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Corrosion resistance of amorphous and polycrystalline FeCuNbSiB alloys in sulphuric acid solution
Journal of Non-Crystalline Solids 247 (1999) 69±73
Corrosion resistance of amorphous and polycrystalline FeCuNbSiB alloys in sulphuric acid solution C.A.C. Souza *, S.E. Kuri, F.S. Politti, J.E. May, C.S. Kiminami Materials Engineering Department, Federal University of S~ ao Carlos, P.O. Box 676 13.565-905, S~ ao Carlos, SP, Brazil
Abstract The in¯uence of structural changes such as structural relaxation and crystallization on the corrosion resistance of the amorphous Fe74 Cu1 Nb3 Si13:5 B8:5 alloy in 0.1M sulphuric acid was investigated. A controlled crystallization of such amorphous alloy leads to nanocrystalline microstructure which increases the corrosion resistance, whereas structural relaxation of amorphous alloy results in reduced corrosion resistance. Dierent conditions of relaxation (330°C and 380°C) and structural crystallization (554°C and 610°C) were analyzed. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The FeCuNbSiB metallic glasses have attracted attention in recent years [1]. This attention is due to the fact that after partial crystallization they have soft magnetic properties attributed to a twophase microstructure, which consists of bcc aFe(Si) crystals, with a grain size in the nanometre scale, surrounded by a residual amorphous phase [2]. Chattoraj et al [3] report that the soft-magnetic properties of FeBSi alloys are aected by corrosion. This report is important because, in some cases, magnetic materials are corroded in atmospheric conditions [4]. The corrosion resistance of soft magnetic alloys depends on their thermal history. Several authors [5±8] have reported that structural changes, such as structural relaxation and crystallization caused by annealing, aect corrosion properties. However, the eect of structural changes on the cor* Corresponding author. Tel.: +55-16 262 8250; fax: +55-16 261 5404; e-mail: [email protected]
rosion resistance of amorphous FeCuNbSiB alloys is not well studied. The purpose of this paper is therefore to study the eect of structural changes such as structural relaxation and crystallization on corrosion resistance of the amorphous Fe74 Cu1 Nb3 Si13:5 B8:5 alloy. 2. Experimental procedure Samples of Fe74 Cu1 Nb3 Si13:5 B8:5 amorphous ribbon of 0.024 mm thickness and 50 mm width (supplied by Vacuumschmelze Germany) was used. The samples were studied in the `as-cast' and annealed states, and were analysed by dierential scanning calorimetry (DSC) and X-ray diraction (XRD) using Cu-Ka radiation. In the DSC scans, samples were heated continuously from room temperature to 1000°C, at a constant heating rate of 25°C/min. The `as-cast' ribbons, whose amorphous structure was con®rmed by XRD, were annealed at 554°C, the peak temperature of the ®rst reactions, at 610°C at the end of the ®rst
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 3 4 - 4
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C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
reactions and at 330°C and 380°C, to cause structural relaxation. Annealing treatments were conducted in an argon atmosphere for 1 h, using heating and cooling rates of 25°C/min and 40°C/ min, respectively. The grain size of a-Fe(Si) crystallites was calculated from XRD spectra peak width using the Scherrer equation [9]. Structural relaxation in the amorphous state was monitored by measuring the speci®c heat of both the `as-cast' and annealed samples by DSC in an argon atmosphere, using a heating rate of 40°C/min. The Curie temperature was obtained from speci®c heat curves [10]. Corrosion resistance measurements were carried out by mass loss measurements in aerated 0.1M H2 SO4 solution, on 50 ´ 50 ´ 0.024 mm samples polished to 600 grade silicon carbide paper. The mass loss measurements were done in an analytical balance (Metler AB204), with 10ÿ4 g accuracy. Before each measurement the samples were rinsed with carbon tetrachloride and dried. Each test was performed with the same sample immersed during dierent times and the weight loss corrosion testing were repeated three times for each condition. The error bars correspond to the smallest and largest mass loss obtained from these three tests. The corroded surface of the `as cast' and of 330°C relaxed samples exposed to 0.1M H2 SO4 during 20 h was observed by scanning electron microscopy using a microscope (Zeiss DSM 940). Surface analysis with X-ray photoelectron spectroscopy (XPS) was used, and samples both in the `as-cast' state and annealed at 554°C were analyzed. XPS was performed in a spectrometer (Kratos XSAM HS), using Mg Ka radiation. The energy of the C 1s peak due to adventitious hydrocarbons was used as binding energy reference (284.8 eV). The background was subtracted using the Shirley method [11] and the peaks were ®tted by both Gaussian and mixed Gaussian/Lorentzian functions [12]. Before the XPS analysis these samples were immersed in 0.1M H2 SO4 solution for 8 h.
grain size of a-Fe(Si) phase of 15 and 25 nm respectively, showing that these samples had typical crystalline microstructure of nanocrystalline alloy. Structural relaxation in 330°C and 380°C annealed samples was observed by comparing the variation of the speci®c heat (Cp ) with temperature, in `ascast' and annealed samples. Fig. 1 shows that annealing at 330°C and 380°C caused only structural relaxation in the amorphous structure. The preservation of the full amorphous state was veri®ed by DSC analysis by comparison with the crystallization enthalpy of the `as-cast' samples. Nevertheless, for the annealed samples at temperatures less than the endothermic peak associated with the magnetic transition (the Curie temperature), the apparent speci®c heats increase monotonicly with increasing temperature, while this increase does not occur in `as-cast' samples. This dierence indicates that the annealed samples have a relaxed structure. The Fig. 1 shows that Curie temperature (Tc ) for the `as cast' state is 316.4°C (0.1°C) and that for annealed samples at 330°C and 380°C is 327.8°C (0.1°C) and 331.4°C (0.1°C), respectively. It can be seen that structural relaxation has increased the Curie temperature and that the higher the temperature of annealing, the higher the Curie temperature. Fig. 2 shows that the weight loss measurements in 0.1M H2 SO4 solution is not linear but it
3. Results The X-ray diraction analysis of the 554°C and 610°C annealed samples indicated an average
Fig. 1. Evolution of the apparent speci®c heat with temperature for the `as cast' (1) and annealed at 330°C (2) and 380°C (3) samples.
C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
71
Fig. 2. Values of accumulated mass loss obtained in H2 SO4 0.1M solution in as a result of immersion time. Lines are drawn as guides for the eye.
is still possible to calculate an approximate corrosion rate for each alloy. Thus, the corrosion rate of the polycrystalline samples at 554°C and 610°C are 4.07 ´ 10ÿ5 (10ÿ7 ) and 1.14 ´ 10ÿ6 (10ÿ8 ) g/cm2 h, respectively. Both the `as cast' and the relaxed samples have the same corrosion rate (1.20 ´ 10ÿ4 (10ÿ6 ) g/cm2 h) until 40 h of immersion. After this period the corrosion rates are 1.2 ´ 10ÿ4 (10ÿ5 ); 1.35 ´ 10ÿ4 (10ÿ6 ) and 1.70 ´ 10ÿ4 (10ÿ6 ) g/cm2 h for the `as cast'; the relaxed samples at 330°C and 380°C, respectively. So, the samples annealed at 330°C and 380°C (structurally relaxed) have the smallest corrosion resistance, and the samples annealed at 554°C and 610°C (nanocrystalline samples) a larger corrosion resistance, compared with the `as-cast' samples, as indicated in Fig. 2. Fig. 3 indicate the `as-cast' and the 330°C relaxed samples surface condition, after immersion in 0.1M H2 SO4 during 20 h. It is possible to see that the corroded surface of the `as cast' sample is homogeneous whilst the corroded surface of the relaxed sample has dierent features from place to place. In Table 1 are presented the results obtained from XPS analysis for the samples in the `as-cast' condition, and those crystallized at 554°C. As can be seen in Table 1, in both samples a protective oxide layer of SiO2 is formed, with the same atomic concentration within errors of measurement.
Fig. 3. Corroded surface of the as cast and 330°C relaxed sample analyzed by scanning electron microscopy. (a) as cast; (b) relaxed at 330°C.
Table 1 Binding energies of the principal photoelectron lines (in eV) and atomic concentration (with a relative error of <10%) of silicon in the super®cial ®lm Samples
Si 2p
Compound Atomic concentration (%)
`as cast' Crystallized at 554°C
103.4 SiO2 102.8 SiO2
30.1 29.8
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4. Discussion In Fig. 1, the continuous decrease in Cp before the endothermic peak corresponding to the magnetic transition, observed for the `as-cast' sample, is related to a structural relaxation. This relaxation is an exothermic process which involves heat liberation, and brings about atomic rearrangements which promotes a decrease in the Cp s until the temperature reaches a point at which no further atomic rearrangement occurs. At this point the speci®c heat starts to increase until the material reaches the magnetic transition, or Curie temperature. Previous authors have reported an increase in the corrosion resistance of magnetic soft amorphous alloys, such as FeBSi [5] and FeNiSi [6] alloys, because of the stress relief caused by structural relaxation. This relaxation decreases the free energy of the amorphous system, thereby causing a decrease in the reactivity of its elements and increasing the chemical stability. However, our results show that structural relaxation decreases the corrosion resistance of Fe74 Cu1 Nb3 Si13 .5 B8:5 , thus indicating that the effect of structural relaxation on corrosion of amorphous alloys depends on the composition of these alloys. As shown in Fig. 1, structural relaxation increases the Curie temperature, indicating an increase in chemical short range ordering [13]. This phenomenon in Fe74 Cu1 Nb3 Si13:5 B8:5 alloy, is due to the presence of Cu. There is a repulsive interaction between copper atoms and the other elements in the alloy producing regions rich in each element during the structural relaxation [14]. This eect is responsible for a greater chemical short range ordering [15]. As a consequence, a decrease in the average free volume occurs in the Cu rich region, establishing an inhomogeneity in the amorphous matrix which induces a larger corrosion rate [15]. This fact could be attributed to an increase of internal stress [15]. Observations from scanning electron microscopy, see Fig. 3, show that corrosion takes place at dierent rate, as if a selective corrosion process was taking place. The additional stress causes an increase in the reactivity of the alloy elements and so increases the corrosion rate [6].
An increase in corrosion resistance with crystallization of amorphous alloy leading to nanocrystalline microstructure formation was reported for Fe78 B13 Si9 alloy [8], and was attributed to a larger silicon diusion rate, which leads to a larger rate of deposition of this element on the surface, and the growth of a thicker, more continuous and more protective SiO2 ®lm. Vacancy migration is the most common mechanism of diusion in crystalline metals [16]. However in amorphous alloys, it has been reported [17] that vacancies are unstable and atomic vibrations quickly redistribute the excess vacancy space over a larger volume than that of a vacancy. Thus, it has been frequently assumed that element diusion cannot occur by the mechanism of vacancy migration [18]. The diusion mechanism in amorphous alloys, suggested by several researchers [19,20], must take place by some form of cooperative motion of groups of adjacent atoms. The increase in corrosion resistance promoted by crystallization of amorphous samples may be due to the formation of a large area of interface between the amorphous and crystalline regions. In this area there are larger numbers of defects such as free volumes and microvoids, which cause an increase in the silicon diusion coecient [8]. So a large amount of silicon migrates quickly to the surface at the crystalline condition and oxidizes to form a continuous SiO2 ®lm. However our XPS analysis, as shown in Table 1, indicates that the `as-cast' and the 554°C annealed samples have the same silicon content in the super®cial ®lm, which is independent of the crystallization process. These results could be attributed to the faster diusion of silicon ions in the polycrystalline structure, which would promote the faster SiO2 ®lm formation. This consideration agrees with the larger corrosion resistance of the sample crystallized at 610°C, because of the greater silicon diusion coecient at this temperature.
5. Conclusion The structural relaxation promotes an increase of the Curie temperature of the amorphous
C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
Fe74 Cu1 Nb3 Si13:5 B8:5 alloy, indicating an increase in chemical short range ordering. A controlled crystallization of amorphous alloy leading to nanocrystalline microstructure formation results in improved corrosion resistance, while structural relaxation decreases the corrosion resistance. The corrosion resistance increases with higher crystallization temperatures and decreases with the higher relaxation temperatures investigated. Acknowledgements The authors are indebted to Vacuumschmelze, Germany for supplying the amorphous ribbon. The authors would like to thank also the FAPESP ± `Projeto Tematico' and Ministry of Science and Technology (MCT) ± `PRONEX 1997' for ®nancial support. References [1] K. Lu, Mater. Sci. Eng. R: Reports R 16 (1996) 161. [2] Y. Yoshizawa, S. Oguma, Yamauch, J. Appl. Phys. 64 (1988) 6044. [3] I. Chattoraj, A. Mitra, Scripta Metall. Mater. 26 (1992) 1013.
73
[4] G.N. Maniar, T.A. Debold, Adv. Mater. Proc. 1 (1998) 57. [5] C.A.C. Souza, F.S. Politi, C.S. Kiminami, Scripta Metall. Mater. 39 (1998) 329. [6] V.S. Raja, K. Kishore, S. Ranganathan, in: S. Steeb, H. Warlimont (Eds.), Proceedings of the International Conference on Rapidly Quenched Metals, Elsevier, Amsterdam, 1985, p. 1485. [7] C.A.C. Souza, C.S. Kiminami, J. Non-Cryst. Solids 219 (1997) 155. [8] H.Y. Tong, F.G. Shi, J. Lavernia, Scripta Metall. Mater. 32 (1995) 511. [9] I. Kaeble, in: Handbook of X-rays, McGraw-Hill, New York, 1967. [10] A.L. Greer, PhD thesis, University of Cambridge, 1979. [11] D.A. Shirley, Phys. Rev. B 5 (1972) 4770. [12] G.C. Smith, Surface Analysis by Electron Spectoscopy, Plenum, New York, 1994. [13] H.S. Chen, A. Inoue, T. Masumoto, J. Mater. Sci. 20 (1985) 2417. [14] A.R. Yavari, O. Drbohlav, Mater. Trans. JIM 36 (7) (1995) 896. [15] A.R. Yavari, J. Mater. Res. 1 (1986) 746. [16] J.W. Christian, in: The Theory of Transformations in Metals and Alloys, Pergamon, Oxford, 1975. [17] C.H. Bennet, P. Chaudhari, V. Moruzzi, P. Steinhardt, Philos. Mag. A 40 (1979) 485. [18] B. Cantor, R.W. Cahn, in: F.E. Luborsky, Amorphous Metallic Alloys, 1983. [19] P. Valenta, K. Maier, H. Kronmuler, K. Freitag, Phys. Status Solidi. 106 (1981) 129. [20] C. Birac, D. Lesueur, Phys. Status Solidi 36 (1976) 247.
Corrosion resistance of amorphous and polycrystalline FeCuNbSiB alloys in sulphuric acid solution C.A.C. Souza *, S.E. Kuri, F.S. Politti, J.E. May, C.S. Kiminami Materials Engineering Department, Federal University of S~ ao Carlos, P.O. Box 676 13.565-905, S~ ao Carlos, SP, Brazil
Abstract The in¯uence of structural changes such as structural relaxation and crystallization on the corrosion resistance of the amorphous Fe74 Cu1 Nb3 Si13:5 B8:5 alloy in 0.1M sulphuric acid was investigated. A controlled crystallization of such amorphous alloy leads to nanocrystalline microstructure which increases the corrosion resistance, whereas structural relaxation of amorphous alloy results in reduced corrosion resistance. Dierent conditions of relaxation (330°C and 380°C) and structural crystallization (554°C and 610°C) were analyzed. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The FeCuNbSiB metallic glasses have attracted attention in recent years [1]. This attention is due to the fact that after partial crystallization they have soft magnetic properties attributed to a twophase microstructure, which consists of bcc aFe(Si) crystals, with a grain size in the nanometre scale, surrounded by a residual amorphous phase [2]. Chattoraj et al [3] report that the soft-magnetic properties of FeBSi alloys are aected by corrosion. This report is important because, in some cases, magnetic materials are corroded in atmospheric conditions [4]. The corrosion resistance of soft magnetic alloys depends on their thermal history. Several authors [5±8] have reported that structural changes, such as structural relaxation and crystallization caused by annealing, aect corrosion properties. However, the eect of structural changes on the cor* Corresponding author. Tel.: +55-16 262 8250; fax: +55-16 261 5404; e-mail: [email protected]
rosion resistance of amorphous FeCuNbSiB alloys is not well studied. The purpose of this paper is therefore to study the eect of structural changes such as structural relaxation and crystallization on corrosion resistance of the amorphous Fe74 Cu1 Nb3 Si13:5 B8:5 alloy. 2. Experimental procedure Samples of Fe74 Cu1 Nb3 Si13:5 B8:5 amorphous ribbon of 0.024 mm thickness and 50 mm width (supplied by Vacuumschmelze Germany) was used. The samples were studied in the `as-cast' and annealed states, and were analysed by dierential scanning calorimetry (DSC) and X-ray diraction (XRD) using Cu-Ka radiation. In the DSC scans, samples were heated continuously from room temperature to 1000°C, at a constant heating rate of 25°C/min. The `as-cast' ribbons, whose amorphous structure was con®rmed by XRD, were annealed at 554°C, the peak temperature of the ®rst reactions, at 610°C at the end of the ®rst
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 3 4 - 4
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C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
reactions and at 330°C and 380°C, to cause structural relaxation. Annealing treatments were conducted in an argon atmosphere for 1 h, using heating and cooling rates of 25°C/min and 40°C/ min, respectively. The grain size of a-Fe(Si) crystallites was calculated from XRD spectra peak width using the Scherrer equation [9]. Structural relaxation in the amorphous state was monitored by measuring the speci®c heat of both the `as-cast' and annealed samples by DSC in an argon atmosphere, using a heating rate of 40°C/min. The Curie temperature was obtained from speci®c heat curves [10]. Corrosion resistance measurements were carried out by mass loss measurements in aerated 0.1M H2 SO4 solution, on 50 ´ 50 ´ 0.024 mm samples polished to 600 grade silicon carbide paper. The mass loss measurements were done in an analytical balance (Metler AB204), with 10ÿ4 g accuracy. Before each measurement the samples were rinsed with carbon tetrachloride and dried. Each test was performed with the same sample immersed during dierent times and the weight loss corrosion testing were repeated three times for each condition. The error bars correspond to the smallest and largest mass loss obtained from these three tests. The corroded surface of the `as cast' and of 330°C relaxed samples exposed to 0.1M H2 SO4 during 20 h was observed by scanning electron microscopy using a microscope (Zeiss DSM 940). Surface analysis with X-ray photoelectron spectroscopy (XPS) was used, and samples both in the `as-cast' state and annealed at 554°C were analyzed. XPS was performed in a spectrometer (Kratos XSAM HS), using Mg Ka radiation. The energy of the C 1s peak due to adventitious hydrocarbons was used as binding energy reference (284.8 eV). The background was subtracted using the Shirley method [11] and the peaks were ®tted by both Gaussian and mixed Gaussian/Lorentzian functions [12]. Before the XPS analysis these samples were immersed in 0.1M H2 SO4 solution for 8 h.
grain size of a-Fe(Si) phase of 15 and 25 nm respectively, showing that these samples had typical crystalline microstructure of nanocrystalline alloy. Structural relaxation in 330°C and 380°C annealed samples was observed by comparing the variation of the speci®c heat (Cp ) with temperature, in `ascast' and annealed samples. Fig. 1 shows that annealing at 330°C and 380°C caused only structural relaxation in the amorphous structure. The preservation of the full amorphous state was veri®ed by DSC analysis by comparison with the crystallization enthalpy of the `as-cast' samples. Nevertheless, for the annealed samples at temperatures less than the endothermic peak associated with the magnetic transition (the Curie temperature), the apparent speci®c heats increase monotonicly with increasing temperature, while this increase does not occur in `as-cast' samples. This dierence indicates that the annealed samples have a relaxed structure. The Fig. 1 shows that Curie temperature (Tc ) for the `as cast' state is 316.4°C (0.1°C) and that for annealed samples at 330°C and 380°C is 327.8°C (0.1°C) and 331.4°C (0.1°C), respectively. It can be seen that structural relaxation has increased the Curie temperature and that the higher the temperature of annealing, the higher the Curie temperature. Fig. 2 shows that the weight loss measurements in 0.1M H2 SO4 solution is not linear but it
3. Results The X-ray diraction analysis of the 554°C and 610°C annealed samples indicated an average
Fig. 1. Evolution of the apparent speci®c heat with temperature for the `as cast' (1) and annealed at 330°C (2) and 380°C (3) samples.
C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
71
Fig. 2. Values of accumulated mass loss obtained in H2 SO4 0.1M solution in as a result of immersion time. Lines are drawn as guides for the eye.
is still possible to calculate an approximate corrosion rate for each alloy. Thus, the corrosion rate of the polycrystalline samples at 554°C and 610°C are 4.07 ´ 10ÿ5 (10ÿ7 ) and 1.14 ´ 10ÿ6 (10ÿ8 ) g/cm2 h, respectively. Both the `as cast' and the relaxed samples have the same corrosion rate (1.20 ´ 10ÿ4 (10ÿ6 ) g/cm2 h) until 40 h of immersion. After this period the corrosion rates are 1.2 ´ 10ÿ4 (10ÿ5 ); 1.35 ´ 10ÿ4 (10ÿ6 ) and 1.70 ´ 10ÿ4 (10ÿ6 ) g/cm2 h for the `as cast'; the relaxed samples at 330°C and 380°C, respectively. So, the samples annealed at 330°C and 380°C (structurally relaxed) have the smallest corrosion resistance, and the samples annealed at 554°C and 610°C (nanocrystalline samples) a larger corrosion resistance, compared with the `as-cast' samples, as indicated in Fig. 2. Fig. 3 indicate the `as-cast' and the 330°C relaxed samples surface condition, after immersion in 0.1M H2 SO4 during 20 h. It is possible to see that the corroded surface of the `as cast' sample is homogeneous whilst the corroded surface of the relaxed sample has dierent features from place to place. In Table 1 are presented the results obtained from XPS analysis for the samples in the `as-cast' condition, and those crystallized at 554°C. As can be seen in Table 1, in both samples a protective oxide layer of SiO2 is formed, with the same atomic concentration within errors of measurement.
Fig. 3. Corroded surface of the as cast and 330°C relaxed sample analyzed by scanning electron microscopy. (a) as cast; (b) relaxed at 330°C.
Table 1 Binding energies of the principal photoelectron lines (in eV) and atomic concentration (with a relative error of <10%) of silicon in the super®cial ®lm Samples
Si 2p
Compound Atomic concentration (%)
`as cast' Crystallized at 554°C
103.4 SiO2 102.8 SiO2
30.1 29.8
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C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
4. Discussion In Fig. 1, the continuous decrease in Cp before the endothermic peak corresponding to the magnetic transition, observed for the `as-cast' sample, is related to a structural relaxation. This relaxation is an exothermic process which involves heat liberation, and brings about atomic rearrangements which promotes a decrease in the Cp s until the temperature reaches a point at which no further atomic rearrangement occurs. At this point the speci®c heat starts to increase until the material reaches the magnetic transition, or Curie temperature. Previous authors have reported an increase in the corrosion resistance of magnetic soft amorphous alloys, such as FeBSi [5] and FeNiSi [6] alloys, because of the stress relief caused by structural relaxation. This relaxation decreases the free energy of the amorphous system, thereby causing a decrease in the reactivity of its elements and increasing the chemical stability. However, our results show that structural relaxation decreases the corrosion resistance of Fe74 Cu1 Nb3 Si13 .5 B8:5 , thus indicating that the effect of structural relaxation on corrosion of amorphous alloys depends on the composition of these alloys. As shown in Fig. 1, structural relaxation increases the Curie temperature, indicating an increase in chemical short range ordering [13]. This phenomenon in Fe74 Cu1 Nb3 Si13:5 B8:5 alloy, is due to the presence of Cu. There is a repulsive interaction between copper atoms and the other elements in the alloy producing regions rich in each element during the structural relaxation [14]. This eect is responsible for a greater chemical short range ordering [15]. As a consequence, a decrease in the average free volume occurs in the Cu rich region, establishing an inhomogeneity in the amorphous matrix which induces a larger corrosion rate [15]. This fact could be attributed to an increase of internal stress [15]. Observations from scanning electron microscopy, see Fig. 3, show that corrosion takes place at dierent rate, as if a selective corrosion process was taking place. The additional stress causes an increase in the reactivity of the alloy elements and so increases the corrosion rate [6].
An increase in corrosion resistance with crystallization of amorphous alloy leading to nanocrystalline microstructure formation was reported for Fe78 B13 Si9 alloy [8], and was attributed to a larger silicon diusion rate, which leads to a larger rate of deposition of this element on the surface, and the growth of a thicker, more continuous and more protective SiO2 ®lm. Vacancy migration is the most common mechanism of diusion in crystalline metals [16]. However in amorphous alloys, it has been reported [17] that vacancies are unstable and atomic vibrations quickly redistribute the excess vacancy space over a larger volume than that of a vacancy. Thus, it has been frequently assumed that element diusion cannot occur by the mechanism of vacancy migration [18]. The diusion mechanism in amorphous alloys, suggested by several researchers [19,20], must take place by some form of cooperative motion of groups of adjacent atoms. The increase in corrosion resistance promoted by crystallization of amorphous samples may be due to the formation of a large area of interface between the amorphous and crystalline regions. In this area there are larger numbers of defects such as free volumes and microvoids, which cause an increase in the silicon diusion coecient [8]. So a large amount of silicon migrates quickly to the surface at the crystalline condition and oxidizes to form a continuous SiO2 ®lm. However our XPS analysis, as shown in Table 1, indicates that the `as-cast' and the 554°C annealed samples have the same silicon content in the super®cial ®lm, which is independent of the crystallization process. These results could be attributed to the faster diusion of silicon ions in the polycrystalline structure, which would promote the faster SiO2 ®lm formation. This consideration agrees with the larger corrosion resistance of the sample crystallized at 610°C, because of the greater silicon diusion coecient at this temperature.
5. Conclusion The structural relaxation promotes an increase of the Curie temperature of the amorphous
C.A.C. Souza et al. / Journal of Non-Crystalline Solids 247 (1999) 69±73
Fe74 Cu1 Nb3 Si13:5 B8:5 alloy, indicating an increase in chemical short range ordering. A controlled crystallization of amorphous alloy leading to nanocrystalline microstructure formation results in improved corrosion resistance, while structural relaxation decreases the corrosion resistance. The corrosion resistance increases with higher crystallization temperatures and decreases with the higher relaxation temperatures investigated. Acknowledgements The authors are indebted to Vacuumschmelze, Germany for supplying the amorphous ribbon. The authors would like to thank also the FAPESP ± `Projeto Tematico' and Ministry of Science and Technology (MCT) ± `PRONEX 1997' for ®nancial support. References [1] K. Lu, Mater. Sci. Eng. R: Reports R 16 (1996) 161. [2] Y. Yoshizawa, S. Oguma, Yamauch, J. Appl. Phys. 64 (1988) 6044. [3] I. Chattoraj, A. Mitra, Scripta Metall. Mater. 26 (1992) 1013.
73
[4] G.N. Maniar, T.A. Debold, Adv. Mater. Proc. 1 (1998) 57. [5] C.A.C. Souza, F.S. Politi, C.S. Kiminami, Scripta Metall. Mater. 39 (1998) 329. [6] V.S. Raja, K. Kishore, S. Ranganathan, in: S. Steeb, H. Warlimont (Eds.), Proceedings of the International Conference on Rapidly Quenched Metals, Elsevier, Amsterdam, 1985, p. 1485. [7] C.A.C. Souza, C.S. Kiminami, J. Non-Cryst. Solids 219 (1997) 155. [8] H.Y. Tong, F.G. Shi, J. Lavernia, Scripta Metall. Mater. 32 (1995) 511. [9] I. Kaeble, in: Handbook of X-rays, McGraw-Hill, New York, 1967. [10] A.L. Greer, PhD thesis, University of Cambridge, 1979. [11] D.A. Shirley, Phys. Rev. B 5 (1972) 4770. [12] G.C. Smith, Surface Analysis by Electron Spectoscopy, Plenum, New York, 1994. [13] H.S. Chen, A. Inoue, T. Masumoto, J. Mater. Sci. 20 (1985) 2417. [14] A.R. Yavari, O. Drbohlav, Mater. Trans. JIM 36 (7) (1995) 896. [15] A.R. Yavari, J. Mater. Res. 1 (1986) 746. [16] J.W. Christian, in: The Theory of Transformations in Metals and Alloys, Pergamon, Oxford, 1975. [17] C.H. Bennet, P. Chaudhari, V. Moruzzi, P. Steinhardt, Philos. Mag. A 40 (1979) 485. [18] B. Cantor, R.W. Cahn, in: F.E. Luborsky, Amorphous Metallic Alloys, 1983. [19] P. Valenta, K. Maier, H. Kronmuler, K. Freitag, Phys. Status Solidi. 106 (1981) 129. [20] C. Birac, D. Lesueur, Phys. Status Solidi 36 (1976) 247.