- Email: [email protected]
Nanocrystallization of Fe-Nb(-Zr)-B-Al amorphous alloys
November
1997
Materials Letters 33 (1997) 107- 111
ELSEVIER
Nanocrys.tallization of Fe-Nb( -Zr) -B-Al
amorphous alloys
Mansour Al-Haj av*, John Barry b, Ayo Olofinjana
’
a Physics Department, University of Queensland, Brisbane, QLD 4072, Australia b Centerfor Microscopy and Microanalysis, University of Queensland, Brisbane, QL.D 4072, Australia ’ Department ofMining, Minerals and Materials Engineering, University of Queensland, Brisbane, QLD 4072, Australia Received
17 March 1997; accepted 20 March 1997
Abstract amorphous alloys was studied by differential The crystallization behavior of Fe,,,,Nb,B,,,Al, and Fe,,,, Nb,Zr,B,,,Al, thermal analysis, transmission electron microscopy and X-ray diffractometry. The formation of a single bee nanophase in the first alloy was preceded by the appearance of an intermediate stage containing the E-NbB, phase. In the second alloy the precipitation of intermetallic phases occurred without any change in the grain size of bee a-Fe. 0 1997 Elsevier Science B.V. PACS: 81.05.-t;
81.05.Y~;
Keywords: Amorphous
alloys; Fe-Nb(Zr)B-Al;
Crystallization;
Nanograins;
1. Introduction Nanocrystalline Fe-(Nb, Zrl-B alloys, prepared by primary crystalhzation of melt-spun amorphous precursors, are new soft magnetic materials with low coercivity and high saturation magnetization [ 11. These materials usually crystallize in a two-step crystallization process in which a single bee a-Fe nanophase appears after the first step and the precipitation of intermetallic phases from the amorphous matrix occurs after the second reaction [2]. Both the reduced magneto-crystalline and magneto-elastic anisotropies are responsible for the magnetic softness of these alloys [3]. The nanocrystallization process of Fe-Zr-B alloys was suggested [4] to be controlled
* Corresponding author. [email protected].
Fax:
+61-7-33651242;
e-mail:
Intermetallic
phases
by the nucleation-and-growth model while that of Fe-Nb-B alloys was described [51 by both the nucleation-and-growth and grain-growth models. In this letter we describe the crystallization processes of both Fe,,,,Nb,B,,,Al, and Fe,,,,Nb,Zr,B,,,Al, (at%> amorphous alloys.
2. Experimental The amorphous materials were prepared by the single-roller melt-spinning method. The thickness and width of the resulting ribbons were about 30 pm and 1.5 mm, respectively. The crystallization reaction was followed by a Perkin Elmer-7 differential thermal analyzer (DTA) at a heating rate of lO”C/min. Samples of the amorphous ribbons were annealed in a conventional furnace under argon atmosphere. The
00167-577X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved, PII SO167-577X(97)00084-0
108
M. Al-Haj et al. /Materials
Letters 33 (1997) 107-111
annealed samples were left to cool slowly to room temperature. The microstructure was observed by a JEM-2010 transmission electron microscope (TEM) operating at 200 kV. Thin samples for TEM observation were prepared using a liquid nitrogen cold stage in a Gatan-600 ion-milling machine. X-ray diffraction (XRD) patterns were taken by a Philips D-5000 diffractometer using Cu K, radiation.
3. Results and discussion The DTA curves for both amorphous alloys are shown in Fig. 1. The DTA curve of Fe,,,Nb,B,,,Al, amorphous alloy (Fig. la) reveals three exothermic peaks: the first one is in the range 455-565°C the
1.2-(a)
Fig. 2. The dark-field images and SAD patterns (in insets) of: (a) Fe,,,,Nb,B,,,Al, amorphous alloy annealed at 31X, and (b) Fes,,,Nb,Zr,B,5Al, amorphous alloy annealed at 340°C for 1 h. I.S-
(b)
1.‘-
Fig. 1. The DTA Fe,,,,Nh,Zr,B,,,Al,
curves of: (a) Fes3,,Nb,B,,,A1,, amorphous alloys.
and (b)
second in the range 565-65O”C, and the third in the range 780-800°C. The DTA curve of Fe,,.,Nb,Zr,B,,,Al, amorphous alloy (Fig. lb) reveals only one exothermic peak in the range 490610°C. In order to clarify the structural changes that have occurred in both alloys during beat treatment, TEM observations were done on annealed samples. Fig. 2 shows the dark-field images and the corresponding selected area diffraction (SAD) patterns for samples of the two alloys annealed for 1 h at temperatures below the first crystallization temperatures revealed by the DTA curves. The halo rings in the SAD patterns reveal that both alloys are still amorphous and there is no indication of any crystalliza-
M. Al-Haj et al. /Materials Letters 33 (1997) 107-I 11
109
tion at this stage. Annealing the Fe,&b,B,,,Al, amorphous alloy for 1 h at 560°C which is within the first peak in the DTA curve, causes the appearance of two different types of grains with different sizes (Fig. 3). The larger grains (indicated by arrows> are of size 25-30 nm, while that of the smaller ones is lo-15 nm. The faint reflection ring besides the (110) reflection ring of bee a-Fe (see the insets in Fig. 3 where a magnified part of the reflection ring is shown) was identified as the (101) reflection ring of the phase E-NbB,. So at this stage there exist two phases: cy-Fe and oNbB,. This result seems reasonable since Nb has a small solubility into cu-Fe ( < 1 at% at 600°C [6]) and so tends to form an intermetallit phase with B. Fig. 4 shows the dark-field images and the corresponding SAD patterns of (a> Fe,,,5Nb,B,,,Al, amorphous alloy annealed at 705”C, which is beyond the second peak in the DTA curve, and (b) Fe,,,Nb,Zr,B,,,Al, amorphous alloy annealed at 630°C which is belyond the first peak in the DTA curve, for 1 h. The dark-field images were taken from a part of the (I 10) reflection ring of bee a-Fe. Both alloys are characterized by the appearance of nanograins of size 20-25 nm with a homogeneous distribution embedded in an amorphous matrix. The SAD patterns confirm the appearance of this single bee a-Fe nanophase with the possibility that the Al
Fig. 4. The dark-field images and SAD patterns (in insets) of: (a) Fes,,sNb,Bs.,Al, amorphous alloy annealed at 705°C and (b) Fes,,,Nb,Zs,B,,,Al, amorphous alloy annealed at 630°C for 1 h.
Fig. 3. The dark-field image and SAD pattern (in insets) of Fes&Ib,B,,,Al, amorphous alloy annealed at 560°C for 1 h.
element has been dissolved in it. The amorphous phase surrounding grains from which electrons were scattered outside the intermediate aperture can be seen from the figure. Annealing the Fe,,,,Nb,B,,~Al, amorphous alloy at 81O”C, which is beyond the third peak in the DTA curve, causes an increase of the grain size to 40-50 nm and the precipitation of other intermetallic phases (Fig. 5a), while annealing the Fe,,,Nb,Zr,B&l, amorphous alloy at 790°C causes the precipitation of intermetallic phases (Fig. 5b) without any increase in the grain size of bee Q-Fe (for this reason the DTA was unable to detect the second crystallization reac-
110
M. Al-Haj et d/Materials
Letters 33 (1997) 107-111
tion). We think that this nonchange in the grain size is due to the presence of Nb, besides Zr, since Nb atoms are known [7] to exhibit a sharp concentration profile with the accumulation of Nb at a-Fe nanograin/amorphous phase interfaces and results in no further grain growth. The precipitation of intermetallic phases is thought to occur from the central zone of this amorphous layer. The XRD patterns taken for both alloys (Fig. 6) reveal, in addition to the (110) a-Fe line, the (211) and (112) lines of orthorhombic Fe,B (indicated by * and 0, respectively) and the (103) line of hexagonal Fe,Nb (indicated by X > for the first alloy, the (111) line of orthorhombic FeB (indicated by + ) and the (120)
Y
3s
*
40 S-theta
.
45
50
(dq)
Fig. 6. The XRD patterns of: (a) Fe,3.5Nb,Bs.,A1, alloy annealed at 81O”C, and (b) Fe,, ,Nb,Zr,B,,Al, alloy annealed at 790°C for 1 h.
amorphous amorphous
line of orthorhombic Fe,B (indicated by 0) for the second one and other unidentified fines.
4. summary
Fig. 5. The dark-field images and SAD patterns (in insets) of: (a) Fe,,,Nb,Bs,,Al, amorphous alloy annealed at 8lO”C, and (b) ,A], amorphous alloy annealed at 790°C for 1 h. Fe ss,sNb,Zr,B,
The formation mechanism of nanophases in Fes,.sh%Bs.sA& and Fes5.5h%%B5.5A& amorphous alloys during annealing proceeds as follows: For Fes&b,B,, Al, alloy: bee cy-Fe nanophase + intermetallic phases + single bee cw-Fenanophase. For Fe ss,sNb,Zr,B,~,Al, alloy: single bee a-Fe nanophase + bee a-Fe nanophase + intermetallic phases.
M. Al-Haj et al. /Materials Letters 33 (1997) 107-111
References [l] K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 74 (1993) 3316. [2] A. Makino, K. Suzuki, A. moue, T. Masumoto, Mater. Sci. Eng. A 179-180 (1994) 127. [3] G. Herzer, Phys. Ser. T 49 (19931 307.
111
[4] K. Suzuki, A. Makino, A.P. Tsai, A. Inoue, T. Masumoto, Mater. Sci. Eng. A 179-180 (1994) 501. [5] K. Suzuki, J.M. Cadogan, J.B. Dunlop, V. Sahajwalla, Appl. Phys. Len. 67 (1995) 1369. [6] T.B. Massalski @cl.), Binary Alloy Phase Diagrams, ASM, OH, 1986. [7] A.R. Yavari, 0. Drbohlav, Mater. Trans. JIM 36 (19951 896.
1997
Materials Letters 33 (1997) 107- 111
ELSEVIER
Nanocrys.tallization of Fe-Nb( -Zr) -B-Al
amorphous alloys
Mansour Al-Haj av*, John Barry b, Ayo Olofinjana
’
a Physics Department, University of Queensland, Brisbane, QLD 4072, Australia b Centerfor Microscopy and Microanalysis, University of Queensland, Brisbane, QL.D 4072, Australia ’ Department ofMining, Minerals and Materials Engineering, University of Queensland, Brisbane, QLD 4072, Australia Received
17 March 1997; accepted 20 March 1997
Abstract amorphous alloys was studied by differential The crystallization behavior of Fe,,,,Nb,B,,,Al, and Fe,,,, Nb,Zr,B,,,Al, thermal analysis, transmission electron microscopy and X-ray diffractometry. The formation of a single bee nanophase in the first alloy was preceded by the appearance of an intermediate stage containing the E-NbB, phase. In the second alloy the precipitation of intermetallic phases occurred without any change in the grain size of bee a-Fe. 0 1997 Elsevier Science B.V. PACS: 81.05.-t;
81.05.Y~;
Keywords: Amorphous
alloys; Fe-Nb(Zr)B-Al;
Crystallization;
Nanograins;
1. Introduction Nanocrystalline Fe-(Nb, Zrl-B alloys, prepared by primary crystalhzation of melt-spun amorphous precursors, are new soft magnetic materials with low coercivity and high saturation magnetization [ 11. These materials usually crystallize in a two-step crystallization process in which a single bee a-Fe nanophase appears after the first step and the precipitation of intermetallic phases from the amorphous matrix occurs after the second reaction [2]. Both the reduced magneto-crystalline and magneto-elastic anisotropies are responsible for the magnetic softness of these alloys [3]. The nanocrystallization process of Fe-Zr-B alloys was suggested [4] to be controlled
* Corresponding author. [email protected].
Fax:
+61-7-33651242;
e-mail:
Intermetallic
phases
by the nucleation-and-growth model while that of Fe-Nb-B alloys was described [51 by both the nucleation-and-growth and grain-growth models. In this letter we describe the crystallization processes of both Fe,,,,Nb,B,,,Al, and Fe,,,,Nb,Zr,B,,,Al, (at%> amorphous alloys.
2. Experimental The amorphous materials were prepared by the single-roller melt-spinning method. The thickness and width of the resulting ribbons were about 30 pm and 1.5 mm, respectively. The crystallization reaction was followed by a Perkin Elmer-7 differential thermal analyzer (DTA) at a heating rate of lO”C/min. Samples of the amorphous ribbons were annealed in a conventional furnace under argon atmosphere. The
00167-577X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved, PII SO167-577X(97)00084-0
108
M. Al-Haj et al. /Materials
Letters 33 (1997) 107-111
annealed samples were left to cool slowly to room temperature. The microstructure was observed by a JEM-2010 transmission electron microscope (TEM) operating at 200 kV. Thin samples for TEM observation were prepared using a liquid nitrogen cold stage in a Gatan-600 ion-milling machine. X-ray diffraction (XRD) patterns were taken by a Philips D-5000 diffractometer using Cu K, radiation.
3. Results and discussion The DTA curves for both amorphous alloys are shown in Fig. 1. The DTA curve of Fe,,,Nb,B,,,Al, amorphous alloy (Fig. la) reveals three exothermic peaks: the first one is in the range 455-565°C the
1.2-(a)
Fig. 2. The dark-field images and SAD patterns (in insets) of: (a) Fe,,,,Nb,B,,,Al, amorphous alloy annealed at 31X, and (b) Fes,,,Nb,Zr,B,5Al, amorphous alloy annealed at 340°C for 1 h. I.S-
(b)
1.‘-
Fig. 1. The DTA Fe,,,,Nh,Zr,B,,,Al,
curves of: (a) Fes3,,Nb,B,,,A1,, amorphous alloys.
and (b)
second in the range 565-65O”C, and the third in the range 780-800°C. The DTA curve of Fe,,.,Nb,Zr,B,,,Al, amorphous alloy (Fig. lb) reveals only one exothermic peak in the range 490610°C. In order to clarify the structural changes that have occurred in both alloys during beat treatment, TEM observations were done on annealed samples. Fig. 2 shows the dark-field images and the corresponding selected area diffraction (SAD) patterns for samples of the two alloys annealed for 1 h at temperatures below the first crystallization temperatures revealed by the DTA curves. The halo rings in the SAD patterns reveal that both alloys are still amorphous and there is no indication of any crystalliza-
M. Al-Haj et al. /Materials Letters 33 (1997) 107-I 11
109
tion at this stage. Annealing the Fe,&b,B,,,Al, amorphous alloy for 1 h at 560°C which is within the first peak in the DTA curve, causes the appearance of two different types of grains with different sizes (Fig. 3). The larger grains (indicated by arrows> are of size 25-30 nm, while that of the smaller ones is lo-15 nm. The faint reflection ring besides the (110) reflection ring of bee a-Fe (see the insets in Fig. 3 where a magnified part of the reflection ring is shown) was identified as the (101) reflection ring of the phase E-NbB,. So at this stage there exist two phases: cy-Fe and oNbB,. This result seems reasonable since Nb has a small solubility into cu-Fe ( < 1 at% at 600°C [6]) and so tends to form an intermetallit phase with B. Fig. 4 shows the dark-field images and the corresponding SAD patterns of (a> Fe,,,5Nb,B,,,Al, amorphous alloy annealed at 705”C, which is beyond the second peak in the DTA curve, and (b) Fe,,,Nb,Zr,B,,,Al, amorphous alloy annealed at 630°C which is belyond the first peak in the DTA curve, for 1 h. The dark-field images were taken from a part of the (I 10) reflection ring of bee a-Fe. Both alloys are characterized by the appearance of nanograins of size 20-25 nm with a homogeneous distribution embedded in an amorphous matrix. The SAD patterns confirm the appearance of this single bee a-Fe nanophase with the possibility that the Al
Fig. 4. The dark-field images and SAD patterns (in insets) of: (a) Fes,,sNb,Bs.,Al, amorphous alloy annealed at 705°C and (b) Fes,,,Nb,Zs,B,,,Al, amorphous alloy annealed at 630°C for 1 h.
Fig. 3. The dark-field image and SAD pattern (in insets) of Fes&Ib,B,,,Al, amorphous alloy annealed at 560°C for 1 h.
element has been dissolved in it. The amorphous phase surrounding grains from which electrons were scattered outside the intermediate aperture can be seen from the figure. Annealing the Fe,,,,Nb,B,,~Al, amorphous alloy at 81O”C, which is beyond the third peak in the DTA curve, causes an increase of the grain size to 40-50 nm and the precipitation of other intermetallic phases (Fig. 5a), while annealing the Fe,,,Nb,Zr,B&l, amorphous alloy at 790°C causes the precipitation of intermetallic phases (Fig. 5b) without any increase in the grain size of bee Q-Fe (for this reason the DTA was unable to detect the second crystallization reac-
110
M. Al-Haj et d/Materials
Letters 33 (1997) 107-111
tion). We think that this nonchange in the grain size is due to the presence of Nb, besides Zr, since Nb atoms are known [7] to exhibit a sharp concentration profile with the accumulation of Nb at a-Fe nanograin/amorphous phase interfaces and results in no further grain growth. The precipitation of intermetallic phases is thought to occur from the central zone of this amorphous layer. The XRD patterns taken for both alloys (Fig. 6) reveal, in addition to the (110) a-Fe line, the (211) and (112) lines of orthorhombic Fe,B (indicated by * and 0, respectively) and the (103) line of hexagonal Fe,Nb (indicated by X > for the first alloy, the (111) line of orthorhombic FeB (indicated by + ) and the (120)
Y
3s
*
40 S-theta
.
45
50
(dq)
Fig. 6. The XRD patterns of: (a) Fe,3.5Nb,Bs.,A1, alloy annealed at 81O”C, and (b) Fe,, ,Nb,Zr,B,,Al, alloy annealed at 790°C for 1 h.
amorphous amorphous
line of orthorhombic Fe,B (indicated by 0) for the second one and other unidentified fines.
4. summary
Fig. 5. The dark-field images and SAD patterns (in insets) of: (a) Fe,,,Nb,Bs,,Al, amorphous alloy annealed at 8lO”C, and (b) ,A], amorphous alloy annealed at 790°C for 1 h. Fe ss,sNb,Zr,B,
The formation mechanism of nanophases in Fes,.sh%Bs.sA& and Fes5.5h%%B5.5A& amorphous alloys during annealing proceeds as follows: For Fes&b,B,, Al, alloy: bee cy-Fe nanophase + intermetallic phases + single bee cw-Fenanophase. For Fe ss,sNb,Zr,B,~,Al, alloy: single bee a-Fe nanophase + bee a-Fe nanophase + intermetallic phases.
M. Al-Haj et al. /Materials Letters 33 (1997) 107-111
References [l] K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 74 (1993) 3316. [2] A. Makino, K. Suzuki, A. moue, T. Masumoto, Mater. Sci. Eng. A 179-180 (1994) 127. [3] G. Herzer, Phys. Ser. T 49 (19931 307.
111
[4] K. Suzuki, A. Makino, A.P. Tsai, A. Inoue, T. Masumoto, Mater. Sci. Eng. A 179-180 (1994) 501. [5] K. Suzuki, J.M. Cadogan, J.B. Dunlop, V. Sahajwalla, Appl. Phys. Len. 67 (1995) 1369. [6] T.B. Massalski @cl.), Binary Alloy Phase Diagrams, ASM, OH, 1986. [7] A.R. Yavari, 0. Drbohlav, Mater. Trans. JIM 36 (19951 896.