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(11) | EP 0 443 647 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Hot-worked rare earth-iron-carbon magnets |
| (57) Anisotropic permanent magnets consisting essentially of a magnetic phase of RE₂TM₁₄C
are prepared by hot-working suitable iron-neodymium/ praseodymium-carbon-containing
alloys so as to produce aligned fine grains of the above essential magnetic phase. |
[0001] This invention relates to permanent magnets based on rare earth elements and iron. More particularly, this invention relates to permanent magnets based on iron, neodymium and/or praseodymium and carbon as specified in the preamble of claim 1, for example, as disclosed in US-A-4,849,035.
[0002] Permanent magnets based on the RE₂Fe₁₄B-type structure have gained wide commercial acceptance. Such magnets can be made by a sintering practice, and they can be made by rapidly solidifying a melt of suitable composition and producing bonded magnets or hot-pressed magnets or hot-pressed and hot-worked magnets from the quenched material.
[0003] Recently, rare earth-iron-carbon compositions have been formed in a RE₂Fe₁₄C structure which is analogous to the above-mentioned iron-rare earth-boron structure. Stadelmaier and Liu, c.f., US-A-4,849,035, cast iron-dysprosium-carbon compositions and iron-dysprosium-neodymium-carbon-boron compositions in the form of ingots and through a prolonged annealing cycle at 900°C produced the magnetically-hard tetragonal 2-14-1 structure. The casting displayed permanent magnet properties as did comminuted particles produced from the casting. The comminuted particles were disclosed as suitable for use in a bonded magnet. Whilst such materials displayed appreciable magnetic coercivity, they displayed relatively low magnetic remanence.
[0004] Coehoorn et al, "Permanent Magnetic Materials Based on Nd₂Fe₁₄C Prepared by Melt Spinning", Journal of Applied Physics, Vol. 62, No. 2, 15 January 1989, pp. 704-709, produced melt-spun ribbon particles of neodymium, iron and carbon which, when annealed at a suitable temperature, produced a permanent magnet of the 2-14-1 structure. Such particles could also be used to make a resin-bonded magnet.
[0005] An anisotropic permanent magnet according to the present invention is characterised by the features specificied in the characterising portion of claim 1.
[0006] It is an object of the present invention to provide hot-worked magnets, e.g., hot-pressed or hot-pressed and die-upset magnets, of the Nd₂Fe₁₄C-type structure that have very fine grains, have permanent magnet characteristics and are magnetically anisotropic. It is another object of the invention to provide a method of making such hot-worked magnets.
[0007] In accordance with a preferred embodiment of the present invention, these and other objects and advantages are accomplished as follows.
[0008] A melt is prepared comprising neodymium and/or praseodymium, iron and carbon, or carbon and boron, that is suitable, upon hot-working, for forming the 2-14-1 type structure with a minor portion of one or more second phases. This molten composition is very rapidly solidified, such as by melt-spinning, to produce an amorphous composition or a composition of very fine grain size, for example, no greater than about 40 nm in average grain size. The melt-spun material is initially in the form of friable, magnetically-isotropic ribbon fragments which may be readily broken into a powder suitable for hot-pressing and/or other hot-working in a die cavity.
[0009] Such powder particles are amorphous or contain many very fine, substantially spherical grains. The particles are magnetically isotropic. They are hot-pressed at a suitable elevated temperature of about, e.g., 700°C to 900°C for a period from 20 to 30 seconds to a few minutes to form a fully-dense, fine-grain Nd₂Fe₁₄C-type tetragonal crystal structure. The hot-pressed body may then be further hot-worked at an elevated temperature, e.g., 750°C to 900°C, to promote the growth of platelet-like grains and to plastically deform the body to align the platelets such that their c-axes are generally parallel and the resultant body is magnetically anisotropic. The body is still fine-grained although the grains are flattened and aligned and its preferred direction of magnetization is in the direction of pressing, i.e., perpendicular to the direction of material flow during hot-working. In general, it is preferred that the largest average dimension of the flat grains be no more than about 1000 nm and that they be no more than 200 nm thick. The microstructure of the hot-worked material is characterized by a predominance of these flattened 2-14-1 grains with one or more minor phases of intergranular material that is typically composed of iron and the rare earth element(s) present.
[0010] Iron is preferably the transition metal element used although mixtures of iron and cobalt may also be employed. Neodymium and/or praseodymium is preferably used as the rare earth element although up to 40 percent of the total rare earth content may include other rare earth elements. Carbon or mixtures of carbon and boron is preferred for the third constituent of the 2-14-1 structure. In the practice of the present invention, the proportions of iron (or iron and cobalt), rare earth elements and carbon must be balanced so that the predominant crystalline phase formed is the 2-14-1 tetragonal structure. If this crystal structure is not formed, the hot-worked product will have low magnetic coercivity or no permanent magnetic characteristics at all.
[0011] The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which:
[0012] Figure 1 consists of two scanning electron microscope (SEM) photographs [Figure 1(a) and Figure 1(b)] from the fracture surface of a die-upset Nd13.75Fe80.25C₆ magnet. The press direction lies vertically in the photographs. Two magnifications of the same region are provided.
[0013] Figure 2 consists of three graphs of process parameters measured during the hot-pressing of melt-spun ribbons with the composition Nd₁₆Fe₇₈C₉.
[0014] Figure 3 consists of three graphs of process parameters measured during the die-upsetting of a hot-pressed precursor with the composition Nd₁₆Fe₇₈C₉.
[0015] Figure 4 consists of demagnetization curves for hot-pressed and die-upset magnets. The compositions are indicated in each panel under the respective curves.
[0016] The product of the present invention is a permanent magnet. It has a coercivity greater than 1000 Oersteds.
Example 1
[0017] An ingot was prepared whose composition on an atomic percent basis was neodymium, 13.75 percent; iron, 80.25 percent; and carbon, 6 percent. This material was re-melted by induction melting in a quartz crucible under argon atmosphere at a super-atmospheric pressure of 6.89 - 20.68 kPa (1-3 psi) and melt-spun by ejecting the molten material through a 0.65 mm orifice at the bottom of the crucible onto the perimeter of a 254 mm (10 inch) diameter chromium-plated copper wheel rotating at a speed of 28 meters per second. The ejected molten stream was instantaneously quenched as it hit the rim of the spinning wheel and thrown off as ribbon fragments.
[0018] An X-ray diffraction analysis of the ribbon particles confirmed that they were substantially amorphous. The ribbon fragments were crushed to powder to facilitate handling. A portion was then placed in the cavity of a 12.7 mm (0.5 inch) diameter graphite die. The ribbon fragments were preheated therein in vacuum to 450°C. The die temperature was then rapidly increased to 750°C. When the die temperature exceeded 640°C, pressure was applied by boron nitride-lubricated tungsten carbide-titanium carbide punches. A pressure cycle was initiated, causing the applied load to rapidly increase to a maximum load of 100 MPa. The load was held at maximum load for 30 seconds to ensure full compaction of the fragments before the punches were withdrawn and the sample ejected. The entire process was done in a vacuum. A fully-densified cylindrical body was thus formed.
[0019] The resulting hot-pressed body had a density of about 7.74 g/cc and contained the Nd₂Fe₁₄C tetragonal crystal phase with small amounts of intergranular phases of uncertain composition believed to be largely neodymium and iron. The lattice parameters of this tetragonal phase were determined to be a = 8.797 angstroms and c = 12.001 angstroms.
[0020] The magnetic properties of this hot-pressed body were derived from a demagnetization curve measured with a hysteresisgraph. The body displayed magnetic anisotropy. The relevant properties in the direction parallel to pressing were as follows: Br = 7.7 kG, Hci = 10.7 kOe and (BH)max = 11.4 MGOe. In the direction perpendicular to pressing, the magnetic properties were: Br = 6.8 kG, Hci = 11.3 kOe and (BH)max = 8.1 MGOe.
Example 2
[0021] A hot-pressed cylinder from Example 1 was pressed a second time in the same direction in vacuum using an oversized (19.05 mm (0.75 inch) internal diameter) graphite die that permitted the cylinder to plastically deform at a die temperature of 750°C to 800°C to about 40 percent of its original height. The resulting die-upset, flat cylindrical magnet was sectioned with a high-speed diamond saw to produce a 2 mm cube for measurement of its magnetic properties in a vibrating sample magnetometer. The cube was cut so that two opposite faces were perpendicular to the direction of pressing and die-upsetting, and the other four faces were parallel to the direction of pressing and die-upsetting.
[0022] The demagnetization curves for the neodymium-iron-carbon die-upset magnet revealed a higher magnetic remanence in the press direction (Br = 12.3 kG) than in the direction perpendicular to the press direction where Br = 1.7 kG. This magnetic anisotropy is indicative of the alignment of the c-axis of the individual die-upset grains along the press direction. The magnetic coercivity of the sample in the press direction was 2.8 kOe.
[0023] Figures 1(a) and 1(b) are two SEM photographs at different magnifications of the same region of a fracture surface of this die-upset specimen. The grains of the Nd₂Fe₁₄B tetragonal crystals are seen to be aligned flat platelets. The grains are about 100 nm thick and up to about 700 to 800 nm in their largest dimension. The short dimension of the grains, the c-axis, the preferred direction of magnetization, lies along the direction of applied stress.
Example 3
[0024] A family of four alloys was prepared so as to be composed as follows: Nd13.75Fe80.25(B1-xCx)₆ where x in the four samples was respectively 0.2, 0.4, 0.6 and 0.8.
[0025] The four samples were individually melt-spun to form amorphous ribbon fragments as in Example 1. The four lots of ribbon fragments were pulverized and hot-pressed into cylindrical bodies in accordance with the practice of Example 1. They contained fine grains of the tetragonal phase Nd₂Fe₁₄CxB1-x where the values of x were as indicated above. The densities and the magnetic properties of the cylindrical magnetic bodies were as follows:
Example 4
[0026] The relatively low magnetic coercivity and high resistance to deformation of the die-upset Nd13.75Fe80.25C₆ magnets suggested the need for higher neodymium concentrations. Several alloys were prepared as described in Example 1 using the formula Nd13.75+xFe80.25-xC₆. The respective compositions were melt-spun as described in Example 1 except that a wheel speed of 30 m/s was used. The samples were hot-pressed and most were die-upset. These hot-working steps were carried out using graphite dies and tungsten carbide-titanium carbide punches also as described in Example 1.
[0027] Typical process parameters used for hot-pressing these Nd-Fe-r ribbons are shown in Figure 2. The ribbons were heated to 650°C in about 5.75 minutes, at which point the pressure was applied (see panels A and B of Figure 2). The time interval required to reach full (or nearly full) density was between 1 and 2 minutes at maximum pressure (about 65 MPa), as the lower two panels in Figure 2 show. The final hot-press temperature was around 850°C for the hot-pressed carbide magnets, compared to about 800°C for Nd-Fe-B magnets.
[0028] The hot-pressed magnets were removed from the die and cooled to room temperature. Magnetic measurements were then made as described below. The data is reported in Table I below. Some of the hot-pressed magnets were then re-heated and die-upset in a larger die as described in Example 2.
[0029] The temperature reached 700°C in about 8.25 minutes of heating. An initial die-upsetting pressure of about 15 MPa was applied at about 800°C (see Figure 3). This pressure was maintained until the sample height had decreased at least by about 5 percent, at which point the pressure was increased to 20 to 25 MPa. Starting with 15 MPa ensured that deformation could be induced without cracking the precursor; however, the strain rate at 15 MPa was too slow. Increasing the pressure to 20 to 25 MPa enhanced the strain rate to levels comparable to those observed for Nd-Fe-B alloys (about 1 min-¹). Higher temperatures were required to produce fully die-upset carbide magnets; the final temperature (about 900°C) was 50 to 100 degrees higher than that used for die-upsetting boride magnets. All die-upset magnets discussed here were reduced to 45 percent of their original height (i.e., 55 percent die-upset).
[0030] Magnetic measurements of the hot-pressed and die-upset magnets were made using a Walker Model MH-5020 hysteresisgraph; the results are summarized in Tables I and II. X-ray (Cu K ) diffraction patterns were obtained for powdered ribbons after annealing for about 30 minutes at 700°C.
[0031] Surprisingly, at neodymium concentrations above 14.5 atomic percent with the carbon concentration at 6 atomic percent, the magnetic coercivity of the hot-pressed magnets decreased sharply compared to similar boride compositions. The magnetic coercivity apparently vanishes altogether at Nd₁₆Fe₇₈C₆ due to the formation of the phase Nd₂Fe₁₇. The major diffraction peaks are easily accounted for when compared to the calculated pattern for the 2-17 phase. It is quite possible that the observed 2-17 phase contained dissolved carbon, as reported by others studying annealed ingots.
[0032] To suppress the formation of the 2-17 phase, higher concentrations of carbon were tried using the composition formula Nd₁₆Fe78-yC6+y. With increasing carbon levels, the magnetic coercivity of hot-pressed magnets increased sharply, exceeding 12 kOe for concentrations at or above 9 percent. Powder X-ray diffraction patterns for annealed Nd₁₆Fe₇₅C₉ ribbons revealed strong intensities from the tetragonal 2-14-1 phase with lattice parameters of a=0.8803 nm and c=1.2010 nm. Comparing the observed reflections to the calculated pattern for Nd₂Fe₁₄C confirmed that the 2-14-1 phase was the major phase, but it was still by no means the only phase present. In addition to the possibility of small amounts of the 2-17 phase, the presence of elemental iron (α-Fe) was also indicated.
[0033] The presence of phases such as α-Fe and 2-17 in these alloys was made more apparent by adjusting the neodymium concentration whilst maintaining high carbon levels of 9 percent and 10 percent. Increasing the neodymium levels above 16 percent (up to about 17 percent) reduced the magnetic coercivity in these hot-pressed magnets, and again the X-ray diffraction patterns of the annealed ribbons revealed the presence of the 2-17 phase. Reducing the neodymium levels below 16 percent (to about 14 percent) also lowered the magnetic coercivity, but this time the decrease can be attributed to α-Fe.
[0034] The demagnetization properties of the Nd13.75+xFe80.25-xC₆ and Nd₁₆Fe78-yC6+y alloys of the present invention are summarized in the following Table I.
[0035] The three hot-pressed magnets with the highest magnetic coercivities (≧12 kOe) were die-upset using the process parameters already described (see Table II for compositions). Demagnetization curves for the three die-upset magnets and their hot-pressed precursors appear in Figure 4; in each case, die-upsetting increased the magnetic remanence by just over 40 percent. More importantly, the magnetic coercivity of these die-upset magnets was sufficient to permit much higher energy products (about 18 MGOe to about 22 MGOe) than those observed with lower neodymium and carbon concentrations (see Example 2).
[0036] In accordance with the practice of the present invention, rapidly-solidified compositions of rare earth elements, iron (or iron and cobalt) and carbon (or carbon and boron) are hot-worked to form fully-densified, fine-grained bodies in which the fine grains are wrought into magnetic alignment such that the body is magnetically anisotropic. By hot-working is meant a process such as hot-pressing, hot die-upsetting, extrusion, hot-isostatic compaction, or rolling, so long as the specified resultant hot-worked microstructure is attained. Generally, if the hot-working process comprises more than one step, such as the combination of hot-pressing and die-upsetting, all steps can be carried out without an intervening cooling step.
[0037] The compositions selected, the rapid solidification practice and the practice of rapid solidification and hot-working are controlled and carried out so that the microstructure of the resultant body consists essentially of the magnetic phase Re₂TM₁₄CxB1-x together with a minor portion of intergranular material. The hot-working aligns the fine platelet-like grains of the principal phase such that the c-axes of the grains are aligned and the resultant body is magnetically anisotropic. The melt-spun (rapidly solidified) material is preferably amorphous or suitably extremely fine-grained such that the average grain size is no greater than about 40 nm. Following severe hot-working, flattened grains are obtained and it is preferred that, on the average, their greatest dimension be no greater than about 1000 nm.
[0038] Preferably the overall composition of the anisotropic magnets of the invention comprise on an atomic percent basis 50 to 90 percent iron, 6 to 20 percent neodymium and/or praseodymium, and 0.5 to 18 percent carbon or carbon and boron. Neodymium and/or praseodymium contents of 13 to 17 atomic percent and a carbon content of 6 to 12 atomic percent are especially preferred. Consistent with these ranges and referring to the formula for the tetragonal crystal structure RE₂TM₁₄CxB1-x, RE is neodymium and/or praseodymium or mixtures of these rare earths with other rare earths provided that the other rare earths make up no more than about 40 percent of the total rare earth content, TM is iron or mixtures of iron with cobalt, and x has a value in the range of 0.2 to 1.0. Cobalt may make up about half of the TM content of the alloy.
[0039] The hot-worked, anisotropic magnets of the invention can be comminuted to an anisotropic magnetic powder for use in bonded magnets. The pulverized powder is mixed with an epoxy resin or other suitable bonding material, magnetically aligned, and pressed or moulded. This resin is cured by heating, if appropriate.
1. An anisotropic permanent magnet containing one or more rare earth elements consisting
of neodymium, praseodymium or mixtures of neodymium and/or praseodymium with one or
more other rare earth elements that make up no more than 40 percent of the total rare
earth content, iron or mixtures of iron with cobalt, carbon, and, optionally, boron,
characterised in that said magnet comprises a principal phase of hot-work-aligned, flat, fine grains of
a tetragonal crystal phase RE₂TM₁₄CxB1-x and an intergranular minor phase, where RE is one or more of said rare earth elements,
TM is iron or mixtures of iron with cobalt, and where the value of x is from 0.2 to
1.0; and the flat grains are on the average no greater than 1000 nm in greatest dimension.
2. An anisotropic permanent magnet according to claim 1, comprising, on an atomic percent
basis, 50 to 90 percent iron, 6 to 20 percent neodymium and/or praseodymium, and 0.5
to 18 percent carbon.
3. An anisotropic permanent magnet according to claim 2, in which x is 1 and the neodymium
and/or praseodymium content is in the range of about 13 to 17 atomic percent and the
carbon content is in the range of about 6 to 12 percent.
4. A method of making a fine-grained, anisotropic permanent magnet according to claim
1, characterised in that the method comprises hot-pressing rapidly-solidified amorphous or very fine-grained
particles of a composition comprising, on an atomic percent basis, 40 to 90 percent
iron, 6 to 20 percent neodymium and/or praseodymium, and 0.5 to 18 percent carbon,
at an elevated temperature to consolidate said particles into a substantially fully-densified
body consisting essentially of aligned, flat grains of said principal phase with one
or more intergranular phases, the maximum dimension of said grains being on the average
no greater than 1000 nm.
5. A method of making a fine-grained, anisotropic permanent magnet according to claim
1, characterised in that the method comprises hot-pressing rapidly-solidified particles that are initially
amorphous or of very fine spherical-grained microstructure, that are no more than
about 40 nm in diameter, and that are of a composition comprising, on an atomic percent
basis, 40 to 90 percent iron, 6 to 20 percent neodymium and/or praseodymium, and 0.5
to 18 percent carbon, at an elevated temperature to consolidate said particles into
a substantially fully-densified body; and further hot-working the body at an elevated
temperature by application of pressure in the same direction as the direction of hot-pressing
so as to produce a microstructure of said principal phase of aligned, generally flat
grains with one or more intergranular phases, the maximum dimension of said grains
being on the average no greater than 1000 nm, the resultant body displaying anisotropic
permanent magnet properties and having a preferred direction of magnetization parallel
to the direction of the applied pressure during the further hot-working.