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Substructural developments during recovery in EC aluminum
Scripta METALLURGICA
Vol. 13, pp. 345-348, Printed in the U.S.A.
1979
Pergamon Press Ltd. All rights reserved,
SUBSTRUCTURAL DEVELOPMENTSDURING RECOVERY IN EC ALUMINUM
S. K. Varma Mechanical Engineering Department, Texas A&M University College Station, Texas 77843, USA (Received February i, 1979)
Introduction I t is now f a i r l y well recognized that substructural strengthening plays an important role in the applications of many metals (particularly for high stacking fault energy metals like aluminum). One of the convenient ways of controlling the subgrain or cell size is a strainanneal method. Sandstrom has proposed a model for subgrain growth in metals and alloys during annealing (1,2). He has also shown experimentally, with the help of insitu experiments in HVEN, that parabolic growth of subgrain size during annealing of aluminum is in agreement with his proposed model (3). The operating mechanism in pure aluminum of four nine purity was considered to be the dissolution of subgrain boundaries without their migration at lower recovery temperatures. On the other hand, both migration and dissolution of the subgrain boundaries control the subgrain growth at higher recovery temperatures. The l a t t e r conclusion is in agreement with the observations e a r l i e r made by Fujita (4). The purpose of this paper is to add some more experimental evidence to the v a l i d i t y of Sandstrom's model. Isochronal annealing has been carried out from 150 to 370°C for one hour to study the mechanisms involved for subgrain growth in EC aluminum. The results have been compared with those obtained by Sandstrom in a semi-quantitative manner. Experimental Details Electrical conductor (EC) grade aluminum was used for the study and the main impurities were Si and Fe (0.05% and 0.075% by weight respectively) which were present in soluble and insoluble forms respectively. A rod of EC aluminum with a diameter of 9.52 mmwas annealed for 3 hours at 425°C and cooled to room temperature in air. This rod was cold drawn to 2.59 mm wire in silicon carbide dies using several successive reductions. The wires were annealed at different temperatures between 150 and 370°C for one hour in a salt bath followed by a i r cooling to room temperature. A JEOL (lOOC) electron microscope was used to characterize the microstructures on the cross section of the wires at lO0 KV. The details of thin f o i l preparation have been described elsewhere (5). The subgrain sizes were measured by the linear intercept method on enlarged prints of the electron micrographs. The intercept length has been reported as subgrain in size here, though actual subgrain size is obtained by multiplying the intercept length by 1.68 assuming a tetrakaidecahedron geometry of subgrains (5). A minimum of 500 subgrains were used for the subgrain size measurements at each one of the annealing temperatures. Results and Discussion I t has been observed that subgrain size increases continuously from 150 to 230°C during isochronal annealing of EC aluminum with a prior true wire drawing strain of 2.6. The values
345 0036-9748/79/130345~04502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
346
RECOVERY
IN ALUMINUM
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of subgrain sizes at 150, 175, 200, 230 and 260°C were 0.329, 0.418, 0.0442, 0.551 and 0.965 um respectively. The scatter in the data for subgrain size measurements was not more than ± I0%. The onset of recrystallization that occurs between 230 and 260°C was confirmed by the measurements of mechanical properties. Representative micrographs showing the substructures developed at 175, 200, 230 and 260°C is shown in Figure I. The figure also includes the substructures observed in cold worked EC aluminum (cell size = 0.247 um) showing the presence of cells with sharp boundaries and almost free of internal dislocations. I t is an indication of the extent of dynamic recovery that EC aluminum has undergone during wire drawing process. The onset of recrystallization is marked by the presence of large nuclei and a complete recrystallization with the absence of subgrains has been observed for temperatures equal to or greater than 290°C. According to Sandstrom (3), the increase in subgrain size during annealing can be described by a parabolic growth law, D2 = D2 + kt (1) o where DO = i n i t i a l cell size t = time of annealing k = temperature dependent constant also (a) k ~ k I = 3MTs, when the subgrain growth occurs by boundary migration, and (b) k-~k 2 = 8M~, when the subgrain growth occurs by dislocation migration in the boundary (6)T where M = mobility of the dislocations ~s = line tension of the dislocations Since subgrain boundary migration and dislocation migration in sub-boundary usually appear together, Sandstrom has added the two k's ~i.e. kI and k2) so that k = lIMTs. The line tension of the dislocations in the subgrain boundaries has been estimated to be Gb2 Ts = 3.5 (2) where G = shear modulus b = Burgers vector The values of G and b for aluminum are 2.618 x lOlO N/m2 and 2.86 x lO-lO m respectively. Putting these values in equation (2) yields a value of Ts = 6.12 x lO-lO N. In order to predict the values of subgrain sizes at different temperatures for a fixed time for comparing with the measured experimental values in this study, the dislocation mobility factor, H, mu~ be calculated. The.values of M calculated by Sandstrom (3) for pure aluminum are 8.5 x lO- H and 3.5 x lO-9 m2N-Isec- ! at 150 and 200°C respectively. Using these values of M and the calculated value of Ts, i t is possible to make the calculation for the subgrain size that should develop in 60 minutes of annealing at these temperatures. The calculated values of subgrain sizes have been found to be 0.25 and 0.38 ~m when the i n i t i a l diameter of the subgrains in cold worked condition is taken as 0.247 ~m. The actual measurements show these values to be 0.33 and 0.44 ~m at 150 and 200°C respectively as mentioned earlier. I t is speculated that maximum error is due to the uncertainty in the values of the diffusion coefficient of the boundaries which is involved in the M factor (3). Obviously, this value is also influenced by the purity of aluminum. However, the calculated values of the subgrain sizes are close enough to the measured values to indicate that perhaps the mechanisms assumed in the model of Sandstrom are correct to a f i r s t approximation. I t must be noted, however, that only two temperatures have been analyzed for one value of t. Further work is being carried out to determine more precisely i f this analysis can hold true for different values of t also. The results w i l l be presented later. A qualitative explanation of the micrographs shown in Figure l at different temperatures indicates that even though the i n i t i a l cold worked microstructure is nearly free of internal dislocations, the subgrains contain increasing number of internal dislocations (increases with increasing temperature) up to 200°C. The subgrain boundaries become more and more perfect (free of tangles) as the annealing temperature increases, with the result that at higher temperatures the subgrains are almost completely void of internal dislocations. Considerableevidence of the dissolution of subgrain boundaries at higher temperatures can be seen in Figure I . This indicates that the dissolution of subgrain boundaries together with boundary migration is perhaps the mechanism involved for the growth of subgrains during isochronal annealing of EC aluminum at higher recovery temperatures. This is in agreement with the observations made by
Vol.
13, No.
5
RECOVERY
IN A L U M I N U M
347
Sandstrom (3) and Fujita (4). However, the presence of increasing number of internal dislocations in the subgrains at lower recovery temperatures needs a further explanation at least qualitatively. The extraction and/or emission of dislocations from the subgrain boundaries is considered to be a good possibility. Conclusions I t has been shown that Sandstrom's approach for the explanation of subgrain growth is quantitatively applicable to the subgrain sizes developed during isochronal annealing of EC aluminum in the present study. The qualitative representation of the mechanisms involved are, in general, in agreement with Sandstrom's observations in pure aluminum. References I. 2. 3. 4. 5. 6.
R. Sandstrom, Acta Met. 25 (1977) 877. R. Sandstrom, Acta Met. 25 (1977) 905. R. Sandstrom, B. Lehtinen, E. Hedman, I. Groza and S. Karlsson, J. Mat. Sci. 13 (1978) 1229. H. Fujita, J. Phys. Soc. Japan 26 (1969) 1437. D. Kalish and B. G. LeFevre, Metall. Trans. (A) 6 (1975) 1319. J . C . M . Li, J. Appl. Phys. 33 (1962) 2958.
Figure I.
The Transmission Electron Micrographs of the Substructural Development in EC Aluminum During Isochronal Annealing at (a) Cold Worked Condition, (b) 175, (c) 200, (d) 230 and (e) 260°C for one hour.
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Vol. 13, pp. 345-348, Printed in the U.S.A.
1979
Pergamon Press Ltd. All rights reserved,
SUBSTRUCTURAL DEVELOPMENTSDURING RECOVERY IN EC ALUMINUM
S. K. Varma Mechanical Engineering Department, Texas A&M University College Station, Texas 77843, USA (Received February i, 1979)
Introduction I t is now f a i r l y well recognized that substructural strengthening plays an important role in the applications of many metals (particularly for high stacking fault energy metals like aluminum). One of the convenient ways of controlling the subgrain or cell size is a strainanneal method. Sandstrom has proposed a model for subgrain growth in metals and alloys during annealing (1,2). He has also shown experimentally, with the help of insitu experiments in HVEN, that parabolic growth of subgrain size during annealing of aluminum is in agreement with his proposed model (3). The operating mechanism in pure aluminum of four nine purity was considered to be the dissolution of subgrain boundaries without their migration at lower recovery temperatures. On the other hand, both migration and dissolution of the subgrain boundaries control the subgrain growth at higher recovery temperatures. The l a t t e r conclusion is in agreement with the observations e a r l i e r made by Fujita (4). The purpose of this paper is to add some more experimental evidence to the v a l i d i t y of Sandstrom's model. Isochronal annealing has been carried out from 150 to 370°C for one hour to study the mechanisms involved for subgrain growth in EC aluminum. The results have been compared with those obtained by Sandstrom in a semi-quantitative manner. Experimental Details Electrical conductor (EC) grade aluminum was used for the study and the main impurities were Si and Fe (0.05% and 0.075% by weight respectively) which were present in soluble and insoluble forms respectively. A rod of EC aluminum with a diameter of 9.52 mmwas annealed for 3 hours at 425°C and cooled to room temperature in air. This rod was cold drawn to 2.59 mm wire in silicon carbide dies using several successive reductions. The wires were annealed at different temperatures between 150 and 370°C for one hour in a salt bath followed by a i r cooling to room temperature. A JEOL (lOOC) electron microscope was used to characterize the microstructures on the cross section of the wires at lO0 KV. The details of thin f o i l preparation have been described elsewhere (5). The subgrain sizes were measured by the linear intercept method on enlarged prints of the electron micrographs. The intercept length has been reported as subgrain in size here, though actual subgrain size is obtained by multiplying the intercept length by 1.68 assuming a tetrakaidecahedron geometry of subgrains (5). A minimum of 500 subgrains were used for the subgrain size measurements at each one of the annealing temperatures. Results and Discussion I t has been observed that subgrain size increases continuously from 150 to 230°C during isochronal annealing of EC aluminum with a prior true wire drawing strain of 2.6. The values
345 0036-9748/79/130345~04502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
346
RECOVERY
IN ALUMINUM
Vol.
13, No.
5
of subgrain sizes at 150, 175, 200, 230 and 260°C were 0.329, 0.418, 0.0442, 0.551 and 0.965 um respectively. The scatter in the data for subgrain size measurements was not more than ± I0%. The onset of recrystallization that occurs between 230 and 260°C was confirmed by the measurements of mechanical properties. Representative micrographs showing the substructures developed at 175, 200, 230 and 260°C is shown in Figure I. The figure also includes the substructures observed in cold worked EC aluminum (cell size = 0.247 um) showing the presence of cells with sharp boundaries and almost free of internal dislocations. I t is an indication of the extent of dynamic recovery that EC aluminum has undergone during wire drawing process. The onset of recrystallization is marked by the presence of large nuclei and a complete recrystallization with the absence of subgrains has been observed for temperatures equal to or greater than 290°C. According to Sandstrom (3), the increase in subgrain size during annealing can be described by a parabolic growth law, D2 = D2 + kt (1) o where DO = i n i t i a l cell size t = time of annealing k = temperature dependent constant also (a) k ~ k I = 3MTs, when the subgrain growth occurs by boundary migration, and (b) k-~k 2 = 8M~, when the subgrain growth occurs by dislocation migration in the boundary (6)T where M = mobility of the dislocations ~s = line tension of the dislocations Since subgrain boundary migration and dislocation migration in sub-boundary usually appear together, Sandstrom has added the two k's ~i.e. kI and k2) so that k = lIMTs. The line tension of the dislocations in the subgrain boundaries has been estimated to be Gb2 Ts = 3.5 (2) where G = shear modulus b = Burgers vector The values of G and b for aluminum are 2.618 x lOlO N/m2 and 2.86 x lO-lO m respectively. Putting these values in equation (2) yields a value of Ts = 6.12 x lO-lO N. In order to predict the values of subgrain sizes at different temperatures for a fixed time for comparing with the measured experimental values in this study, the dislocation mobility factor, H, mu~ be calculated. The.values of M calculated by Sandstrom (3) for pure aluminum are 8.5 x lO- H and 3.5 x lO-9 m2N-Isec- ! at 150 and 200°C respectively. Using these values of M and the calculated value of Ts, i t is possible to make the calculation for the subgrain size that should develop in 60 minutes of annealing at these temperatures. The calculated values of subgrain sizes have been found to be 0.25 and 0.38 ~m when the i n i t i a l diameter of the subgrains in cold worked condition is taken as 0.247 ~m. The actual measurements show these values to be 0.33 and 0.44 ~m at 150 and 200°C respectively as mentioned earlier. I t is speculated that maximum error is due to the uncertainty in the values of the diffusion coefficient of the boundaries which is involved in the M factor (3). Obviously, this value is also influenced by the purity of aluminum. However, the calculated values of the subgrain sizes are close enough to the measured values to indicate that perhaps the mechanisms assumed in the model of Sandstrom are correct to a f i r s t approximation. I t must be noted, however, that only two temperatures have been analyzed for one value of t. Further work is being carried out to determine more precisely i f this analysis can hold true for different values of t also. The results w i l l be presented later. A qualitative explanation of the micrographs shown in Figure l at different temperatures indicates that even though the i n i t i a l cold worked microstructure is nearly free of internal dislocations, the subgrains contain increasing number of internal dislocations (increases with increasing temperature) up to 200°C. The subgrain boundaries become more and more perfect (free of tangles) as the annealing temperature increases, with the result that at higher temperatures the subgrains are almost completely void of internal dislocations. Considerableevidence of the dissolution of subgrain boundaries at higher temperatures can be seen in Figure I . This indicates that the dissolution of subgrain boundaries together with boundary migration is perhaps the mechanism involved for the growth of subgrains during isochronal annealing of EC aluminum at higher recovery temperatures. This is in agreement with the observations made by
Vol.
13, No.
5
RECOVERY
IN A L U M I N U M
347
Sandstrom (3) and Fujita (4). However, the presence of increasing number of internal dislocations in the subgrains at lower recovery temperatures needs a further explanation at least qualitatively. The extraction and/or emission of dislocations from the subgrain boundaries is considered to be a good possibility. Conclusions I t has been shown that Sandstrom's approach for the explanation of subgrain growth is quantitatively applicable to the subgrain sizes developed during isochronal annealing of EC aluminum in the present study. The qualitative representation of the mechanisms involved are, in general, in agreement with Sandstrom's observations in pure aluminum. References I. 2. 3. 4. 5. 6.
R. Sandstrom, Acta Met. 25 (1977) 877. R. Sandstrom, Acta Met. 25 (1977) 905. R. Sandstrom, B. Lehtinen, E. Hedman, I. Groza and S. Karlsson, J. Mat. Sci. 13 (1978) 1229. H. Fujita, J. Phys. Soc. Japan 26 (1969) 1437. D. Kalish and B. G. LeFevre, Metall. Trans. (A) 6 (1975) 1319. J . C . M . Li, J. Appl. Phys. 33 (1962) 2958.
Figure I.
The Transmission Electron Micrographs of the Substructural Development in EC Aluminum During Isochronal Annealing at (a) Cold Worked Condition, (b) 175, (c) 200, (d) 230 and (e) 260°C for one hour.
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