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Motion of bubbles in the mushy zone
Scripta Materialia 55 (2006) 871–874 www.actamat-journals.com
Motion of bubbles in the mushy zone Qingyou Han* Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083, United States Received 24 June 2006; accepted 30 July 2006 Available online 24 August 2006
Radical bubble motion in the mushy zone during solidification of transparent material was observed. Often, bubbles jump at great speeds, from location to location towards higher temperature regions in the mushy zone. This type of radical bubble motion may also affect the final distribution and size of pores in a solidifying casting. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Solidification microstructure; Casting; Porosity formation
This paper reports the behavior of bubbles precipitated during solidification of alloys having a large solubility of gas in the liquid but a negligible solubility of gas in the solid. Typical gas elements that fall in this category are hydrogen in aluminum and magnesium alloys [1], oxygen (forming CO bubbles) in irons and steels [2] and hydrogen in copper alloys [1]. Due to the solubility difference of the gas element in the solid and liquid phases, the gas element is rejected by the solid and enriched in the liquid during solidification. Bubbles form when the gas concentration is higher than its solubility in the liquid. Bubbles that are entrapped by the growing solid and remain in the solid after solidification are termed pores. Porosity is a microstructure defect deteriorating the mechanical properties, notably the fatigue life of the alloys [3]. A considerable amount of research has been carried out to investigate bubble formation during solidification. Using X-rays, Lee and Hunt [4] observed the formation of worm-like bubbles during solidification of aluminum alloys. They found that the number of bubbles increased and their size decreased with increasing solidification rates and initial hydrogen concentration in the melt. Carte [5] studied air bubble formation during the solidification of ice and found that the size, morphology and density of bubbles were strongly affected by the solidification rate and gas saturation in water. He also found that bubbles are formed in a wave-like fashion at high solidification rates and interpreted this phenomenon in terms of the depletion of the supersaturation of gas by bubbles * Tel.: +1 865 574 4352; fax: +1 865 574 4357; e-mail: [email protected]
inhibiting nucleation until the supersaturation builds up again. Potschke [6], using tetrachlorine, benzol and water system, and Golovach [7], using phenyl salicylate and azobenzene, observed bubble formation in situ and found that bubbles nucleated at the solid–liquid interface where the gaseous solute has the greatest concentration during solidification. These researchers have provided useful information on the sites where bubbles nucleate and the parameters that affect the final pore size and distribution. Little is known of the behavior of bubbles in the semisolid region. This study uses a transparent material to observe the behavior of bubbles in the mushy zone. Cyclohexane, a transparent organic material, was used because it contains dissolved gases that precipitate as bubbles during solidification. A long glass tube of rectangular cross section, 0.05 · 5 mm, was filled with liquid cyclohexane. The tube was then placed on a temperature stage [8] with a cold box at one side and hot box on the other. A translation device was employed to withdraw the tube through the cool box of the temperature stage under controlled rates. As a result, directional solidification of the cyclohexane took place in the tube. The separation between the cold and hot boxes was 10 mm, above which a stereomicroscope, attached to a video camera, was used to observe the behavior of bubbles that formed during solidification. The growth rate of the solid and the rate of bubble migration were measured at the output of the video camera. During the experiments, the temperature gradient was about 5 C/mm and the growth rates of the solid were varied in the range of 10–140 lm/s. On solidification, an array of dendrites was formed, see Figure 1. Bubbles were formed at the roots of the
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.07.052
872
Q. Han / Scripta Materialia 55 (2006) 871–874
Figure 1. A dendrite array formed during directional solidification of cyclohexane.
dendrites due to the fact that the concentration of the dissolved gas was highest there [9]. The dissolved gas elements diffused to the bubbles and the bubbles grew. Since the bubble had to grow in a space that was already occupied by a dendrite array, the behavior of the bubble has to be determined by the morphology of the dendrites, the solid fraction, and the supersaturation of the dissolved gases in the liquid of the mushy zone. A general trend observed in our experiments is that the bubbles move/jump toward the regions of higher liquid fractions (Fig. 2). Three patterns of bubble motion have been observed. They are bubble eruption, bubble jumping, and coupled growth of bubble with dendrites. Figures 3 and 4 illustrate a pattern of bubble eruption. During bubble eruption, a large number of bubbles escaped out of the mushy zone. The eruption was initiated with bubbles formed or entrapped in the dendrite network. As the supersaturated dissolved gases diffused to the bubbles continuously, the bubbles need to grow but were restrained in the dendrites. The pressure within the bubble increased until it was so high that the dendrites were deformed suddenly (sometimes we observed that the dendrites were either deformed or broken). As soon as the dendrites yielded, the bubble shot to locations of higher local liquid fractions. The sudden release of restriction made the bubble travel at great speeds. It also produced a sudden pressure drop that sucked bubbles nearby to migrate to the location where the bubbles were. Meanwhile the passage that the first bubble trav-
Figure 2. Eruption of bubbles at the dendritic front. Some of the dendrites were fragmented and brought away by the escaping bubbles, resulting in a highly irregular dendritic front.
Figure 3. Escape of bubbles from the dendritic front during bubble eruption. These photos were taken at various times: (a) 0 s (b) 0.017 s (c) 0.067 s and (d) 0.183 s.
Figure 4. A photo showing bubble jumping in the mushy zone. Two early bubbles, termed bubble source in this article, are marked with arrows. These bubble sources release multi bubbles towards the dendritic front.
eled through served as the passage for the following bubbles to travel. As a result, a number of bubbles traveled through the passage to the dendrite front, making a series of bubble eruptions at the dendritic solidification front. Sometimes fragments of dendrites were also brought out of the mushy zone with the eruption of bubbles. Figure 3 shows that bubbles escape the dendritic front at great speeds. Figure 3a shows a small bubble escaping from the dendritic front. The bubble traveled to the right side on Figure 3b within 0.017 s. This translates into a speed around 14 cm/s. A larger bubble attached to the dendritic front may collect smaller
Q. Han / Scripta Materialia 55 (2006) 871–874
bubbles escaping the mushy zone. Figure 3a shows a bubble at the freezing front. It becomes larger in Figure 3b, dislodges from the freezing front during the bubble eruption shown in Figure 3c, and finally travels away from the dendritic front, shown at the right side of Figure 3d. Bubble jumping occurred frequently in the mushy zone. Bubbles jumped from one location to another towards the regions of higher temperature. In each jump, the bubble could travel over a few secondary dendrites. Unlike the eruption of the bubbles, this pattern of bubble motion involved the discontinuous jumping of individual bubbles in the mushy zone. When a bubble jumped towards the freezing front, it could move across a few primary/secondary dendrites, stay for a while until pressure within the bubble is built up, and jump again. Often, when a bubble jumped to another location, part of the bubble remained in its original location. As more dissolved gas diffused to the remaining bubble, it grew once again and released another bubble as the pressure is being built up. This is an important feature of bubble multiplication. Evidence of this important feature is shown in Figure 4. A bubble in the mushy zone may serve as a bubble source for a number of new bubbles at regions with larger liquid fractions. The jumping of bubbles also leads to the coalescence of two bubbles or multi bubbles since the pathway that the first bubble travels is usually used by other bubbles formed later on the same bubble source/location. In some of the locations in the specimen, coupled growth of bubbles and dendrites could be observed, resulting in the formation of worm-like bubbles that grew side by side with the neighboring dendrites at the dendritic front, as shown in Figure 5. The worn-like bubble may initiate on a spherical bubble, shown in Figure 5a, and then grow side by side with
dendrites. In case there is a misalignment between the dendrites and the worm-like bubble, the dendrite can be overgrown by the bubble, shown in Figure 5b. The coupled growth of bubble and dendrites is the result of a continuous supply of dissolved gas to the bubble to maintain its growth. Coupled growth of bubbles and dendrites usually occurred at small growth rates at some locations where the supply of dissolved gas to the bubble is sufficient. However, too much supply of dissolved gas led to the formation of a spherical bubble at the front of the worm-like bubble. In cases where there was insufficient supply of dissolved gas into the worm-like bubble, the bubble will be entrapped by the growing dendritic front. The reason that bubbles jump in the mushy zone or erupt out of the mushy zone can be understood considering the growth of a bubble in the mushy zone which consists of liquid pockets between solid dendrites. In most cases when the liquid fraction is higher than 0.25, the liquid pockets are linked by liquid channels. When the dissolved gas is saturated in the liquid in a mushy zone, bubbles can nucleate on foreign particles near the solid–liquid interface and grow into the nearby liquid pockets. The tiny bubble is usually spherical before it totally fills the liquid pocket. As more dissolved gas diffuses into the tiny bubble, pressure is built up in the bubble and the bubble is forced to grow into the space defined by the preexisting dendrites. Usually at this stage, the bubble becomes elongated. As the pressure within the bubble is further increased, the pressure can deform the neighboring dendrites or even break them, leading to a sudden release of pressure and the jump of the bubble. The pressure can also force the bubble to squeeze through the liquid channel connecting other liquid pockets. When the bubble reaches another liquid pocket, the radius at the bubble tip is suddenly increased and the internal pressure of the bubble is released, leading to the jump of a bubble from one smaller liquid pocket to a larger pocket. Usually a bubble jumps towards regions of higher temperatures because the liquid fractions there are higher and the liquid channels are larger. To understand the driving forces for bubble eruption/ jumping in the mushy zone, we can consider bubble formation during directional solidification. Due to the existence of a temperature gradient, the liquid fraction is higher near the dendritic front than that at the primary dendrite roots. A bubble entrapped in the mushy zone would have larger curvature (smaller radius) at its lower temperature side than that at its higher temperature side. Assuming the curvatures at both sides are 1/r1 and 1/r2 as illustrated in Figure 6, respectively, the pressure applied at both sides is given by the following equations: P1 ¼
Figure 5. Coupled growth of bubble with dendrites growth at a rate of about 100 lm: (a) a worm-like bubble initiated on a spherical bubble, and (b) a dendrite is overgrown by the bubble.
873
2r r1
and
P2 ¼
2r r2
ð1Þ
where r is the surface energy of the melt/bubble interface. Since P1 is larger than P2, a force, F, in the order of oP 2r 1 1 F ¼ ð2Þ ox l r1 r2
874
Q. Han / Scripta Materialia 55 (2006) 871–874
Figure 6. Schematic illustration of the shape of a bubble in a dendritic array. The radius of the bubble is smaller at the left side where the solid fraction is larger than that at its right side where the solid fraction is less.
is applied on the bubble, where l is the length of the bubble. Under the influence of this driving force due to curvature difference, the bubble tends to move towards regions of large liquid fraction but the secondary dendrites in front of the bubble can block the bubble from motion. Another driving force arises from the diffusion of the dissolved gas into the bubble to increase the internal pressure of the bubble. It is equivalent to the squeezing of the bubble from the primary dendrites so that the bubble has to be pushed to more open regions. The contraction of the solid may also tend to squeeze the bubble out of the liquid pocket. These driving forces tend to make bubble jump from one location to another when the restrictions from the secondary dendrites are suddenly removed if the primary or secondary dendrites are deformed or fractured. High growth rate, high temperature gradient, and high content of the dissolved gas in the melt promote bubble jumping in the mushy zone. The motion of bubbles in the mushy zone brings about redistribution of the dissolved gas during the solidification process of alloys. It is expected that the final size and the distribution of bubbles in a casting must be affected by the bubble motion.
Worm-like bubbles could sometimes be observed in the mushy zone. However, in most cases, bubbles jumped in the mushy zone. Eruption of bubbles also occurred frequently at the freezing front of the mushy zone. During bubble eruption, bubbles traveled out of the mushy zone at great speeds. The driving forces for bubble migration are correlated to the curvature difference of a bubble in a temperature gradient. Forces resisting the bubble motion arise from the preexisting dendrites in the mushy zone. When the driving forces on the bubble is larger than the drag force restricting the bubble motion, radial bubble motion such as jumping and eruption occurs. Research sponsored by the US Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCar and Vehicle Technologies, Automotive Lightweighting Materials Transportation Technology Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. [1] R.J. Fruehan, Gases in Metals, Metals Handbook, ninth ed., vol. 15 (Casting), ASM International, Metals Park, OH, 1988, p. 82. [2] K. Kobo, D.A. Pehkle, Metall. Trans. 16B (1985) 359–366. [3] J. Campbell, Castings, Butterworth-Heinemann, Oxford, 1991, 282. [4] P.D. Lee, J.D. Hunt, Acta Metall. 45 (10) (1997) 4155– 4169. [5] A.E. Carte, Proc. Phys. Soc. 77 (1960) 757–768. [6] J. Potschke, Arch. Eisenhu¨ttenwes 50 (7) (1979) 277–283. [7] Y.Y. Golovach, Izv. VUZ Chernaya Metall. 5 (1980) 128– 132. [8] J.D. Hunt, K.A. Jackson, H. Brown, Review Sci. Instrum. 37 (6) (1966) 805. [9] Q. Han, S. Viswanathan, Metall. Mater. Trans. 33A (2002) 2067–2072.
Motion of bubbles in the mushy zone Qingyou Han* Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083, United States Received 24 June 2006; accepted 30 July 2006 Available online 24 August 2006
Radical bubble motion in the mushy zone during solidification of transparent material was observed. Often, bubbles jump at great speeds, from location to location towards higher temperature regions in the mushy zone. This type of radical bubble motion may also affect the final distribution and size of pores in a solidifying casting. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Solidification microstructure; Casting; Porosity formation
This paper reports the behavior of bubbles precipitated during solidification of alloys having a large solubility of gas in the liquid but a negligible solubility of gas in the solid. Typical gas elements that fall in this category are hydrogen in aluminum and magnesium alloys [1], oxygen (forming CO bubbles) in irons and steels [2] and hydrogen in copper alloys [1]. Due to the solubility difference of the gas element in the solid and liquid phases, the gas element is rejected by the solid and enriched in the liquid during solidification. Bubbles form when the gas concentration is higher than its solubility in the liquid. Bubbles that are entrapped by the growing solid and remain in the solid after solidification are termed pores. Porosity is a microstructure defect deteriorating the mechanical properties, notably the fatigue life of the alloys [3]. A considerable amount of research has been carried out to investigate bubble formation during solidification. Using X-rays, Lee and Hunt [4] observed the formation of worm-like bubbles during solidification of aluminum alloys. They found that the number of bubbles increased and their size decreased with increasing solidification rates and initial hydrogen concentration in the melt. Carte [5] studied air bubble formation during the solidification of ice and found that the size, morphology and density of bubbles were strongly affected by the solidification rate and gas saturation in water. He also found that bubbles are formed in a wave-like fashion at high solidification rates and interpreted this phenomenon in terms of the depletion of the supersaturation of gas by bubbles * Tel.: +1 865 574 4352; fax: +1 865 574 4357; e-mail: [email protected]
inhibiting nucleation until the supersaturation builds up again. Potschke [6], using tetrachlorine, benzol and water system, and Golovach [7], using phenyl salicylate and azobenzene, observed bubble formation in situ and found that bubbles nucleated at the solid–liquid interface where the gaseous solute has the greatest concentration during solidification. These researchers have provided useful information on the sites where bubbles nucleate and the parameters that affect the final pore size and distribution. Little is known of the behavior of bubbles in the semisolid region. This study uses a transparent material to observe the behavior of bubbles in the mushy zone. Cyclohexane, a transparent organic material, was used because it contains dissolved gases that precipitate as bubbles during solidification. A long glass tube of rectangular cross section, 0.05 · 5 mm, was filled with liquid cyclohexane. The tube was then placed on a temperature stage [8] with a cold box at one side and hot box on the other. A translation device was employed to withdraw the tube through the cool box of the temperature stage under controlled rates. As a result, directional solidification of the cyclohexane took place in the tube. The separation between the cold and hot boxes was 10 mm, above which a stereomicroscope, attached to a video camera, was used to observe the behavior of bubbles that formed during solidification. The growth rate of the solid and the rate of bubble migration were measured at the output of the video camera. During the experiments, the temperature gradient was about 5 C/mm and the growth rates of the solid were varied in the range of 10–140 lm/s. On solidification, an array of dendrites was formed, see Figure 1. Bubbles were formed at the roots of the
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.07.052
872
Q. Han / Scripta Materialia 55 (2006) 871–874
Figure 1. A dendrite array formed during directional solidification of cyclohexane.
dendrites due to the fact that the concentration of the dissolved gas was highest there [9]. The dissolved gas elements diffused to the bubbles and the bubbles grew. Since the bubble had to grow in a space that was already occupied by a dendrite array, the behavior of the bubble has to be determined by the morphology of the dendrites, the solid fraction, and the supersaturation of the dissolved gases in the liquid of the mushy zone. A general trend observed in our experiments is that the bubbles move/jump toward the regions of higher liquid fractions (Fig. 2). Three patterns of bubble motion have been observed. They are bubble eruption, bubble jumping, and coupled growth of bubble with dendrites. Figures 3 and 4 illustrate a pattern of bubble eruption. During bubble eruption, a large number of bubbles escaped out of the mushy zone. The eruption was initiated with bubbles formed or entrapped in the dendrite network. As the supersaturated dissolved gases diffused to the bubbles continuously, the bubbles need to grow but were restrained in the dendrites. The pressure within the bubble increased until it was so high that the dendrites were deformed suddenly (sometimes we observed that the dendrites were either deformed or broken). As soon as the dendrites yielded, the bubble shot to locations of higher local liquid fractions. The sudden release of restriction made the bubble travel at great speeds. It also produced a sudden pressure drop that sucked bubbles nearby to migrate to the location where the bubbles were. Meanwhile the passage that the first bubble trav-
Figure 2. Eruption of bubbles at the dendritic front. Some of the dendrites were fragmented and brought away by the escaping bubbles, resulting in a highly irregular dendritic front.
Figure 3. Escape of bubbles from the dendritic front during bubble eruption. These photos were taken at various times: (a) 0 s (b) 0.017 s (c) 0.067 s and (d) 0.183 s.
Figure 4. A photo showing bubble jumping in the mushy zone. Two early bubbles, termed bubble source in this article, are marked with arrows. These bubble sources release multi bubbles towards the dendritic front.
eled through served as the passage for the following bubbles to travel. As a result, a number of bubbles traveled through the passage to the dendrite front, making a series of bubble eruptions at the dendritic solidification front. Sometimes fragments of dendrites were also brought out of the mushy zone with the eruption of bubbles. Figure 3 shows that bubbles escape the dendritic front at great speeds. Figure 3a shows a small bubble escaping from the dendritic front. The bubble traveled to the right side on Figure 3b within 0.017 s. This translates into a speed around 14 cm/s. A larger bubble attached to the dendritic front may collect smaller
Q. Han / Scripta Materialia 55 (2006) 871–874
bubbles escaping the mushy zone. Figure 3a shows a bubble at the freezing front. It becomes larger in Figure 3b, dislodges from the freezing front during the bubble eruption shown in Figure 3c, and finally travels away from the dendritic front, shown at the right side of Figure 3d. Bubble jumping occurred frequently in the mushy zone. Bubbles jumped from one location to another towards the regions of higher temperature. In each jump, the bubble could travel over a few secondary dendrites. Unlike the eruption of the bubbles, this pattern of bubble motion involved the discontinuous jumping of individual bubbles in the mushy zone. When a bubble jumped towards the freezing front, it could move across a few primary/secondary dendrites, stay for a while until pressure within the bubble is built up, and jump again. Often, when a bubble jumped to another location, part of the bubble remained in its original location. As more dissolved gas diffused to the remaining bubble, it grew once again and released another bubble as the pressure is being built up. This is an important feature of bubble multiplication. Evidence of this important feature is shown in Figure 4. A bubble in the mushy zone may serve as a bubble source for a number of new bubbles at regions with larger liquid fractions. The jumping of bubbles also leads to the coalescence of two bubbles or multi bubbles since the pathway that the first bubble travels is usually used by other bubbles formed later on the same bubble source/location. In some of the locations in the specimen, coupled growth of bubbles and dendrites could be observed, resulting in the formation of worm-like bubbles that grew side by side with the neighboring dendrites at the dendritic front, as shown in Figure 5. The worn-like bubble may initiate on a spherical bubble, shown in Figure 5a, and then grow side by side with
dendrites. In case there is a misalignment between the dendrites and the worm-like bubble, the dendrite can be overgrown by the bubble, shown in Figure 5b. The coupled growth of bubble and dendrites is the result of a continuous supply of dissolved gas to the bubble to maintain its growth. Coupled growth of bubbles and dendrites usually occurred at small growth rates at some locations where the supply of dissolved gas to the bubble is sufficient. However, too much supply of dissolved gas led to the formation of a spherical bubble at the front of the worm-like bubble. In cases where there was insufficient supply of dissolved gas into the worm-like bubble, the bubble will be entrapped by the growing dendritic front. The reason that bubbles jump in the mushy zone or erupt out of the mushy zone can be understood considering the growth of a bubble in the mushy zone which consists of liquid pockets between solid dendrites. In most cases when the liquid fraction is higher than 0.25, the liquid pockets are linked by liquid channels. When the dissolved gas is saturated in the liquid in a mushy zone, bubbles can nucleate on foreign particles near the solid–liquid interface and grow into the nearby liquid pockets. The tiny bubble is usually spherical before it totally fills the liquid pocket. As more dissolved gas diffuses into the tiny bubble, pressure is built up in the bubble and the bubble is forced to grow into the space defined by the preexisting dendrites. Usually at this stage, the bubble becomes elongated. As the pressure within the bubble is further increased, the pressure can deform the neighboring dendrites or even break them, leading to a sudden release of pressure and the jump of the bubble. The pressure can also force the bubble to squeeze through the liquid channel connecting other liquid pockets. When the bubble reaches another liquid pocket, the radius at the bubble tip is suddenly increased and the internal pressure of the bubble is released, leading to the jump of a bubble from one smaller liquid pocket to a larger pocket. Usually a bubble jumps towards regions of higher temperatures because the liquid fractions there are higher and the liquid channels are larger. To understand the driving forces for bubble eruption/ jumping in the mushy zone, we can consider bubble formation during directional solidification. Due to the existence of a temperature gradient, the liquid fraction is higher near the dendritic front than that at the primary dendrite roots. A bubble entrapped in the mushy zone would have larger curvature (smaller radius) at its lower temperature side than that at its higher temperature side. Assuming the curvatures at both sides are 1/r1 and 1/r2 as illustrated in Figure 6, respectively, the pressure applied at both sides is given by the following equations: P1 ¼
Figure 5. Coupled growth of bubble with dendrites growth at a rate of about 100 lm: (a) a worm-like bubble initiated on a spherical bubble, and (b) a dendrite is overgrown by the bubble.
873
2r r1
and
P2 ¼
2r r2
ð1Þ
where r is the surface energy of the melt/bubble interface. Since P1 is larger than P2, a force, F, in the order of oP 2r 1 1 F ¼ ð2Þ ox l r1 r2
874
Q. Han / Scripta Materialia 55 (2006) 871–874
Figure 6. Schematic illustration of the shape of a bubble in a dendritic array. The radius of the bubble is smaller at the left side where the solid fraction is larger than that at its right side where the solid fraction is less.
is applied on the bubble, where l is the length of the bubble. Under the influence of this driving force due to curvature difference, the bubble tends to move towards regions of large liquid fraction but the secondary dendrites in front of the bubble can block the bubble from motion. Another driving force arises from the diffusion of the dissolved gas into the bubble to increase the internal pressure of the bubble. It is equivalent to the squeezing of the bubble from the primary dendrites so that the bubble has to be pushed to more open regions. The contraction of the solid may also tend to squeeze the bubble out of the liquid pocket. These driving forces tend to make bubble jump from one location to another when the restrictions from the secondary dendrites are suddenly removed if the primary or secondary dendrites are deformed or fractured. High growth rate, high temperature gradient, and high content of the dissolved gas in the melt promote bubble jumping in the mushy zone. The motion of bubbles in the mushy zone brings about redistribution of the dissolved gas during the solidification process of alloys. It is expected that the final size and the distribution of bubbles in a casting must be affected by the bubble motion.
Worm-like bubbles could sometimes be observed in the mushy zone. However, in most cases, bubbles jumped in the mushy zone. Eruption of bubbles also occurred frequently at the freezing front of the mushy zone. During bubble eruption, bubbles traveled out of the mushy zone at great speeds. The driving forces for bubble migration are correlated to the curvature difference of a bubble in a temperature gradient. Forces resisting the bubble motion arise from the preexisting dendrites in the mushy zone. When the driving forces on the bubble is larger than the drag force restricting the bubble motion, radial bubble motion such as jumping and eruption occurs. Research sponsored by the US Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCar and Vehicle Technologies, Automotive Lightweighting Materials Transportation Technology Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. [1] R.J. Fruehan, Gases in Metals, Metals Handbook, ninth ed., vol. 15 (Casting), ASM International, Metals Park, OH, 1988, p. 82. [2] K. Kobo, D.A. Pehkle, Metall. Trans. 16B (1985) 359–366. [3] J. Campbell, Castings, Butterworth-Heinemann, Oxford, 1991, 282. [4] P.D. Lee, J.D. Hunt, Acta Metall. 45 (10) (1997) 4155– 4169. [5] A.E. Carte, Proc. Phys. Soc. 77 (1960) 757–768. [6] J. Potschke, Arch. Eisenhu¨ttenwes 50 (7) (1979) 277–283. [7] Y.Y. Golovach, Izv. VUZ Chernaya Metall. 5 (1980) 128– 132. [8] J.D. Hunt, K.A. Jackson, H. Brown, Review Sci. Instrum. 37 (6) (1966) 805. [9] Q. Han, S. Viswanathan, Metall. Mater. Trans. 33A (2002) 2067–2072.