PERMANENT MAGNET MANUFACTURE FROM VERY LOW COERCIVITY CRYSTALLINE RARE EARTH-TRANSITION METAL-BORON ALLOY

Abstract: A method is disclosed of hot-working an alloy, comprising a magnetically-soft, cast, crystalline material containing grains of RE₂TM₁₄B, and an RE metal-rich intergranular second phase, into anisotropic permanent magnet bodies.

Description: This invention relates to a method of making high coercivity, high energy product magnets from fully crystalline alloys containing one or more light rare earth (RE) elements, one or more transition metals (TM) and boron (B), which alloys are not permanently magnetic as cast. The invention relates particularly to the manufacture of strong permanent magnets by hot-working large particles or billets of non-permanently magnetic, crystalline, cast alloys consisting predominantly of a RE₂TM₁₄B phase where the TM includes iron. Background The invention of high coercivity, high energy product light rare earth-iron (RE-Fe) based permanent magnets is the subject matter of US-A-4,496,395, European Patent Application 0 108 474 (General Motors Corporation) and European Patent Application 0 144 112 (General Motors Corporation). The preferred magnet compositions are based on the rare earth elements neodymium (Nd) or praseodymium (Pr) or both, the transition metal iron (Fe) or mixtures of iron and cobalt (Co), and boron (B). The preferred compositions contain a large proportion of a RE₂TM₁₄B phase where TM is one or more transition metal elements including iron. An article by Stadelmaier et al in Materials Letters, Vol 2, No.5A, June 1984, pages 411-415 relating to the metallurgy of the iron-neodymium-boron permanent magnet system reveals that the iron-neodymium-boron alloys comprise a tetragonal magnetic phase of Fe₁₄ Nd₂B in the form of grains separated by a neodymium-rich phase. The preferred method of processing such alloys, as disclosed in the above noted patent and patent applications, involves rapidly solidifying molten alloy to achieve a substantially amorphous to very finely crystalline microstructure that has isotropic, permanently magnetic properties. It is believed that the crystallographic ordering range in the highest energy product alloys is commensurate with optimum single magnetic domain size. Overquenched alloys can be annealed at suitable temperatures to cause grain growth and thereby induce magnetic coercivity. The maximum magnetic energy product to date for an as-quenched Nd-Fe-B-based alloy is about 119,400 tesla.amps/metre (15 megaGaussOersted). European Patent Application No. 0 133 758 (General Motors Corporation) relates to a method of introducing anisotropic magnetic properties in rapidly solidified RE-Fe-B-based alloys by hot-working. Alloys with overquenched,substantially amorphous microstructures are worked at elevated temperatures to cause grain growth and crystallite orientation which result in substantially higher energy products than in the best as-rapidly-solidified alloys. The maximum energy product to date for hot-worked, melt-spun Nd-Fe-B alloy is about 358,200 T.A/m (45 MGOe), although energy products as high as 509,440 T.A/m (64 mGOe) are theoretically possible. The inventor, Mr R.W. Lee, of the invention disclosed in and claimed in European Patent Application No.0 133 758, published a paper entitled "Hot-pressed neodymium-iron-boron magnets" in Applied Physics Letters 46 (1985) No.8, pages 790-791, in which he discussed his method for producing neodymium-iron-boron permanent magnet materials from melt-spun ribbons of the alloy, and reveals a two-stage hot-press procedure for such melt-spun ribbons which produces an anisotropic magnet of 100% density and significant magnetic alignment. It is also known that high coercivities and energy products can be induced in light RE-Fe-B-based alloys by the conventional orient-press-sinter (OPS) method which has been used for more than twenty years to make samarium cobalt and other rare earth cobalt permanent magnets. This method has severe commercial limitations because it entails many processing steps which must be carried out in a controlled, non-oxidizing atmosphere. High coercivities and energy products (greater than 79,600 T.A/m (10 mGOe) are obtained only by grinding crystalline alloys into very fine (less than 5 micrometres) powder, magnetically aligning and compacting the powder in a strong magnetic field, and sintering the compacts. Magnets made by the OPS process are brittle and generally require a substantial amount of finish-grinding to achieve any but the simplest small shapes. It is also known that by substituting copper for some of the cobalt in RECo₅ and RE₂Co₁₇ compositions, coercivity can be induced in as-cast alloys by suitable heat treatment between 400 and 500°C. The coercivity is believed to be caused by a domain wall pinning phenomenon. Analogous precipitation hardening of cast RE-Fe-B-based compositions has not been achieved to date and the presence of copper in more than trace amounts has been found to drastically reduce the energy product of RE-Fe-B-based alloys. It is also well known that certain as-cast Cr-Co-Fe compositions which do not contain rare earth elements can be uniaxially worked at cold temperatures to achieve energy products up to about 39,800 T.A/m (5 mGOe). While the rapid solidification and hot-working of RE-Fe-B-based compositions appears to be a more practical and less expensive process than OPS, it would also be desirable to have a means of making high coercivity, high energy product RE-Fe-B-based permanent magnets without initial rapid solidification. By rapid solidificiation herein is meant solidification of an alloy initially at a temperature above its melting point at a cooling rate of about l0⁵ °C per minute or greater. That is, it would be advantageous to make permanent RE-Fe-B magnets from conventionally cast, fully crystalline alloys but without the fine grinding and aligning steps of the OPS process. Conventional casting results in cooling rates substantially lower than l0⁵ °C per minute. However, before this invention, no such method was known or had been suggested. Brief Summary In accordance with a preferred practice of the invention, suitable proportions of one or more rare earth elements, one or more transition metals including iron, and boron are melted together in a crucible. The ratio of constituents should be chosen to result in a cast material consisting predominantly of RE₂TM₁₄B grains with a RE-containing minor phase(s) present as a layer at the grain boundaries. It is particularly preferred that the RE₂TM₁₄B grains be thin, flat platelets. The preferred rare earth elements are Nd and Pr, and the preferred transition metal (TM) is iron or mixtures of iron and cobalt. Nd 16.5 Fe 76.3 B 7.2 is the nominal overall composition of one such preferred material. The minor second phase(s) of this alloy is relatively neodymium and boron rich. The molten metal is cast onto a chill block or into a chill mould of highly heat-conductive material. The chill rate is orders of magnitude slower than the chill rate of melt-spinning or other rapid solidification processes. For a 6-12 mm thick cast billet, cooling on a relatively thick chill block results in platelet-shaped crystals with a smallest dimension of at least 5 micrometres and generally not greater than 50 micrometres. The crystallographic c-axis of each platelet is perpendicular to its major flat surfaces. In the smallest dimension, these single crystal platelets are larger than the maximum size of ground particles useful in the OPS process. The platelets, in turn, align flat-surface to flat-surface in lamellar regions comprised of several (as few as 4 or as many as 30 or more) in a group, all oriented with smallest dimension (thickness of platelets) perpendicular to the chill direction. These lamellar regions, hereafter called "packets", have a long range order from about 50 to several hundred micrometres in the smallest dimension to the overall thickness of the chill-cast ingot in the largest dimension. The orientation of the platelets is caused by directional cooling from the chill surface on which they are cast. The crystallographic c-axes of the platelets lie parallel to the chill surface. The preferred direction for magnetic alignment of RE₂TM₁₄B crystal is along the c-axis. The coercivity of this cast material is very low -- less than 39,800 A/m (500 Oe), if detectable at all. In one preferred method, the cast alloy may be crudely broken up into particles averaging between about 50 and 600 micrometres average dimension. The billet fractures preferentially at the boundaries between packets so that each particle is primarily made up of a single packet. Thus, there is substantially uniform crystallographic orientation in each particle. The particles may be precompacted to about 80 percent density in a conventional cold press or may be poured into a compacting vessel or can without precompaction. The can is preferably made of an easily deformable metal such as dead soft iron or copper. Once the particles are in the can, it is sealed. The can and contents are then heated to a temperature above the melting point of the second phase. Typically, temperatures above 650°C and below 800°C are suitable. In the case of the treatment of the Nd₁₆.₅Fe₇₆.₃B₇.₂ composition, a temperature of 730° + 30°C is preferred. Once the material is at temperature, it is deformed to cause suitable hot-working which introduces coercivity and increased magnetic alignment in the material. The hot-working causes the initially magnetically-soft alloy to develop good permanent magnetic properties including coercivities of several thousand Oersteds and remanence of several thousand Gauss and energy products over 79,600 T.A/m (10 MGOe). In another preferred practice, molten RE-Fe-B metal is cast to result in directionally-solidified alloy which may be cut into suitably sized and shaped pieces for further processing without grinding or containing the sample in a can. Directionally-solidified samples are preferably hot-worked in a non-oxidizing atmosphere at a temperature of about 730 ± 30°C in a direction parallel to the crystallographic c-axis of the alloy; i.e., in a direction transverse to the direction of cooling. Another processing method involves cutting an alloy sample from a cast billet having partially oriented platelets (i.e., not fully directionally-solidified) and hot-working it as described for directionally-solidified alloy. The practice and scope of the subject invention will be better understood in view of the detailed description which follows, and the accompanying drawings, in which: Figure l is an exploded view of a crystal of Nd₂Fe₁₄B alloy. Figure 2 is a diagrammatic flow chart for a preferred method of hot-working fully crystalline RE-Fe-B alloy to obtain a permanent magnet. Figure 3 is an optical micrograph of Nd l6.5 Fe 76.3 B 7.2 alloy cast into a steel chill mould. Figures 4-7 are second quadrant demagnetization curves for crystalline Nd-Fe-B alloy samples that were canned, heated and then hot-rolled. Figure 4 shows the effect of hot-working temperature; Figure 5 shows the effect of thickness reduction; Figure 6 shows the effect of different forms of precursor alloy; and Figure 7 shows the effect of hot-working a sample in two different mutually perpendicular directions. Figure 8 is a diagrammatic flow chart for hot-working fully crystalline, directionally-solidified or partially directionally-solidified alloy to obtain a permanent magnet. Figure 9 shows second quadrant demagnetization curves for crystalline Nd-Fe-B alloy billet samples hot-worked perpendicular to the chill direction of the billet in a punch and die in a vacuum press. Detailed Description In accordance with a preferred practice of the invention, rare earth-transition metal-boron (RE-TM-B) alloys belonging to the family of compositions particularly disclosed and claimed in European Patent Application Nos.0 l08 474 and 0 l44 ll2 (referenced above and incorporated herein by reference, are particularly useful in the practice of the present invention. The transition metal component is iron or iron and one or more of cobalt, nickel (Ni), chromium (Cr) or manganese (Mn). Cobalt is interchangeable with the iron up to about 40 atomic percent to maintain high energy product alloys. Chromium, manganese and nickel are interchangeable in lower amounts, preferably less than l0 atomic percent. Small amounts of zirconium (Zr), titanium (Ti), silicon (Si) and carbon (C) may be added to tailor the shape of the second quadrant demangetization curves. The composition preferably comprises at least 50 atomic percent iron based on the total alloy composition. The composition also comprises at least l0 and preferably l3 to 30 atomic percent rare earth component. Neodymium and/or praseodymium are preferred rare earths and may be used interchangeably. Smaller amounts of other rare earth elements, such as samarium (Sm), lanthanum (La) and cerium (Ce), may be mixed with neodymium and praseodymium without substantial loss of the desirable magnetic properties. Small amounts of heavy rare earth elements such as terbium (Tb) and dysprosium (Dy) may be added to increase coercivity. Preferably rare earth elements other than Nd and/or Pr make up no more than about 40 atomic percent of the rare earth component. Preferably, enough rare earth elements should be contained in the alloy to provide a rare earth component in the secondary phase in a proportion greater than it is present in the predominant RE₂TM₁₄B phase. The composition preferably contains at least 0.5 and preferably 6 to l0 atomic percent boron. The predominant magnetic phase of the subject alloys is RE₂TM₁₄B where the TM is predominantly Fe which phase has a tetragonal crystal structure with room temperature lattice constants of a = 0.878 nanometre and c = l.2l8 nanometres. RE₂Fe₁₄B crystals have been indexed to the P4₂/mnm space group of the International Tables for X-Ray Crystallography , N Henry et al., ed., Kynock, Birmingham, England (l952). An exploded view of a Nd₂Fe₁₄B crystal showing the crystallographic a and c axes is shown in Figure l. The following table summarizes the symmetry sites and positions for a unit cell Nd₈Fe₅₆B₄ of the Nd₂Fe₁₄B phase shown in Figure l. The formulas RE₂TM₁₄B and RE₂Fe₁₄B as used herein include all compositions having the tetragonal crystal structure above including small amounts of any other elements such as Si, C, calcium (Ca), strontium (Sr), magnesium (Mg), so long as these other elements do not destroy the RE₂TM₁₄B crystal phase. About l0 volume percent or less of the alloy preferably consists of one or more minor rare earth-rich phases. Fe₄B₄Nd has been tentatively identified as one such phase. Another phase which is probably close in composition to the rare earth-iron eutectic is also believed to be present. At least one of the secondary phases has a lower melting temperature than the predominant phase. For purposes of illustration, the invention will be described using compositions of approximately the following atomic proportions: Nd 16.5 Fe 76.3 B 7.2 However, it is to be understood that the method of the invention is applicable to other compositions as described above. In the past, achievement of high energy (greater than 79,600 T.A/m (10 MGOe)) product RE-Fe-B magnets depended critically on the submicron crystallite size of rapidly solidified alloy or the less than five micrometre particle size of ground alloy powder in the OPS method. European Patent Application No.0 133 758 relates to hot-working substantially amorphous to very finely crystalline alloys to arrive at alloys with fine grain microstructures where the grains have a maximum dimension of 50 to 500 nanometres (0.05 to 0.5 micrometres). The OPS process generally starts with fully crystalline alloy, but requires that the alloy be ground to a powder size no greater than about 5 micrometres. Such fine powders must be processed away from any oxygen to prevent degradation of magnetic properties and spontaneous combustion of the powders. The powder must then be magnetically aligned in an externally applied field, pressed and sintered. The present invention starts with fully crystalline alloy with primary phase grains of elongated platelet shape. The shortest average dimension of the platelet is at least 5 micrometres -- comparable to the maximum OPS ground powder size. The crystallographic c-axes of the platelets is along the shortest dimension; i.e., transverse to the platelet faces. In chill-cast alloys, the platelets form as lamellar regions with platelet faces parallel to one another. Thus the c-axes of the individual platelets are also parallel to one another; and an entire lamellar region, or packet, is characterized by a single c-axis direction which is parallel to the chill surface and transverse to the direction of cooling. The terms "platelet", "grain", and "cystal" may be used interchangeably herein. In accordance with this invention the crystalline alloy may initially be coarsely attrited to break the aggregate platelet packets apart. This results in relatively large particles averaging about 200 to 600 micrometres across with very few fine particles under 50 micrometres. These particles do not combust spontaneously in air. Alternatively, chunks of alloy can simply be cut from a chilled billet and hot-pressed directly. Figure 2 is a schematic representation for a preferred embodiment of the practice of this invention. The first step shown in Figure 2(a) entails casting a molten RE-Fe-B alloy 2 into chill mould 4 to form a layer of alloy 6 that is preferably at least l mm thick. This causes large platelets 8 to form in the cooled alloy which are parallel to one another over regions of a few platelets up to several hundred platelets. Boundaries ll between platelets 8 are shown larger than scale for purposes of illustration. The crystllographic c-axes are parallel to primary chill surface l0 as shown by the arrows. Platelets 8 consist essentially of a crystalline RE₂TM₁₄B phase where the RE is predominantly Nd and/or Pr and the TM is predominantly Fe. An average platelet formed by casting molten alloy into a room temperature steel mould would be about l0-30 micrometres by about 50-l00 micrometres by the thickness of the alloy layer. Figure 3 is an optical micrograph looking down at a polished top surface (i.e., opposite the face of the ingot against the primary chill suface of the steel chill mould) of a Nd l6.5 Fe 76.3 B 7.2 alloy ingot about 0.64 cm thick. The average smallest platelet dimension is about l0 micrometres across. In contrast, a melt-spun, rapidly solidified alloy of like composition has grains which are less than 50 nanometres across. Such tiny grains cannot be seen using optical micrographic techniques. The small crystallite size of melt-spun alloy is the result of cooling at a rate generally at least l000 times faster than the rate at which an alloy cools in a steel chill mould. It is also apparent from the micrograph that the platelets form packets of similarly aligned platelets. The regions may extend l00 micrometres or more across (arrow 32) and are generally many platelet layers thick as indicated by arrow 30. A much smaller amount of a second phase indicated by the dark regions on the micrograph forms around the RE₂TM₁₄B platelets. Electron microprobe analysis indicates that this phase is RE-rich compared to the predominant RE₂TM₁₄B phase and has a composition near the RE-Fe eutectic. The minor phase(s) has (have) a lower melting point than the RE₂TM₁₄B phase. Figure 2(b) shows particles l2 formed by roughly grinding chill-cast alloy 6. The alloy 6 fractures preferentially at the boundaries of the packets or at the boundaries ll between platelets 8. Any suitable attriting equipment may be used to rough-grind the billet although care should be taken to avoid excessive moisture or heat which can cause oxidation of the particles. Figure 2(b) shows fracture of alloy 6 between crusher rollers l3. The preferred particle size range is greater than 50 micrometres and less than 600 micrometres average diameter. It has been found that grinding the alloy in a jaw and roller crusher (not shown) results in fairly uniformly-sized particles over a normally distributed particle size range. The crushing process produced few fine particles smaller than 50 micrometres. Figure 2(c) shows dispensing particles l2 in a soft iron can l4 or other suitable airtight container preparatory to hot-working. Particles l2 may be compacted to about 80 percent density in an ordinary cold press either before or after insertion in the can, if desired, but they may also be loosely packed in can l4. Can l4 is preferably made of a soft, deformable metal which does not degrade the magnetic properties of the alloy. Soft steel and copper have been found to be suitable although other malleable materials may be used which are not adversely affected at elevated hot-working temperatures. After particles l2 are in can l4, it is sealed with a cover l6 or simply pinched closed or welded shut to prevent any significant oxidation of the contained alloy as it is hot-worked. Unless otherwise noted, the cans used for the examples set forth below were soft steel blocks about l0 cm long, 5.7 cm wide and l.9 mm thick. A slot about 7.5 cm deep, 3.2 cm wide and l.3 cm thick was machined into the block. A precompact (about 80 percent dense) of roughly ground particles about 6.4 cm long, 3.l cm wide and l.25 cm thick was placed in the slot and a plug was welded in place to close the slot. It did not seem to matter whether or not the slot was evacuated before the plug was sealed in place. It is believed that much thinner-walled cans can be used. For example, powder could be continuously disposed in thin copper tubes which could be heated quickly, rolled to reduce thickness and cut, hot stamped or otherwise suitably fabricated into magnet shapes. The hot-working step of Figure 2(d) entails elevating the temperature of the workpiece to above about 650°C, preferably about 730°C + 30°C in the case of the Nd 16.5 Fe 76.3 B 7.2 alloy. This temperature is above the melting point of the minor phase(s) that form as a layer between the RE₂TM₁₄B platelets. Said temperature will vary depending upon composition of the alloy. At this temperature, suitable hot-working produces magnetic coercivity in the workpiece. Referring to Figure 2(d), the preferred method of hot-working particles l2 in can l4 is to preheat them to about 740°C and pass them through calender rollers l8 to effect a reduction in thickness of the material in the can l4 by about 30 percent to 70 percent after correcting for full densification of the material. The reduction may take place in one pass or several although the integrity of densified compact 20 which results seems to be better with a single pass. Rollers l8 may or may not be heated although it may be necessary to heat the workpiece between passes if they are not. During this rolling, the large, flat platelets of RE₂TM₁₄B in the hot material are caused to move so that their c-axes become aligned perpendicular to the rolling plane. This introduces magnetic alignment perpendicular to the rolling plane. However, the critical aspect of such hot-working is that it introduces coercivity into the material. As shown in Figure 2(e), after the hot-worked alloy compact 20 is cooled below its Curie temperature, it can be magnetized in a suitable magnetizer 22. If the can is magnetic, as in the case of iron or steel, it should be removed from the compact before the magnetizing step. Otherwise, the can may be left in place. Neither the alignment nor coercivity mechanism(s) produced by the subject method are completely understood at this time, but there are two physical changes which occur during hot-working which are believed to be contributory: (l) the RE₂TM₁₄B platelets are fragmented which reduces particle size, increases surface area, and specifically exposes fresh surfaces not bounded by the Nd-rich phases which separate the platelets, and (2) the Nd-rich, low-melting, secondary phase eutectic mixtures flow at the hot-press temperatures and substantially coat or separate these individual fragmented particles of the RE₂TM₁₄B phase. It is believed that one or more of the physical changes induced by hot-working produces a favourable condition for domain wall trapping. In the large, as-cast, RE₂TM₁₄B platelets there is ample volume for multiple magnetic domains which are easily reversed in a reverse magnetic field. The fact that the cast alloy has soft magnetic properties supports this. The hot-working method of the invention probably results in domain wall trapping, most likely at the RE₂TM₁₄B/minor phase boundaries. Domain wall trapping promotes resistance to demagnetization and much higher coercivity. EXAMPLE l About l500 grams of an alloy with the nominal composition (in atomic percent) Nd 16.5 Fe 76.3 B 7.2 was heated in a vacuum furnace to about l50°C above its melting temperature. The alloy was cast into a room temperature cylindrical solid steel mould (chill block) having an inside diameter of about l5.2 cm and a thickness at the bottom of 2.54 cm and a 6 mm thick lip about 2 cm high around the sides. The resultant ingot was about l.27 cm thick. Because most of the heat was withdrawn through the thick bottom of the mould, the platelets formed in clusters or packets with their crystallographic c-axes substantially parallel to the surface of the chill plate as shown, for example, at Figure 2(a). Each platelet had a generally ovoid, flat plate shape. The smallest average platelet dimension (i.e., its thickness) was about 30 micrometres and its longest about l.27 cm, the thickness of the cast ingot. The ingot was crushed in a jaw and roller crusher to a maximum particle size of about 600 micrometres, an average size of about 200 micrometres with a normal distribution of smaller sized particles. There were very few fine particles smaller than 50 micrometres. The crushed particles did not combust spontaneously in air and seemed to be fairly resistant to oxidation. Each particle contained a plurality of crystals. Each crystal contained a plurality of magnetic domains. No more than a few Oersteds coercivity could be induced in the particles at this stage of the process by the application of a magnetic field. That is, the particles had magnetically-soft properties. The particles were pre-compacted to 80 percent density and inserted in a slot in a soft steel can as described above. The can and compact were heated in a resistance-heated furnace to a temperature of about 740°C. The can was removed from the furnace and immediately passed through a pair of 22.2 cm diameter calender rollers originally at room temperature. The can decreased in temperature by 30-40°C during each roll and was reheated to about 740°C for subsequent rolling steps. The total time at temperatures above about 700°C for each sample was approximately 20 minutes. After rolling of each sample was completed, the can was cut away and small samples were taken from the resultant compacted block of fully densified magnetic material. Some stress cracking appeared at the edges of samples subjected to multiple rolling steps. However, it is believed the cracks can be eliminated by using better equipment and making minor processing adjustments. An unworked "as-cast" control and the hot-worked samples were magnetized in a 1512.4 kA/m (19 kOe) magnetic field and the second quadrant demagnetization curves were measured in a Princeton Applied Research vibrating sample magnetometer at room temperature. Curve 46 in Figure 4 is the demagnetization curve of the as-cast material. For each hot-worked sample, the preferred direction of magnetization was transverse to the rolling plane (i.e., parallel to the direction in which the thickness of the canned sample was reduced). Figure 4 shows the effect of can temperature prior to each rolling step on the magnetic properties of hot-worked alloy. The thickness (after correction for full densification) of the alloy in the can was reduced from about 1.25 cm to 0.67 cm by rolling the can through the calender rollers three times, effecting about 1/3 of the total thickness reduction on each pass. The sample rolled after being heated to a temperature of 730-740°C (curve 40) had the highest remanence, coercivity and energy product. Rolling at 7l0°C (curve 42) and 770°C (curve 44) resulted in the creation of permanent magnetism in the samples but at lower values. Thus, hot-working may be carried out over a substantial elevated temperature range but about 740°C appears to be optimum when working with cold rollers. Figure 5 shows the effect of the degree of deformation on magnetic properties for samples heated to about 740°C before each pass. In the examples, deformation is shown as a percent reduction of the original thickness of the alloy sample, after correcting for densification. Even a small l5 percent reduction on a single pass through the rollers results in the creation of permanent magnetism in the sample. A 30 percent reduction on a single pass improves the magnetic remanence and coercivity. The highest remanence and squarest curve were obtained for a sample whose thickness was reduced 50 percent in three passes. A 70 percent reduction in three passes created higher coercivity but lower remanence. Thus, it appears that the shape of the second quadrant hysteresis curve can be tailored to some degree by making changes in the hot-working parameters. Figure 6 shows the effect of the condition of the precursor alloy on demagnetization of l.25 cm thick alloy samples. Each corrected sample thickness was reduced by 50 percent in three passes when heated to about 740°C before each pass. Curve 50 corresponds to a sample can initially containing particles having a minimum platelet dimension of about 30 micrometres comprising particles (consisting of a plurality of platelets) precompacted to 80 percent density. Curve 52 represents a sample processed in like manner using an ingot having a finer grain microstructure where the average platelet thickness was about l0 micrometres rather than 30. This resulted in a higher coercivity than the coarser grain sample with a slightly reduced remanence. Curve 54 shows the effect of reducing the full density thickness of the alloy by 50 percent in three passes where the particles (30 micrometre smallest platelet dimension) were simply poured into the can and the plug tapped into place. The initial powder density was about 60 percent of theoretical density. Curve 56 shows the effect of reducing the thickness of a block of alloy 3.2 cm wide by 2.5 cm long by l.25 cm thick by 50 percent (30 micrometre smallest platelet dimension). The block of ingot was encapsulated in a steel can and oriented so that the c-axes of the platelets were generally perpendicular to the rolling direction. Figure 7, Curve 60, shows that making the first of three hot-rolling passes in a direction perpendicular to the other two resulted in slightly better magnetic properties for a 50 percent thickness reduction than in a sample where all three passes are made in the same direction (Curve 62). All passes were made after heating or reheating the sample to about 740°C. Figure 8(a) to 8(d) show a diagrammatic flow chart for another preferred method of practicing the present invention. Molten alloy is either cast into a conventional chill mould as described above and shown in Figure 2(a) or is directionally-solidified to cause parallel alignment of all grains in the alloy. Figure 8(a) shows one well known method for rapidly solidifying metals which entails pouring molten alloy 2′ into a mould 4′ which is chilled in the primary cooling direction through mould bottom l0′ and heated in the transverse direction through mould side wall ll′ by means of cooling coils l5′. In the solid alloys thus produced, the c-axis of directionally-solidified alloy 6′ would be substantially parallel to the chill surface l0′. Conventionally cast alloy has a high proportion of grains perpendicular to the chill direction but there are also some grain regions with other crystal orientations. Referring to Figure 8(b), the second step entails positioning directionally-solidified segment(s) 85 of alloy 6′ in hot-press 80 so that crystallographic c-axes are oriented in the direction of pressing as indicated. A suitable hot-working apparatus consists of a conventional hot-press having a steel die 82. Heating coils 84 surround die 82. The segment(s) 85 of several directionally-soldified platelets of alloy 6′ are shown resting on the top 87 of bottom punch 88. The crystallographic c-axes of the segments are parallel to the direction of pressing as indicated. Referring to Figure 8(c), heating coils 84 are activated until the RE-TM-B alloy softens substantially at a temperature of about 700°C. Top punch 86 and bottom punch 88 are then forced together. Alloy segments 85 fracture and flow in a direction generally perpendicular to the crystallographic c-axes of the segments indicated by the arrow. Simply heating a large-grained crystalline RE-TM-B sample to an elevated temperture does not introduce coercivity. However, hot-working results in the creation of significant coercivity and remanence with the preferred direction of magnetization transverse to the direction of sample flow. In the preferred practice of the invention, the cast alloy is directionally-chilled so as to produce platelet-like grains of RE₂TM₁₄B. Directional-chilling further tends to produce regions in which the platelets 8′ are aligned in the cast block 6′. It is much preferred to press (during hot working) in a direction generally parallel to the c-axes of the platelets because pressure in this direction is most effective in introducing coercivity into the magnet. When pressure is applied in the direction perpendicular to the c-axes of the platelets, a much smaller introdution of coercivity is observed. Coercivity can be induced in overquenched rapidly solidified (e.g., melt-spun) alloy by heating alone. Therefore, the mechanism by which coercivity is introduced into fully crystalline RE₂TM₁₄B alloy by the method of the present invention is different and is related to hot-working. Referring to Figure 8(d), hot-pressed alloy 6' is magnetized in suitable equipment shown graphically as 22' after it cools. EXAMPLE 2 A sample about 6 mm on a side and having the nominal composition Nd 16.5 FE 76.3 B 7.2 was cut from a billet cast as described in Example 1 above but only about 6 mm thick and the chill mould was made of copper. Most (but not all) of the grains were aligned with their crystallographic c-axes perpendicular to the chill direction (parallel to the primary chill surface). The sample was positioned in a hot-press so that its c-axes were preferentially oriented parallel to the direction of pressing. A vacuum was established in the press and heating coils were activated to heat the sample to about 725°C. A pressure of about 103 megaPascals (15,000 psi) was exerted on the sample between carbide punches. The thickness of the sample was reduced by about 50 percent and the sample flowed in a direction transverse to the press direction. The sample was at a temperature above about 700°C for a total time of about 10 minutes. The sample was magnetized in a pulsed 7960 kA/m (100 kOe) field. It was magnetized first in the direction parallel to the c-axes, which also was parallel to the direction of pressing. The magnetic properties of the hot-pressed sample were measured on the Princeton Applied Research vibrating sample magnetometer. Demagnetization curves are shown in Figure 9. Referring to Figure 9, the sample was found to have a maximum coercivity of about 477.6 kA/m (6 kOe), a remanence of about 0.96 tesla (9.6 kGauss), and an energy product of about 123,380 T.A/m (15.5 MGOe) (curve 90). The sample was then magnetized in the same field but in a direction perpendicular to the c-axes and to the direction of pressing. Curve 92 shows that, when so magnetized, the sample had much lower coercivity and remanence. The hot-working introduced considerable magnetic anisotropy into the sample as well as coercivity. The sample had less than 7960 A/m (100 Oe) coercivity before it was hot-worked. In conclusion, a novel method of hot-working fully crystalline RE₂TM₁₄B alloys to make permanent magnets has been discovered. The method is much easier to practice and less costly than prior art methods. Hot-working randomly-oriented, roughly-ground, chill-cast particles results in alignment of at least some of the crystallographic c-axes in the direction of pressing, transverse to the direction of flow. Directionally-solidified alloy can be intentionally worked parallel to the crystallographic c-axis of the RE₂TM₁₄B crystals to ensure desired alignment. While heat treating chill-cast crystalline ingot induces no magnetic coercivity, simultaneous application of heat and pressure develops develops significant coercivity. That is, hot-working transforms the crystalline alloy from being magnetically-soft to magnetically-hard. Thus it is possible, by using the method of the present invention, to produce magnetically-hard alloy products, using cast ingots of alloy as the starting material rather than melt-spun alloy or finely-ground alloy.

Claims: A method of making an anisotropic permanent magnet which comprises forming an alloy containing rare earth elements, iron and boron which comprises a crystalline material consisting essentially of grains of RE₂TM₁₄B where RE is one or more rare earth elements, at least 60 percent of which is neodymium and/or praseodymium, TM is a transition metal, at least 60 percent of which is iron, and B is the element boron, and then hot-working the alloy at an elevated temperature and pressure to produce a final body which is magnetically anisotropic with a magnetic coercivity in excess of 79,600 A/m (1000 Oe), and exhibits permanent magnetic characteristics, characterised in that the method comprises chill-casting a molten metal mixture (2,2') containing said rare earth elements, iron and boron in suitable proportions to form a solidified, magnetically-soft alloy (6,6') initially having a magnetic coercivity of no more than 39,800 A/m (500 Oe), in which said grains of RE₂TM₁₄B that form the predominant phase present in the alloy (6,6') each have a crystallographic c-axis and are aggregated in regions where such axes are substantially parallel to one another, said grains are separated by an intergranular layer of an alloy phase which is richer in rare earth elements than are the grains of RE₂TM₁₄B, and said alloy phase is a minor second phase which has a lower melting point than the RE₂TM₁₄B phase; said chill-cast alloy (6,6') is heated to an elevated temperature above the melting point of the second phase, and then said alloy is mechanically worked and deformed so that, upon cooling the alloy, the resulting body (20,85) is magnetically anisotropic and has said permanent magnet characteristics. A method of making an anisotropic permanent magnet according to claim 1, characterised in that the predominant phase produced by the chill-casting of the molten metal mixture (2) consists essentially of thin, flat platelets (8) of RE₂TM₁₄B, said platelets each having said crystallographic c-axis lying perpendicular to the flat faces thereof, and being aggregated in regions where the respective crystallographic c-axes are substantially parallel to one another; and the method includes crushing said magnetically-soft alloy (6) to form coarse particles (12) which are greater than 50 micrometres in size, enclosing said particles (12) in a malleable metal container (14,16), heating said container (14,16) and alloy (6) to said elevated temperature to melt the second phase, and then rolling said container (14,16) to deform it to such an extent that the particles (12) therein are compacted to near 100 percent density. A method of making an anisotropic permanent magnet according to claim 1, characterised in that the method comprises directionally-solidifying the molten metal mixture (2') to form said solidified, low-coercivity alloy, the predominant phase produced by the directional chill-casting of the molten metal mixture (2') consists essentially of thin, flat platelets (8') of RE₂TM₁₄B, said platelets (8') each having said crystallographic c-axis lying perpendicular to the flat faces thereof, and being aggregated in regions where the respective crystallographic c-axes are substantially parallel to one another; and said alloy (6') is heated to said elevated temperature and mechanically worked until it flows so as to align the crystallographic c-axes of the platelets (8') therein, the body (85) resulting upon cooling of the hot-worked alloy being magnetically anisotropic and having said permanent magnet characteristics. A method of making an anisotropic permanent magnet according to claim 3, characterised in that the alloy (6') is mechanically worked by pressing the alloy (6') in a direction substantially parallel to the c-axis of the platelets (8'). A method of making an anisotropic permanent magnet according to any one of claims 1 to 4, characterised in that said grains of RE₂TM₁₄B measure at least 5 micrometres along their smallest average dimension.

Inventors: LEE, ROBERT WEIR, VERNIA, PETER
Applicants: GENERAL MOTORS CORPORATION
Application year: 1986
Main classification: H01F1
IPC codes: C21D8, C22C1, C22C38, H01F1, B22F9, C22C33, C21D6