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Pressure dependence of nanocrystallization in amorphous Fe86B14 and Fe85Cu1B14 alloys
Materials Science and Engineering A286 (2000) 193 – 196 www.elsevier.com/locate/msea
Pressure dependence of nanocrystallization in amorphous Fe86B14 and Fe85Cu1B14 alloys B. Varga a,b,* , A. Lovas a,b, F. Ye c, X.J. Gu c, K. Lu c a
Department of Mechanical Engineering Technology, Technical Uni6ersity of Budapest, H-1111 Budapest Bertalan L. u. 2., Hungary b Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B. 49, Hungary c State Key Lab for RSA, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110015, PR China
Abstract Pressure effect on the nanocrystallization of amorphous Fe86B14 and Fe85Cu1B14 alloys has been investigated by means of piston-cylinder measurements within a pressure range of 0 – 1.0 GPa. It was found that the pressure does not alter the crystallization products but decreases the crystallization temperature of each crystallization product for both samples. Crystallization products of these amorphous alloys are bcc-Fe, metastable Fe3B and Fe2B, respectively. The crystallization and precipitation temperatures of both alloys decrease with increasing pressure but not in same manner. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous alloy; Nanocrystallization; Pressure effect
1. Introduction The effect of pressure on the thermodynamic stability of metastable materials has been an interesting subject in the high pressure research field. Many experimental results demonstrate that the applied pressure has a significant effect on crystallization process of amorphous solids, e.g. the variation of the crystallization temperature or mode with the pressure increase [1–3]. In the recent years rising attention has been focused on Fe–B based amorphous and nanocrystalline alloys because of their excellent soft magnetic properties. The pressure effect on crystallization of binary Fe –B alloys has been closely studied [4,5]. These investigations have been carried out on eutectic and hyper-eutectic compositions and found an appreciable retardation of crystallization due to the pressure, e.g. an increase of about 10 15 K GPa − 1 in the crystallization temperature was observed. However, to the authors’ knowledge, the pressure dependence of crystallization in hypoeutectic binary Fe–B and ternary Fe – Cu – B amorphous alloys has not been studied yet. * Corresponding author. E-mail address: [email protected] (B. Varga)
The Fe86B14 and Fe85Cu1B14 alloys were selected as targets in present work because of their similarity to the FINEMET-system (Fe73.5Cu1Nb3Si13.5B9), which is a very attractive soft magnetic material. This paper gives a demonstration of the pressure effect on the nanocrystallization of both the amorphous materials. We found the applied pressure decreases the crystallization temperature, which is quite different from those in Fe80B20 [4] and Fe83B17 [5].
2. Experimental The Fe86B14 and Fe85Cu1B14 master alloys were prepared by arc melting the mixture of pure Fe (99.99 wt.%), B (99.99 wt.%) and Cu (99.99 wt.%) rods under purified argon atmosphere on water-cooled hearth. The amorphous ribbons about 2.0 mm wide and 20 mm thick were produced by single roller melt-spinning technique. The amorphous nature of the ribbons was confirmed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments. The XRD analyses were carried out on a Rigaku X-ray diffractometer with Cu Ka radiation, and the DSC experiments on Perkin-Elmer DSC-7.
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 6 3 4 - 1
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B. Varga et al. / Materials Science and Engineering A286 (2000) 193–196
A piston-cylinder pressure vessel with an internal diameter of 8 mm was used at pressures up to 1.0 GPa in a vacuum chamber (3 ×10 − 3 Pa). The experimental setup and measurement procedures were described in detail previously [3]. The amorphous ribbons with a total weight of about 0.8 g were placed into the vessel randomly. The applied heating rate was 5 K min − 1. A thermocouple was placed under the vessel to monitor the sample temperature. The difference between the actual temperature of the specimen in the vessel and the temperature measured by the thermocouple was less than 3 K, and all the measured temperatures have been corrected. The applied pressure accuracy is about 9 1%. Prior to heating the sample under pressure, it was compressed at 1.0 GPa for 60 min at ambient temperature in order to densify the ribbons. Fig. 3. The differential ( dP/dT− T) curves for heating Fe85Cu1B14 amorphous alloy under different preset pressures at a heating rate of 5 K min − 1. The dotted line indicates the crystallization temperature deduced from XRD experiments.
3. Results
Fig. 1. Typical experimental curve and its differential curve for the crystallization process of the Fe85Cu1B14 amorphous alloy under a preset pressure of 0.15 GPa at a heating rate of 5 K min − 1.
Fig. 2. The XRD patterns of the Fe85Cu1B14 amorphous alloy samples after annealing at different temperatures at a preset pressure of 0.15 Gpa.
DSC analyses combined with XRD experiments showed that both of the Fe86B14 and Fe85Cu1B14 amorphous alloys crystallize in a typical two-step mechanism, as similar as described previously [6,7]. The first step, so-called a primary reaction, results in a dispersion of nanometer sized bcc-Fe grains in the residual amorphous matrix, which is considered as a diffusion controlled process. This structure ensures excellent magnetic softness. The second step is the crystallization of Fe borides. The sequence of this crystallization resembles to the amorphous-nanocrystalline reactions in the FINEMET type alloys. Fig. 1 shows a typical experimental result of the pressure applied to the sample as a function of temperature with a preset pressure of 0.15 GPa for Fe85Cu1B14 sample. One can see that upon heating pressure rises continuously because of the thermal expansion of the sample and the piston. When a densification process takes place due to a phase transformation with a negative volume change, a decrease in the slope appears in the P–T curve, which corresponds to a peak in the dP/dT − T differential curve. The XRD analyses proved that this peak is originated from the formation of Fe-borides (see Fig. 2). The primary reaction, which leads to the precipitation of bcc-Fe grains from the amorphous phase cannot be detected by this apparatus. Therefore, the onset temperature of the primary reaction was determined on the basis of XRD analyses of samples heated up to different temperatures under different pressures. The XRD results also showed that the applied pressure (within the range of 0–1.0 GPa) does not alter the crystallization product, which are bcc-Fe,
B. Varga et al. / Materials Science and Engineering A286 (2000) 193–196
Fe3B and Fe2B, respectively. Fig. 3 gives the dP/dT versus temperature plots in the preset pressure range of 00.8 GPa for Fe85Cu1B14 sample. One can find that at higher pressures both the onset and the peak temperature decrease with increasing pressure but not linearly. On the basis of the high pressure measurements and the combined XRD phase identification results the crystallization temperature was determined and summarized in the Figs. 4 and 5, for Fe86B14 and Fe85Cu1B14, respectively. The data points under ambient pressure were determined by DSC measurement as well and they showed a good agreement with the results given by pressure measurements at low pressure. Fig. 4 shows that the crystallization temperatures of Fe86B14 amorphous alloy decrease with the applied pressure increase. The onset temperature of the primary crystallization process for the Fe86B14 alloy decreases in the pressure range of 0 0.3 GPa (by about 15 K) but shows a slightly increase of about 5 K in the pressure range of 0.3–1.0 GPa. One can see from Fig. 4 that
Fig. 4. The pressure dependence of the crystallization temperatures for the Fe86B14 amorphous alloy.
Fig. 5. The pressure dependence of the crystallization temperatures for the Fe85Cu1B14 amorphous alloy.
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after a relative fast decrease (by about 15–20 K) in the pressure range of 0 0.3 GPa both the onset and the peak temperatures of the second crystallization process for Fe86B14 change slightly from 0.3 to 0.9 GPa. In the case of the Fe85Cu1B14 amorphous alloy (see Fig. 5) it is clear that the crystallization temperatures for both crystallization processes decrease with increasing pressure but not in linear manner. The onset temperature of the primary crystallization process for the Fe85Cu1B14 alloy shows a decrease of about 30 K in the pressure range of 0 1.0 GPa. Both the onset and the peak temperatures of the second crystallization step for the Fe85Cu1B14 show a decrease of about 40 K in the pressure range of 0 1.0 GPa. It is also noted from Fig. 5 that the temperature values for the second crystallization has only a slightly change in the pressure range of 0.30.7 GPa.
4. Discussion Our previous work showed that the applied pressure either enhances [8] or retards [3] the crystallization process of amorphous solids. It was found the simultaneously formation of the crystalline/amorphous (c/a) interface during the nucleation process is the governing factor in the crystallization kinetics of the amorphous solids under pressure [9]. It is known that the c/a interface is lower coordinated than the crystalline and the amorphous phase, and its formation will result in a volume expansion. Thus the pressure will obstacle the interface formation. However, this effect can be dominant only at the initial stage of nucleation process. Our calculation suggests that for amorphous selenium and Ni80P20 alloy, the volume change for forming critical nucleus is positive, resulting in an increase in the nucleation work with an increment of pressure. Hence, the crystallization temperature of the amorphous phase rises when a pressure is applied. While for Al–La–Ni amorphous alloy, probably because of highly dense atomic configuration of the interface, the volume change is negative for forming critical Al nuclei, and the nucleation barrier decreases at higher pressures. The phenomena that the pressure decreases the crystallization temperature for the two Fe-based amorphous alloys in this work might be interpreted similarly as for Al–La–Ni amorphous alloy. It has been mentioned that the applied pressure increases the crystallization temperature of the Fe80B20 and Fe83B17 amorphous alloys, while in this work the Fe86B14 and Fe85Cu1B14 amorphous alloy crystallize at lower temperatures under higher pressures. These different phenomena might come from their different composition. Due to the relative high Fe concentration in Fe86B14 and Fe85Cu1B14 amorphous alloy, the interfacial energy might be smaller than those in Fe80B20 and Fe83B17
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amorphous alloys, which means the c/a interface formed in Fe86B14 and Fe85Cu1B14 amorphous phase possess relative high density. Hence, the total volume change during the critical nucleus formation might be negative so that the nucleation barrier decreases when pressure is applied and the crystallization temperature decreases.
Acknowledgements This work was partially supported by the Hungarian Scientific Research Fund (OTKA) through grant T 022124 and by the Research and Development Fund for Higher Education (1999–2002). The financial supports from the Chinese Academy of Sciences and the National Science Foundation of China (grant no. 59431021 and 59625101) are also acknowledged.
5. Conclusions The external pressure (01.0 GPa) decreases the thermal stability of both the Fe86B14 and Fe85Cu1B14 amorphous alloys but not in the same manner. In the applied pressure range the temperature of the first crystallization process for the Fe86B14 showed a decrease of about 15 K, having a minimum value at about 0.3–0.4 GPa. On the other hand in the case of Fe85Cu1B14 the temperature of the first crystallization step decreased continuously about 30 K. In the same pressure range the crystallization temperature of the second process also decreased, by about 20 K for Fe86B14 and about 40 K for Fe85Cu1B14, respectively.
.
References [1] H. Iwasaki, T. Masumoto, J. Mater. Sci. 13 (1978) 2171. [2] B. Yao, F.S. Li, X.M. Lin, B.Z. Ding, W.H. Su, Z.Q. Hu, J. Non-Cryst. Sol. 217 (1997) 317. [3] F. Ye, K. Lu, Acta Mater. 46 (1998) 5965. [4] M. Cedergen, G. Backstrom, J. Non-Cryst. Sol. 30 (1978) 69. [5] W.K. Wang, H. Iwasaki, K. Fukamichi, J. Mater. Sci. 15 (1980) 2701. [6] G. Herzer, Scripta Metall. Mater. 33 (1995) 1741. [7] A. Makino, A. Inoue, T. Masumoto, Mater. Trans. JIM. 36 (1995) 909. [8] F. Ye, K. Lu, Acta Mater. 47 (1998) 2449. [9] F. Ye, K. Lu, Phys. Rev. B (in press).
Pressure dependence of nanocrystallization in amorphous Fe86B14 and Fe85Cu1B14 alloys B. Varga a,b,* , A. Lovas a,b, F. Ye c, X.J. Gu c, K. Lu c a
Department of Mechanical Engineering Technology, Technical Uni6ersity of Budapest, H-1111 Budapest Bertalan L. u. 2., Hungary b Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B. 49, Hungary c State Key Lab for RSA, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110015, PR China
Abstract Pressure effect on the nanocrystallization of amorphous Fe86B14 and Fe85Cu1B14 alloys has been investigated by means of piston-cylinder measurements within a pressure range of 0 – 1.0 GPa. It was found that the pressure does not alter the crystallization products but decreases the crystallization temperature of each crystallization product for both samples. Crystallization products of these amorphous alloys are bcc-Fe, metastable Fe3B and Fe2B, respectively. The crystallization and precipitation temperatures of both alloys decrease with increasing pressure but not in same manner. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous alloy; Nanocrystallization; Pressure effect
1. Introduction The effect of pressure on the thermodynamic stability of metastable materials has been an interesting subject in the high pressure research field. Many experimental results demonstrate that the applied pressure has a significant effect on crystallization process of amorphous solids, e.g. the variation of the crystallization temperature or mode with the pressure increase [1–3]. In the recent years rising attention has been focused on Fe–B based amorphous and nanocrystalline alloys because of their excellent soft magnetic properties. The pressure effect on crystallization of binary Fe –B alloys has been closely studied [4,5]. These investigations have been carried out on eutectic and hyper-eutectic compositions and found an appreciable retardation of crystallization due to the pressure, e.g. an increase of about 10 15 K GPa − 1 in the crystallization temperature was observed. However, to the authors’ knowledge, the pressure dependence of crystallization in hypoeutectic binary Fe–B and ternary Fe – Cu – B amorphous alloys has not been studied yet. * Corresponding author. E-mail address: [email protected] (B. Varga)
The Fe86B14 and Fe85Cu1B14 alloys were selected as targets in present work because of their similarity to the FINEMET-system (Fe73.5Cu1Nb3Si13.5B9), which is a very attractive soft magnetic material. This paper gives a demonstration of the pressure effect on the nanocrystallization of both the amorphous materials. We found the applied pressure decreases the crystallization temperature, which is quite different from those in Fe80B20 [4] and Fe83B17 [5].
2. Experimental The Fe86B14 and Fe85Cu1B14 master alloys were prepared by arc melting the mixture of pure Fe (99.99 wt.%), B (99.99 wt.%) and Cu (99.99 wt.%) rods under purified argon atmosphere on water-cooled hearth. The amorphous ribbons about 2.0 mm wide and 20 mm thick were produced by single roller melt-spinning technique. The amorphous nature of the ribbons was confirmed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments. The XRD analyses were carried out on a Rigaku X-ray diffractometer with Cu Ka radiation, and the DSC experiments on Perkin-Elmer DSC-7.
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 6 3 4 - 1
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B. Varga et al. / Materials Science and Engineering A286 (2000) 193–196
A piston-cylinder pressure vessel with an internal diameter of 8 mm was used at pressures up to 1.0 GPa in a vacuum chamber (3 ×10 − 3 Pa). The experimental setup and measurement procedures were described in detail previously [3]. The amorphous ribbons with a total weight of about 0.8 g were placed into the vessel randomly. The applied heating rate was 5 K min − 1. A thermocouple was placed under the vessel to monitor the sample temperature. The difference between the actual temperature of the specimen in the vessel and the temperature measured by the thermocouple was less than 3 K, and all the measured temperatures have been corrected. The applied pressure accuracy is about 9 1%. Prior to heating the sample under pressure, it was compressed at 1.0 GPa for 60 min at ambient temperature in order to densify the ribbons. Fig. 3. The differential ( dP/dT− T) curves for heating Fe85Cu1B14 amorphous alloy under different preset pressures at a heating rate of 5 K min − 1. The dotted line indicates the crystallization temperature deduced from XRD experiments.
3. Results
Fig. 1. Typical experimental curve and its differential curve for the crystallization process of the Fe85Cu1B14 amorphous alloy under a preset pressure of 0.15 GPa at a heating rate of 5 K min − 1.
Fig. 2. The XRD patterns of the Fe85Cu1B14 amorphous alloy samples after annealing at different temperatures at a preset pressure of 0.15 Gpa.
DSC analyses combined with XRD experiments showed that both of the Fe86B14 and Fe85Cu1B14 amorphous alloys crystallize in a typical two-step mechanism, as similar as described previously [6,7]. The first step, so-called a primary reaction, results in a dispersion of nanometer sized bcc-Fe grains in the residual amorphous matrix, which is considered as a diffusion controlled process. This structure ensures excellent magnetic softness. The second step is the crystallization of Fe borides. The sequence of this crystallization resembles to the amorphous-nanocrystalline reactions in the FINEMET type alloys. Fig. 1 shows a typical experimental result of the pressure applied to the sample as a function of temperature with a preset pressure of 0.15 GPa for Fe85Cu1B14 sample. One can see that upon heating pressure rises continuously because of the thermal expansion of the sample and the piston. When a densification process takes place due to a phase transformation with a negative volume change, a decrease in the slope appears in the P–T curve, which corresponds to a peak in the dP/dT − T differential curve. The XRD analyses proved that this peak is originated from the formation of Fe-borides (see Fig. 2). The primary reaction, which leads to the precipitation of bcc-Fe grains from the amorphous phase cannot be detected by this apparatus. Therefore, the onset temperature of the primary reaction was determined on the basis of XRD analyses of samples heated up to different temperatures under different pressures. The XRD results also showed that the applied pressure (within the range of 0–1.0 GPa) does not alter the crystallization product, which are bcc-Fe,
B. Varga et al. / Materials Science and Engineering A286 (2000) 193–196
Fe3B and Fe2B, respectively. Fig. 3 gives the dP/dT versus temperature plots in the preset pressure range of 00.8 GPa for Fe85Cu1B14 sample. One can find that at higher pressures both the onset and the peak temperature decrease with increasing pressure but not linearly. On the basis of the high pressure measurements and the combined XRD phase identification results the crystallization temperature was determined and summarized in the Figs. 4 and 5, for Fe86B14 and Fe85Cu1B14, respectively. The data points under ambient pressure were determined by DSC measurement as well and they showed a good agreement with the results given by pressure measurements at low pressure. Fig. 4 shows that the crystallization temperatures of Fe86B14 amorphous alloy decrease with the applied pressure increase. The onset temperature of the primary crystallization process for the Fe86B14 alloy decreases in the pressure range of 0 0.3 GPa (by about 15 K) but shows a slightly increase of about 5 K in the pressure range of 0.3–1.0 GPa. One can see from Fig. 4 that
Fig. 4. The pressure dependence of the crystallization temperatures for the Fe86B14 amorphous alloy.
Fig. 5. The pressure dependence of the crystallization temperatures for the Fe85Cu1B14 amorphous alloy.
195
after a relative fast decrease (by about 15–20 K) in the pressure range of 0 0.3 GPa both the onset and the peak temperatures of the second crystallization process for Fe86B14 change slightly from 0.3 to 0.9 GPa. In the case of the Fe85Cu1B14 amorphous alloy (see Fig. 5) it is clear that the crystallization temperatures for both crystallization processes decrease with increasing pressure but not in linear manner. The onset temperature of the primary crystallization process for the Fe85Cu1B14 alloy shows a decrease of about 30 K in the pressure range of 0 1.0 GPa. Both the onset and the peak temperatures of the second crystallization step for the Fe85Cu1B14 show a decrease of about 40 K in the pressure range of 0 1.0 GPa. It is also noted from Fig. 5 that the temperature values for the second crystallization has only a slightly change in the pressure range of 0.30.7 GPa.
4. Discussion Our previous work showed that the applied pressure either enhances [8] or retards [3] the crystallization process of amorphous solids. It was found the simultaneously formation of the crystalline/amorphous (c/a) interface during the nucleation process is the governing factor in the crystallization kinetics of the amorphous solids under pressure [9]. It is known that the c/a interface is lower coordinated than the crystalline and the amorphous phase, and its formation will result in a volume expansion. Thus the pressure will obstacle the interface formation. However, this effect can be dominant only at the initial stage of nucleation process. Our calculation suggests that for amorphous selenium and Ni80P20 alloy, the volume change for forming critical nucleus is positive, resulting in an increase in the nucleation work with an increment of pressure. Hence, the crystallization temperature of the amorphous phase rises when a pressure is applied. While for Al–La–Ni amorphous alloy, probably because of highly dense atomic configuration of the interface, the volume change is negative for forming critical Al nuclei, and the nucleation barrier decreases at higher pressures. The phenomena that the pressure decreases the crystallization temperature for the two Fe-based amorphous alloys in this work might be interpreted similarly as for Al–La–Ni amorphous alloy. It has been mentioned that the applied pressure increases the crystallization temperature of the Fe80B20 and Fe83B17 amorphous alloys, while in this work the Fe86B14 and Fe85Cu1B14 amorphous alloy crystallize at lower temperatures under higher pressures. These different phenomena might come from their different composition. Due to the relative high Fe concentration in Fe86B14 and Fe85Cu1B14 amorphous alloy, the interfacial energy might be smaller than those in Fe80B20 and Fe83B17
196
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amorphous alloys, which means the c/a interface formed in Fe86B14 and Fe85Cu1B14 amorphous phase possess relative high density. Hence, the total volume change during the critical nucleus formation might be negative so that the nucleation barrier decreases when pressure is applied and the crystallization temperature decreases.
Acknowledgements This work was partially supported by the Hungarian Scientific Research Fund (OTKA) through grant T 022124 and by the Research and Development Fund for Higher Education (1999–2002). The financial supports from the Chinese Academy of Sciences and the National Science Foundation of China (grant no. 59431021 and 59625101) are also acknowledged.
5. Conclusions The external pressure (01.0 GPa) decreases the thermal stability of both the Fe86B14 and Fe85Cu1B14 amorphous alloys but not in the same manner. In the applied pressure range the temperature of the first crystallization process for the Fe86B14 showed a decrease of about 15 K, having a minimum value at about 0.3–0.4 GPa. On the other hand in the case of Fe85Cu1B14 the temperature of the first crystallization step decreased continuously about 30 K. In the same pressure range the crystallization temperature of the second process also decreased, by about 20 K for Fe86B14 and about 40 K for Fe85Cu1B14, respectively.
.
References [1] H. Iwasaki, T. Masumoto, J. Mater. Sci. 13 (1978) 2171. [2] B. Yao, F.S. Li, X.M. Lin, B.Z. Ding, W.H. Su, Z.Q. Hu, J. Non-Cryst. Sol. 217 (1997) 317. [3] F. Ye, K. Lu, Acta Mater. 46 (1998) 5965. [4] M. Cedergen, G. Backstrom, J. Non-Cryst. Sol. 30 (1978) 69. [5] W.K. Wang, H. Iwasaki, K. Fukamichi, J. Mater. Sci. 15 (1980) 2701. [6] G. Herzer, Scripta Metall. Mater. 33 (1995) 1741. [7] A. Makino, A. Inoue, T. Masumoto, Mater. Trans. JIM. 36 (1995) 909. [8] F. Ye, K. Lu, Acta Mater. 47 (1998) 2449. [9] F. Ye, K. Lu, Phys. Rev. B (in press).