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Study on continuous casting of bulk metallic glass
Materials Letters 65 (2011) 2257–2260
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Study on continuous casting of bulk metallic glass Tao Zhang a, Xingguo Zhang a,⁎, Wei Zhang a,b, Fei Jia a, Akihisa Inoue c, Hai Hao a, Yuejiao Ma a a b c
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan WPI, Advanced Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan
a r t i c l e
i n f o
Article history: Received 17 February 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Cast Solidification Amorphous materials Continuous
a b s t r a c t Continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. In this study, a new continuous casting method was developed for the massive production of bulk metallic glass ingot with centimeter-scale diameter without length limitation. An intermittent withdrawal procedure was practiced for continuous casting of bulk glassy alloy. The new developed continuous casting method can provide a cooling speed as high as that provided by the stationary mold casting method. A Zr-based glassy alloy rod with a diameter of about 10 mm and length of tens of centimeters was prepared by the continuous casting method for the first time. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) possess high strength, high hardness, high elastic properties, low modulus, high corrosion resistance, good magnetic properties etc. They are thus used in many fields, such as electric power industry, biomedical field, micro electro-mechanical systems and information technology [1]. Lots of glassy alloys with critical diameter on centimeter scale were discovered in the past two decades. The superplastic forming of glassy alloy has been widely investigated. The superplastic forming has the potential to replace die casting as a net-shaping processing method for BMGs and might even lead to new applications for the use of BMGs [2]. Superplastic forming of an amorphous alloy involves heating it into the supercooled liquid region and forming it under an applied pressure. However, the glassy alloy starting ingots must be provided before the superplastic forming process. Copper mold casting and quartz container water quenching technologies are mostly used to produce BMG ingots [3,4]. However, these methods are not suitable for the massive production of glassy alloy ingot because of their low productivity. The methods for continuous production of BMGs should be proposed. As we all know, continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. A lot of attention also has been paid to the continuous casting of BMGs. Several methods for continuous fabrication of BMGs have been adopted to produce amorphous wires, strips and rods. Bridgman solidification method is used to produce glass alloy rods or
⁎ Corresponding author. Tel.: + 86 411 84706183; fax: + 86 411 84706183. E-mail address: [email protected] (X. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.033
composites with varied structure and volume of crystalline phases by tailoring the withdrawal speeds [5,6]. A rotating disk casting method is developed for producing long amorphous alloy wires with diameters of 0.5 mm to 1.5 mm [7]. Twin-roll strip continuous casting technology has been utilized to fabricate the sheet products of bulk amorphous alloys with thickness of less than 2 mm [8,9]. The critical diameters or thicknesses of these samples produced using the methods mentioned above are not more than 3 mm. Although zone melting method is also borrowed to prepare BMG strip with a thickness of 10 mm, a width of 12 mm and a length of 170 mm, the regions contacted with the copper base are crystallized areas [10]. In this study, we developed a novel continuous casting method for the massive production of amorphous alloy ingots of large size, and prepared a BMG long rod with a diameter of 10 mm, length of tens of centimeters. The glassy nature of the sample was characterized carefully, and the effects of continuous casting parameters on the surface morphology and inner microstructure of the ingot were discussed. 2. Experimental procedure A horizontal continuous casting system has been built up in our laboratory [11]. The schematic illustration of the continuous casting setup is shown in Fig. 1a. A thermal open-ended mould surrounded by a heater is connected to a water-cooled open-ended mould coaxially. A thermocouple is placed at the exit end of the thermal mould. The thermocouple and the heater are adopted to adjust the temperature of the thermal mould. The glassy alloy melt is kept in a crucible for continuous casting process. The crucible is connected to the thermal mould. A starting block is inserted into the thermal mould for initiation of casting. The starting block is moved by a roller device which is not
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Fig. 1. The schematic illustration of the continuous casting setup (a) and the map of the withdrawal procedure (b).
shown in Fig. 1a. All the units are put into a stainless steel vacuum chamber. At the beginning of the experiment, the melt is guided by the starting block passing through the thermal mould into the water-cooled mould and then cooled to form glassy alloy (or supercooled liquid) in the water-cooled mould. At last the alloy is gradually pulled out of the water-cooled mould by the roller device. An intermittent withdrawal procedure in which a withdrawal cycle is employed is adopted for the continuous casting of BMG. Fig. 1b shows a map of an intermittent withdrawal procedure. The “down” and the “go” periods are represented by pd and pg, respectively. The drawing speed during pd is represented by vg. Zr48Cu36Ag8Al8 (ZC1) alloy was chosen for the continuous casting of BMG rod [12]. The chamber was vacuumed to 10−1 Pa and refilled with high pure Ar gas. The temperature of the thermal mould was maintained above 1140 K. The parameters of the withdrawal procedure are 2 mm/s, 5 s and 2 s for vg, pg and pd, respectively. The glassy phase of the ingot was identified by X-ray diffraction (XRD), and the thermal stability was characterized by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s with Ar atmosphere. The microstructure of the sample was determined by optical microscopy (OM) and JEOL 2100 transmission electron microscopy (TEM) after etching and twin-jet electropolishing, respectively. Vickers hardness (Hv) of the sample was measured by a HVS-5 Vickers microhardness tester with a 50 N (5 kgf) load for 15 s. 3. Experimental results Fig. 2a shows a part of the continuous cast ZC1 rod with a diameter of 10 mm. The continuous cast glassy alloy rod has metallic luster and peripheral bands. One peripheral band is formed on the rod each time the rod is stopped during the intermittent casting procedure. The bands appear to form on that part of the ingot in the mould that shrinks away from contact with the mould wall during the “down” period of the intermittent withdrawal cycle. It is believed that the distance between the two adjacent bands corresponds to the distance moved by the rod in the water-cooled mould during the “go” period of the cycle employed in the intermittent withdrawal procedure, for 10 mm distance between the two adjacent bands is equal to the result of the vg (2 mm/s) multiplied by the pg (5 s). The Fig. 2b shows a cross-sectional XRD pattern of the rod. No sharp peaks corresponding to crystalline phases are imposed to a main halo peak, indicating the good amorphous nature of the ingot. Fig. 2c and d show cross-sectional OM images of the ingot. The specimen is taken from the center of the ingot. It can be seen from Fig. 2c that only several spherical phases with diameters of 100–200 μm
precipitate from the glassy matrix, which are not identified by the XRD analysis. Fig. 2e and f show microstructure of the longitudinal section of the peripheral band. No spherical phases and other precipitated phases are found in the longitudinal section. The glassy structure of the ingot along the longitudinal direction is distributed homogeneously. TEM observation was carried out for the identification of glassy nature and the joint area of peripheral band BMG ingot was specially observed. No difference was found for the samples taken from different areas of the ingot. The featureless high-resolution TEM image and corresponding selected-area electron diffraction (SAED) with a maze pattern typical for an amorphous structure are shown in Fig. 3a and the inset. It is believed that the periodically changed peripheral bands on the rod caused by the intermittent withdrawal procedure do not affect the inner glassy nature of the rod. The new continuous casting technology together with the practiced withdrawal procedure can be used for the continuous production of BMG rod. The glassy nature of the resultant ZC1 rod is further identified DSC analysis. Several specimens are prepared for the thermal analysis. The spherical phases shown in Fig. 2c are not included in the prepared specimens. Fig. 3b shows a normalized DSC curve of the alloy. The glass transition behavior and crystallization reaction are identified clearly in Fig. 3b. The Tg (glass transition temperature), Tx (onset crystallization temperature), △Tx (difference between Tx and Tg) and total heats of crystallization for the sample are 707 K, 801 K, 94 K and 53 J/g, respectively, which are corresponding to the results of ribbon and copper mold casting samples [12]. It is confirmed that the continuous cast rod is of good glassy nature except for only a few spherical phases precipitated from the matrix. These results indicate that continuous casting method could provide a cooling speed as high as that provided by the stationary mold casting method. The Vickers hardness of the glassy phase is also measured on the cross section of the specimens. No cracks along the corner or the face of the indentation are observed. It is found that the alloy shows nearly invariable Hv with measured positions and the average Hv is about 506 which is compared to that of Cu–Zr based glassy alloys, implying that the ingot might have good mechanical properties [13]. The mechanical properties will be studied in details in the future. 4. Discussion The solidification control of the alloy during continuous casting is discussed based on the continuous cooling transformation (CCT) curve, although the real CCT curve of the ZC1 alloy is unknown. Two processes affect the cooling of the molten alloy: heat transfer through the melt and heat transfer through the melt–mould interface, the latter described by the heat-transfer coefficient [14]. The heat transfer through the melt is out of consideration for the same composition of the alloy used in the experiment. The heat-transfer coefficient is supposed to be constant at the same size of the stationary mould cast sample and the part of the continuous cast sample formed during the “down” period of the cycle employed in the intermittent withdrawal procedure. Fig. 4 shows the illustration of cooling curves of the continuous cast alloy and the stationary mould cast alloy. The Tm, Td, and Tg shown in the Fig. 4 are the melting temperature of the alloy, the temperature of the glassy alloy (or supercooled liquid) at the end of each “down” period (Td is determined by pd but exact value is unknown) and the glass transition temperature of the alloy, respectively. The cooling process of the glassy alloy produced by stationary mould casting is a single-step cooling operation, while that produced by the new developed continuous casting is a two-step cooling operation. The continuous cast alloy is produced by the two steps: initially cooling the liquid alloy to Td in the water-cooled mould (the first step described by CC0 curve) and then pulling the alloy out of the mould (the second step described by CC2 curve). The single-step cooling operation by stationary mould casting can be described deliberately by two steps (described by CC0 and CC1 curves) for comparison with continuous casting process. It can be
T. Zhang et al. / Materials Letters 65 (2011) 2257–2260
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Fig. 2. The picture (a), XRD patterns (b) and OM images of continuous cast rod. Images (c) and (d) show the cross section; Images (e) and (f) show the longitudinal section of peripheral band.
considered that the CC0 curve represents the cooling curve of the alloy contact with stationary mould during mould casting or water-cooled mould during each “down” period of the intermittent casting procedure, when the liquid cooled rapidly from liquid above melting temperature to the glassy alloy (or supercooled liquid) at Td. The CC1 and CC2 curves below Td shown in Fig. 4 represent the cooling curves of the alloys that shrink away form contact with the stationary and water-cooled mould wall, respectively. The two curves should be below the CCT curve for the formation of fully glassy alloy.
It is known that the solidification shrinkage during casting of a Zr-based BMG former is approximately only 0.5% for the absence of a first-order phase transition during solidification [15]. The effect of solidification shrinkage on the heat-transfer coefficient cannot be ignored during the second step. The solidification shrinkage is considered to be the main factor that leads to a low heat-transfer coefficient through the ingot/mould interface. The solidification shrinkage is also helpful for the movement of the ingot. However, the heat-transfer coefficients at temperature below Td are different for the
Fig. 3. High-resolution TEM image (a) and SAED pattern (inset) of the sample taken from the joint area of peripheral band BMG and its DSC curve (b).
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related experiments will be done in the future and the effect of the continuous casting withdrawal procedure on the mechanical properties of glassy alloy will also be studied in detail later. 5. Conclusion
Fig. 4. The illustration of cooling curves of the continuous cast and stationary mould cast alloys.
In summary, a novel method for continuous casting of BMG was proposed. An intermittent withdrawal procedure was practiced for the continuous casting of BMG. A Zr48Cu36Ag8Al8 glassy alloy rod with a diameter of 10 mm was produced by the continuous casting method. The parameters of the drawing procedure are 2 mm/s, 5 s and 2 s for vg, pg and pd, respectively. The solidification of glassy alloy was actually subjected. The success of producing BMG rod with centimeter-scale diameter by the continuous casting method can effectively reduce production costs and accelerate the developments and applications of bulk glassy alloys. Acknowledgements
two methods. As we all know, the alloy is not removed from the mould during the stationary mould casting process and the cooling time can be represented by Δt1 shown in Fig. 4, while the alloy is gradually pulled out of the exit end of the water-cooled mould under the intermittent casting procedure and the cooling time can be presented by Δt2 also shown in Fig. 4. The resultant glassy alloy (or supercooled liquid) produced by stationary mould casting or continuous casting methods can retain steady amorphous state for a long time above Tg because of the high thermal stability of supercooled liquid cooled from the liquid [16]. It is thus believed that although Δt2 is greater than Δt1, the crystalline phases can not precipitate from the glassy alloy or supercooled liquid. Theoretically, the same cooling effect as stationary mould casting can be obtained by the new continuous casting method with an optimum withdrawal procedure. In this study, the ZC1 ingot at Td was moved out of the water-cooled mould and glassy alloy still could be formed. The temperature Td decreases with the increase of the time period pd, so that the cooling speed of the alloy could be adjusted by continuous casting method by tailoring the withdrawal parameters. The
The research was supported by the National Natural Science Foundation of China (Grant No. 50875031). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Inoue A, Takeuchi A. Acta Mater 2011;59:2243–67. Schroers J. JOM 2005;57:35–9. Inoue A. Acta Mater 2000;48:279–306. Johnson WL. MRS Bull 1999;24:42–56. Li Y, Liu HY, Davies HA, Jones H. Mater Sci Eng A 1994;179:628–31. Qiao JW, Wang S, Zhang Y, Liaw PK, Chen GL. Appl Phys Lett 2009:94. Zhang T, Inoue A. Mater Trans JIM 2000;41:1463–6. Lee JG, Lee H, Oh YS, Lee S, Kim NJ. Intermetallics 2006;14:987–93. Urata A, Nishiyama N, Amiya K, Inoue A. Mater Sci Eng A 2007;449:269–72. Inoue A, Yokoyama Y, Shinohara Y, Masumoto T. Mater Trans JIM 1994;35:923–6. X.G. Zhang, F. Jia, W. Zhang, H. Hao, C.F. Fang, T. Zhang, Patent CN20091011405, 2009. Zhang QS, Zhang W, Inoue A. Mater Trans JIM 2007;48:629–31. Zhang W, Zhang QS, Qin CL, Inoue A. J Mater Res 2009;24:2935–40. Louzguine-Luzgin DV, Saito T, Saida J, Inoue A. J Mater Res 2008;23:2283–7. Schroers J. Adv Mater 2010;22:1566–97. Schroers J, Busch R, Bossuyt S, Johnson WL. Mater Sci Eng A 2001;304:287–91.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Study on continuous casting of bulk metallic glass Tao Zhang a, Xingguo Zhang a,⁎, Wei Zhang a,b, Fei Jia a, Akihisa Inoue c, Hai Hao a, Yuejiao Ma a a b c
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan WPI, Advanced Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan
a r t i c l e
i n f o
Article history: Received 17 February 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Cast Solidification Amorphous materials Continuous
a b s t r a c t Continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. In this study, a new continuous casting method was developed for the massive production of bulk metallic glass ingot with centimeter-scale diameter without length limitation. An intermittent withdrawal procedure was practiced for continuous casting of bulk glassy alloy. The new developed continuous casting method can provide a cooling speed as high as that provided by the stationary mold casting method. A Zr-based glassy alloy rod with a diameter of about 10 mm and length of tens of centimeters was prepared by the continuous casting method for the first time. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) possess high strength, high hardness, high elastic properties, low modulus, high corrosion resistance, good magnetic properties etc. They are thus used in many fields, such as electric power industry, biomedical field, micro electro-mechanical systems and information technology [1]. Lots of glassy alloys with critical diameter on centimeter scale were discovered in the past two decades. The superplastic forming of glassy alloy has been widely investigated. The superplastic forming has the potential to replace die casting as a net-shaping processing method for BMGs and might even lead to new applications for the use of BMGs [2]. Superplastic forming of an amorphous alloy involves heating it into the supercooled liquid region and forming it under an applied pressure. However, the glassy alloy starting ingots must be provided before the superplastic forming process. Copper mold casting and quartz container water quenching technologies are mostly used to produce BMG ingots [3,4]. However, these methods are not suitable for the massive production of glassy alloy ingot because of their low productivity. The methods for continuous production of BMGs should be proposed. As we all know, continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. A lot of attention also has been paid to the continuous casting of BMGs. Several methods for continuous fabrication of BMGs have been adopted to produce amorphous wires, strips and rods. Bridgman solidification method is used to produce glass alloy rods or
⁎ Corresponding author. Tel.: + 86 411 84706183; fax: + 86 411 84706183. E-mail address: [email protected] (X. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.033
composites with varied structure and volume of crystalline phases by tailoring the withdrawal speeds [5,6]. A rotating disk casting method is developed for producing long amorphous alloy wires with diameters of 0.5 mm to 1.5 mm [7]. Twin-roll strip continuous casting technology has been utilized to fabricate the sheet products of bulk amorphous alloys with thickness of less than 2 mm [8,9]. The critical diameters or thicknesses of these samples produced using the methods mentioned above are not more than 3 mm. Although zone melting method is also borrowed to prepare BMG strip with a thickness of 10 mm, a width of 12 mm and a length of 170 mm, the regions contacted with the copper base are crystallized areas [10]. In this study, we developed a novel continuous casting method for the massive production of amorphous alloy ingots of large size, and prepared a BMG long rod with a diameter of 10 mm, length of tens of centimeters. The glassy nature of the sample was characterized carefully, and the effects of continuous casting parameters on the surface morphology and inner microstructure of the ingot were discussed. 2. Experimental procedure A horizontal continuous casting system has been built up in our laboratory [11]. The schematic illustration of the continuous casting setup is shown in Fig. 1a. A thermal open-ended mould surrounded by a heater is connected to a water-cooled open-ended mould coaxially. A thermocouple is placed at the exit end of the thermal mould. The thermocouple and the heater are adopted to adjust the temperature of the thermal mould. The glassy alloy melt is kept in a crucible for continuous casting process. The crucible is connected to the thermal mould. A starting block is inserted into the thermal mould for initiation of casting. The starting block is moved by a roller device which is not
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Fig. 1. The schematic illustration of the continuous casting setup (a) and the map of the withdrawal procedure (b).
shown in Fig. 1a. All the units are put into a stainless steel vacuum chamber. At the beginning of the experiment, the melt is guided by the starting block passing through the thermal mould into the water-cooled mould and then cooled to form glassy alloy (or supercooled liquid) in the water-cooled mould. At last the alloy is gradually pulled out of the water-cooled mould by the roller device. An intermittent withdrawal procedure in which a withdrawal cycle is employed is adopted for the continuous casting of BMG. Fig. 1b shows a map of an intermittent withdrawal procedure. The “down” and the “go” periods are represented by pd and pg, respectively. The drawing speed during pd is represented by vg. Zr48Cu36Ag8Al8 (ZC1) alloy was chosen for the continuous casting of BMG rod [12]. The chamber was vacuumed to 10−1 Pa and refilled with high pure Ar gas. The temperature of the thermal mould was maintained above 1140 K. The parameters of the withdrawal procedure are 2 mm/s, 5 s and 2 s for vg, pg and pd, respectively. The glassy phase of the ingot was identified by X-ray diffraction (XRD), and the thermal stability was characterized by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s with Ar atmosphere. The microstructure of the sample was determined by optical microscopy (OM) and JEOL 2100 transmission electron microscopy (TEM) after etching and twin-jet electropolishing, respectively. Vickers hardness (Hv) of the sample was measured by a HVS-5 Vickers microhardness tester with a 50 N (5 kgf) load for 15 s. 3. Experimental results Fig. 2a shows a part of the continuous cast ZC1 rod with a diameter of 10 mm. The continuous cast glassy alloy rod has metallic luster and peripheral bands. One peripheral band is formed on the rod each time the rod is stopped during the intermittent casting procedure. The bands appear to form on that part of the ingot in the mould that shrinks away from contact with the mould wall during the “down” period of the intermittent withdrawal cycle. It is believed that the distance between the two adjacent bands corresponds to the distance moved by the rod in the water-cooled mould during the “go” period of the cycle employed in the intermittent withdrawal procedure, for 10 mm distance between the two adjacent bands is equal to the result of the vg (2 mm/s) multiplied by the pg (5 s). The Fig. 2b shows a cross-sectional XRD pattern of the rod. No sharp peaks corresponding to crystalline phases are imposed to a main halo peak, indicating the good amorphous nature of the ingot. Fig. 2c and d show cross-sectional OM images of the ingot. The specimen is taken from the center of the ingot. It can be seen from Fig. 2c that only several spherical phases with diameters of 100–200 μm
precipitate from the glassy matrix, which are not identified by the XRD analysis. Fig. 2e and f show microstructure of the longitudinal section of the peripheral band. No spherical phases and other precipitated phases are found in the longitudinal section. The glassy structure of the ingot along the longitudinal direction is distributed homogeneously. TEM observation was carried out for the identification of glassy nature and the joint area of peripheral band BMG ingot was specially observed. No difference was found for the samples taken from different areas of the ingot. The featureless high-resolution TEM image and corresponding selected-area electron diffraction (SAED) with a maze pattern typical for an amorphous structure are shown in Fig. 3a and the inset. It is believed that the periodically changed peripheral bands on the rod caused by the intermittent withdrawal procedure do not affect the inner glassy nature of the rod. The new continuous casting technology together with the practiced withdrawal procedure can be used for the continuous production of BMG rod. The glassy nature of the resultant ZC1 rod is further identified DSC analysis. Several specimens are prepared for the thermal analysis. The spherical phases shown in Fig. 2c are not included in the prepared specimens. Fig. 3b shows a normalized DSC curve of the alloy. The glass transition behavior and crystallization reaction are identified clearly in Fig. 3b. The Tg (glass transition temperature), Tx (onset crystallization temperature), △Tx (difference between Tx and Tg) and total heats of crystallization for the sample are 707 K, 801 K, 94 K and 53 J/g, respectively, which are corresponding to the results of ribbon and copper mold casting samples [12]. It is confirmed that the continuous cast rod is of good glassy nature except for only a few spherical phases precipitated from the matrix. These results indicate that continuous casting method could provide a cooling speed as high as that provided by the stationary mold casting method. The Vickers hardness of the glassy phase is also measured on the cross section of the specimens. No cracks along the corner or the face of the indentation are observed. It is found that the alloy shows nearly invariable Hv with measured positions and the average Hv is about 506 which is compared to that of Cu–Zr based glassy alloys, implying that the ingot might have good mechanical properties [13]. The mechanical properties will be studied in details in the future. 4. Discussion The solidification control of the alloy during continuous casting is discussed based on the continuous cooling transformation (CCT) curve, although the real CCT curve of the ZC1 alloy is unknown. Two processes affect the cooling of the molten alloy: heat transfer through the melt and heat transfer through the melt–mould interface, the latter described by the heat-transfer coefficient [14]. The heat transfer through the melt is out of consideration for the same composition of the alloy used in the experiment. The heat-transfer coefficient is supposed to be constant at the same size of the stationary mould cast sample and the part of the continuous cast sample formed during the “down” period of the cycle employed in the intermittent withdrawal procedure. Fig. 4 shows the illustration of cooling curves of the continuous cast alloy and the stationary mould cast alloy. The Tm, Td, and Tg shown in the Fig. 4 are the melting temperature of the alloy, the temperature of the glassy alloy (or supercooled liquid) at the end of each “down” period (Td is determined by pd but exact value is unknown) and the glass transition temperature of the alloy, respectively. The cooling process of the glassy alloy produced by stationary mould casting is a single-step cooling operation, while that produced by the new developed continuous casting is a two-step cooling operation. The continuous cast alloy is produced by the two steps: initially cooling the liquid alloy to Td in the water-cooled mould (the first step described by CC0 curve) and then pulling the alloy out of the mould (the second step described by CC2 curve). The single-step cooling operation by stationary mould casting can be described deliberately by two steps (described by CC0 and CC1 curves) for comparison with continuous casting process. It can be
T. Zhang et al. / Materials Letters 65 (2011) 2257–2260
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Fig. 2. The picture (a), XRD patterns (b) and OM images of continuous cast rod. Images (c) and (d) show the cross section; Images (e) and (f) show the longitudinal section of peripheral band.
considered that the CC0 curve represents the cooling curve of the alloy contact with stationary mould during mould casting or water-cooled mould during each “down” period of the intermittent casting procedure, when the liquid cooled rapidly from liquid above melting temperature to the glassy alloy (or supercooled liquid) at Td. The CC1 and CC2 curves below Td shown in Fig. 4 represent the cooling curves of the alloys that shrink away form contact with the stationary and water-cooled mould wall, respectively. The two curves should be below the CCT curve for the formation of fully glassy alloy.
It is known that the solidification shrinkage during casting of a Zr-based BMG former is approximately only 0.5% for the absence of a first-order phase transition during solidification [15]. The effect of solidification shrinkage on the heat-transfer coefficient cannot be ignored during the second step. The solidification shrinkage is considered to be the main factor that leads to a low heat-transfer coefficient through the ingot/mould interface. The solidification shrinkage is also helpful for the movement of the ingot. However, the heat-transfer coefficients at temperature below Td are different for the
Fig. 3. High-resolution TEM image (a) and SAED pattern (inset) of the sample taken from the joint area of peripheral band BMG and its DSC curve (b).
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related experiments will be done in the future and the effect of the continuous casting withdrawal procedure on the mechanical properties of glassy alloy will also be studied in detail later. 5. Conclusion
Fig. 4. The illustration of cooling curves of the continuous cast and stationary mould cast alloys.
In summary, a novel method for continuous casting of BMG was proposed. An intermittent withdrawal procedure was practiced for the continuous casting of BMG. A Zr48Cu36Ag8Al8 glassy alloy rod with a diameter of 10 mm was produced by the continuous casting method. The parameters of the drawing procedure are 2 mm/s, 5 s and 2 s for vg, pg and pd, respectively. The solidification of glassy alloy was actually subjected. The success of producing BMG rod with centimeter-scale diameter by the continuous casting method can effectively reduce production costs and accelerate the developments and applications of bulk glassy alloys. Acknowledgements
two methods. As we all know, the alloy is not removed from the mould during the stationary mould casting process and the cooling time can be represented by Δt1 shown in Fig. 4, while the alloy is gradually pulled out of the exit end of the water-cooled mould under the intermittent casting procedure and the cooling time can be presented by Δt2 also shown in Fig. 4. The resultant glassy alloy (or supercooled liquid) produced by stationary mould casting or continuous casting methods can retain steady amorphous state for a long time above Tg because of the high thermal stability of supercooled liquid cooled from the liquid [16]. It is thus believed that although Δt2 is greater than Δt1, the crystalline phases can not precipitate from the glassy alloy or supercooled liquid. Theoretically, the same cooling effect as stationary mould casting can be obtained by the new continuous casting method with an optimum withdrawal procedure. In this study, the ZC1 ingot at Td was moved out of the water-cooled mould and glassy alloy still could be formed. The temperature Td decreases with the increase of the time period pd, so that the cooling speed of the alloy could be adjusted by continuous casting method by tailoring the withdrawal parameters. The
The research was supported by the National Natural Science Foundation of China (Grant No. 50875031). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Inoue A, Takeuchi A. Acta Mater 2011;59:2243–67. Schroers J. JOM 2005;57:35–9. Inoue A. Acta Mater 2000;48:279–306. Johnson WL. MRS Bull 1999;24:42–56. Li Y, Liu HY, Davies HA, Jones H. Mater Sci Eng A 1994;179:628–31. Qiao JW, Wang S, Zhang Y, Liaw PK, Chen GL. Appl Phys Lett 2009:94. Zhang T, Inoue A. Mater Trans JIM 2000;41:1463–6. Lee JG, Lee H, Oh YS, Lee S, Kim NJ. Intermetallics 2006;14:987–93. Urata A, Nishiyama N, Amiya K, Inoue A. Mater Sci Eng A 2007;449:269–72. Inoue A, Yokoyama Y, Shinohara Y, Masumoto T. Mater Trans JIM 1994;35:923–6. X.G. Zhang, F. Jia, W. Zhang, H. Hao, C.F. Fang, T. Zhang, Patent CN20091011405, 2009. Zhang QS, Zhang W, Inoue A. Mater Trans JIM 2007;48:629–31. Zhang W, Zhang QS, Qin CL, Inoue A. J Mater Res 2009;24:2935–40. Louzguine-Luzgin DV, Saito T, Saida J, Inoue A. J Mater Res 2008;23:2283–7. Schroers J. Adv Mater 2010;22:1566–97. Schroers J, Busch R, Bossuyt S, Johnson WL. Mater Sci Eng A 2001;304:287–91.