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Surface crystallization of Fe80B20 metallic glass ribbons
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Materials Science and Engineering, AI33 ( 1991 ) 630-635
Surface crystallization of Fe8oB20metallic glass ribbons D Oleszak, P. Glijer and H. Matyja Warsaw University of Technology, Institute of Materials Science and Engineering, 02-524 Warsaw, Narbutta 85 (Poland)
Abstract The surface crystallization of Fe80B20 metallic glass was studied using DSC, optical microscopy and TEM. It was shown that the contact surface layer with retarded crystallization occurs when a sufficiently high cooling rate is applied during the melt spinning process. The existence of the layer is due to the lack of quenched-in nuclei in this part of the ribbon and it is caused by the cooling rate distribution at the ribbon thickness. Differences in microstructure were found at the cross-section of the ribbon.
1. Introduction In recent years the surface crystallization of metallic glasses has become a subject of increasing research effort. Several reports have been published on surface crystallization in metallic glass ribbons, in which a variety of crystallization behaviours, depending on alloying, quenching conditions, surface treatment or different annealing atmospheres, have been described [1-3]. In the case of Fe80B20 metallic glass it has been found that crystallization occurs preferentially on the top side of the ribbon leaving a region near the contact side where crystallization starts later than in the remaining part of the ribbon [4, 5]. The aim of the present work is to show the influence of cooling rate applied in the meltspinning process on the occurrence of the layer exhibiting retarded crystallization effect and to confirm (or not) the microstructural differences between the contact side layer and the remaining part of the ribbon after completion of the crystallization process.
2. Experiment Fes0B20 glassy ribbons were prepared in air by the melt-spinning technique using copper and steel quenching wheels. The use of different wheel materials allowed us to achieve different cooling rates during the melt-spinning operation. The amorphicity of as-quenched samples was confirmed by X-ray diffraction measurements. 0921-5093/91/$3.50
DSC experiments were carried out in a Perkin Elmer DSC2 calorimeter, and a heating rate of 20 K min- t was used. To reveal the structure of the cross-sections of the investigated ribbons, a Neophot 21 optical microscope was employed. Using a Philips E M 300 electron microscope two different T E M experiments were performed: (a) the crystallization process of the near surface areas of the samples was monitored in situ via hot-stage electron microscopy; and (b) the structural observations at different distances from the surface of the samples previously annealed in the DSC were performed (ex post experiment). To obtain the samples for both experiments the onesided electrolytical polishing technique was employed.
3. Results DSC curves recorded during continuous heating at 20 K rain-1 of the samples of both examined ribbons are shown in Fig. 1. The second small peak for the ribbon melt spun onto a copper wheel is visible. In the case of the ribbon cast onto a steel wheel (i.e. prepared at a lower cooling rate) the second exothermic effect does not appear. The arrows in Fig. 1 indicate the temperatures to which the samples submitted to the microscopic observations were heated. A series of optical micrographs of the ribbon melt spun onto a copper wheel are shown at Figs. © Elsevier Sequoia/Printed in The Netherlands
631
710
8~£
733 a
79o
89o TEMPERATURE (K)
Fig. 1. DSC curves of the ribbons melt spun onto different wheels: (a) copper wheel; (b) steel wheel. The heating rate is 20 K min- ~. The arrows indicate the temperatures to which the samples submitted to the microscopic observations were heated.
2(a)-(d). Different stages of the crystallization process are presented. After heating the sample to temperatures corresponding to the peak temperature at the DSC curve (724 K) the continuous contact side free of a crystal layer is clearly visible. After heating to 733 K the layer is still present and its width is almost constant during the crystallization process. Heating to the higher temperature (830 K) causes the remaining glass to crystallize, and after complete crystallization the layer is indistinguishable. Figure 2(e) show the result of a similar investigation of a ribbon cast onto a steel wheel. In this case the lack of a surface layer with retarded crystallization effect is evident. The crystalliza-
Fig. 2. A series of optical micrographs of the ribbons heated to different temperatures: (a) 710 K, (b) 724 K, (c) 733 K, (d) 830 K (copper wheel); (e) 710 K (steel wheel).
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Fig. 3. Electron diffractions patterns at various stages of crystallization (in situ heating experiment) of the near contact surface areas of the sample. T h e sample was heated to 470 K at a rate of 80 K rain- J, then a slow heating rate (5 K m i n - l ) was applied.
Fig. 4. Electron diffractions patterns at various stages of crystallization (in situ heating experiment) of the near top surface areas of the sample. Heating conditions were the same as in Fig. 3.
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tion products are uniformly distributed throughout the cross-section of the ribbon. Figure 3 shows a temperature sequence of electron diffraction patterns from an area near the contact surface of the sample. Crystallization starts at about 620 K. In this temperature weak 110 and 211 Fe-a diffraction rings appear. At 723 K reflections corresponding to the Fe3B phase are detected for the first time. The higher the temperature of the sample, the weaker the amorphous halo, and at 773 K it is already not visible. At 873 K the diffraction pattern indicates a fully crystalline alloy. The existence of the Fe-a and Fe3B phases is found. At the same time it can be concluded from the diffraction patterns that boride grains are finer than iron grains. A similar crystallization scheme, concerning the top surface area, is exemplified in Fig. 4. The crystallization process starts at 573 K. The first phase to appear is Fe-a. A weak 110 ring is found, overlapping the still strong amorphous halo. The amorphous halo is visible up to 723 K. At 773 K the sample is fully crystallized. Diffraction spots from the Fe3B phase and rings from Fe-a are observed. Such a diffraction pattern testifies to the existence of large crystals of the Fe3B phase and highly dispersed iron crystals.
The structure of the samples heated in the DSC to 650 K appears to be fully amorphous. Any traces of crystallinity are not observed. An initial crystallization stage near the top surface areas of the sample heated to 710 K is shown in Fig. 5(a). The crystallization process occurs by the appearance of uniformly distributed spherulitic crystals. The same crystallization behaviour is observed at depths of 7 pm, 15 p m and 22 ~m from the top surface of the ribbon. Spherulites have a complex structure. They consist of Fe3B crystals and small amounts of needle-like crystals of Fe-a. The contact surface areas of the sample heated to the same temperature (710 K) reveal an entirely amorphous structure (Fig. 5(b)). The microstructure of the samples heated up to 724 K is uniform throughout the thickness of the ribbon, except the contact surface layer. Figure 6(a) shows a micrograph taken at a depth of 15 p m from the top surface of the sample. The phase composition of spherulites is the same as for the former heating temperature. Finely distributed Fe-a crystals and Fe3B crystals are detected. The contact side areas of this sample are only partially crystallized. The samples submitted to heating up to 733 K show no differences in microstructure
Fig. 5. Micrographs and electron diffractions patterns (insets) of the samples heated in the DSC to 710 K: (a) top surface area; (b) contact surface area.
Fig. 6. Micrographs and electron diffraction patterns (insets) of the samples heated in the DSC to (a) 724 K and (b) 733 K; the distance of 15 pm from the top surface of the ribbon.
634
t9
?¢ , , 1,o distance from wheel surface(pm)
Fig. 8. Hypothetical cooling rate distribution at the crosssection of the ribbon during the melt-spinning process.
Fig. 7. Micrographs and electron diffraction pattern (inset) of the sample heated in the DSC to 830 K: (a) top surface area; (b) contact surfacearea. and phase composition compared with the lower heat treatment temperature (Fig. 6(b)). After heating to 830 K the structure of the samples is fully crystalline and of uniform thickness (Fig. 7).
4. Discussion The results of optical microscopic observations are in good agreement with the results of DSC experiments. The occurrence of the contact side layer with delayed crystallization is confirmed for the ribbon for which the second small peak at the DSC continuous heating curve is observed. The presence of this peak can be attributed to the delayed crystallization of the contact side layer [6]. Structural differences between both sides of the ribbon have been revealed by in situ crystallization studies. The diffused halo characteristic of the amorphous structure disappears in electron diffraction patterns at 723 K for the top surface of the sample and at 773 K for the contact surface. For all samples investigated the phase composition during the crystallization process is the same (Fe-a and Fe3B), but differences in microstructure of arising phases are found. The
contact side layer crystallizes into a coarsegrained structure of Fe-a crystals and smaller Fe3B phase crystals. In the remaining part of the ribbon large spherulitic crystals of Fe3B and small Fe-a crystals are found. The spherulites can result from the growth of quenched-in nuclei of Fe3B. In light of the results obtained it seems reasonable that the lack of quenched-in nuclei (due to the higher cooling rate) is responsible for the existence of the layer exhibiting delayed crystallization effect. Taking into account the strong dependence of the number of quenched-in nuclei on the cooling rate [6], it can be assumed that observed effect is a result of the characteristic cooling rate distribution at the crosssection of the ribbon during the melt-spinning operation (Fig. 8). Considering different cooling rate distributions one can state that the shape of the curve in Fig. 8 corresponds to the Biot number Bi>2 [7]. Taking Bi=ht/~c, where h = the heat flow coefficient, t = the thickness of the ribbon, 7¢= the thermal conductivity of the alloy, and assuming t = 29 # m and r -- 30 W m- 1 K -1, we can calculate h > 2 x l 0 6 W m -2 K -1. This result is in agreement with values of the heat flow coefficient for the melt-spinning process [7].
5. Conclusions It was shown that in a FesoB20 metallic glass ribbon the contact surface layer with retarded crystallization occurs when a sufficiently high cooling rate is applied during the melt-spinning process. The existence of the layer is due to the lack of quenched-in nuclei in this part of the ribbon and it is caused by the cooling rate distribution at the ribbon thickness. Differences in microstructure were found at the cro,~s-section of
635
the ribbon: large spherulites of Fe3B and highly dispersed Fe-a crystals in the main part of the ribbon and, on the other hand, fine crystals of boride in the contact surface layer. This fact can be explained by different crystallization temperatures in the two parts of the ribbon. References 1 U. Koster. Mater. Sci. Eng., 97(1988) 233.
2 M. H. Zuercher and D. G. Morris, J. Mater Sci., 23 (1988) 515. 3 M. A. Gibson and G. W. Delamore, J. Mater Sci., 23 (1988) 1164. 4 U. Herold and U. Koster, in B. Cantor (ed.), Proc. 3rd Int. Conf. on Rapidly Quenched Metals, Vol 1, Brighton, 1978, p. 281. 5 A. L. Greer and J. A. Leake, in B. Cantor (ed.), Proc. 3rd Int. ( b n f on RapMly Quenched Metals, 11101. 1, Brighton, 1978, p. 299. 6 A. L. Greer, Acta Metall., 30(1982) 171. 7 L. Granasy, Mater. Sci. Eng., A l l l (1989) 129.
Materials Science and Engineering, AI33 ( 1991 ) 630-635
Surface crystallization of Fe8oB20metallic glass ribbons D Oleszak, P. Glijer and H. Matyja Warsaw University of Technology, Institute of Materials Science and Engineering, 02-524 Warsaw, Narbutta 85 (Poland)
Abstract The surface crystallization of Fe80B20 metallic glass was studied using DSC, optical microscopy and TEM. It was shown that the contact surface layer with retarded crystallization occurs when a sufficiently high cooling rate is applied during the melt spinning process. The existence of the layer is due to the lack of quenched-in nuclei in this part of the ribbon and it is caused by the cooling rate distribution at the ribbon thickness. Differences in microstructure were found at the cross-section of the ribbon.
1. Introduction In recent years the surface crystallization of metallic glasses has become a subject of increasing research effort. Several reports have been published on surface crystallization in metallic glass ribbons, in which a variety of crystallization behaviours, depending on alloying, quenching conditions, surface treatment or different annealing atmospheres, have been described [1-3]. In the case of Fe80B20 metallic glass it has been found that crystallization occurs preferentially on the top side of the ribbon leaving a region near the contact side where crystallization starts later than in the remaining part of the ribbon [4, 5]. The aim of the present work is to show the influence of cooling rate applied in the meltspinning process on the occurrence of the layer exhibiting retarded crystallization effect and to confirm (or not) the microstructural differences between the contact side layer and the remaining part of the ribbon after completion of the crystallization process.
2. Experiment Fes0B20 glassy ribbons were prepared in air by the melt-spinning technique using copper and steel quenching wheels. The use of different wheel materials allowed us to achieve different cooling rates during the melt-spinning operation. The amorphicity of as-quenched samples was confirmed by X-ray diffraction measurements. 0921-5093/91/$3.50
DSC experiments were carried out in a Perkin Elmer DSC2 calorimeter, and a heating rate of 20 K min- t was used. To reveal the structure of the cross-sections of the investigated ribbons, a Neophot 21 optical microscope was employed. Using a Philips E M 300 electron microscope two different T E M experiments were performed: (a) the crystallization process of the near surface areas of the samples was monitored in situ via hot-stage electron microscopy; and (b) the structural observations at different distances from the surface of the samples previously annealed in the DSC were performed (ex post experiment). To obtain the samples for both experiments the onesided electrolytical polishing technique was employed.
3. Results DSC curves recorded during continuous heating at 20 K rain-1 of the samples of both examined ribbons are shown in Fig. 1. The second small peak for the ribbon melt spun onto a copper wheel is visible. In the case of the ribbon cast onto a steel wheel (i.e. prepared at a lower cooling rate) the second exothermic effect does not appear. The arrows in Fig. 1 indicate the temperatures to which the samples submitted to the microscopic observations were heated. A series of optical micrographs of the ribbon melt spun onto a copper wheel are shown at Figs. © Elsevier Sequoia/Printed in The Netherlands
631
710
8~£
733 a
79o
89o TEMPERATURE (K)
Fig. 1. DSC curves of the ribbons melt spun onto different wheels: (a) copper wheel; (b) steel wheel. The heating rate is 20 K min- ~. The arrows indicate the temperatures to which the samples submitted to the microscopic observations were heated.
2(a)-(d). Different stages of the crystallization process are presented. After heating the sample to temperatures corresponding to the peak temperature at the DSC curve (724 K) the continuous contact side free of a crystal layer is clearly visible. After heating to 733 K the layer is still present and its width is almost constant during the crystallization process. Heating to the higher temperature (830 K) causes the remaining glass to crystallize, and after complete crystallization the layer is indistinguishable. Figure 2(e) show the result of a similar investigation of a ribbon cast onto a steel wheel. In this case the lack of a surface layer with retarded crystallization effect is evident. The crystalliza-
Fig. 2. A series of optical micrographs of the ribbons heated to different temperatures: (a) 710 K, (b) 724 K, (c) 733 K, (d) 830 K (copper wheel); (e) 710 K (steel wheel).
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Fig. 3. Electron diffractions patterns at various stages of crystallization (in situ heating experiment) of the near contact surface areas of the sample. T h e sample was heated to 470 K at a rate of 80 K rain- J, then a slow heating rate (5 K m i n - l ) was applied.
Fig. 4. Electron diffractions patterns at various stages of crystallization (in situ heating experiment) of the near top surface areas of the sample. Heating conditions were the same as in Fig. 3.
633
tion products are uniformly distributed throughout the cross-section of the ribbon. Figure 3 shows a temperature sequence of electron diffraction patterns from an area near the contact surface of the sample. Crystallization starts at about 620 K. In this temperature weak 110 and 211 Fe-a diffraction rings appear. At 723 K reflections corresponding to the Fe3B phase are detected for the first time. The higher the temperature of the sample, the weaker the amorphous halo, and at 773 K it is already not visible. At 873 K the diffraction pattern indicates a fully crystalline alloy. The existence of the Fe-a and Fe3B phases is found. At the same time it can be concluded from the diffraction patterns that boride grains are finer than iron grains. A similar crystallization scheme, concerning the top surface area, is exemplified in Fig. 4. The crystallization process starts at 573 K. The first phase to appear is Fe-a. A weak 110 ring is found, overlapping the still strong amorphous halo. The amorphous halo is visible up to 723 K. At 773 K the sample is fully crystallized. Diffraction spots from the Fe3B phase and rings from Fe-a are observed. Such a diffraction pattern testifies to the existence of large crystals of the Fe3B phase and highly dispersed iron crystals.
The structure of the samples heated in the DSC to 650 K appears to be fully amorphous. Any traces of crystallinity are not observed. An initial crystallization stage near the top surface areas of the sample heated to 710 K is shown in Fig. 5(a). The crystallization process occurs by the appearance of uniformly distributed spherulitic crystals. The same crystallization behaviour is observed at depths of 7 pm, 15 p m and 22 ~m from the top surface of the ribbon. Spherulites have a complex structure. They consist of Fe3B crystals and small amounts of needle-like crystals of Fe-a. The contact surface areas of the sample heated to the same temperature (710 K) reveal an entirely amorphous structure (Fig. 5(b)). The microstructure of the samples heated up to 724 K is uniform throughout the thickness of the ribbon, except the contact surface layer. Figure 6(a) shows a micrograph taken at a depth of 15 p m from the top surface of the sample. The phase composition of spherulites is the same as for the former heating temperature. Finely distributed Fe-a crystals and Fe3B crystals are detected. The contact side areas of this sample are only partially crystallized. The samples submitted to heating up to 733 K show no differences in microstructure
Fig. 5. Micrographs and electron diffractions patterns (insets) of the samples heated in the DSC to 710 K: (a) top surface area; (b) contact surface area.
Fig. 6. Micrographs and electron diffraction patterns (insets) of the samples heated in the DSC to (a) 724 K and (b) 733 K; the distance of 15 pm from the top surface of the ribbon.
634
t9
?¢ , , 1,o distance from wheel surface(pm)
Fig. 8. Hypothetical cooling rate distribution at the crosssection of the ribbon during the melt-spinning process.
Fig. 7. Micrographs and electron diffraction pattern (inset) of the sample heated in the DSC to 830 K: (a) top surface area; (b) contact surfacearea. and phase composition compared with the lower heat treatment temperature (Fig. 6(b)). After heating to 830 K the structure of the samples is fully crystalline and of uniform thickness (Fig. 7).
4. Discussion The results of optical microscopic observations are in good agreement with the results of DSC experiments. The occurrence of the contact side layer with delayed crystallization is confirmed for the ribbon for which the second small peak at the DSC continuous heating curve is observed. The presence of this peak can be attributed to the delayed crystallization of the contact side layer [6]. Structural differences between both sides of the ribbon have been revealed by in situ crystallization studies. The diffused halo characteristic of the amorphous structure disappears in electron diffraction patterns at 723 K for the top surface of the sample and at 773 K for the contact surface. For all samples investigated the phase composition during the crystallization process is the same (Fe-a and Fe3B), but differences in microstructure of arising phases are found. The
contact side layer crystallizes into a coarsegrained structure of Fe-a crystals and smaller Fe3B phase crystals. In the remaining part of the ribbon large spherulitic crystals of Fe3B and small Fe-a crystals are found. The spherulites can result from the growth of quenched-in nuclei of Fe3B. In light of the results obtained it seems reasonable that the lack of quenched-in nuclei (due to the higher cooling rate) is responsible for the existence of the layer exhibiting delayed crystallization effect. Taking into account the strong dependence of the number of quenched-in nuclei on the cooling rate [6], it can be assumed that observed effect is a result of the characteristic cooling rate distribution at the crosssection of the ribbon during the melt-spinning operation (Fig. 8). Considering different cooling rate distributions one can state that the shape of the curve in Fig. 8 corresponds to the Biot number Bi>2 [7]. Taking Bi=ht/~c, where h = the heat flow coefficient, t = the thickness of the ribbon, 7¢= the thermal conductivity of the alloy, and assuming t = 29 # m and r -- 30 W m- 1 K -1, we can calculate h > 2 x l 0 6 W m -2 K -1. This result is in agreement with values of the heat flow coefficient for the melt-spinning process [7].
5. Conclusions It was shown that in a FesoB20 metallic glass ribbon the contact surface layer with retarded crystallization occurs when a sufficiently high cooling rate is applied during the melt-spinning process. The existence of the layer is due to the lack of quenched-in nuclei in this part of the ribbon and it is caused by the cooling rate distribution at the ribbon thickness. Differences in microstructure were found at the cro,~s-section of
635
the ribbon: large spherulites of Fe3B and highly dispersed Fe-a crystals in the main part of the ribbon and, on the other hand, fine crystals of boride in the contact surface layer. This fact can be explained by different crystallization temperatures in the two parts of the ribbon. References 1 U. Koster. Mater. Sci. Eng., 97(1988) 233.
2 M. H. Zuercher and D. G. Morris, J. Mater Sci., 23 (1988) 515. 3 M. A. Gibson and G. W. Delamore, J. Mater Sci., 23 (1988) 1164. 4 U. Herold and U. Koster, in B. Cantor (ed.), Proc. 3rd Int. Conf. on Rapidly Quenched Metals, Vol 1, Brighton, 1978, p. 281. 5 A. L. Greer and J. A. Leake, in B. Cantor (ed.), Proc. 3rd Int. ( b n f on RapMly Quenched Metals, 11101. 1, Brighton, 1978, p. 299. 6 A. L. Greer, Acta Metall., 30(1982) 171. 7 L. Granasy, Mater. Sci. Eng., A l l l (1989) 129.