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The effect of experimental variables on the levels of melt undercooling
Materials Science and Engineering A 375–377 (2004) 485–487
The effect of experimental variables on the levels of melt undercooling Kalin I. Dragnevski, Andrew M. Mullis∗ , Robert F. Cochrane Department of Materials, University of Leeds, Leeds LS2 9JT, UK
Abstract A melt encasement (fluxing) technique has been used to study the effect of experimental variables, such as cooling rate, superheating time and superheating temperature, on the levels of undercooling in Cu, Cu–O and Cu–Sn melts. The maximum undercoolings achieved were 352 K for pure Cu and 328 and 320 K for Cu–O and Cu–Sn, respectively. The present results are higher than the ones previously reported for these particular systems. It was found that a minimum superheating temperature of 40 K and a minimum superheating time of 600 s are required in order to achieve undercooling of the melt prior to nucleation. However, the most efficient parameter that affected the undercoolability of the studied metal systems proved to be the number of thermal cycles applied prior to solidification. Our results clearly demonstrate the importance of the experimental parameters on the levels of undercooling of metallic melts by using a fluxing technique. © 2003 Elsevier B.V. All rights reserved. Keywords: Undercooling; Fluxing; Solidification
1. Introduction Rapid solidification of metals and alloys has developed in the past few decades from an insignificant activity into major industrial practice and important process for research studies [1]. Rapid solidification is generally brought about by one of the following two methods. The first is by rapid quenching from the melt, e.g. melt spinning, planar flow casting, splat cooling, etc. In this way, the heat is extracted through a steep gradient in the solid and so the sample is required to be thin in one dimension [2]. Even though a variety of novel products have emerged from these methods, they are not suitable for direct analysis of the processes involved in the solidification of the melt. Moreover, rapid quenching is a method that allows little control over the conditions during processing. Consequently, conclusions of the development of the system are usually deduced post mortem from an examination of the as solidified products [3]. The second method involves undercooling the liquid by a large degree below its melting temperature prior to nucleation. In this way, a large driving force for rapid solidification accumulates due to the large difference in Gibbs free energy between the solid and the liquid state. The exper∗
Corresponding author. Fax: +44-113-242-2531. E-mail address: [email protected] (A.M. Mullis).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.050
imental techniques are essentially based on the avoidance of any potential sites for heterogeneous nucleation, such as crucible walls, which allows the preservation of the liquid in the metastable undercooled condition for extended periods of time. These alternative methods for rapid dendritic growth include melt fluxing, drop tube processing and the levitation techniques and are commonly known as containerless processes or denucleation techniques [3]. The main advantage of these alternative methods for non-equilibrium solidification is that they allow direct monitoring of rapid solidification and even external stimulation of the process itself, thus enabling investigations in the early stages of crystallization. In this study, a melt encasement fluxing technique was used to study the effect of experimental variables on the levels of melt undercooling in pure Cu, Cu–O alloy (dilute alloy) and Cu–3 wt.% Sn alloy (more concentrated alloy). These particular systems were chosen also with the aim to determine whether alloy composition influences the levels of undercooling achieved during the fluxing experiments.
2. Experimental procedure Melt fluxing was chosen as the most suitable means to study the alloy systems under consideration, as the melting
486
K.I. Dragnevski et al. / Materials Science and Engineering A 375–377 (2004) 485–487
3. Results
Undercooling/ K
point of Cu-based alloys is generally too low for significant undercoolings to be achieved by electromagnetic levitation. The method allows high undercoolings to be achieved as nucleation on container walls is prevented by isolating the melt from these surfaces. The flux can also aid the removal of oxide impurities from the melt by dissolution in the flux and protects the surface from oxidation. Undercooling experiments were performed within a stainless steel vacuum chamber evacuated to a pressure of 10−6 mbar and backfilled to 500 mbar with N2 gas. Samples were heated, in silica crucibles, by induction heating of a graphite susceptor contained within an alumina radiation shield. A commercial soda-lime glass was employed as the flux to reduce the number of potential sites for heterogeneous nucleation. The temperature was monitored by means of a K-type thermocouple positioned at the base of the crucible, which had been thinned, thus reducing the thermal lag between the sample and the thermocouple. Heating and cooling curves were obtained by using a chart recorder. A schematic diagram of the fluxing apparatus is shown in Fig. 1. By heating the sample to its melting temperature, cooling and repeating this procedure, it was found that the temperatures were reproducible to within 5 K. By using this method it was possible to achieve temperatures of up to 1473 K, thus easily accommodating melting and superheating of the alloy systems that were to be investigated, ensuring that the samples were completely contained within the glass flux. Copper samples of 99.9999% purity (metal basis) were obtained from ALFA (Johnson Matthey). However, these samples contain a residual level of oxygen. Based on the work of previous researchers [4], we have defined Cu–O alloys as Cu containing at least 200 ppm of oxygen after undercooling and oxygen free copper as containing less than 200 ppm of oxygen. A Cu–3 wt.% Sn alloy was prepared by arc melting under argon. All starting materials were obtained from ALFA (Johnson Matthey) and were of 99.9999% purity. In order to ensure complete mixing of the elements and to eliminate any segregation the alloy was re-melted and finally annealed at 1023 K for 1800 s.
300 250 200 150 100 50 0 1300
1320
1340
1360
1380
1400
1420
1440
Temperature/ K
Fig. 2. Undercooling as function of superheating temperature for Cu– 3 wt.% Sn alloy.
300 250 Undercooling/ K
Fig. 1. Schematic diagram of the fluxing apparatus.
In order to determine the influence of superheating time and temperature on the levels of melt undercooling, two different experiments were conducted. The first involved superheating the samples by increasing amounts, cooling and recording the undercooling at which nucleation occurred spontaneously (Fig. 2). In the cases of both Cu and Cu–Sn alloys, it was found that a minimum superheating temperature of 40 K was required in order to achieve any undercooling prior to nucleation. Above this superheating temperature the undercooling increases following almost a linear relationship. The second experiment involved the study of the effect of superheating time on the levels of melt undercooling. Superheating the samples by a preliminary known amount above their melting point and then holding the melt at that temperature for different periods of time before cooling until spontaneous nucleation occurred, monitored the effect of this particular experimental variable. The results from this experiment are shown in Fig. 3 for Cu–O alloy. As seen from the graph, a minimum superheating time of 600 s is required to achieve any undercooling. Above this holding time, the undercooling increases smoothly, as in the case for the previously described parameter, following almost a linear relationship. The effect of cooling rate on the levels of undercooling was studied in an experiment that involved superheating
200 150 100 50 0 0
300 600 900 1200 1500 1800 2100 2400 2700 3000 Superheating time/s
Fig. 3. Effect of superheating time on the levels of undercooling for Cu–O alloy.
K.I. Dragnevski et al. / Materials Science and Engineering A 375–377 (2004) 485–487
487
4. Discussion
160 Undercooling/ K
140 120 100 80 60 40 20 0 0
0.1
0.2
0.3
0.4
Cooling rate/K s
0.5
0.6
-1
Undercooling/K
Fig. 4. Variation of undercooling with cooling rate for Cu–3 wt.% Sn alloy.
400 300 200 100 0 0
2
4
6
8
10
Number of cycles Fig. 5. Undercooling vs. number of cycles for pure Cu.
the sample above its equilibrium melting point and holding at this temperature for a certain period of time, followed by cooling at a rate from 0.08 to 0.5 K s−1 . From the results shown in Fig. 4 for the Cu–3 wt.% Sn alloy, it can be seen that the lower the cooling rate the higher the undercooling achieved before spontaneous nucleation took place. However, the most efficient parameter that affected the undercoolability of the studied systems, proved to be the number of thermal cycles applied prior to nucleation. The undercooling experiments involved three main stages: heating to melting the sample, superheating up to 1473 K for 2400 s and cooling at a rate of 0.1 K s−1 . Additional thermal cycles, following the established procedure, were performed in order to try and increase the undercooling of the melt. After the desired number of cycles had been reached the system was cooled down at a preliminary determined cooling rate until nucleation occurred spontaneously. The results presented in Fig. 5 for pure Cu reveal a very interesting trend. Yet again, as in the previously described parameters, the undercooling increases smoothly, following a linear relationship, up to the sixth cycle, but then remains constant or slightly decreases.
Several factors were identified to be influential on the levels of melt undercooling that can be achieved by using the fluxing technique: superheating time and temperature, cooling rate and thermal cycling. It was found that a minimum superheating temperature of 40 K and a minimum superheating time of 600 s are required in order to achieve any undercooling. This may be explained by considering that full melting of some of the contaminants present in the sample does not occur below these temperatures and times. The further increase in undercooling is most probably due to the dissolution of any impurities present in the melt and complete encasement of the sample by the glass flux. During the study of the effect of experimental variables on the levels of undercooling it also became clear that thermal cycling in a combination with any other parameter, was the most influential factor in the fluxing experiments. Costa Agra Mello and Kiminami [5] used a similar technique to examine the effect of thermal cycling on the undercoolability of copper melts and observed similar trends. However, they did not suggest any reasons why the maximum undercooling is always achieved after six cycles. One possible answer to this question might be that by re-heating (cycling) the sample further formation of surface oxides could be expected, that would not be dissolved in the glass flux, as it is already saturated with the ones formed during earlier cycles. 5. Conclusions In conclusion, it can be said that the presented results clearly demonstrate the importance of experimental parameters on the levels of undercooling of metallic melts by using the fluxing approach. Thus, it was possible to achieve undercooling of up to 352 K in pure Cu and 328 K and 320 K in Cu–O and Cu3 wt.% Sn alloys, respectively. The present values are significantly higher than the ones reported previously in the literature for these particular metal systems. References [1] H. Jones, Rapid Solidification of Metals and Alloys, Institute of Metals, London, 1982, p. 3. [2] Z. Esslinger, Metals 57 (1996) 106. [3] D.M. Herlach, R.F. Cochrane, I. Ergy, H.J. Fecht, A.L. Greer, Int. Mater. Rev. 38 (1996) 6. [4] S.E. Battersby, R.F. Cochrane, A.M. Mullis, J. Mater. Sci. 35 (2000) 1365. [5] M. Costa Agra Mello, C.S. Kiminami, J. Mater. Sci. Lett. 8 (1989) 1416.
The effect of experimental variables on the levels of melt undercooling Kalin I. Dragnevski, Andrew M. Mullis∗ , Robert F. Cochrane Department of Materials, University of Leeds, Leeds LS2 9JT, UK
Abstract A melt encasement (fluxing) technique has been used to study the effect of experimental variables, such as cooling rate, superheating time and superheating temperature, on the levels of undercooling in Cu, Cu–O and Cu–Sn melts. The maximum undercoolings achieved were 352 K for pure Cu and 328 and 320 K for Cu–O and Cu–Sn, respectively. The present results are higher than the ones previously reported for these particular systems. It was found that a minimum superheating temperature of 40 K and a minimum superheating time of 600 s are required in order to achieve undercooling of the melt prior to nucleation. However, the most efficient parameter that affected the undercoolability of the studied metal systems proved to be the number of thermal cycles applied prior to solidification. Our results clearly demonstrate the importance of the experimental parameters on the levels of undercooling of metallic melts by using a fluxing technique. © 2003 Elsevier B.V. All rights reserved. Keywords: Undercooling; Fluxing; Solidification
1. Introduction Rapid solidification of metals and alloys has developed in the past few decades from an insignificant activity into major industrial practice and important process for research studies [1]. Rapid solidification is generally brought about by one of the following two methods. The first is by rapid quenching from the melt, e.g. melt spinning, planar flow casting, splat cooling, etc. In this way, the heat is extracted through a steep gradient in the solid and so the sample is required to be thin in one dimension [2]. Even though a variety of novel products have emerged from these methods, they are not suitable for direct analysis of the processes involved in the solidification of the melt. Moreover, rapid quenching is a method that allows little control over the conditions during processing. Consequently, conclusions of the development of the system are usually deduced post mortem from an examination of the as solidified products [3]. The second method involves undercooling the liquid by a large degree below its melting temperature prior to nucleation. In this way, a large driving force for rapid solidification accumulates due to the large difference in Gibbs free energy between the solid and the liquid state. The exper∗
Corresponding author. Fax: +44-113-242-2531. E-mail address: [email protected] (A.M. Mullis).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.050
imental techniques are essentially based on the avoidance of any potential sites for heterogeneous nucleation, such as crucible walls, which allows the preservation of the liquid in the metastable undercooled condition for extended periods of time. These alternative methods for rapid dendritic growth include melt fluxing, drop tube processing and the levitation techniques and are commonly known as containerless processes or denucleation techniques [3]. The main advantage of these alternative methods for non-equilibrium solidification is that they allow direct monitoring of rapid solidification and even external stimulation of the process itself, thus enabling investigations in the early stages of crystallization. In this study, a melt encasement fluxing technique was used to study the effect of experimental variables on the levels of melt undercooling in pure Cu, Cu–O alloy (dilute alloy) and Cu–3 wt.% Sn alloy (more concentrated alloy). These particular systems were chosen also with the aim to determine whether alloy composition influences the levels of undercooling achieved during the fluxing experiments.
2. Experimental procedure Melt fluxing was chosen as the most suitable means to study the alloy systems under consideration, as the melting
486
K.I. Dragnevski et al. / Materials Science and Engineering A 375–377 (2004) 485–487
3. Results
Undercooling/ K
point of Cu-based alloys is generally too low for significant undercoolings to be achieved by electromagnetic levitation. The method allows high undercoolings to be achieved as nucleation on container walls is prevented by isolating the melt from these surfaces. The flux can also aid the removal of oxide impurities from the melt by dissolution in the flux and protects the surface from oxidation. Undercooling experiments were performed within a stainless steel vacuum chamber evacuated to a pressure of 10−6 mbar and backfilled to 500 mbar with N2 gas. Samples were heated, in silica crucibles, by induction heating of a graphite susceptor contained within an alumina radiation shield. A commercial soda-lime glass was employed as the flux to reduce the number of potential sites for heterogeneous nucleation. The temperature was monitored by means of a K-type thermocouple positioned at the base of the crucible, which had been thinned, thus reducing the thermal lag between the sample and the thermocouple. Heating and cooling curves were obtained by using a chart recorder. A schematic diagram of the fluxing apparatus is shown in Fig. 1. By heating the sample to its melting temperature, cooling and repeating this procedure, it was found that the temperatures were reproducible to within 5 K. By using this method it was possible to achieve temperatures of up to 1473 K, thus easily accommodating melting and superheating of the alloy systems that were to be investigated, ensuring that the samples were completely contained within the glass flux. Copper samples of 99.9999% purity (metal basis) were obtained from ALFA (Johnson Matthey). However, these samples contain a residual level of oxygen. Based on the work of previous researchers [4], we have defined Cu–O alloys as Cu containing at least 200 ppm of oxygen after undercooling and oxygen free copper as containing less than 200 ppm of oxygen. A Cu–3 wt.% Sn alloy was prepared by arc melting under argon. All starting materials were obtained from ALFA (Johnson Matthey) and were of 99.9999% purity. In order to ensure complete mixing of the elements and to eliminate any segregation the alloy was re-melted and finally annealed at 1023 K for 1800 s.
300 250 200 150 100 50 0 1300
1320
1340
1360
1380
1400
1420
1440
Temperature/ K
Fig. 2. Undercooling as function of superheating temperature for Cu– 3 wt.% Sn alloy.
300 250 Undercooling/ K
Fig. 1. Schematic diagram of the fluxing apparatus.
In order to determine the influence of superheating time and temperature on the levels of melt undercooling, two different experiments were conducted. The first involved superheating the samples by increasing amounts, cooling and recording the undercooling at which nucleation occurred spontaneously (Fig. 2). In the cases of both Cu and Cu–Sn alloys, it was found that a minimum superheating temperature of 40 K was required in order to achieve any undercooling prior to nucleation. Above this superheating temperature the undercooling increases following almost a linear relationship. The second experiment involved the study of the effect of superheating time on the levels of melt undercooling. Superheating the samples by a preliminary known amount above their melting point and then holding the melt at that temperature for different periods of time before cooling until spontaneous nucleation occurred, monitored the effect of this particular experimental variable. The results from this experiment are shown in Fig. 3 for Cu–O alloy. As seen from the graph, a minimum superheating time of 600 s is required to achieve any undercooling. Above this holding time, the undercooling increases smoothly, as in the case for the previously described parameter, following almost a linear relationship. The effect of cooling rate on the levels of undercooling was studied in an experiment that involved superheating
200 150 100 50 0 0
300 600 900 1200 1500 1800 2100 2400 2700 3000 Superheating time/s
Fig. 3. Effect of superheating time on the levels of undercooling for Cu–O alloy.
K.I. Dragnevski et al. / Materials Science and Engineering A 375–377 (2004) 485–487
487
4. Discussion
160 Undercooling/ K
140 120 100 80 60 40 20 0 0
0.1
0.2
0.3
0.4
Cooling rate/K s
0.5
0.6
-1
Undercooling/K
Fig. 4. Variation of undercooling with cooling rate for Cu–3 wt.% Sn alloy.
400 300 200 100 0 0
2
4
6
8
10
Number of cycles Fig. 5. Undercooling vs. number of cycles for pure Cu.
the sample above its equilibrium melting point and holding at this temperature for a certain period of time, followed by cooling at a rate from 0.08 to 0.5 K s−1 . From the results shown in Fig. 4 for the Cu–3 wt.% Sn alloy, it can be seen that the lower the cooling rate the higher the undercooling achieved before spontaneous nucleation took place. However, the most efficient parameter that affected the undercoolability of the studied systems, proved to be the number of thermal cycles applied prior to nucleation. The undercooling experiments involved three main stages: heating to melting the sample, superheating up to 1473 K for 2400 s and cooling at a rate of 0.1 K s−1 . Additional thermal cycles, following the established procedure, were performed in order to try and increase the undercooling of the melt. After the desired number of cycles had been reached the system was cooled down at a preliminary determined cooling rate until nucleation occurred spontaneously. The results presented in Fig. 5 for pure Cu reveal a very interesting trend. Yet again, as in the previously described parameters, the undercooling increases smoothly, following a linear relationship, up to the sixth cycle, but then remains constant or slightly decreases.
Several factors were identified to be influential on the levels of melt undercooling that can be achieved by using the fluxing technique: superheating time and temperature, cooling rate and thermal cycling. It was found that a minimum superheating temperature of 40 K and a minimum superheating time of 600 s are required in order to achieve any undercooling. This may be explained by considering that full melting of some of the contaminants present in the sample does not occur below these temperatures and times. The further increase in undercooling is most probably due to the dissolution of any impurities present in the melt and complete encasement of the sample by the glass flux. During the study of the effect of experimental variables on the levels of undercooling it also became clear that thermal cycling in a combination with any other parameter, was the most influential factor in the fluxing experiments. Costa Agra Mello and Kiminami [5] used a similar technique to examine the effect of thermal cycling on the undercoolability of copper melts and observed similar trends. However, they did not suggest any reasons why the maximum undercooling is always achieved after six cycles. One possible answer to this question might be that by re-heating (cycling) the sample further formation of surface oxides could be expected, that would not be dissolved in the glass flux, as it is already saturated with the ones formed during earlier cycles. 5. Conclusions In conclusion, it can be said that the presented results clearly demonstrate the importance of experimental parameters on the levels of undercooling of metallic melts by using the fluxing approach. Thus, it was possible to achieve undercooling of up to 352 K in pure Cu and 328 K and 320 K in Cu–O and Cu3 wt.% Sn alloys, respectively. The present values are significantly higher than the ones reported previously in the literature for these particular metal systems. References [1] H. Jones, Rapid Solidification of Metals and Alloys, Institute of Metals, London, 1982, p. 3. [2] Z. Esslinger, Metals 57 (1996) 106. [3] D.M. Herlach, R.F. Cochrane, I. Ergy, H.J. Fecht, A.L. Greer, Int. Mater. Rev. 38 (1996) 6. [4] S.E. Battersby, R.F. Cochrane, A.M. Mullis, J. Mater. Sci. 35 (2000) 1365. [5] M. Costa Agra Mello, C.S. Kiminami, J. Mater. Sci. Lett. 8 (1989) 1416.