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Properties and structures of Fe-based metallic glasses
Properties and Structures of Fe-Based Metallic Glasses K. Habib,+ K. Moore,* R. Nessler:
V. Eling,t and C. WUf
*lowa Electron Microscopy Center, University Instruments, Inc., Santa Barbara, CA 93227
of Iowa, Iowa City, IA 52240; and +Digital
A fundamental
study has been conducted
on Fe78B13Sig metallic thin ribbon in order to
understand the changes in the properties and structures of the glass as a function of Cr and Ni-Mo additions. The separate additions of Cr and Ni-Mo were successful in producing two new m.etallic thin glasses with the compositions Fe&&B&i, and Fe&&Mo,Br7SiZ. The study was focused on evaluating the physical and magnetic properties of these glasses with respect to the Cr and Ni-Mo additions. In addition, characterization of the internal and the surface structures of the glasses was conducted using transmission electron microscopy
and scanning
tunneling
microscopy,
respectively.
A comparison
between
the
internal and surface structures of the glasses was carried out on both amorphous and crystalline forms. .4s a result, a correlation between the properties and the structures of the glasses was established.
INTRODUCTION Since the invention of metallic glasses in 1960 [l, 21, the alloys have become widely studied because of the many practical values arising from their (extreme homogeneous and disordered atomic structures. For instance, Allied Chelmical Inc. has recently reported results on a FemB&& metallic glass, known commercially as Metglas 2605s [3]. The results showed that the glass exhibits excellent physical and magnetic properties compared to the crystalline alloys. The glass is found suitable for extremely low core loss in distribution, power transformers, and motors. It combines high induction and superb magnetic properties at the frequencies, induction, and operating temperatures of these devices. Furthermore, the glass can be used in inductions, current transformers, and other devices requiring high permeability and low core loss at low frequencies. Subsequent to the development of Fe78Br3Si9
metallic glass, Allied Chemical has succeeded in developing two new metallic glasses, Fe+&B&i=_ known as Metglas 2605S-3A, and Fe74Ni4M03B17Si2, known as Metglas 2605SM, with improved physical and magnetic properties [3]. The development of the new glasses is based on separate additions of Cr and Ni-Mo elements to the FeY8B&i9 glass. More specifically, the properties of the Fe7rCr2B1&i5 are found to exhibit very low core loss at high frequencies (>lkHz) when annealed to obtain a round B-H loop. Its saturation induction is much higher than that of ferrites and it can be used at elevated temperatures without cracking or showing large drops in useable flux density. In toroidal cores, losses are reduced as the temperature increases. In general, the properties of Fe#Zr2Bi6Si5 can be substantially tailored by annealing treatment. In addition, the alloy offers an improved high-frequency annealing cycle that makes field annealing optional. High153
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(1995)
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K. Habib et al.
154 frequency losses can be lowered without use of a field, whereas field annealing will increase permeability. Other anneals for low frequencies yield exceptionally high 60Hz permeabilities. This is useful in a current transformer or ground fault protection device. On the other hand, the properties of the Fer4Ni&IoaB&i2 glass are found in low core loss at high frequencies (>lkHz), concurrent with low exciting power. The alloy has exceptionally high-frequency permeability with only a slightly higher core loss than that of the Fe&&B&i, glass. Also, the glass has a high saturation induction and a relatively square B-H loop, making it suitable for use in medium frequency magnetic amplifiers. In addition, the Fe7fii@03B17Si2 glass has been developed in a way to offer a wide annealing window that facilitates processing. Thus, annealing treatments can provide very high-frequency or low-frequency permeabilities such as those required in ground fault protection devices and current transformers. In the present report, the results are given of a fundamental study on the materials. The study was conducted in order to understand the changes in the properties of the glasses as a function of Cr and Ni-Mo additions. Also, the study evaluated and compared the properties and structures of the glasses with respect to the element additions. As a result, a correlation between the obtained results is established.
EXPERIMENTAL
different glasses. The annealing process was carried out in an electric furnace at nearly 560°C for 1 h, after which the samples were slow cooled in the furnace. These steps were conducted in order to ensure the production of stable crystalline microstructures. Another three sets of samples were used without heat treatment and will be referred to as being in the as-received conditions, with asreceived structures. The difference between the as-received and crystalline structures was nondestructively determined by showing the contrast between the electron and X-ray diffraction patterns of both microstructures and macrostructures for the three different glasses. The internal structure of the glasses was examined by a transmission electron microscope (TEM). Prior to the examinations, the following steps were performed: 1. A number of samples of different glasses were cut to a suitable size (5mm) before milling. 2. The samples were thinned by ion milling at 4.5kV for times ranging from 10 to 24 h, depending on the type of the glasses. 3. Finally, the samples were examined in the TEM (a Hitachi 7000 made by Nissei Sangyo American) at 125kV. The surface structures of the glasses were examined by Nanoscope 11 scanning tunneling microscope (STM), made by Digital Instruments. Surface images were obtained based on a tunneling current ranged between 1.5 and 2.6nA.
DETAILS RESULTS
The metallic glasses used in the investigation were produced by a parallel flow casting method, details on which are given elsewhere [2]. Three different sets of samples were prepared from thin ribbons of FemB&&,, Fe+&.B&i5, and Fe7fi&Mo3B17Si2 glasses in a rectangular shape (25 x 20mm). The sample thicknesses were 16.7pm, 16.7pm, and 20pm for Fe78B13Si9, Fe,Cr,B&i,, and Fer4Ni&Io3B1& glasses, respectively. Three different sets of samples were annealed above the crystallization temperatures of the
AND DISCUSSION
The physical and magnetic properties of the glass were obtained from Allied Chemical [3] and are presented in Table 1. One can readily compare between the properties from the table. It is clear that the saturation induction, the saturation magnetostriction, and the maximum permeability of the FemB&ig glass decrease as a function of Cr and Ni-Mo additions. In fact, the decrease of the saturation induction and saturation magneto&i&on is greater with the addition of Ni-Mo to the Fe-B-Si glass than with the
155
Structures of Fe-Based Metallic Glasses Table 1
Physical and Magnetic Properties of the Metallic Glasses in As-Received Condition Metallic
glass property
%&&
Saturation induction (B,-Tesla) Saturation magnetostriction (h, - x10m6)
1.5 27
1.4
600,000
35,000
137
138
128
57
>60
860
58 860
>7 x 10s
>7 x 10s
>7 x 10s
7.18
7.29
4763
4732
7.50 4911
20
1.3 19
Maximum permeability in annealed conditions, (u,,,,,-d.c.) Electrical resistivity (p-uLncm) Elastic modulus (x 109N/m2) Vickers hardness (Hv50g
load)
Yield strength (N/m2) Density (glcm3) Molecular weight (10m3 x Kg/mole)
addition
of Cr. In contrast,
the decrease
of
is more pronounced with the addition of Cr than Ni-Mo to the Fe-B-Si glass. Furthermore, the electrical resistivity remains practically unchanged as Cr is ad.ded whereas it is decreased by the addition of Ni-Mo to the Fe-B-Si glass. This indicates that the FemB&i9 and Fe&-, B16Si5 glasses probably exhibit better corrosion resistance than the Fe74Ni&Io3B17Si2 glass in aqueous solutions. In general, the physical properties of the glasses, that is, elastic moldulus, Vickers hardness, yield strength, dlensity, and molecular weight, are found to be the same or slightly to increase with the addition of Cr. On the other hand, the physical properties are found to increase in the case of the Ni-Mo addition. This occurred because of the introduction of heavier elements such as Ni and MO to the structure of the Fe-B-Si glass. As a result, the molecular weigh.t of the Fe74Ni4M03B17Si2 is increased in comparison with Fe78B13Si+ All attempts to evaluate the properties of the glasses in annealing conditions above the crystallizat:ion temperatures of the glasses [3] were unsuccessful because of the extreme brittleness that the glasses suffered during the annealing process. The results from the study on the internal and surface structures of the glasses are shown in Figs. 1-6. Figures l(a), 3(a), and 5(a) illustrate internal structures of the Fe78B13Si9,Fe+&B&is, and Fe&Ii&IoaB1+Si2 glasses, respectively, in the as-received
the
maximum
permeability
100,000
990
condition. Figure l(a) indicates that the Fe7sBnSi9 has a homogeneous and completely disordered structure. There is no visible distinction observed in the structure. On the other hand, Figs. 3(a) and 5(a) indicate that the Fe,,Cr2B16Sis and Fe74Ni&Ios B&i2 glasses have grainy structures with submicrograins ranging in size from 0.4pm to 1.6um. Also, second-phase precipitates are visible in Figs. 3(a) and 5(a). Figures l(b), 3(b), and 5(b) are the electron diffraction patterns which respectively correspond to Figs. l(a), 3(a), and 5(a). That of FemBi3Si9 [Fig. l(b)] represents a typical amorphous structure. By contrast, the patterns of Fen Cr2B16Si5 [Fig. 3(b)] and Fe74Ni4M03B17Si2 [Fig. 5(b)] glasses represent typical microcrystalline structures. Figures l(c), 3(c), and 5(c) are the X-ray diffraction patterns which respectively correspond to Figs. l(a), 3(a), and 5(a). These represent general amorphous macrostructures, in contrast with the electron diffraction patterns [Figs. l(b), 3(b), and 5(b)]. It is clear from the internal structures that the separate addition of the Cr and Ni-Mo to the Fe-B-Si glass has produced microcrystalline structures in the as-received condition, even though the glasses attained amorphous macrostructures as a result of the rapid solidification process [Figs. 3(c) and 5(c)]. In other words, the additions of Cr and Ni-Mo slowed the freezing process of the molten metal of the Fe-B-Si glass to the point at which the nucleation and growth phenomena had sufficient time to take place
K. Habib et al.
156
(b)
300-
250 -
200-
100
50
1
1
k-0
FIG. 1. Fe78B13Si9in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures of Fe-Based Metallic Glasses
157
(a) 1000
...- --~~~~~~
..--.-.
(b) ~_.___.._~
.._
.
[counts] 900 I
i 600
1 -I
500 i
1 400
i
1 300200loo-
0.0 t
I
Z!O
I
40 (4
FIG. 2. Fe,sB13Sig in the annealed condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
I
$0
[’
K. Habib et al.
158
W
(a) 350
[countsl 300
250
200
150
IOO-
50-
0.0
’
I
d0
I
40
I
$0
[
!I31
(c)
(d)
FIG. 3. Fe&2B16Si5 in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures
[counts]
ofFe-Bused Metallic
Glasses
159
T---‘
FIG. 4. Fe~CqB& in the annealed condition: (a) internal structure, {b) electron diffraction pattern, (c) X-ray diEration pattern, and (d) surface &mcture.
n (d)
K. Habib et al.
160
lntsl
-
350 -
300-
250 -
200-
150
100
I
i’
7
(4
FIG. 5. Fe7QN&Mo3B17Si2in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures of Fe-Based Metallic Glasses
161
(a)
‘OOr------.-
(W
__-.
_
[countsl 600 1
500
300
1 i
FIG. 6. Fe74Ni,,Mo3B17Si2 in the annealed condition: (a) internal structure. (b\ electron diffraction pattern, (cj X-ray diffraction ~a&& and (d) surface stkture.
(df
K. Habib et al.
162 during the solidification of the glasses on a microscopic scale. Figures 2(a), 4(a), and 6(a) illustrate the internal structures of FersBuSi9, Fe&r2B1&i5, and Fer4N&Mo3Bi7Si2, respectively, in the annealed condition. It is obvious from the figures that the annealed samples have sharp grainy structures, subgrains, especially in the Fe7sBuSi9 and Fe74Ni4M03B1rSi2 glasses. The average size of the subgrains ranged from 1.6 to 5u.m. However, the subgrain size in the Fe&r2B1&i5 glass is in the range of 0.4 to 1.6um. This indicates that the addition of Cr to the Fe-B-Si glass has refined the structure of the glass as compared to Ni-Mo addition. Second-phase precipitates are visible in all the annealed samples. Similar structures were observed in an Fe-Ni metallic glass [4]. In addition, no crystallographic defects are observed at the given resolution of the TEM micrographs. Figures 2(b), 4(b), and 6(b) are the electron diffraction patterns which respectively correspond to Figs. 2(a), 4(a), and 6(a). These represent typical crystalline FCC structures. Figures 2(c), 4(c), and 6(c) are the respective X-ray diffraction patterns corresponding to Figs. 2(a), 4(a), and 6(a). Such patterns are typical of those obtained from macrocrystalline structures. It is well known that the mean size of a microcrystallite (i.e.,
D=Kh p cos
8
where D is the mean size of a microcrystallite, K is the shape factor which usually takes a value of about 0.9 [6], h is the wavelength of the radiation, B is the line broadening, and 0 is the diffraction angle. B = B - b, where B is the breadth of the diffraction line at its half-intensity maximum and b is the instrumental broadening. Details of the derivation of Eq. (1) can be found in Scherrer [5] and Klug and Alexander [6]. It is clear from Eq. (1) that D is inversely proportional to B, the line broadening. In
other words, as the line broadening increases, the size of the microcrystallite decreases. This kind of behavior can be seen in Figs. 3(c) and 5(c) where the line broadening is large at a diffraction angle 20 = 45” while in Figs. 4(c) and 6(c)B is small at the same angle. This implies that, even though the macrostructures of Figs. 3(c) and 5(c) are generally amorphous, microcrystallinity is present with the mean diameter of the microcrystallites being O.&m [from Figs. 3(a) and 5(a)]. In contrast, B is found to be small [for example, at 20 = 45” and 65” in Figs. 4(c) and 6(c)], indicating that large microcrystallites were developed. In fact, the mean diameter of the microcrystallite size is 2.5um in Figs. 2(a) and 6(a), except in Fig. 4(a) where the mean diameter remains the same (D = 0.8u.m) as in the as-received structure, Fig. 3(a). This observation occurred as a result of the addition of Cr to the glass, which causes refinement to the structure of the glass. This is in agreement with the established effect of chromium as a grain-refining element to the structure of steels [7]. The surface structures of the glasses were obtained without prior surface treatment. The surfaces of all the glasses have mirrorlike finishes, despite the surfaces not having been treated in either the as-received or the annealed forms. Results on the surface structures of the glasses in different conditions are shown in Figs. l(d)-6(d). The asreceived structures of Fe7sB1&, FenCr2Bi6 Si5, and Fer4Ni4M03Bi7Si2 are given in Figs. l(d), 3(d), and 5(d), respectively. These figures are basically three-dimensional line plots of the surface profile in which completely distorted to partially distorted structures are observed on a 200 x 200nm scanning area. For instance, the surface structure of FemB&i9 glass is completely distorted in comparison with the partially distorted structures of the other glasses. These observations are in agreement with previous ones made by the authors about other metallic glasses [&lo]. In fact, the authors have demonstrated that the STM can be utilized to measure the surface roughness of an optically thin film on a nanoscopic scale [lo]. The results show that the value of the sur-
Structures
163
of Fe-Based Metallic Glasses
face roughness represents approximately the entire surface area, especially if the samples have a mirrorlike finish, such as with metallic glasses. In general, it was found that the Fe78B1& glass exhibited the highest range of the surface roughness among the glasses examined. The surface roughness of this particular glass ranged from 5 to 20nm [Fig. l(d)]. By contrast, the surface roughness of the Fe&r2 B&i5 [Fig. 3(d)] and FeY4N&Mo3Bi7Si2 [Fig. 5(d)] glasses varied from 2 to 4nm and from 4 to 8nm, respectively. In other words, the additions of Cr and Ni-Mo to the FeB-Si reduce the surface roughness of the glass. Howlever, the surface structure of the Fe74N&Mo31B17Si2glass appears to have a number of atomic clusters with a size equivalent to 25-50nm in diameter. This can probably be attributed to the introduction of the heavier elelments (Ni and MO) to the structure of the Fe-B-Si glass. The molecular weight of the Fe-B-Si glass is increased as a result of the addition of Ni and MO (Table l), whereas there is little change in the molecular weight with the addition of Cr. This indicates that the molecular weight of the Fe7&J&Mo3B&i2 is overbalanced as far as the surface structure is concerned compared to the other metallic glasses. In other words, the addition of Ni and MO caused a misbalance on a nanoscopic scale. Similar observations have been previously documented in the Fe&oisBisSi metallic glass as a consequence of the addition of Co to the Fe-B-Si glass [ll]. The surfmace structures of the annealed samples of the Fe78B&i9, Fe7#&B&i5, and Fe74Ni4M031B17Si2are shown in Figs. 2(d), 4(d), and 6(d), respectively. These figures indicate th,at the surface structures of the glasses attain less surface distortion in the annealed condition than in the as-received condition. This probably occurred as a consequence of the surface atomic arrangement during the annealing process. It is worth noting that the annealing process dissolved away the atomic clusters which had been observed in the as-received structure of the Fe74Ni4Mo313&i2 glass [Fig. 6(d)]. On the other hand, a partial degree of surface re-
covery is noted in the other samples [Figs. 2(d) and 4(d)]. These observations can be explained as a result of the high atomic mobility of the Fe74N&Mo3B1$Si2 glass relative to the other samples during the annealing process.
CONCLUSIONS
The following conclusions the present study:
are drawn from
1. Fe7sBi3Si9 attains lower saturation
2.
3.
4.
5.
6.
induction, saturation magnetostriction, and maximum permeability with Cr and Ni-Mo additions. The effect of Cr addition is less than that of Ni-Mo in lowering the saturation induction and saturation magnetostriction but not in the case of the maximum permeability in which a pronounced decrease occurs. The electrical resistivity remains the same with the addition of Cr to the FeB-Si glass, but, by contrast, the addition of Ni-Mo causes a decrease. As a result, one may suggest that the addition of NiMO to the Fe-B-Si glass probably decreases the corrosion resistance of the glass in aqueous solutions. In general, the physical properties of the Fe-B-Si glass are found to be the same or slightly increased with the addition of Cr. On the other hand, the addition of Ni-Mo causes increases in them. The internal structure of the as-received Fe7gB1&i9 glass seems to have a homogeneous and completely disordered structure. There is no visible distinction observed. In contrast, the internal structures of the as-received Fe77Cr2B16Si5 and FeT4Ni4 Mo3Bi7Si2 glasses are found to be grainy, with submicrograins ranging in size from 0.4 to 1.6pm. For all the annealed samples, the internal structures are found to possess sharp grainy structures, with the average subgrain size in the range 0.4 to 5pm. The subgrains in Fe77Cr2B16Si5 are smaller than the other samples, an indication
K. Habib
164
7.
8.
9.
10.
11.
12.
that the addition of Cr to the Fe-B-Si refined the structure of the glass. The electron diffraction patterns show that the addition of Cr and Ni-Mo has microcrystallized the structure of the FeB-Si glass in the as-received condition. In addition, the patterns demonstrate that all the glasses have a FCC structure. The X-ray diffraction patterns of the asreceived samples show that the addition of Cr and Ni-Mo to the structure of FeB-Si glass induced microcrystallinities even though the general structures of these samples remained amorphous. The surface structures of the glasses are revealed in three-dimensional line plots. The plots represent the surface topography in which completely distorted to partially distorted structures were observed along a 200 x 200nm scanning area. In general, the surface roughness of the glasses is higher in the as-received condition than in the annealed condition. The Fe7sB&i9 has the highest surface roughness of the three glasses in the asreceived condition. The addition of Cr and Ni-Mo decreases the surface roughness, with the addition of the latter producing atomic clusters with diameters ranging in size from 25 to 50mm. The surface structures of the annealed samples show less surface distortion compared with the as-received structures. In fact, the atomic clusters found in the Fe74Ni4M03B17Si2 glass dissolved away during the annealing process.
et al.
The authors gratefully acknowledge the assistance provided by Allied Chemical Inc. in supplying the Metglas samples.
References 1. W. Klement, R. Willen, and l? Duwez, Noncrystalline structure in solidified gold-silicon alloys, Nature 1871869-870 (1960). 2. H. Beck and H. Guntherodt, Glass Metal Springer-Verlag, Berlin, pp. 167, 343 (1983).
II,
3. Metglas magnetic alloys, Data Sheet on Magnetic Alloys, Allied Chemical Inc., Prispery, N.J. (1983). 4. K. Habib, K. Moore, and l? Fritz, A TEM study of an Fe-Ni metallic glass, 1. Muter. Sci. Mt. 9:852-853 (1990). 5. l? Scherrer, Gtittinger Nachrichten,
pp. 2, 92 (1918).
6. H. l? Klug and L. E. Alexander, X-my puwder difiaction procedures, John Wiley and Sons, New York, pp. 24, 512 (1954). 7. W. Smith, Principles ofMaterialsScience Engineering, McGraw-Hill, New York, p. 471 (1986). 8. K. Habib, V. Eling, C. Wu, K. Moore, and R. Mehalik, The effect of C and Co additions on the properties of a Fe-B-Si metallic glass, Scripta Metall. Muter. 24:1057-1062 (1990). 9. K. Habib, K. Moore, and R. Mehalik, Correlation between data obtained by TEM and STM for a cobalt-based metallic glass in amorphous and crystalline forms, Microstruct. Sci. l&371-379 (1989). 10. K. Habib, V. Eling, and C. Wu, Measuring surface roughness of and optical thin film with scanning tunneling microscopes, J. Muter. Sci. Lett. 9:1194 (1990). 11. K. Habib, K. Moore, and R. Nessler, Nanoscopic structures of Fe-B-Si metallic glasses, Thin Solid FiZilms 215:162-165 (1992).
Received September 1994; accepted March 199.5.
V. Eling,t and C. WUf
*lowa Electron Microscopy Center, University Instruments, Inc., Santa Barbara, CA 93227
of Iowa, Iowa City, IA 52240; and +Digital
A fundamental
study has been conducted
on Fe78B13Sig metallic thin ribbon in order to
understand the changes in the properties and structures of the glass as a function of Cr and Ni-Mo additions. The separate additions of Cr and Ni-Mo were successful in producing two new m.etallic thin glasses with the compositions Fe&&B&i, and Fe&&Mo,Br7SiZ. The study was focused on evaluating the physical and magnetic properties of these glasses with respect to the Cr and Ni-Mo additions. In addition, characterization of the internal and the surface structures of the glasses was conducted using transmission electron microscopy
and scanning
tunneling
microscopy,
respectively.
A comparison
between
the
internal and surface structures of the glasses was carried out on both amorphous and crystalline forms. .4s a result, a correlation between the properties and the structures of the glasses was established.
INTRODUCTION Since the invention of metallic glasses in 1960 [l, 21, the alloys have become widely studied because of the many practical values arising from their (extreme homogeneous and disordered atomic structures. For instance, Allied Chelmical Inc. has recently reported results on a FemB&& metallic glass, known commercially as Metglas 2605s [3]. The results showed that the glass exhibits excellent physical and magnetic properties compared to the crystalline alloys. The glass is found suitable for extremely low core loss in distribution, power transformers, and motors. It combines high induction and superb magnetic properties at the frequencies, induction, and operating temperatures of these devices. Furthermore, the glass can be used in inductions, current transformers, and other devices requiring high permeability and low core loss at low frequencies. Subsequent to the development of Fe78Br3Si9
metallic glass, Allied Chemical has succeeded in developing two new metallic glasses, Fe+&B&i=_ known as Metglas 2605S-3A, and Fe74Ni4M03B17Si2, known as Metglas 2605SM, with improved physical and magnetic properties [3]. The development of the new glasses is based on separate additions of Cr and Ni-Mo elements to the FeY8B&i9 glass. More specifically, the properties of the Fe7rCr2B1&i5 are found to exhibit very low core loss at high frequencies (>lkHz) when annealed to obtain a round B-H loop. Its saturation induction is much higher than that of ferrites and it can be used at elevated temperatures without cracking or showing large drops in useable flux density. In toroidal cores, losses are reduced as the temperature increases. In general, the properties of Fe#Zr2Bi6Si5 can be substantially tailored by annealing treatment. In addition, the alloy offers an improved high-frequency annealing cycle that makes field annealing optional. High153
MATERIALS C:HARACTERIZATION 34153-161 0 Elsevier Science Inc., 1995
(1995)
655 Avenue of the Americas, New York, NY 10010
1044-5803/95/$9.50 SSDI 10445803(95)00081-9
K. Habib et al.
154 frequency losses can be lowered without use of a field, whereas field annealing will increase permeability. Other anneals for low frequencies yield exceptionally high 60Hz permeabilities. This is useful in a current transformer or ground fault protection device. On the other hand, the properties of the Fer4Ni&IoaB&i2 glass are found in low core loss at high frequencies (>lkHz), concurrent with low exciting power. The alloy has exceptionally high-frequency permeability with only a slightly higher core loss than that of the Fe&&B&i, glass. Also, the glass has a high saturation induction and a relatively square B-H loop, making it suitable for use in medium frequency magnetic amplifiers. In addition, the Fe7fii@03B17Si2 glass has been developed in a way to offer a wide annealing window that facilitates processing. Thus, annealing treatments can provide very high-frequency or low-frequency permeabilities such as those required in ground fault protection devices and current transformers. In the present report, the results are given of a fundamental study on the materials. The study was conducted in order to understand the changes in the properties of the glasses as a function of Cr and Ni-Mo additions. Also, the study evaluated and compared the properties and structures of the glasses with respect to the element additions. As a result, a correlation between the obtained results is established.
EXPERIMENTAL
different glasses. The annealing process was carried out in an electric furnace at nearly 560°C for 1 h, after which the samples were slow cooled in the furnace. These steps were conducted in order to ensure the production of stable crystalline microstructures. Another three sets of samples were used without heat treatment and will be referred to as being in the as-received conditions, with asreceived structures. The difference between the as-received and crystalline structures was nondestructively determined by showing the contrast between the electron and X-ray diffraction patterns of both microstructures and macrostructures for the three different glasses. The internal structure of the glasses was examined by a transmission electron microscope (TEM). Prior to the examinations, the following steps were performed: 1. A number of samples of different glasses were cut to a suitable size (5mm) before milling. 2. The samples were thinned by ion milling at 4.5kV for times ranging from 10 to 24 h, depending on the type of the glasses. 3. Finally, the samples were examined in the TEM (a Hitachi 7000 made by Nissei Sangyo American) at 125kV. The surface structures of the glasses were examined by Nanoscope 11 scanning tunneling microscope (STM), made by Digital Instruments. Surface images were obtained based on a tunneling current ranged between 1.5 and 2.6nA.
DETAILS RESULTS
The metallic glasses used in the investigation were produced by a parallel flow casting method, details on which are given elsewhere [2]. Three different sets of samples were prepared from thin ribbons of FemB&&,, Fe+&.B&i5, and Fe7fi&Mo3B17Si2 glasses in a rectangular shape (25 x 20mm). The sample thicknesses were 16.7pm, 16.7pm, and 20pm for Fe78B13Si9, Fe,Cr,B&i,, and Fer4Ni&Io3B1& glasses, respectively. Three different sets of samples were annealed above the crystallization temperatures of the
AND DISCUSSION
The physical and magnetic properties of the glass were obtained from Allied Chemical [3] and are presented in Table 1. One can readily compare between the properties from the table. It is clear that the saturation induction, the saturation magnetostriction, and the maximum permeability of the FemB&ig glass decrease as a function of Cr and Ni-Mo additions. In fact, the decrease of the saturation induction and saturation magneto&i&on is greater with the addition of Ni-Mo to the Fe-B-Si glass than with the
155
Structures of Fe-Based Metallic Glasses Table 1
Physical and Magnetic Properties of the Metallic Glasses in As-Received Condition Metallic
glass property
%&&
Saturation induction (B,-Tesla) Saturation magnetostriction (h, - x10m6)
1.5 27
1.4
600,000
35,000
137
138
128
57
>60
860
58 860
>7 x 10s
>7 x 10s
>7 x 10s
7.18
7.29
4763
4732
7.50 4911
20
1.3 19
Maximum permeability in annealed conditions, (u,,,,,-d.c.) Electrical resistivity (p-uLncm) Elastic modulus (x 109N/m2) Vickers hardness (Hv50g
load)
Yield strength (N/m2) Density (glcm3) Molecular weight (10m3 x Kg/mole)
addition
of Cr. In contrast,
the decrease
of
is more pronounced with the addition of Cr than Ni-Mo to the Fe-B-Si glass. Furthermore, the electrical resistivity remains practically unchanged as Cr is ad.ded whereas it is decreased by the addition of Ni-Mo to the Fe-B-Si glass. This indicates that the FemB&i9 and Fe&-, B16Si5 glasses probably exhibit better corrosion resistance than the Fe74Ni&Io3B17Si2 glass in aqueous solutions. In general, the physical properties of the glasses, that is, elastic moldulus, Vickers hardness, yield strength, dlensity, and molecular weight, are found to be the same or slightly to increase with the addition of Cr. On the other hand, the physical properties are found to increase in the case of the Ni-Mo addition. This occurred because of the introduction of heavier elements such as Ni and MO to the structure of the Fe-B-Si glass. As a result, the molecular weigh.t of the Fe74Ni4M03B17Si2 is increased in comparison with Fe78B13Si+ All attempts to evaluate the properties of the glasses in annealing conditions above the crystallizat:ion temperatures of the glasses [3] were unsuccessful because of the extreme brittleness that the glasses suffered during the annealing process. The results from the study on the internal and surface structures of the glasses are shown in Figs. 1-6. Figures l(a), 3(a), and 5(a) illustrate internal structures of the Fe78B13Si9,Fe+&B&is, and Fe&Ii&IoaB1+Si2 glasses, respectively, in the as-received
the
maximum
permeability
100,000
990
condition. Figure l(a) indicates that the Fe7sBnSi9 has a homogeneous and completely disordered structure. There is no visible distinction observed in the structure. On the other hand, Figs. 3(a) and 5(a) indicate that the Fe,,Cr2B16Sis and Fe74Ni&Ios B&i2 glasses have grainy structures with submicrograins ranging in size from 0.4pm to 1.6um. Also, second-phase precipitates are visible in Figs. 3(a) and 5(a). Figures l(b), 3(b), and 5(b) are the electron diffraction patterns which respectively correspond to Figs. l(a), 3(a), and 5(a). That of FemBi3Si9 [Fig. l(b)] represents a typical amorphous structure. By contrast, the patterns of Fen Cr2B16Si5 [Fig. 3(b)] and Fe74Ni4M03B17Si2 [Fig. 5(b)] glasses represent typical microcrystalline structures. Figures l(c), 3(c), and 5(c) are the X-ray diffraction patterns which respectively correspond to Figs. l(a), 3(a), and 5(a). These represent general amorphous macrostructures, in contrast with the electron diffraction patterns [Figs. l(b), 3(b), and 5(b)]. It is clear from the internal structures that the separate addition of the Cr and Ni-Mo to the Fe-B-Si glass has produced microcrystalline structures in the as-received condition, even though the glasses attained amorphous macrostructures as a result of the rapid solidification process [Figs. 3(c) and 5(c)]. In other words, the additions of Cr and Ni-Mo slowed the freezing process of the molten metal of the Fe-B-Si glass to the point at which the nucleation and growth phenomena had sufficient time to take place
K. Habib et al.
156
(b)
300-
250 -
200-
100
50
1
1
k-0
FIG. 1. Fe78B13Si9in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures of Fe-Based Metallic Glasses
157
(a) 1000
...- --~~~~~~
..--.-.
(b) ~_.___.._~
.._
.
[counts] 900 I
i 600
1 -I
500 i
1 400
i
1 300200loo-
0.0 t
I
Z!O
I
40 (4
FIG. 2. Fe,sB13Sig in the annealed condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
I
$0
[’
K. Habib et al.
158
W
(a) 350
[countsl 300
250
200
150
IOO-
50-
0.0
’
I
d0
I
40
I
$0
[
!I31
(c)
(d)
FIG. 3. Fe&2B16Si5 in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures
[counts]
ofFe-Bused Metallic
Glasses
159
T---‘
FIG. 4. Fe~CqB& in the annealed condition: (a) internal structure, {b) electron diffraction pattern, (c) X-ray diEration pattern, and (d) surface &mcture.
n (d)
K. Habib et al.
160
lntsl
-
350 -
300-
250 -
200-
150
100
I
i’
7
(4
FIG. 5. Fe7QN&Mo3B17Si2in the as-received condition: (a) internal structure, (b) electron diffraction pattern, (c) X-ray diffraction pattern, and (d) surface structure.
Structures of Fe-Based Metallic Glasses
161
(a)
‘OOr------.-
(W
__-.
_
[countsl 600 1
500
300
1 i
FIG. 6. Fe74Ni,,Mo3B17Si2 in the annealed condition: (a) internal structure. (b\ electron diffraction pattern, (cj X-ray diffraction ~a&& and (d) surface stkture.
(df
K. Habib et al.
162 during the solidification of the glasses on a microscopic scale. Figures 2(a), 4(a), and 6(a) illustrate the internal structures of FersBuSi9, Fe&r2B1&i5, and Fer4N&Mo3Bi7Si2, respectively, in the annealed condition. It is obvious from the figures that the annealed samples have sharp grainy structures, subgrains, especially in the Fe7sBuSi9 and Fe74Ni4M03B1rSi2 glasses. The average size of the subgrains ranged from 1.6 to 5u.m. However, the subgrain size in the Fe&r2B1&i5 glass is in the range of 0.4 to 1.6um. This indicates that the addition of Cr to the Fe-B-Si glass has refined the structure of the glass as compared to Ni-Mo addition. Second-phase precipitates are visible in all the annealed samples. Similar structures were observed in an Fe-Ni metallic glass [4]. In addition, no crystallographic defects are observed at the given resolution of the TEM micrographs. Figures 2(b), 4(b), and 6(b) are the electron diffraction patterns which respectively correspond to Figs. 2(a), 4(a), and 6(a). These represent typical crystalline FCC structures. Figures 2(c), 4(c), and 6(c) are the respective X-ray diffraction patterns corresponding to Figs. 2(a), 4(a), and 6(a). Such patterns are typical of those obtained from macrocrystalline structures. It is well known that the mean size of a microcrystallite (i.e.,
D=Kh p cos
8
where D is the mean size of a microcrystallite, K is the shape factor which usually takes a value of about 0.9 [6], h is the wavelength of the radiation, B is the line broadening, and 0 is the diffraction angle. B = B - b, where B is the breadth of the diffraction line at its half-intensity maximum and b is the instrumental broadening. Details of the derivation of Eq. (1) can be found in Scherrer [5] and Klug and Alexander [6]. It is clear from Eq. (1) that D is inversely proportional to B, the line broadening. In
other words, as the line broadening increases, the size of the microcrystallite decreases. This kind of behavior can be seen in Figs. 3(c) and 5(c) where the line broadening is large at a diffraction angle 20 = 45” while in Figs. 4(c) and 6(c)B is small at the same angle. This implies that, even though the macrostructures of Figs. 3(c) and 5(c) are generally amorphous, microcrystallinity is present with the mean diameter of the microcrystallites being O.&m [from Figs. 3(a) and 5(a)]. In contrast, B is found to be small [for example, at 20 = 45” and 65” in Figs. 4(c) and 6(c)], indicating that large microcrystallites were developed. In fact, the mean diameter of the microcrystallite size is 2.5um in Figs. 2(a) and 6(a), except in Fig. 4(a) where the mean diameter remains the same (D = 0.8u.m) as in the as-received structure, Fig. 3(a). This observation occurred as a result of the addition of Cr to the glass, which causes refinement to the structure of the glass. This is in agreement with the established effect of chromium as a grain-refining element to the structure of steels [7]. The surface structures of the glasses were obtained without prior surface treatment. The surfaces of all the glasses have mirrorlike finishes, despite the surfaces not having been treated in either the as-received or the annealed forms. Results on the surface structures of the glasses in different conditions are shown in Figs. l(d)-6(d). The asreceived structures of Fe7sB1&, FenCr2Bi6 Si5, and Fer4Ni4M03Bi7Si2 are given in Figs. l(d), 3(d), and 5(d), respectively. These figures are basically three-dimensional line plots of the surface profile in which completely distorted to partially distorted structures are observed on a 200 x 200nm scanning area. For instance, the surface structure of FemB&i9 glass is completely distorted in comparison with the partially distorted structures of the other glasses. These observations are in agreement with previous ones made by the authors about other metallic glasses [&lo]. In fact, the authors have demonstrated that the STM can be utilized to measure the surface roughness of an optically thin film on a nanoscopic scale [lo]. The results show that the value of the sur-
Structures
163
of Fe-Based Metallic Glasses
face roughness represents approximately the entire surface area, especially if the samples have a mirrorlike finish, such as with metallic glasses. In general, it was found that the Fe78B1& glass exhibited the highest range of the surface roughness among the glasses examined. The surface roughness of this particular glass ranged from 5 to 20nm [Fig. l(d)]. By contrast, the surface roughness of the Fe&r2 B&i5 [Fig. 3(d)] and FeY4N&Mo3Bi7Si2 [Fig. 5(d)] glasses varied from 2 to 4nm and from 4 to 8nm, respectively. In other words, the additions of Cr and Ni-Mo to the FeB-Si reduce the surface roughness of the glass. Howlever, the surface structure of the Fe74N&Mo31B17Si2glass appears to have a number of atomic clusters with a size equivalent to 25-50nm in diameter. This can probably be attributed to the introduction of the heavier elelments (Ni and MO) to the structure of the Fe-B-Si glass. The molecular weight of the Fe-B-Si glass is increased as a result of the addition of Ni and MO (Table l), whereas there is little change in the molecular weight with the addition of Cr. This indicates that the molecular weight of the Fe7&J&Mo3B&i2 is overbalanced as far as the surface structure is concerned compared to the other metallic glasses. In other words, the addition of Ni and MO caused a misbalance on a nanoscopic scale. Similar observations have been previously documented in the Fe&oisBisSi metallic glass as a consequence of the addition of Co to the Fe-B-Si glass [ll]. The surfmace structures of the annealed samples of the Fe78B&i9, Fe7#&B&i5, and Fe74Ni4M031B17Si2are shown in Figs. 2(d), 4(d), and 6(d), respectively. These figures indicate th,at the surface structures of the glasses attain less surface distortion in the annealed condition than in the as-received condition. This probably occurred as a consequence of the surface atomic arrangement during the annealing process. It is worth noting that the annealing process dissolved away the atomic clusters which had been observed in the as-received structure of the Fe74Ni4Mo313&i2 glass [Fig. 6(d)]. On the other hand, a partial degree of surface re-
covery is noted in the other samples [Figs. 2(d) and 4(d)]. These observations can be explained as a result of the high atomic mobility of the Fe74N&Mo3B1$Si2 glass relative to the other samples during the annealing process.
CONCLUSIONS
The following conclusions the present study:
are drawn from
1. Fe7sBi3Si9 attains lower saturation
2.
3.
4.
5.
6.
induction, saturation magnetostriction, and maximum permeability with Cr and Ni-Mo additions. The effect of Cr addition is less than that of Ni-Mo in lowering the saturation induction and saturation magnetostriction but not in the case of the maximum permeability in which a pronounced decrease occurs. The electrical resistivity remains the same with the addition of Cr to the FeB-Si glass, but, by contrast, the addition of Ni-Mo causes a decrease. As a result, one may suggest that the addition of NiMO to the Fe-B-Si glass probably decreases the corrosion resistance of the glass in aqueous solutions. In general, the physical properties of the Fe-B-Si glass are found to be the same or slightly increased with the addition of Cr. On the other hand, the addition of Ni-Mo causes increases in them. The internal structure of the as-received Fe7gB1&i9 glass seems to have a homogeneous and completely disordered structure. There is no visible distinction observed. In contrast, the internal structures of the as-received Fe77Cr2B16Si5 and FeT4Ni4 Mo3Bi7Si2 glasses are found to be grainy, with submicrograins ranging in size from 0.4 to 1.6pm. For all the annealed samples, the internal structures are found to possess sharp grainy structures, with the average subgrain size in the range 0.4 to 5pm. The subgrains in Fe77Cr2B16Si5 are smaller than the other samples, an indication
K. Habib
164
7.
8.
9.
10.
11.
12.
that the addition of Cr to the Fe-B-Si refined the structure of the glass. The electron diffraction patterns show that the addition of Cr and Ni-Mo has microcrystallized the structure of the FeB-Si glass in the as-received condition. In addition, the patterns demonstrate that all the glasses have a FCC structure. The X-ray diffraction patterns of the asreceived samples show that the addition of Cr and Ni-Mo to the structure of FeB-Si glass induced microcrystallinities even though the general structures of these samples remained amorphous. The surface structures of the glasses are revealed in three-dimensional line plots. The plots represent the surface topography in which completely distorted to partially distorted structures were observed along a 200 x 200nm scanning area. In general, the surface roughness of the glasses is higher in the as-received condition than in the annealed condition. The Fe7sB&i9 has the highest surface roughness of the three glasses in the asreceived condition. The addition of Cr and Ni-Mo decreases the surface roughness, with the addition of the latter producing atomic clusters with diameters ranging in size from 25 to 50mm. The surface structures of the annealed samples show less surface distortion compared with the as-received structures. In fact, the atomic clusters found in the Fe74Ni4M03B17Si2 glass dissolved away during the annealing process.
et al.
The authors gratefully acknowledge the assistance provided by Allied Chemical Inc. in supplying the Metglas samples.
References 1. W. Klement, R. Willen, and l? Duwez, Noncrystalline structure in solidified gold-silicon alloys, Nature 1871869-870 (1960). 2. H. Beck and H. Guntherodt, Glass Metal Springer-Verlag, Berlin, pp. 167, 343 (1983).
II,
3. Metglas magnetic alloys, Data Sheet on Magnetic Alloys, Allied Chemical Inc., Prispery, N.J. (1983). 4. K. Habib, K. Moore, and l? Fritz, A TEM study of an Fe-Ni metallic glass, 1. Muter. Sci. Mt. 9:852-853 (1990). 5. l? Scherrer, Gtittinger Nachrichten,
pp. 2, 92 (1918).
6. H. l? Klug and L. E. Alexander, X-my puwder difiaction procedures, John Wiley and Sons, New York, pp. 24, 512 (1954). 7. W. Smith, Principles ofMaterialsScience Engineering, McGraw-Hill, New York, p. 471 (1986). 8. K. Habib, V. Eling, C. Wu, K. Moore, and R. Mehalik, The effect of C and Co additions on the properties of a Fe-B-Si metallic glass, Scripta Metall. Muter. 24:1057-1062 (1990). 9. K. Habib, K. Moore, and R. Mehalik, Correlation between data obtained by TEM and STM for a cobalt-based metallic glass in amorphous and crystalline forms, Microstruct. Sci. l&371-379 (1989). 10. K. Habib, V. Eling, and C. Wu, Measuring surface roughness of and optical thin film with scanning tunneling microscopes, J. Muter. Sci. Lett. 9:1194 (1990). 11. K. Habib, K. Moore, and R. Nessler, Nanoscopic structures of Fe-B-Si metallic glasses, Thin Solid FiZilms 215:162-165 (1992).
Received September 1994; accepted March 199.5.