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Surface and bulk crystallization mechanisms in the Metglas Fe78B13Si9 investigated by Mössbauer spectroscopy
M. g
Journal of Magnetism and Magnetic Materials 117 (1992) 93-101 North-Holland
Surface and bulk crystallization mechanisms in the Metglas FeTsB138i 9 investigated by M6ssbauer spectroscopy N. R a n d r i a n a n t o a n d r o a, J.M. G r e n e c h e a and F. Varret b Laboratoire de Physique des MatJriaux, URA, CNRS, 807 Universit# du Maine, BP 535, F 72017 Le Mans Cedex, France b DRP., URA CNRS 71, UniversitJ Pierre et Marie Curie (Paris VI) F 75252 Paris Cedex 05, France a
Received 5 Mei 1992
The amorphous-to-crystalline transformation of the as-received Metglas FeTsB13Si 9 (Metglas ® 2605S2) was studied by transmission and reflection Mfssbauer spectroscopy in order to compare shiny and dull surfaces and bulk crystallization mechanisms. In agreement with previous results, the onset of crystallization occurs on both surfaces, simultaneously to the out-of-plane anisotropy. Higher kinetics of crystallization is observed at the dull surface. Then, primary and eutectic crystallization processes seem to be evidenced for the shiny and dull surfaces respectively. It is proposed that these different phenomena originate from the presence of structural and chemical inhomogeneities induced during the solidification process.
1. Introduction
The amorphous-to-crystalline transformation of metallic glasses has received large attention because of fundamental interest in understanding their structural and physical properties and because of potential applications in relation to their thermodynamic properties. Numerous studies were carried out on structural relaxation and on the evolution of the magnetic properties as a function of the annealing time and of the annealing temperature; examples are given on binary [1] and on ternary [2-7] iron-based metallic glasses. For the F e - B - S i amorphous system, the nature of the crystalline phases is strongly dependent upon the chemical composition and upon the rapid quenching conditions; in other respect, differences in crystallization morphology were observed [8] with contradictory results reported in the literature concerning the crystallization process [9,10]. Correspondence to: Dr. J.M. Greneche, Laboratoire de Physique des Mat6riaux, URA, CNRS, 807 Universit6 du Maine, BP 535, F 72017 Le Mans Cedex, France.
Annealing treatments were performed on the metallic glass FeTaB13Si9 under different conditions in order to study the evolution of magnetic properties [11,12] and the surface crystallization process [12,13]. It has been found that the nature of the atmosphere has no significant effect on the crystallization mechanisms [13]. More recently, the microstructures and kinetics of the crystallization of the Metglas F e 7 8 B l 3 S i 9 w e r e followed by X-ray diffraction, transmission electron microscopy, differential thermal analysis and differential scanning calorimetry (on a 10 mm wide and 25 Ixm thick ribbon): the simultaneous formation of ot(Fe-Si) and F e 3 B crystalline phases was evidenced and the transformation of the Fe3B phase into a stable crystalline phase of Fe2B was observed at higher temperature [4]. In complement to calorimeter measurements, M6ssbauer spectroscopy proved to be an excellent tool to follow the thermal evolution of the amorphous material under annealing, because of its local probe character: it provides fruitful information relative to (i) the structural relaxation, (ii) the transition from the amorphous to crystalline state through the evolution of the hyperfine data
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
94
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13Si 9
and the increase of the recoil-free fraction [2,14,15], and (iii) the magnetic texture through the line intensities of the magnetic spectra [16]. The bulk and the surfaces can be investigated separately by using, respectively, transmission geometry (~/-rays) and scattering geometry (conversion electrons) M6ssbauer techniques. So, some recent studies reveal that the kinetics of surface crystallization differs from that of the bulk with a weakly higher activation energy for surface crystallization [17,18]. In addition, it must be kept in mind that the as-quenched amorphous ribbons prepared by the melt-spinning technique exhibit structural and chemical inhomogeneities originating from thermal stresses induced during the solidification process. It can be suspected that such inhomogeneities will affect the surface properties and the crystallization process (through nucleation); this could explain the diversity of published experimental data in that field. In the present paper, we study the transformation of the Metglas 2605S2 prepared by the meltspinning technique, from the as-quenched amorphous to the crystalline state. We report the structural evolution and the changes in the magnetic texture of the bulk and of the surfaces from transmission and reflection M/Sssbauer techniques in order to characterize the different crystallization processes.
2. Experimental section
The 57Fe M6ssbauer spectra were recorded at room temperature in transmission and reflection geometry with a constant acceleration spectrometer, using a 57Co source diffused in a rhodium matrix. Conversion-electron M6ssbauer spectra were recorded at room temperature using a continuous H e - C H 4 gas flow proportional counter providing average information over the range 0200 nm in depth. The samples were cut from an as-received ribbon roll of amorphous Fe78B13Si9 (Metglas ® 2605S2) prepared by Allied Chemical Company with a preliminary thermal treatment (2 h at 380°C) under influence of a longitudinal external
magnetic field. The absorbers were made of a single piece of ribbon, 17 Ixm thick, oriented perpendicularly to the -y-beam, so that thickness effects can be neglected in the treatment of the transmission spectra. The samples were annealed in a helium atmosphere (in order to avoid surface oxidation) in a classical furnace with a temperature stability of + 2°C. The annealing temperatures ranged from 300 to 900 K. The heating rate was 20°C/min; after 60 min annealing, the sample was removed from the furnace and cooled down to room temperature.
3. Results
Some transmission M6ssbauer spectra are given in fig. 1 for different annealing temperatures. Below TA = 760 K, these spectra exhibit six well resolved and broad lines; sharp lines typical of crystalline phases appear as TA is raised above 760 K. Also, the relative intensities of the intermediate lines strongly differ for TA > 720 K. Figs. 2 and 3 exhibit conversion-electron M6ssbauer spectra recorded respectively on shiny and dull surfaces of 2605S2 ribbons at several annealing temperatures. Firstly, before any quantitative analysis, these spectra give evidence that the surfaces crystallize before the bulk (in agreement with results of literature data [3,19-21]). Both transmission and reflection M6ssbauer spectra were analyzed using the Mosfit program [22]. The typical amorphous spectra were fitted well by using a discrete distribution of hyperfine fields linearly correlated to an isomer shift distribution in order to reproduce the asymmetry of the sextet. The refined mean values of hyperfine parameters, including the isomer shift, the quadrupole shift, the hyperfine field, and the angle /3 defined by the direction of the ~/-beam and that of the hyperfine field, are reported in table 1. The mean values of the hyperfine field and the angle/3 are plotted as a function of the annealing temperature in fig. 4. The hyperfine field distribution determined from these spectra
N. Randrianantoandro et al. / Crystallization in Metglas Fe 78 BI3S[ o
exhibits a close to Gaussian shape: some examples are reported in fig. 5. The other spectra result from the superposition of four crystalline components and one amorphous contribution if any, assumed to result from a pure hyperfine field distribution similar to the previous ones. One can assign three of the -I0
0
95
- I0 &
0 I
+10
~
T A = 300 K
~I0
'K
ZK
-I0
~K
0
VELOCITY ( l / s
+10
}
Fig. 2. Conversion-electron M6ssbauer spectra recorded at room temperature on shiny surface of amorphous ribbon Fe78B13Si 9 after annealing at TA during 60 min. 9K
#
TA m 883 K
~
-10
~
VELOCITY ( t ~ t / s
+I0 )
Fig. 1. Transmission M6ssbauer spectra recorded at room temperature on amorphous ribbon FeTsB13Si 9 after annealing at TA during 60 min.
crystalline sextets to the three iron sites in et-FeSi alloy (with respectively 6, 7 and 8 Fe nearest neighbours) and the last component to Fe2B. The refined values of the hyperfine parameters, reported for different annealing temperatures, are reported in table 2 together with those previously obtained on crystalline samples [2]. To support this partial surface crystallization, we performed small- and large-angle X-ray diffraction: well defined peaks corresponding to the a - ( F e - S i ) crystalline phase were observed. In addition, an Auger electron Spectroscopy experiment with a sputtering system using argon ions, has revealed the presence of a few very small crystalline particles such as boron and iron oxides in agreement with ref. [12].
96
N. Randrianantoandro et al. / Crystallization in Metglas Fe z8 B13Si 9 -10
0 |
•
+;0 |
275
!
270 -
!
7
265"0 o
260255i.¢
250-
o
245 ~ _ . . - 24~00
/
400
500
600
700
800
900
Annealing temperature (K) 70 ~2 K
60-
u
50 i
9K
m -!0
i.,
0 YELOClTY
÷10 (
mm/s
3
)
40o
i 500
60o
700
800
Annealing temperature (K)
Fig. 3. Conversion-electron M6ssbauer spectra recorded at room temperature on dull surface of amorphous ribbon Fe7sB13Si 9 after annealing at TA during 60 min.
Fig. 4. Evolution of the mean values of the hyperfine field and the angle fl as a function of the annealing temperature.
Table 1 Refined mean values of the hyperfine parameters characterizing the amorphous contribution of the Fe7sB13Si 9 ribbon at room temperature, for several temperatures TA (IS values are referred to et-Fe at 300 K) TA ( + 2 ) [K]
Site
(H) (+2) [kOe]
IS (+0.01) [mm s - 1]
2e (+0.02) [mm s - i]
/3 ( + 3 ) [o]
300
shiny bulk dull
249 243 250
0.09 0.10 0.09
0.00 0.00 0.00
77 68 73
642
shiny bulk dull
253 250 253
0.09 0.09 0.09
- 0.03 - 0.03 0.00
85 66 77
722
shiny bulk dull
275 249 269
0.09 0.09 0.13
0.00 - 0.02 0.05
65 34 64
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13S~i9
TA
DULL FACE
BULK
97
SHINY FACE
A, A 722K
I
100
250
4.00
Hi-tjp (kOe)
I
100
250
Fig. 5. Normalized hyperfine field distributions calculated from transmission and from conversion-electron M6ssbauer spectra recorded on bulk and shiny and surfaces of the amorphous ribbon FeTsB13Si9 after annealing at TA during 60 min.
4. Data analysis
4.1. Structural relaxation
One can observe three annealing temperature steps from the different hyperfine structures provided by transmission and reflection M6ssbauer spectroscopy.
The transmission and reflection M6ssbauer spectra exhibit rather similar patterns for TA < 720 K. However, the mean hyperfine field value is slightly increased on increasing the annealing
Table 2 Hyperfine data of the different crystalline phases (a-Fe-Si alloy with n ffi 8, 7 and 6 nearest neighbors, and Fe2B) at room temperature on the amorphous FeTsBt3Si9, for several annealing temperatures TA (IS values are referred to ~t-Fe at 300 K) a-Fe-Si
Fe2B
n=8
n=7
nffi6
x in Fel-x-Six
Ref. [2]
( H ) [kOe] IS [mm s -1]
331 0.02
308 0.05
277 0.10
-
237 0.12
Present study
( H ) [kOe] IS [mm s -1]
332 0.03
311 0.07
285 0.13
-
238 0.12
TA = 722 K
shiny [%] bulk [%] dull [%]
6 0 6
2 0 2
0 0 3
0.04 0 0.04
0 0 6
769 K
shiny [%] bulk [%] dull [%]
32 16 40
14 18 15
4 2 4
0.05 0.12 0.05
33 32 28
bulk [%]
19
21
10
0.12
50
TA =
TA = 883 K
98
N. Randrianantoandro et al. / Crystallization in Metglas Fe78BlsSi 9
temperature while the isomer shift to hyperfine field correlation is weakly reduced. These variations reveal a significant structural relaxation before the onset of crystallization, in agreement with previous observations established on other metallic glasses [19]. In other respects, the mean hyperfine field value of the bulk is smaller than that of the surfaces for the as-received ribbon. The present result agrees well with that evidenced on Fe40Ni40P14B6 from simultaneous transmission and scattering Mfssbauer experiments [23]. This difference is opposite to that expected from a thermodynamic description of the amorphous state: during the solidification of the ribbon, the quenching rates differ for the shiny side, the dull side and the bulk. The dull side, which is directly in contact with the rotating wheel, undergoes a faster quenching rate, and consequently has a larger free volume; on the contrary, the bulk undergoes the slowest quenching rate, and has the smallest free volume. This viewpoint is supported by recent observations of the curvature of the ribbons under annealing [24]. Consequently, the magnetic hyperfine field of the bulk should be larger than those of the surfaces. Such a difference might be due to the preliminary thermal treatment a n d / o r to the presence of chemical inhomogeneity induced during the solidification process, for instance a boron discrepancy of the surfaces. The exact nominal composition of the superficial amorphous state cannot be determined from literature data giving the hyperfine field values as a function of the composition. Indeed, these data are too scattered because of the calculation method used to fit the spectra and of the nature of the amorphous state which can differ according to the quenching conditions.
4.2. Partial surface crystallization Between TA = 720 and 760 K, conversion-electron Mfssbauer spectroscopy allows differences to be observed in the behaviour of the surfaces (see table 2): (i) a crystalline component attributed to the a-iron silicone phase is detectable on shiny and dull ribbon faces, (ii) the Fe2 B crystalline component appears first on the dull
face at TA = 722 K. These observations are discussed in the next section. Finally, from further results obtained by transmission M6ssbauer spectroscopy, one can observe the magnetic anisotropy with an easy direction perpendicular to the ribbon plane (see fig. 4). This strongly differs from the preferentially inplane magnetic texture evidenced by the asquenched ribbon (in agreement with ref. [25]) and by annealed ribbons with TA < 720 K. The bulk spin reorientation results from the surface crystallization: it is related to the presence of compressive stresses induced by the formation of higherdensity crystalline surface layers, through the positive magnetostriction. Such a mechanism was also evidenced on other amorphous ribbons prepared as well by melt-spinning technique [3,2628].
4.3. Crystallization products When TA > 760 K, the hyperfine structures evidenced on both transmission and reflection M6ssbauer spectra reveal an increase of the ot(Fe-Si) and Fe2B crystalline phases while the amorphous component decreases in the shiny and the dull surfaces as in the bulk.
5. Discussion
In this section, we focus attention on the phenomena which occur during the partial crystallization of surfaces. First, the crystallization kinetics will be discussed in thermodynamic aspects of the as-quenched ribbon; then, we present a quantitative analysis of the amorphous-to-crystalline transformation. The present study showed that (i) the onset of crystallization occurred first on the surfaces, and (ii) the kinetics of crystallization was higher at the dull surface. As the presence of a large free volume favours the crystallization process, one can attribute the higher stability of the bulk and the lower stability of the dull surface against crystallization to their different thermodynamic behaviour induced during the solidification process (see the discussion in the above section).
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N. Randrianantoandro et al. / Crystallization in Metglas FersB13Si 9
In addition, the present study showed first that the crystallization of the Metglas Fe78B13Si 9 leads to the simultaneous formation of ot-(Fe-Si) and Fe2B crystalline phases: this result differs from that observed in ref. [4] on a thicker analogous metallic glass where the crystallization products are the ot-(Fe-Si) compound and the metastable tetragonal Fe3B phase. In the composition range Fe81Bt9_~Six (with 3 at% < x < 11 at%), some authors have observed the amorphous-to-crystalline transformation in the Fe3B phase formed by a polymorphic reaction for x < 4% and the Fe2B phase for x > 4% [7]. In order to discuss the crystallization mechanism, the Si-atoms associated with the transition metal atoms in a solid solution are assumed to follow a metallic-like behavior, because of the similarity of their atomic size [29]. Consequently, the crystallization of the present Metglas Fe78B13 Si 9 can be identified to that of (Fe-Si)s7B13 , with different mechanisms on the surfaces, described as follows. The general considerations concerning the crystallization processes previously established in the case of binary amorphous F e - B compounds [30,31] that the amorphous-to-crystalline transformation can proceed by one of the three following micromechanisms according to the boron content: (i) the polymorphous crystallization corresponds to a reaction without any change in concentration into a supersaturated alloy or a metastable or stable crystalline phase, (ii) the primary crystallization leads to c~-Fe microcrystals embedded in a boron-enriched amorphous phase until reaching the metastable equilibrium while (iii) the eutectic process is associated with a simultaneous crystallization into two crystalline
phases. Consequently, the present study reveals a primary crystallization process on the shiny surface and an eutectic process on the dull surface (see table 2). In addition, one can mention that an annealing treatment under vacuum makes boron sublimation easier, contrarily to that under argon gas which limits boron sublimation and so favors a primary crystallization (as observed in ref. [17]). In other respects, one can correlate the present results to those obtained from annealed amorphous ribbons Fel_xB x with 0.12 < x < 0.20 prepared by the melt-spinning technique, showing that primary and eutectic crystallization are observed when x < 0.175 and x > 0.175 respectively [1,32]. Consequently, primary crystallization should be observed in the present study with the Metglas Fe78B13Si 9. In the light of all these considerations, one can discuss the present results obtained from M6ssbauer experiments in terms of nominal composition. For TA = 883 K, the process of crystallization is completely achieved. By using the following crystallization equation: Fe7sBlaSi 9 ~ a F e x S i l o o _ x + Fe2B, one can determine immediately the nominal composition of the F e - S i precipitated crystalline phase. The equilibrium condition which is established as Fe78BlaSi 9 ~ 0.61FessSi15 + 0.13Fe2B, reveals that 67(33)% of iron nuclei belong to the Fe85Sils (Fe2B) phase. This result strongly differs from the present experimental data (50:50). This disagreement can be explained by the uncertainty
Table 3 Normalized occupation probabilities for different values of the concentration x of Si atoms and for n Fe nearest neighbors of Fe atoms in the case of a random distribution of Si atoms in bcc-FeSi lattice P(x, n)
8 7 6
3 78 20 2
4 72 24 4
5 67 28 5
6 62 31 7
12 38 42 20
15 30 43 27
100
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13Si 9
of the estimation of the different phase contents due to the line broadenings originating from the lack of pure crystallinity; let us mention some further minor reasons as the thickness effects or the weakly different values of recoil-free fraction for the two crystalline phases. A second method for estimating the nominal F e - S i composition, consists in assuming a random distribution of Si-atoms in the bcc-lattice sites. Consequently, the probability for an Fe atom to have n Fe nearest neighbors is given by the binomial law, expressed as P ( x , n) = C8(1 - x ) " x (8-"),
where x is the Si atomic concentration. In table 3, we report some values of the normalized occupation probabilities for 8, 7 and 6 nearest neighbors. The comparison of these values with the experimental data (see table 2) leads to a Si concentration of 12 at% in the bulk, which is not too far from the previous value (15) at%) deduced from the crystallization equation. The onset of crystallization occurs with a rather poor Si-atom F e - S i phase (Fe95Si 5) which precipitates out on the two faces. For higher annealing temperatures, one can observe (i) an increase of Si content in the F e - S i phase, and consequently (ii) an increase of B content in the amorphous state until the eutectic composition. In addition, it can be concluded that the B content of the dull face is weaker than that of the shiny face at TA = 769 K (see table 2). In other words, a eutectic crystallization mechanism should be evidenced later on the dull face, which is contrary to the p h e n o m e n o n observed, as previously discussed in the present paper. In the light of these results, the occurrence of a eutectic crystallization mechanism of the dull surface should imply the presence of local structural inhomogeneities introduced in the ribbon during the solidification process. Topological and chemical differences result from the distribution of cooling rates: this confirms that higher quenching rates increase the chemical and structural disorder; this also explains the diversity of experimental results reported in the literature for ribbons prepared with different quenching conditions.
6. Conclusion The present results, concerning the onset of crystallization of surfaces before the bulk, and the reorientation of the magnetic anisotropy perpendicular to the ribbon plane, obtained on the metglass Fe78B135i9, clearly confirm those previously observed on other Metglasses prepared by the melt spinning technique. The new essential contribution lies in the different kinetics of crystallization between the shiny and dull faces. Finally, the occurrence of primary and eutectic crystallization mechanisms seems to be evidenced on shiny and dull surfaces respectively. These two p h e n o m e n a probably result from the topological disorder and the chemical local inhomogeneity which differ between the bulk and the surfaces.
Acknowledgements Acknowledgments are due to Dr. A. Gibaud (Universit6 du Mans) for his helpful contribution in X-ray measurements and to Prof. B. Grolleau (Institut des Mat6riaux de Nantes) in ESCA measurements.
References [1] T. Kemeny, I. Vincze, B. Fogarassy and S. Arajs, Phys. Rev. B 20 (1979) 476. [2] H.N. Ok and A.H. Morrish, Phys. Rev. B 22 (1980) 3471. [3] H.N. Ok. and A.H. Morrish, Phys. Rev. B 23 (1981) 2257. [4] J.Y. Bang and R.Y. Lee, J. Mater. Sci. 26 (1991) 4961. [5] S. Enzo, (3. Cocco, I. Soletta, C. Antonione and L. Battezzati, Phys. Stat. Sol. (a) 115 (1989) 459. [6] H.N. Ok, K.S. Back and C.S. Kim, Phys. Rev. B 24 (1981) 6600. [7] C. Antonione, L. Battezzati, G. Cocco and F. Marino, Z. Metalk. 75 (1984) 714. [8] A. Quivy,J. Rzepski, J. Chevalier and Y. Calvayrac,Proc. 5th Int. Conf. Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (Amsterdam, 1985) p. 15. [9] S. Surinach, M.D. Baro and N. Clavaguera, Proc. 5th Int. Conf. Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (Amsterdam, 1985) p. 323. [10] A. Zaluska and H. Mataja, J. Mater. Sci. 18 (1983) 2163. [11] H.Y. Pai, C.H. Lam, S.U. Jen and C.K. Hou, J. Mater. Sci. 25 (1990) 3159. [12] N. Morito, T. Suzuki, C. Maeda, T. Yamashita and Y. Kitano, J. Mater. Sci. 25 (1990) 5166.
N. Randrianantoandro et al. / Crystallization in Metglas [13] Gao Wei and B. Cantor, J. Mater. Sci. Lett. 8 (1989) 399. [14] A.S. Schaafsma, H. Snijders, F. van der Woude, J.W. Drijver and S. Radelaar, Phys. Rev. B 20 (1979) 4423. [15] N. Saegusa and A.H. Morrish, Phys. Rev. B 26 (1982) 305. [16] J.M. Greneche and F. Varret, J. Phys. C 15 (1982) 5333. [17] A. Gupta, M.E. Jayaraj, S. Lal and R.P. Verma, J. Phys. F 18 (1988) 2159. [18] A. Gupta and S. Habibi, Mater. Sci. Eng. A 133 (1991) 375. [19] N. Saegusa and A.H. Morrish, Phys. Rev. B 26 (1982) 6547. [20] H.G. Wagner, M. Ackermann and U. Gonser, J. NonCryst. Solids 61&62 (1984) 8~7. [21] U. Gonser, M. Ackermann, H.J. Bauer, N. Blaes, S.M. Fries, R. Gaa and H.G. Wagner, in: Industrial Applications of the M6ssbauer Effect, eds. G.J. Long and J.G. Stevens (Plenum, New York, 1986) p. 25. [22] J. Teillet, J.M. Grenechc and F. Varret, unpublished MOSFIT program.
F e 78 B I 3 Si 9
101
[23] S. Nasu and U. Gonser, J. de Phys. 41 (1980) C8-690. [24] M. Bourrous and F. Varret, J. Magn. Magn. Mater. 66 (1987) 229. [25] P. Schaaf and U. Gonser, Hyperfine Interactions 57 (1990) 2101. [26] L. Chien and R. Hasegawa, Phys. Rev. B 16 (1977) 3024. [27] P.J. Schurer and A.H. Morrish, J. Magn. Magn. Mater. 15 (1980) 577. [28] I. Nowik, I. Felner, Y. Wolfus and Y. Yeshurun, J. Phys. F 18 (1988) L181. [29] J.M. Dubois, thesis, University of Nancy (1981), unpublished. [30] U. Koster and U. Herold, Topics in Applied Physics: Glassy Metals, vol. 46, eds. H.J. Guntherodt and H. Beck (1981) p. 225. [31] U. Koster and P. Weiss, J. Non-Cryst. Solids 17 (1975) 359. [32] J.A. Cusido and J. Tejada, J. Mater. Sci. Lett. 5 (1986) 75.
Journal of Magnetism and Magnetic Materials 117 (1992) 93-101 North-Holland
Surface and bulk crystallization mechanisms in the Metglas FeTsB138i 9 investigated by M6ssbauer spectroscopy N. R a n d r i a n a n t o a n d r o a, J.M. G r e n e c h e a and F. Varret b Laboratoire de Physique des MatJriaux, URA, CNRS, 807 Universit# du Maine, BP 535, F 72017 Le Mans Cedex, France b DRP., URA CNRS 71, UniversitJ Pierre et Marie Curie (Paris VI) F 75252 Paris Cedex 05, France a
Received 5 Mei 1992
The amorphous-to-crystalline transformation of the as-received Metglas FeTsB13Si 9 (Metglas ® 2605S2) was studied by transmission and reflection Mfssbauer spectroscopy in order to compare shiny and dull surfaces and bulk crystallization mechanisms. In agreement with previous results, the onset of crystallization occurs on both surfaces, simultaneously to the out-of-plane anisotropy. Higher kinetics of crystallization is observed at the dull surface. Then, primary and eutectic crystallization processes seem to be evidenced for the shiny and dull surfaces respectively. It is proposed that these different phenomena originate from the presence of structural and chemical inhomogeneities induced during the solidification process.
1. Introduction
The amorphous-to-crystalline transformation of metallic glasses has received large attention because of fundamental interest in understanding their structural and physical properties and because of potential applications in relation to their thermodynamic properties. Numerous studies were carried out on structural relaxation and on the evolution of the magnetic properties as a function of the annealing time and of the annealing temperature; examples are given on binary [1] and on ternary [2-7] iron-based metallic glasses. For the F e - B - S i amorphous system, the nature of the crystalline phases is strongly dependent upon the chemical composition and upon the rapid quenching conditions; in other respect, differences in crystallization morphology were observed [8] with contradictory results reported in the literature concerning the crystallization process [9,10]. Correspondence to: Dr. J.M. Greneche, Laboratoire de Physique des Mat6riaux, URA, CNRS, 807 Universit6 du Maine, BP 535, F 72017 Le Mans Cedex, France.
Annealing treatments were performed on the metallic glass FeTaB13Si9 under different conditions in order to study the evolution of magnetic properties [11,12] and the surface crystallization process [12,13]. It has been found that the nature of the atmosphere has no significant effect on the crystallization mechanisms [13]. More recently, the microstructures and kinetics of the crystallization of the Metglas F e 7 8 B l 3 S i 9 w e r e followed by X-ray diffraction, transmission electron microscopy, differential thermal analysis and differential scanning calorimetry (on a 10 mm wide and 25 Ixm thick ribbon): the simultaneous formation of ot(Fe-Si) and F e 3 B crystalline phases was evidenced and the transformation of the Fe3B phase into a stable crystalline phase of Fe2B was observed at higher temperature [4]. In complement to calorimeter measurements, M6ssbauer spectroscopy proved to be an excellent tool to follow the thermal evolution of the amorphous material under annealing, because of its local probe character: it provides fruitful information relative to (i) the structural relaxation, (ii) the transition from the amorphous to crystalline state through the evolution of the hyperfine data
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
94
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13Si 9
and the increase of the recoil-free fraction [2,14,15], and (iii) the magnetic texture through the line intensities of the magnetic spectra [16]. The bulk and the surfaces can be investigated separately by using, respectively, transmission geometry (~/-rays) and scattering geometry (conversion electrons) M6ssbauer techniques. So, some recent studies reveal that the kinetics of surface crystallization differs from that of the bulk with a weakly higher activation energy for surface crystallization [17,18]. In addition, it must be kept in mind that the as-quenched amorphous ribbons prepared by the melt-spinning technique exhibit structural and chemical inhomogeneities originating from thermal stresses induced during the solidification process. It can be suspected that such inhomogeneities will affect the surface properties and the crystallization process (through nucleation); this could explain the diversity of published experimental data in that field. In the present paper, we study the transformation of the Metglas 2605S2 prepared by the meltspinning technique, from the as-quenched amorphous to the crystalline state. We report the structural evolution and the changes in the magnetic texture of the bulk and of the surfaces from transmission and reflection M/Sssbauer techniques in order to characterize the different crystallization processes.
2. Experimental section
The 57Fe M6ssbauer spectra were recorded at room temperature in transmission and reflection geometry with a constant acceleration spectrometer, using a 57Co source diffused in a rhodium matrix. Conversion-electron M6ssbauer spectra were recorded at room temperature using a continuous H e - C H 4 gas flow proportional counter providing average information over the range 0200 nm in depth. The samples were cut from an as-received ribbon roll of amorphous Fe78B13Si9 (Metglas ® 2605S2) prepared by Allied Chemical Company with a preliminary thermal treatment (2 h at 380°C) under influence of a longitudinal external
magnetic field. The absorbers were made of a single piece of ribbon, 17 Ixm thick, oriented perpendicularly to the -y-beam, so that thickness effects can be neglected in the treatment of the transmission spectra. The samples were annealed in a helium atmosphere (in order to avoid surface oxidation) in a classical furnace with a temperature stability of + 2°C. The annealing temperatures ranged from 300 to 900 K. The heating rate was 20°C/min; after 60 min annealing, the sample was removed from the furnace and cooled down to room temperature.
3. Results
Some transmission M6ssbauer spectra are given in fig. 1 for different annealing temperatures. Below TA = 760 K, these spectra exhibit six well resolved and broad lines; sharp lines typical of crystalline phases appear as TA is raised above 760 K. Also, the relative intensities of the intermediate lines strongly differ for TA > 720 K. Figs. 2 and 3 exhibit conversion-electron M6ssbauer spectra recorded respectively on shiny and dull surfaces of 2605S2 ribbons at several annealing temperatures. Firstly, before any quantitative analysis, these spectra give evidence that the surfaces crystallize before the bulk (in agreement with results of literature data [3,19-21]). Both transmission and reflection M6ssbauer spectra were analyzed using the Mosfit program [22]. The typical amorphous spectra were fitted well by using a discrete distribution of hyperfine fields linearly correlated to an isomer shift distribution in order to reproduce the asymmetry of the sextet. The refined mean values of hyperfine parameters, including the isomer shift, the quadrupole shift, the hyperfine field, and the angle /3 defined by the direction of the ~/-beam and that of the hyperfine field, are reported in table 1. The mean values of the hyperfine field and the angle/3 are plotted as a function of the annealing temperature in fig. 4. The hyperfine field distribution determined from these spectra
N. Randrianantoandro et al. / Crystallization in Metglas Fe 78 BI3S[ o
exhibits a close to Gaussian shape: some examples are reported in fig. 5. The other spectra result from the superposition of four crystalline components and one amorphous contribution if any, assumed to result from a pure hyperfine field distribution similar to the previous ones. One can assign three of the -I0
0
95
- I0 &
0 I
+10
~
T A = 300 K
~I0
'K
ZK
-I0
~K
0
VELOCITY ( l / s
+10
}
Fig. 2. Conversion-electron M6ssbauer spectra recorded at room temperature on shiny surface of amorphous ribbon Fe78B13Si 9 after annealing at TA during 60 min. 9K
#
TA m 883 K
~
-10
~
VELOCITY ( t ~ t / s
+I0 )
Fig. 1. Transmission M6ssbauer spectra recorded at room temperature on amorphous ribbon FeTsB13Si 9 after annealing at TA during 60 min.
crystalline sextets to the three iron sites in et-FeSi alloy (with respectively 6, 7 and 8 Fe nearest neighbours) and the last component to Fe2B. The refined values of the hyperfine parameters, reported for different annealing temperatures, are reported in table 2 together with those previously obtained on crystalline samples [2]. To support this partial surface crystallization, we performed small- and large-angle X-ray diffraction: well defined peaks corresponding to the a - ( F e - S i ) crystalline phase were observed. In addition, an Auger electron Spectroscopy experiment with a sputtering system using argon ions, has revealed the presence of a few very small crystalline particles such as boron and iron oxides in agreement with ref. [12].
96
N. Randrianantoandro et al. / Crystallization in Metglas Fe z8 B13Si 9 -10
0 |
•
+;0 |
275
!
270 -
!
7
265"0 o
260255i.¢
250-
o
245 ~ _ . . - 24~00
/
400
500
600
700
800
900
Annealing temperature (K) 70 ~2 K
60-
u
50 i
9K
m -!0
i.,
0 YELOClTY
÷10 (
mm/s
3
)
40o
i 500
60o
700
800
Annealing temperature (K)
Fig. 3. Conversion-electron M6ssbauer spectra recorded at room temperature on dull surface of amorphous ribbon Fe7sB13Si 9 after annealing at TA during 60 min.
Fig. 4. Evolution of the mean values of the hyperfine field and the angle fl as a function of the annealing temperature.
Table 1 Refined mean values of the hyperfine parameters characterizing the amorphous contribution of the Fe7sB13Si 9 ribbon at room temperature, for several temperatures TA (IS values are referred to et-Fe at 300 K) TA ( + 2 ) [K]
Site
(H) (+2) [kOe]
IS (+0.01) [mm s - 1]
2e (+0.02) [mm s - i]
/3 ( + 3 ) [o]
300
shiny bulk dull
249 243 250
0.09 0.10 0.09
0.00 0.00 0.00
77 68 73
642
shiny bulk dull
253 250 253
0.09 0.09 0.09
- 0.03 - 0.03 0.00
85 66 77
722
shiny bulk dull
275 249 269
0.09 0.09 0.13
0.00 - 0.02 0.05
65 34 64
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13S~i9
TA
DULL FACE
BULK
97
SHINY FACE
A, A 722K
I
100
250
4.00
Hi-tjp (kOe)
I
100
250
Fig. 5. Normalized hyperfine field distributions calculated from transmission and from conversion-electron M6ssbauer spectra recorded on bulk and shiny and surfaces of the amorphous ribbon FeTsB13Si9 after annealing at TA during 60 min.
4. Data analysis
4.1. Structural relaxation
One can observe three annealing temperature steps from the different hyperfine structures provided by transmission and reflection M6ssbauer spectroscopy.
The transmission and reflection M6ssbauer spectra exhibit rather similar patterns for TA < 720 K. However, the mean hyperfine field value is slightly increased on increasing the annealing
Table 2 Hyperfine data of the different crystalline phases (a-Fe-Si alloy with n ffi 8, 7 and 6 nearest neighbors, and Fe2B) at room temperature on the amorphous FeTsBt3Si9, for several annealing temperatures TA (IS values are referred to ~t-Fe at 300 K) a-Fe-Si
Fe2B
n=8
n=7
nffi6
x in Fel-x-Six
Ref. [2]
( H ) [kOe] IS [mm s -1]
331 0.02
308 0.05
277 0.10
-
237 0.12
Present study
( H ) [kOe] IS [mm s -1]
332 0.03
311 0.07
285 0.13
-
238 0.12
TA = 722 K
shiny [%] bulk [%] dull [%]
6 0 6
2 0 2
0 0 3
0.04 0 0.04
0 0 6
769 K
shiny [%] bulk [%] dull [%]
32 16 40
14 18 15
4 2 4
0.05 0.12 0.05
33 32 28
bulk [%]
19
21
10
0.12
50
TA =
TA = 883 K
98
N. Randrianantoandro et al. / Crystallization in Metglas Fe78BlsSi 9
temperature while the isomer shift to hyperfine field correlation is weakly reduced. These variations reveal a significant structural relaxation before the onset of crystallization, in agreement with previous observations established on other metallic glasses [19]. In other respects, the mean hyperfine field value of the bulk is smaller than that of the surfaces for the as-received ribbon. The present result agrees well with that evidenced on Fe40Ni40P14B6 from simultaneous transmission and scattering Mfssbauer experiments [23]. This difference is opposite to that expected from a thermodynamic description of the amorphous state: during the solidification of the ribbon, the quenching rates differ for the shiny side, the dull side and the bulk. The dull side, which is directly in contact with the rotating wheel, undergoes a faster quenching rate, and consequently has a larger free volume; on the contrary, the bulk undergoes the slowest quenching rate, and has the smallest free volume. This viewpoint is supported by recent observations of the curvature of the ribbons under annealing [24]. Consequently, the magnetic hyperfine field of the bulk should be larger than those of the surfaces. Such a difference might be due to the preliminary thermal treatment a n d / o r to the presence of chemical inhomogeneity induced during the solidification process, for instance a boron discrepancy of the surfaces. The exact nominal composition of the superficial amorphous state cannot be determined from literature data giving the hyperfine field values as a function of the composition. Indeed, these data are too scattered because of the calculation method used to fit the spectra and of the nature of the amorphous state which can differ according to the quenching conditions.
4.2. Partial surface crystallization Between TA = 720 and 760 K, conversion-electron Mfssbauer spectroscopy allows differences to be observed in the behaviour of the surfaces (see table 2): (i) a crystalline component attributed to the a-iron silicone phase is detectable on shiny and dull ribbon faces, (ii) the Fe2 B crystalline component appears first on the dull
face at TA = 722 K. These observations are discussed in the next section. Finally, from further results obtained by transmission M6ssbauer spectroscopy, one can observe the magnetic anisotropy with an easy direction perpendicular to the ribbon plane (see fig. 4). This strongly differs from the preferentially inplane magnetic texture evidenced by the asquenched ribbon (in agreement with ref. [25]) and by annealed ribbons with TA < 720 K. The bulk spin reorientation results from the surface crystallization: it is related to the presence of compressive stresses induced by the formation of higherdensity crystalline surface layers, through the positive magnetostriction. Such a mechanism was also evidenced on other amorphous ribbons prepared as well by melt-spinning technique [3,2628].
4.3. Crystallization products When TA > 760 K, the hyperfine structures evidenced on both transmission and reflection M6ssbauer spectra reveal an increase of the ot(Fe-Si) and Fe2B crystalline phases while the amorphous component decreases in the shiny and the dull surfaces as in the bulk.
5. Discussion
In this section, we focus attention on the phenomena which occur during the partial crystallization of surfaces. First, the crystallization kinetics will be discussed in thermodynamic aspects of the as-quenched ribbon; then, we present a quantitative analysis of the amorphous-to-crystalline transformation. The present study showed that (i) the onset of crystallization occurred first on the surfaces, and (ii) the kinetics of crystallization was higher at the dull surface. As the presence of a large free volume favours the crystallization process, one can attribute the higher stability of the bulk and the lower stability of the dull surface against crystallization to their different thermodynamic behaviour induced during the solidification process (see the discussion in the above section).
99
N. Randrianantoandro et al. / Crystallization in Metglas FersB13Si 9
In addition, the present study showed first that the crystallization of the Metglas Fe78B13Si 9 leads to the simultaneous formation of ot-(Fe-Si) and Fe2B crystalline phases: this result differs from that observed in ref. [4] on a thicker analogous metallic glass where the crystallization products are the ot-(Fe-Si) compound and the metastable tetragonal Fe3B phase. In the composition range Fe81Bt9_~Six (with 3 at% < x < 11 at%), some authors have observed the amorphous-to-crystalline transformation in the Fe3B phase formed by a polymorphic reaction for x < 4% and the Fe2B phase for x > 4% [7]. In order to discuss the crystallization mechanism, the Si-atoms associated with the transition metal atoms in a solid solution are assumed to follow a metallic-like behavior, because of the similarity of their atomic size [29]. Consequently, the crystallization of the present Metglas Fe78B13 Si 9 can be identified to that of (Fe-Si)s7B13 , with different mechanisms on the surfaces, described as follows. The general considerations concerning the crystallization processes previously established in the case of binary amorphous F e - B compounds [30,31] that the amorphous-to-crystalline transformation can proceed by one of the three following micromechanisms according to the boron content: (i) the polymorphous crystallization corresponds to a reaction without any change in concentration into a supersaturated alloy or a metastable or stable crystalline phase, (ii) the primary crystallization leads to c~-Fe microcrystals embedded in a boron-enriched amorphous phase until reaching the metastable equilibrium while (iii) the eutectic process is associated with a simultaneous crystallization into two crystalline
phases. Consequently, the present study reveals a primary crystallization process on the shiny surface and an eutectic process on the dull surface (see table 2). In addition, one can mention that an annealing treatment under vacuum makes boron sublimation easier, contrarily to that under argon gas which limits boron sublimation and so favors a primary crystallization (as observed in ref. [17]). In other respects, one can correlate the present results to those obtained from annealed amorphous ribbons Fel_xB x with 0.12 < x < 0.20 prepared by the melt-spinning technique, showing that primary and eutectic crystallization are observed when x < 0.175 and x > 0.175 respectively [1,32]. Consequently, primary crystallization should be observed in the present study with the Metglas Fe78B13Si 9. In the light of all these considerations, one can discuss the present results obtained from M6ssbauer experiments in terms of nominal composition. For TA = 883 K, the process of crystallization is completely achieved. By using the following crystallization equation: Fe7sBlaSi 9 ~ a F e x S i l o o _ x + Fe2B, one can determine immediately the nominal composition of the F e - S i precipitated crystalline phase. The equilibrium condition which is established as Fe78BlaSi 9 ~ 0.61FessSi15 + 0.13Fe2B, reveals that 67(33)% of iron nuclei belong to the Fe85Sils (Fe2B) phase. This result strongly differs from the present experimental data (50:50). This disagreement can be explained by the uncertainty
Table 3 Normalized occupation probabilities for different values of the concentration x of Si atoms and for n Fe nearest neighbors of Fe atoms in the case of a random distribution of Si atoms in bcc-FeSi lattice P(x, n)
8 7 6
3 78 20 2
4 72 24 4
5 67 28 5
6 62 31 7
12 38 42 20
15 30 43 27
100
N. Randrianantoandro et al. / Crystallization in Metglas Fe78B13Si 9
of the estimation of the different phase contents due to the line broadenings originating from the lack of pure crystallinity; let us mention some further minor reasons as the thickness effects or the weakly different values of recoil-free fraction for the two crystalline phases. A second method for estimating the nominal F e - S i composition, consists in assuming a random distribution of Si-atoms in the bcc-lattice sites. Consequently, the probability for an Fe atom to have n Fe nearest neighbors is given by the binomial law, expressed as P ( x , n) = C8(1 - x ) " x (8-"),
where x is the Si atomic concentration. In table 3, we report some values of the normalized occupation probabilities for 8, 7 and 6 nearest neighbors. The comparison of these values with the experimental data (see table 2) leads to a Si concentration of 12 at% in the bulk, which is not too far from the previous value (15) at%) deduced from the crystallization equation. The onset of crystallization occurs with a rather poor Si-atom F e - S i phase (Fe95Si 5) which precipitates out on the two faces. For higher annealing temperatures, one can observe (i) an increase of Si content in the F e - S i phase, and consequently (ii) an increase of B content in the amorphous state until the eutectic composition. In addition, it can be concluded that the B content of the dull face is weaker than that of the shiny face at TA = 769 K (see table 2). In other words, a eutectic crystallization mechanism should be evidenced later on the dull face, which is contrary to the p h e n o m e n o n observed, as previously discussed in the present paper. In the light of these results, the occurrence of a eutectic crystallization mechanism of the dull surface should imply the presence of local structural inhomogeneities introduced in the ribbon during the solidification process. Topological and chemical differences result from the distribution of cooling rates: this confirms that higher quenching rates increase the chemical and structural disorder; this also explains the diversity of experimental results reported in the literature for ribbons prepared with different quenching conditions.
6. Conclusion The present results, concerning the onset of crystallization of surfaces before the bulk, and the reorientation of the magnetic anisotropy perpendicular to the ribbon plane, obtained on the metglass Fe78B135i9, clearly confirm those previously observed on other Metglasses prepared by the melt spinning technique. The new essential contribution lies in the different kinetics of crystallization between the shiny and dull faces. Finally, the occurrence of primary and eutectic crystallization mechanisms seems to be evidenced on shiny and dull surfaces respectively. These two p h e n o m e n a probably result from the topological disorder and the chemical local inhomogeneity which differ between the bulk and the surfaces.
Acknowledgements Acknowledgments are due to Dr. A. Gibaud (Universit6 du Mans) for his helpful contribution in X-ray measurements and to Prof. B. Grolleau (Institut des Mat6riaux de Nantes) in ESCA measurements.
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F e 78 B I 3 Si 9
101
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