CA1236381A - Iron-rare earth-boron permanent magnets by hot working - Google Patents
Iron-rare earth-boron permanent magnets by hot workingInfo
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- CA1236381A CA1236381A CA000451851A CA451851A CA1236381A CA 1236381 A CA1236381 A CA 1236381A CA 000451851 A CA000451851 A CA 000451851A CA 451851 A CA451851 A CA 451851A CA 1236381 A CA1236381 A CA 1236381A
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- percent
- iron
- rare earth
- permanent magnet
- neodymium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
- H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
- H01F1/047—Alloys characterised by their composition
- H01F1/053—Alloys characterised by their composition containing rare earth metals
- H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
- H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
- H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
- H01F1/0575—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together
- H01F1/0576—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together pressed, e.g. hot working
Abstract
IRON-RARE EARTH-BORON PERMANENT
MAGNETS BY HOT WORKING
Abstract of the Disclosure High energy product, magnetically anisotropic permanent magnets are produced by hot working over-quenched or fine grained, melt spun materials com-prising iron, neodymium and/or praseodymium, and boron to produce a fully densified, fine grained body that has undergone plastic flow.
MAGNETS BY HOT WORKING
Abstract of the Disclosure High energy product, magnetically anisotropic permanent magnets are produced by hot working over-quenched or fine grained, melt spun materials com-prising iron, neodymium and/or praseodymium, and boron to produce a fully densified, fine grained body that has undergone plastic flow.
Description
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D-7,804 C-3,517 IRON-RARE EARTH-BORON PERMANENT
MAGNETS BY HOT WORKING
This invention relates to high temperature strain-anneal pro~`essing of extremely rapidly solidified compositions comprising iron, one or more rare earth metals and boron to produce useful permanent magnets. More particularly, this invention relates to the hot consolidation and hot working of overquenched compositions comprising iron, neodymium and/or praseodymium, and boron to form useful, magnetically aligned permanent magnets.
Background High energy product, high coercivity permanent magnet compositions comprising, for example, iron, neodymium and/or praseodymium, and boron and methods of making them are disclosed in European patent application 0 108 474 published May 16, 1984 by John J. Croat and assigned to the assignee of this application. An illustrative composition, expressed in atomic proportions, is Ndo.13(Fe0 95B0.o5)o.87.
substantially the composition of a specific stable intermetallic phase that possesses high coercivity when formed as fine crystallites about 20 to 400 or 500 nanometers in largest dimension.
Melts of the above family of compositions can be very rapidly quenched, such as by melt spinning, to produce a solid material, e.g., a thin ribbon. When the rate of cooling has been controlled to produce a suitable fine crystalline microstructure (20 nm to 400 or 500 nm), the material has excellent permanent magnet ,~
properties. On the other hand, faster cooling (overquenching) produces a material with smaller crystallites and lower coercivity. However, as disclosed, such overquenched material can be annealed to form the suitable crystal size with the associated high coercivity and high energy product.
An interesting and useful property of this neodymium-iron-boron composition (for example) is that it is magnetically isotropic. A fine grain, melt spun ribbon can be broken up into flat particles. The particles can be pressed in a die at room temperature to form a unitary body of about 85% of the material's density. Bonding agents can be employed before or after the compaction. It is surprising to find that such bonded magnets displayed no preferred magnetic direction. Values of intrinsic coercivity or maximum energy product were not dependent upon the direction of the applied magnetic field. There was no advantage in grinding the ribbon to very fine particles and magnetically aligning the particles before compaction.
Such magnetically isotropic materials are very useful because they can be easily pressed (without magnetic alignment) into bonded shapes. The shapes can be magnetized in the most convenient direction.
It is recognized that the iron-neodymium-boron type compositions might provide still higher energy products if at least a portion of the grains or crystallites in their microstructure could be physically aligned and if such alignment produced at least partial magnetic domain alignment. The material would then have a preferred direction of magneti2ation.
The material would be magnetically anisotropic and would have higher residual magnetization and higher energy product in the preferred direction. I have now accomplished this using overquenched melt spun material by hot working the material to consolidate it to full density and to effect plastic flow that yields magnetic alignment. The same improvement can be accomplished on finely crystalline, high coercivity material (e.g., HCi > 1000 Oe) if the hot work is performed rapidly before excessive grain growth occurs and coercivity decreases.
It is an object of my invention to provide a fully densified fine grain, anisotropic, permanent magnet formed by hot working a suitable material com-prising iron, neodymium and/or praseodymiuml andboron. This anisotropic magnet has higher residual magnetization and energy product than isotropic magnets of like composition.
It is an object of my invention to provide a method of treating overquenched compositions con-taining suitable proportions of iron, neodymium and/or praseodymium, and boron at suitable temperatures and pressures to fully densify the material into a solid mass, to effect the growth of fine, high coercivity crystallites and to cause a flow and orientation of the material sufficient to produce macroscopic mag-netic anisotropy.
It is another object of my invention to treat suitable transition metal-rare earth metal-boron compositions that do not have permanent magnet proper-ties because their microstructure is amorphous or too finely crystalline. The treatment is by a hot working process, such as hot pressing, hot die-upsetting, ex-trusion, forging, rolling or the like, to fully consolidate 36~
pieces of the material, to effect suitable grain growth and to proauce a plastic flow therein that results in a body having magnetic anisotropy. It is found that the maximum magnetic properties in such a hot worked body are oriented parallel to the direction of pressing (perpendicular to the direction of flow. In the direction of preferred magnetic alignment, energy products are obtainable that are significantly greater than those in isotropic magnets of like composition.
Brief Summary In accordance with a preferred embodiment of my invention, these and other objects and advantages are accomplished as follows:
A molten composition comprising iron, neodymium and/or praseodymium, and boron is prepared.
Other constituents may be present, as will be dis-closed below. An example of a preferred composition, expressed in terms of atomic proportions, is 0.13(FeO. 95Bo . 05) o 87- The molten material is cooled extremely rapidly, as by melt spinning, to form a thin ribbon of solid material that does not have permanent magnet properties. Typically, the material is amorphous in microstructure. It will not produce an x-ray pattern containing many discrete diffraction maxima like that obtained from diffraction in crystalline substances. When highly magnified, as in a scanning electron microscope micrograph, no discrete grains (or crystallites) will be apparent.
The ribbon or other thin, solid form may be broken, if necessary, into particles of convenient size for an intended hot working operation. The par-ticles ore heated under argon to a suitable elevated :
Q~
temperature, preferably 700C or higher, and subjected to short term hot working under pressure, preferably at least 10,000 psi. Such processing may be accom-plished by any of a number of known hot working practices. The material may be hot pressed in a die.
It may be extruded, or rolled, or die-upset, or hammered. Whatever the particular form of hot working employed, the several individual particles are pressed and flowed together until the mass achieves full density for the composition. In addition, the hot mass is caused to undergo plastic flow. During the exposure at high temperature the nonpermanent magnet microstructure is converted to a suitable fine grain crystalline material. The flow of the hot, fine grain material produces a body, that upon cooling below its Curie temperature, has preferred direction of magnetization and provides excellent permanent magnet properties.
As suitably practiced, the high temperature working produces a finely crystalline or granular microstructure (for example, up to about 0.4 to 0.5 micrometers in greatest dimension). Care is taken to cool the material before excessive grain growth and loss of coercivity occurs. The preferred direction of magnetization of my hot worked product is typically parallel to the direction of pressing and transverse to the direction of plastic flow. A significantly higher ene-gy product is obtained when the body is magnetized transverse to the direction of plastic flow.
As previously stated, material of like composition and similar microstructure has been made without hot working. Such materials have been mag-netically isotropic and had lower maximum energy product.
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In another embodiment of my invention the starting material may be a high coercivity (> 1000 Oe) isotropic material. Suitable hot working of the material will fully densify it and effect plastic flow to orient the fine crystallites in a magnPtically anisotropic structure. However the duration of the hot working must be short so that the crystallites do not grow so large that the desirable magnetic properties are lost.
'rhese and other objects and advantages of the invention will become more apparent from a de-tailed description thereof, which follows. Reference will be made to the drawings, in which Figure 1 is a cross-sectional view of a hot pressing die for practicing one embodiment of my invention;
Figure 2 is a second quadrant, room temper-ature, 4~M versus H plot of a sample produced by hot pressing;
Figure 3a is a photomicrograph at 6QOX
magnification of a sample compacted to 85% of theoretical density in accordance with earlier work;
Figure 3b is a photomicrograph at 600X
magnification of a sample hot pressed in accordance with my method;
Figure 3c is a photomicrograph at 600X
magnification of a sample extruded it accordance with my method;
Figure 4 is a second quadrant, room temper-ature, 4~M versus H plot of a sample produced byextrusion;
Figure 5 is a Scanning Electron Microscope micrograph at 43,600X magnification, illustrating ~3~3t~3~
the texture of the fracture surface of an extruded sample prepared in accordance with my method;
Figure 6 is a second quadrant, room tem-perature, 4~ versus H plot of a sample produced by die upsetting in accordance with my method; and Figure 7 is a second quadrant, room tem-perature, 4~M versus H plot of a sample produced by a different die upsetting practice in accordance with my method.
Detailed Des`cription My method is applicable to compositions comprising a suitable transition metal component, a suitable rare earth component, and boron.
The transition metal component is iron or iron and (one or more of) cobalt, nickel, chromium or manganese. Cobalt is interchangeable with iron up to about 40 atomic percent. Chromium, manganese and nickel are interchangeable in lower amounts, prefer-ably less than about 10 atomic percent. Zirconium and/or titanium in small amounts (up to about 2 atomic percent of the iron) can be substituted for iron.
Very small amounts of carbon and silicon can be tolerated where low carbon steel is the source of iron for the composition. The composition preferably comprises about 50 atomic percent to about 90 atomic percent transition metal component -- largely iron.
The composition also comprises prom about 10 atomic percent to about 50 atomic percent rare earth component. Neodymium and/or praseodymium are the essential rare earth constituents. As indicated,-they may be used interchangeably. Relatively small amounts of other rare earth elements, such as samarium, lanthanum, cerium, terbium and dysprosium, may be mixed with neodymium and praseodymium without substan-tial loss of the desirable magnetic properties. Pref-erably, they make up no more than about 40 atomic percent of the rare earth component. It is expected that there will be small amounts of impurity elements with the rare earth component.
The overquenched composition contains about 1 to 10 atomic percent boron.
The overall composition may be expressed by the formula REl X(TMl yBy)x. The rare earth (RE) component makes up 10 to 50 atomic percent of the composition (x = 0.5 to 0.9), with at least 60 atomic percent of the rare earth component being neodymium and/or praseodymium. The transition metal (TM) as used herein makes up about 50 to 90 atomic percent of the overall composition, with iron representing about 80 atomic percent of the transition metal con-tent. The other constituents, such as cobalt, nickel, chromium or manganese, are called "transition metals"
insofar as the above empirical formula is concerned.
Boron is present in an amount of about 1 to 10 atomic percent (y = about 0.01 to 0.11) of the total composition.
For convenience, the compositions have been expressed in terms of atomic proportions. Obviously these specifications can be readily converted to weight proportions for preparing the composition mixtures.
For purposes of illustration, my invention will be described using compositions of approximatelv the following atomic proportions:
0.13( 0.95 0.05)0.87 However, it is to be understood that my method is applicable to a family of compositions as described above.
Depending on the rate of cooling, molten transition mekal-rare earth-boron compositions can be solidified to have microstructures ranging from:
(a) amorphous (glassy) and extremely fine grained microstructures (e.g., Tess than 20 nm in largest dimension) through (b) very fine (micro) grained microstructures (e.g., 20 nm to about 400 or 500 nm) to (c) larger grained microstructures.
Thus far, large grained microstructure melt spun materials have not been produced with useful permanent magnet properties. Fine grain microstructures, where the grains have a maximum dimension of about 20 to 400 or 500 nm, have useful permanent magnet properties.
Amorphous materials do not. However, some of the glassy microstructure materials can be annealed to i convert them to fine grain permanent magnets having I isotropic magnetic properties. My invention is 1 20 applicable to such overquenched, glassy materials. It ¦ is also applicable to "as-quenched" high coercivity, fine grain materials provided the materials are exposed only for short times, c less than five minutes, at high temperatures, over 700C, during the hot working.
Suitable overquenched compositions can be made by melt spinning. In my melt spinning experiments the material is contained in a suitable vessel, such as a quartz crucible. The composition is melted by induction or resistance heating in the crucible under argon. At the bottom of the crucible is provided a small, circular ejection orifice about 500 microns in diameter. Provision is made to close the I' ' : ' . , .
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top of the crucible so that the argon can be pressurized to eject the melt from the vessel in a very fine stream.
The molten stream is directed onto a mov-ing chill surface located about one-quarter inch below the ejection orifice. In examples described herein the chi]l surface is a 25 cm diameter, 1.3 cm thick copper wheel. The circumferential surface is chrome plated. The crucible and wheel are contained in a box that is evacuated of air and backfilled with argon. In my experiments the wheel is not cooled.
Its mass is so much greater than the amount of melt impinging on it in any run that its temperature does not appreciably change. When the melt hits the turning wheel, it flattens, almost instantaneously solidifies and is thrown off as a ribbon. The thick-ness of the ribbon and the rate of coolin,~ are largely determined by the circumferential speed of the wheel. In this work, the speed can be varied to produce an amorphous ribbon, a fine grained ribbon or a large grained ribbon.
In the practice of my method, the cooling rate or speed of the chill wheel preferably is such that an amorphous or extremely fine crystal structure is produced. Such a structure will be amorphous or will have finer crystals than that which produces a permanent magnet as is, for example, less than about 20 nanometers in largest dimension. As a practical matter, the distinction between an amorphous micro-structure and such an extremely fine crystallinemicrostructure is probably not discernible. What is desired is an overquenched material that has less than optimum permanent magnetic properties but that ~3~31~3~
can be annealed to produce improved permanent magnet properties. However, in accordance with my practice, the material is not separately annealed. It is, in effect, annealed while it is hot worked to produce a magnetic microstructure that has anistropic magnetic properties.
A few examples will further illustrate the practice of my invention.
Example 1 10 An overquenched, melt spun ribbon was prepared. A molten mixture was prepared in accordance with the following formula: Ndo.13(FeO 95Bo 05)0.87 About 40 grams of the mixture was melted in a quartz tube that was about 10 cm long and ~,54 em in diameter.
The quartz tube had an ejection orifice in the bottom, which was round and about 600 em in diameter. The top of the tube was sealed and adapted to supply pressurized argon gas to the tube above the molten alloy. The alloy was actually melted in the tube using induction heating. When the melt was at 1400C, an argon ejection pressure of about 3 psig was applied.
An extremely fine stream of the molten metal was ejected down onto the rim of the above described wheel. The wheel was made of copper and the peri-meter surface was slated with chromium. The wheelwas initially at room temperature and was neither heated nor cooled during the experiment, except from contact with the molten metal ejected onto it. The wheel was rotated at a rim velocity of about 35 meters per second (m/s).
A solidified melt spun ribbon came off the wheel. It was about 30 em thick and about one mm wide.
This material was cooled too rapidly to have useful permanent magnet properties. In other words, it was overquenched. Tad the wheel been rotated slightly slower, the ribbon could have been produced to have a microstructure affording useful hard magnetic properties The ribbon was broken into short pieces and they were placed into the cylindrical cavity 12 of a round die 10 like that depicted in Figure 1. The cavity was 3/8 inch in diameter and the material was contained by upper and lower punches 14. The die was made of a high temperature nickel alloy with a tool steel liner, and the punches were tungsten carbide.
The die and the contents were rapidly hea~e~
under argon with an induction coil 16 to a maximum temperature of 750C. The temperature was measured using a thermocouple (not shown) in the die adjacent the cavity. The upper punch was then actuated to exert a maximum pressure of 32,000 psi on the broken-up ribbon particles. Heating and pressure were stopped.
The workpiece was cooled to room temperature on the die. However, the total time that the workpiece was at a temperature above 700~C was only about five minutes. The consolidated workpiece was removed from the die. The resulting cylinder was hard and strong.
It had a density of about 7.5 grams per cubic centi-meter, which is substantially its full density.
The magnetic properties of the material weredetermined by cutting a piece from the cylinder and grinding a small sphere, about 2 mm in diameter, from the cut off piece. The sphere was magnetized in a known direction by subjecting it to a pulsed magnetic field having a strength of about forty kilo gauss-. -The sphere was then placed in a vibrating sample magnetometer with-the positive magnetic pole of the sphere aligned with the positive pole of the magne-tome~er. The sample was subjected to a gradually decreasing magnetic field from ~lOkOe to -20kOe that produced corresponding decreasing sample magnetization (4~1). In this manner the second quadrant demagneti-zation plot (4~ versus H) was obtained for the par-tic-ular direction magnet;zat;on.
The sample was removed from the magnetometer and magnetized in a pulsed field as before in a dif-ferent direction. It was returned to the magnetometer and a new demagnetization curve determined. This process was again repeated and the respective curves compared. The sample displayed magnetic anistropy.
Figure 2 contains four different second quadrant plots of 4~M versus H. The second quadrant portion of a hysteresis loop provides useful informa-tion regarding permanent magnet properties. Threeof these plots in Figure 2 represent good properties.
The residual magnetization at zero field (H=0) is high and the intrinsic coercivity, i.e., the reverse field to demagnetize the sample (4~M = 0) is high. The upper curve 18 represents a favorable direction of magnetization obtained in the spherical sample. The lowest curve 20 represents the data obtained from a direction relatively far removed from the aligned direction of the hot pressed compact. The middle line 22 is the demagnetization plot also generated in the vibrating sample magnetometer of an isotropic array of the same ribbon from which its hot compact was made.
These ribbon samples were heated (annealed) at a rate of 160 per minute to a temperature of 727C, and then cooled at the same rate to room temperature. The data obtained was normalized to a sample density of 100%. Thus plot 22 is of an isotropic magnet of the same composition as the anisotropic magnet produced in this example.
~3~3~ -A hysteresis curve was also prepared from a sample of the original overquenched ribbon. The second quadrant portion is produced as curve 24 in Figure 2. It has relatively low intrinsic coercivity and residual magnetization.
Thus, the hot pressing operation produced a fully densified body and also produced flow in the material that oriented the microstructure so that it became magnetically anisotropic. In the preferred direction of magnetization represented by curve 18 the residual magnetization and energy product are greater than in the isotropic material.
In addition to having excellent permanent properties at room temperature the hot pressed body retains its properties during exposure at high tem-peratures in air. A hot pressed body of this example was exposed at 160~C in air to a reverse field of 4kOe for 1507 hours. It suffered only minimal loss in permanent magnet properties.
Figure 3a is a pho-tomicrograph of a cross-section of a bonded magnet that was compacted at room temperature to 85% of full density, The plate-like sections of the original ribbon are seen to line up and be preserved in the bonded magnet. Fig-ure 3b is a photomicrograph at the same magnification of a hot pressed specimen fully densified in accor-dance with my invention. The flat ribbon fragments are still perceptible at about the same size as in the bonded magnet, but there are no voids in this fully densified specimen.
Example 2 Another overquenched, melt spun ribbon was prepared by the method described in Example l The nominal composition of the ribbon was in accordance with the empirical formula Ndo 13 Leo 94Bo 06~0 87.
The ribbons were produced by quenching the melt on a chill wheel rotating at a velocity of 32 m/s. The thickness of the ribbon was approximately 30~m and the width approximately one millimeter. This cooling rate produced a microstructure that could not be magnetized to form a magnet having useful permanent magnet properties.
Ribbon pieces were compacted at room temperature in a die to form a precompacted body of about 85% full density. The precompact was then placed in the cavity of a high temperature alloy die similar to that described in Example 1. However, the die had a graphite liner. Carbide punches con-fined the precompact in the die cavity. The die and its contents were quickly heated under argon to 740C
and a ram pressure of 10 kpsi was applied in an attempt to extrude the preform. An unexpected form of back-ward extrusion was obtained as the precompacted material flowed out from between the punches and displaced graphite die liner to form a cup-like piece, After cooling to room temperature this piece was removed from the die and it was found that the extruded por-tion of the sample was of sufficient dimensions to allow density measurement as well as magnetic measure-ment. The extruded portion was fully densified.
A 2 mm cube was ground from a portion ofthe extruded metal and it was tested in a vibrating sample magnetometer. By magnetizing and demagnetizing the sample transverse to the cube faces it was ob- --served that the specimen displayed magnetic anisotropy.Three orthogonal directions are displayed in Figure by curves 26, 28 and 30. The separations of these second quadrant plots from different directions of magnetization results from physical alignment of magnetic domains withi'n the''sample.' The greater the 3~
separation of the plots, the greater the degree of magnetic alignment. It is seen that the alignment for the extruded sample was even more pronounced khan for the sample of Figure l. The demagnetization curves for the annealed ribbon 22 and the overquenched ribbon 24 are also included in this figure as in Figure 2.
It is seen that the coercivity of the extruded sample is even higher than that of the annealed ribbons pre-sumably hecause a more appropriate crystallite size was achieved during the extrusion. The magnetization of the extruded sample in its most preferred direction is higher and results in higher energy product than that obtainable in isotropic annealed ribbons.
Figure 3c is a photomicrograph at 600X
magnification of a cross-section of the extruded sample. It is seen that greater plastic flow occurred in the extruded sample as evidenced by the reduction in thickness of the original ribbon particles. It is believed that this plastic flow is essential to align-ment of the magnetic moments within the material andthat this alignment is genérally transverse to the plastic flow. In other words, with respect to this sample, the magnetic alignment is transverse to the long dimension of the extruded ribbons (i.e., up and down in Figure 3c).
Figure 5 is a scanning electron microscope micrograph at nearly 44,000X magnification of a frac-ture surface of the extruded sample. It shows the wine grain texture.
Additional hot press tests, like Example 1, and modified extrusion tests, like Example 2, were-carried out at various die temperatures in the range of 700 to 770C and pressures in the range of ln,000 to 30,000 psi. These tests showed that full densi-fication could be realiæed even at the lower pressures 3~
and temperatures. However, the samples prepared at the lower temperatures and pressures appeared to be more brittle. Optical micrographs revealed the ribbon pieces to have cracks similar to those present in Figure 3a. Evidently, higher pressure is required at temperatures of 750C and lower before such cracks disappear as in Figure 3b. The preferred magnetiza-tion direction for the hot pressed samples is parallel to the press direction and perpendicular to the direc-tion of plastic flow. Greater directional anisotropydevelops when more plastic flow is allowed, as in the extrusion tests.
Example 3 This example illustrates a die upsetting practice Overquenched ribbon fragments of Example 2 wexe hot pressed under argon in a heated die, like that in Figuxe 1, at a maximum die temperature of 770C and pressure of 15 kpsi. A 3/8" cylindrical body, 100~ density, was formed. This hot pressed cylinder was sanded to a smaller cylinder (diameter less than 1 cm) with its cylindrical axis transverse to the axis of the original cylinder. This cylinder was re-hot pressed in the original diameter cavity ~5 along its axis (perpendicular to the original press direction) so that it was free to deform to a shorter cylinder o 3/8" dial~Rter (i.e., die upsetting). The die upsetting operation was conducted at a maximum temperature of 770C and a pressure of 16 kpsi. As in previous examples the part was cooled in the die.
A cubic specimen was machined from the die upset body and its magnetic properties measured parallel and transverse to the press direction in a vibrating sample magnetometer, as in the above Exa~,ples 1 and 2.
Second quadrant, room temperature 4~M versus H plots for these two directions are depicted in Figure 60 Curve 32 was obtainer in the direction parallel to the die upset press direction and curve 34 in the direction transverse thereto and thus parallel to the direction of material slow. It is seen that this die upset practice produced greater anisotropy than the single hot pressing operation or the extrusion test.
This translates to a Br of 9.2 kG and an energy produced of 18 MGOe compared with isotropic rïbbon values of Br = 8kG and energy product of about 12 MGOe.
Example 4 This example illustrates a die upsetting practice similar to Example 3, except a fully dense, hot pressed sample was die-upset with pressure applied in the same direction as the original hot press pressure.
Overquenched ribbon fragments ox Example 2 were hot pressed under argon in a heated die, like that depicted in Figure 1, at a maximum temperature of 760C and pressure of 15 kpsi. A 3/8 inch cylin-drical body, 100% density, was formed. This hot pressed piece was sanded to a smaller diameter (less than about 1 cm) and die upset in the same diameter cavity in a direction parallel to.the first press ~5 direction. The die upset operation was conducted at a maximum temperature of 750C and a pressure ox 12 kpsi. The sample was cooled in the die.
A cubic specimen was machined from the die upset body and its magnetic properties measured in a vibrating sample magnetometer parallel and transverse to the d;e upset press direction as in the above example. Second quadrant, room temperature, 4~M
versus plots for these two directions are depicted in Figure 7. Curve 36 was obtained in the direction parallel to the die upset press directions and curve 38 in the direction txanSYerse thereto. It is seen that this pxactice of hot pressing followed by die upsetting in the same direction produced greater anisotropy than was obtained in any of k preYïous samples. It is seen in figure 7 that in the preferred d;rection of magnetization (curve 36) toe remnant magnetization was greater than 11 kG, while the intrinsic coercivity was still greater than 7 kOe. The maximum energy product of this sample was 27 MGOe.
It is believed that still greater alignment can be obtained by a practice that provides greater plastic flow at elevated temperature. One may define an alignment factor by (Br)parallel/(Br)perpendicular~
where Br is residual induction (at H = 0) measured parallel to and perpendicular to, respectively, the press direction. An alignment factor of 2.46 was obtained in Example 4. An alignment factor of 1.32 has been achieved by die upsettïng (like in Example 3). An alignment factor of 1.18 has been achieved for extrusion (like in Example 2).
My practice of high temperature consolida-tion and plastic flow can be viewed as a strain-anneal process. This process produces magnetic alignment of the grains of the workpiece and grain growth. However, if the grain yrowth is excessive, coercivity is de-creased. Therefore, consideration tand probably trial and error testing) must be given to the grain size of the starting material in conjunction with the time that the matexial is at a temperature-at which grain growth can occur. If, as is preferred, the starting material is overquenched, the workpiece can be held at a relatively high temperature for a longer time because some grain growth is desired. It one starts with near ~3~
optimal train size material, the hot working must be rapid and subsequent cooling prompt to retard excessiYe grain growth. For example, I have carried out hot pressing experiments on enodymium-iron-boron-melt spun compositions thaw haze teen optimally quenched to produce optimal grain size for achieving the highest magnetic product. During such hot pressing the material was over 700C for more than five minutes.
The material was held too long at such temperature because the coerci~ity was always reduced although not completely eliminated. Therefore, optimal benefits were not obtained.
I also conducted hot pressing experiments on annealed ingot that had a homogenized, large grain microstructure. When magneti2ed, such ingots con-tained very low coercivity, less than 500 oersted. My hot pressing strain-anneal practice produced a signif-icant directional dependence of Br in the ingot samples, but no coercivity increase. It had been hoped that the strain-anneal practice would induce recrystallization in the ingot which would allow for development of the optimal grain size. The failure to obtain a coercivity increase in these experiments indicates that the strain-anneal practice is not bene-ficially applicable to large grained materials.
Thus, my high temperature-high pressure consolidation and hot working of suitable, transition metal, rare earth metal, boron compositions yields magnetically anisotropic product of excellent perman-ent magnet properties. For purposes of illustration,the practice of my invention has been described, using specific composition of neodymium, iron and boron.
However, other materials may be substituted or present in suitably small amounts. Prafieodymium may be sub-stituted for neodymium or used in combination with it.
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Other rare earth metals may be used with neodymiumand/or praseodymium. Likewise, other metals, such as cobalt, nickel, manganese and chromium, in suitably small amounts, may be used in combination with iron.
The preferred compositional ranges are described above.
While my invention has been described in terms of preferred embodiments thereof, it Jill be appreciated that other embodiments could readily be adapted by those skilled in the art. Accordingly, the scope of my invention is to be considered limited only by the following claims.
. . .
D-7,804 C-3,517 IRON-RARE EARTH-BORON PERMANENT
MAGNETS BY HOT WORKING
This invention relates to high temperature strain-anneal pro~`essing of extremely rapidly solidified compositions comprising iron, one or more rare earth metals and boron to produce useful permanent magnets. More particularly, this invention relates to the hot consolidation and hot working of overquenched compositions comprising iron, neodymium and/or praseodymium, and boron to form useful, magnetically aligned permanent magnets.
Background High energy product, high coercivity permanent magnet compositions comprising, for example, iron, neodymium and/or praseodymium, and boron and methods of making them are disclosed in European patent application 0 108 474 published May 16, 1984 by John J. Croat and assigned to the assignee of this application. An illustrative composition, expressed in atomic proportions, is Ndo.13(Fe0 95B0.o5)o.87.
substantially the composition of a specific stable intermetallic phase that possesses high coercivity when formed as fine crystallites about 20 to 400 or 500 nanometers in largest dimension.
Melts of the above family of compositions can be very rapidly quenched, such as by melt spinning, to produce a solid material, e.g., a thin ribbon. When the rate of cooling has been controlled to produce a suitable fine crystalline microstructure (20 nm to 400 or 500 nm), the material has excellent permanent magnet ,~
properties. On the other hand, faster cooling (overquenching) produces a material with smaller crystallites and lower coercivity. However, as disclosed, such overquenched material can be annealed to form the suitable crystal size with the associated high coercivity and high energy product.
An interesting and useful property of this neodymium-iron-boron composition (for example) is that it is magnetically isotropic. A fine grain, melt spun ribbon can be broken up into flat particles. The particles can be pressed in a die at room temperature to form a unitary body of about 85% of the material's density. Bonding agents can be employed before or after the compaction. It is surprising to find that such bonded magnets displayed no preferred magnetic direction. Values of intrinsic coercivity or maximum energy product were not dependent upon the direction of the applied magnetic field. There was no advantage in grinding the ribbon to very fine particles and magnetically aligning the particles before compaction.
Such magnetically isotropic materials are very useful because they can be easily pressed (without magnetic alignment) into bonded shapes. The shapes can be magnetized in the most convenient direction.
It is recognized that the iron-neodymium-boron type compositions might provide still higher energy products if at least a portion of the grains or crystallites in their microstructure could be physically aligned and if such alignment produced at least partial magnetic domain alignment. The material would then have a preferred direction of magneti2ation.
The material would be magnetically anisotropic and would have higher residual magnetization and higher energy product in the preferred direction. I have now accomplished this using overquenched melt spun material by hot working the material to consolidate it to full density and to effect plastic flow that yields magnetic alignment. The same improvement can be accomplished on finely crystalline, high coercivity material (e.g., HCi > 1000 Oe) if the hot work is performed rapidly before excessive grain growth occurs and coercivity decreases.
It is an object of my invention to provide a fully densified fine grain, anisotropic, permanent magnet formed by hot working a suitable material com-prising iron, neodymium and/or praseodymiuml andboron. This anisotropic magnet has higher residual magnetization and energy product than isotropic magnets of like composition.
It is an object of my invention to provide a method of treating overquenched compositions con-taining suitable proportions of iron, neodymium and/or praseodymium, and boron at suitable temperatures and pressures to fully densify the material into a solid mass, to effect the growth of fine, high coercivity crystallites and to cause a flow and orientation of the material sufficient to produce macroscopic mag-netic anisotropy.
It is another object of my invention to treat suitable transition metal-rare earth metal-boron compositions that do not have permanent magnet proper-ties because their microstructure is amorphous or too finely crystalline. The treatment is by a hot working process, such as hot pressing, hot die-upsetting, ex-trusion, forging, rolling or the like, to fully consolidate 36~
pieces of the material, to effect suitable grain growth and to proauce a plastic flow therein that results in a body having magnetic anisotropy. It is found that the maximum magnetic properties in such a hot worked body are oriented parallel to the direction of pressing (perpendicular to the direction of flow. In the direction of preferred magnetic alignment, energy products are obtainable that are significantly greater than those in isotropic magnets of like composition.
Brief Summary In accordance with a preferred embodiment of my invention, these and other objects and advantages are accomplished as follows:
A molten composition comprising iron, neodymium and/or praseodymium, and boron is prepared.
Other constituents may be present, as will be dis-closed below. An example of a preferred composition, expressed in terms of atomic proportions, is 0.13(FeO. 95Bo . 05) o 87- The molten material is cooled extremely rapidly, as by melt spinning, to form a thin ribbon of solid material that does not have permanent magnet properties. Typically, the material is amorphous in microstructure. It will not produce an x-ray pattern containing many discrete diffraction maxima like that obtained from diffraction in crystalline substances. When highly magnified, as in a scanning electron microscope micrograph, no discrete grains (or crystallites) will be apparent.
The ribbon or other thin, solid form may be broken, if necessary, into particles of convenient size for an intended hot working operation. The par-ticles ore heated under argon to a suitable elevated :
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temperature, preferably 700C or higher, and subjected to short term hot working under pressure, preferably at least 10,000 psi. Such processing may be accom-plished by any of a number of known hot working practices. The material may be hot pressed in a die.
It may be extruded, or rolled, or die-upset, or hammered. Whatever the particular form of hot working employed, the several individual particles are pressed and flowed together until the mass achieves full density for the composition. In addition, the hot mass is caused to undergo plastic flow. During the exposure at high temperature the nonpermanent magnet microstructure is converted to a suitable fine grain crystalline material. The flow of the hot, fine grain material produces a body, that upon cooling below its Curie temperature, has preferred direction of magnetization and provides excellent permanent magnet properties.
As suitably practiced, the high temperature working produces a finely crystalline or granular microstructure (for example, up to about 0.4 to 0.5 micrometers in greatest dimension). Care is taken to cool the material before excessive grain growth and loss of coercivity occurs. The preferred direction of magnetization of my hot worked product is typically parallel to the direction of pressing and transverse to the direction of plastic flow. A significantly higher ene-gy product is obtained when the body is magnetized transverse to the direction of plastic flow.
As previously stated, material of like composition and similar microstructure has been made without hot working. Such materials have been mag-netically isotropic and had lower maximum energy product.
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In another embodiment of my invention the starting material may be a high coercivity (> 1000 Oe) isotropic material. Suitable hot working of the material will fully densify it and effect plastic flow to orient the fine crystallites in a magnPtically anisotropic structure. However the duration of the hot working must be short so that the crystallites do not grow so large that the desirable magnetic properties are lost.
'rhese and other objects and advantages of the invention will become more apparent from a de-tailed description thereof, which follows. Reference will be made to the drawings, in which Figure 1 is a cross-sectional view of a hot pressing die for practicing one embodiment of my invention;
Figure 2 is a second quadrant, room temper-ature, 4~M versus H plot of a sample produced by hot pressing;
Figure 3a is a photomicrograph at 6QOX
magnification of a sample compacted to 85% of theoretical density in accordance with earlier work;
Figure 3b is a photomicrograph at 600X
magnification of a sample hot pressed in accordance with my method;
Figure 3c is a photomicrograph at 600X
magnification of a sample extruded it accordance with my method;
Figure 4 is a second quadrant, room temper-ature, 4~M versus H plot of a sample produced byextrusion;
Figure 5 is a Scanning Electron Microscope micrograph at 43,600X magnification, illustrating ~3~3t~3~
the texture of the fracture surface of an extruded sample prepared in accordance with my method;
Figure 6 is a second quadrant, room tem-perature, 4~ versus H plot of a sample produced by die upsetting in accordance with my method; and Figure 7 is a second quadrant, room tem-perature, 4~M versus H plot of a sample produced by a different die upsetting practice in accordance with my method.
Detailed Des`cription My method is applicable to compositions comprising a suitable transition metal component, a suitable rare earth component, and boron.
The transition metal component is iron or iron and (one or more of) cobalt, nickel, chromium or manganese. Cobalt is interchangeable with iron up to about 40 atomic percent. Chromium, manganese and nickel are interchangeable in lower amounts, prefer-ably less than about 10 atomic percent. Zirconium and/or titanium in small amounts (up to about 2 atomic percent of the iron) can be substituted for iron.
Very small amounts of carbon and silicon can be tolerated where low carbon steel is the source of iron for the composition. The composition preferably comprises about 50 atomic percent to about 90 atomic percent transition metal component -- largely iron.
The composition also comprises prom about 10 atomic percent to about 50 atomic percent rare earth component. Neodymium and/or praseodymium are the essential rare earth constituents. As indicated,-they may be used interchangeably. Relatively small amounts of other rare earth elements, such as samarium, lanthanum, cerium, terbium and dysprosium, may be mixed with neodymium and praseodymium without substan-tial loss of the desirable magnetic properties. Pref-erably, they make up no more than about 40 atomic percent of the rare earth component. It is expected that there will be small amounts of impurity elements with the rare earth component.
The overquenched composition contains about 1 to 10 atomic percent boron.
The overall composition may be expressed by the formula REl X(TMl yBy)x. The rare earth (RE) component makes up 10 to 50 atomic percent of the composition (x = 0.5 to 0.9), with at least 60 atomic percent of the rare earth component being neodymium and/or praseodymium. The transition metal (TM) as used herein makes up about 50 to 90 atomic percent of the overall composition, with iron representing about 80 atomic percent of the transition metal con-tent. The other constituents, such as cobalt, nickel, chromium or manganese, are called "transition metals"
insofar as the above empirical formula is concerned.
Boron is present in an amount of about 1 to 10 atomic percent (y = about 0.01 to 0.11) of the total composition.
For convenience, the compositions have been expressed in terms of atomic proportions. Obviously these specifications can be readily converted to weight proportions for preparing the composition mixtures.
For purposes of illustration, my invention will be described using compositions of approximatelv the following atomic proportions:
0.13( 0.95 0.05)0.87 However, it is to be understood that my method is applicable to a family of compositions as described above.
Depending on the rate of cooling, molten transition mekal-rare earth-boron compositions can be solidified to have microstructures ranging from:
(a) amorphous (glassy) and extremely fine grained microstructures (e.g., Tess than 20 nm in largest dimension) through (b) very fine (micro) grained microstructures (e.g., 20 nm to about 400 or 500 nm) to (c) larger grained microstructures.
Thus far, large grained microstructure melt spun materials have not been produced with useful permanent magnet properties. Fine grain microstructures, where the grains have a maximum dimension of about 20 to 400 or 500 nm, have useful permanent magnet properties.
Amorphous materials do not. However, some of the glassy microstructure materials can be annealed to i convert them to fine grain permanent magnets having I isotropic magnetic properties. My invention is 1 20 applicable to such overquenched, glassy materials. It ¦ is also applicable to "as-quenched" high coercivity, fine grain materials provided the materials are exposed only for short times, c less than five minutes, at high temperatures, over 700C, during the hot working.
Suitable overquenched compositions can be made by melt spinning. In my melt spinning experiments the material is contained in a suitable vessel, such as a quartz crucible. The composition is melted by induction or resistance heating in the crucible under argon. At the bottom of the crucible is provided a small, circular ejection orifice about 500 microns in diameter. Provision is made to close the I' ' : ' . , .
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top of the crucible so that the argon can be pressurized to eject the melt from the vessel in a very fine stream.
The molten stream is directed onto a mov-ing chill surface located about one-quarter inch below the ejection orifice. In examples described herein the chi]l surface is a 25 cm diameter, 1.3 cm thick copper wheel. The circumferential surface is chrome plated. The crucible and wheel are contained in a box that is evacuated of air and backfilled with argon. In my experiments the wheel is not cooled.
Its mass is so much greater than the amount of melt impinging on it in any run that its temperature does not appreciably change. When the melt hits the turning wheel, it flattens, almost instantaneously solidifies and is thrown off as a ribbon. The thick-ness of the ribbon and the rate of coolin,~ are largely determined by the circumferential speed of the wheel. In this work, the speed can be varied to produce an amorphous ribbon, a fine grained ribbon or a large grained ribbon.
In the practice of my method, the cooling rate or speed of the chill wheel preferably is such that an amorphous or extremely fine crystal structure is produced. Such a structure will be amorphous or will have finer crystals than that which produces a permanent magnet as is, for example, less than about 20 nanometers in largest dimension. As a practical matter, the distinction between an amorphous micro-structure and such an extremely fine crystallinemicrostructure is probably not discernible. What is desired is an overquenched material that has less than optimum permanent magnetic properties but that ~3~31~3~
can be annealed to produce improved permanent magnet properties. However, in accordance with my practice, the material is not separately annealed. It is, in effect, annealed while it is hot worked to produce a magnetic microstructure that has anistropic magnetic properties.
A few examples will further illustrate the practice of my invention.
Example 1 10 An overquenched, melt spun ribbon was prepared. A molten mixture was prepared in accordance with the following formula: Ndo.13(FeO 95Bo 05)0.87 About 40 grams of the mixture was melted in a quartz tube that was about 10 cm long and ~,54 em in diameter.
The quartz tube had an ejection orifice in the bottom, which was round and about 600 em in diameter. The top of the tube was sealed and adapted to supply pressurized argon gas to the tube above the molten alloy. The alloy was actually melted in the tube using induction heating. When the melt was at 1400C, an argon ejection pressure of about 3 psig was applied.
An extremely fine stream of the molten metal was ejected down onto the rim of the above described wheel. The wheel was made of copper and the peri-meter surface was slated with chromium. The wheelwas initially at room temperature and was neither heated nor cooled during the experiment, except from contact with the molten metal ejected onto it. The wheel was rotated at a rim velocity of about 35 meters per second (m/s).
A solidified melt spun ribbon came off the wheel. It was about 30 em thick and about one mm wide.
This material was cooled too rapidly to have useful permanent magnet properties. In other words, it was overquenched. Tad the wheel been rotated slightly slower, the ribbon could have been produced to have a microstructure affording useful hard magnetic properties The ribbon was broken into short pieces and they were placed into the cylindrical cavity 12 of a round die 10 like that depicted in Figure 1. The cavity was 3/8 inch in diameter and the material was contained by upper and lower punches 14. The die was made of a high temperature nickel alloy with a tool steel liner, and the punches were tungsten carbide.
The die and the contents were rapidly hea~e~
under argon with an induction coil 16 to a maximum temperature of 750C. The temperature was measured using a thermocouple (not shown) in the die adjacent the cavity. The upper punch was then actuated to exert a maximum pressure of 32,000 psi on the broken-up ribbon particles. Heating and pressure were stopped.
The workpiece was cooled to room temperature on the die. However, the total time that the workpiece was at a temperature above 700~C was only about five minutes. The consolidated workpiece was removed from the die. The resulting cylinder was hard and strong.
It had a density of about 7.5 grams per cubic centi-meter, which is substantially its full density.
The magnetic properties of the material weredetermined by cutting a piece from the cylinder and grinding a small sphere, about 2 mm in diameter, from the cut off piece. The sphere was magnetized in a known direction by subjecting it to a pulsed magnetic field having a strength of about forty kilo gauss-. -The sphere was then placed in a vibrating sample magnetometer with-the positive magnetic pole of the sphere aligned with the positive pole of the magne-tome~er. The sample was subjected to a gradually decreasing magnetic field from ~lOkOe to -20kOe that produced corresponding decreasing sample magnetization (4~1). In this manner the second quadrant demagneti-zation plot (4~ versus H) was obtained for the par-tic-ular direction magnet;zat;on.
The sample was removed from the magnetometer and magnetized in a pulsed field as before in a dif-ferent direction. It was returned to the magnetometer and a new demagnetization curve determined. This process was again repeated and the respective curves compared. The sample displayed magnetic anistropy.
Figure 2 contains four different second quadrant plots of 4~M versus H. The second quadrant portion of a hysteresis loop provides useful informa-tion regarding permanent magnet properties. Threeof these plots in Figure 2 represent good properties.
The residual magnetization at zero field (H=0) is high and the intrinsic coercivity, i.e., the reverse field to demagnetize the sample (4~M = 0) is high. The upper curve 18 represents a favorable direction of magnetization obtained in the spherical sample. The lowest curve 20 represents the data obtained from a direction relatively far removed from the aligned direction of the hot pressed compact. The middle line 22 is the demagnetization plot also generated in the vibrating sample magnetometer of an isotropic array of the same ribbon from which its hot compact was made.
These ribbon samples were heated (annealed) at a rate of 160 per minute to a temperature of 727C, and then cooled at the same rate to room temperature. The data obtained was normalized to a sample density of 100%. Thus plot 22 is of an isotropic magnet of the same composition as the anisotropic magnet produced in this example.
~3~3~ -A hysteresis curve was also prepared from a sample of the original overquenched ribbon. The second quadrant portion is produced as curve 24 in Figure 2. It has relatively low intrinsic coercivity and residual magnetization.
Thus, the hot pressing operation produced a fully densified body and also produced flow in the material that oriented the microstructure so that it became magnetically anisotropic. In the preferred direction of magnetization represented by curve 18 the residual magnetization and energy product are greater than in the isotropic material.
In addition to having excellent permanent properties at room temperature the hot pressed body retains its properties during exposure at high tem-peratures in air. A hot pressed body of this example was exposed at 160~C in air to a reverse field of 4kOe for 1507 hours. It suffered only minimal loss in permanent magnet properties.
Figure 3a is a pho-tomicrograph of a cross-section of a bonded magnet that was compacted at room temperature to 85% of full density, The plate-like sections of the original ribbon are seen to line up and be preserved in the bonded magnet. Fig-ure 3b is a photomicrograph at the same magnification of a hot pressed specimen fully densified in accor-dance with my invention. The flat ribbon fragments are still perceptible at about the same size as in the bonded magnet, but there are no voids in this fully densified specimen.
Example 2 Another overquenched, melt spun ribbon was prepared by the method described in Example l The nominal composition of the ribbon was in accordance with the empirical formula Ndo 13 Leo 94Bo 06~0 87.
The ribbons were produced by quenching the melt on a chill wheel rotating at a velocity of 32 m/s. The thickness of the ribbon was approximately 30~m and the width approximately one millimeter. This cooling rate produced a microstructure that could not be magnetized to form a magnet having useful permanent magnet properties.
Ribbon pieces were compacted at room temperature in a die to form a precompacted body of about 85% full density. The precompact was then placed in the cavity of a high temperature alloy die similar to that described in Example 1. However, the die had a graphite liner. Carbide punches con-fined the precompact in the die cavity. The die and its contents were quickly heated under argon to 740C
and a ram pressure of 10 kpsi was applied in an attempt to extrude the preform. An unexpected form of back-ward extrusion was obtained as the precompacted material flowed out from between the punches and displaced graphite die liner to form a cup-like piece, After cooling to room temperature this piece was removed from the die and it was found that the extruded por-tion of the sample was of sufficient dimensions to allow density measurement as well as magnetic measure-ment. The extruded portion was fully densified.
A 2 mm cube was ground from a portion ofthe extruded metal and it was tested in a vibrating sample magnetometer. By magnetizing and demagnetizing the sample transverse to the cube faces it was ob- --served that the specimen displayed magnetic anisotropy.Three orthogonal directions are displayed in Figure by curves 26, 28 and 30. The separations of these second quadrant plots from different directions of magnetization results from physical alignment of magnetic domains withi'n the''sample.' The greater the 3~
separation of the plots, the greater the degree of magnetic alignment. It is seen that the alignment for the extruded sample was even more pronounced khan for the sample of Figure l. The demagnetization curves for the annealed ribbon 22 and the overquenched ribbon 24 are also included in this figure as in Figure 2.
It is seen that the coercivity of the extruded sample is even higher than that of the annealed ribbons pre-sumably hecause a more appropriate crystallite size was achieved during the extrusion. The magnetization of the extruded sample in its most preferred direction is higher and results in higher energy product than that obtainable in isotropic annealed ribbons.
Figure 3c is a photomicrograph at 600X
magnification of a cross-section of the extruded sample. It is seen that greater plastic flow occurred in the extruded sample as evidenced by the reduction in thickness of the original ribbon particles. It is believed that this plastic flow is essential to align-ment of the magnetic moments within the material andthat this alignment is genérally transverse to the plastic flow. In other words, with respect to this sample, the magnetic alignment is transverse to the long dimension of the extruded ribbons (i.e., up and down in Figure 3c).
Figure 5 is a scanning electron microscope micrograph at nearly 44,000X magnification of a frac-ture surface of the extruded sample. It shows the wine grain texture.
Additional hot press tests, like Example 1, and modified extrusion tests, like Example 2, were-carried out at various die temperatures in the range of 700 to 770C and pressures in the range of ln,000 to 30,000 psi. These tests showed that full densi-fication could be realiæed even at the lower pressures 3~
and temperatures. However, the samples prepared at the lower temperatures and pressures appeared to be more brittle. Optical micrographs revealed the ribbon pieces to have cracks similar to those present in Figure 3a. Evidently, higher pressure is required at temperatures of 750C and lower before such cracks disappear as in Figure 3b. The preferred magnetiza-tion direction for the hot pressed samples is parallel to the press direction and perpendicular to the direc-tion of plastic flow. Greater directional anisotropydevelops when more plastic flow is allowed, as in the extrusion tests.
Example 3 This example illustrates a die upsetting practice Overquenched ribbon fragments of Example 2 wexe hot pressed under argon in a heated die, like that in Figuxe 1, at a maximum die temperature of 770C and pressure of 15 kpsi. A 3/8" cylindrical body, 100~ density, was formed. This hot pressed cylinder was sanded to a smaller cylinder (diameter less than 1 cm) with its cylindrical axis transverse to the axis of the original cylinder. This cylinder was re-hot pressed in the original diameter cavity ~5 along its axis (perpendicular to the original press direction) so that it was free to deform to a shorter cylinder o 3/8" dial~Rter (i.e., die upsetting). The die upsetting operation was conducted at a maximum temperature of 770C and a pressure of 16 kpsi. As in previous examples the part was cooled in the die.
A cubic specimen was machined from the die upset body and its magnetic properties measured parallel and transverse to the press direction in a vibrating sample magnetometer, as in the above Exa~,ples 1 and 2.
Second quadrant, room temperature 4~M versus H plots for these two directions are depicted in Figure 60 Curve 32 was obtainer in the direction parallel to the die upset press direction and curve 34 in the direction transverse thereto and thus parallel to the direction of material slow. It is seen that this die upset practice produced greater anisotropy than the single hot pressing operation or the extrusion test.
This translates to a Br of 9.2 kG and an energy produced of 18 MGOe compared with isotropic rïbbon values of Br = 8kG and energy product of about 12 MGOe.
Example 4 This example illustrates a die upsetting practice similar to Example 3, except a fully dense, hot pressed sample was die-upset with pressure applied in the same direction as the original hot press pressure.
Overquenched ribbon fragments ox Example 2 were hot pressed under argon in a heated die, like that depicted in Figure 1, at a maximum temperature of 760C and pressure of 15 kpsi. A 3/8 inch cylin-drical body, 100% density, was formed. This hot pressed piece was sanded to a smaller diameter (less than about 1 cm) and die upset in the same diameter cavity in a direction parallel to.the first press ~5 direction. The die upset operation was conducted at a maximum temperature of 750C and a pressure ox 12 kpsi. The sample was cooled in the die.
A cubic specimen was machined from the die upset body and its magnetic properties measured in a vibrating sample magnetometer parallel and transverse to the d;e upset press direction as in the above example. Second quadrant, room temperature, 4~M
versus plots for these two directions are depicted in Figure 7. Curve 36 was obtained in the direction parallel to the die upset press directions and curve 38 in the direction txanSYerse thereto. It is seen that this pxactice of hot pressing followed by die upsetting in the same direction produced greater anisotropy than was obtained in any of k preYïous samples. It is seen in figure 7 that in the preferred d;rection of magnetization (curve 36) toe remnant magnetization was greater than 11 kG, while the intrinsic coercivity was still greater than 7 kOe. The maximum energy product of this sample was 27 MGOe.
It is believed that still greater alignment can be obtained by a practice that provides greater plastic flow at elevated temperature. One may define an alignment factor by (Br)parallel/(Br)perpendicular~
where Br is residual induction (at H = 0) measured parallel to and perpendicular to, respectively, the press direction. An alignment factor of 2.46 was obtained in Example 4. An alignment factor of 1.32 has been achieved by die upsettïng (like in Example 3). An alignment factor of 1.18 has been achieved for extrusion (like in Example 2).
My practice of high temperature consolida-tion and plastic flow can be viewed as a strain-anneal process. This process produces magnetic alignment of the grains of the workpiece and grain growth. However, if the grain yrowth is excessive, coercivity is de-creased. Therefore, consideration tand probably trial and error testing) must be given to the grain size of the starting material in conjunction with the time that the matexial is at a temperature-at which grain growth can occur. If, as is preferred, the starting material is overquenched, the workpiece can be held at a relatively high temperature for a longer time because some grain growth is desired. It one starts with near ~3~
optimal train size material, the hot working must be rapid and subsequent cooling prompt to retard excessiYe grain growth. For example, I have carried out hot pressing experiments on enodymium-iron-boron-melt spun compositions thaw haze teen optimally quenched to produce optimal grain size for achieving the highest magnetic product. During such hot pressing the material was over 700C for more than five minutes.
The material was held too long at such temperature because the coerci~ity was always reduced although not completely eliminated. Therefore, optimal benefits were not obtained.
I also conducted hot pressing experiments on annealed ingot that had a homogenized, large grain microstructure. When magneti2ed, such ingots con-tained very low coercivity, less than 500 oersted. My hot pressing strain-anneal practice produced a signif-icant directional dependence of Br in the ingot samples, but no coercivity increase. It had been hoped that the strain-anneal practice would induce recrystallization in the ingot which would allow for development of the optimal grain size. The failure to obtain a coercivity increase in these experiments indicates that the strain-anneal practice is not bene-ficially applicable to large grained materials.
Thus, my high temperature-high pressure consolidation and hot working of suitable, transition metal, rare earth metal, boron compositions yields magnetically anisotropic product of excellent perman-ent magnet properties. For purposes of illustration,the practice of my invention has been described, using specific composition of neodymium, iron and boron.
However, other materials may be substituted or present in suitably small amounts. Prafieodymium may be sub-stituted for neodymium or used in combination with it.
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Other rare earth metals may be used with neodymiumand/or praseodymium. Likewise, other metals, such as cobalt, nickel, manganese and chromium, in suitably small amounts, may be used in combination with iron.
The preferred compositional ranges are described above.
While my invention has been described in terms of preferred embodiments thereof, it Jill be appreciated that other embodiments could readily be adapted by those skilled in the art. Accordingly, the scope of my invention is to be considered limited only by the following claims.
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Claims (15)
1. A method of making an iron-rare earth metal permanent magnet comprising hot pressing magnetically isotropic particles of an amorphous or finely crystalline material having an average grain size up to about 500 nanometers and comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of which is iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and at least one percent boron, at an elevated temperature and pressure for a time sufficient to consolidate the particulate material into a fully densified body and cooling the body, whereby the resulting hot pressed body is magnetically anisotropic and upon magnetization is a permanent magnet.
2. A method of making an anisotropic iron-rare earth metal permanent magnet comprising hot working an amorphous to finely crystalline solid material having a grain size up to about 500 nanometers and comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of the total transition metal being iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and at least one percent boron, to produce a plastically deformed body having a fine grain microstructure and cooling the body, the duration of hot working and rate of cooling being such that the resultant body is magnetically anisotropic and upon magnetization is a permanent magnet.
3. A method of making an anisotropic iron-rare earth metal magnet comprising hot die upsetting an amorphous to finely crystalline, rapidly solidified material having a grain size up to about 500 nanometers and comprising, on an atomic percent basis, 50 to 90 percent iron, 10 to 50 percent neodymium and/or praseodymium, and at least one percent boron at an elevated temperature and pressure to cause plastic flow in the material and to yield a body having a fine grained, crystalline microstructure, and cooling the body, whereby the resulting body upon cooling is magnetically anisotropic and capable of being magnetized to a permanent magnet in which the preferred magnetic direction lies parallel to the die upset press direction.
4. A method of making an anisotropic iron-rare earth metal magnet comprising quenching a molten mixture comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of the total transition metal being iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and at least one percent boron, at a rate to form an overquenched, thin, solid ribbon material having an average grain size less than about 500 nanometers, hot working magnetically isotropic pieces of the ribbon material at an elevated temperature and pressure to consolidate the pieces into a fully densified body, to cause plastic flow of at least a portion of the body and to form a fine grained, crystalline microstructure, and cooling the body, whereby the resulting body upon cooling is magnetically anisotropic and capable of being magnetized to make a permanent magnet.
5. A method of making an iron-rare earth metal permanent magnet comprising hot pressing magnetically isotropic particles of an amorphous or finely crystalline material comprising, on an atomic percent basis, 50 to 90 percent iron, 10 to 50 percent of neodymium and/or praseodymium, and at least one percent boron, at an elevated temperature and pressure for a time sufficient to consolidate the material into a fully densified body and cooling the body, whereby the resulting hot pressed body is a permanent magnet.
6. A fully densified, fine grain, anisotropic permanent magnet formed by hot consolidation and hot working of amorphous or fine grain material comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of which is iron, 10 to 50 percent of rare earth metal, at least 60 percent of which is neodymium and/or praseodymium, and at least 1 percent boron.
7. A hot die upset, fine grain, anisotropic permanent magnet comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of which is iron, 10 to 50 percent of rare earth metal, at least 60 percent of which is neodymium and/or praseodymium, and at least 1 percent boron, in which magnet the preferred magnetization direction is parallel to the die upset press direction.
8. An anisotropic permanent magnet formed by hot plastic deformation of an amorphous or fine grained alloy consisting essentially of, on an atomic percent basis, 50 to 90 percent iron, 10 to 50 percent neodymium and/or praseodymium, and about 1 to 10 percent boron, the preferred magnetization direction being substantially transverse to the directions of material flow during said deformation.
9. A fully densified, fine grain, permanent magnet formed by hot pressing amorphous or fine grain material comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of which is iron, 10 to 50 percent of rare earth metal, at least 60 percent of which is neodymium and/or praseodymium, and at least 1 percent boron.
10. A hot pressed particulate, fully densified, fine grain, permanent magnet comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of the total transition metal and at least 50 percent of the magnet being iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and 1 to 10 percent boron.
11. A fully densified, hot deformed grain structure, anisotropic permanent magnet comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of the total transition metal and at least 50 percent of the magnet being iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and 1 to 10 percent boron.
12. A fully densified, hot worked, deformed grain structure, anisotropic permanent magnet comprising, on an atomic percent basis, 50 to 90 percent of transition metal, at least 60 percent of the total transition metal and at least 50 percent of the magnet being iron, 10 to 50 percent of rare earth metal, at least 60 percent of the total of which is neodymium and/or praseodymium, and 1 to 10 percent boron, in which magnet the preferred magnetization direction is substantially transverse to directions of material flow during said deformation.
13. A fully densified, permanent magnet formed by hot pressing rapidly solidified particulate material and comprising, on an atomic percent basis, 70 to 85 percent iron, 10 to 30 percent of rare earth material, at least 60 percent of which is neodymium and/or praseodymium, and 1 to 10 percent boron.
14. A fully densified, fine grain, permanent magnet formed by hot pressing rapidly solidified particulate material and comprising, on an atomic percent basis, 70 to 85 percent iron, 10 to 30 percent of rare earth material, at least 60 percent of which is neodymium and/or praseodymium, and 1 to 10 percent boron.
15. A fully densified, hot worked, deformed grain structure, anisotropic permanent magnet comprising, on an atomic percent basis, 10 to 85 percent iron, 10 to 30 percent of rare earth material, at least 60 percent of which is neodymium and/or praseodymium, and 1 to 10 percent boron, in which magnet the preferred magnetization direction is substantially transverse to directions of material flow during said deformation.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US52017083A | 1983-08-04 | 1983-08-04 | |
| US520,170 | 1983-08-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1236381A true CA1236381A (en) | 1988-05-10 |
Family
ID=24071457
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000451851A Expired CA1236381A (en) | 1983-08-04 | 1984-04-12 | Iron-rare earth-boron permanent magnets by hot working |
Country Status (8)
| Country | Link |
|---|---|
| EP (1) | EP0133758B1 (en) |
| JP (1) | JPS60100402A (en) |
| AU (1) | AU574697B2 (en) |
| BR (1) | BR8403875A (en) |
| CA (1) | CA1236381A (en) |
| DE (1) | DE3483226D1 (en) |
| ES (1) | ES534879A0 (en) |
| MX (1) | MX167658B (en) |
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| CA1216623A (en) * | 1983-05-09 | 1987-01-13 | John J. Croat | Bonded rare earth-iron magnets |
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| US5529854A (en) * | 1984-09-12 | 1996-06-25 | Seiko Epson Corporation | Magneto-optic recording systems |
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| EP0229946B1 (en) * | 1986-01-10 | 1991-10-16 | Ovonic Synthetic Materials Company, Inc. | Permanent magnetic alloy |
| CA1269029A (en) * | 1986-01-29 | 1990-05-15 | Peter Vernia | Permanent magnet manufacture from very low coercivity crystalline rare earth-transition metal-boron alloy |
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| JP2546989B2 (en) * | 1986-04-30 | 1996-10-23 | 株式会社 トーキン | Permanent magnet with excellent oxidation resistance |
| JP2546990B2 (en) * | 1986-04-30 | 1996-10-23 | 株式会社 トーキン | Permanent magnet with excellent oxidation resistance |
| JP2611221B2 (en) * | 1986-05-01 | 1997-05-21 | セイコーエプソン株式会社 | Manufacturing method of permanent magnet |
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| IT1206056B (en) * | 1986-06-20 | 1989-04-05 | Seiko Epson Corp | PROCEDURE FOR THE PREPARATION OF PERMANENT MAGNETS AND PRODUCT OBTAINED |
| US4778542A (en) * | 1986-07-15 | 1988-10-18 | General Motors Corporation | High energy ball milling method for making rare earth-transition metal-boron permanent magnets |
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| JPH01297807A (en) * | 1988-05-26 | 1989-11-30 | Daido Steel Co Ltd | Manufacture of permanent magnet |
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| US4919732A (en) * | 1988-07-25 | 1990-04-24 | Kubota Ltd. | Iron-neodymium-boron permanent magnet alloys which contain dispersed phases and have been prepared using a rapid solidification process |
| US4895607A (en) * | 1988-07-25 | 1990-01-23 | Kubota, Ltd. | Iron-neodymium-boron permanent magnet alloys prepared by consolidation of amorphous powders |
| JP2623731B2 (en) * | 1988-07-29 | 1997-06-25 | 三菱マテリアル株式会社 | Manufacturing method of rare earth-Fe-B based anisotropic permanent magnet |
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| JPH0257662A (en) * | 1988-08-23 | 1990-02-27 | M G:Kk | Rapidly cooled thin strip alloy for bond magnet |
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| EP0369097B1 (en) * | 1988-11-14 | 1994-06-15 | Asahi Kasei Kogyo Kabushiki Kaisha | Magnetic materials containing rare earth element iron nitrogen and hydrogen |
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| JPH02247304A (en) * | 1989-03-17 | 1990-10-03 | Nippon Steel Corp | Nozzle for pouring molten nd alloy |
| US5098486A (en) * | 1989-05-23 | 1992-03-24 | Hitachi Metals, Ltd. | Magnetically anisotropic hotworked magnet and method of producing same |
| US5026419A (en) * | 1989-05-23 | 1991-06-25 | Hitachi Metals, Ltd. | Magnetically anisotropic hotworked magnet and method of producing same |
| US4952251A (en) * | 1989-05-23 | 1990-08-28 | Hitachi Metals, Ltd. | Magnetically anisotropic hotworked magnet and method of producing same |
| JPH0344904A (en) * | 1989-07-12 | 1991-02-26 | Matsushita Electric Ind Co Ltd | Manufacture of rare earth element-iron permanent magnet |
| JP2596835B2 (en) * | 1989-08-04 | 1997-04-02 | 新日本製鐵株式会社 | Rare earth anisotropic powder and rare earth anisotropic magnet |
| US5201963A (en) * | 1989-10-26 | 1993-04-13 | Nippon Steel Corporation | Rare earth magnets and method of producing same |
| JPH03241705A (en) * | 1989-11-14 | 1991-10-28 | Hitachi Metals Ltd | Magnetically anisotropic magnet and manufacture thereof |
| JP2780429B2 (en) * | 1990-03-30 | 1998-07-30 | 松下電器産業株式会社 | Rare earth-iron magnet manufacturing method |
| US5178691A (en) * | 1990-05-29 | 1993-01-12 | Matsushita Electric Industrial Co., Ltd. | Process for producing a rare earth element-iron anisotropic magnet |
| JP2010222601A (en) | 2009-03-19 | 2010-10-07 | Honda Motor Co Ltd | Rare earth permanent magnet and method for producing the same |
| JP2012023190A (en) * | 2010-07-14 | 2012-02-02 | Toyota Motor Corp | Manufacturing method of anisotropic rare earth magnet |
| JP5413383B2 (en) * | 2011-02-23 | 2014-02-12 | トヨタ自動車株式会社 | Rare earth magnet manufacturing method |
| JP6221978B2 (en) * | 2014-07-25 | 2017-11-01 | トヨタ自動車株式会社 | Rare earth magnet manufacturing method |
| JP6447395B2 (en) * | 2015-07-09 | 2019-01-09 | トヨタ自動車株式会社 | Forging method |
| US10460871B2 (en) | 2015-10-30 | 2019-10-29 | GM Global Technology Operations LLC | Method for fabricating non-planar magnet |
| CN108242305B (en) * | 2016-12-27 | 2020-03-27 | 有研稀土新材料股份有限公司 | Rare earth permanent magnetic material and preparation method thereof |
| CN108428541B (en) * | 2017-02-14 | 2020-05-22 | 中国科学院宁波材料技术与工程研究所 | Preparation method of superfine-crystal high-performance anisotropic neodymium-iron-boron permanent magnet |
| JP7021035B2 (en) | 2018-09-18 | 2022-02-16 | 株式会社東芝 | Permanent magnets, rotary electric machines, and cars |
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|---|---|---|---|---|
| CH525547A (en) * | 1970-12-21 | 1972-07-15 | Bbc Brown Boveri & Cie | Process for the production of fine particle permanent magnets |
| US4358419A (en) * | 1980-11-12 | 1982-11-09 | The United States Of America As Represented By The United States Department Of Energy | Method of making amorphous metal composites |
| JPS57141901A (en) * | 1981-02-26 | 1982-09-02 | Mitsubishi Steel Mfg Co Ltd | Permanent magnet powder |
| JPS58123853A (en) * | 1982-01-18 | 1983-07-23 | Fujitsu Ltd | Rare earth metal-iron type permanent magnet and its manufacture |
| CA1316375C (en) * | 1982-08-21 | 1993-04-20 | Masato Sagawa | Magnetic materials and permanent magnets |
| EP0108474B2 (en) * | 1982-09-03 | 1995-06-21 | General Motors Corporation | RE-TM-B alloys, method for their production and permanent magnets containing such alloys |
| JPS59177346A (en) * | 1983-03-25 | 1984-10-08 | Sumitomo Special Metals Co Ltd | Alloy of rare earth metal for magnet material |
| CA1216623A (en) * | 1983-05-09 | 1987-01-13 | John J. Croat | Bonded rare earth-iron magnets |
| AU572120B2 (en) * | 1983-06-24 | 1988-05-05 | General Motors Corporation | High energy product rare earth transition metal magnet alloys |
-
1984
- 1984-04-12 CA CA000451851A patent/CA1236381A/en not_active Expired
- 1984-07-11 EP EP84304731A patent/EP0133758B1/en not_active Expired - Lifetime
- 1984-07-11 DE DE8484304731T patent/DE3483226D1/en not_active Expired - Lifetime
- 1984-07-23 AU AU30962/84A patent/AU574697B2/en not_active Ceased
- 1984-08-03 MX MX2660784A patent/MX167658B/en unknown
- 1984-08-03 BR BR8403875A patent/BR8403875A/en not_active IP Right Cessation
- 1984-08-03 ES ES534879A patent/ES534879A0/en active Granted
- 1984-08-04 JP JP59163486A patent/JPS60100402A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| EP0133758B1 (en) | 1990-09-19 |
| AU3096284A (en) | 1985-02-07 |
| ES8507289A1 (en) | 1985-08-16 |
| MX167658B (en) | 1993-04-01 |
| JPS60100402A (en) | 1985-06-04 |
| DE3483226D1 (en) | 1990-10-25 |
| JPH0420242B2 (en) | 1992-04-02 |
| AU574697B2 (en) | 1988-07-14 |
| BR8403875A (en) | 1985-07-09 |
| EP0133758A3 (en) | 1987-05-20 |
| EP0133758A2 (en) | 1985-03-06 |
| ES534879A0 (en) | 1985-08-16 |
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