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Solidification of aluminium alloys in aerogel moulds
J O U R N A L OF
NON.C] " T,T,IISO S ELSEVIER
Journal of Non-Crystalline Solids 186 (1995) 395-401
Solidification of aluminium alloys in aerogel moulds J. Alkemper "'*, T. Buchholz
K. Murakami
a
b
L. Ratke a
a German Aerospace Research Center, Cologne, 51140 Germany b NASDA, Japan
Abstract A new application of silica aerogels is presented, using them, for the first time, as a mould material for the casting of metals. Aluminium alloy melts were cast into forms machined from thick silica aerogel plates. The excellent thermal insulation properties of aerogels lead to a nearly one-dimensional cooling and solidification process. This is reflected by the microstructure of the solidified samples. Owing to the transparency of the aerogels, it is also possible to study the cooling process with the help of optical measurements and video techniques. Using an infrared camera, the velocity of the solidification process and the temperature gradient ahead of the solid-liquid interface were obtained from video signals. These can be directly correlated with the microstructure. The successful results of these casting experiments have initiated a more detailed study of the potential offered by aerogels for researching solidification processes. One example is a new type of Bridgeman furnace using aerogels as a crucible material and a special feedback loop controlling the solidification processes without movable samples or heaters like in a classical Bridgeman configuration. The concept of such a furnace is presented and first experimental results are shown.
1. Introduction
The microstructure of a casting depends on its composition with respect to the phase diagram, the cooling process, and thus the solidification velocity. From an industrial point of view, the quality of a casting is characterized by its hardness, toughness, strength, etc., and these properties are solely determined by the microstructure. From this relationship between the solidification process, microstructures mechanical and physical properties, it becomes clear that there are two important things to control in order to obtain a good casting: the composition and the
* Corresponding author. Tel: +49-2203 601 4576. Telefax: +49-2203 617 68.
heat flow. Since every casting of an alloy is done into specifically shaped forms, called moulds, the mould materials must have some special properties: their melting temperature should be higher than the casting temperature of the metals under consideration, and no chemical reaction between the mould and the alloy should take place. The wetting behaviour of the metal on the mould is also important. If the mould is wetted by the metal, it is difficult to remove the solidified sample from it and if it is not wetted the temperature flow often becomes uncontrolable. Typical mould materials are ceramics such as sintered alumina, zirconia, mixed silicates, boron nitride and graphite. All these materials have a high thermal conductivity compared with aerogels and the thermal contact between the melt and the mould cannot be characterized quantitatively.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 6 0 - 7
396
J. AIkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
Transparent silica aerogels are an interesting new mould material for the casting of aluminium alloys. From a large number of castings performed during the past year, we know that aluminium alloys do not wet silica aerogels (although aluminium wets and reacts chemically with dense silica glass) [1]. Owing to the transparency of the aerogel, it is possible to measure the surface temperatures with optical methods such as pyrometry. The low thermal conductivity of the aerogel means that the temperature distribution in the sample is not influenced by the mould. Therefore calculations of the temperature field can be done with a higher accuracy and they can be compared with the optical measurements. The cooling of the melt inside an aerogel mould is dominated by heat losses due to radiation and can be controlled via special cooling devices inserted into the mould.
2. Experimental Casting devices were developed and employed for several types of A1 alloy. The first is a chill cast method. A melt is cast into an aerogel tube, which is closed with a graphite foil at the bottom (see Fig. 1). Below there is a movable copper plate, which allows one to switch the cooling on and off. The procedure of an experiment can be described as follows.
Fig. 1. Experimental set-up for casting experiments with AI-Pb alloys.
The alloy is cast into the aerogel tube, with the copper rod in a position so that heat flow through the bottom is low. After 5 s convection in the melt is damped out owing to the viscosity of the melt and the copper rod is moved so that the melt rapidly cools by the intimate contact with the copper chill plate. This type of experiment was used to investigate the microstructure evolution of aluminium-lead and aluminium-bismuth alloys. A1-Pb or A1-Bi exhibit a liquid miscibility gap in their phase diagram. During cooling of an alloy melt, which is homogeneous at the casting temperature of around 1000°C, a separation into two different liquids occurs [3]. This process starts with the creation of small droplets rich in lead or bismuth within the aluminium matrix. During further cooling these droplets grow, move and settle because of their density difference with the A1 matrix. The separations and moving of the droplets stops with the solidification of the aluminium matrix at 659°C. The objective of these experiments was to study the entire separation process in the liquid state. For this task, it was important to have a nearly one-dimensional heat extraction from the melt and to know the temperature distribution inside the sample with high spatial and temporal resolution. The onedimensional solidification was realized by cooling from the bottom and the excellent thermal insulation properties of the aerogel mould. The temperatures were measured with a chargecoupled detector (CCD) camera. The camera is able to detect light in the range of 1000-1600 nm and produces true grey signals. The camera works with a frequency of 25 Hz and a resolution of 255 × 255 pixels. The output signal is recorded by a videotape-recorder and later digitized to 8 bits by an image analysing system. The digitized brightness can then be compared with a reference measurement to convert the grey levels to temperatures. The accuracy of the results depends on the local surface emissivity, because the brightness directly scales with the emission coefficient. The second facility developed is a so-called transient-furnace (see Fig. 2). Samples of 8 mm diameter and 100 mm length are placed vertically in a furnace, which has a constant temperature of 980°C along its length. The bottom of the sample sticks in a cooling device and the upper part of the sample is mounted
397
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
:ube device flingwater Fig. 2. Transient furnace for directional solidification.
inside an aerogel tube. During an experiment the furnace is initially heated and after holding the temperature for 5 min moved upward. Being inside the furnace, the sample is heated by radiation and directionally cooled by the cooling block at its bottom. After 5 min approximately 50 mm of the sample is completely molten and solidifies in a directional way once the furnace is removed. The temperatures are also measured with the CCD camera mentioned above. The third method is a Bridgeman-like facility (see Fig. 3). A typical sample is 10 mm in diameter and 100 mm in length. The free center is surrounded by an aerogel tube, which tightly fits to a boron nitride core of the furnaces. The furnaces are controlled by a computer in such a way that, for example, a constant temperature gradient can be imposed in a sample and be held and moved through it in a controlled way. The cooling rate, d T / d t , the solidification velocity, u and the temperature difference between the furnaces and thus the temperature gradient, G, in the sample are connected by d T / d t = Gu. With a line CCD camera, the computer is able to measure the temperatures of the sample surface in the region of the aerogel tube. At the beginning of an experiment a sample is completely molten to optimize the thermal contact between the furnaces and the sample. The next step is to set the starting conditions. For example, the upper furnace is heated to a temperature of
900°C and the lower one to 700°C. The furnace temperatures are held for several minutes to establish a stationary temperature profile. Both furnaces are then cooled at a constant cooling rate, d T / d t , and the temperature difference between the furnaces is kept constant. Since the solidification velocity is given by the cooling rate of the furnaces used, the velocities can be varied between 2 and 0.05 m m / m i n in the present set-up of the facility. In the classical Bridgeman process a very long sample ( 1 0 - 3 0 cm) is moved through a furnace such that its bottom part moves into or through a cooling liquid. The sample sits in a thick ceramic or glass tube and the temperature profile is measured with encapsulated thermocouples at a few positions in the solid and liquid part. The second and the third facility were used with A1-Ni alloys of exact eutectic composition. At the solidification of the alloys, two solid phases grow in a coupled manner, since the liquid decomposes at the eutectic temperature of about 640°C into two solid phases simultaneously [2,4]. The solid body then consists of an aluminium matrix with A13Ni fibers of about 0.3 Ixm diameter and about 2 Ixm distance arranged parallel to the heat flux. The diameter and the distance of the fibers vary with the cooling conditions during the solidification in a well known
m 8ulat.~
.~
~
heater
/ aero ~eltube
oceuput.~ Fig. 3. Bridgeman-like furnace for directional solidification with different velocities.
398
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
Fig. 4. Lower part of a vertical cut through a cast A1-Pb sample. The bright phase is lead-rich;the surroundingdark phase is the aluminium matrix.
way. This alloy is therefore a good test material to investigate both new solidification procedures.
3. Results and discussion Figs. 4 - 6 show some results of the casting experiments. Fig. 4 shows the lower part of a vertical cut through an a l u m i n i u m - l e a d sample. The bright phase is the lead-rich one and the surrounding darker phase
is pure aluminium. One clearly can see that the lead droplets have settled and formed a layer at the bottom of the sample. Above this layer there are large droplets which were created from coagulation events in the liquid state. In the upper part of the picture one can see small lead droplets 1 0 - 2 0 p,m in diameter. The plots in Fig. 5 show the brightness distribution over the total sample length of 27 mm for 11 different times ( 0 - 4 0 s). This gives a good representation of the cooling process, but there are many
S o
3: .£:1
"z3
0
bottom
5
10
15
20
position [ram]
25
30
top
Fig. 5. Brightnessdistributionof the cooling sample in the casting facility for different times measured by a CCD camera.
5
10
15
20 time [s]
25
30
35
Fig. 6. Motionof solidificationfront in a casted sample.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
399
Fig. 7. Horizontal cut through a eutectic A1-Ni sample solidified in the transient furnace.
disturbances in the signal. The surface of the samples is not homogeneous and the emissivity changes with the position, yielding these disturbances. From analyses of a large number of such curves the plot in Fig. 6 was obtained, which shows the position of the solidification front versus time. One can sees that the solidification process needed 20 s and the velocity of the solidification front was nearly constant (0.6
m m / s ) . In the plot, a linear fit is drawn to emphasize this fact. In the transient-furnace, the cooling behaviour (Figs. 8 and 9) was different. In Fig. 7 a horizontal cut through an A I - N i sample of eutectic composition is shown. The bright dots are the AI3Ni fibres in the darker aluminium matrix. Fig. 8 shows the brightness of the lower part of the furnace and Fig. 9 shows again the position of the solidification front
1200 14 1000
g
13
8OO
'-E
12
~
10
s
z 600
g
..o 400 200
~
9
~
8 7
0 4
6
8 10 length [ram]
12
14
Fig. 8. Brightness distribution of the cooling sample in the transient furnace for 20 different times measured by a C C D camera.
6 0
10
20
30
40
50
time [s]
Fig. 9. Motion of solidification front in a sample inside the transient furnace.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
400
Fig. 10. H o r i z o n t a l cut t h r o u g h a e u t e c t i c A 1 - N i s a m p l e s o l i d i f i e d in the B r i d g e m a n - l i k e f u r n a c e .
versus time. In the calibration used the brightness cannot exceed a value of 1025. At this value, the pixels at the CCD chip of the camera were oversaturated and the real brightness of the sample may have been higher. The velocity of the solidification front was around 0.2 m m / s , but the motion was not a linear one. The continuous line shows a square-root fit, which seems to describe the results in a good way. This result can be rationalized quantitatively by
solving the heat conduction equation with suitable boundary conditions. Results of the Bridgeman-like facility are shown in Figs. 10-12. Fig. 10 shows again a horizontal cut through a sample, in which A13Ni fibres (bright dots) are distributed in an A1 matrix. The picture looks like Fig. 7, but the diameters and the distances of the fibres are larger because the solidification rate was slower. This can be seen in Fig. 12. The velocity 35
680
60
!gOb
i
!
E 25
~
• 660
......
i. . . . . . . !. . . . . . ~! ....
~. . . . .
650
4'----
_~.,.,~ i
!f
~
~'~:'
o
20
i m
t ' .~- 10
-- 630 620
~
0
4
. . . . . . . . .
8
12
16 20 24 length [mm]
- --
28
32
_J
I
_,5
.....
a. 640
!
30
~ ' ~
Y
f
J
5
600
800
1000
1200 1400 time Is]
1600
1800
F i g . 11. B r i g h t n e s s d i s t r i b u t i o n o f the s a m p l e in the B r i d g e m a n - l i k e
Fig. 12. M o t i o n o f s o l i d i f i c a t i o n front in a s a m p l e in the B r i d g e -
f u r n a c e for 20 d i f f e r e n t t i m e s .
m a n - l i k e furnace.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
of the solidification front was constant at 0.03% m m / s . In Fig. 11 some temperature distributions are shown to clarify that the shape of the temperature profile did not change during processing. Every part of the sample solidified with the same thermal conditions.
401
temperatures below 900°C. For industrial applications, the casting of copper, nickel or iron alloys would be of much more interest but the only transparent and monolithic aerogels available in suitable sizes are silica aerogels, which restrict us to alloys of low melting point. For steel casting, alumina or zirconia aerogels would be necessary.
4. Conclusions References
Silica aerogels are a good alternative to the usual mould or crucible materials. The important properties of silica aerogels with respect to solidification are good optical transparency, low thermal conductivity and the fact that they are not wetted by the liquid alloys. Silica aerogels can be used for short times (1 min) with sample temperatures above 1000°C and for long times (hours) with sample
[1] J. Alkemper, S. Diefenbach and L. Ratke, Scripta Metall. 29 (1993) 1495. [2] W. Kurz and P.R. Sahm, Gerichtet erstarrte eutektische Werkstoffe (Springer, Heidelberg, 1975). [3] L. Ratke, ed., Immiscible Liquid Metals and Organics (DGM, Oberursel, 1993). [4] W. Kurz and D.J. Fisher, Fundamentals on Solidification (TransTech, Aedermannsdorf, Switzerland, 1986).
NON.C] " T,T,IISO S ELSEVIER
Journal of Non-Crystalline Solids 186 (1995) 395-401
Solidification of aluminium alloys in aerogel moulds J. Alkemper "'*, T. Buchholz
K. Murakami
a
b
L. Ratke a
a German Aerospace Research Center, Cologne, 51140 Germany b NASDA, Japan
Abstract A new application of silica aerogels is presented, using them, for the first time, as a mould material for the casting of metals. Aluminium alloy melts were cast into forms machined from thick silica aerogel plates. The excellent thermal insulation properties of aerogels lead to a nearly one-dimensional cooling and solidification process. This is reflected by the microstructure of the solidified samples. Owing to the transparency of the aerogels, it is also possible to study the cooling process with the help of optical measurements and video techniques. Using an infrared camera, the velocity of the solidification process and the temperature gradient ahead of the solid-liquid interface were obtained from video signals. These can be directly correlated with the microstructure. The successful results of these casting experiments have initiated a more detailed study of the potential offered by aerogels for researching solidification processes. One example is a new type of Bridgeman furnace using aerogels as a crucible material and a special feedback loop controlling the solidification processes without movable samples or heaters like in a classical Bridgeman configuration. The concept of such a furnace is presented and first experimental results are shown.
1. Introduction
The microstructure of a casting depends on its composition with respect to the phase diagram, the cooling process, and thus the solidification velocity. From an industrial point of view, the quality of a casting is characterized by its hardness, toughness, strength, etc., and these properties are solely determined by the microstructure. From this relationship between the solidification process, microstructures mechanical and physical properties, it becomes clear that there are two important things to control in order to obtain a good casting: the composition and the
* Corresponding author. Tel: +49-2203 601 4576. Telefax: +49-2203 617 68.
heat flow. Since every casting of an alloy is done into specifically shaped forms, called moulds, the mould materials must have some special properties: their melting temperature should be higher than the casting temperature of the metals under consideration, and no chemical reaction between the mould and the alloy should take place. The wetting behaviour of the metal on the mould is also important. If the mould is wetted by the metal, it is difficult to remove the solidified sample from it and if it is not wetted the temperature flow often becomes uncontrolable. Typical mould materials are ceramics such as sintered alumina, zirconia, mixed silicates, boron nitride and graphite. All these materials have a high thermal conductivity compared with aerogels and the thermal contact between the melt and the mould cannot be characterized quantitatively.
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 6 0 - 7
396
J. AIkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
Transparent silica aerogels are an interesting new mould material for the casting of aluminium alloys. From a large number of castings performed during the past year, we know that aluminium alloys do not wet silica aerogels (although aluminium wets and reacts chemically with dense silica glass) [1]. Owing to the transparency of the aerogel, it is possible to measure the surface temperatures with optical methods such as pyrometry. The low thermal conductivity of the aerogel means that the temperature distribution in the sample is not influenced by the mould. Therefore calculations of the temperature field can be done with a higher accuracy and they can be compared with the optical measurements. The cooling of the melt inside an aerogel mould is dominated by heat losses due to radiation and can be controlled via special cooling devices inserted into the mould.
2. Experimental Casting devices were developed and employed for several types of A1 alloy. The first is a chill cast method. A melt is cast into an aerogel tube, which is closed with a graphite foil at the bottom (see Fig. 1). Below there is a movable copper plate, which allows one to switch the cooling on and off. The procedure of an experiment can be described as follows.
Fig. 1. Experimental set-up for casting experiments with AI-Pb alloys.
The alloy is cast into the aerogel tube, with the copper rod in a position so that heat flow through the bottom is low. After 5 s convection in the melt is damped out owing to the viscosity of the melt and the copper rod is moved so that the melt rapidly cools by the intimate contact with the copper chill plate. This type of experiment was used to investigate the microstructure evolution of aluminium-lead and aluminium-bismuth alloys. A1-Pb or A1-Bi exhibit a liquid miscibility gap in their phase diagram. During cooling of an alloy melt, which is homogeneous at the casting temperature of around 1000°C, a separation into two different liquids occurs [3]. This process starts with the creation of small droplets rich in lead or bismuth within the aluminium matrix. During further cooling these droplets grow, move and settle because of their density difference with the A1 matrix. The separations and moving of the droplets stops with the solidification of the aluminium matrix at 659°C. The objective of these experiments was to study the entire separation process in the liquid state. For this task, it was important to have a nearly one-dimensional heat extraction from the melt and to know the temperature distribution inside the sample with high spatial and temporal resolution. The onedimensional solidification was realized by cooling from the bottom and the excellent thermal insulation properties of the aerogel mould. The temperatures were measured with a chargecoupled detector (CCD) camera. The camera is able to detect light in the range of 1000-1600 nm and produces true grey signals. The camera works with a frequency of 25 Hz and a resolution of 255 × 255 pixels. The output signal is recorded by a videotape-recorder and later digitized to 8 bits by an image analysing system. The digitized brightness can then be compared with a reference measurement to convert the grey levels to temperatures. The accuracy of the results depends on the local surface emissivity, because the brightness directly scales with the emission coefficient. The second facility developed is a so-called transient-furnace (see Fig. 2). Samples of 8 mm diameter and 100 mm length are placed vertically in a furnace, which has a constant temperature of 980°C along its length. The bottom of the sample sticks in a cooling device and the upper part of the sample is mounted
397
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
:ube device flingwater Fig. 2. Transient furnace for directional solidification.
inside an aerogel tube. During an experiment the furnace is initially heated and after holding the temperature for 5 min moved upward. Being inside the furnace, the sample is heated by radiation and directionally cooled by the cooling block at its bottom. After 5 min approximately 50 mm of the sample is completely molten and solidifies in a directional way once the furnace is removed. The temperatures are also measured with the CCD camera mentioned above. The third method is a Bridgeman-like facility (see Fig. 3). A typical sample is 10 mm in diameter and 100 mm in length. The free center is surrounded by an aerogel tube, which tightly fits to a boron nitride core of the furnaces. The furnaces are controlled by a computer in such a way that, for example, a constant temperature gradient can be imposed in a sample and be held and moved through it in a controlled way. The cooling rate, d T / d t , the solidification velocity, u and the temperature difference between the furnaces and thus the temperature gradient, G, in the sample are connected by d T / d t = Gu. With a line CCD camera, the computer is able to measure the temperatures of the sample surface in the region of the aerogel tube. At the beginning of an experiment a sample is completely molten to optimize the thermal contact between the furnaces and the sample. The next step is to set the starting conditions. For example, the upper furnace is heated to a temperature of
900°C and the lower one to 700°C. The furnace temperatures are held for several minutes to establish a stationary temperature profile. Both furnaces are then cooled at a constant cooling rate, d T / d t , and the temperature difference between the furnaces is kept constant. Since the solidification velocity is given by the cooling rate of the furnaces used, the velocities can be varied between 2 and 0.05 m m / m i n in the present set-up of the facility. In the classical Bridgeman process a very long sample ( 1 0 - 3 0 cm) is moved through a furnace such that its bottom part moves into or through a cooling liquid. The sample sits in a thick ceramic or glass tube and the temperature profile is measured with encapsulated thermocouples at a few positions in the solid and liquid part. The second and the third facility were used with A1-Ni alloys of exact eutectic composition. At the solidification of the alloys, two solid phases grow in a coupled manner, since the liquid decomposes at the eutectic temperature of about 640°C into two solid phases simultaneously [2,4]. The solid body then consists of an aluminium matrix with A13Ni fibers of about 0.3 Ixm diameter and about 2 Ixm distance arranged parallel to the heat flux. The diameter and the distance of the fibers vary with the cooling conditions during the solidification in a well known
m 8ulat.~
.~
~
heater
/ aero ~eltube
oceuput.~ Fig. 3. Bridgeman-like furnace for directional solidification with different velocities.
398
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
Fig. 4. Lower part of a vertical cut through a cast A1-Pb sample. The bright phase is lead-rich;the surroundingdark phase is the aluminium matrix.
way. This alloy is therefore a good test material to investigate both new solidification procedures.
3. Results and discussion Figs. 4 - 6 show some results of the casting experiments. Fig. 4 shows the lower part of a vertical cut through an a l u m i n i u m - l e a d sample. The bright phase is the lead-rich one and the surrounding darker phase
is pure aluminium. One clearly can see that the lead droplets have settled and formed a layer at the bottom of the sample. Above this layer there are large droplets which were created from coagulation events in the liquid state. In the upper part of the picture one can see small lead droplets 1 0 - 2 0 p,m in diameter. The plots in Fig. 5 show the brightness distribution over the total sample length of 27 mm for 11 different times ( 0 - 4 0 s). This gives a good representation of the cooling process, but there are many
S o
3: .£:1
"z3
0
bottom
5
10
15
20
position [ram]
25
30
top
Fig. 5. Brightnessdistributionof the cooling sample in the casting facility for different times measured by a CCD camera.
5
10
15
20 time [s]
25
30
35
Fig. 6. Motionof solidificationfront in a casted sample.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
399
Fig. 7. Horizontal cut through a eutectic A1-Ni sample solidified in the transient furnace.
disturbances in the signal. The surface of the samples is not homogeneous and the emissivity changes with the position, yielding these disturbances. From analyses of a large number of such curves the plot in Fig. 6 was obtained, which shows the position of the solidification front versus time. One can sees that the solidification process needed 20 s and the velocity of the solidification front was nearly constant (0.6
m m / s ) . In the plot, a linear fit is drawn to emphasize this fact. In the transient-furnace, the cooling behaviour (Figs. 8 and 9) was different. In Fig. 7 a horizontal cut through an A I - N i sample of eutectic composition is shown. The bright dots are the AI3Ni fibres in the darker aluminium matrix. Fig. 8 shows the brightness of the lower part of the furnace and Fig. 9 shows again the position of the solidification front
1200 14 1000
g
13
8OO
'-E
12
~
10
s
z 600
g
..o 400 200
~
9
~
8 7
0 4
6
8 10 length [ram]
12
14
Fig. 8. Brightness distribution of the cooling sample in the transient furnace for 20 different times measured by a C C D camera.
6 0
10
20
30
40
50
time [s]
Fig. 9. Motion of solidification front in a sample inside the transient furnace.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
400
Fig. 10. H o r i z o n t a l cut t h r o u g h a e u t e c t i c A 1 - N i s a m p l e s o l i d i f i e d in the B r i d g e m a n - l i k e f u r n a c e .
versus time. In the calibration used the brightness cannot exceed a value of 1025. At this value, the pixels at the CCD chip of the camera were oversaturated and the real brightness of the sample may have been higher. The velocity of the solidification front was around 0.2 m m / s , but the motion was not a linear one. The continuous line shows a square-root fit, which seems to describe the results in a good way. This result can be rationalized quantitatively by
solving the heat conduction equation with suitable boundary conditions. Results of the Bridgeman-like facility are shown in Figs. 10-12. Fig. 10 shows again a horizontal cut through a sample, in which A13Ni fibres (bright dots) are distributed in an A1 matrix. The picture looks like Fig. 7, but the diameters and the distances of the fibres are larger because the solidification rate was slower. This can be seen in Fig. 12. The velocity 35
680
60
!gOb
i
!
E 25
~
• 660
......
i. . . . . . . !. . . . . . ~! ....
~. . . . .
650
4'----
_~.,.,~ i
!f
~
~'~:'
o
20
i m
t ' .~- 10
-- 630 620
~
0
4
. . . . . . . . .
8
12
16 20 24 length [mm]
- --
28
32
_J
I
_,5
.....
a. 640
!
30
~ ' ~
Y
f
J
5
600
800
1000
1200 1400 time Is]
1600
1800
F i g . 11. B r i g h t n e s s d i s t r i b u t i o n o f the s a m p l e in the B r i d g e m a n - l i k e
Fig. 12. M o t i o n o f s o l i d i f i c a t i o n front in a s a m p l e in the B r i d g e -
f u r n a c e for 20 d i f f e r e n t t i m e s .
m a n - l i k e furnace.
J. Alkemper et al. /Journal of Non-Crystalline Solids 186 (1995) 395-401
of the solidification front was constant at 0.03% m m / s . In Fig. 11 some temperature distributions are shown to clarify that the shape of the temperature profile did not change during processing. Every part of the sample solidified with the same thermal conditions.
401
temperatures below 900°C. For industrial applications, the casting of copper, nickel or iron alloys would be of much more interest but the only transparent and monolithic aerogels available in suitable sizes are silica aerogels, which restrict us to alloys of low melting point. For steel casting, alumina or zirconia aerogels would be necessary.
4. Conclusions References
Silica aerogels are a good alternative to the usual mould or crucible materials. The important properties of silica aerogels with respect to solidification are good optical transparency, low thermal conductivity and the fact that they are not wetted by the liquid alloys. Silica aerogels can be used for short times (1 min) with sample temperatures above 1000°C and for long times (hours) with sample
[1] J. Alkemper, S. Diefenbach and L. Ratke, Scripta Metall. 29 (1993) 1495. [2] W. Kurz and P.R. Sahm, Gerichtet erstarrte eutektische Werkstoffe (Springer, Heidelberg, 1975). [3] L. Ratke, ed., Immiscible Liquid Metals and Organics (DGM, Oberursel, 1993). [4] W. Kurz and D.J. Fisher, Fundamentals on Solidification (TransTech, Aedermannsdorf, Switzerland, 1986).