommended that it be used for Alnico 9 production.
As far as the author knows, there has been very little success in growing large columnar crystal structures of Alnico 9 with a cored hole included in the final casting. For some reason not fully understood, the results of pouring this material in thin-wall cylindrical sections
ENERGy PROOUCT YIELD FOR VARIOUS F"ORMS OF ALNICO CRYSTAL TEXTURE AND CHEMICAL COMPOSITION.
MtASUREO ENERGY PROOUCr:-~~De
MATERIAL STRuCTURE TYPE EOUIAXIAl COLuMNAR MONOCRV$TAL
ALNICO 5.25 7.5 10.0
5
ALN 1 C 0 4.0 6,0 1.5
6
AL Nt c 0 AlNlcoa ALNICO 9 12.5-15.0
8 89 5.0 10.0 {ESTIMATEDj
FIG. 6. Tabulation of measured and predicted energy product development for various alloy compositions and crystal textures.
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1100 EDWARD R. CRONK
are not as good as those obtained with Alnico V-7 (Columax) and, therefore, at the moment, it is highly recommended that all parts be designed with a solid mass, either in cubical or cylindrical form. If holes are absolutely essential, they should be placed in the final casting by EDM (electro-discharge machining) methods.
graph of several monocrystalline magnets, and Fig. 6 gives tabulated data of the best results to date for this research project. At the moment, recrystallized Alnico 9 has not indicated energy products better than that from the polycrystalline columnar structure. However, improvement ratios experienced for Alnico 5 and Alnico 6 when converted to monocrystals would indicate that energy product levels on the order of 12.5 MG·Oe are technically achievable and work towards this end is progressing steadily.
Development work is now proceeding on a program of introducing secondary recrystallization in a columnar Alnico 9 polycrystalline magnet. Figure 5 is a photo-
JOURNAL OF APPLIED PHYSICS VOLUME 37, NUMBER
Microstructure of Alnico Alloys
K. J. DE Vos
Metallurgical Laboratory, N. V. Philips' Gloeilampenjabrieken, Eindhoven, the Netherlands
1 MARCH 1966
During the last few years it has been contended repeatedly that the a+a' duplex structure of Alnico alloys is a consequence of spinodal decomposition. Electron micrographs have been made of numerous alloys of the pseudobinary Ff<NiAl system, after various isothermal heat treatments. The results strengthen the view that there exist two different mechanisms of decomposition for each Alni alloy: spinodal decomposition at a temperature remote from, and a conventional nucleation and growth transformation at a temperature adjacent to the solubility curve.
The influence of the relative volume fraction of the a and a' phases on the morphology of the microstructure and thus on the magnetic properties was also investigated. Due to a distinctly asymmetric form of the miscibility gap in the Fe-NiAl system, the relative amounts of a and a' are very temperature dependent. It was found that as a consequence of this the microstructure of an Alni alloy arising from spinodal decomposition at a relatively high temperature during continuous cooling is considerably Qifferent from the microstructure arising from quenching followed by tempering. In alloys with an Fe content higher than approximately 45 at. %, optimum properties can be obtained by continuous cooling. On the contrary, in alloys with less than 45 at. % Fe optimum coercivity is achieved by quenching followed by tempering.
A third aspect considered is the formation of an elongated precipitate in Alnico alloys by magnetic annealing. According to Cahnl the presence of a magnetic field prevents the formation of the plane wave perpendicular to the field direction, thus resulting in the formation of elongated particles parallel to this direction from the initial stage of spinodal decomposition. It was shown for Ticonal X that perpendicular waves are present even during the early stages of decomposition. It was further shown that the Ni-AI phase perpendicular to the field direction diminishes with increasing annealing time; in other words, elongated Fe-Co particles develop during the isothermal annealing. It can therefore be stated that it is very probable that during the isothermal heat treatment of Ticonal X the magnetic field does not have the influence attributed to it by Cahn.
Some information about the mechanism by which elongated particles are formed was gained from the electron micrographs. Many imperfections are visible disturbing the periodicity of the microstructure. The imperfections give a visual impression of dislocation lines. The assumption of Cahn that the increase of the interphase spacing in periodic microstructures might be attributed to the "climb" of this sort of "dislocation" thus becomes more admissible. In this mechanism the increase of interphase distance (increase of wavelength) and the elongation of the Fe-Co phase parallel to a (100 > direction are interconnected. If annealing takes place in a magnetic field, the magnetic dipolar energy may have a selected effect, resulting in a preferential elongation of the Fe-Co phase. It is quite possible that the whole process can be described by the diffusion equations worked out by Zijlstra2 if only surface and magnetic free energies have to be taken into account.
Tempering of Alnico alloys after optimum cooling does not result in significant changes in the morphology of the microstructure. The increase of coercivity is then very probably connected with an exchange of atoms between a and a', resulting in an increase in the difference in magnetization between the phases. However, when alloys which have undergone spinodal decomposition in the temperature region close to the solubility curve are rapidly cooled, the subsequent tempering does not result in optimum properties. In this case a secondary spinodal decomposition in the Ni-AI phase was observed.
) J. W. Cahn, J. App!. Phys. 34, 3581 (1963). 2 H. Zijlstra, University of Amsterdam, thesis (1960).
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