ISSN 1318-0010 KZLTET 32(3-515)165(1998) HIGH COERCIVITIES IN HDDR PROCESSED Sm-Fe-Ta-N MAGNETS VISOKE KOERCITIVNOSTI Sm-Fe-Ta-N MAGNETOV DOBLJENIH PO HDDR POSTOPKU KRISTINA ŽUŽEK, P. J. MCGUINESS, S. KOBE Institut Jožef Stefan, Jamova 39, 1001 Ljubljana, Slovenija Prejem rokopisa - received: 1998-11-16; sprejem za objavo - accepted for publication: 1998-12-07 An alloy of composition Sm13,8Fe82,2Ta4,0 was compared with a conventional Sm13,7Fe86,3 material in order to assess its potential as a permanent magnet material. The optimum conditions necessary to provide the highest coercivities using the HDDR process, and for the HDDR process combined with milling were investigated. The coercivities obtained after using the HDDR process and subsequent nitriding were 676,6 kA/m for the Sm13,8Fe82,2Ta4,0Nx and 358,2 kA/m for the Sm13,8Fe86,3Nx samples. Coercivites of 1003,0 kA/m for Sm13,8Fe86,3Nx and 1273,6 kA/m Sm13,8Fe82,2Ta4,0Nx were achieved by reducing the particle size with milling prior to the HDDR process. Magnetic results were compared with the phase composition, determined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The better coercivities obtained with the Ta containing sample were found to be due to the presence of a much smaller amount of aFe, a soft magnetic phase, with the TaFe2 phase forming in its place. The milling prior to the HDDR treatment improves the magnetic properties because of the small particle size which prevents the grains growing too large, with their consequent very negative effect on the coercivity. Key words: HDDR, SmFeN, Ta, coercivity, milling Primerjali smo zlitino sestave Sm13,8Fe82,2Ta4,0 z že poznano zlitino sestave Sm13,8Fe86,3. Želeli smo ovrednotiti potencial zlitine, z dodatkom Ta, z vidika permanentnega magnetnega materiala. Poiskali smo optimalne pogoje postopka HDDR, ki so nam dali najboljše rezultate na področju magnetnih lastnosti. Koercitivnosti prahov dobljenih po tem postopku in nadaljnem nitriranju, so 676,6 kA/m za Sm13,8Fe82,2Ta4,0Nx in 358,2 kA/m za Sm13,8Fe86,3Nx. Višje koercitivnosti 1003,0 kA/m za Sm13,8Fe86,3Nx in 1273,6 kA/m za Sm13,8Fe82,2Ta4,0Nx pa smo dosegli z mletjem materiala pred HDDR postopkom. Rezultate magnetnih lastnosti smo primerjali tudi s fazno sestavo, ki smo jo določili z elektronsko vrstično mikroskopijo (SEM) in rentgensko difrakcijo (XRD). Razliko v višji koercitivnosti sestave s Ta, gre pripisati manjši vsebnosti aFe, ki kot mehkomagnetna faza znižuje magnetne lastnosti. aFe s tantalom tvori TaFe2 fazo, ki pa je paramagnetna in nima negativnega vpliva na magnetne lastnosti. Tudi predmletje materiala vpliva pozitivno na magnetne lastnosti saj z zmanjšanjem velikost delcev preprečimo grobo zrnatost, ki negativno vpliva na magnetne lastnosti. Ključne besede: HDDR, SmFeN, Ta, koercitivnost, mletje 1 INTRODUCTION SmFeN magnets have received considerable attention since their discovery in 19911. Their intrinsic properties are comparable with, or better than, those of magnets based on Nd2FewB, and so they have the potential of taking a significant share of the rare earth permanent magnet market. SmFeN magnets are open to a number of possible processing routes, for example, mechanical al-loying2,3, melt spinning4,5 and conventional powder met-allurgy1,6 however the HDDR process7,8 appears to be the most promising. In this paper we have investigated the effects of a Ta addition on the magnetic properties and microstructure of a conventional SmFe cast alloy. Also the influence of milling, prior to the HDDR process, on the magnetic properties, will be discussed. 2 EXPERIMENTAL The SmFe(A) and SmFeTa(B) cast alloys were produced by conventional induction melting methods in 5 kg batches by Less-Common Metals Ltd. The composition of the SmFeTa(B) alloy was chosen on the basis of our previous results9,10. The ingot material was then crushed to a particle size of less than 1 mm and the particle size reduced further by milling. Milling was carried out in an attritor mill for 30 minutes, under hexane in the inert atmosphere of a glove box. The HDDR processing was carried out in a rotating furnace operating between 1 bar over pressure and a vacuum of 10-2 mbar. The non pre-milled samples were introduced into the furnace as alloy lumps of less than 5 mm and the milled material as powder of approximately 5 mm particle size. The resulting non pre-milled HDDR powder was crushed by hand in a mortar and pestle to less than 200 mm. All samples were subsequently nitrided at 450°C for 4 hours in a flow of nitrogen gas. Permanent magnet bonded samples were produced by mixing the powder with epoxy resin, these samples were measured at room temperature in a conventional permeameter after pulsing the magnets in a field of 5 T. 3 RESULTS The overall composition of the alloys is given in Table 1. Small pieces of approximately 0.5 cm3 were cut from similar parts of each alloy, mounted and polished for metallographic examination. KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 3-4 16 5 K. ŽUŽEK ET AL.: HIGH COERCIVITIES IN HDDR PROCESSED.. Table 1 : Chemical composition of cast alloys Tabela 1: Kemijska sestava zlitin Sm (at%) Fe (at%) Ta (at%) SmFe(A) 13.7 86.3 - SmFeTa(B) 13.8 82.2 4.0 The resulting microstructures of the SmFe(A) and SmFeTa(B) materials in the as-cast condition can be seen in Figure 1 and in Figure 2. The conventional SmFe(A) (Figure 1) binary alloy exhibits the characteristic microstructure with dendrites of peritectically formed a-Fe, a Sm2Fen phase which forms the majority phase, with SmFe2 and SmFe3 observed to form at the grain boundaries. The results of the analysis confirm the existence of Sm2Fen, SmFe2, SmFe3 and a-Fe. The SmFeTa(B) alloy (Figure 2) is characterized by a very different microstructure. No a-Fe is observed. Rather, it appears that the primary phase in this case is TaFe2 which forms throughout the alloy in grains of 1-10 mm. Sm2Fe17 is formed as the major phase and SmFe2 and SmFe3 are also observed to exist. The detected phases and their composition is given in a Table 2. Table 2: Phases detected in both alloys, Sm-Fe(A) and Sm-Fe-Ta (B) and their composition Tabela 2: Detektirane faze v obeh zlitinah Sm-Fe (A) in Sm-Fe-Ta (B) in njihova sestava SmFe (A) SmFeTa (B) Sm2Fei7 phase SmnFe89 SmFe3 phase Sm25,2Fe74,8 SmFe2 phase Sm32,2Fe67,8 aFe phase Sm0,1Fe99,9 Sm2Fei7 phase Smi0Fe88Ta2 SmFe3 phase Sm23Fe75Ta2 SmFe2 phase Sm30Fe69Tai TaFe2 phase Ta70Fe30 In order to provide confirmation of the EDX data, XRD studies were undertaken on powdered samples of Figure 1: SEM micrograph of the Sm2Fej7 cast alloy showing dendrites of primary Fe (dark), Sm2Fej7 (gray), SmFe3 (light gray) and SmFe2 (white) phases Slika 1: SEM posnetek zlitine Sm2Fej7, ki prikazuje dendritsko Fe fazo (~rno), Sm2Fej7 fazo (sivo), SmFe3 fazo (svetlo sivo), SmFe2 (belo) 516 Figure 2: SEM micrograph of the Sm-Fe-Ta cast alloy with 4 at.% of Ta showing Sm2Fej7 (dark), SmFe3 (gray), SmFe2 (light gray), TaFe2 (white) phases Slika 2: SEM posnetek zlitine Sm-Fe-Ta s 4 at.% Ta, ki prikazuje Sm2Fe17 fazo (~rno), SmFe3 fazo (sivo), SmFe2 fazo (sv. sivo) in TaFe2 fazo (belo) the cast SmFe(A) and SmFeTa(B) materials. Figure 3 shows the two spectra obtained using Cu Ka radiation. The lower spectrum from SmFe(A) shows clear evidence for the Sm2Fe17, SmFe2, SmFe3 phases, as well as for a-Fe. The upper spectrum, for the SmFeTa(B) material, again confirms the EDX results with the Sm2Fen, SmFe2, SmFe3 and TaFe2 phases being detected. In order to process materials with hard magnetic properties both samples, pre-milled and not pre-milled, SmFe(A) and SmFeTa(B) were HDDR processed according to the scheme in Figure 4. 3.1 Influence of Recombination Temperature on Coerciv-ity Non pre-milled material Both types of samples exhibit peak coercivities at a recombination temperature of 750°C (Figure 5), but the SmFeTa(B) sample (676,6 kA/m) has almost twice the coercivity of the Ta free SmFe(A) binary alloy (358,2 Sm-Fe-Ta cast 20