US3919004A

US3919004A - Liquid sintered cobalt-rare earth intermetallic product

Info

Publication number: US3919004A
Application number: US442870A
Authority: US
Inventors: Mark G Benz
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-04-30
Filing date: 1974-02-15
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11

Abstract

A process for preparing novel sintered cobalt-rare earth intermetallic products which can be magnetized to form permanent magnets having stable improved magnetic properties. A particulate mixture is formed of a base CoR alloy and an additive CoR alloy, where R is a rare earth metal. The base CoR alloy is one which, at sintering temperature, exists as a solid Co5R intermetallic single phase. The additive CoR alloy is richer in rare earth metal than the base CoR alloy, and at sintering temperature is at least partly liquid. The base and additive alloys, in particulate form, are each used in an amount to form a mixture which has a cobalt and rare earth metal content substantially corresponding to that of the final desired sintered product. The mixture is pressed into compacts and sintered to the desired sintered product phase composition and density. At sintering temperature, the final sintered product has a phase composition lying outside the Co5R single phase on the rare earth richer side. Specifically, the final sintered product contains a major amount of the Co5R solid intermetallic phase and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth content than the Co5R phase.

Description

United States Patent i 1 Benz [ LIQUID SINTERED COBALT-RARE EARTH INTERMETALLIC PRODUCT Mark G. Benz. Burnt Hills. NY.

[73} Assignee: General Electric Company,

Schenectady. NY.

[22] Filed: Feb. 15. 1974 Appl. No.1442.870

Related U.S. Application Data [60] Continuation of Ser. No. 209.373. Dec. [7. l97l. abandoned. which is a di\ision of Ser. No. 33.347. April 30. I970. Pat. No. 3.655.464.

[75] Inventor:

OTHER PU BLlCATlONS Westendorp, F.. et al. Perm. Mag. With Energy Products of 20 MGO. in Soc. State Comm, 7. i969. pp. 639-640. Das. D.. Influence of Sint. Temp. on Mag. Prop. of Sm-Co;" in IEEE Trans. Mag. l97l. pp. 432-435. Young. J.. Materials and Processes, New York, l954. pp. 690-692.

[ Nov. 11. 1975 Primary E.\umi'm'r-Walter R. Satterfield Allurnc). Agent. or Firm-Jane M. Binkouski'. Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A process for preparing novel sintered cobaltrare earth intermetallic products which can be magnetized to form permanent magnets having stable improved magnetic properties. A particulate mixture is formed of a base COR alloy and an additive CoR alloy. where R is a rare earth metal. The base CoR allo} is one which. at sintering temperature. exists as a solid Co R intermetallic single phase. The additive CoR alloy is richer in rare earth metal than the base COR ullo). and at sintering temperature is at least partl liquid. The base and additive alloys. in particulate form. are each used in an amount to form a mixture which has a cobalt and rare earth metal content substantiall corresponding to that of the final desired sintered product. The mixture is pressed into compacts and sintcred to the desired sintered product phase Composition and density. At sintering temperature. the final sintered product has a phase composition lying outside the Co R single phase on the rare earth richer side. Specifically. the final sintered product contains a major amount of the (0 R solid intermetallic phase and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth content than the Co.-,R phase.

1 Claim. 2 Drawing Figures US. Patent Nov. 11, 1975 Sheet 2 on 3,919,004

(ssn 2190701) 14/1; wvou V211 a/vs vw \mQwk MES 68% hi Quwt M 1 LIQUID SINTERED COBALT-RARE EARTH INTERMETALLIC PRODUCT This is a continuation of application Ser. No. 209,373, filed Dec. 17, I971, now abandoned, which is a division of application Ser. No. 33,347 filed Apr. 30, I970, now U.S. Pat. No. 3,655,464.

The present invention relates generally to the art of permanent magnets and is more particularly concerned with novel sintered cobalt-rare earth intermetallic products having unique characteristics and with a sintering method for producing such products.

Permanent magnets, i.e., hard magnetic materials such as the cobalt-rare earth intermetallic compounds, are of technological importance because they can maintain a high, constant magnetic flux in the absence of an exciting magnetic field or electrical current to bring about such a field.

Cobalt-rare earth intermetallic compounds exist in a variety of phases, but the Co R intermetallic single phase compounds (in each occurrence R designates a rear earth metal) have exhibited the best magnetic properties. The permanent magnet properties of bulk Co R, as well as cobalt-rare earth intermetallic magnetic materials generally, can be enhanced by reducing the bulk bodies to powders, but in such finely divided form these materials are unstable in air and their magnetic properties deteriorate after a short period of time.

It is an object of the present invention to provide cobalt-rare earth intermetallic magnets which have superior magnetic properties and are stable in air.

Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with the FIGURES accompanying and forming a part of the specification, in which:

FIG. 1 is the cobalt-samarium phase diagram. It is assumed herein, that the phase diagram at 300C, which is the lowest temperature shown in the FIGURE, is substantially the same at room temperatures.

FIG. 2 is a chart bearing curves which illustrate the effect of samarium content on the magnetic properties of permanent magnets including two produced in accordance with the present invention.

Briefly stated, the process of the present invention comprises the steps of forming a particulate mixture of a base cobalt-rare earth alloy and an additive cobaltrare alloy, compacting the mixture to produce a green body, and sintering the green body to produce an ultimate sintered body containing a major amount of Co k and up to 35 percent of other cobalt-rare earth phases richer in rare earth content than Co R.

The base alloy is one which at sintering temperature exists as a solid C0,,R intermetallic single phase where R is a rare earth metal. The additive cobalt-rare earth alloy is richer in rare earth metal than the base alloy and at sintering temperature it is at least partly in liquid form and thus increases the sintering rate. The base and additive alloys, in particulate form, are each used in an amount to form a mixture which has a cobalt and rare earth metal content substantially corresponding to that of the final desired sintered product.

The mixture is pressed into compacts, preferably in an aligning magnetic field, and sintered to the desired sintered product phase composition and desired density. At sintering temperature, the final sintered product has a composition lying outside the Co R single phase region on the rare earth-rich side. Specifically. at sintering temperature, as well as at room temperatures, the final sintered product contains a major amount of the Co R solid intermetallic phase and up to about 35 percent by weight of the product of a second CoR intermetallic phase which is richer in rare earth content than C0,,R phase.

The base cobalt-rare earth alloy used in the present process is one which at sintering temperature exists as a Co R single intermetallic phase. Since the Co R single phase may vary in composition, the base alloy may vary in composition which can be determined from the phase diagram for the particular cobalt-rare earth system, or empirically. For example, FIG. I shows that for the cobalt-samarium system, the base alloy at room temperature may vary in samarium content from about 32 to 36 percent by weight since this particular composition is single phase at sintering temperatures ranging from about 950 to I200C. Preferably, for simplicity, the base alloy at room temperature is a Co R intermetallic phase.

The additive cobalt-rare earth alloy is one which is richer in rare earth metal content than the base alloy. It must also be one that exists at least partly in a liquid form at sintering temperature. The additive alloy may vary in composition and can be determined from the phase diagram for the particular cobalt-rare earth system or it can be determined empirically. For example, FIG. I shows that for the cobalt-samarium system, there are phases which are partly or completely liquid at the temperature ranging from about 950 to 1200C which is a suitable sintering temperature range for Co-Sm in the present process. Any alloy within the range shown in FIG. 1 which forms at least a partly liquid phase at the particular sintering temperature would be a satisfactory additive alloy in the present process. For example, as illustrated in FIG. I, the Co-Sm additive alloy can vary upward in samarium content from about 46 percent by weight of the additive. If desired, an additive alloy can be empirically selected by a number of methods, such as by means ofa composition scan at the sintering temperatures, i.e., heating samples of various additive alloy compositions to the desired sintering temperature and observing the extent of the development of the liquid phase.

Although suitable additive CoR alloys fall within a general composition range, the preferred ones are comparatively low in rare earth metal content so that undesirable characteristics of the pure rare earth metal in the additive alloy are minimized. Specifically, for example, pure samarium is both pyrophoric and very ductile and consequently difficult to crush and to blend with the base alloy since it has a tendency to separate out and fall to the bottom of the container. On the other hand, the additive CoSm alloy of samarium con tent preferably less than '70 percent by weight, is substantially non-reactive at room temperature in air, it can be crushed by conventional techniques, and being slightly magnetic, it clings to the base alloy resulting in a substantially thorough stable mixture. However, as the samarium content of the additive Co-Srn alloy is increased, the additive alloy becomes more reactive and more difficult to blend. 0n the other hand, the higher the cobalt content of the additive alloy, the stronger are its magnetic properties and the more stable is the particulate mixture it forms with the base alloy.

The rare earth metals useful in forming the present cobalt-rare earth alloys and intermetallic compounds are the l5 elements of the lanthanide series having atomic numbers 57 to 71 inclusive. The element yttrium (atomic number 39) is commonly included in this group of metals and, in this specification, is considered a rare earth metal. A plurality of rare earth metals can also be used to form the present desired cobalt-rare earth alloys or intermetallic compounds which, for example may be ternary, quartenary or which may contain an even greater number of rare earth metals as de sired.

Representative of the cobaltrare earth alloys useful as base and additive alloys in the present invention are cobalt-cerium, cobalt-praseodymium, cobaltneodymium, cobalt-promethium, cobalt-samarium, cobalt-europium, cobalt-gadolinium, cobalt-terbium, c0- balt-dysprosium, cobalt-holmium, cobalt-erbium, cobalt-thulium, cobalt-ytterbium, cobalt-lutecium, cobalt-yttrium, cobalt-lanthanum and cobalt-misch metal. Misch metal is the most common alloy of the rare earth metals which contains the metals in the approximate ratio in which they occur in their most common naturally occurring ores. Examples of specific ternary alloys include cobalt-samarium-misch metal, cobalt-cerium-praseodymium, cobalt-yttriumpraseodymium, and cobalt-praseodymium-misch metal.

in carrying out the present process, the base and additive cobalt-rare earth alloys can be formed by a number of methods. For example, each can be prepared by arc-melting the cobalt and rare earth metal together in the proper amounts under a substantially inert atmosphere such as argon and allowing the melt to solidify. Preferably, the melt is cast into an ingot.

The solid base and additive alloys can be converted to particulate form in a conventional manner. Such conversion can be carried out in air at room temperature since the alloys are substantially non-reactive. For example, each alloy can be crushed by mortar and pestle and then pulverized to a finer form by jet milling.

The particle size of the base and additive cobalt-rare earth alloys used in forming the mixture of the present process may vary. Each can be in as finely divided a form as desired. For most applications, average particle size will range from about 1 micron or less to about microns. Larger sized particles can be used, but as the particle size is increased, the maximum coercive force obtainable is lower because the coercive force generally varies inversely with particle size. In addition, the smaller the particle size, the lower is the sintering temperature which may be used.

In forming the mixture in the present process, the base and additive alloys are each used in an amount so that the resulting mixture has a cobalt and rare earth metal content substantially corresponding to that of the final desired sintered product phase composition. In addition, however, in forming the mixture, the alloy additive should be used in an amount sufficient to promote sintering. This amount depends largely on the specific composition of the alloy additive and can be determined empirically, but generally, the additive alloy should be used in an amount of at least 0.5 percent by weight of the base-additive alloy mixture. Specifically, the larger the rare earth metal component of the additive alloy, the less is the amount of the additive alloy which need be used.

In the present invention, at sintering temperature, the final sintered product should have a phase composition lying outside the Co -,R single phase on the rare earthrich side. Magnetization of such a product results in a permanent magnet with superior magnetic properties. On the other hand, if a final sintered product at sintering or room temperatures consists only of a single Co R intermetallic phase, or if it contains a second cobaltrare earth intermetallic phase of lesser rare earth content than the Co R phase, a permanent magnet of only inferior magnetic properties can be produced no matter how the magnetization step is carried out.

Specifically, in the present invention at sintering temperature, as well as at room temperatures, the final sintered product contains a major amount of the Co -,R solid intermetallic phase, generally at least about percent by weight of the product, and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth metal content than the (10 R phase. Traces of other cobaltrare earth intermetallic phases, in most instances less than 1 percent by weight of the product, may also be present. Sintered products having the highest energy products are those having the smallest content of the second CoR phase. The preferred final sintered product, therefore, is comprised predominantly of the Co,R intermetallic phase, i.e. about percent by weight or higher but less than percent, with only a detectable content of the second CoR phase, i.e., 5 percent or lower by weight of the product. If desired, for a particular cobalt-rare earth metal system of the present invention, a composition scan, i.e., a series of runs at the same sintering temperature with proportionately varying mixtures of base and additive alloys, may be made to determine the specific sintered product composition which produces the best magnetic properties. Determination of the second CoR phase can be made by a number of techniques, such as for example, X-ray diffraction as well as standard metallographic analysis. As the content of the C0,,R intermetallic single phase is decreased in the present sintered product, the magnetic properties obtainable decrease correspondingly. Furthermore, when the content of the C0,, R intermetallic phase is below about 65 percent by weight of the present sintered product, its permanent magnet properties are sharply reduced.

In carrying out the process of this invention, the base alloy is admixed with the additive alloy in any suitable manner to produce a substantially thorough particulate mixture. The particulate mixture can then be compressed into a green body of the desired size and density by any of a number of techniques such as hydro static pressing or methods employing steel dies. Preferably, the mixture is compressed in the presence of an aligning magnetizing field to magnetically align the particles along their easy axis, or if desired, the mixture may be compressed after magnetically aligning the particles. The greater the magnetic alignment of the particles, the better are the resulting magnetic properties. Preferably also, compression is carried out to produce a green body with as high a density as possible, since the higher its density, the greater the sintering rate. Green bodies having a density of about 40 percent or higher of theoretical are preferred.

The green body is sintered to produce a sintered body of desired density. Preferably, the green body is sintered to produce a sintered body wherein the pores are substantially non-interconnecting. Such non-inter- S connectivity stabilizes the permanent magnet properties of the product because the interior of the sintered product or magnet is protected against exposure to the ambient atmosphere.

The sintering temperature used in the present pro cess depends largely on the particular cobalt-rare earth mixture to be sintered, and to a lesser degree, on particle size. The minimum sintering temperature must be sufficiently high for sintering to occur in a particular cobalt-rare earth system, i.e., it must be high enough to coalesce the component particles. Preferably, sintering is carried out so that the pores in the sintered product are substantially non-interconnecting. A sintered body having a density or packing of at least about 87 percent of theoretical is generally one wherein the pores are substantially non-interconnecting. Such non-interconnectivity is determinable by standard metallographic techniques, as for example, by means of transmission electron micrographs of a cross-section of the sintered product. The maximum sintering temperature is preferably one at which significant growth of the component particles or grains does not occur, since too large an increase in grain size deteriorates magnetic properties such as coercive force. The green body is sintered 'in a substantially inert atmosphere such as argon, and upon completion of the sintering, it is preferably cooled to room temperatures in a substantially inert atmosphere.

The particular sintering temperature range can be determined empirically, as for example, carrying out a series of runs at successively higher sintering tempera tures and then determining the magnetic properties of the sintered products. For the cobalt-Samarium alloy mixture of the present invention, a sintering temperature ranging from about 950C up to about I200C is suitable with a sintering temperature of l IC being particularly satisfactory.

The density of the sintered product may vary. The particular density depends largely on the particular permanent magnet properties desired. Preferably, to obtain a product with substantially stable permanent magnet properties, the density of the sintered product should be one wherein the pores are substantially noninterconnecting and this occurs usually at a density or packing of about 87 percent. Generally, for a number of applications, the density may range from about 80 percent to I00 percent. For example, for low temperature applications, a sintered body having a density ranging down to about 80 percent may be satisfactory. The preferred density of the sintered product is one which is the highest obtainable without producing a growth in grain size which would deteriorate magnetic properties significantly, since the higher the density the better are the magnetic properties. For cobaltsamarium sintered products of the present invention, a density of at least about 87 percent of theoretical, i.e. of full density, and as high as about 96 percent of theoretical is preferred to produce permanent magnets with suitable magnetic properties which are substantially stable.

The sintered product of the present invention has the appearance of having been liquid sintered. Examination of a polished cross-section of the sintered bulk product under an x-ray microprobe or light microscope shows that its grains differ significantly in appearance from the original particles used in forming the green body. Specifically, the original particles have an angular rough surface structure. In contrast, substantially all of the grains of the present sintered product are rounded and have a smooth surface. i.e.. the appear ance is that of a liquid-sintered-smooth surface. In addition, under a light microscope as well as under an X-ray microprobe, there can be seen a material in a number of the pores that appears to have been liquid at high temperatures. Apparently, during sintering, some of the liquid becomes trapped in closing the pores. The pores of the sintered product are preferably substantially non-interconnecting. Generally, for the sintered product to have good magnetic properties, the component grains of the bulk product should preferably not have an average size larger than about 30 microns.

The sintered product of the present invention is use ful as a permanent magnet. lts permanent magnet properties can be significantly enhanced, however, by subjecting it to a magnetizing field. The resulting permanent magnet is substantially stable in air and has a wide variety of uses. For example, the permanent magnets of the present invention are useful in telephones, electric clocks, radios, television, and phonographs. They are also useful in portable appliances, such as electric toothbrushes and electric knives, and to operate automobile accessories. ln industrial equipment, the present invention magnets can be used in such diverse applications as meters and instruments, magnetic separators, computers and microwave devices.

If desired, the sintered bulk product of the present invention can be crushed to a desired particle size preferably a powder, which is particularly suitable for alignment and matrix bonding to give a stable permanent magnet. The matrix material may vary widely and may be plastic, rubber or metal such as, for example, lead, tin, zinc, copper or aluminum. The powder-containing matrix can be cast, pressed or extruded to form the desired permanent magnet.

All parts and percentages used herein are by weight unless otherwise noted.

The invention is further illustrated by the following examples in which, unless otherwise noted, the conditions and procedure were as follows:

The aligning magnetizing field was used to magnetically align along the easy axis.

The sintering furnace was a ceramic tube.

All sintering was carried out in an inert atmosphere of purified argon and upon completion of the sintering, the sintered product was cooled in the same purified argon atmosphere.

Particle size was determined by a standard metallographic method.

The density of the green body as well as the sintered product is given as packing. Packing is the relative density of the material, i.e., it is a percent of theoretical. Packing was determined by a standard method using the following equation:

Weight Volume 8, X l00=% Packing 7 point were connected with a straight line. The rest of the demagnetization curve was approximated by a straight line from the open-circuit point to the H z axis with a slope of 45. The maximum energy product 8 pressed into a circular disc. Compression was carried out in a steel die in an aligning magnetizing field ranging from 7 to kilo-oersteds provided by an iron electromagnet. Each disc was then sintered and its properon this line-segment demagnetization curve is the esti- 5 ties determined after sintering. After magnetization at mated energy product. room temperatures in a field of 25 kilooersteds, the

The intrinsic coercive force H,.,- or ,,,H,. is the field magnetic properties of the disc were determined. strength at which the magnetization (B-H) or Mr M is In Runs 11 through [6, the procedure was substanzero. tially the same as in Runs 1 through i0 except that the Normal coercive force H is the field strength at sample for each run was taken from a master batch which the induction B becomes zero. formed of 173 grams of the base alloy and 37 grams of The maximum energy product (BHL represents the additive alloy to give a mixture composed of 62 the maximum product of the magnetic field H and the weight percentcobalt and 38 weight percentsamarium. induction B determined on the demagnetization curve. Specifically, in Runs 1 l through 16, each sample was EXAMPLE l placed in a rubber tube and magnetically aligned therein. For Runs 11 through [3 the aligning magnetiz- A base alloy melt and an additive alloy melt of coing field was provided by an electromagnet and ranged balt-samarium were made under purified argon by arc from 7 to l5 kilooersteds. In Runs l4 through 16, the melting and cast into ingots. The base alloy was formed aligning magnetizing field was provided by a superconfrom 33.3 weight percent samarium and 66.7 weight ducting coil which in Runs 14 and l5.was [O0 kilo-oerpercent cobalt. The additive alloy was formed from 60 steds and in Run 16 was 60 kilo-oersteds. After magweight percent samarium and 40 weight percent cobalt. netic alignment in Runs 1 1 through l6. the tube was Each ingot was initially crushed by means of mortar evacuated to freeze the alignment and then it was and pestle and then reduced by fluid energy jet" millpressed hydrostatically to form the green body. ing to a powder ranging in size from approximately I to The tabulated sintering temperature of [050C or 10 microns in diameter and generally had an average l.iO0C is one at which the additive is in a partly liquid particle size of about 6 microns. phase.

The samples for Run Nos. 1 through 10 were taken To prepare the controlof Run No. 17, 13.95 grams of from 12 gram mixtures formed by admixing 10 grams the base alloy powder and 0.41 grams of an alloy powof the base alloy powder with 2.14gram of the additive der formed from 77% by weight cobalt and 23% by alloy powder by tumbling to form a substantially thorweight samarium, the alloy powder being the same size ough mixture composed of 62 weight percent cobalt and being prepared in the same manner as the base and 38 weight percent samarium. Since the additive alloy powder. were used to form a mixture of 67% by alloy was substantially non-reactive in air and was weight cobalt and 33% by weight Samariumv A portion slightly magnetic, a stable mixture of the two powders of the control mixture was placed in a rubber tube and was produced. Wet chemical analysis of portions of the magnetically aligned therein in an aligning magnetizing mixture showed 37.4 t 0.3% Sm. Portions of the mixfield of I00 kilo-oersteds provided by a superconducture were then compressed into green bodies which ting coil. The tube was then evacuated to freeze the were sintered and their magnetic properties deter alignment and then it was pressed hydrostatically to mined. form the green body. The estimated packing of the con- Table l tabulates the specific procedure used for trol before sintering was based on those measured for each run made. Specifically, in Runs 1 through l0 a similar samples prepared in the same manner. portion of the mixture was weighed and then com- TABLE 1 Magnetic Properties of Compressed Powder Disc Sintering Sintered Sintered Product After or Green Body Method Product Magnetization Max. Energy Prod. Intrinsic Compac- Size Tern Size (BH),,,,, Coercive lion (inches) per- (inches) Packl0 Force Pressures Wt. Thickness Packing Time ature Wt. Thickness ing gauss mH Runs (K psi) (gms) Diam. or Length (3%) Hrs. [C) (gms) Diam. or Length (3(2) X oersteds] (K oersteds) 1 200 1 0.319 0.115 78.4 1 1100 1 .3055 .109 90 8(est.) -19?! 2 200 2.5 0.507 0.113 79.1 1 1100 2.5 .482 .108 91 :51. ll.6 3 Product of Run No. 2 sintered longer 2 [100 2.5 .480 .106 94 8(es1.) -l5.5 4 Product of Run No. 3 sintered longer 3 [100 2.5 sample .l055 sample l3.5

broken broken 5 200 2.5 0.507 0.113 79.2 1 1100 2.5 .484 .1075 91 7.5(esi.) 20.8 6 Product of Run No. 5 sintered longer 2 H00 2.5 .482 .105 94 7.5(esL) l9.2 7 200 2.5 0.508 0.111 82.2 1 1100 2.5 .485 .105 93 8(est.) 20.0 B 200 2.5 0.507 0.111 80.1 a 1100 2.5 .483 .106 93 7.51m. 17.5 9 200 2.5 0.507 0.114 78. 15 1050 2.5 .496 .1115 33 7.50151.) 20.8 10 200 2.5 0.505 0.113 79.3 6' 1100 2.5 .484 .108 8(est.) 21.0 11 196 8.32 0.277 1.23 80.5 a 1100 3.319 .268 1.15 92 15 12 12 196 5.98 0.251 1.074 81. h 1100 5.966 .241 1.029 91.4 10.8 -12 13 60 7.24 0.318 1.1 41. 1% 1100 7.24 .2815 .995 as 13.4 12 i4 200 Did not measure 2 l I00 Did not measure 15.2 15 200 6.42 0.249 1.16 81.6 vs 1100 0.42 .241 1.11 91 14.3 12 16 200 6.92 0.249 1.25 8L6 is 1100 6.92 .242 1.196 90 16 12 17 200 Did not measure 8l(est.) Vb 6.73 .265 L055 33 6 l.7

TABLE l-continued Magnetic Properties of Compressed Powder Disc Sintering Sintered Sintered Product After or Green Body Method Product Magnetization Max. Energy Prod. Intrinsic Compac- Size Tem- Size (BH),,,,,, Coerci e lion (inches) per- (inches) Pack- Force Pressures Wt. Thickness Packing Time ature Wt. Thickness ing gauss m r Run (K psi) (gmsl Diam. or Length (7%) Hrs. (C) (gms) Diam. or Length (76) X oersteds) (K oersteds) l C ontrol] Table 1 shows that sintering of the green body produces a sinte red product which weighs about the same as the green body indicating no loss in the cobalt and samarium components. However, the green body does undergo some shrinkage during sintering as illustrated by the packing of the sintered product which was significantly higher in every run than that of the green body. In Runs 1 through 10 the energy product was estimated due to the geometric limitations of the product.

Runs Nos. 1 through 16 specifically illustrate the significantly better magnetic properties obtained by 'the present process as compared to Run No. 17, the control, where no additive alloy of the present invention was used.

Examination of the sintered products of Run Nos. 5 and 16, by standard metallographic analysis showed that two phases were present in each product. Micro graphs of a polished cross-section of each product showed that the pores were substantially non-interconnecting and that a major amount of one phase was present along with a minor amount of a second phase and that traces of a few other phases also appeared to be present. In addition, under a light microscope as well as under an X-ray microprobe, there can be seen material deposits in a number of the pores that appear to have been liquid at high temperatures. Also, substantially all of the grains of each sintered product were rounded and had a smooth surface which appearedto be a liquid sintered smooth surface. The average grain size of each sintered product was about 7 microns.

The sintered product of Run No. 14 of Example 1 had an intrinsic coercive force of 15,200 oersteds. This product was used to provide the particulate samples of the present example. lnitially, the product was crushed to a particle size ranging from 210 to 420 microns by mortar and pestle. The coercive force of a portion of this crushed material was determined. A second portion of the crushed material was placed in air in an oven at a temperature of 150C for 17 hours. lts coercive force was then determined and the results are shown in Run No. 18 in Table 11.

For Run No. 19 of Table [1, the remaining portion of the crushed material was further crushed in the same manner to produce a powder ranging in size from 42 to 210 microns, and the same procedure was then followed as in Run No. 18.

For Run No. 20 the remaining material was crushed to a size less than 42 microns and again the same procedure was followed as in Run No. 18.

In all of the runs of Table ll, the intrinsic coercive force of the sintered cobalt-rare earth powder, was measured at room temperature in the same manner. Specifically, a specimen of the powder was prepared for magnetic measurement by introducing it into a body of molten paraffin wax in a small glass tube and cooling the wax in an aligning magnetic field of about 17,500 oersteds until the paraffin solidified. The intrinsic coercive force of each such prepared sample was then measured after magnetization in a field of 17,500 oersteds. The results are shown in Table II.

TABLE II Sintered Intrinsic Intrinsic Coercive Force Product Coercive Force I-I Roe.) Particle Size, H Koe.) Afier 17 hours. Run No. Microns As Crushed at 150C in Air Examination of a polished cross-section of the sintered product of Run No. 17, the control, by standard metallographic analysis showed only a single intermetallic phase. Standard wet chemical analysis of the control showed it to contain 33 percent by weight samarrum.

After aging the sintered product of Run No. l for 400 hours at a temperature of 150C in air, its intrinsic coercive force was determined and found to be unchanged. This illustrates the highly stable properties of the permanent magnets of the present invention.

EXAMPLE 2 In this example the stability of magnetic properties of the sintered product of Example 1 was determined.

Table 11 illustrates the good stability of the permanent magnet materials produced by the present process. Specifically, after extensive heat aging in air, Run

magnetic properties.

The sintered particulate material of Table 11 is particularly suitable for magnetic alignment and matrix bonding to give a stable permanent magnet.

EXAMPLE 3 In this example the magnetic properties of sintered products formed from three different cobaltsumarium mixtures were determined.

Portions of the base alloy (66.7 percent by weight cobalt and 33.3 percent by weight samarium) powder and the additive alloy (40 percent by weight cobalt and 60 percent by weight samarium) powder disclosed in Example l were used to prepare the three mixtures of this example. Specifically, for Run No. 21, 13.12 grams of the base alloy were admixed with 0.88 grams of the additive alloy to form a mixture composed of 65 percent by weight cobalt and 35 percent by weight samarium. For Run No. 22, 12.08 grams of the base alloy were admixed with l.92 grams of the additive alloy to form a mixture composed of 63 percent by weight cobalt and 37 percent by weight samarium. For Run No. 23, 11.04 grams of the base alloy were admixed with 2.96 grams of the additive alloy to form a mixture composed of 61 percent by weight cobalt and 39 percent by weight samarium. Each mixture was formed in the same manner and into substantially the same sized bar as disclosed for Run No. l 1 of Example 1. Each bar was sintered at a temperature of 1 100C for one-half hour. The pack ing of Run No. 21 (35 percent Sm) was 84 percent, of Run No. 22 (37 percent Sm) 89 percent and of Run No. 23 (39 percent Sm) 87 percent. Each bar was magnetized at room temperatures in a field of 100 kilo-oersteds. Each bar was then demagnetized using the specific magnetizing fields shown in FIG. 2 and its magnetization 41rM in such field was determined. Since none of the sintered bars showed a weight loss, its cobalt and samarium content was the same as the mixture it was formed from.

in FIG. 2, the abscissa of the graph is the magnetic field (H) in kilo-oersteds and the ordinate is magnetization 41rM in kilo-gauss. From the demagnetization curves of FIG. 2, it can be seen that the product of Run No. 22 containing 37 percent by weight samarium has the best magnetic properties. This is particularly illustrated by its high intrinsic coercive force. As can be seen from the phase diagram of FIG. 1, this product at the sintering temperature of 1,100C as well as at room temperature is comprised of a major amount of the Co Sm single intermetallic phase, i.e., about 95 percent by weight of the product, and a minor amount of the Co Sm phase, i.e., about 5 percent by weight of the product. X-ray diffraction analysis of this product showed it to have two phases.

FIG. 2 shows poor magnetic properties for the sintered product of Run No. 21 composed of 65 percent cobalt and 35 percent by weight samarium which according to FIG. 1 is the composition for a single intermetallic phase. Microscopic analysis of the product of Run No. 21 showed it to be a single phase.

EXAMPLE 4 In this example, the procedure and materials used were substantially the same as that set forth in Example 1 for preparing the compressed disc of Run No. 1 and a number of sintering runs were carried out in substantially the same manner as Run No. 1 of Example 1.

Standard wet chemical analysis of the initial powder cobalt-samarium mixture as well as the sintered product of each run showed that the sintered products contained the same amount of samarium by weight as the starting powder.

EXAMPLE 5 In this example, a permanent magnet was formed comprised of the sintered powder of the present invention distributed in a metal matrix.

In order to produce a thorough mixture, the sintered powder of Run No. 18 of Example 2 was initially demagnetized by heating it to a temperature of 900C for 10 minutes. The demagnetized powder was then mixed with a l00 mesh (US. Standard Screen Size) aluminum powder to produce a mixture of volume percent of the sintered powder and 20 volume percent of the aluminum powder.

A slurry of the mixture was made with isopropyl alcohol and placed in a die press in an aligning magnetizing field of 15 kilo-oersteds and pressed under a pressure of 200 K psi. The resulting compressed body had a diameter of 0.336 inch and a length of 0.351 inch. After it was magnetized in a field of 15 kilo-oersteds, the resulting magnet had an open circuit induction 8,, of 2395 gauss.

EXAMPLE 6 A base alloy melt and an additive alloy melt were formed under purified argon by arc-melting and were cast into ingots. The base alloy was formed from 68 weight percent cobalt, 1 6 weight percent samarium and 16 weight percent cerium-misch metal. The additive alloy was formed from 40.8 weight percent cobalt and 59.2 weight percent samarium. Each ingot was formed into a powder in the same manner as disclosed as in Example ranging in size from approximately 1 to about 10 microns in diameter with an average particle size of about 6 microns.

24.48 grams of the base alloy were admixed with 5 .51 grams of the additive alloy by tumbling to form a substantially thorough mixture of 63 weight percent cobalt and 37 weight percent rare earth.

7.83 grams of the mixture were aligned in an aligning magnetic field of 60 kilo-oersteds and then compressed under a pressure of 200 K psi to form a bar which was 0.286 inch in diameter and 1.075 inches long and had a packing of 79 percent. The bar was sintered for onehalf hour at a temperature of 1050C. Upon completion of the sintering, the sintered bar had a 0.284 inch diameter and a length of 1.020 inches and a packing of 87 percent. It also weighed 7.83 grams which indicates that none of the material was lost during sintering. After the sintered bar was magnetized in a field of 16.5 kilo-oersteds, it had an intrinsic coercive force H, of 4600 oersteds and an open circuit induction B of 6 l 60 gauss in a self-demagnetizing field of 300 oersteds.

All of the following cited copending patent applications are, by reference, made part of the disclosure of the present application.

In copending US. Pat. application Ser. No. 33,348, now US. Pat. No. 3,655,463, entitled Sintered Cohalt-Rare Earth lntermetallic Product And Process Using Solid Sintering Additive, filed of even date herewith in the name of Mark G. Benz and assigned to the assignee hereof, there is disclosed a process for preparing novel sintered cobalt-rare earth interrnetallic products by forming a particulate mixture of a base cobalt-rare earth alloy, which at sintering temperature exists as a solid C0,,R intermetallic phase, and an additive cobalt-rare earth alloy, which at sintering temperature is a solid, compacting the mixture to produce a green body and sintering the green body to produce a sinl3 tered product containing a major amount of C0,,R and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth metal content than the C0,,R phase.

ln copending US. Pat. application Ser. No. 33,224, now US. Pat. No. 3,695,945, entitled Sintered Cobalt-Rare Earth lnterrnetallic Product," filed of even date herewith in the name of Mark G. Benz and assigned to the assignee hereof, there is disclosed a process for preparing novel sintered cobalt-rare earth intermetallic products by forming a particulate cobaltrare earth metal alloy which contains a major amount of the Co R intermetallic phase and a second solid CoR phase which is richer in rare earth metal content than the Co R phase, compacting the alloy particles to produce a green body, and sintering the green body to produce a sintered body containing a major amount of Co R phase and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth metal content than the Co R phase.

In US. Pat. No. 3,684,593 which is a continuationin-part of copending US. Patent application Ser. No. 33,315, now abandoned, entitled Heat-Aged Sintered Cobalt-Rare Earth lntermetallic Product and Process," filed of even date herewith in the names of Mark G. Benz and Donald L. Martin and assigned to the assignee hereof, there is disclosed a process for preparing heat-aged novel sintered cobalt-rare earth intermetallic products by providing a sintered cobalt-rare earth intermetallic product composed of C0,,R intermetallic phase or a major amount of Co R intermetallic phase, and heat-aging said product to precipitate out of the C0,,R phase a CoR phase richer in rare earth metal content than the Co R phase in an amount sufficient to increase either its intrinsic coercive force H and/or normal coercive force H by at least percent.

What l claim as new and desire to secure by Letters Patent of the United States is:

l. A sintered product having significantly enhanced permanent magnet properties which are substantially stable in air at room temperature consisting essentially of compacted particulate cobalt-rare earth alloy consisting essentially of a Co R intermetallic phase and a CoR intermetallic phase which is richer in rare earth metal content than said Co R phase, said Co R intermetallic phase being present in an amount of at least 65% by weight of said sintered product and said CoR intermetallic phase which is richer in rare earth metal content than said Co R phase being present in a positive amount having a value ranging up to 35% by weight of said product, where R is a rare earth metal or metals, said sintered product having a density of at least 87 percent of theoretical and having pores which are substantially non-interconnecting and wherein the component grains have an average size less than 30 microns, said sintered product being produced by admixing particles of a base cobalt-rare earth metal alloy and an additive cobalt-rare earth metal alloy, said particles having an average size ranging up to about 10 microns, said base alloy existing at sintering temperature as a solid Co R intermetallic phase and said additive alloy existing at sintering temperature as at least a partly liquid phase and being richer in rare earth metal content than said base alloy, said base alloy and said additive alloy each being used to form a mixture which has a cobalt and rare earth metal content substantially corresponding to that of the final sintered product with said additive alloy being present in an amount of at least 0.5 percent by weight of said mixture, pressing said mixture into a green body, and sintering said green body in a substantially inert atmosphere to produce said sintered product.

i l l

Claims

1. A SINTERED PRODUCT HAVING SIGNIFICANTLY ENCHANCED PERMANENT MAGNET PROPERTIES WHICH ARE SUBSTANTIALLY STABLE IN AIR AT ROOM TEMPERATURE CONSISTING ESSENTIALLY OF COMPACTED PARTICULATE COBALT-RARE EARTH ALLOY CONSISTING ESSENTIALLY OF A CO5R INTERMETALLIC PHASE AND A COR INTERMETALLIC PHASE WHICH IS RICHER IN RARE EARTH METAL CONTENT THAN SAID CO5R PHASE, SAID CO5R INTERMETALLIC PHASE BEING PRESENT IN AN AMOUNT OF AT LEAST 65% BY WEIGHT OF SAID SINTERED PRODUCT AND SAID COR INTERMETALLIC PHASE WHICH IS RICHER IN RARE EARTH METAL CONTENT THAN SAID CO5R PHASE BEING PRESENT IN A POSITIVE AMOUNT HAVING A VALUE RANGING UP TO 35% BY WEIGHT OF SAID PRODUCT WHERE R IS A RARE EARTH METAL OR METALS, SAID SINTERED PRODUCT HAVING A DENSITY OF AT LEAST 87 PERCENT OF THEORETICAL AND HAVING PORES WHICH ARE SUBSTANTIALLY NON-INTERCONNECTING AND WHEREIN THE COMPONENT GRAINS HAVE AN AVERAGE SIZE LESS THAN PS 30 MICRONS, SAID SINTERED PRODUCT BEING PRODUCED BY ADMIXING PARTICLES OF A BASE COBALT-RARE EARTH METAL ALLOY AND AN ADDITIVE COBALT-RARE EARTH METAL ALLOY, SAID PARTICLES HAVING AN AVERAGE SIZE RANGING TO ABOUT 10 MICRONS, SAID BASE ALLOY EXISTING AT SINTERING TEMPERATURE AS A SOLID CO5R INTERMETALLIC PHASE AND SAID ADDITIVE ALLOY EXISTING AT SINTERING TEMPERATURE AS AT LEAST A PARTLY PHASE AND BEING RICHER IN RARE EARTH METAL CONTENT THAN SAID BASE ALLOY, SAID BASE ALLOY AND SAID ADDITIVE ALLOY EACH BEING USED TO FORM A MIXTURE WHICH HAS A COBALT AND RARE EARTH METAL CONTENT SUBSTANTIALLY CORRESPONDING TO THAT OF THE FINAL SINSTERED PRODUCT WITH SAID ADDITIVE ALLOY BEING PRESENT IN AN AMOUNT OF AT LEAST 0.5 PERCENT BY WEIGHT OF SAID MIXTURE, PRESSING AND MIXING INTO A GREEN BODY, AND SINSTERING SAID GREEN BODY IN A SUBSTANTIALLY INERT ATMOSPHERE TO PRODUCE SAID SINSTERED PRODUCT.