Feedstock and heterogeneous structure for tough rare earth permanent magnets and production therefor

Abstract

New types of particle feedstocks and heterogeneous grain structures are provided for rare earth permanent magnets (REPMs) and their production in a manner to significantly enhance toughness of the magnet with little or no sacrifice in the hard magnetic properties. The novel tough REPMs made from the feedstock have heterogeneous grain structures, such as bi-modal, tri-modal, multi-modal, laminated, gridded, gradient fine/coarse grain structures, or other microstructural heterogeneity and configurations, without changing the chemical compositions of magnets.

Claims

1. A rare earth SmCo.sub.5 permanent magnet comprised of binder-free consolidated and sintered powder particles and having a magnet composition and sintered microstructure characterized by a heterogeneous multi-modal grain structure that is present throughout the sintered microstructure without added grain boundary modifier composition different from the magnet composition wherein the multi-modal grain structure of the sintered microstructure includes at least two distinct grain regions having the same composition as the magnet composition but having at least one most coarse grain region having a plurality of relatively most coarse grains aggregated together with an average most coarse grain size of about 10 to about 30 microns and at least one fine grain region having a plurality of relatively fine grains aggregated together with an average fine grain size that is about 25% to about 50% of the average coarse grain size wherein the at least one fine grain region is present as a strengthening microstructural feature in the sintered microstructure with a periphery thereof forming a fine grain-to-coarse grain boundary with the at least one most coarse grain region and is present in controlled proportion of the sintered microstructure to increase a mechanical property comprising at least one of fracture toughness and mechanical strength of said magnet having the heterogeneous multi-modal grain structure as compared to a sintered magnet having said magnet composition but a single modal grain structure with a single average grain size of about 40 microns, without altering the magnet composition, wherein the SmCo.sub.5 permanent magnet has a flexural strength of 182 to 214 MPa at 20 C.

2. The magnet of claim 1 wherein the heterogeneous grain structure comprises a bi-modal grain size distribution.

3. The magnet of claim 1 wherein the heterogeneous grain structure comprises a tri-modal or multi modal grain size distribution.

4. The magnet of claim 1 wherein the heterogeneous grain structure comprises a gradient distribution of grain sizes across the microstructure.

5. A rare earth permanent magnet comprised of binder-free consolidated and sintered powder particles and having a magnet composition and sintered microstructure characterized by a heterogeneous multi-modal grain structure that is present throughout the sintered microstructure without added grain boundary modifier composition different from the magnet composition wherein the multi-modal grain structure of the sintered microstructure includes at least two distinct grain regions having the same composition as the magnet composition but having at least one most coarse grain region having a plurality of relatively most coarse grains aggregated together with an average most coarse grain size of about 10 to about 30 microns and at least one fine grain region having a plurality of relatively fine grains aggregated together with an average fine grain size that is less than the average most coarse grain size wherein the at least one fine grain region has a periphery forming a fine grain-to-coarse grain boundary with the at least one most coarse grain region in the sintered microstructure and present in controlled proportion of the sintered microstructure to produce the permanent magnet, without altering the magnet composition wherein the multi-modal grain structure comprises laminated most coarse grain regions and fine grain regions in the sintered microstructure.

6. The magnet of claim 5 comprising a samarium-cobalt (SmCo.sub.5 and Sm.sub.2Co.sub.17) magnet, a neodymium-iron-boron (Nd.sub.2Fe.sub.14B) magnet, a neodymium-iron-carbon magnet (R.sub.2Fe.sub.14C, R=rare earth, La or yttrium), a R-iron-nitrogen magnet (R.sub.2Fe.sub.17X.sub.8, R=rare earth, La or Y; X=H, C, and/or N), or a R-iron-M-nitrogen magnet (R (Fe, M).sub.12X.sub.8, R=rare earth, La or Y; M=Mo, V, Ti, Si, Al, Cr, Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N).

7. A rare earth Sm.sub.2(CoFeCuZr).sub.17 permanent magnet comprised of binder-free consolidated and sintered powder particles and having a magnet composition and sintered microstructure characterized by a heterogeneous multi-modal grain structure that is present throughout the sintered microstructure without added grain boundary modifier composition different from the magnet composition wherein the multi-modal grain structure of the sintered microstructure includes at least two distinct grain regions having the same composition as the magnet composition but having at least one most coarse grain region having a plurality of relatively most coarse grains aggregated together with an average most coarse grain size of about 10 to about 30 microns and at least one fine grain region having a plurality of relatively fine grains aggregated together with an average fine grain size that is about 25% to about 50% of the average coarse grain size wherein the at least one fine grain region is present as a strengthening microstructural feature in the sintered microstructure with a periphery thereof forming a fine grain-to-coarse grain boundary with the at least one most coarse grain region and is present in controlled proportion of the sintered microstructure to increase a mechanical property comprising at least one of fracture toughness and mechanical strength of said magnet having the heterogeneous multi-modal grain structure as compared to a sintered magnet having said magnet composition but a single modal grain structure with a single average grain size of about 40 microns, without altering the magnet composition, wherein the Sm.sub.2(CoFeCuZr).sub.17 permanent magnet has a flexural strength of 159 to 202 MPa at 20 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows morphology (SEM images) of SmCo.sub.5 powders (1500) (Jet milled powders shown in top row) and further cryomilled in LN.sub.2 (liquid nitrogen) for 5 hrs (shown in bottom row), according to one embodiment of the invention. The right column is the corresponding enlarged images (5000).

(2) FIG. 2 shows grain size distribution of the SmCo.sub.5 powders (Jet milled powders (shown in top graph) and further cryomilled in LN.sub.2 for 5 hr (shown bottom graph) with average (mean) sizes of about 1.5 m and 2.0 m, respectively, where the terms average and mean are used interchangeably (synonymously) herein with respect to values of particle sizes and grain sizes. These results were obtained from the SEM images analyzed by the image J software.

(3) FIG. 3 presents demagnetization curves of 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from feedstock of (100-x) wt. % 2:17 type jet milled (JM) powders+x wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40, according to an embodiment of the process of the invention.

(4) FIG. 4 illustrates demagnetization curves of 1:5 type SmCo sintered magnets made from feedstock of (100-x) wt. % 1:5 type jet milled (JM) SmCo powders+x wt. % 1:5 type cryomilled in liquid nitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40, in accordance with an embodiment of the process of the invention.

(5) FIG. 5 shows an optical photomicrograph (200) of cross-section microstructure of the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % 2:17 type jet milled (JM) powders. Typical single-modal grain size structure was observed. The specimen was mechanically polished then etched with 2% nital etchant for the metallographic examination.

(6) FIG. 6 shows optical photomicrographs (500) of cross-section microstructure from selected areas 1 and 2 as shown in FIG. 5, for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % 2:17 type jet milled (JM) powders. Typical single-modal grain size structure was observed.

(7) FIG. 7 shows grain size distribution of the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % 2:17 type jet milled (JM) powders. Typical single-modal grain size structure was observed with an average grain size of about 40 m.

(8) FIG. 8 shows optical photomicrograph (200) of cross-section microstructure of the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders. The specimen was mechanically polished then etched with 2% nital etchant for the metallographic examination. A typical bio-modal grain size structure was observed in the sintered magnet. The fine grains (FG) formed cluster areas (marked by white ovals) those uniformly distributed within the coarse grain (CG) matrix. The SmCo sintered magnet with a 3D gradient harmonic structure of controlled bimodal grain size distribution exhibited considerably higher flexural strength and comparable magnetic properties, compared with the magnet with a single-modal grained structure as shown in FIGS. 5-7.

(9) FIG. 9 shows optical photomicrograph (200) of cross-section microstructure of the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders. The areas 1, 2, 3 and 4 marked by squares were selected for further enlarged observation.

(10) FIG. 10 shows optical photomicrographs (500) of cross-section microstructure from selected areas 1, 2, 3 and 4 as shown in FIG. 9, for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders. A typical bio-modal grain size structure was observed in the sintered magnet. The fine grains (FG) formed cluster areas (marked by white ovals) those uniformly distributed within the coarse grain (CG) matrix.

(11) FIG. 11 shows optical photomicrographs (1000) of the cross-section microstructure from selected areas for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders. A typical bio-modal grain size structure was observed in the sintered magnet. The fine grains (FG) formed cluster areas (marked by white ovals) in FIG. 10 and uniformly distributed within the coarse grain (CG) matrix.

(12) FIGS. 12a and 12b show grain size distribution of fine grain areas (FG areas, Fi. 12b) and coarse grain matrix (CG areas, FIG. 12a) of 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % jet milled (JM) powders+30 wt. % cryomilled in liquid nitrogen (CM) powders. The average (or mean) grain sizes were about 24 m and 6 m for the FG areas and CG matrix, respectively. These results were obtained from the optical images analyzed by the image J software.

(13) FIG. 13 shows scanning electron microscope (SEM) images of the fracture surface of Sm.sub.2(CoFeCuZr).sub.17 type sintered magnets made from raw materials of (100-x) wt. % 2:17 type jet milled powders+x wt. % 2:17 type cryomilled powders. Right column shows the enlarged images from the selected areas (marked by the black-line ovals). Top images: x=0 wt. % sample, a single-modal coarse grain size (about 40 m) microstructure; and Bottom images: x=30 wt. % sample having a 3D gradient harmonic microstructure with a bi-modal grain size distribution. A higher density while smaller size of river patterns and cleavage steps were observed in the heterogeneous magnets (bottom) with a bi-modal grain size distribution compared with the x=0 wt. % sample with a single-modal coarse grain size (top).

(14) FIG. 14 shows SEM images of the fracture surface of Sm.sub.2(CoFeCuZr).sub.17 type sintered magnets made from raw materials of 70 wt. % jet milled powders+30 wt. % cryomilled powders. Bottom-left and top-right images are the enlarged images from the selected areas: area 1fine grain (FG) area, and area 2coarse grain (CG) matrix (marked by the black-line ovals). A higher density while smaller size of river patterns and cleavage steps were observed in the heterogeneous magnets, especially in the finer grain cluster regions on the fracture surface.

(15) Table 1 lists the flexural strength of sintered 2:17 type and 1:5 type SmCo magnets made from feedstock of (100-x) wt. % 2:17 or 1:5 type jet milled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled in liquid nitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40, according to the embodiment of the invention.

(16) Table 2 lists magnetic properties and density of sintered 2:17 type and 1:5 type SmCo magnets made from feedstock of (100-x) wt. % 2:17 or 1:5 type jet milled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled in liquid nitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40. RemanenceB.sub.r, intrinsic coercivityH.sub.cl, maximum energy product(BH).sub.max, value of H.sub.c at 0.9B.sub.rH.sub.k, coercivityH.sub.c magnet density.

(17) Table 3 lists mean grain sizes of the fine grain (FG) areas and the coarse grain (CG) matrix, and overall mean gain sizes of both for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled (JM) powders+x wt. % (x=0, 10, 15, 20, 30, 40 wt. %) cryomilled (CM) powders.

DETAILED DESCRIPTION OF THE INVENTION

(18) The present invention relates to rare earth permanent magnets (REPMs) having a heterogeneous microstructure and their production in a manner to provide significantly enhanced toughness; i.e. resistance-to-fracture as evidenced by enhanced mechanical toughness property such as flexural strength and/or fracture toughness, while maintaining or with a minimum sacrifice in the hard magnetic properties. The REPMs made pursuant to embodiments of the invention have heterogeneous grain microstructures, such as bi-modal, tri-modal, multi-modal, laminated, gridded, or gradient coarse/fine grain structures, or other microstructural heterogeneity, etc., without the need for changing the chemical compositions of magnets. To increase flexural strength and/or fracture toughness of the REPMs, particle sizes or grain sizes of the particle feedstock are modified with fixed chemical feedstock compositions in this invention. For purposes of illustration and not limitation the typical feedstock comprises 1-99 wt. % modified finer (average particle size from less than 1 micron; i.e. submicron to about 1.5 micron) SmCo or NdFeB particle powders made from the commercial jet-milled SmCo or NdFeB microparticle powders and 99-1 wt. % commercial jet-milled SmCo or NdFeB microparticle powders (average particle size about 2-10 micron) where the terms average and mean are used interchangeably (synonymously) herein with respect to values of particle sizes and grain sizes. As a result, practice of the invention can be easily integrated with the current industry production line for sintered REPMs. In certain embodiments, the powders can be mixed together under argon or other non-reactive atmosphere in a mixer or mill for greater than 0 to 100 hours or more as needed. The powders can be formed into a green compact and consolidated by techniques that include, but are not limited to, powder metallurgy processing, hot pressing, friction consolidation extrusion, 3D printing, surface mechanical attrition treatment (SMAT), equal channel angular extrusion (ECAE), hot accumulative roll bonding (ARB), hot asymmetric rolling, high pressure torsion (HPT), hot drawing, and mechanical milling. The REPMs with heterogeneous microstructures and fixed chemical compositions pursuant to embodiments of the invention can maintain the hard magnetic properties without substantial degradation of the hard magnetic properties such as BH.sub.max and B.sub.r, can be less dependent on the critical element resources, and can facilitate separation, sorting and recycling at the end of life.

(19) The following examples are offered for purposes of further illustration, but not limitation, with respect to the present invention:

(20) Fine SmCo (both 2:17 and 1:5 types) powders (average particle size about 1 m) were synthesized by cryomilling in liquid nitrogen (LN.sub.2), using commercial jet milled microparticle powders as the precursors and a SPEX 6875D Freezer/Mill. FIGS. 1 and 2 show typical morphology and grain size distribution of jet milled SmCo.sub.5 powders with/without (w/o) further ball milling in LN.sub.2 for 5 hrs. The jet milled precursor powders were mainly composed of particles with a size of about 1.0-2.5 m with an average (mean) particle size of about 2.0 m. The particle size range was within about 0.7-8.0 m. With increasing the cryomilling time from 1 hr to 5 hrs, the average particle size continuously decreased. The SmCo powders after cryomilled for 5 hrs were mainly composed of particles with a size of about 1.0 m or less, which had average (mean) particle size of about 1.5 m. The particle size range was within about 0.5-6.0 m. Similar results were obtained for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 jet milled and cryomilled powders. The jet milled Sm.sub.2(CoFeCuZr).sub.17 powders were composed of irregular microparticles with a particle size mainly in the range of about 0.7-3.0 m and average (mean) particle size of 2.3 m. The overall particle size range of the jet milled powders was within about 0.7-8.0 m. Whereas, the Sm.sub.2(CoFeCuZr).sub.17 powders further cryomilled for 2 hrs were composed of finer irregular microparticles with a particle size mainly in the range of about 0.5-1.5 m and average (mean) particle size of 1.3 m and less-sharp edges. The overall particle size range of the cryomilled powders was within about 0.5-8.0 m. Both the conventional jet milled microparticles and the further cryomilled finer particles had a single-crystal structure. These particle size results were obtained from the SEM images analyzed by image J software. Both the cryomilled finer particles and the commercial jet milled microparticles were mainly single-crystal structure. Besides cryomilling in liquid nitrogen, the other finer powder preparation methods wherein the particle size ranging from nanometer, submicron, and micron scale or their mixtures, that is smaller than that of jet-milled powders with a typical average particle size of about 2-5 micron, include but are not limited to, some top-down and bottom-up approaches, such as, multiple jet milling, low or high energy ball milling at room temperature in inert gas (Ar, N.sub.2, or He) or in solvent media (ethanol, hexane, heptane, toluene, etc.), organic surfactant-assisted high energy ball milling at room temperature, inert gas atomization, gas condensation, spark erosion, chemical precipitation, sol-gel, pyrolysis and hydrothermal synthesis, plasma arcing, chemical vapor deposition (CVD), physical vapor deposition (PVD), electrodeposition, atomic layer deposition (ALD), etc.

(21) The cryomilled SmCo sub-micron powders produced pursuant to embodiments of this invention were then mixed with the jet milled precursor powders under an argon atmosphere in a SPEX 8000M Mixer/Mill without any milling balls for a time of 2 or 3 minutes, which more generally can be up to 15 minutes or more or other suitable blending time. The particle mixture then is subjected to conventional powder metallurgy method (i.e. sintering a pressed compact) to produce a bulk magnet with grain boundary engineering or modified microstructure. The particular illustrative powder metallurgy steps typically include cold compaction of the magnetically aligned powder mixture to form a green compact and then sintering the green compact, although the powders can be formed into a green compact and consolidated by techniques that include, but are not limited to, powder metallurgy processing, hot pressing, friction consolidation extrusion, 3D printing, surface mechanical attrition treatment (SMAT), equal channel angular extrusion (ECAE), hot accumulative roll bonding (ARB), hot asymmetric rolling, high pressure torsion (HPT), hot drawing, and mechanical milling. The powder metallurgy method optionally can include preparation of ingot chips by strip casting or bulk ingot by induction melting or arc melting, hydrogen decrepitation or crushing into coarse powders of about 100 micron or less sizes, jet milling or ball milling into fine microparticles of the average particle sizes described above, magnetically aligning by a 4 or 7 Tesla pulsed magnetic field and pre-pressing powder mixtures into green compacts by a pressure of 35,000 psi (about 241 MPa) using a Nikisso CL15-45-30 iso-static press, and subsequent heat treatment procedure, including sintering, solution, temper, and aging.

(22) In the examples below, the green compacts were pre-pressed by a pressure of 241 MPa using the above Nikisso CL15-45-30 iso-static press and sintered. The 2:17 type Sm.sub.2(CoFeCuZr).sub.17 were sintered at 1190-1220 C. for 1-2 hrs, solution tempered at 1150-1185 C. for 1-7 hrs, and aged at 800-850 C. for 5-10 hrs then cooling to 400 C. at a ramp rate of 0.7-1.0 C./min., further aging at 400 C. for 1-10 hrs. Sintered 1:5 type SmCo.sub.5 were sintered at 1130-1150 C. for 1-2 hrs, solution tempered at 850-900 C. for 5-7 hrs, and then quenched in argon.

(23) Sintered 2:17 type Sm.sub.2(CoFeCuZr).sub.17 magnet density p was 8.4 g/cc, which was about 99% of theoretical value. Demagnetization curves and magnetic properties are shown in FIG. 3 and Table 2 for the sintered 2:17 type magnets made from feedstock of (100-x) wt. % 2:17 type jet milled (JM) powders+x wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders. The remanence B.sub.r was about 10.6 kGs, and intrinsic coercivity H.sub.ci was higher than 24 kOe for all the 2:17 type magnets. Maximum energy product (BH).sub.max was about 26 MGOe for x=0 sample while its value decreased a little with increasing of x values.

(24) Table 1 below lists the flexural strength of sintered 2:17 type and 1:5 type SmCo magnets made from feedstock of (100-x) wt. % 2:17 or 1:5 type jet milled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled in liquid nitrogen (CM) powders. By engineering the grain size and grain-boundary microstructure, the flexural strength of the sintered Sm.sub.2Co.sub.17 type magnets were enhanced by 50%, 58%, and 73% (175 MPa, 185 MPa and 202 MPa for the samples with x=20, 30, and 40, respectively) compared to 117 MPa for the sample with x=0. A flexural strength value of 120 MPa for the sample with x=0 was reported for a commercial Sm.sub.2Co.sub.17 type magnets (http://www.electronenergy.com/products/materials). Excellent magnetic properties were maintained with the maximum energy product (BH).sub.max (about 24 MGOe) decreased by only 7.7% (less than 8%), respectively, and almost no decrease of remanence B.sub.r values. Flexural strengths reported above were measured using the 3-point bending ASTM flexure test no. C1161-13. Fracture toughness, another mechanical toughness property, can be measured by the Charpy V-notch or IZOD ASTM tests no. C1421-16.

(25) FIG. 4 and Table 2 below show demagnetization curves and magnetic properties of the sintered 1:5 type SmCo.sub.5 magnets made from feedstock of (100-x) wt. % 1:5 type jet milled (JM) powders+x wt. % 1:5 type cryomilled in liquid nitrogen (CM) powders. By engineering the grain size and grain-boundary microstructure, the flexural strength of new sintered SmCo.sub.5 type magnets were enhanced by 37% and 21%, which was 214 MPa and 189 MPa for the samples with x=20 and 30, respectively, compared to a flexural strength value of 156 MPa for the sample with x=0. A flexural strength value of 130 MPa was reported for commercial SmCo.sub.5 magnets (http://www.electronenergy.com/products/materials). The higher flexural strength value of 156 MPa for the present sample with x=0 than that of the reported commercial magnet was likely due to the variable particles sizes of feedstocks and thus grain sizes of the sintered magnets. The values of both remanence B.sub.r and the maximum energy product (BH).sub.max of the samples with modified microstructures pursuant to the invention were even higher than those of the commercial counterpart x=0 sample. That is, (BH).sub.max (about 21 and 22 MGOe) increased by 10.5% and 15.8%, B.sub.r (about 9.3 and 9.4 KG) increased by 6.9% and 8.0% for the samples with x=20 and 30, respectively.

(26) With respect to sintered microstructures, a typical single-modal grain size structure with average (mean) grain size of about 40 m was observed in the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % 2:17 type jet milled (JM) powders, as shown in FIGS. 5-7. In contrast, the SmCo bulk magnets made from the feedstock mixtures pursuant to the invention have a bi-modal grain size microstructure, as shown in the typical FIGS. 8-12a, 12b. The fine grains (FG) formed cluster areas (localized regions) and uniformly distributed within the coarse grain (CG) matrix. FIGS. 12a, 12b show grain size distribution of fine grain areas (FG) and coarse grain (CG) matrix of 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of 70 wt. % jet milled (JM) powders+30 wt. % cryomilled in liquid nitrogen (CM) powders. The average (mean) sizes were about 6 m and 24 m for the FG areas and CG matrix, respectively. The sizes of the respective FG localized areas were in the range of about 60-190 m. The other SmCo sintered magnets developed by practice of embodiments of the present invention showed similar heterogeneous microstructure with a bi-modal grain size distribution. Table 3 lists mean grain sizes of the fine grain (FG) areas and the coarse grain (CG) matrix, and overall mean gain sizes for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled (JM) powders+x wt. % (x=0, 10, 15, 20, 30, 40 wt. %) cryomilled (CM) powders. With increasing x values, the mean grain sizes of both FG areas and CG matrix, and overall mean gain sizes decreased monotonously. These results were obtained from the optical images analyzed by image J software. This novel 3D gradient harmonic microstructure with a bi-modal grain size resulted in considerably higher flexural strength and higher fracture toughness, and comparable magnetic properties, compared with the magnet with a single-modal grain sized microstructure as shown in FIGS. 5-7.

(27) The enhancement of flexural strength of SmCo sintered magnets resulted from the grain size refinement with the contributions from both localized fine grain cluster regions and a general grain size reduction from the coarse grain matrix, i.e. bimodal grain size engineering. The localized fine grain clusters are more effective in preventing crack nucleation or propagation via acting as strengthening sites and therefore they have higher flexural strength. Since it is known that the sintered SmCo magnets have a brittle intragranular cleavage fracture mechanism under normal stress, the characteristic morphology of cleavage fracture, such as river patterns and cleavage steps, was observed on the fracture surface of the Sm.sub.2(CoFeCuZr).sub.17 type sintered magnets as shown in FIGS. 13 and 14. No section shrink, fiber region, or shear lip was observed on the fracture surface. A higher density while smaller size of river patterns and cleavage steps were observed in the fine grain clusters on the fracture cross-section surface as shown in FIGS. 13 and 14, which increased the energy needed for nucleation or propagation of the main crack. This is a structure proof of the strengthening effect from the fine grain clusters.

(28) The magnets developed pursuant to embodiments of the invention can be expected to find similar applications in various industries as those of commercial sintered or die-upset REPMS. Applications include, but are not limited to, e.g., telecommunication, magnetic storage, biomedical equipment, consumer electronics, sensors, power and propulsion applications such as high performance motors and generators and ion engines, inertial devices such as gyroscopes and accelerometers, and traveling wave tubes, and many more.

(29) Tables 1, 2 and 3 appear below:

(30) TABLE-US-00001 TABLE 1 Flexural strength of sintered 2:17 type and 1:5 type SmCo magnets made from feedstock of (100 x) wt. % 2:17 or 1:5 type jet milled (JM) powders + x wt. % 2:17 or 1:5 type cryomilled in liquid nitrogen (CM) powders, x = 0, 10, 15, 20, 25, 30, 40. CM/JM Flexural weight ratio strength Increase Samples x/(100 x) (MPa) STD by (%) SmCo.sub.5 0/100 156 (130*) 14 10/90 183 6 17 15/85 176 4 13 20/80 214 2 37 25/75 201 9 29 30/70 189 14 21 40/60 182 5 17 Sm.sub.2(CoFeCuZr).sub.17 0/100 117 (120*) 3 10/90 159 10 36 15/85 161 17 38 20/80 175 20 50 30/70 185 25 58 40/60 202 10 73 Note: Values marked as * were from the commercial SmCo magnets (http://www.electronenergy.com/products/materials).

(31) TABLE-US-00002 TABLE 2 Magnetic properties and density of sintered 2:17 type and 1:5 type SmCo magnets made from feedstock of (100 x) wt. % 2:17 or 1:5 type jet milled (JM) powders + x wt. % 2:17 or 1:5 type cryomilled in liquid nitrogen (CM) powders, x = 0, 10, 15, 20, 25, 30, 40. Remanence-B.sub.r, intrinsic coercivity-H.sub.ci, maximum energy product-(BH).sub.max, value of H.sub.c at 0.9B.sub.r- H.sub.k, coercivity-H.sub.c, density-. x/(100 x) B.sub.r H.sub.ci (BH).sub.max H.sub.k H.sub.c Samples ratio (kG) (kOe) (MGOe) (kOe) (kOe) (g/cc) SmCo.sub.5 0/100 8.7 >24 19 22.0 8.6 8.1 10/90 9.1 >24 21 21.3 9.0 8.4 15/85 9.1 >23 21 20.9 9.0 8.3 20/80 9.3 >23 21 20.7 9.1 8.3 25/75 9.4 >23 22 20.2 9.3 8.5 30/70 9.4 >23 22 15.5 9.2 8.5 40/60 9.4 18.7 22 12.6 9.1 8.5 Sm.sub.2(CoFeCuZr).sub.17 0/100 10.6 >24 26 12.0 9.8 8.4 10/90 10.6 >24 26 11.5 9.7 8.4 15/85 10.6 >24 26 10.1 9.6 8.4 20/80 10.6 >24 26 8.8 9.5 8.4 25/75 10.6 >24 25 7.6 9.3 8.4 30/70 10.6 >24 24 5.9 9.0 8.4 40/60 10.9 >23 24 4.2 8.7 8.4

(32) TABLE-US-00003 TABLE 3 Mean grain sizes of the fine grain (FG) areas and the coarse grain (CG) matrix, and overall mean gain sizes for the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100 x) wt. % jet milled (JM) powders + x wt. % (x = 0, 10, 15, 20, 30, 40 wt. %) cryomilled powders. Mean Mean Overall CM/JM grain grain mean powder size from size from grain weight ratio FG areas CG matrix size D Samples x/(100 x) % (m) (m) (m) Sm.sub.2(CoFeCuZr)17 0/100 40 40 10/90 14 29 28 15/85 12 26 24 20/80 8 25 22 30/70 6 24 19 40/60 5 12 9

(33) From Table 3, it is apparent that practice of embodiments of the invention can produce fine grain (FG) areas that have an average (mean) grain size in the range of about 5 to about 15 m, while the associated CG matrix can have an average (mean) grain size in the range of about 10 to about 30 m wherein the average grain size of the FG areas can be about 25% to about 50% of the average grain size of the CG matrix. Practice of the invention is not limited to the particular permanent magnet materials set forth above and can include, but is not limited to, samarium-cobalt type (SmCo.sub.5 and Sm.sub.2Co.sub.17 types) magnets, neodymium-iron-boron type (Nd.sub.2Fe.sub.14B type) magnets, neodymium-iron-carbon type magnets (R.sub.2Fe.sub.14C type, R=rare earth, La or yttrium), R-iron-nitrogen type magnets (R.sub.2Fe.sub.17.sub. type, R=rare earth, La or Y; X=H, C, and/or N), or R-iron-M-nitrogen type magnets (R(Fe, M).sub.12.sub. type, R=rare earth, La or Y; M=Mo, V, Ti, Si, Al, Cr, Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N). The permanent magnet materials also can comprise a stable or metastable rare-earth-transition metal based magnetic compound, having the formula of R.sub.2TM.sub.14A, RTM.sub.5, RT.sub.2M.sub.17, R.sub.2TM.sub.17A, RTM.sub.7, RTM.sub.7A, RTM.sub.12, RTM.sub.12A, R.sub.3TM.sub.29, and R.sub.3TM.sub.29A, wherein R is one or a combination of rare earths, La or yttrium, TM is one or a mixture of transition metals, and A is one or a combination of the following elements: Be, B, C, N, S, Mg, Al, Si, P, Ga, Ge, As, Se, In, Sn, Sb, Te, I, Pb, and Bi.

(34) References incorporated herein by reference: E. P. Wohlfarth, K. H. J. Buschow, Ferromagnetic materials, North Holland, 1988. http://www.electronenergy.com/products/materials. J. F. Liu, P. Vora, M. H. Walmer, E. Kottcamp, S. A. Bauser, A. Higgins, and S. Liu, Journal of Applied Physics, 97 (2005) 10H101. W. Li, A. H. Li, H. J. Wang, W. Pan, H. W. Chang, Journal of Applied Physics, 105 (2009) 07A703. S. Q. Liu, J. F. Liu, US Patent Pub. No.: US 2005/0081960 A1 X. Y. Li, K. Lu, Nature Materials 16 (2017) 700. X. L. Wu, M. X. Yang, F. P. Yuan, G. L. Wu, Y. J. Wei, X. X. Huang, and Y. T. Zhu, Proceedings of the National Academy of Sciences of the United States of America, 112 (2015) 14501. X. L. Wu, P. Jiang, L. Chen, F. P. Yuan, and Y. T. Zhu, Proceedings of the National Academy of Sciences of the United States of America, 111 (2014) 7197. Y. M. Wang, M. W. Chen, F. H. Zhou, E. Ma, Nature 419 (2002) 912. P. F. Cesar, H. N. Yoshimura, W. G. Jr Miranda, C. L. Miyazaki, L. M. Muta, L. E. Rodrigues Filho, Journal of Biomedical Materials Research Part B Applied Biomaterials 78 (2006) 265.

(35) While the exemplary embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention as set forth in the following claims.