Fine grain structures for tough rare earth permanent magnets

Abstract

The present invention provides fine grain structures for rare earth permanent magnets (REPMs) and their production in a manner to significantly enhance flexural strength and fracture toughness of the magnets with no or little sacrifice in the hard magnetic properties. The tough REPMs can have either homogeneous or heterogeneous refined grain microstructural architectures achieved by introducing a small amount of additive particle materials into the magnet matrix, such as fine-sized, insoluble, chemically stable, and non-reactive with the magnet matrix. These additive materials can act effectively as both heterogeneous nuclei sites and grain growth inhibitors during the heat treatment processes, which in turn resulting in refined grain structures of the REPMs. Alternatively, the fine grain structures were also achieved by using magnet alloy feedstock powders with finer particle sizes. The fine grains acting as the strengthening sites can inhibit the crack nucleation and can also slow down the propagation of micro-cracks, which in turn increasing magnet's fracture toughness.

Claims

1. A rare earth permanent magnet having at least one region having a sufficiently refined grain microstructure that improves a mechanical toughness and/or a mechanical strength property of the magnet.

2. The magnet of claim 1 that exhibits a flexural strength increase of 30% or greater at 20 C. with no or little reduction of (BH).sub.max, B.sub.r and H.sub.ci magnetic properties.

3. The magnet of claim 1 wherein the magnet has a sufficiently refined grain structure comprising a homogenous single-modal grain size distribution.

4. The magnet of claim 1 wherein the magnet has a heterogeneous grain structure that comprises regions having relatively fine grain size and coarser grain size wherein at least one of the regions has the sufficiently refined grain microstructure.

5. The magnet of claim 1 comprising R-cobalt type (mainly including RCo.sub.5 and R.sub.2Co.sub.17 types, R=rare earth, Lanthanum, or Yttrium) magnets, R-iron-boron type (R.sub.2Fe.sub.14B type or R-TM-B, TM is selected from a group of transition metals consisting essentially of Fe, Co and other transition metal elements) magnets, a R-TM-carbon type magnet (R.sub.2Fe.sub.14C type), a R-TM-nitrogen type magnet (R.sub.2Fe.sub.17X.sub.5 type, R=rare earth, La, or Y; XH, C, N, B, F, P, and/or S), or a, R-TM-M-nitrogen type magnet (R(Fe, M).sub.12X.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; XH, C, N, B, F, P, and/or S), and other stable or metastable rare-earth-transition metal based magnetic compounds.

6. The magnet of claim 1 comprising a stable or metastable rare-earth-transition metal based magnetic compounds, 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 Y, TM is one or a mixture of transition metals, 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.

7. The magnet of claim 1 which comprises consolidated powders.

8. The magnet of claim 1 which is sintered.

9. Feedstock comprising a mixture of rare earth-bearing powders and a percentage of additive particle material wherein the additive particle material includes at least one of metal carbides, fluorides, nitrides, oxides, sulfides, and combinations of any of these materials.

10. The feedstock of claim 9 wherein morphology of the additive particle material includes at least one of particles, fibers, rods, tubes, dendrites, whiskers, mesoporous structures in either nano-, submicron-, and/or micron-scales, and combinations of any of these material morphologies and the mixture forms a homogeneous or heterogeneous refined grain structure when the mixture is consolidated as a rare earth permanent magnet.

11. The feedstock of claim 9 wherein the metal carbide particle material includes at least one of: BC, BaC, BeC, AlC, CaC, CeC, CrC, FeC, LaC, LiC, MoC, SiC, TiC, WC, YC, ZrC, etc., and a combination of any of these materials; wherein the fluoride particle material includes at least one of: AlF, BF, BaF, BiF, CaF, CeF, CrF, CoF, CuF, DyF, FeF, HfF, LaF, MoF, NdF, NbF, PF, SmF, TiF, VF, WF, ZnF, ZrF, and a combination of any of these materials; wherein the nitride particle material includes at least one of: AlN, BN, LiN, CuN, FeN, MN (M=alkaline-earth metals: Ba, Be, Ca, Mg, Sr, Ra), NaN, NbN, PN, SN, TaN, TiN, WN, VN, YN, ZnN, ZrN, and a combination of any of these materials; wherein the oxide particle material includes at least one of: AlO, BaO, BO, BiO, CaO, CeO, CoO, CrO, CuO, DyO, ErO, FeO, GaO, HfO, InO, LaO, LiO, MgO, MnO, MoO, NaO, NdO, NiO, NbO, PrO, SiO, SmO, TaO, TiO, VO, WO, YO, ZnO, ZrO, and combinations of any of these materials; wherein the sulfide particle material includes at least one of: AlS, BaS, BeS, BiS, BS, CaS, CeS, CrS, CoS, CuS, DyS, ErS, GdS, GaS, GeS, FeS, HfS, LaS, LiS, MgS, MnS, MoS, NaS, NdS, NiS, KS, PrS, SmS, SiS, SnS, TiS, WS, YS, ZnS, ZrS, and a combination of any of these materials.

12. The feedstock of claim 10 for making the rare earth permanent magnet that is selected from R-cobalt type (mainly including RCo.sub.5 and R.sub.2Co.sub.17 types, R=rare earth, Lanthanum or Yttrium) magnets, R-iron-boron type (R.sub.2Fe.sub.14B type or R-TM-B, TM is selected from a group of transition metals consisting essentially of Fe, Co and other transition metal elements) magnets, a R-TM-carbon type magnet (R.sub.2Fe.sub.14C type), a R-TM-nitrogen type magnet (R.sub.2Fe.sub.17X.sub. type, R=rare earth, La or Y; XH, C, N, B, F, P, and/or S), or a, R-TM-M-nitrogen type magnet (R(Fe, M).sub.12X.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; XH, C, N, B, F, P, and S), and other stable or metastable rare-earth-transition metal based magnetic compounds, 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 Y, TM is one or a mixture of transition metals, 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.

13. The feedstock of claim 9 that includes relatively smaller additive particles wherein the relatively smaller additive particles are cryomilled in liquid nitrogen for a time using micro-sized jet milled particle powders as the precursor powders.

14. The method of claim 9 that includes relatively smaller additive particles that are blended with commercial jet-milled fine powders present in an amount greater than 90% to 99.9% by weight of the mixture.

15. The method of claim 14 wherein the blending is conducted in argon, nitrogen, or other inert or non-reactive gases for a time from greater than 0 to 10 hrs or more blending time.

16. Feedstock comprising fine magnet alloy powders having an average particle size of 0.1 micron to less than 1.5 microns wherein the alloy powders comprises 100% of said feedstock.

17. The method of claim 16 wherein the magnet alloy powders are prepared by a method that includes at least one of multiple-cycle jet milling in nitrogen gas, low or high energy ball milling at room temperature in inert gas (Ar, N.sub.2, or He) or in solvent media (acetone, ethanol, hexane, heptane, toluene, etc.), surfactant-assisted high energy ball milling at room temperature or immersed in the liquid nitrogen, inert gas atomization, gas condensation, spark erosion, chemical precipitation, sol-gel, pyrolysis and hydrothermal synthesis, thermal decomposition, plasma arcing, chemical reduction or oxidization, gas-solid reaction, vapor-liquid-solid (VLS) process, carburizing, carbonitriding, nitriding, chemical vapor deposition (CVD), physical vapor deposition (PVD), hydrogen decrepitation (HD), hydrogen decrepitation deabsorbation recombination (HDDR) process, severe plastic deformation (SPD), electrodeposition, colloidal lithography, and atomic layer deposition (ALD).

18. A method of producing a rare earth permanent magnet that possesses flexural strength increased by 30% or above at room temperature (20 C.), said method comprising the steps of: preparing a feedstock comprising rare earth containing powders alone or mixed with additive particles, and consolidating the feedstock to form a rare earth permanent magnet having at least one region with a sufficiently refined grain microstructure to increase the flexure strength of the magnet.

19. The method of claim 18 that produces a microstructure having a homogeneous grain structure of the sufficiently refined grain microstructure structure or a heterogeneous grain structure that comprises regions having different relatively finer grain size and coarser grain size wherein at least one of the regions has the sufficiently refined grain microstructure.

20. The method of claim 18 wherein the consolidating step includes at least one of powder metallurgy processing, hot pressing, die-upset, friction consolidation extrusion, hot 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 shows morphology (SEM images) and corresponding particle size distribution of Sm.sub.2O.sub.3 submicron powders made by cryomilling in immersed LN.sub.2 for 2 hrs (5000, shown in the left image), according to one embodiment of the invention. The right column is the corresponding enlarged image (10,000) from the selected area (marked by the red oval). The average particle size was about 0.35 m. These and other particle size measurements and results described herein were obtained from the SEM images of the particles analyzed by the Image J software.

[0025] FIG. 2 shows morphology (SEM images) and corresponding particle size distributions of Sm.sub.2Co.sub.17 type Sm.sub.2(CoFeCuZr).sub.17 jet milled powder (1500, shown in top row) and further cryomilled powder in LN.sub.2 for 2 hrs (shown in bottom row), according to one embodiment of the invention. The right column is the corresponding enlarged images (5000) from the selected areas (marked by the red ovals). The average particle sizes were about 2.3 m and 1.3 m for the jet milled powder and cryomilled powder, respectively.

[0026] FIG. 3 presents typical flexural stress-strain curves for selected Sm.sub.2O.sub.3added Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled powders+x wt. % (x=0, 1, 3) Sm.sub.2O.sub.3 cryomilled submicron powders, according to an embodiment of the process of the invention. The highest flexural strength value of about 199 MPa was achieved for selected specimen with the addition of x=3 wt. % Sm.sub.2O.sub.3 submicron particles that was enhanced by about 75%.

[0027] FIG. 4 presents demagnetization curves of selected Sm.sub.2O.sub.3added Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled powders+x wt. % (x=0, 1, 3) Sm.sub.2O.sub.3 cryomilled submicron powders, according to an embodiment of the process of the invention.

[0028] FIG. 5 illustrates typical flexural stress-strain curves for laminated coarse/fine/coarse grain Sm.sub.2(CoFeCuZr).sub.17 sintered magnets (ie. laminated magnet) made from the feedstock of a well-mixed mixture of 97 wt. % jet milled powders+3 wt. % Sm.sub.2O.sub.3 cryomilled submicron powders as 30 wt. % of the magnet that was put in the middle region of magnet while 35 wt. % of jet milled powder was put at each of both side regions of the magnet, according to an embodiment of the process of the invention. As a comparison, a typical flexural stress-strain curve of commercial type magnet (ie. reference magnet) with a single modal coarse grain distribution made from 100 wt. % jet milled powder was also shown in FIG. 5.

[0029] FIG. 6 illustrates typical demagnetization curve for laminated coarse/fine/coarse grain Sm.sub.2(CoFeCuZr).sub.17 sintered magnets (ie. laminated magnet) made from the feedstock of a well-mixed mixture of 97 wt. % jet milled powders+3 wt. % Sm.sub.2O.sub.3 cryomilled submicron powders as 30 wt. % of the magnet that was put in the middle region of magnet, while 35 wt. % of jet milled powder was put at each of both side regions of the magnet, according to an embodiment of the process of the invention. As a comparison, a typical demagnetization curve of commercial type magnet (ie. reference magnet) made from 100 wt. % jet milled powder was also shown in FIG. 6.

[0030] FIG. 7 shows typical flexural stress-strain curve of refined grain Sm.sub.2(CoFeCuZr).sub.17 magnet (ie. refined grain magnet) made from 100 wt. % finer particle powder cryomilled in LN.sub.2 for 2 hrs, according to an embodiment of the process of the invention. The highest flexural strength value of 244 MPa was achieved in this specimen that was enhanced by about 114%. As a comparison, a typical flexural stress-strain curve of commercial type magnet (ie. reference magnet) made from 100 wt. % jet milled powder with a flexural strength value of 114 MPa was also shown in FIG. 7.

[0031] FIG. 8 shows demagnetization curves of refined grain Sm.sub.2(CoFeCuZr).sub.17 magnet (i.e. refined grain magnet) made from 100 wt. % cryomilled powder in LN.sub.2 for 2 hrs and the commercial type magnet made from 100 wt. % jet milled powder, according to an embodiment of the process of the invention. As a comparison, a typical demagnetization curve of commercial type magnet (ie. reference magnet) made from 100 wt. % jet milled powder was also shown in FIG. 8.

[0032] FIG. 9 shows morphology (optical photomicrographs, 100, shown in the top left images) and corresponding grain size distribution (bottom images) of cross-section microstructures of selected Sm.sub.2O.sub.3added Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled powders+x wt. % (x=0, 3) Sm.sub.2O.sub.3 cryomilled submicron particles. The right column is the corresponding enlarged images (400) from the selected areas (marked by the red ovals). The average grain size was about 22 m for the magnet with the addition of 3 wt. % Sm.sub.2O.sub.3 submicron particles. Whereas, the average grain size of the commercial counterpart magnet (x=0 wt. %) was about 45 m. Both magnets have a single-modal grain size distribution. The specimens were mechanically polished then etched with 2% nital etchant for the metallographic examination. Grain size set forth herein and elsewhere in the specification were measured from optical images of the respective microstructures analyzed by Image J software.

[0033] FIG. 10 shows morphology of cross-section microstructures (optical photomicrographs, 100, shown in the top image) and corresponding grain size distribution (bottom images) from selected fine and coarse grain areas (marked by the red ovals) of the laminated coarse/fine/coarse grain Sm.sub.2(CoFeCuZr).sub.17 sintered magnet. The middle column is the corresponding enlarged images (1000) from the selected coarse and fine areas (marked by the red ovals), respectively. The magnet was made from the feedstock of a well-mixed mixture of 97 wt. % jet milled powders+3 wt. % Sm.sub.2O.sub.3 cryomilled submicron powders as 30 wt. % of the magnet that was put in the middle region of magnet while 35 wt. % of jet milled powder was put at each of both side regions of the magnet. Novel laminated coarse/fine/coarse grain microstructural architecture was formed in this magnet. The finer average grain size in the magnet central part was about 22 m. The two coarse grain side regions had an average grain size of about 34 m. Whereas, the average grain size of the commercial counterpart magnet was about 45 m as shown in FIG. 9.

[0034] FIG. 11 shows morphology (optical photomicrographs, 100, shown in the left image) and corresponding grain size distribution of cross-section microstructures of refined grain Sm.sub.2(CoFeCuZr).sub.17 magnet made from 100 wt. % cryomilled powder in LN.sub.2 for 2 hrs. The right column is the corresponding enlarged image (400) from the selected area (marked by the red oval). The average grain size of the refined grain magnets was about 15 m with a single-modal grain size distribution. The specimen was mechanically polished then etched with 2% nital etchant for the metallographic examination.

[0035] FIG. 12 shows morphology (optical photomicrographs, 1000) and particle size distribution of Sm.sub.2O.sub.3 microparticles (in gray color) for the Sm.sub.2(CoFeCuZr).sub.17 refined grain magnet (shown in the bottom image) and commercial reference magnet (shown in the top image) those also shown in FIGS. 9 and 11, respectively. These particle size results were obtained from the optical photomicrographs of the particles analyzed by the Image J software.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Embodiments of the present invention relates to rare earth permanent magnets (REPMs) having a sufficiently refined grain microstructure 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, and the method of their manufacture. Embodiments of the present invention can be employed to make REPMs that include, but are not limited to, SmCo, NdFeB and other REPMs.

[0037] The REPMs made pursuant to certain embodiments of the invention have refined homogeneous or heterogeneous grain microstructures. To increase flexural strength and/or fracture toughness of the REPMs, the sufficiently refine grain structures were achieved in one embodiment by introducing a small amount of fine-sized, insoluble, chemically stable, and non-reactive additive particle material into the magnet matrix, such as, carbides, fluorides, nitrides, oxides, sulfides, and/or their mixtures, or, alternatively, by using feedstock powders with finer particle sizes than that of conventional ones.

[0038] For purposes of the present invention, the carbide-based additive particle material can include, but is not limited to: B.sub.4C, BC.sub.3, BaC.sub.2, Be.sub.2C, Al.sub.4C.sub.3, CaC.sub.2, CeC.sub.2, Cr.sub.3C.sub.2, Cr.sub.4C, Cr.sub.8C.sub.2, Fe.sub.3C, LaC.sub.2, Li.sub.2C.sub.2, Mo.sub.2C, MoC, Mn.sub.3C, SiC, SrC.sub.2, TaC, ThC, TiC, U.sub.2C.sub.3, WC, (W,Ti)C, W.sub.2C, YC.sub.2, ZrC, ZrC.sub.2, or a combination of any of these materials. The fluoride-based additive particle material can include, but is not limited to: AcF.sub.3, AlF.sub.3, AuF.sub.7, AuF.sub.3, AuF.sub.5, AuF, BaF.sub.2, BeF.sub.2, BiF.sub.5, BiF.sub.3, BF, BF.sub.3, CdF.sub.2, CaF.sub.2, CeF.sub.3, CrF.sub.6, CrF.sub.5, CrF.sub.4, CrF.sub.3, CrF.sub.2, CoF.sub.2, CoF.sub.3, CuF, CuF.sub.2, DyF.sub.3, GaF.sub.3, FeF.sub.2, FeF.sub.3, GeF.sub.2, HfF.sub.4, InF.sub.3, IrF.sub.6, KAlF.sub.4, K.sub.3CuF.sub.6, K.sub.2NiF.sub.6, LaF.sub.3, LiF, LiBeF, LiNaKF, MgF.sub.2, MnF.sub.2, MnF.sub.3, MnF.sub.4, Hg.sub.2F.sub.2, HgF.sub.2, HgF.sub.4, MoF.sub.6, MoF.sub.4, MoF.sub.4, NdF.sub.3, NiF.sub.2, NbF.sub.4, NbF.sub.5, OsF.sub.6, OsF.sub.5, PF.sub.5, PF.sub.3, PbF.sub.2, PbF.sub.4, PdF.sub.2, PdF.sub.4, PdF.sub.3, PtF.sub.6, PtF.sub.5, PtF.sub.4, ReF.sub.7, ReF.sub.6, RhF.sub.3, RuF.sub.6, SbF.sub.5, SbF.sub.3, SmF.sub.3, SnF.sub.2, SnF.sub.4, TaF.sub.5, TeF.sub.6, TeF.sub.4, TiF4, TiF.sub.3, TiF.sub.3, UF.sub.4, VF.sub.5, VF.sub.4, VF.sub.3, WF.sub.6, WF.sub.5, WF.sub.4, WOF.sub.4, YbF.sub.3, ZnF.sub.2, ZrF.sub.4, or a combination of any of these additive materials. The nitride-based additive particle material can include, but not limited to: AlN, Ag.sub.3N, BN, Li.sub.3N, (CN).sub.2, Cu.sub.3N, Fe.sub.2N, Fe.sub.4N, Fe.sub.16N.sub.2, GaN, Hg.sub.3N.sub.2, InN, M.sub.3N.sub.2 (M=alkaline-earth metals: Ba, Be, Ca, Mg, Sr, Ra), Na.sub.3N, NbN, P.sub.3N.sub.5, S.sub.2N.sub.2, S.sub.4N.sub.4, ScN, Se.sub.4N.sub.4, Si.sub.3N.sub.4, TaN, Ta.sub.2N, Ta.sub.3N.sub.5, TiN, TiAlN, TiCN, Tl.sub.3N, W.sub.2N, WN, WN.sub.2, YN, VN, Zn.sub.3N.sub.2, ZrN, or a combination of any of these additive materials. The oxide-based additive particle material can include, but is not limited to: Ac.sub.2O.sub.3, Ag.sub.2O, Al.sub.2O.sub.3, Al.sub.1813.sub.4O.sub.33, Al.sub.6BeO.sub.10, Al.sub.2MgO.sub.4, Au.sub.2O, Au.sub.2O.sub.3, BaO, BeO, Bi.sub.2O.sub.3, Bi.sub.2O.sub.5, B.sub.2O.sub.3, CaO, Ce.sub.2O.sub.3, CeO.sub.2, CdO, COO, CrO, Cr.sub.2O.sub.3, CrO.sub.2, CrO.sub.3, Co.sub.2O.sub.3, Cs.sub.2O, Cu.sub.2O.sub.5Yb.sub.2, CuFe.sub.2O.sub.4, Cu.sub.2O, CuO, Dy.sub.2O.sub.3, Er.sub.2O.sub.3, Eu.sub.2O.sub.3, Fr.sub.2O, Gd.sub.2O.sub.3, GaO, Ga.sub.2O.sub.3, GeO, GeO.sub.2, HfO.sub.2, In.sub.2O, InO, In.sub.2O.sub.3, Ir.sub.2O.sub.3, Fe.sub.3O.sub.4, FeO, Fe.sub.2O.sub.3, Hg.sub.2O, HgO, K.sub.2O, K.sub.2MnO.sub.4, K.sub.2Ti.sub.6O.sub.13, La.sub.2O.sub.3, Li.sub.2O, Lu.sub.2O.sub.3, Mg.sub.2B.sub.2O.sub.5, MgO, Mn.sub.3O.sub.4, MnO, Mn.sub.2O.sub.3, MnO.sub.2, Mn.sub.2O.sub.5, Mn.sub.2O.sub.7, MoO.sub.2, Mo.sub.2O.sub.5, Na.sub.2O, Nd.sub.2O.sub.3, NiFe.sub.2O.sub.4, NiO, Ni.sub.2O.sub.3, Nb.sub.2O.sub.3, Os.sub.2O.sub.3, OsO.sub.3, OsO.sub.4, PbO, PbO.sub.2, PdO, PdO.sub.2, Pr.sub.6O.sub.11, Pt.sub.3O.sub.4, PtO, Pt.sub.2O.sub.3, PuO.sub.2, Pu.sub.2O.sub.5, RaO, Rh.sub.2O.sub.3, Rb.sub.2O, RuO.sub.2, RuO.sub.4, Sb.sub.2O.sub.5, Sc.sub.2O.sub.3, Se.sub.3O.sub.4, SiO.sub.2, Sm.sub.2O.sub.3, SnO, SnO.sub.2, SrO, Ta.sub.2O.sub.3, Ta.sub.2O.sub.5, Tb.sub.4O.sub.7, TcO, TiO, Ti.sub.2O.sub.3, TiO.sub.2, Tl.sub.2O.sub.3, Tm.sub.2O.sub.3, U.sub.2O.sub.5, VO, V.sub.2O.sub.3, VO.sub.2, V.sub.2O.sub.5, VOCl.sub.2, Yb.sub.2O.sub.3, WCl.sub.2O.sub.2, W.sub.2O.sub.3, WO.sub.2, W.sub.2O.sub.5, YBa.sub.2Cu.sub.3O.sub.7, Y.sub.2O.sub.3, ZnO, ZrO.sub.2, or combinations of any of these additive materials. The sulfide-based additive particle material can include, but is not limited to: Al.sub.2S.sub.3, Ag.sub.2S, As.sub.2S.sub.3, BaS, BeS, Bi.sub.2S.sub.3, B.sub.2S.sub.3, CdS, CaS, CeS, Ce.sub.2S.sub.3, Cr.sub.2S.sub.3, CoS, CoS.sub.2, Cu.sub.2S, CuS, Dy.sub.2S.sub.3, Er.sub.2S.sub.3, EuS, Gd.sub.2S.sub.3, Ga.sub.2S.sub.3, GeS, GeS.sub.2, HfS.sub.2, Ho.sub.2S.sub.3, In.sub.2S, InS, FeS, FeS.sub.2, La.sub.2S.sub.3, LaS.sub.2, La.sub.2O.sub.2S, Li.sub.2S, MgS, MnS, HgS, MoS.sub.2, Na.sub.2S, Nd.sub.2S.sub.3, NiS, NdS, K.sub.2S, PbS,

[0039] Pr.sub.2S.sub.3, Sb.sub.2S.sub.3, Sm.sub.2S.sub.3, Sc.sub.2S.sub.3, SiS.sub.2, SnS, SnS.sub.2, SrS, Tb.sub.2S, ThS.sub.2, Tm.sub.2S.sub.3, TiS.sub.2, US.sub.2, V.sub.25.sub.3, WS, WS.sub.2, Yb.sub.2S.sub.3, Y.sub.2S.sub.3, Y.sub.2O.sub.2S, ZnS, and ZrS.sub.2, or a combination of any of these additive materials.

[0040] For purposes of illustration and not limitation, one exemplary feedstock comprises of 99.5-90 wt. % commercial jet-milled SmCo or NdFeB microparticle powders (average particle size of about 2.3-5 m), and 0.5-10 wt. % fine-sized additive particle material comprising for example, Sm.sub.2O.sub.3 or Nd.sub.2O.sub.3 particles with an average particle size of about 0.3-1 m wherein the particle size measurements described here and elsewhere in this application were obtained from the SEM images of the particles analyzed by the Image J software. As a result, practice of the invention can be easily integrated with the current industry production line for sintered REPMs.

[0041] In certain embodiments, the powders can be mixed together under nitrogen, argon or other non-reactive atmosphere in a mixer or mill for greater than 0 to 1 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, hot 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 refined grain microstructures 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, H.sub.ci and B.sub.r. The tough REPMs will be more robust for energy applications and can be less dependent on the critical element resources.

[0042] The following examples are offered for purposes of further illustration, but not limitation, with respect to the present invention:

[0043] Dry samarium (III) oxide powder (Sm.sub.2O.sub.3, Alfa Aesar, REacton, 99.99% (REO), Stock No. 11230-14) after cryomilled for 2 hrs in LN.sub.2 with a SPEX 6875 Freezer/Mill were composed of fine irregular particles with a particle size mainly in the range of about 0.1-0.5 m and an average particle size of about 0.35 m and few-sharp edges, as shown in FIG. 1. The dry samarium oxide microparticle powders to be cryomilled were first sealed in a polycarbonate grinding vial under either gaseous Ar or N.sub.2 atmosphere inside of a glove box, an Ar glove box being used for the examples. The entire cryomilling process was conducted with the grinding vial immersed in liquid nitrogen (LN.sub.2) on the SPEX 6875D Freezer/Mill, which has a LN.sub.2 tank that is connected to a dewar container as a continuous LN.sub.2 source. The cryomilling cycle sequence was cryomilling for 10 minutes and then pausing for 2 minutes to cool down the powders. Ten (10) such cryomilling cycle sequences were applied in this and the other examples herein, the total multiple-cycle cryomilling time being 2 hours excluding the pause times.

[0044] The overall particle size range of the Sm.sub.2O.sub.3 cryomilled powders was within about 0.05-1 m. The Sm.sub.2Co.sub.17 type Sm.sub.2(CoFeCuZr).sub.17 conventional jet milled powders were composed of irregular microparticles with a particle size mainly in the range of about 0.7-3.0 m and an average particle size of about 2.3 m, as shown in FIG. 2. The overall particle size range of the jet milled powders was within about 0.7-8 m. However, 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 an average particle size of about 1.3 m and few sharp edges. The overall particle size range of the cryomilled powders was within about 0.5-8 m (see FIG. 2). These particle size results were obtained from the SEM images analyzed by image J software. Both the jet milled microparticles and the further cryomilled finer SmCo particles had a single-crystal structure.

[0045] 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 commercial jet-milled powders (that have a typical average particle size of about 2.3-5 micron), include but are not limited to, some top-down and bottom-up approaches, such as, multiple-cycle jet milling in nitrogen (N.sub.2) gas atmosphere, low or high energy ball milling at room temperature in inert gas (Ar, N.sub.2, or He) or in solvent media (acetone, ethanol, hexane, heptane, toluene, etc.), surfactant-assisted high energy ball milling at room temperature or immersed in the liquid nitrogen, inert gas atomization, gas condensation, spark erosion, chemical precipitation, sol-gel, pyrolysis and hydrothermal synthesis, thermal decomposition, plasma arcing, chemical reduction or oxidization, gas-solid reaction, vapor-liquid-solid (VLS) process, carburizing, carbonitriding, nitriding, chemical vapor deposition (CVD), physical vapor deposition (PVD), hydrogen decrepitation (HD), hydrogen decrepitation deabsorbation recombination (HDDR) process, severe plastic deformation (SPD), electrodeposition, colloidal lithography, atomic layer deposition (ALD), etc.

[0046] The cryomilled Sm.sub.2O.sub.3 sub-micron powders produced in this invention were then mixed with the jet milled precursor powders under a nitrogen atmosphere in a SPEX 8000M Mixer/Mill without any milling balls for a time of 7 minutes, which more generally can be up to 15 minutes or more or other suitable blending time. The particle mixtures then are subjected to conventional powder metallurgy method (i.e. pressing to form a compact, sintering the pressed compact followed by solution heat treating, tempering, and aging) to produce a bulk magnet with grain size and grain boundary engineering or modified microstructural architectures.

[0047] 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, hot 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 (HD) or crushing into coarse powders of about 200-500 m 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) or higher using a Nikisso CL15-45-30 iso-static press, and subsequent heat treatment procedure, including sintering, solution, temper, and aging.

[0048] In the examples and results described above and 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 Sm2(CoFeCuZr)17 were sintered at 1190-1250 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.

[0049] FIG. 3 shows the typical flexural stress-strain curves for selected Sm.sub.2O.sub.3-added Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % jet milled powders+x wt. % (x=0, 1, 3) Sm.sub.2O.sub.3 cryomilled submicron powders. By engineering the grain size and grain-boundary microstructure, the average flexural strength values of the sintered Sm.sub.2Co.sub.17 type magnets were enhanced by about 30% and 62% (about 148 MPa and 185 MPa for the samples with x=1 and 3 wt. %, respectively) compared to that of 114 MPa for the reference sample with x=0. Whereas, the highest flexural strength value of about 199 MPa was achieved for selected specimens with the addition of x=3 wt. % Sm.sub.2O.sub.3 submicron particles that was enhanced by about 75%. Flexural strength values 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.

[0050] Demagnetization curves are shown in FIG. 4 for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of (100-x) wt. % 2:17 type jet milled (JM) powders+x wt. % (x=0, 1, 3) Sm.sub.2O.sub.3 cryomilled submicron particle powders. Excellent hard magnetic properties were maintained with the maximum energy product (BH).sub.max values of about 26 MGOe, 26 MGOe, and 24.5 MGOe for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets added with x=0, 1, and 3 wt. % Sm.sub.2O.sub.3 submicron particles, respectively. (BH).sub.max decreased (about 24.5 MGOe) by only about 5.8% (less than 6%) for the magnet with the addition of x=3 wt. % Sm.sub.2O.sub.3 while no decrease of (BH).sub.max for the magnet with the addition of 1 wt. % Sm.sub.2O.sub.3. There was almost no decrease of remanence B.sub.r values, those were about 10.6 kGs, 10.6 kGs, and 10.5 kGs for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets with the addition of 0, 1, 3 wt. % Sm.sub.2O.sub.3 submicron particles, respectively. The intrinsic coercivity H.sub.ci values were about 32.7 kOe, 32.9 kOe, 33.4 kOe for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets with the addition of 0, 1, 3 wt. % Sm.sub.2O.sub.3 submicron particles, respectively. The H.sub.ci values were slightly increased for the Sm.sub.2O.sub.3added magnets due to the grain refinement with the addition of Sm.sub.2O.sub.3 submicron particles. The density of these Sm.sub.2(CoFeCuZr).sub.17 sintered magnets with the addition of 0, 1, 3 wt. % Sm.sub.2O.sub.3 was about 8.4 g/cc, which was about 99% of the theoretical value.

[0051] FIG. 5 shows typical flexural stress-strain curves for laminated coarse/fine/coarse grain Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from the feedstock of a well-mixed mixture of 97 wt. % jet milled powders+3 wt. % Sm.sub.2O.sub.3 cryomilled submicron particle powders as 30 wt. % of the magnet that was put in the middle region of magnet. Whereas, 35 wt. % of 100% jet milled powder was put at each of both side regions of the magnet. By engineering the grain size, grain-boundary and microstructural architecture, the average flexural strength values of the laminated Sm.sub.2(CoFeCuZr).sub.17 sintered magnets were enhanced by 62% (about 185 MPa) relative to a flexural strength value of 114 MPa for the commercial reference magnet.

[0052] FIG. 6 shows demagnetization curves of the laminated Sm.sub.2(CoFeCuZr).sub.17 sintered magnets. Excellent magnetic properties were maintained in the laminated magnet. The values of (BH).sub.max B.sub.r and H.sub.ci of the laminate magnet and commercial type reference magnet were, 26 MGOe; 26MGOe; 10.6 kGs, 10.6 kGs; 34.0 kOe, 32.7 kOe, respectively. There was no decrease of (BH).sub.max and B.sub.r values while an even slightly increased H.sub.ci by 4% for the laminated magnet made with the modified microstructures pursuant to the invention compared with those of the commercial counterpart magnet. These Sm.sub.2(CoFeCuZr).sub.17 sintered magnets have a density of about 8.4 g/cc, which was about 99% of the theoretical value.

[0053] FIG. 7 shows typical flexural stress-strain curves of refined grain Sm.sub.2(CoFeCuZr).sub.17 magnet made from 100 wt. % finer particle powder cryomilled in LN.sub.2 for 2 hrs and the commercial type magnet made from 100 wt. % jet milled powder. By engineering the grain size and grain-boundary microstructure, the average flexural strength value of Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % cryomilled powder was enhanced by about 96% (about 223 MPa) while the highest flexural strength value of about 244 MPa was achieved for the selected specimen, which was enhanced by about 114%, compared to 114 MPa for the commercial counterpart sample made from 100 wt. % jet milled powder.

[0054] FIG. 8 shows demagnetization curves of the refined grain Sm.sub.2(CoFeCuZr).sub.17 magnet made from 100 wt. % finer powder cryomilled for 2 hrs. Excellent magnetic properties were maintained with the maximum energy product (BH).sub.max (about 25 MGOe) decreased by only 3.8% (less than 4%) while with a degrade squareness on the 2.sup.nd quadrant of demagnetization curve while an increased B.sub.r value (about 10.9 kGs), and almost no decrease of H.sub.d value (about 32.2 kOe) respectively. These are relative to values of (BH).sub.max (about 26 MGOe), B.sub.r (about 10.6 kGs), and H.sub.ci (about 32.7 kOe) for the commercial reference magnet. Sm.sub.2(CoFeCuZr).sub.17 sintered magnet density was about 8.4 g/cc, which was about 99% of the theoretical value.

[0055] With respect to sintered microstructures, a typical single-modal coarse grain size structure with an average grain size of about 45 m was observed in the 2:17 type Sm.sub.2(CoFeCuZr).sub.17 commercial-type sintered magnet made from 100 wt. % 2:17 type jet milled powders, as shown in FIG. 9. In contrast, the SmCo sintered magnets made from the mixture feedstocks of Sm.sub.2O.sub.3submicron particles and jet milled magnet alloy powders pursuant to the invention have a refined single-modal grain size microstructure, as shown in FIG. 9. The average grain sizes of the Sm.sub.2O.sub.3added sintered magnets were about 32 m and 22 m for the magnets with the addition of 1 wt. % and 3 wt. % Sm.sub.2O.sub.3 submicron particles, respectively. These grain size results were obtained from the optical images of the microstructures analyzed by Image J software. This refined grain sized microstructure resulted in considerably higher flexural strength and higher fracture toughness, and comparable magnetic properties, compared with the commercial reference magnet with a single-modal coarse grain sized microstructure as shown in FIG. 9. The fine grains acting as the strengthening sites can inhibit the crack nucleation and also slow down the propagation of micro-cracks, which in turn increasing magnet's flexural strength and fracture toughness.

[0056] FIG. 10 shows morphology of cross-section microstructures and corresponding grain size distribution from selected fine and coarse regions of the laminated coarse/fine/coarse grain Sm.sub.2(CoFeCuZr).sub.17 sintered magnet. The magnet was made from the feedstock of a well-mixed mixture of 97 wt. % jet milled powders+3 wt. % Sm.sub.2O.sub.3 cryomilled submicron powders as 30 wt. % of the magnet that was put in the middle region of magnet. The 35 wt. % of jet milled powder was put at each of both side regions of the magnet. Novel laminated coarse/fine/coarse grain microstructural architecture was formed in this magnet. The finer average grain size in the magnet central (middle) region was about 22 m. The two coarse grain areas at each side region of the magnet had an average grain size of about 34 m. Whereas, the average grain size of the commercial counterpart magnet was about 45 m as shown in FIG. 9. The enhancement of flexural strength of the laminated SmCo sintered magnets resulted from the grain size refinement with the contributions from both localized finer grain central regions (about 30 wt. % of the magnet) and a general grain size reduction from the coarse grain matrix. The localized finer grain region is more effective in preventing or propagation via acting as strengthening sites than the coarser matrix.

[0057] As shown in FIGS. 11 and 12, the average grain size of the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % finer cryomilled feedstock powder was about 15 m with a refined single-modal grain size microstructure. There were about 0.65 wt. % well-dispersed Sm.sub.2O.sub.3 microparticles with an average particle size of about 1.5 m in this refined grain magnet. Whereas, the average grain size of the commercial counterpart magnet was about 45 m. There were about 0.5 wt. % Sm.sub.2O.sub.3 microparticles with an average particle size of about 3.5 m in the commercial sintered SmCo reference magnets while the agglomerates of a few of these Sm.sub.2O.sub.3 microparticles were also commonly observed. A higher volume fraction, a finer average particle size, and better-dispersion of Sm.sub.2O.sub.3 microparticles, which has larger pinning force, lead to finer grain size in the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets made from 100 wt. % finer cryomilled feedstock powder, compare to that of the commercial counterpart magnet. These Sm.sub.2O.sub.3 microparticles mentioned above are naturally formed through a partial oxidization during the multi-step magnet fabrication processes, especially during the powder fabrication and handling processes, sintering and heat treatment processes. It should be noticed that, this source of Sm.sub.2O.sub.3 microparticles is different with that of the well-controlled, artificially Sm.sub.2O.sub.3-added magnets, according to an embodiment of the process of the invention. There are two sources of Sm.sub.2O.sub.3 particles in the sintered magnet matrix. One source is the small amount (1 or 3 wt. %) of Sm.sub.2O.sub.3 submicron particles those being artificially added into the magnet matrix, which maintain their original fine particle size in the magnet matrix. The other is the naturally formed Sm.sub.2O.sub.3 micron scale particles through a partial oxidization during the magnet fabrication processes which is similar to the non-Sm.sub.2O.sub.3-added magnets.

[0058] Further experimental samples were made in accordance with the parameters set forth in the examples described above. These further samples comprised sintered Sm(CoFeCuZr).sub.17 that included 0.5 wt. % Sm.sub.2O.sub.3 submicron particles; 2 wt. % Sm.sub.2O.sub.3 submicron particles, and 0.5 wt. % La.sub.2O.sub.3 submicron particles, respectively, (where the Sm.sub.2O.sub.3 particles were 0.35 microns in average particle size and the La.sub.2O.sub.3 particles were 0.2 microns in average particle size). These further samples were tested for mechanical and magnetic properties as described above in the prior examples, and produced similar results in terms of similar mechanical improvements while maintaining their excellent magnetic properties.

[0059] For example, the average flexural strength values were enhanced by about 11%, 52%, and 45% (about 127, 173, and 165 MPa) for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets added with x=0.5 and 2 wt. % of Sm.sub.2O.sub.3, and 0.5 wt. % of La.sub.2O.sub.3 submicron particles, respectively, compared to that of 114 MPa for the reference magnet with x=0. Excellent hard magnetic properties were maintained with (BH).sub.max values of about 26, 26, 25, and 26 MGOe; B.sub.r values of about 10.6, 10.6, 10.5, and 10.6 kGs; H.sub.ci values of about 32.7, 32.5, 34.9, and 35.5 kOe for the Sm.sub.2(CoFeCuZr).sub.17 sintered magnets added with x=0, 0.5, and 2 wt. % Sm.sub.2O.sub.3, and 0.5 wt. % of La.sub.2O.sub.3 submicron particles, respectively.

[0060] The magnets made pursuant to embodiments of the invention can be expected to find similar applications in various industries as those of commercial sintered, die-upset or bonded 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.

[0061] While 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.

[0062] Relevant literature references incorporated herein by reference include: [0063] E. P. Wohlfarth, K. H. J. Buschow, Ferromagnetic materials, North Holland, 1988. [0064] http://www.electronenergy.com/products/materials. [0065] 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. [0066] W. Li, A. H. Li, H. J. Wang, W. Pan, H. W. Chang, Journal of Applied Physics,105 (2009) 07A703. [0067] S. Q. Liu, J. F. Liu, US Patent Pub. No.: US 2005/0081960 A1 [0068] X. Y. Li, K. Lu, Nature Materials 16 (2017) 700. [0069] 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. [0070] Y. M. Wang, M. W. Chen, F. H. Zhou, E. Ma, Nature 419 (2002) 912. [0071] 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. [0072] L. C. Stearns, M. P. Harmer, Journal of the American Ceramic Society 79 (1996) 3020. [0073] A. V. Karasev, H. Suito, ISIJ International 48 (2000) 658. [0074] Jun Cui, Baozhi Cui, Feedstock and heterogeneous structure for tough rare earth permanent magnets and production process therefor, US patent publication No. 2019/0115128 based on U.S. Ser. No. 16/350,215, filed Oct. 15, 2018.