US3219495A - Method of effecting gamma phase precipitation to produce a monocrystalline growth in permanent magnets - Google Patents

Description

E. STEINORT Nov. 23, 1965 3,219,495 ODUCE A METHOD OF EFFECTING GAMMA PHASE PRECIPITATION TO FR MONOCRYSTALLINE GROWTH IN PERMANENT MAGNETS Filed April 5, 1965 2 Sheets-Sheet 1 ffiflfl 6------ I N VE N TOR. [EiZ/MED Spa/meg;

E. STEINORT Nov. 23, 1965 3,219,495 MMA PHASE PRECIPITATION TO PRO LINE GROWTH IN PERMANENT MAGNETS DUCE A METHOD OF EFFECTING GA MONOCRYSTAL Filed April 5, 1963 2 Sheets-Sheet 2 v, w 6 5 M M W w w M flames I N VEN TOR. fame 4w firm/oz;

yI/IIII United States Patent 3,219,495 METHOD OF EFFECTING GAMMA PHASE PRE- CIPITATION TO PRODUCE A MONOCRYSTAL- LINE GROWTH IN PERMANENT MAGNETS Eberhard Steinort, Milan, ltaly, assignor to Centro Magneti Permanenti, S.p.A., Milan, Italy, a corporation of Italy Filed Apr. 5, 1963, Ser. No. 276,684 Claims priority, application Italy, Apr. 6, H62, 6,949/62 24 Claims. (Cl. 148-191) This invention is a modification and improvement of the subject matter of my prior copending application Serial No. 14,348, filed March 11, 1960, now Patent No. 3,085,036, dated April 9, 1963. This invention relates to a process of making permanent magnets having a monocrystalline structure, and to certain products produced in such method.

It is the general object of the invention to provide a modified and improved method of making monocrystalline magnets, which will be better adapted for commercial use, will avoid the necessity for certain conditions and steps which are preferably used in the method disclosed in my prior application, and which will reliably produce monocrystalline structure in originally polycrystalline magnet castings.

In the preferred method of said prior application, monocrystalline structure is produced in normally polycrystalline magnet castings by (a) including in the magnet alloy a gamma-phase precipitant, (b) casting such alloy into molds to produce magnet castings which contain the gamma-phase precipitant throughout their entire bodies, (c) heat treating such gamma-phase-containing castings at a gamma-phase precipitating temperature, e.g. l0001050 C., to produce gamma-phase precipitation therein, and thereby produce critical strain conditions therein, and (d) heat-treating the castings at a recrystallizing temperature, e.g., 1260-l310 C., with gamma-phase precipitate present therein, for a time sulficient to permit monocrystal growth and to redissolve the gamma-phase precipitate. To secure monocrystal orientation relative to the preferred diection of magnetization in the magnets produced, the recrystallization heat-treatment is carried out in a manner to produce a temperature gradient from one end of the casting, so that the monocrystal growth is nucleated at such end and progresses in a direction normal to such end.

The presence of gamma-phase precipitate is highly undesirable in finished magnets because of its deleterious effeet on magnetic characteristics, and after the recrystallization treatment in the prior process, it is necessary to cool the treated magnet bodies rapidly through the gammaphase precipitating range in order to avoid the presence of gamma-phase precipitate in the magnets.

In accordance with the present invention, the magnet alloy from which the magnet bodies are cast contains no added gamma-phase precipitant and is of a normal composition selected solely for its magnetic and other characteristics. Instead of producing in the entire magnet body the composition and conditions needed to cause gammaphase precipitation and resulting critical strain, the present invention contemplates the production of such composition and conditions in only a thin layer at a selected face of the magnet casting. The monocrystal growth is started in this layer and, once sufiiciently started therein, will progress throughout the normal alloy composition of the main body of the casting. The thin layer in which the gamma-phase precipitate and strain conditions are produced is provided by enrichment with suitable components, as will be explained below. Such layer may be removed in the normal finishing operations on the magnets, so that the entire magnet produced will be of the normal alloy composition.

Since the body of the magnet will not be of a composition which tends to produce gamma-phase precipitate, special rapid cooling need not be used, and standard production procedures can be followed. Moreover, the enriched layer on a selected face of the magnet has the effect itself of controlling the location of monocrystal nucleation and the direction of monocrystal growth, so that no particular temperature gradient need be maintained during the recrystallization heat-treatment.

The present invention, like that of said prior application, is applicable to the known high-strength iron-nickelaluminum type alloys basically composed of iron, nickel and aluminum, and commonly including cobalt, copper, and various addition elements. These comprise the alnico-type alloys containing 10 to 30 percent nickel, 6 to 14 percent aluminum, 5 to 42 percent cobalt, up to 8 percent copper, up to 10 percent titanium, with the balance substantially all iron but permissibly containing up to 5 percent of silicon, zirconium, columbium, or other known addition elements. The preferred alnico alloys contain 12 to 20 percent nickel, 6 to 11 percent aluminum, 16 to 30 percent cobalt, 2 to 6 percent copper, and from a trace to 7 percent titanium;

The prior application discloses that monocrystal growth is favored by the presence of certain elements which widen the gamma-phase loop in the iron phase-diagram and which promote gamma-phase precipitation and stabilize gamma-phase precipitate. These were listed as including carbon, nitrogen, manganese, ruthenium, rhodium, rhenium, osmium, iridium, platinum, and gold. They also include cobalt and nickel which in normal alnico alloys have their action suppressed by co-present aluminum.

In alnico alloys these elements have the effect of displacing to the right the line representing that alloy in the phase diagram, and hence of enlarging both upward and downward the temperature range within which gamma-phase is precipitated and is stable.

In the process of the prior application, the monocrystalline growth takes place starting with crystals already present as part of the originally cast polycrystalline structure, but which have their axis parallel to What will subsequently be the direction of preferred magnetic orientation. Such crystals occur at particular outer faces of the magnet castings. Accordingly, in the recrystallization treatment it is necessary to take care that such particular faces are heated in an especially intensive way to develop a temperature gradient in the direction of the (100) axis of such crystals, and thereby cause the monocrystalline growth to develop from such crystals and to progress in that same direction. This need for a directed temperature gradient is an inconvenience.

Moreover, the addition and control of the gammaphase precipitants in the entire alloy in the prior process was somewhat critical, for if excess amounts were present, either from an original formulation or by reuse of scrap, or if cooling from recrystallization temperature was not sufiiciently rapid in the magnet castings or in parts thereof, the danger existed that gamma-phase precipitation would occur during such cooling, and that gamma-phase precipitate would therefore be present in the magnets and would adversely affect their magnetic properties.

Further studies and research about the mechanism of crystal growth in magnet castings have brought about results which extend beyond the disclosure of the prior application and permit decisive improvements.

It was possible to establish that, for granule growth, particular importance resides in the energies existing at the granule or grain boundaries, which depend to a large extent on the internal stresses within the crystal grains, which in turn are susceptible to influence by the shape or size of the granules and by the quantity of gamma-phase precipitate present.

Considerable difference of energy at the grain boundaries occur as the result of the presence in the structure of the casting of grains of large size in direct proximity to grains of small size.

Now with respect to the volume of granules, those of small size have relatively larger surface areas than those of larger size. Since the crystal grain boundaries correspond to the external surfaces of the granules, if equal magnitude of absolute energy in different crystal grains is assumed, the grains of smaller dimensions will have higher grain-boundary energies per unit of area than the larger grains. In any system involving energy differences, the basic tendency is for change to go in the direction which gives the minimum value of total energy. Thus, in a crystalline structure, the trend is in the direction to relieve or destroy the elevated energy levels existing at the grain boundaries, and is toward a condition in which there is no longer any energy at the grain boundary, which is the condition of a monocrystal which has no grain boundary with any other crystal.

The elimination of the energy differences at the grain boundaries takes place by means of displacements of the grain boundaries where the granules of smaller size are disintegrated and absorbed by the adjacent granules of larger size. That however is possible only by a corresponding mobility of the atoms, which occurs only at higher temperatures.

If a big granule has caused the disintegration and the absorption of a smaller one, then the first big granule, now still larger, comes into proximity with another granule and again a difference occurs between the energies existing at the grain boundaries, so that again the adjacent smaller granule is absorbed. In consequence of such constant growth of a granule, the difference of the relative energies existing at the boundaries of said granules, being proportional to the granule volume differences, becomes greater and greater and the speed of growth of the granule increases continuously. Accordingly, if monocrystal growth is once started, it tends to proceed to completion.

The differences of volume of adjacent granules formed by normal alnico alloys, cast normally, are not of sufficient magnitude to cause with certainty the beginning of the first process of growth. Hence the formation of monocrystal structure in such alloys, in the absence of a heat-treatment to precipitate gamma-phase, takes place only incidentally and only when by chance a very big granule and a very small granule happen to be adjacent to each other, and where then the difference of the relative energies existing at the grain boundaries is higher than a critical value.

Now, by measurement of micro-hardness, it can be shown that gamma-phase precipitate in the basic alpha structure produces a substantial increase of the microhardness of said alpha phase. Further it has been possible to establish an inter-relationship between the microhardness of the basic structure and internal stresses, and an inter-relationship between the internal stresses and the absolute energy existing at the boundaries of the granules per unit of surface area. This may be explained theoretically by the fact that the alpha phase has a body-centered cubic structure whilst the gamma-phase has a face-centered cubic structure and that the two reticular structures present constant differences of reticle and different coefficients of heat expansion.

If by means of gamma segregation the absolute values of the energies at the boundaries of the granules are increased, then the process of granule growth to form a monocrystal will take place with certainty even with smaller diflerences of volume between adjacent granules. From the diagram of phases illustrated in my prior application, FIG. 1 in the accompanying drawings, it can be seen that the region with two phases Alpha+ gamma, for the standard alloy of composition: 24% Co, 14% Ni, 8% Al, 3% Cu and with about 0.015% of C extends only over an interval of temperature approximately ranging from 30 C. to 1,200 C., where the gamma-phase again returns to solution at temperatures of from 1,175 C. to 1,200 C. However, since as already set forth, a certain mobility of the atoms is necessary for displacement of the grain boundaries, and this is produced only above temperatures of 1,220 C. approximately, it is evident that it is necessary to keep the gamma-phase stable at higher temperatures. Otherwise, the gamma-phase becomes dissolved before the necessary mobility of the atoms is attained. Such stabilization is possible by addition of the elements mentioned in my co-pending Patent No. 3,085,036, the use of which as alloying elements in the entire magnet casting, however, involves certain sources of danger, as already set forth.

On the basis of my discovery that the crystal growth continues without the presence of gamma-phase precip itate, if the difference of volume of adjacent granules once becomes sufficiently gerat, I have now found according to the present invention that it suffices to start the growing process of the granules in a layer with stable gammaphase, and that to start this process it sufiices to enrich a thin layer of the magnet with an element which produces a stable gamma-phase precipitate therein. In the layer containing stable gamma-phase precipitate, the process of granule growth can be reliably started, and a certain crystal in such layer will grow in the layer until it attains a size that is sufiicient to ensure continued progressive growth to disintegrate and absorb the granules of the unenriched main body of the magnet, where, in the absence of growth-stimulating gamma-phase precipitate, the granules have remained small.

Accordingly, the process of the present invention is characterized in that the body of the magnet is cast of standard or normal alloy, to produce a crystal structure in which the polycrystalline grains are of normal small size and in which the occurrence of gamma-phase precipitate tends to be suppressed, in accordance with normal magnet producing practices; and that the process of producing gamma-phase precipitate and stabilizing the same, as disclosed in said prior application, is applied to only a thin layer of the magnet casting, in a thickness of the order of from 0.5 to 1 mm. or so. This may be done by enriching the layer by the addition to that layer of the known gamma-phase precipitants listed in said prior application, or by enriching the layer with the elements nickel or cobalt, which have the same effect and which are already present in selected proportions for other purposes.

The action of the gamma-phase precipitants, especially that of nickel and cobalt, depends on the amount of aluminum present, since aluminum has a gamma-phase suppressing effect. Accordingly, the desired gammaphase-producing layer can be obtained both by adding a gamma-phase precipitant thereto, and by preferentially removing the gamma-phase-suppressing aluminum while leaving a gamma-phase precipitant, such as nickel and cobalt, which is already present.

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a reproduction of the phase diagram showing phase changes of a typical Alnico alloy;

FIG. 2 is a diagrammatic sectional view of a magnet casting at an intermediate stage of the process; and

FIG. 3 is a diagram of a heat-treatment cycle used in a process wherein aluminum is removed from a surface layer of a magnet casting, as described in Example 9 below.

In the diagram of FIG. 1, the vertical dotted line rep-.

resents an Alnico alloy of the composition indicated. The diagram shows that this alloy has a pure alpha structure both above 1200 C. as well as in the range of 900930 C. To obtain good magnetic properties, this alloy must be magnetically heat-treated, i.e., brought under the influence of the magnetic field While it is in the pure alphaphase state. Any magnetic heat-treatment from temperatures between 930 and 1180" C. leads to poor results because the structure is not pure alpha-phase in this temperature range. At temperatures between 930 C. and about 1150 C. a second or gamma-phase is precipitated, first at the grain boundaries and then also within the crystals. From about 1175 C. to 1200 C. the gammaphase again enters into solution. The presence of this gamma-phase in a magnet casting during heat-treatment in a magnetic field has a very marked deleterious efiect on the formation of a preferred magnetic orientation. Even the presence of very small amounts of gamma-phase is enough to decrease the magnetic values to such a point that the magnets are useless. A 7% content of gammaphase will lower energy values by about Recent studies indicate that preferred magnetic orientation is the result of the directional precipitation of submicroscopic particles of a second alpha-phase, referred to as alpha-prime, at temperatures below 900 C. However, the presence of even minor quantities of the gammaphase obstructs the proper establishment of a preferred magnetic orientation. These gamma-phase precipitates also cause major lattice distortions with associated large internal mechanical strains, and these serve to induce large-crystal growth under suitable recrystallization heat treatment.

FIG. 2 of the drawing represents a magnet casting 10 at an intermediate stage of the present process. The main body 12 of the casting, constituting substantially the entire casting, is in its original as-cast condition. The magnet is assumed to have been cast from a standard magnet alloy such as an alnico alloy of the preferred composition given above, and to have been cast by standard casting procedures. The main body 12 is represented as having the completely polycrystalline and desirably fine-grained structure normally produced by such casting, and the granules are represented as having no particular orientation, since the present invention requires none, although in practice the crystallites adjacent the surfaces which were cooled by contact with the mold would normally be oriented with their (100) axes normal to such surfaces. It is known that certain addition elements such as titanium produce fine-grained structure, and such structure is desirably present for purposes of the present invention.

The top and side surfaces of the casting 10 are coated with a protective coating 14 of a character commonly used in metal processing operations to protect the underlying surface from a carburizing, nitriding, or other surface treatment. The bottom surface 16 of the casting is uncoated, and left exposed, and a thin layer 18 of the casting at this surface is shown as modified from its as cast condition. In accordance with the invention such a layer is produced at a selected face of the magnet casting, and in such layer the composition is enriched with a gammaphase precipitant or otherwise modified to promote the precipitation of gamma-phase and to stabilize the precipitate produced. As shown, such layer desirably has its crystallites oriented with their (100) axes in a predetermined relationship to the body of the casting, for example, normal to the end face 16 of the casting.

The modified layer may be produced in various ways, as by carburizing or nitriding, by casting the bodies in contact with a deposit of the desired enrichment component, by depleting the layer of a gamma-phase suppressant, etc., as illustrated by the following examples.

Example 1 Polycrystalline magnets weighing 100 grammes each were cast in the usual way, of a standard Alnico composition, without any C, Mn or other additions adapted to promote gamma-phase precipitation. The castings so produced were divided into two groups.

The magnets of group 1 were heat-treated for a period of 30 minutes at the temperature of l,0-00 C., for the purpose of obtaining precipitation of gamma-phase, and were then heat-treated for a period of three hours at the recrystallization temperature of 1,300" C. The treated castings were then cooled down and crushed. Examination of the crushed castings showed that there had been no formation of monocrystals and no granule growth.

The magnets of group 2 were covered on all their surfaces, with the exception of one end surface, and were carburized for a period of 30 minutes at the temperature of l,000 C., as with standard hardening processes. The carburization of the exposed end surface produced a hardened layer having a depth of hardening about equal to 0.3 mm. and in this layer produced a carbon content approximately equal to 0.08%. The carburization heattreatment also served to precipitate gamma-phase, and no further heat-treatment for this purpose was used.

The magnets of group 2 were subsequently brought to a temperature of 1,300 C. and, after diiferent times of stay, were removed from the furnace, cooled down and crushed. The results were as follows:

Stay for 2 minutes at 1,300 C.: Growth of the granules in the carburized layer, no growth of the granules in the non-carburized mass.

Stay for 5 minutes at 1,300 C.: Formation of a large crystal, which had developed in and grown from the carburized layer, and had absorbed into itself parts of the mass of the magnet casting which had not received added carbon from the carburization.

Stay for 10 minutes at 1,300 C.: A large crystal, which had developed in and grown from the carburized layer, occupied /a of the volume of the magnet;

Stay for 30 minutes at 1,300 C.: Formation of monocrystal in all of the tests.

Example 2 In a plural-cavity sand mold, a thin layer of carbon dust was deposited on that surface portion of each cavity which would define an end surface of the magnet body cast therein, and the carbon dust was compressed to hold it in place. A magnet alloy melt was then poured and cast in the mold in the normal way.

Magnet castings so produced were examined. Microscopic examination of the metallurgical structure showed that the castings had absorbed carbon in a thin layer at the surfaces cast in contact with the deposits of carbon dust.

The magnet castings produced in this way were heattreated for 30 minutes at a gamma-phase precipitating temperature of 1,000 C., were subsequently heat-treated for one hour at a recrystallizing temperature of 1,300 C. and were then cooled and crushed. Examination showed the formation of monocrystals had occurred in all the magnet castings.

Example 3 Magnets of grammes weight cast in normal manner, with polycrystalline structure, without any addition of C, N, Mn or other gamma-phase precipitant to the standardized composition, were divided into two groups.

The magnets of group 1 were at first heat-treated at a tem erature of 1,000 C. to promote precipitation of gamma-phase, then heat-treated at a recrystallization temperature of 1,300 C., and then cooled and crushed. The results were as follows: In 19 pieces no formation of monocrystals, in one piece formation of a large crystal that had absorbed about /z of the volume of the magnet.

The magnets of group 2 were coated on all surfaces except one end face with a protective coating, as shown in FIG. 2, and were then subjected to nitriding in a suitable bath at a temperature of 1,000 C. The nitriding treatment, which served simultaneously for the precipitation of gamma-phase, lasted 30 minutes.

By microscopic examination, it was found that the nitriding treatment had produced a modified layer at the uncoated end face of the castings having a depth of about 0.2 mm.

The magnets of group 2 were subsequently heat-treated at a temperature of 1,300 C., and after different times of stay at that temperature they were removed from the furnace, cooled down and crushed. The results were as follows.

Stay for 2 minutes at 1,300 C.: Growth of the crystal granules had developed and progressed from the nitrided layer into the non-nitrided mass of the magnet.

Stay for 8 minutes at 1,3000" C.: Formation of a large crystal that had already absorbed about A of the volume of the magnet.

Stay for 25 minutes at 1,300 C.: Formation of monocrystal in all of the tests.

The nitrided layer was removed with the grinder and analyzed. It was found that besides the nitrogen absorption in the nitriding bath, also a carburizing effect was produced.

Example 4 As in Example 2, in the cavities of the molds, prior to casting, there was spread a powdery material, which in this case was not carbon dust but a fine iron powder with a maximum N content. The magnets then were cast in normal manner, then heat-treated as in Example 2, and research was carried out to determine the formation of monocrystals.

About 80% of all the samples showed the formation of monocrystals. In the remaining 20% of the samples no substantial growth of the granules took place, and it was found that no nitrogen-enriched layer had been formed, apparently for the reason that no intimate contact had taken place between the iron powder with maximum nitrogen content and the alloy of the magnet.

Examples 1 to 4 appear to show that it is by no means necessary to react the entire mass of the magnet with elements adapted to precipitate and stabilize the gammaphase (and that, therefore, it is by no means necessary to enrich the alloy in its entirety with elements such as C, N, Mn, etc.), but that it is quite suflicient to proceed by enrichment of a thin layer only with those elements. In that way, there is started in that layer the formation of monocrystals which propagate further in the pure alloy of the magnet.

Since the layer in which the added elements have been absorbed is removed by grinding in the mechanical working which is necessary for other purposes, there is no longer any danger of contamination of the alloy which gives the desired magnetic properties, so that according to the present invention it is possible to eliminate the cause of danger stated at the beginning that affects the process described in the prior application.

As set forth previously, the elements nickel and cobalt already present in the alloy also have the effect of Widening the zone of the gamma-phase, and consequently enrichment with these elements also serves to precipitate and stabilize gamma-phase. However, if higher contents of cobalt or of nickel are included in the whole alloy composition, the magnetic values that can be attained may be reduced below those obtained with the standard composition of the alloy. Hence, according to the present invention, one proceeds to enrich with nickel or with cobalt only a thin layer of the mass of the magnet, as was done with additions of carbon and nitrogen.

8 Example 5 In the cavities of sand molds, prior to casting, were placed small discs of pure nickel 0.2 mm. thick and of a size corresponding to the end face of the magnets. A standard magnet alloy was then cast in the molds, and the discs became solidly joined to the end faces of the magnet castings.

It was found by microscopic observation of castings so produced that the small disc of nickel joined to the end face of the magnet was still constituted of nearly pure nickel and that an alloy with the mass of the magnet had been formed for a thickness of only about 0.1 mm. For that reason, the magnets were brought to a temperature of 1,250 C. in order to obtain at that temperature an energetic diffusion of the atoms, and in order to realize in that way a better formation of a nickel-enriched alloy layer with the use of the entire nickel disc.

Subsequently, the magnets were allowed slowly to cool down in a furnace to favour the segregation of gammaphase, with the result that the castings remained in the alpha-plus-gamma phase temperature range for a considerable period, cooling in 50 minutes time from 1,250 C. down to 800 C.

Subsequently, the castings were subjected to recrystallization treatment for a period of one hour at a temperature of 1,300 C.

Result: Complete formation of monocrystals in all of the samples.

Example 6 The procedure of Example 5 was accurately followed, with the only difference that instead of the nickel discs a thin cobalt sheet was employed.

Result: Complete formation of monocrystals in all of the samples.

Example 7 The procedure of Example 5 was followed, with the only exception that a sheet of commercial type was used having a thickness of 0.25 mm., formed of 50% of cobalt and of 50% of iron.

Result: Monocrystals formed in 72% of the castings; in the remainder there was a vigorous growing of the granules, however, such growth had not progressed far enough to form complete monocrystals of the whole castings.

On the basis of theoretical considerations, it appears that higher contents of nickel or of cobalt have the effect of relatively lowering the aluminum content, and that it is the relative contents of these which controls gammaphase precipitation and stabilization. A reduction of the aluminum content in the alloy, therefore, ought to act in the same way to stabilize the gamma-phase, especially since aluminum is known to be one of the elements that suppresses the formation of gamma-phase.

It is known that the magnetic characteristics of alnicotype alloys drop very rapidly with reduction of the aluminum content, and that at an aluminum content of 7.6% they attain magnetic values which are too low to be used. Moreover, the desired degree of stabilizing of gammaphase by a reduction in aluminum content is produced only with values lower than 7.4%. In consequence, a reduced aluminum content should be limited exclusively to a thin layer of the magnet body. The present invention permits this to be done.

As an excellent means for the partial reduction of aluminum from the composition of a magnet casting, and hence of the formation of a layer poor in aluminum, oxidation processes proved to be useful. However it was found very soon that it is not possible to obtain selective oxidation of only the aluminum if the temperatures used are too high. At temperatures of about l,250 C. all of the components of the alloy of the magnet become oxidized and a layer of oxidized material is formed that is not solidly joined with the mass of the magnet. As a conclusion of a long series of researches according to the invention, a method of oxidation treatment was developed that produces an oxidation of the aluminum only, while it does not involve any appreciable oxidation of the other components of the alloy.

Moreover, the course of the temperature relative to time is also used to produce the desired segregation of gamma-phase to the correct extent. This is illustrated in the following example.

Example 8 Magnets of standardized alloy, cast in normal man ner, were covered on all their surfaces except one end face with a protective coating as shown in FIG. 2. The magnets were then subjected to heat-treatment in a furnace containing normal atmosphere, without any excess oxygen and without any protective atmosphere. The cycle of heat-treatment is shown by FIG. 3 in the accompanying drawings. This produced a decrease in the aluminum content of the magnet alloy at the exposed faces of the magnet castings, sufiicient to stabilize the gammaphase precipitate produced during the early stages of the treatment, so that such precipitate created the internal strain conditions desired in the subsequent stage of heat-treatment at the recrystallization temperature of 1300 C.

The treatment resulted in the complete formation of a monocrystal in all of the samples.

In all the foregoing examples, it proved to be very convenient and favorable for the formation of monocrystals to cool the magnets, after the heat-treatment for gamma-phase precipitation, to temperatures lower than 900 (3., preferably to temperatures of 800 0, prior to the recrystallization treatment. The structure of the gamma-phase in the alpha-plus-gamma temperature range is with cubic reticle with centered faces, and lowering of the temperature to below the 900 limit ensures sufiicient cooling to carry the alloy into the alpha-phase temperature range, the reticle passes from the cubic with centered faces to become a cubic reticle with centered cubes. As is apparent, in passing from one structure to the other of cubic reticle, by means of the change of the reticle constants, a contribution is made to the increase of the internal stresses and to the increase of the energy at the boundaries of the granules.

Further, in carrying out the invention, it is found that there is an interdependence between the casting temperature and the certainty of realization of the monocrystal. This can be explained by the fact that as a general rule elevated casting temperatures produce coarser granules, while lower casting temperatures produce fine granules in the alnico-type alloys. The large granules possess by their nature a quantity of energy at the boundaries of the granules that is relatively small and, therefore, they do not display as great a tendency as the small granules toward the reduction of energy at the boundaries of the granules by means of the formation of monocrystals. Lower casting temperatures and the smaller grain sizes produced thereby are preferred, as is more fully shown by the following example.

Example 9 Casting Probability of Temperature, C.: monocrystals, percent 1,460 100 1,550 96 From these results it appears that the casting tempera ture of crude magnets should be as low as possible in practicing the present invention and preferably should not be higher than the melting temperature increased by C.

I claim as my invention: 1. The process of producing monocrystalline structure in a normally polycrystalline magnet casting of iron-nickel-aluminum type permanent-magnet alloy, which comprises enriching a thin layer of the magnet casting at one face thereof with a component which is a gammaphase precipitant and stabilizer, to form in such layer a composition in which gamma-phase precipitate will form and will remain stable at recrystallization temperature, subjecting the casting to substantially uniform heating at a temperature and for a time sufiicient to induce gamma-phase precipitation in said layer in an amount sufficient to produce critical strain in the layer,

subjecting the casting, with the gamma-phase precipitate present in said layer to heating at a recrystallization temperature for a time sufiicient to initiate monocrystal growth in said layer,

and continuing such recrystallization heating to cause said monocrystal growth to progress from said layer into the main body of the casting.

2. The process as defined in claim 1 in which said layer is enriched with a member of the class consisting of carbon, nitrogen, manganese, ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, gold, cobalt, and nickel.

3. The process as defined in claim 1 in which said layer is enriched with carbon.

4. The process as defined in claim 1 in which said layer is enriched with nitrogen.

5. The process as defined in claim 1 in which said layer is enriched with manganese.

6. The process as defined in claim 1 in which said layer is enriched with cobalt.

7. The process as defined in claim 1 in which said layer is enriched with nickel.

8. The process as defined in claim 1 in which said layer is enriched by casting the magnet alloy into contact with a deposit containing the component which is a gamma-phase precipitant and stabilizer.

9. The process as defined in claim 1 in which said layer is enriched by subjecting the casting to a surface absorption treatment which diffuses said component into said layer.

10. The process as defined in claim 9 in which the surface absorption treatment is confined to a selected surface portion of the casting.

11. The process as defined in claim 1 in which said layer is enriched by a surface absorption treatment at a temperature and for a time which both diffuses said component into said layer and also induces gamma-phase precipitation in said layer.

12. The process as defined in claim 1 in which the enriched layer is formed by carburizing at a gamma-phase precipitating temperature.

13. The process as defined in claim 1 in which the enriched layer is formed by nitriding at a gamma-phase precipitating temperature.

14. The process as defined in claim 1 in which the magnet alloy contains a gamma-phase precipitating component and sufficient aluminum to suppress gamma-phase precipitation, and said enriched layer is produced by removing aluminum therefrom to enrich the layer proportionally in said gamma-phase precipitating component.

15. The process as defined in claim 14 in which the aluminum is removed by oxidation at a low gamma-phase precipitation temperature and such oxidation is continued for a time to produce sufficient gamma-phase precipitate for the subsequent recrystallization step.

16. The process as defined in claim 14 in which the gamma-phase precipitating component contained in the magnet alloy is an element of the group consisting of co balt and nickel.

17. The process as defined in claim 1 in which the casting is cooled to below 900 C. after precipitation of gamma-phase in said layer and prior to said treatment at recrystallization temperature.

18. The process of producing magnets having substantial monocrystalline structure and of an alloy composition in which the formation of gamma-phase precipitate is suppressed, which comprises casting said alloy to form a magnet casting having said alloy and a normally polycrystalline structure throughout substantially the entire main body thereof, enriching a thin layer of the magnet casting at one face thereof with a gamma-phase precipitant to form in such layer a composition in which gamma-phase precipitate will form and will remain stable at recrystallization temperature, subjecting the casting to substantially uniform heating at a temperature and for a time sufiicient to induce gamma-phase precipitation in said layer in an amount sufiicient to produce critical strain in the layer,

subjecting the casting, with such precipitate and strain in said layer, to heating at a recrystallization temperature for a time suflicient to cause monocrystal growth to start in said layer,

and continuing said recrystallization heating to cause said monocrystal growth to progress substantially into the main body of the casting.

19. The process as defined in claim 18 in which said alloy composition in an Alnico alloy containing 10 to 30 percent nickel, 6 to 14 percent aluminum, to 42 percent cobalt, up to 8 percent copper, up to 10 percent titanium, with the balance substantially all iron.

20. The process as defined in claim 19, in which said layer is enriched with a gamma-phase precipitant of the group consisting of carbon, nitrogen, manganese, ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, gold, cobalt, and nickel.

21. The process as defined in claim 18 in which said casting step is carried out at a temperature not substantially more than 100 C. above the melting temperature of the alloy.

22. The process as defined in claim 18 in which enrichment of said layer is produced by casting the alloy into contact with a deposit containing the gamma-phase precipitant.

23. The process as defined in claim 22 in which the alloy is cast into contact with a deposit containing one of the metals of the group consisting of nickel, cobalt, and manganese.

24. A permanent magnet casting having a main body portion composed of an alloy consisting of 10 to 30 percent nickel, 6 to 14 percent aluminum, 5 to 42 percent cobalt, up to 8 percent copper, up to 10 percent titanium, and with the balance substantially all iron, said alloy having a composition which suppresses the formation of gamma-phase precipitate and in which gamma-phase precipitate is substantially unstable at recrystallization temperature,

said casting having a thin layer at a limited portion of its surface in which the composition of the casting is enriched with a gamma-phase precipitant in an amount sufficient to cause gamma-phase precipitate to form and to remain stable at recrystallization temperature, and

said casting having a monocrystalline structure extending through said layer and substantially into the main body portion of the casting.

References Cited by the Examiner UNITED STATES PATENTS 1,738,307 12/1929 McKeehan 148--12l 2,032,912 3/1936 Corson 148100 2,617,723 11/1952 Studders et al 148-101 2,791,517 5/1957 Becker et a1. -123 2,943,007 6/1960 Walker et al. l48-1.6 2,970,075 1/ 1961 Grenoble 14831.55 3,085,036 4/1963 Steinort 14831.57

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,219,495 November 23, 1965 Eberhard Steinort It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 43, for "diection" read direction column 4, line 7, for "30 C." read 930 C. line 25, for "gerat" read great column 6, line 2, for "C, Mn" read C, N, Mn column 7, line 20, for "1,3000 C." read Signed and sealed this 4th day of October 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. THE PROCESS OF PRODUCING MONOCRYSTALLINE STRUCTURE IN A NORMALLY POLYCRYSTALLINE MAGNET CASTING OF IRON-NICKEL-ALUMINUM TYPE PERMANENT-MAGNET ALLOY, WHICH COMPRISES ENRICHING A THIN LAYER OF THE MAGNET CASTING AT ONE FACE THEREOF WITH A COMPONENT WHICH IS A GAMMAPHASE PRECIPITANT AND STABILIZER, TO FORM IN SUCH LAYER A COMPOSITION IN WHICH GAMMA-PHASE PRECIPITATE WILL FORM AND WILL REMAIN STABLE AT RECRYSTALLIZATION TEMPERATURE, SUBJECTING THE CASTING TO SUBSTANTIALLY UNIFORM HEATING AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO INDUCE GAMMA-PHASE JPRECIPITATION IN SAID LAYER IN AN AMOUNT SUFFICIENT TO PRODUCE CRITICAL STRAIN IN THE LAYER, SUBJECTING THE CASTING, WITH THE GAMMA-PHASE PRECIPITATE PRESENT IN SAID LAYER TO HEATING AT A RECRYSTALLIZATION TEMPERATURE FOR A TIME SUFFICIENT TO INITIATE MONOCRYSTAL GROWTH IN SAID LAYER, AND CONTINUING SUCH RECRYSTALLIZATION HEATINT TO CAUSE SAID MONOCRYSTAL GROWTH TO PROGRESS FROM SAID LAYER INTO THE MAIN BODY OF THE CASTING.

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