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Rapid solidification of AgCu and AgPb alloys
Materials Science and Engineering, A133 (1991 ) 574-576
574
Rapid solidification of Ag-Cu and Ag-Pb alloys Nils Jacobson Department of Casting of Metals, Royal Institute of Technology, Stockholm (Sweden)
Abstract A series of experiments on Ag-Cu and Ag-Pb alloys were performed in a wide range of compositions from 10 wt.% Cu to 25 wt.% Cu and from 1.2 wt.% Pb to 18 wt.% Pb. Each composition was solidified by three different cooling rates 103, 3 x 103 K s- 1 and 105 K s- 1. The Ag-Cu system showed a dendritic solidification with the lowest cooling rate and a diffusionless solidification with the highest cooling rate. The Ag-Pb system solidified in a cellular or dendritic morphology at all cooling rates and no diffusionless solidification was detected.
1. Introduction In rapidly solidified materials different types of micro-structural changes can be observed. Metals with one or two alloying elements often show a dendritic type of structure at low cooling rates. When increasing the cooling rate, the structure changes to a cellular type and in some cases to a diffusionless solidification type. These types of transition had earlier been observed in the Ag-Cu alloys [1-5]. However, in many other systems this type of transition is not observed. It has been reported [6] that it is difficult to obtain this type of transition in A g - P b alloys. The A g - P b system has the same character as the Ag-Cu system, therefore it would be interesting to make comparable experiments in the two systems to obtain more information about factors influencing the transition from cellular to diffusionless growth.
(2) Levitation melting and casting into a silver mold with a 0.5 mm thick rectangular crosssection. (3) Melt-spinning on a copper-wheel with a circumferential velocity of 10 m s- 1. The experimental procedures in the two first modes were as follows. A sample of 1.5 g was melted in an argon atmosphere and thereafter quenched into the molds. An open thermocouple made of 0.1 mm thin wires of cromel-alurnel type was placed in the bottom of the mold. The cooling curve of the sample was recorded with a memory oscilloscope. The cooling rates were l × 103 K s -1 and 3 x 103 K s - l , respectively. In the last mode the cooling rate was approximately l 0 s K s-~. The samples were metallurgically prepared for observation in optical and scanning electron microscopes where the final compositions of all alloys were analysed with EDS.
2. Experimental
3. Results
The eight alloys investigated were made by melting elements with 99.9 wt.% purity in a low frequency induction furnace. All eight alloys were quenched with three different cooling rates in three different quenching modes achieved by the following methods [11]. (1) Levitation melting and casting into a copper mold with a 5 mm thick cylindrical crosssection.
Ag-l.2wt.%Pb. This sample was solidified dendritically in the two first quenching modes. In the third mode a cellular structure was formed. A small microsegregation could be observed in the first mode, but not in the second or third quenching modes. Ag-4.3wt. %Pb. The structure in this aUoywas dendritic in the two first quenching modes and in the third mode the structure was more cellular
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Fig. 1. SEM picture, back-scattered electron image, Ag-6.4wt.%Pb, mode I, coolingrate 1 0 3 K s- 1.
Fig. 2. SEM picture, back-scattered electron image, Ag-18wt.% Pb, mode I, cooling rate 1 0 3 K s- 1.
than dendritic. In all modes one could observe a secondary phase rich in lead in the inter-dendritic areas. Ag-6.4wt. %Pb. In this alloy one finds a welldeveloped dendritic structure in all quenching modes. Microsegregation could be detected in all modes. The alloy consists of two phases and the secondary phase was rich in lead (Fig. 1). Ag-18.0wt.%Pb. This alloy showed a similar structure to the one discussed above but the fraction of the secondary phase increased (Fig. 2). Ag-lOwt.%Cu. This alloy shows a dendritic structure in the two first quenching modes, and microsegregation was also detected in both sampies. A secondary phase rich in copper could be detected in these samples. In the third mode the structure was hard to identify but it is best described as irregular cellular. No microsegregation could be observed in this sample.
Fig. 3. SEM picture, back-scattered electron image, Ag-20wt.%Cu, mode II, cooling rate 3 x 1 0 3 K s 1.
Ag-15wt. %Cu. These samples show identical structures to the ones mentioned above in all modes. One could clearly detect microsegregation in the first two modes but none in the third. It is hard to say if a eutectic phase has been formed in the inter-dendritic areas or if it is just the a-phase rich in copper. Ag-2Owt. %Cu. In this alloy the primary silverrich phase has grown dendritically; the interdendritic areas in the two first quenching modes have a high copper content and consist of a eutectic secondary phase (Fig. 3). The amount of the eutectic is higher in mode I than in mode II. In the third mode the alloy has solidified cellularly. No difference in composition was found between the cells and their boundaries. Ag-25wt. %Cu. These samples show the same types of structure in the first two modes as in the ones earlier discussed, and the fraction of the secondary eutectic phase increased. The third mode showed a cellular solidification with a change in coarseness in the middle of the ribbon. No microsegregation could be detected. However, a precipitation probably formed during the solid state cooling could be detected (Fig. 4). 4. Discussion
A number of investigations have been performed with rapid solidification techniques on Ag-Cu alloys and on A g - P b alloys [1-6]. Our experiments confirm earlier observations that it is easier to obtain diffusionless solidification in Ag-Cu alloys than in A g - P b alloys. In the latter case, this transition was only observed in the
576
from the two systems shows a clear difference in solidification behaviour. In the Cu-Ag system the primary silver phase has a greater tendency to extend its maximum solubility and to solidify diffusionless compared with the Ag-Pb system. The thermodynamics of these two systems do not differ much and therefore the different solidification behaviour cannot be easily explained by the theories existing today. In our opinion the different behaviour of the two systems can be influenced by the interface kinetics and the solid-liquid interface structure [11]. Fig. 4. SEM picture, back-scattered electron Ag-25wt.%Cu, mode Ill, cooling rate 105 K s- i.
image,
alloys with the lowest lead content. A number of theoretical investigations concerned with the transition from cellular to diffusionless solidification have been performed [7-10]. One of the latest was performed on melt-spun ribbons by JSnsson [10]. According to J6nsson one should obtain a banded structure of silver cells and diffusionless material in melt-spun Ag-Cu alloys in a similar way as observed in other works [1-3] made in electron beam or laser experiments. However, in our experiments we have not observed a banded structure formed by solidification, but a structure similar to the banded structure that Thoma [4] has seen in melt-spun Ag-Cu alloys could be observed in one of the samples. This banded structure is probably formed by a solid-state reaction and shows similarity with the structure that Boettinger et al. [1] describe as spinodal decomposition. In the experiments a gradual transition from cellular to a diffusionless solidification was observed and not a sharp one. As stressed earlier, a comparison of the results
Acknowledgment The author is grateful to Professor H. Fredriksson for valuable help.
References 1 W.J. Boettinger, D. Shechtman, R. J. Schaefer and E S. Biancaniello, Metall. Trans. A, 15A (1984) 55. 2 D. G. Beck, S. M. Copley and M. Bass, Metall. Trans. A, 13A (1982)1879. 3 D. G. Beck, S. M. Copley and M. Bass, Metall. Trans. A, 12A (1981) 1981. 4 W.A. Elliott, E P. Gagliano and G. Krauss, Metall. Trans. A, 4 (1973) 2031. 5 H. Fredriksson, B. Ramaeus and I. Svensson, Solidification Processing ( 1987) 518. 6 D.J. Thoma, T. K. Glasgow, S. N. Tewari, J. H. Perepezko and N. Jayaraman, Mater. Sci. Eng., 98(1988) 89-93. 7 M. Hilert and B. Sundman, Metall. Trans. A, 25 (1977) 11. 8 J. Agren, Metall. Trans. A, 37(1989) 181. 9 W. J. Boettinger and M. J. Aziz, Metall. Trans. A, 37 (1989) 3379. 10 B. J6nsson, TRITA-MAC-0415, Internal report, Div. of Phys. Met., Royal Institute of Technology, Stockholm, Sweden, January, 1990. 11 J. Liu, N. Jacobson and H. Fredriksson, TRITA-MAC0395, Internal report, Div. of Casting of Metals, Royal Institute of Technology, Stockholm, Sweden, March 1989.
574
Rapid solidification of Ag-Cu and Ag-Pb alloys Nils Jacobson Department of Casting of Metals, Royal Institute of Technology, Stockholm (Sweden)
Abstract A series of experiments on Ag-Cu and Ag-Pb alloys were performed in a wide range of compositions from 10 wt.% Cu to 25 wt.% Cu and from 1.2 wt.% Pb to 18 wt.% Pb. Each composition was solidified by three different cooling rates 103, 3 x 103 K s- 1 and 105 K s- 1. The Ag-Cu system showed a dendritic solidification with the lowest cooling rate and a diffusionless solidification with the highest cooling rate. The Ag-Pb system solidified in a cellular or dendritic morphology at all cooling rates and no diffusionless solidification was detected.
1. Introduction In rapidly solidified materials different types of micro-structural changes can be observed. Metals with one or two alloying elements often show a dendritic type of structure at low cooling rates. When increasing the cooling rate, the structure changes to a cellular type and in some cases to a diffusionless solidification type. These types of transition had earlier been observed in the Ag-Cu alloys [1-5]. However, in many other systems this type of transition is not observed. It has been reported [6] that it is difficult to obtain this type of transition in A g - P b alloys. The A g - P b system has the same character as the Ag-Cu system, therefore it would be interesting to make comparable experiments in the two systems to obtain more information about factors influencing the transition from cellular to diffusionless growth.
(2) Levitation melting and casting into a silver mold with a 0.5 mm thick rectangular crosssection. (3) Melt-spinning on a copper-wheel with a circumferential velocity of 10 m s- 1. The experimental procedures in the two first modes were as follows. A sample of 1.5 g was melted in an argon atmosphere and thereafter quenched into the molds. An open thermocouple made of 0.1 mm thin wires of cromel-alurnel type was placed in the bottom of the mold. The cooling curve of the sample was recorded with a memory oscilloscope. The cooling rates were l × 103 K s -1 and 3 x 103 K s - l , respectively. In the last mode the cooling rate was approximately l 0 s K s-~. The samples were metallurgically prepared for observation in optical and scanning electron microscopes where the final compositions of all alloys were analysed with EDS.
2. Experimental
3. Results
The eight alloys investigated were made by melting elements with 99.9 wt.% purity in a low frequency induction furnace. All eight alloys were quenched with three different cooling rates in three different quenching modes achieved by the following methods [11]. (1) Levitation melting and casting into a copper mold with a 5 mm thick cylindrical crosssection.
Ag-l.2wt.%Pb. This sample was solidified dendritically in the two first quenching modes. In the third mode a cellular structure was formed. A small microsegregation could be observed in the first mode, but not in the second or third quenching modes. Ag-4.3wt. %Pb. The structure in this aUoywas dendritic in the two first quenching modes and in the third mode the structure was more cellular
0921-5093/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
575
Fig. 1. SEM picture, back-scattered electron image, Ag-6.4wt.%Pb, mode I, coolingrate 1 0 3 K s- 1.
Fig. 2. SEM picture, back-scattered electron image, Ag-18wt.% Pb, mode I, cooling rate 1 0 3 K s- 1.
than dendritic. In all modes one could observe a secondary phase rich in lead in the inter-dendritic areas. Ag-6.4wt. %Pb. In this alloy one finds a welldeveloped dendritic structure in all quenching modes. Microsegregation could be detected in all modes. The alloy consists of two phases and the secondary phase was rich in lead (Fig. 1). Ag-18.0wt.%Pb. This alloy showed a similar structure to the one discussed above but the fraction of the secondary phase increased (Fig. 2). Ag-lOwt.%Cu. This alloy shows a dendritic structure in the two first quenching modes, and microsegregation was also detected in both sampies. A secondary phase rich in copper could be detected in these samples. In the third mode the structure was hard to identify but it is best described as irregular cellular. No microsegregation could be observed in this sample.
Fig. 3. SEM picture, back-scattered electron image, Ag-20wt.%Cu, mode II, cooling rate 3 x 1 0 3 K s 1.
Ag-15wt. %Cu. These samples show identical structures to the ones mentioned above in all modes. One could clearly detect microsegregation in the first two modes but none in the third. It is hard to say if a eutectic phase has been formed in the inter-dendritic areas or if it is just the a-phase rich in copper. Ag-2Owt. %Cu. In this alloy the primary silverrich phase has grown dendritically; the interdendritic areas in the two first quenching modes have a high copper content and consist of a eutectic secondary phase (Fig. 3). The amount of the eutectic is higher in mode I than in mode II. In the third mode the alloy has solidified cellularly. No difference in composition was found between the cells and their boundaries. Ag-25wt. %Cu. These samples show the same types of structure in the first two modes as in the ones earlier discussed, and the fraction of the secondary eutectic phase increased. The third mode showed a cellular solidification with a change in coarseness in the middle of the ribbon. No microsegregation could be detected. However, a precipitation probably formed during the solid state cooling could be detected (Fig. 4). 4. Discussion
A number of investigations have been performed with rapid solidification techniques on Ag-Cu alloys and on A g - P b alloys [1-6]. Our experiments confirm earlier observations that it is easier to obtain diffusionless solidification in Ag-Cu alloys than in A g - P b alloys. In the latter case, this transition was only observed in the
576
from the two systems shows a clear difference in solidification behaviour. In the Cu-Ag system the primary silver phase has a greater tendency to extend its maximum solubility and to solidify diffusionless compared with the Ag-Pb system. The thermodynamics of these two systems do not differ much and therefore the different solidification behaviour cannot be easily explained by the theories existing today. In our opinion the different behaviour of the two systems can be influenced by the interface kinetics and the solid-liquid interface structure [11]. Fig. 4. SEM picture, back-scattered electron Ag-25wt.%Cu, mode Ill, cooling rate 105 K s- i.
image,
alloys with the lowest lead content. A number of theoretical investigations concerned with the transition from cellular to diffusionless solidification have been performed [7-10]. One of the latest was performed on melt-spun ribbons by JSnsson [10]. According to J6nsson one should obtain a banded structure of silver cells and diffusionless material in melt-spun Ag-Cu alloys in a similar way as observed in other works [1-3] made in electron beam or laser experiments. However, in our experiments we have not observed a banded structure formed by solidification, but a structure similar to the banded structure that Thoma [4] has seen in melt-spun Ag-Cu alloys could be observed in one of the samples. This banded structure is probably formed by a solid-state reaction and shows similarity with the structure that Boettinger et al. [1] describe as spinodal decomposition. In the experiments a gradual transition from cellular to a diffusionless solidification was observed and not a sharp one. As stressed earlier, a comparison of the results
Acknowledgment The author is grateful to Professor H. Fredriksson for valuable help.
References 1 W.J. Boettinger, D. Shechtman, R. J. Schaefer and E S. Biancaniello, Metall. Trans. A, 15A (1984) 55. 2 D. G. Beck, S. M. Copley and M. Bass, Metall. Trans. A, 13A (1982)1879. 3 D. G. Beck, S. M. Copley and M. Bass, Metall. Trans. A, 12A (1981) 1981. 4 W.A. Elliott, E P. Gagliano and G. Krauss, Metall. Trans. A, 4 (1973) 2031. 5 H. Fredriksson, B. Ramaeus and I. Svensson, Solidification Processing ( 1987) 518. 6 D.J. Thoma, T. K. Glasgow, S. N. Tewari, J. H. Perepezko and N. Jayaraman, Mater. Sci. Eng., 98(1988) 89-93. 7 M. Hilert and B. Sundman, Metall. Trans. A, 25 (1977) 11. 8 J. Agren, Metall. Trans. A, 37(1989) 181. 9 W. J. Boettinger and M. J. Aziz, Metall. Trans. A, 37 (1989) 3379. 10 B. J6nsson, TRITA-MAC-0415, Internal report, Div. of Phys. Met., Royal Institute of Technology, Stockholm, Sweden, January, 1990. 11 J. Liu, N. Jacobson and H. Fredriksson, TRITA-MAC0395, Internal report, Div. of Casting of Metals, Royal Institute of Technology, Stockholm, Sweden, March 1989.