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Grain elongation in a superplastic 7075 Al alloy
Scripta Materialia, Vol. 41, No. 3, pp. 269 –274, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter
Pergamon
PII S1359-6462(99)00160-8
GRAIN ELONGATION IN A SUPERPLASTIC 7075 Al ALLOY Dong Hyuk Shin1, Yeon Jun Joo2, Chong Soo Lee3 and Kyung-Tae Park4 1
Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-Do, Korea 425-791 2 Division of Metals, Korea Institute of Science and Technology, Seoul, Korea 130-650 3 Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, Korea 790-784 4 Department of Advanced Materials Science and Engineering, Taejon National University of Technology, Taejon, Korea 300-717 (Received February 26, 1999) (Accepted in revised form May 4, 1999)
Introduction Grain growth during superplastic deformation (SPD) of various fine-grained materials (1– 6) has often been reported to be significant. Besides grain growth, it was reported recently (5,6) that noticeable grain elongation along the stress axis takes place in the initial stage of SPD of fine-grained Al alloys. In addition, such alloys with elongated grain structures showed superplastic behavior. Almost all analytical treatments (7) developed for the mechanisms of micrograin superplasticity are based on the fact that the initially equiaxed fine grains remain nearly unchanged during SPD. Accordingly, an examination of the grain elongation behavior during SPD of fine-grained Al alloys may provide an insight into the mechanisms of micrograin superplasticity. In the present investigation, the microstructural evolution during SPD of a superplastic 7075 Al alloy was carefully examined and a plausible mechanism of grain elongation, associated with the observed microstructural evolution, was demonstrated.
Experimental A high strength 7075 Al alloy in the form of non-superplastic rolled plate was used in the present investigation. The chemical composition (in wt.%) of the alloy was 5.96 Zn, 2.4 Mg, 1.43 Cu, 0.2 Cr, 0.06 Fe, 0.05 Si and the balance, Al. In order to obtain a superplastic fine grained structure, a modified thermomechanical treatment (8) was applied to the alloy. The resultant linear intercept grain size of the alloy was 5.5 ⫾ 1.2 m. It is worth noting that the thermomechanical treatment resulted in a slightly pancake-like grain structure: the linear intercept grain size in the short transverse section was 3.9 ⫾ 0.8 m. Tensile specimens with a gauge length of 8 mm were machined from the thermomechanically treated plate such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at 789 K and a true strain rate range from 2 ⫻ 10⫺4 s⫺1 to 6 ⫻ 10⫺3 s⫺1. The grain elongation behavior was observed using specimens deformed to predetermined strains. The detailed procedure for the microstructural examination is described elsewhere (8). In order to obtain information regarding the 269
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Figure 1. The optical micrographs showing grain elongation behavior of the present 7075 Al alloy deformed at T ⫽ 798 K and ˙ I ⫽ 2 ⫻ 10⫺3 s⫺1. The stress axis is horizontal: (a) e ⫽ 0 %, (b) e ⫽ 100 %, (c) e ⫽ 200 %, (d) e ⫽ 400%, (e) e ⫽ 800 % and (f) e ⫽ 1000 %.
mechanisms associated with grain elongation in the initial stage of SPD, the strain rate sensitivity (m) was measured as a function of strain () from true stress () ⫺ true strain rate (˙ ) curves plotted on a logarithmic scale: at small strains, the uninterrupted technique was used while the strain rate change technique (9) was utilized at large strains. Results and Discussion Microstructural Evolution Figures 1 (a)⬃(f) represent the optical micrographs showing the grain elongation behavior with strain in the alloy. Grain elongation along the stress axis was significant even at relatively low strains, for example, e ⬇ 100 % (Figure 1 (b)) and 200 % (Figure 1 (c)). The grain elongation behavior was
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Figure 2. Grain aspect ratio () as a function of true strain for the present 7075 Al alloy deformed at T ⫽ 798 K and various initial strain rates.
quantified with the grain aspect ratio () plotted against true strain in Figure 2: ⫽ dL/dT where dL and dT are the linear intercept grain sizes measured along the longitudinal (i.e. parallel to the stress axis) and transverse directions, respectively. increased with strain from the initial stage of SPD and became saturated at about ⬇ 1.6: ⬇ 1.1 at ⫽ 0 and ⬇ 1.7 at ⬎ 1.6. The saturated value of (⬇ 1.7) was comparable to that reported by Ravinovich and Trifonov (5) and indicated that, at least for the present alloy, grain elongation might contribute considerably to deformation in the initial stage of SPD. From Figure 2, it is evident that grain elongation was virtually independent of strain rate. Although Li et al. (6) addressed that grain elongation is totally ascribed to dislocation creep, this finding suggests that diffusional creep is likely to be responsible for grain elongation by the following reasons. First, the test temperatures of the present study and Li et al.’s study (6) were above 0.9 Tm (Tm is the melting point of the alloy; the incipient Tm of the alloys used in both investigations is about 850 K) at which diffusional creep is dominant over dislocation creep. Second, if grain elongation is attributed to dislocation creep, would depend on strain rate since rates of dislocation viscous glide and climb which occur sequentially in dislocation creep depend on the applied stress or imposed strain rate. The formation of dispersoid free zone (DFZ) in the vicinity of grain boundaries perpendicular to the stress axis, shown in Figures 3 (a) and (b), was also observed. The formation of DFZ during SPD of fine grained Al alloys has been reported previously (8,12–15). At present, the origin of DFZ and its role on SPD remain unclear. However, the present experimental observations indicate that the origin of DFZ seems to be associated with diffusional processes since: (a) as mentioned above, the test temperature
Figure 3. An example of DFZ in the vicinity of grain boundaries perpendicular to the stress axis: T ⫽ 798 K, ˙ I ⫽ 2 ⫻ 10⫺4 s⫺1 and e ⫽ 40 %: (a) Optical micrograph and (b) TEM micrograph.
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Figure 4. Strain rate sensitivity (m) vs. true strain for the present 7075 Al alloy at T ⫽ 798 K.
was high enough to operate diffusional processes rather than dislocation creep and (b) the DFZ resides in the vicinity of grain boundaries perpendicular to the stress axis as similar to the grain shape change associated with diffusional creep. In addition, Shin et al. (15) observed the existence of DFZ at a very early stage of SPD of a 7075 Al alloy, ⬇ 20 %, under similar experimental conditions as those used in the present study. The flow stresses at such low strains are much lower than the maximum stress that is obtained in the uniaxial tensile SPD and that is used to construct the typical true stress () ⫺ true strain rate (˙ ) curves characterizing the superplastic behavior. This also supports the hypothesis that diffusional processes would be responsible for the formation of DFZ since diffusional creep and dislocation creep are independent of each other and creep rate of the former is faster than that of the latter at low stresses. Strain Rate Sensitivity In order to examine the possibility that the present microstructural evolution, i.e. grain elongation and the formation of DFZ, is attributed to diffusional processes, the strain rate sensitivity (m) was measured as a function of strain by utilizing the uninterrupted test and the strain rate change test. As shown in Figure 4, at small strains ( ⬍ 1.5), m decreased gradually with strain while m remained nearly unchanged at high strains ( ⬎ 1.5). This finding indicates that diffusional processes may play an important role on deformation of the alloy in the initial stage of SPD under the present experimental conditions. It is of interest to note that a strain exhibiting the above transition nearly coincides with that at which becomes saturated in Figure 2. It implies that, after grain structure reaches its quasi-stable state (5), other deformation mechanisms such as grain boundary sliding would be operative dominantly. Grain Elongation Li et al. (6) suggested that dislocation creep results in grain elongation along the stress axis and concurrent grain contraction along the transverse and short transverse directions. Contrarily, Ravinovich and Trifonov (5) argued that grain elongation is primarily attributed to faster migration rate of grain boundaries perpendicular to the stress axis by accelerated self diffusion and that it slows down due to an action of counter-force restoring equiaxed grain shape. Their explanation (5) is partly in harmony with the present findings. That is, the accelerated self diffusion leads to the formation of DFZ in the vicinity of grain boundaries perpendicular to the stress axis. Once the DFZ is formed, migration of grain boundaries encompassed by the DFZ would be faster than those parallel to the stress axis due to a lack of obstacles pinning grain boundary in that region, leading grain elongation. The presence of DFZ would make an additional contribution to grain elongation during SPD. Since the DFZ is a relatively
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Figure 5. The schematic illustration of grain elongation by cooperative process of the formation of dispersoid free zone due to diffusional process and locally enhanced plastic deformation at dispersoid free zone.
soft region, strain at the DFZ can be larger than that at other parts of the specimen. As SPD proceeds, stress increases until it reaches its maximum value and results in the locally enhanced plastic strain at the DFZ. Then, after grain structure reaches its quasi-stable state as suggested by Ravinovich and Trifonov (5), grain elongation ceases and grain boundary sliding becomes a dominant deformation mechanism leading to a large strain. Accordingly, it is quite possible that grain elongation observed in the initial stage of SPD in the alloy is ascribed to the cooperative process of the formation of DFZ due to diffusional processes and locally enhanced plastic deformation at the DFZ. This cooperative process is illustrated schematically in Figure 5. Summary Microstructural evolution during superplastic deformation of a fine grained 7475 Al alloy is manifested by considerable grain elongation along the stress axis and the formation of dispersoid free zone in the vicinity of grain boundaries perpendicular to the stress axis. The present experimental findings and analysis reveal that the above microstructural evolution is associated with diffusional processes. In addition, it is suggested that grain elongation is mainly attributed to the cooperative process of the formation of dispersoid free zone due to diffusional processes and locally enhanced plastic deformation at relatively soft dispersoid free zone. Acknowledgment This research was supported by the Korea Ministry of Education Research Fund for Advanced Materials in 1998.
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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
M. A. Clark and T. H. Alden, Acta Metall. 21, 1195 (1973). N. O. Senkov and M. M. Myshlyaev, Acta Metall. 34, 97 (1986). A. K. Ghosh and C. H. Hamilton, Metall. Trans. 10A, 669 (1979). E. Sato and K. Kuribayashi, ISIJ Int. 33, 825 (1993). M. Kh. Rabinovich and V. G. Trifonof, Acta Mater. 44, 2073 (1996). F. Li, H. Bae and A. K. Ghosh, Acta Mater. 45, 3887 (1997). J. Pilling and N. Ridley, in Superplasticity in Crystalline Solids, p. 70, The Institute of Metals, London (1989). D. H. Shin, C. S. Lee, and W. J. Kim, Acta Mater. 45, 5195 (1997). D. H. Shin, K. T. Park, and E. J. Lavernia, Mater. Sci. Eng. A201, 118 (1995). C. H. Caceres and D. S. Wilkinson, Acta Metall. 32, 415 (1984). D. S. Wilkinson and C. H. Caceres, Acta Metall. 34, 1335 (1986). D. H. Shin, K. S. Kim, D. W. Kum, and S. W. Nam, Metall. Trans., 21A, 2729 (1990). U. Koch, in Superplasticity in Aerospace, ed. H. C. Heikkenen and T. R. McNelly, p. 115, TMS, Warrendale, PA (1988). J. J. Blandin, B. Hong, A. Varloteaux, M. Suery, and G. L’esperance, Acta Mater. 44, 2317 (1996). D. H. Shin, Y. J. Joo, W. J. Kim, and C. S. Lee, J. Mater. Sci. 33, 3073 (1998).
Pergamon
PII S1359-6462(99)00160-8
GRAIN ELONGATION IN A SUPERPLASTIC 7075 Al ALLOY Dong Hyuk Shin1, Yeon Jun Joo2, Chong Soo Lee3 and Kyung-Tae Park4 1
Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-Do, Korea 425-791 2 Division of Metals, Korea Institute of Science and Technology, Seoul, Korea 130-650 3 Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, Korea 790-784 4 Department of Advanced Materials Science and Engineering, Taejon National University of Technology, Taejon, Korea 300-717 (Received February 26, 1999) (Accepted in revised form May 4, 1999)
Introduction Grain growth during superplastic deformation (SPD) of various fine-grained materials (1– 6) has often been reported to be significant. Besides grain growth, it was reported recently (5,6) that noticeable grain elongation along the stress axis takes place in the initial stage of SPD of fine-grained Al alloys. In addition, such alloys with elongated grain structures showed superplastic behavior. Almost all analytical treatments (7) developed for the mechanisms of micrograin superplasticity are based on the fact that the initially equiaxed fine grains remain nearly unchanged during SPD. Accordingly, an examination of the grain elongation behavior during SPD of fine-grained Al alloys may provide an insight into the mechanisms of micrograin superplasticity. In the present investigation, the microstructural evolution during SPD of a superplastic 7075 Al alloy was carefully examined and a plausible mechanism of grain elongation, associated with the observed microstructural evolution, was demonstrated.
Experimental A high strength 7075 Al alloy in the form of non-superplastic rolled plate was used in the present investigation. The chemical composition (in wt.%) of the alloy was 5.96 Zn, 2.4 Mg, 1.43 Cu, 0.2 Cr, 0.06 Fe, 0.05 Si and the balance, Al. In order to obtain a superplastic fine grained structure, a modified thermomechanical treatment (8) was applied to the alloy. The resultant linear intercept grain size of the alloy was 5.5 ⫾ 1.2 m. It is worth noting that the thermomechanical treatment resulted in a slightly pancake-like grain structure: the linear intercept grain size in the short transverse section was 3.9 ⫾ 0.8 m. Tensile specimens with a gauge length of 8 mm were machined from the thermomechanically treated plate such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at 789 K and a true strain rate range from 2 ⫻ 10⫺4 s⫺1 to 6 ⫻ 10⫺3 s⫺1. The grain elongation behavior was observed using specimens deformed to predetermined strains. The detailed procedure for the microstructural examination is described elsewhere (8). In order to obtain information regarding the 269
270
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Vol. 41, No. 3
Figure 1. The optical micrographs showing grain elongation behavior of the present 7075 Al alloy deformed at T ⫽ 798 K and ˙ I ⫽ 2 ⫻ 10⫺3 s⫺1. The stress axis is horizontal: (a) e ⫽ 0 %, (b) e ⫽ 100 %, (c) e ⫽ 200 %, (d) e ⫽ 400%, (e) e ⫽ 800 % and (f) e ⫽ 1000 %.
mechanisms associated with grain elongation in the initial stage of SPD, the strain rate sensitivity (m) was measured as a function of strain () from true stress () ⫺ true strain rate (˙ ) curves plotted on a logarithmic scale: at small strains, the uninterrupted technique was used while the strain rate change technique (9) was utilized at large strains. Results and Discussion Microstructural Evolution Figures 1 (a)⬃(f) represent the optical micrographs showing the grain elongation behavior with strain in the alloy. Grain elongation along the stress axis was significant even at relatively low strains, for example, e ⬇ 100 % (Figure 1 (b)) and 200 % (Figure 1 (c)). The grain elongation behavior was
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Figure 2. Grain aspect ratio () as a function of true strain for the present 7075 Al alloy deformed at T ⫽ 798 K and various initial strain rates.
quantified with the grain aspect ratio () plotted against true strain in Figure 2: ⫽ dL/dT where dL and dT are the linear intercept grain sizes measured along the longitudinal (i.e. parallel to the stress axis) and transverse directions, respectively. increased with strain from the initial stage of SPD and became saturated at about ⬇ 1.6: ⬇ 1.1 at ⫽ 0 and ⬇ 1.7 at ⬎ 1.6. The saturated value of (⬇ 1.7) was comparable to that reported by Ravinovich and Trifonov (5) and indicated that, at least for the present alloy, grain elongation might contribute considerably to deformation in the initial stage of SPD. From Figure 2, it is evident that grain elongation was virtually independent of strain rate. Although Li et al. (6) addressed that grain elongation is totally ascribed to dislocation creep, this finding suggests that diffusional creep is likely to be responsible for grain elongation by the following reasons. First, the test temperatures of the present study and Li et al.’s study (6) were above 0.9 Tm (Tm is the melting point of the alloy; the incipient Tm of the alloys used in both investigations is about 850 K) at which diffusional creep is dominant over dislocation creep. Second, if grain elongation is attributed to dislocation creep, would depend on strain rate since rates of dislocation viscous glide and climb which occur sequentially in dislocation creep depend on the applied stress or imposed strain rate. The formation of dispersoid free zone (DFZ) in the vicinity of grain boundaries perpendicular to the stress axis, shown in Figures 3 (a) and (b), was also observed. The formation of DFZ during SPD of fine grained Al alloys has been reported previously (8,12–15). At present, the origin of DFZ and its role on SPD remain unclear. However, the present experimental observations indicate that the origin of DFZ seems to be associated with diffusional processes since: (a) as mentioned above, the test temperature
Figure 3. An example of DFZ in the vicinity of grain boundaries perpendicular to the stress axis: T ⫽ 798 K, ˙ I ⫽ 2 ⫻ 10⫺4 s⫺1 and e ⫽ 40 %: (a) Optical micrograph and (b) TEM micrograph.
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Figure 4. Strain rate sensitivity (m) vs. true strain for the present 7075 Al alloy at T ⫽ 798 K.
was high enough to operate diffusional processes rather than dislocation creep and (b) the DFZ resides in the vicinity of grain boundaries perpendicular to the stress axis as similar to the grain shape change associated with diffusional creep. In addition, Shin et al. (15) observed the existence of DFZ at a very early stage of SPD of a 7075 Al alloy, ⬇ 20 %, under similar experimental conditions as those used in the present study. The flow stresses at such low strains are much lower than the maximum stress that is obtained in the uniaxial tensile SPD and that is used to construct the typical true stress () ⫺ true strain rate (˙ ) curves characterizing the superplastic behavior. This also supports the hypothesis that diffusional processes would be responsible for the formation of DFZ since diffusional creep and dislocation creep are independent of each other and creep rate of the former is faster than that of the latter at low stresses. Strain Rate Sensitivity In order to examine the possibility that the present microstructural evolution, i.e. grain elongation and the formation of DFZ, is attributed to diffusional processes, the strain rate sensitivity (m) was measured as a function of strain by utilizing the uninterrupted test and the strain rate change test. As shown in Figure 4, at small strains ( ⬍ 1.5), m decreased gradually with strain while m remained nearly unchanged at high strains ( ⬎ 1.5). This finding indicates that diffusional processes may play an important role on deformation of the alloy in the initial stage of SPD under the present experimental conditions. It is of interest to note that a strain exhibiting the above transition nearly coincides with that at which becomes saturated in Figure 2. It implies that, after grain structure reaches its quasi-stable state (5), other deformation mechanisms such as grain boundary sliding would be operative dominantly. Grain Elongation Li et al. (6) suggested that dislocation creep results in grain elongation along the stress axis and concurrent grain contraction along the transverse and short transverse directions. Contrarily, Ravinovich and Trifonov (5) argued that grain elongation is primarily attributed to faster migration rate of grain boundaries perpendicular to the stress axis by accelerated self diffusion and that it slows down due to an action of counter-force restoring equiaxed grain shape. Their explanation (5) is partly in harmony with the present findings. That is, the accelerated self diffusion leads to the formation of DFZ in the vicinity of grain boundaries perpendicular to the stress axis. Once the DFZ is formed, migration of grain boundaries encompassed by the DFZ would be faster than those parallel to the stress axis due to a lack of obstacles pinning grain boundary in that region, leading grain elongation. The presence of DFZ would make an additional contribution to grain elongation during SPD. Since the DFZ is a relatively
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Figure 5. The schematic illustration of grain elongation by cooperative process of the formation of dispersoid free zone due to diffusional process and locally enhanced plastic deformation at dispersoid free zone.
soft region, strain at the DFZ can be larger than that at other parts of the specimen. As SPD proceeds, stress increases until it reaches its maximum value and results in the locally enhanced plastic strain at the DFZ. Then, after grain structure reaches its quasi-stable state as suggested by Ravinovich and Trifonov (5), grain elongation ceases and grain boundary sliding becomes a dominant deformation mechanism leading to a large strain. Accordingly, it is quite possible that grain elongation observed in the initial stage of SPD in the alloy is ascribed to the cooperative process of the formation of DFZ due to diffusional processes and locally enhanced plastic deformation at the DFZ. This cooperative process is illustrated schematically in Figure 5. Summary Microstructural evolution during superplastic deformation of a fine grained 7475 Al alloy is manifested by considerable grain elongation along the stress axis and the formation of dispersoid free zone in the vicinity of grain boundaries perpendicular to the stress axis. The present experimental findings and analysis reveal that the above microstructural evolution is associated with diffusional processes. In addition, it is suggested that grain elongation is mainly attributed to the cooperative process of the formation of dispersoid free zone due to diffusional processes and locally enhanced plastic deformation at relatively soft dispersoid free zone. Acknowledgment This research was supported by the Korea Ministry of Education Research Fund for Advanced Materials in 1998.
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Vol. 41, No. 3
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
M. A. Clark and T. H. Alden, Acta Metall. 21, 1195 (1973). N. O. Senkov and M. M. Myshlyaev, Acta Metall. 34, 97 (1986). A. K. Ghosh and C. H. Hamilton, Metall. Trans. 10A, 669 (1979). E. Sato and K. Kuribayashi, ISIJ Int. 33, 825 (1993). M. Kh. Rabinovich and V. G. Trifonof, Acta Mater. 44, 2073 (1996). F. Li, H. Bae and A. K. Ghosh, Acta Mater. 45, 3887 (1997). J. Pilling and N. Ridley, in Superplasticity in Crystalline Solids, p. 70, The Institute of Metals, London (1989). D. H. Shin, C. S. Lee, and W. J. Kim, Acta Mater. 45, 5195 (1997). D. H. Shin, K. T. Park, and E. J. Lavernia, Mater. Sci. Eng. A201, 118 (1995). C. H. Caceres and D. S. Wilkinson, Acta Metall. 32, 415 (1984). D. S. Wilkinson and C. H. Caceres, Acta Metall. 34, 1335 (1986). D. H. Shin, K. S. Kim, D. W. Kum, and S. W. Nam, Metall. Trans., 21A, 2729 (1990). U. Koch, in Superplasticity in Aerospace, ed. H. C. Heikkenen and T. R. McNelly, p. 115, TMS, Warrendale, PA (1988). J. J. Blandin, B. Hong, A. Varloteaux, M. Suery, and G. L’esperance, Acta Mater. 44, 2317 (1996). D. H. Shin, Y. J. Joo, W. J. Kim, and C. S. Lee, J. Mater. Sci. 33, 3073 (1998).