- Email: [email protected]
Influence of additive elements Nb and Mo on the crystallization process of amorphous alloy Fe76.5Cu1Si13.5B9
July 1998
Materials Letters 36 Ž1998. 223–228
Influence of additive elements Nb and Mo on the crystallization process of amorphous alloy Fe 76.5 Cu 1Si 13.5 B 9 J.W. Zhang a , X.Y. Zhang a
a,b,)
, F.R. Xiao a , J.H. Liu a , Y.Z. Zheng
a
Department of Materials Engineering, Yanshan UniÕersity, Qinhuangdao, 066004, China b Institute of Physics, Chinese Academy of Science, Beijing, 100080, China Received 9 June 1997; revised 24 September 1997; accepted 23 January 1998
Abstract The influence of additive elements Nb and Mo on the crystallization process of the amorphous alloy Fe 76.5 Cu 1Si 13.5 B 9 have been investigated by means of a Differential Scanning Calorimeter ŽDSC., X-ray Diffraction ŽXRD. and Transmission Electron Microscopy ŽTEM.. Compared with the Fe 76.5 Cu 1Si 13.5 B 9 alloy, the alloy with addition of Nb or Mo, Fe 73.5 Cu 1 M 3 Si 13.5 B 9 ŽM s Nb, Mo., has a unique crystallization process. The activation energy of crystallization of the alloy with addition of Nb, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , during the beginning stage of crystallization is the smallest, it increases with increasing crystallized fraction and then shows a larger value Ž500 kJrmol. in the range of crystallized fraction of 25%–90%. The crystallization process of the alloy with addition of Mo, Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, but the increase of activation energy in the alloy with crystallized fraction is smaller. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline alloy; Activation energy of crystallization; Additive elements
1. Introduction Many studies in recent years have been devoted to a new class of nanocrystalline alloys made by crystallizing metallic glasses since they exhibit excellent soft magnetic properties. The classical alloy is Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 developed by Yoshizawa et al. w1 x. It has been shown that w 2 – 15 x the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy having optimum soft magnetic properties consists primarily of an a-FeŽSi.
)
Corresponding author.
crystalline phase and a residual amorphous phase; the a-FeŽSi. crystalline phase has a grain size of 10 nm and a DO 3 superstructure. Such an ultra-fine microstructure can be also obtained by the replacement of Nb with Mo or W; but the alloy with Nb, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , has the smallest grain size, which is very significant for the excellent soft magnetic properties of the alloy. However, the reason why the alloy with Nb has a smaller grain size than the alloy with Mo or W has not been known clearly so far. Since these nanocrystalline alloys were prepared by crystallization of amorphous alloys, it is quite likely to better understand this cause by studying the crystallization process of these amorphous alloys.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 3 2 - 9
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
224
In the present paper, the influence of additive element Nb and Mo on the crystallization process of amorphous alloy Fe 76.5 Cu 1 Si 13.5 B 9 was investigated in detail using the Differential Scanning Calorimeter ŽDSC., X-ray Diffraction ŽXRD. and Transmission Electron Microscopy ŽTEM.. The difference in the crystallization process for these alloys was also discussed.
2. Experimental procedure Amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žat.%. ribbons being 20–30-m m thick and 5-mm wide were prepared by the melt-spinning technique on a single copper roller. The amorphous nature was confirmed by X-ray diffraction ŽXRD.. The thermal analysis experiments of the crystallization process of the three kinds of amorphous alloys were carried out using a Differential Scanning Calorimeter ŽDSC. at different heating rates, 58Crmin, 108Crmin, 158Crmin and 258Crmin, respectively. The X-ray diffraction of the alloy which had been measured by DSC was carried out using a DrMAX-rB diffractometer with a graphite monochromator and Cu K a radiation. The microstructure of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloys annealed at the temperature of 5208C in Ar atmosphere for 1 h was observed using an H800 Transmission Electron Microscope ŽTEM.. Using the DSC experimental data, we calculated the average activation energy of crystallization Ž E . of amorphous alloy by the Kissinger method w16x: ln
B
ž / Tp2
E sy RTp
q constant
DSC curve, T is the temperature, x is the crystallized fraction, A is the frequency factor, and F Ž x . is the function which only relates to the crystallized fraction Ž x .. According to Eq. Ž1., by using the value of Tp at different heating rates, a plot of lnŽ BrTp2 . vs. 1rTp yields approximate straight line. From the slope of the line, the average activation energy of crystallization, E, is obtained. From the exothermal peak of the DSC curve, we can determine the curves of crystallized fraction Ž x . vs. temperature ŽT . at different heating rates. Given a certain crystallized fraction x, we can obtain different temperatures T which correspond to different heating rates B. According to Eq. Ž2., we can make a plot of log B vs. 1rT, and from the slope of this plot the activation energy of crystallization at the certain crystallized fraction x, Ec Ž x ., is obtained. The Ec Ž x . is defined as the local activation energy of crystallization.
3. Experimental results and discussion Fig. 1 shows the DSC curves of crystallization process for amorphous alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 . Two exothermal peaks can be seen clearly from Fig. 1 as these amorphous alloy are heated to 6708C. The
Ž 1.
and the activation energy of crystallization at a certain crystallized fraction x, Ec Ž x ., by the Doyle method w17x: log B s Ig
A = Ec Ž x . R=FŽ x.
y 2.315 y 0.4567 =
Ec Ž x . R=T
.
Ž 2. Where B is the heating rate, R is the gas constant, Tp is the peak temperature of the exothermal peak of
Fig. 1. DSC curves of crystallization process for amorphous alloys F e 7 6 .5 C u 1 S i 1 3 .5 B 9 , F e 7 3 .5 C u 1 N b 3 S i 1 3 .5 B 9 a n d Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žat a heating rate of 108Crmin. ŽA. Fe 7 3 .5 Cu 1 Nb 3 Si 1 3 .5 B 9 , Ž B . Fe 7 3 .5 Cu 1 Mo 3 Si 1 3 .5 B 9 , Ž C . Fe 76.5 Cu 1 Si 13.5 B 9 .
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
result of XRD shows that the first exothermal peak corresponds to the formation of iron–silicon crystallites, and the second corresponds to the formation of Fe–B compounds. The crystallization process of the iron–silicon crystallites in these alloys is investigated in detail below for better understanding why the nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy has a smaller size for iron–silicon crystallite. Table 1 Summarize the average activation energy of crystallization Ž E . of amorphous alloys Fe 76.5Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1Mo 3 Si 13.5 B 9 . For Fe 76.5 Cu 1 Si 13.5 B 9 alloy. The average activation energy E is 182.4 " 4.0 kJrmol, which is smaller than that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy Ž452.4 " 4.8 kJrmol. and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy Ž350.7 " 4.2 kJrmol.. This indicates that the addition of Nb or Mo to Fe 76.5 Cu 1Si 13.5 B 9 alloy results in an increase of the activation energy of crystallization of the alloy. The changes of the local activation energy of crystallization Ec Ž x ., the activation energy of crystallization at a certain crystallized fraction x, with crystallized fraction Ž x . for the amorphous alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 are shown in Fig. 2. The activation energy of crystallization for Fe 76.5 Cu 1 Si 13.5 B 9 alloy increases slightly with crystallized fraction between 2% and 90%. However, the alloy with addition of Nb or Mo, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , exhibits a unique crystallization process. For Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, the activation energy of crystallization at the beginning stage of crystallization is the smallest, it increases quickly with the increase of crystallized fraction and shows the maximum Ž; 500 kJrmol. in a large range of crystallized fraction Ž25%–90%.. It can be seen clearly that the crystallization process of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy. However, the increase of the activation energy of crystallization in
225
Fig. 2. Changes of the local activation energy of crystallization w ith cry stallized fractio n fo r F e 7 6 .5 C u 1 S i 1 3 .5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy.
Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 with the increase of crystallized fraction is smaller than that in Fe 73.5Cu 1 Nb 3 Si 13.5 B 9 alloy. The maximum of activation energy of crystallization of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy, 370 kJrmol, on the other hand, is also smaller than that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, 500 kJrmol. All these imply that the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy has a stronger constraint for the growth of iron–silicon crystallite than Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Fig. 3 shows the TEM micrograph of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloys annealed at a temperature of 5208C for 1 h. Analysis shows that the crystallites in these annealed alloys are iron–silicon, and that the alloy with addition of Nb or Mo has the iron–silicon crystallites with much smaller grain size in comparison with Fe 76.5 Cu 1 Si 13.5 B 9 alloy, but the size of iron–silicon crystallites in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is the smallest, 10–12 nm. We suggest that the microstructural features shown in Fig. 3 for annealed alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 re-
Table 1 Average activation energy of crystallization of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy Alloy
Average activation energy, E ŽkJrmol.
Fe 76.5 Cu 1 Si 13.5 B 9
Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9
Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9
182.4 " 4.0
452.4 " 4.8
350.7 " 4.2
226
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
Fig. 3. TEM micrograph of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy annealed at 5208C for 1 h. Ža. Fe 76.5 Cu 1 Si 13.5 B 9 , Žb. Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žc. Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 .
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
sult from their crystallization behaviour. At the beginning stage of crystallization, the lower activation energy of Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, which results from the separation of the Fe, Cu atoms during the crystallization process w1x, benefits the formation of the nuclei of iron–silicon crystallites in these alloys. However, for Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the iron–silicon crystallites increase in size and form a large grain due to the lower activation energy of crystallization during the crystallization process. For the alloy Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , the stability of the residual amorphous regions around iron–silicon crystallites becomes stronger due to the formation of Nb-Žor Mo-. and B-rich regions in these amorphous regions in the crystallization process w1,8,10x. So the activation energy of crystallization increases with increasing fraction of crystallites. This constrains the growth of the iron–silicon crystallites and results in the formation of ultral fine grains in the end. It was indicated that w18,19x the size and the electronegativity of the atom consisting of an amorphous alloy are two important factors that affect the stability of the alloy. The larger the difference in the size and electronegativity between the constituents of the amorphous alloy, the more stable is the alloy. Nb and B have a larger size difference and electronegativity difference than Mo and B, and consequently, the residual amorphous phase which enriches in Nb and B in the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is more stable than that which enriches in Mo and B in the Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Therefore, the activation energy of crystallization of Fe 7 3 .5 Cu 1 Nb 3 Si 13.5 B 9 alloy increases more quickly during the crystallization process, and shows a larger value in a large range of the crystallized fraction than that of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. We suggest that this is very significant for constraining the growth of iron–silicon crystallites and leads to the formation of the smaller iron–silicon grains in the annealed Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy.
4. Conclusions Ž1. For the amorphous Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the average activation energy of crystallization calcu-
227
lated by the Kissinger method is 182.4 " 4.0 kJrmol, and it increases to 350.7 " 4.2 kJrmol and 452.4 " 4.8 kJrmol, respectively, with the addition of Mo and Nb for the alloy Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 . Ž2. For the amorphous Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the local activation energy of crystallization calculated by the Doyle method increases slightly from 224 to 234 kJrmol with the increase of crystallized fraction. The local activation energy of crystallization of the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy at the beginning stage of crystallization is the smallest, it increases with the crystallized fraction and shows the largest Ž500 kJrmol. in the range of crystallized fraction of 25%–90%. The crystallization process of the Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, but the increase of activation energy in the alloy with crystallized fraction is smaller than that in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy. Ž3. The reason why the size of iron–silicon crystallite in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is smaller than that in Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is that the activation energy of crystallization of the alloy increases more quickly with crystallized fraction and exhibits a larger value in a larger crystallized fraction than that of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Acknowledgements The work was supported by the Natural Science Foundation of Hebei Province of China.
References w1x Y. Yoshizawa, S. Oguchi, K. Yamauchi, J. Appl. Phys. 64 Ž1988. 6044. w2x G. Herzer, IEEE Trans. Magn. 25 Ž1989. 3327. w3x G. Herzer, IEEE Trans. Magn. 26 Ž1990. 1397. w4x K. Hono, K. Hiraga, Q. Wang, A. Inoue, T. Sakurai, Acta Metall. Mater. 40 Ž1992. 2137. w5x H. Kim, M. Matsuura, M. Sakurai, K. Suzuki, Jpn. J. Appl. Phys. 32 Ž1993. 676. w6x M. Sakurai, M. Matsuura, S.H. Kim, Y. Yoshizawa, K. Ymamauchi, K. Suzuki, Mater. Sci. Eng. 469 Ž1994. A179– A180. w7x J.D. Ayers, V.G. Harris, J.A. Sprague, W.T. Elam, Appl. Phys. Lett. 64 Ž1994. 974. w8x Y. Yoshizawa, K. Yamauchi, Mater. Sci. Eng. A 133 Ž1991. 176.
228
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
w9x M. Muller, N. Mattern, J. Magn. Mater. 136 Ž1994. 79. ¨ w10x X.Y. Zhang, J.W. Zhang, J.H. Liu, Y.Z. Zheng, R.P. Liu, J.H. Zhao, Mater. Lett. 29 Ž1996. 31. w11x X.Y. Zhang, J.W. Zhang, F.R. Xiao, J.H. Liu, Y.Z. Zheng, Mater. Lett., 34 Ž1,2., 85, 1998. w12x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, Y.Z. Zheng, J. Magn. Magn. Mater. 172 Ž1997. 301. w13x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, J.H. Liu, Y.Z. Zheng, NanoStructured Mater. 8 Ž1997. 337.
w14x J.H. Liu, X.Y. Zhang, Y.Z. Zheng, J.H. Zhao, R.P. Liu, J. Mater. Sci. Technol. 13 Ž1997. 37. w15x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, J.H. Liu, Y.Z. Zheng, J. Mater. Sci. Lett. 16 Ž1997. 1745. w16x H.E. Kissinger, Anal. Chem. 29 Ž1975. 1702. w17x C.D. Doyle, J. Appl. Polym. Sci. 5 Ž1961. 285. w18x A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 31 Ž1990. 177. w19x A. Inoue, Mater. Trans. JIM 36 Ž1995. 866.
Materials Letters 36 Ž1998. 223–228
Influence of additive elements Nb and Mo on the crystallization process of amorphous alloy Fe 76.5 Cu 1Si 13.5 B 9 J.W. Zhang a , X.Y. Zhang a
a,b,)
, F.R. Xiao a , J.H. Liu a , Y.Z. Zheng
a
Department of Materials Engineering, Yanshan UniÕersity, Qinhuangdao, 066004, China b Institute of Physics, Chinese Academy of Science, Beijing, 100080, China Received 9 June 1997; revised 24 September 1997; accepted 23 January 1998
Abstract The influence of additive elements Nb and Mo on the crystallization process of the amorphous alloy Fe 76.5 Cu 1Si 13.5 B 9 have been investigated by means of a Differential Scanning Calorimeter ŽDSC., X-ray Diffraction ŽXRD. and Transmission Electron Microscopy ŽTEM.. Compared with the Fe 76.5 Cu 1Si 13.5 B 9 alloy, the alloy with addition of Nb or Mo, Fe 73.5 Cu 1 M 3 Si 13.5 B 9 ŽM s Nb, Mo., has a unique crystallization process. The activation energy of crystallization of the alloy with addition of Nb, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , during the beginning stage of crystallization is the smallest, it increases with increasing crystallized fraction and then shows a larger value Ž500 kJrmol. in the range of crystallized fraction of 25%–90%. The crystallization process of the alloy with addition of Mo, Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, but the increase of activation energy in the alloy with crystallized fraction is smaller. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline alloy; Activation energy of crystallization; Additive elements
1. Introduction Many studies in recent years have been devoted to a new class of nanocrystalline alloys made by crystallizing metallic glasses since they exhibit excellent soft magnetic properties. The classical alloy is Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 developed by Yoshizawa et al. w1 x. It has been shown that w 2 – 15 x the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy having optimum soft magnetic properties consists primarily of an a-FeŽSi.
)
Corresponding author.
crystalline phase and a residual amorphous phase; the a-FeŽSi. crystalline phase has a grain size of 10 nm and a DO 3 superstructure. Such an ultra-fine microstructure can be also obtained by the replacement of Nb with Mo or W; but the alloy with Nb, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , has the smallest grain size, which is very significant for the excellent soft magnetic properties of the alloy. However, the reason why the alloy with Nb has a smaller grain size than the alloy with Mo or W has not been known clearly so far. Since these nanocrystalline alloys were prepared by crystallization of amorphous alloys, it is quite likely to better understand this cause by studying the crystallization process of these amorphous alloys.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 3 2 - 9
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
224
In the present paper, the influence of additive element Nb and Mo on the crystallization process of amorphous alloy Fe 76.5 Cu 1 Si 13.5 B 9 was investigated in detail using the Differential Scanning Calorimeter ŽDSC., X-ray Diffraction ŽXRD. and Transmission Electron Microscopy ŽTEM.. The difference in the crystallization process for these alloys was also discussed.
2. Experimental procedure Amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žat.%. ribbons being 20–30-m m thick and 5-mm wide were prepared by the melt-spinning technique on a single copper roller. The amorphous nature was confirmed by X-ray diffraction ŽXRD.. The thermal analysis experiments of the crystallization process of the three kinds of amorphous alloys were carried out using a Differential Scanning Calorimeter ŽDSC. at different heating rates, 58Crmin, 108Crmin, 158Crmin and 258Crmin, respectively. The X-ray diffraction of the alloy which had been measured by DSC was carried out using a DrMAX-rB diffractometer with a graphite monochromator and Cu K a radiation. The microstructure of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloys annealed at the temperature of 5208C in Ar atmosphere for 1 h was observed using an H800 Transmission Electron Microscope ŽTEM.. Using the DSC experimental data, we calculated the average activation energy of crystallization Ž E . of amorphous alloy by the Kissinger method w16x: ln
B
ž / Tp2
E sy RTp
q constant
DSC curve, T is the temperature, x is the crystallized fraction, A is the frequency factor, and F Ž x . is the function which only relates to the crystallized fraction Ž x .. According to Eq. Ž1., by using the value of Tp at different heating rates, a plot of lnŽ BrTp2 . vs. 1rTp yields approximate straight line. From the slope of the line, the average activation energy of crystallization, E, is obtained. From the exothermal peak of the DSC curve, we can determine the curves of crystallized fraction Ž x . vs. temperature ŽT . at different heating rates. Given a certain crystallized fraction x, we can obtain different temperatures T which correspond to different heating rates B. According to Eq. Ž2., we can make a plot of log B vs. 1rT, and from the slope of this plot the activation energy of crystallization at the certain crystallized fraction x, Ec Ž x ., is obtained. The Ec Ž x . is defined as the local activation energy of crystallization.
3. Experimental results and discussion Fig. 1 shows the DSC curves of crystallization process for amorphous alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 . Two exothermal peaks can be seen clearly from Fig. 1 as these amorphous alloy are heated to 6708C. The
Ž 1.
and the activation energy of crystallization at a certain crystallized fraction x, Ec Ž x ., by the Doyle method w17x: log B s Ig
A = Ec Ž x . R=FŽ x.
y 2.315 y 0.4567 =
Ec Ž x . R=T
.
Ž 2. Where B is the heating rate, R is the gas constant, Tp is the peak temperature of the exothermal peak of
Fig. 1. DSC curves of crystallization process for amorphous alloys F e 7 6 .5 C u 1 S i 1 3 .5 B 9 , F e 7 3 .5 C u 1 N b 3 S i 1 3 .5 B 9 a n d Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žat a heating rate of 108Crmin. ŽA. Fe 7 3 .5 Cu 1 Nb 3 Si 1 3 .5 B 9 , Ž B . Fe 7 3 .5 Cu 1 Mo 3 Si 1 3 .5 B 9 , Ž C . Fe 76.5 Cu 1 Si 13.5 B 9 .
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
result of XRD shows that the first exothermal peak corresponds to the formation of iron–silicon crystallites, and the second corresponds to the formation of Fe–B compounds. The crystallization process of the iron–silicon crystallites in these alloys is investigated in detail below for better understanding why the nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy has a smaller size for iron–silicon crystallite. Table 1 Summarize the average activation energy of crystallization Ž E . of amorphous alloys Fe 76.5Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1Mo 3 Si 13.5 B 9 . For Fe 76.5 Cu 1 Si 13.5 B 9 alloy. The average activation energy E is 182.4 " 4.0 kJrmol, which is smaller than that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy Ž452.4 " 4.8 kJrmol. and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy Ž350.7 " 4.2 kJrmol.. This indicates that the addition of Nb or Mo to Fe 76.5 Cu 1Si 13.5 B 9 alloy results in an increase of the activation energy of crystallization of the alloy. The changes of the local activation energy of crystallization Ec Ž x ., the activation energy of crystallization at a certain crystallized fraction x, with crystallized fraction Ž x . for the amorphous alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 are shown in Fig. 2. The activation energy of crystallization for Fe 76.5 Cu 1 Si 13.5 B 9 alloy increases slightly with crystallized fraction between 2% and 90%. However, the alloy with addition of Nb or Mo, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , exhibits a unique crystallization process. For Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, the activation energy of crystallization at the beginning stage of crystallization is the smallest, it increases quickly with the increase of crystallized fraction and shows the maximum Ž; 500 kJrmol. in a large range of crystallized fraction Ž25%–90%.. It can be seen clearly that the crystallization process of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy. However, the increase of the activation energy of crystallization in
225
Fig. 2. Changes of the local activation energy of crystallization w ith cry stallized fractio n fo r F e 7 6 .5 C u 1 S i 1 3 .5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy.
Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 with the increase of crystallized fraction is smaller than that in Fe 73.5Cu 1 Nb 3 Si 13.5 B 9 alloy. The maximum of activation energy of crystallization of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy, 370 kJrmol, on the other hand, is also smaller than that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, 500 kJrmol. All these imply that the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy has a stronger constraint for the growth of iron–silicon crystallite than Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Fig. 3 shows the TEM micrograph of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloys annealed at a temperature of 5208C for 1 h. Analysis shows that the crystallites in these annealed alloys are iron–silicon, and that the alloy with addition of Nb or Mo has the iron–silicon crystallites with much smaller grain size in comparison with Fe 76.5 Cu 1 Si 13.5 B 9 alloy, but the size of iron–silicon crystallites in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is the smallest, 10–12 nm. We suggest that the microstructural features shown in Fig. 3 for annealed alloys Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 re-
Table 1 Average activation energy of crystallization of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy Alloy
Average activation energy, E ŽkJrmol.
Fe 76.5 Cu 1 Si 13.5 B 9
Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9
Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9
182.4 " 4.0
452.4 " 4.8
350.7 " 4.2
226
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
Fig. 3. TEM micrograph of amorphous Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy annealed at 5208C for 1 h. Ža. Fe 76.5 Cu 1 Si 13.5 B 9 , Žb. Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 Žc. Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 .
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
sult from their crystallization behaviour. At the beginning stage of crystallization, the lower activation energy of Fe 76.5 Cu 1 Si 13.5 B 9 , Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 , and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, which results from the separation of the Fe, Cu atoms during the crystallization process w1x, benefits the formation of the nuclei of iron–silicon crystallites in these alloys. However, for Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the iron–silicon crystallites increase in size and form a large grain due to the lower activation energy of crystallization during the crystallization process. For the alloy Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , the stability of the residual amorphous regions around iron–silicon crystallites becomes stronger due to the formation of Nb-Žor Mo-. and B-rich regions in these amorphous regions in the crystallization process w1,8,10x. So the activation energy of crystallization increases with increasing fraction of crystallites. This constrains the growth of the iron–silicon crystallites and results in the formation of ultral fine grains in the end. It was indicated that w18,19x the size and the electronegativity of the atom consisting of an amorphous alloy are two important factors that affect the stability of the alloy. The larger the difference in the size and electronegativity between the constituents of the amorphous alloy, the more stable is the alloy. Nb and B have a larger size difference and electronegativity difference than Mo and B, and consequently, the residual amorphous phase which enriches in Nb and B in the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is more stable than that which enriches in Mo and B in the Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Therefore, the activation energy of crystallization of Fe 7 3 .5 Cu 1 Nb 3 Si 13.5 B 9 alloy increases more quickly during the crystallization process, and shows a larger value in a large range of the crystallized fraction than that of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. We suggest that this is very significant for constraining the growth of iron–silicon crystallites and leads to the formation of the smaller iron–silicon grains in the annealed Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy.
4. Conclusions Ž1. For the amorphous Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the average activation energy of crystallization calcu-
227
lated by the Kissinger method is 182.4 " 4.0 kJrmol, and it increases to 350.7 " 4.2 kJrmol and 452.4 " 4.8 kJrmol, respectively, with the addition of Mo and Nb for the alloy Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 . Ž2. For the amorphous Fe 76.5 Cu 1 Si 13.5 B 9 alloy, the local activation energy of crystallization calculated by the Doyle method increases slightly from 224 to 234 kJrmol with the increase of crystallized fraction. The local activation energy of crystallization of the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy at the beginning stage of crystallization is the smallest, it increases with the crystallized fraction and shows the largest Ž500 kJrmol. in the range of crystallized fraction of 25%–90%. The crystallization process of the Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is similar to that of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy, but the increase of activation energy in the alloy with crystallized fraction is smaller than that in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy. Ž3. The reason why the size of iron–silicon crystallite in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy is smaller than that in Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy is that the activation energy of crystallization of the alloy increases more quickly with crystallized fraction and exhibits a larger value in a larger crystallized fraction than that of Fe 73.5 Cu 1 Mo 3 Si 13.5 B 9 alloy. Acknowledgements The work was supported by the Natural Science Foundation of Hebei Province of China.
References w1x Y. Yoshizawa, S. Oguchi, K. Yamauchi, J. Appl. Phys. 64 Ž1988. 6044. w2x G. Herzer, IEEE Trans. Magn. 25 Ž1989. 3327. w3x G. Herzer, IEEE Trans. Magn. 26 Ž1990. 1397. w4x K. Hono, K. Hiraga, Q. Wang, A. Inoue, T. Sakurai, Acta Metall. Mater. 40 Ž1992. 2137. w5x H. Kim, M. Matsuura, M. Sakurai, K. Suzuki, Jpn. J. Appl. Phys. 32 Ž1993. 676. w6x M. Sakurai, M. Matsuura, S.H. Kim, Y. Yoshizawa, K. Ymamauchi, K. Suzuki, Mater. Sci. Eng. 469 Ž1994. A179– A180. w7x J.D. Ayers, V.G. Harris, J.A. Sprague, W.T. Elam, Appl. Phys. Lett. 64 Ž1994. 974. w8x Y. Yoshizawa, K. Yamauchi, Mater. Sci. Eng. A 133 Ž1991. 176.
228
J.W. Zhang et al.r Materials Letters 36 (1998) 223–228
w9x M. Muller, N. Mattern, J. Magn. Mater. 136 Ž1994. 79. ¨ w10x X.Y. Zhang, J.W. Zhang, J.H. Liu, Y.Z. Zheng, R.P. Liu, J.H. Zhao, Mater. Lett. 29 Ž1996. 31. w11x X.Y. Zhang, J.W. Zhang, F.R. Xiao, J.H. Liu, Y.Z. Zheng, Mater. Lett., 34 Ž1,2., 85, 1998. w12x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, Y.Z. Zheng, J. Magn. Magn. Mater. 172 Ž1997. 301. w13x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, J.H. Liu, Y.Z. Zheng, NanoStructured Mater. 8 Ž1997. 337.
w14x J.H. Liu, X.Y. Zhang, Y.Z. Zheng, J.H. Zhao, R.P. Liu, J. Mater. Sci. Technol. 13 Ž1997. 37. w15x X.Y. Zhang, J.W. Zhang, R.P. Liu, J.H. Zhao, J.H. Liu, Y.Z. Zheng, J. Mater. Sci. Lett. 16 Ž1997. 1745. w16x H.E. Kissinger, Anal. Chem. 29 Ž1975. 1702. w17x C.D. Doyle, J. Appl. Polym. Sci. 5 Ž1961. 285. w18x A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 31 Ž1990. 177. w19x A. Inoue, Mater. Trans. JIM 36 Ž1995. 866.