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New structural state in melt-spun NdFeB alloys
Journal of
ALLOYS AND C O M ~ U N D S ELSEVIER
Journal of Alloysand Compounds226 ( 1995) 158-160
New structural state in melt-spun Nd-Fe-B alloys L.A. Pamyatnykh *, V.I. Pushkarsky, S.V. Andreev, G.F. Korznikova, N.V. Kudrevatykh, B.N. Lobanov Institute of Physics and Applied Mathematics, Ural State University, 620083 Ekaterinburg, Russia
Received25 August 1994;in finalform 13 January 1995
Abstract
Microstructure and magnetic properties of the Nd-Fe-B ribbons prepared by melt-spinning technique have been studied. A new structural state of the material was observed. This state is characterizedby large crystallites of about 1/zm size which are composed of aligned precipitates of approximately 50 nm, separated by amorphous layers of 3-5 nm width forming a regular net. The ability to form this structural state promises the development of an industrial scale production method of anisotropic hard-magnetic R-Fe-B powders suitable for bonded magnets. Keywords: Microstructure;Amorphousribbons;Melt spinning
1. Introduction
The melt-spinning technique method is a basic method to generate excellent magnetic hysteresis properties in R-Fe-B alloys [ 1]. The microstructure of such materials have been studied in many papers, for example in Refs. [ 1-4]. This paper deals with the magnetic properties and the microstructure of rapidly quenched (melt-spun) Nd-Fe-B alloys (ribbons) obtained by melt quenching on the inner surface of a spinning wheel under conditions optimal for the formation of magnetic hardness (surface wheel speed 14 m s - l ) when the ribbon is annealed in vacuum at 700 °C for 10 min. An initial alloy, Fe-36wt.%Nd-l.2%B, with small impurities of Pr, Cu, Ni and Ca, was centrifugally quenched on the inner surface of the steel disc. The samples obtained had the form of ribbon strips with 35/~m thickness, 2 mm width and several meters length. The microstructure was studied using scanning electron (SEM) and transmission electron microscopy (TEM) methods by means of JEM-1000, ISM840 and ISM-2000 EX electron microscopes. Specimens for TEM study were prepared by electrochemical thinning in a solution ofCrO 3 and H3PO4. The presence of magnetic phases was checked by the thermal magnetic analysis method (TMA) in the temperature interval 31)0-1100 K. Magnetic hysteresis parameters were measured on a vibrating sample * Correspondingauthor. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
SSDI0925-8388(95)01581-7
magnetometer in magnetic fields up to 2 T after preliminary sample magnetizing by pulse fields of 10 T.
2. Results 2.1. Melt-spun ribbons as-cast
As-cast ribbons of the above mentioned alloy composition had the following magnetic parameters: coercive force iHc = 24 kOe, specific remanent magnetization o'r = 70 emu g - 1, maximal energy product B H r ~ = 8.10 6 GOe. The magnetic measurements along the length of the ribbons and along the direction perpendicular to the ribbon surface showed anisotropy to be absent. As observed by SEM, the crystallite size is changed from 1.5/zm (near the contact surface) to 0.1 ftm (free surface). Figs. 1 and 2 show the TEM micrographs. One can clearly see the crystallites of the main phase of 0.5-1.0 p m size separated by a different minority phase. Each crystallite looks like an assembly of very small precipitates forming a regular net. The large magnification micrograph of a crystallite (Fig. 2) shows a regular and fine substructure consisting of small grains about 30-50 nm in size separated by interlayers of 3-5 nm width. The regular form of electron diffraction patterns taken from an area of 0.65/zm diameter (insertion in Fig. 2) indicates good crystallographic alignment of these small grains (precipitates). The method of defocusing which was used to check the nature of the observed contrast does
L.A. Pamyatnykh et al. /Journal of Alloys and Compounds 226 (1995) 158-160
159
contrast. Thus the conclusion that the observed net exists within the sample volume (but not on the sample surface) can surely be drawn. Experiments with changing foil slope also showed that the observed contrast are not dislocations or a dislocation (subgrain) network. This metallographic study allows us therefore to conclude that the observed net contrast originates from the crystallite phase inhomogeneity itself and the observed network consists of a new phase formed inside the matrix phase 2-14-1. Electron diffraction patterns from separate crystallites confirm this conclusion, showing beside the regular spot system from the 2-14-1 phase two homogeneous rings usually occurring for quasi-amorphous atomic structures. The TMA data (curve 1 in Fig. 3) indicate also in as-cast melt-spun ribbons the presence of two magnetic phases: a main 2-14-1 phase having a Curie point (Tc) value of 305 °C and a new unknown phase with Tc = 275 °C.
Fig. 1. TEM imageof the foil of as-cast Nd-Fe-B ribbon.
2.2. Melt-spun ribbons after heat treatment
Fig. 2. TEM image and electron diffraction pattern for as-cast Nd-Fe-B ribbon at i:argemagnification. I
.~80
The microstmcture of the melt-spun ribbons after heat treatment is drastically changed. It is seen from Fig. 4 that the net contrast inside the crystallite has disappeared. There is only a very fine structure with atomic scale irregularities (up to 5 nm) of unknown composition. The mean size of the main phase (crystallites) is practically unchanged, whereas the intracrystalline layers of the minority phase, which takes a relatively large volume in as-cast ribbons (Fig. 1 ), can no longer be detected. The crystalline borders become straight and homogeneous (Fig. 4). It is necessary to note that after the heat treatment the net contrast occasionally still exists near some crystallite borders. As is seen from Fig. 3 (curve 2) only one magnetic phase, with Tc = 305 °C, is detected by TMA in annealed ribbons. The phase with Tc = 275 °C does not exist. The hysteresis loop changes its form, including coercivity and remanence.
I
I
,~, 4 o
0
400
:
450
500
550
600
Temperature[K] ~g. 3. ~ m ~ r a m ~ dependences of inifi~ sus~ptibility of ~-cast (cu~e 1) ~ d he~-t~ated ~ 7 ~ °C (cu~e 2) melt-spunNd-Fe-B ri~ons.
not lead to its disappearance or its inversion. This seems to refute the magnetic origins in its formation [ 4,5 ]. Moreover, change of the sample slope does not influence the picture
Fig. 4. TEM image of melt-spunNd-Fe-B ribbon after the heat treatment in vacuumat 700 °C for 10 min.
160
L.A. Pamyatnykh et al. / Journal of Alloys and Compounds 226 (1995) 158-160
T h u s it can be c o n c l u d e d that the ability to form this structural state opens the possibility to design an industrial scale production m e t h o d for anisotropic hard-magnetic R - F e - B powders for b o n d e d magnets.
References [ 1] J.J. Croat, J.F. Herbst, R.W. Lee and F.E. Pinkerton, J. Appl. Phys., 55 (1984) 2078.
[ 2] P.J. Grundy, D.G. Lord, S.F.H. Parker and R.J. Pollard, in I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsch (eds.), Concerted European Action on Magnets, Elsevier, London, 1989, p. 405. [3] I. Ahmad, M.A. AI-Khafal, H.A. Davies, R.A. Buckley and W.M. Rainforth, in Proc. 8th Int. Symposium on Magnetic Anisotropy and Coercivity in RE-TM Alloys, Birmingham, UK, 15 September 1994, p. 145. [4] J. Fidler, in G.C. Hadjipanayis and G.A. Prinz (eds.), Science and Technology of Nanostructured Magnetic Materials, Plenum Press, New York, 1991, p. 627. [5] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworths, London, 1965.
ALLOYS AND C O M ~ U N D S ELSEVIER
Journal of Alloysand Compounds226 ( 1995) 158-160
New structural state in melt-spun Nd-Fe-B alloys L.A. Pamyatnykh *, V.I. Pushkarsky, S.V. Andreev, G.F. Korznikova, N.V. Kudrevatykh, B.N. Lobanov Institute of Physics and Applied Mathematics, Ural State University, 620083 Ekaterinburg, Russia
Received25 August 1994;in finalform 13 January 1995
Abstract
Microstructure and magnetic properties of the Nd-Fe-B ribbons prepared by melt-spinning technique have been studied. A new structural state of the material was observed. This state is characterizedby large crystallites of about 1/zm size which are composed of aligned precipitates of approximately 50 nm, separated by amorphous layers of 3-5 nm width forming a regular net. The ability to form this structural state promises the development of an industrial scale production method of anisotropic hard-magnetic R-Fe-B powders suitable for bonded magnets. Keywords: Microstructure;Amorphousribbons;Melt spinning
1. Introduction
The melt-spinning technique method is a basic method to generate excellent magnetic hysteresis properties in R-Fe-B alloys [ 1]. The microstructure of such materials have been studied in many papers, for example in Refs. [ 1-4]. This paper deals with the magnetic properties and the microstructure of rapidly quenched (melt-spun) Nd-Fe-B alloys (ribbons) obtained by melt quenching on the inner surface of a spinning wheel under conditions optimal for the formation of magnetic hardness (surface wheel speed 14 m s - l ) when the ribbon is annealed in vacuum at 700 °C for 10 min. An initial alloy, Fe-36wt.%Nd-l.2%B, with small impurities of Pr, Cu, Ni and Ca, was centrifugally quenched on the inner surface of the steel disc. The samples obtained had the form of ribbon strips with 35/~m thickness, 2 mm width and several meters length. The microstructure was studied using scanning electron (SEM) and transmission electron microscopy (TEM) methods by means of JEM-1000, ISM840 and ISM-2000 EX electron microscopes. Specimens for TEM study were prepared by electrochemical thinning in a solution ofCrO 3 and H3PO4. The presence of magnetic phases was checked by the thermal magnetic analysis method (TMA) in the temperature interval 31)0-1100 K. Magnetic hysteresis parameters were measured on a vibrating sample * Correspondingauthor. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
SSDI0925-8388(95)01581-7
magnetometer in magnetic fields up to 2 T after preliminary sample magnetizing by pulse fields of 10 T.
2. Results 2.1. Melt-spun ribbons as-cast
As-cast ribbons of the above mentioned alloy composition had the following magnetic parameters: coercive force iHc = 24 kOe, specific remanent magnetization o'r = 70 emu g - 1, maximal energy product B H r ~ = 8.10 6 GOe. The magnetic measurements along the length of the ribbons and along the direction perpendicular to the ribbon surface showed anisotropy to be absent. As observed by SEM, the crystallite size is changed from 1.5/zm (near the contact surface) to 0.1 ftm (free surface). Figs. 1 and 2 show the TEM micrographs. One can clearly see the crystallites of the main phase of 0.5-1.0 p m size separated by a different minority phase. Each crystallite looks like an assembly of very small precipitates forming a regular net. The large magnification micrograph of a crystallite (Fig. 2) shows a regular and fine substructure consisting of small grains about 30-50 nm in size separated by interlayers of 3-5 nm width. The regular form of electron diffraction patterns taken from an area of 0.65/zm diameter (insertion in Fig. 2) indicates good crystallographic alignment of these small grains (precipitates). The method of defocusing which was used to check the nature of the observed contrast does
L.A. Pamyatnykh et al. /Journal of Alloys and Compounds 226 (1995) 158-160
159
contrast. Thus the conclusion that the observed net exists within the sample volume (but not on the sample surface) can surely be drawn. Experiments with changing foil slope also showed that the observed contrast are not dislocations or a dislocation (subgrain) network. This metallographic study allows us therefore to conclude that the observed net contrast originates from the crystallite phase inhomogeneity itself and the observed network consists of a new phase formed inside the matrix phase 2-14-1. Electron diffraction patterns from separate crystallites confirm this conclusion, showing beside the regular spot system from the 2-14-1 phase two homogeneous rings usually occurring for quasi-amorphous atomic structures. The TMA data (curve 1 in Fig. 3) indicate also in as-cast melt-spun ribbons the presence of two magnetic phases: a main 2-14-1 phase having a Curie point (Tc) value of 305 °C and a new unknown phase with Tc = 275 °C.
Fig. 1. TEM imageof the foil of as-cast Nd-Fe-B ribbon.
2.2. Melt-spun ribbons after heat treatment
Fig. 2. TEM image and electron diffraction pattern for as-cast Nd-Fe-B ribbon at i:argemagnification. I
.~80
The microstmcture of the melt-spun ribbons after heat treatment is drastically changed. It is seen from Fig. 4 that the net contrast inside the crystallite has disappeared. There is only a very fine structure with atomic scale irregularities (up to 5 nm) of unknown composition. The mean size of the main phase (crystallites) is practically unchanged, whereas the intracrystalline layers of the minority phase, which takes a relatively large volume in as-cast ribbons (Fig. 1 ), can no longer be detected. The crystalline borders become straight and homogeneous (Fig. 4). It is necessary to note that after the heat treatment the net contrast occasionally still exists near some crystallite borders. As is seen from Fig. 3 (curve 2) only one magnetic phase, with Tc = 305 °C, is detected by TMA in annealed ribbons. The phase with Tc = 275 °C does not exist. The hysteresis loop changes its form, including coercivity and remanence.
I
I
,~, 4 o
0
400
:
450
500
550
600
Temperature[K] ~g. 3. ~ m ~ r a m ~ dependences of inifi~ sus~ptibility of ~-cast (cu~e 1) ~ d he~-t~ated ~ 7 ~ °C (cu~e 2) melt-spunNd-Fe-B ri~ons.
not lead to its disappearance or its inversion. This seems to refute the magnetic origins in its formation [ 4,5 ]. Moreover, change of the sample slope does not influence the picture
Fig. 4. TEM image of melt-spunNd-Fe-B ribbon after the heat treatment in vacuumat 700 °C for 10 min.
160
L.A. Pamyatnykh et al. / Journal of Alloys and Compounds 226 (1995) 158-160
T h u s it can be c o n c l u d e d that the ability to form this structural state opens the possibility to design an industrial scale production m e t h o d for anisotropic hard-magnetic R - F e - B powders for b o n d e d magnets.
References [ 1] J.J. Croat, J.F. Herbst, R.W. Lee and F.E. Pinkerton, J. Appl. Phys., 55 (1984) 2078.
[ 2] P.J. Grundy, D.G. Lord, S.F.H. Parker and R.J. Pollard, in I.V. Mitchell, J.M.D. Coey, D. Givord, I.R. Harris and R. Hanitsch (eds.), Concerted European Action on Magnets, Elsevier, London, 1989, p. 405. [3] I. Ahmad, M.A. AI-Khafal, H.A. Davies, R.A. Buckley and W.M. Rainforth, in Proc. 8th Int. Symposium on Magnetic Anisotropy and Coercivity in RE-TM Alloys, Birmingham, UK, 15 September 1994, p. 145. [4] J. Fidler, in G.C. Hadjipanayis and G.A. Prinz (eds.), Science and Technology of Nanostructured Magnetic Materials, Plenum Press, New York, 1991, p. 627. [5] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworths, London, 1965.