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Liquid phase separation and monotectic structure evolution of ternary Al62.6Sn28.5Cu8.9 immiscible alloy within ultrasonic field
Materials Letters 141 (2015) 221–224
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Liquid phase separation and monotectic structure evolution of ternary Al62.6Sn28.5Cu8.9 immiscible alloy within ultrasonic field W. Zhai, H.M. Liu, B. Wei n Department of Applied Physics, Northwestern Polytechnical University, P.O. Box 624, 127, Youyi West Road, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2014 Accepted 15 November 2014 Available online 26 November 2014
Power ultrasound was introduced to investigate its effects on the liquid phase separation and monotectic solidification process of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. As ultrasound power increases, the macrosegregation induced by liquid demixing is strikingly reduced. Meanwhile, a large number of spherical ternary (Alþ Snþ θ(Al2Cu)) monotectic cells are formed within ultrasonic field. Cavitation effect, which produces strong shockwaves and facilities the radial symmetry of temperature, concentration, and flow fields in front of liquid–solid interface accounts for these microstructure variations. & 2014 Published by Elsevier B.V.
Keywords: Immiscible alloy Solidification Microstructure Liquid phase separation Power ultrasound
1. Introduction Aluminum is easy to form immiscible alloys with heavier elements such as Pb, Sn and Bi. When solidifying, these alloys initially separate into two immiscible liquids L1 and L2, which are subsequently followed by a monotectic reaction. In the final solidification structures, if the soft particles with small volume fraction disperse homogeneously within the harder Al, Al–Si or Al–Cu matrix, they become potential candidates for advanced bearings in automotive applications [1]. However, such fine dispersion composite materials are always difficult to be produced under conventional condition due to the large density difference between L1 and L2 phases [2]. Therefore, it is highly desirable to search for effective ways to suppress the macrosegregation of two immiscible liquids and hence promotes the formation of uniform monotectic structures. The application of power ultrasound during liquid to solid transformation is proved to be an effective way to improve the solidification microstructures and mechanical properties [3]. The ultrasonic wave brings about such nonlinear effects as cavitation and acoustic streaming, which greatly affect the crystal nucleation and growth process. Most recent work reports on dendritically solidified alloys, and the common finding is that the previously coarse (Al) and (Mg) dendrites turn into refined equiaxed or globular grains in the presence of ultrasound [4,5]. However, there are few literatures on the effect of ultrasound on liquid phase separation and monotectic solidification. It may be optimistically speculated that the exertion of ultrasound on monotectic alloy influences the liquid
n
Corresponding author. Tel./fax: þ 86 29 88431666. E-mail address: [email protected] (B. Wei).
http://dx.doi.org/10.1016/j.matlet.2014.11.087 0167-577X/& 2014 Published by Elsevier B.V.
phase separation process and produces novel monotectic structures. Alternatively, it has also been demonstrated that the severe plastic deformation method can be one of the effective ways to produce uniform structure. By high pressure torsion (HPT), Straumal et al. [6] have shown that two glassy phases and two other nanocrystalline NiY and Nb15Ni2 phases about 20 nm in size are produced in ternary Ni50Nb20Y30 monotectic alloys, which differs greatly from the as-cast alloy composed of 25 μm Ni3Y grains and several minor phases of 3–5 μm size. In fact, such microstructural refinement effects by HPT method are also found in other types of system such as Cu–Co peritectic alloys [7]. In the present work, power ultrasound with a frequency of 20 kHz is introduced into the liquid phase separation and solidification process of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. The effects of ultrasound on liquid phase segregation and monotectic structures are investigated.
2. Materials and methods The experiments were performed with a solidification apparatus incorporated with ultrasonic generator. Al62.6Sn28.5Cu8.9 alloy samples were ∅25 20 mm in size, and were melted by an electrical resistance furnace. The ultrasonic generator consists of two parts: a KNbO3 piezoelectric transducer with a resonant frequency of 20 kHz and a horn with an end plane of ∅20 mm. When alloy melt temperature dropped to 100 K higher than its liquidus temperature, the ultrasonic transducer was turned on and longitudinal ultrasonic wave was introduced from the top of the alloy sample until it solidified completely. Different exciting currents were input to the
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Fig. 1. Thermal analysis and phase constitution of ternary Al62.6Sn28.5Cu8.9 alloy: (a) DSC curves and (b) XRD patterns.
ultrasonic transducer. The corresponding ultrasound power P is estimated to be 66, 132, 198, 264, 330 and 396 W. After experiments, the alloys samples were vertically sectioned and polished. The phase constitution, microstructure and solute distribution were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).
3. Results and discussion Fig. 1(a) presents the DSC traces of ternary Al62.6Sn28.5Cu8.9 alloy at a scan rate of 5 K/min. During cooling, if temperature decreases to 873 K, the homogenous liquid L starts to separate into two immiscible liquids L1 (Al-rich) and L2 (Sn-rich). When temperature drops to 803 K, a very small exothermic peak appears, denoting that a small amount of primary (Al) solid phase as well as L2 phase precipitates from L1 phase. The sharp exothermic peak at 796 K corresponds to the four-phase monotectic reaction in L1 phase: L1-L2 þ(Al) þ θ(Al2Cu). Finally, invariant ternary eutectic transformation L2-(Sn) þ (Al) þ θ(Al2Cu) takes place at 500 K. XRD analyses are performed on the solidified samples at various ultrasound powers. As shown in Fig. 1(b), the solidification microstructures obtained under static condition and within ultrasonic field are both composed of (Al), (Sn) and θ(Al2Cu) phases. The introduction of power ultrasound does not change the phase constitution. Fig. 2 illustrates the macroscopic structural pattern variation of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. Under static condition, as shown in Fig. 2(a)–(c), two distinct zones, a dark Al-rich zone and a white Sn-rich zone appear from sample top to bottom, which take up height fraction of 52% and 48%, respectively. Undoubtedly, the Stokes motion and coalescence of heavier L2 droplets during liquid phase separation account for this layered structure. The introduction of power ultrasound alters the macrosegregation pattern. Fig. 2(d)–(f) shows the microstructures under the highest ultrasound power of 396 W. At the upper part, a large number of spherical monotectic cells are distributed on Sn-rich matrix. As ultrasound wave propagates, as displayed in Fig. 2(e), a clear boundary appears, below which a Sn accumulated region is formed once again. Metallographic observation shows that this bottom Sn-rich zone always exists irrespective of ultrasound power. Define the macrosegregation degree SH as SH ¼ H s =H t
ð1Þ
where Hs stands for the height of bottom Sn-rich zone, and Ht denotes the total height of alloy sample. Fig. 3(a) plots the
macrosegregation degree versus ultrasound power. The segregation degree decreases as ultrasound power rises, which reduces to only about 10% under the highest ultrasound power. Furthermore, the solute distribution profile for both Sn and Cu elements in the whole vertical section of alloy samples solidified under static solidification and ultrasound power 396 W are determined by EDS method. Each sample is analyzed by dividing into six isometric areas from top to bottom, and the measurement results are shown in Fig. 3(b) and (c). Though desirable uniform distribution is not achieved, the segregation of both Sn and Cu elements is improved considerably as compared with that during static solidification. This demonstrates that ultrasound suppresses spatial separation of liquid phases in a confined region of alloy melt near the radiator, which may also indicate that bulk convection within the whole melt induced by acoustic streaming plays a minor role in preventing segregation. In this case, cavitation effect is presumably to be the main factor affecting liquid phase separation. It has been reported that cavitation is probably most efficient in the region immediately below the horn and less intense with progressive exponential attenuation with distance [8]. In the intensive cavitation region, the strong shockwaves produced by cavitation counterbalance the Stokes motion of L2 droplets, whereas outside of this location, ultrasound has no impact on liquid phase separation. Consequently, gravity dominates the movement of L2 droplets again, resulting in a severe segregation of Sn element as formed during static solidification. As ultrasound power increases, the cavitation region enlarges obviously, leading to a shorter Sn-rich phase region at sample bottom. Fig. 4 presents the morphologies of ternary monotectic structures. Under static condition, (Al), θ and (Sn) phases grow along a certain direction by coupled manner to form a fine and regular structure, as shown in Fig. 4(a) and (b). The average spacing between θ and (Al) phase is only about 1.7 μm. When ultrasonic field is applied, a large number of spherical ternary monotectic cells are formed on L2 matrix. Fig. 4(c) shows those cells formed under ultrasound power of 396 W, and an enlarged view of one typical monotectic cell is presented in Fig. 4(d). Clearly, the (Alþ θ) lamellar structure grows epitaxially from the center, within which very fine white (Sn) particles are dispersed. The interlamellar spacing between (Al) and θ phases is about 3.2 μm, which is about twice as large as that under static condition. This indicates that power ultrasound field also brings about a coarsening effect to the monotectic structure. The formation of such spherical monotectic cells may also be attributed to the cavitation effect. It is known that cavitation sites are potential nucleation sites by raising the local undercooling level [9]. Once nucleation takes place there, the cavitation site
W. Zhai et al. / Materials Letters 141 (2015) 221–224
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Fig. 2. Microstructural characteristics in different zones of Al62.6Sn28.5Cu8.9 alloy samples solidified at static and ultrasonic conditions: (a)–(c) after static solidification and (d)–(f) solidified under 396 W ultrasound.
Fig. 3. Macrosegregation of ternary Al62.6Sn28.5Cu8.9 immiscible alloy: (a) segregation degree versus ultrasound power; (b) solute distribution of Sn element and (c) Cu element under static and ultrasonic condition (P¼ 396 W).
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4. Conclusions In summary, the effects of ultrasound on liquid phase separation pattern and monotectic structure of ternary Al62.6Sn28.5Cu8.9 immiscible alloy have been investigated. It is found that the macrosegregation degree decreases as ultrasound power rises. Meanwhile, a large number of spherical ternary (Al þ Snþ θ (Al2Cu)) monotectic cells are formed within ultrasonic field, in which nucleation takes place at the center and the three monotectic phases grow cooperatively and radially into lamellar structure. This work demonstrates that power ultrasound is an effective and promising way in preventing the spatial liquid phase separation and in promoting novel monotectic structures.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 51327901, 51271150 and 51201136), Natural Science Foundation of Shanxi Province, China (No. 2013JQ6011), Aviation Foundation of China (No. 2012ZF53069), Fundamental Research Fund of Northwestern Polytechnical University (No. JCQ01091) and Excellent Personnel Supporting Project for Ao Xiang New Star of Northwestern Polytechnical University. References
Fig. 4. Ternary (Al þθ þSn) monotectic structure in Al62.6Sn28.5Cu8.9 alloy: (a) regular structure during static solidification; (b) enlarged view of (a); (c) spherical monotectic structure formed within ultrasonic field (P ¼396 W); (d) an enlarged view of one spherical monotectic cell and (e) average diameter of spherical monotectic cell versus ultrasound power.
always acts as center points of the circulation flow in the liquid adjacent to the crystals [10], which facilitates the radial symmetry of temperature, concentration, and flow fields, resulting in the formation of spherical monotectic cells. Fig. 4(e) plots the average diameter of the spherical monotectic cells versus ultrasound power, which shows a decreasing tendency as ultrasound power rises. The relationship between them can be fitted by exponential regression: D ¼ 142:56 exp ð 0:072PÞ þ 78:68
ð2Þ
This can be explained by the fact the number of cavitation sites increases as ultrasound power rises, which leads to more nucleation points of ternary monotectic cell at high ultrasound power.
[1] Dai R, Zhang SG, Li YB, Guo X, Li JG. Phase separation and formation of coretype microstructure of Al–65.5 mass% Bi immiscible alloys. J Alloy Compd 2011;509:2289–93. [2] Wang WL, Li ZQ, Wei B. Macrosegregation pattern and microstructure feature of ternary Fe–Sn–Si immiscible alloy solidified under free fall condition. Acta Mater 2011;59:5482–93. [3] Silva AP, Spinelli JE, Garcia A. Microstructural evolution during upward and downward transient directional solidification of hypomonotectic and monotectic Al–Bi alloys. J Alloy Compd 2009;480:485–93. [4] Li YL, Feng HK, Cao FR, Chen YB, Gong LY. Effect of high density ultrasonic on the microstructure and refining property of Al–5Ti–0.25C grain refiner alloy. Mater Sci Eng A 2008;487:518–23. [5] Bantibhai P, Chaudhari GP, Bhingole PP. Microstructural evolution in ultrasonicated AS41 magnesium alloy. Mater Lett 2012;66:335–8. [6] Mazilkin AA, Abrosimova GE, Protasova SG, Straumal BB, Schütz G, Dobatkin SV, et al. Transmission electron microscopy investigation of boundaries between amorphous “grains” in Ni50Nb20Y30 alloy. J Mater Sci 2011;46:4336–42. [7] Straumal BB, Mazilkin AA, Baretzky B, Schütz G, Rabkin E, Valiev RZ. Accelerated diffusion and phase transformations in CoCu alloys driven by the severe plastic deformation. Mater Trans 2012;53:63–71. [8] Price GJ, Harris NK, Stewart AJ. Direct observation of cavitation fields at 23 and 515 kHz. Ultrason Sonochem 2010;17:30–3. [9] Zhai W, Hong ZY, Xie WJ, Wei B. Dynamic solidification of Sn–38.1%Pb eutectic alloy within ultrasonic field. Chin Sci Bull 2010;56:89–95. [10] Chow R, Blindt R, Chivers R, Povey M. A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics 2005;43:227–30.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Liquid phase separation and monotectic structure evolution of ternary Al62.6Sn28.5Cu8.9 immiscible alloy within ultrasonic field W. Zhai, H.M. Liu, B. Wei n Department of Applied Physics, Northwestern Polytechnical University, P.O. Box 624, 127, Youyi West Road, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2014 Accepted 15 November 2014 Available online 26 November 2014
Power ultrasound was introduced to investigate its effects on the liquid phase separation and monotectic solidification process of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. As ultrasound power increases, the macrosegregation induced by liquid demixing is strikingly reduced. Meanwhile, a large number of spherical ternary (Alþ Snþ θ(Al2Cu)) monotectic cells are formed within ultrasonic field. Cavitation effect, which produces strong shockwaves and facilities the radial symmetry of temperature, concentration, and flow fields in front of liquid–solid interface accounts for these microstructure variations. & 2014 Published by Elsevier B.V.
Keywords: Immiscible alloy Solidification Microstructure Liquid phase separation Power ultrasound
1. Introduction Aluminum is easy to form immiscible alloys with heavier elements such as Pb, Sn and Bi. When solidifying, these alloys initially separate into two immiscible liquids L1 and L2, which are subsequently followed by a monotectic reaction. In the final solidification structures, if the soft particles with small volume fraction disperse homogeneously within the harder Al, Al–Si or Al–Cu matrix, they become potential candidates for advanced bearings in automotive applications [1]. However, such fine dispersion composite materials are always difficult to be produced under conventional condition due to the large density difference between L1 and L2 phases [2]. Therefore, it is highly desirable to search for effective ways to suppress the macrosegregation of two immiscible liquids and hence promotes the formation of uniform monotectic structures. The application of power ultrasound during liquid to solid transformation is proved to be an effective way to improve the solidification microstructures and mechanical properties [3]. The ultrasonic wave brings about such nonlinear effects as cavitation and acoustic streaming, which greatly affect the crystal nucleation and growth process. Most recent work reports on dendritically solidified alloys, and the common finding is that the previously coarse (Al) and (Mg) dendrites turn into refined equiaxed or globular grains in the presence of ultrasound [4,5]. However, there are few literatures on the effect of ultrasound on liquid phase separation and monotectic solidification. It may be optimistically speculated that the exertion of ultrasound on monotectic alloy influences the liquid
n
Corresponding author. Tel./fax: þ 86 29 88431666. E-mail address: [email protected] (B. Wei).
http://dx.doi.org/10.1016/j.matlet.2014.11.087 0167-577X/& 2014 Published by Elsevier B.V.
phase separation process and produces novel monotectic structures. Alternatively, it has also been demonstrated that the severe plastic deformation method can be one of the effective ways to produce uniform structure. By high pressure torsion (HPT), Straumal et al. [6] have shown that two glassy phases and two other nanocrystalline NiY and Nb15Ni2 phases about 20 nm in size are produced in ternary Ni50Nb20Y30 monotectic alloys, which differs greatly from the as-cast alloy composed of 25 μm Ni3Y grains and several minor phases of 3–5 μm size. In fact, such microstructural refinement effects by HPT method are also found in other types of system such as Cu–Co peritectic alloys [7]. In the present work, power ultrasound with a frequency of 20 kHz is introduced into the liquid phase separation and solidification process of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. The effects of ultrasound on liquid phase segregation and monotectic structures are investigated.
2. Materials and methods The experiments were performed with a solidification apparatus incorporated with ultrasonic generator. Al62.6Sn28.5Cu8.9 alloy samples were ∅25 20 mm in size, and were melted by an electrical resistance furnace. The ultrasonic generator consists of two parts: a KNbO3 piezoelectric transducer with a resonant frequency of 20 kHz and a horn with an end plane of ∅20 mm. When alloy melt temperature dropped to 100 K higher than its liquidus temperature, the ultrasonic transducer was turned on and longitudinal ultrasonic wave was introduced from the top of the alloy sample until it solidified completely. Different exciting currents were input to the
222
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Fig. 1. Thermal analysis and phase constitution of ternary Al62.6Sn28.5Cu8.9 alloy: (a) DSC curves and (b) XRD patterns.
ultrasonic transducer. The corresponding ultrasound power P is estimated to be 66, 132, 198, 264, 330 and 396 W. After experiments, the alloys samples were vertically sectioned and polished. The phase constitution, microstructure and solute distribution were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).
3. Results and discussion Fig. 1(a) presents the DSC traces of ternary Al62.6Sn28.5Cu8.9 alloy at a scan rate of 5 K/min. During cooling, if temperature decreases to 873 K, the homogenous liquid L starts to separate into two immiscible liquids L1 (Al-rich) and L2 (Sn-rich). When temperature drops to 803 K, a very small exothermic peak appears, denoting that a small amount of primary (Al) solid phase as well as L2 phase precipitates from L1 phase. The sharp exothermic peak at 796 K corresponds to the four-phase monotectic reaction in L1 phase: L1-L2 þ(Al) þ θ(Al2Cu). Finally, invariant ternary eutectic transformation L2-(Sn) þ (Al) þ θ(Al2Cu) takes place at 500 K. XRD analyses are performed on the solidified samples at various ultrasound powers. As shown in Fig. 1(b), the solidification microstructures obtained under static condition and within ultrasonic field are both composed of (Al), (Sn) and θ(Al2Cu) phases. The introduction of power ultrasound does not change the phase constitution. Fig. 2 illustrates the macroscopic structural pattern variation of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. Under static condition, as shown in Fig. 2(a)–(c), two distinct zones, a dark Al-rich zone and a white Sn-rich zone appear from sample top to bottom, which take up height fraction of 52% and 48%, respectively. Undoubtedly, the Stokes motion and coalescence of heavier L2 droplets during liquid phase separation account for this layered structure. The introduction of power ultrasound alters the macrosegregation pattern. Fig. 2(d)–(f) shows the microstructures under the highest ultrasound power of 396 W. At the upper part, a large number of spherical monotectic cells are distributed on Sn-rich matrix. As ultrasound wave propagates, as displayed in Fig. 2(e), a clear boundary appears, below which a Sn accumulated region is formed once again. Metallographic observation shows that this bottom Sn-rich zone always exists irrespective of ultrasound power. Define the macrosegregation degree SH as SH ¼ H s =H t
ð1Þ
where Hs stands for the height of bottom Sn-rich zone, and Ht denotes the total height of alloy sample. Fig. 3(a) plots the
macrosegregation degree versus ultrasound power. The segregation degree decreases as ultrasound power rises, which reduces to only about 10% under the highest ultrasound power. Furthermore, the solute distribution profile for both Sn and Cu elements in the whole vertical section of alloy samples solidified under static solidification and ultrasound power 396 W are determined by EDS method. Each sample is analyzed by dividing into six isometric areas from top to bottom, and the measurement results are shown in Fig. 3(b) and (c). Though desirable uniform distribution is not achieved, the segregation of both Sn and Cu elements is improved considerably as compared with that during static solidification. This demonstrates that ultrasound suppresses spatial separation of liquid phases in a confined region of alloy melt near the radiator, which may also indicate that bulk convection within the whole melt induced by acoustic streaming plays a minor role in preventing segregation. In this case, cavitation effect is presumably to be the main factor affecting liquid phase separation. It has been reported that cavitation is probably most efficient in the region immediately below the horn and less intense with progressive exponential attenuation with distance [8]. In the intensive cavitation region, the strong shockwaves produced by cavitation counterbalance the Stokes motion of L2 droplets, whereas outside of this location, ultrasound has no impact on liquid phase separation. Consequently, gravity dominates the movement of L2 droplets again, resulting in a severe segregation of Sn element as formed during static solidification. As ultrasound power increases, the cavitation region enlarges obviously, leading to a shorter Sn-rich phase region at sample bottom. Fig. 4 presents the morphologies of ternary monotectic structures. Under static condition, (Al), θ and (Sn) phases grow along a certain direction by coupled manner to form a fine and regular structure, as shown in Fig. 4(a) and (b). The average spacing between θ and (Al) phase is only about 1.7 μm. When ultrasonic field is applied, a large number of spherical ternary monotectic cells are formed on L2 matrix. Fig. 4(c) shows those cells formed under ultrasound power of 396 W, and an enlarged view of one typical monotectic cell is presented in Fig. 4(d). Clearly, the (Alþ θ) lamellar structure grows epitaxially from the center, within which very fine white (Sn) particles are dispersed. The interlamellar spacing between (Al) and θ phases is about 3.2 μm, which is about twice as large as that under static condition. This indicates that power ultrasound field also brings about a coarsening effect to the monotectic structure. The formation of such spherical monotectic cells may also be attributed to the cavitation effect. It is known that cavitation sites are potential nucleation sites by raising the local undercooling level [9]. Once nucleation takes place there, the cavitation site
W. Zhai et al. / Materials Letters 141 (2015) 221–224
223
Fig. 2. Microstructural characteristics in different zones of Al62.6Sn28.5Cu8.9 alloy samples solidified at static and ultrasonic conditions: (a)–(c) after static solidification and (d)–(f) solidified under 396 W ultrasound.
Fig. 3. Macrosegregation of ternary Al62.6Sn28.5Cu8.9 immiscible alloy: (a) segregation degree versus ultrasound power; (b) solute distribution of Sn element and (c) Cu element under static and ultrasonic condition (P¼ 396 W).
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W. Zhai et al. / Materials Letters 141 (2015) 221–224
4. Conclusions In summary, the effects of ultrasound on liquid phase separation pattern and monotectic structure of ternary Al62.6Sn28.5Cu8.9 immiscible alloy have been investigated. It is found that the macrosegregation degree decreases as ultrasound power rises. Meanwhile, a large number of spherical ternary (Al þ Snþ θ (Al2Cu)) monotectic cells are formed within ultrasonic field, in which nucleation takes place at the center and the three monotectic phases grow cooperatively and radially into lamellar structure. This work demonstrates that power ultrasound is an effective and promising way in preventing the spatial liquid phase separation and in promoting novel monotectic structures.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 51327901, 51271150 and 51201136), Natural Science Foundation of Shanxi Province, China (No. 2013JQ6011), Aviation Foundation of China (No. 2012ZF53069), Fundamental Research Fund of Northwestern Polytechnical University (No. JCQ01091) and Excellent Personnel Supporting Project for Ao Xiang New Star of Northwestern Polytechnical University. References
Fig. 4. Ternary (Al þθ þSn) monotectic structure in Al62.6Sn28.5Cu8.9 alloy: (a) regular structure during static solidification; (b) enlarged view of (a); (c) spherical monotectic structure formed within ultrasonic field (P ¼396 W); (d) an enlarged view of one spherical monotectic cell and (e) average diameter of spherical monotectic cell versus ultrasound power.
always acts as center points of the circulation flow in the liquid adjacent to the crystals [10], which facilitates the radial symmetry of temperature, concentration, and flow fields, resulting in the formation of spherical monotectic cells. Fig. 4(e) plots the average diameter of the spherical monotectic cells versus ultrasound power, which shows a decreasing tendency as ultrasound power rises. The relationship between them can be fitted by exponential regression: D ¼ 142:56 exp ð 0:072PÞ þ 78:68
ð2Þ
This can be explained by the fact the number of cavitation sites increases as ultrasound power rises, which leads to more nucleation points of ternary monotectic cell at high ultrasound power.
[1] Dai R, Zhang SG, Li YB, Guo X, Li JG. Phase separation and formation of coretype microstructure of Al–65.5 mass% Bi immiscible alloys. J Alloy Compd 2011;509:2289–93. [2] Wang WL, Li ZQ, Wei B. Macrosegregation pattern and microstructure feature of ternary Fe–Sn–Si immiscible alloy solidified under free fall condition. Acta Mater 2011;59:5482–93. [3] Silva AP, Spinelli JE, Garcia A. Microstructural evolution during upward and downward transient directional solidification of hypomonotectic and monotectic Al–Bi alloys. J Alloy Compd 2009;480:485–93. [4] Li YL, Feng HK, Cao FR, Chen YB, Gong LY. Effect of high density ultrasonic on the microstructure and refining property of Al–5Ti–0.25C grain refiner alloy. Mater Sci Eng A 2008;487:518–23. [5] Bantibhai P, Chaudhari GP, Bhingole PP. Microstructural evolution in ultrasonicated AS41 magnesium alloy. Mater Lett 2012;66:335–8. [6] Mazilkin AA, Abrosimova GE, Protasova SG, Straumal BB, Schütz G, Dobatkin SV, et al. Transmission electron microscopy investigation of boundaries between amorphous “grains” in Ni50Nb20Y30 alloy. J Mater Sci 2011;46:4336–42. [7] Straumal BB, Mazilkin AA, Baretzky B, Schütz G, Rabkin E, Valiev RZ. Accelerated diffusion and phase transformations in CoCu alloys driven by the severe plastic deformation. Mater Trans 2012;53:63–71. [8] Price GJ, Harris NK, Stewart AJ. Direct observation of cavitation fields at 23 and 515 kHz. Ultrason Sonochem 2010;17:30–3. [9] Zhai W, Hong ZY, Xie WJ, Wei B. Dynamic solidification of Sn–38.1%Pb eutectic alloy within ultrasonic field. Chin Sci Bull 2010;56:89–95. [10] Chow R, Blindt R, Chivers R, Povey M. A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics 2005;43:227–30.