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Superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy AA7010 containing Sc
Materials Science and Engineering A 527 (2010) 854–857
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy AA7010 containing Sc A. Kumar, A.K. Mukhopadhyay ∗ , K.S. Prasad Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India
a r t i c l e
i n f o
Article history: Received 29 April 2009 Accepted 4 September 2009
Keywords: Al–Zn–Mg–Cu–Zr ally 7010 Sc addition Thermo-mechanical process Strain rate–temperature combination Recrystallization Superplasticity
a b s t r a c t The influence of small addition of Sc on the superplastic potential of Al–Zn–Mg–Cu–Zr alloy AA 7010 has been examined. The alloy was subjected to a three-step thermo-mechanical process (TMP) and the resultant material (having essentially an unrecrystallized grain structure) was subjected to a strain rate–temperature combination of 1.9 × 10−2 s−1 , 475 ◦ C to develop recrystallized grain structure amenable to superplastic deformation. This resulted in a total elongation of 650%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In a recent study involving a Zr-containing Al–Zn–Mg–Cu alloy AA 7449, it was demonstrated that the alloy could be suitably thermo-mechanically processed and subjected to superplastic tensile deformation using appropriate combinations of strain rates and temperatures [1]. The alloy exhibited high values of total elongation, and the corresponding final grain structures were fully recrystallized. The implication is that the presence of Zr in the alloy did not inhibit the process of recrystallization during the superplastic deformation. It is the purpose of this study to examine the superplastic behaviour of an Al–Zn–Mg–Cu alloy containing both Zr and Sc, wherein recrystallization is known to be extremely difficult to occur [2]. The incentive for the present investigation is two fold. Firstly, Al–Zn–Mg–Cu–Zr alloy containing Sc is worthy of a study, because Sc additions to such alloys bring about improvements in a variety of properties such as strength (through refinement of as-cast grain size and subgrain size, suppression of recrystallization and grain growth and refinement of dispersoids and strengthening precipitates) [3], elevated temperature compressive strength [4], stress corrosion cracking resistance [5] and weldability [6]. Secondly, in Al alloys containing both Zr and Sc, the Al3 Scx Zr1−x dispersoids are fine and more uniformly distributed compared to either Al3 Sc or Al3 Zr, i.e., when Sc or Zr is added singly [7]. The Al3 Scx Zr1−x dispersoids have proven most effective in pinning grain and subgrain bound-
∗ Corresponding author. E-mail address: ashim [email protected] (A.K. Mukhopadhyay). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.09.010
aries inhibiting recrystallization during mechanical and thermal processing of 7xxx series Al alloys post homogenization [2–5]. It would be, therefore, important to reveal how and at what stage recrystallization occurs during the superplastic tensile deformation of these alloys.
2. Experimental procedure The chemical compositions of the alloys, i.e., the base alloy AA 7010 (hereafter termed the base alloy) and the base alloy containing Sc (hereafter termed the Sc-bearing alloy) investigated are shown in Table 1. The alloys were prepared in an induction furnace under argon atmosphere, homogenized and scalped. The resultant slabs (395 mm × 300 mm × 85 mm) were then soaked at 435 ◦ C (708 K) for 4 h followed by rolling to produce plates of thickness 15 mm. While rolling down the slabs from 85 to 15 mm thickness, intermediate annealing was given each time at 435 ◦ C (708 K) for 25 min following 7% reductions in thickness. The resultant 15 mm thick plates were then subjected to a three-step thermo-mechanical process (TMP) in the following sequence: (1) solutionizing at 465 ◦ C (738 K) for 1 h followed by quenching in water, (2) over aging at 400 ◦ C (673 K) for 8 h, and (3) hot rolling at 400 ◦ C (673 K) to produce 2.3 mm thick sheets (without any intermediate annealing). Flat tensile samples were machined from 2.3 mm thick sheets to carry out incremental velocity tensile tests (jump tests) at temperatures and strain rates ranging from 425 to 475 ◦ C (698–748 K) and 3 × 10−5 to 1.9 × 10−2 s−1 , respectively to evaluate the strain rate sensitivity index ‘m’ [8,9]. Constant strain rate tensile tests were then performed at appropriate combinations of temperatures and strain rates to determine the total elongation.
A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857
855
Table 1 Chemical compositions (wt.%) of Al alloys (Fe and Si, associated with the primary aluminium, are present as impurities) examined in the present investigation. Alloy
Zn
Mg
Cu
Zr
Sc
Fe
Si
Base alloy 7010 Sc-bearing alloy
6.3 6.3
2.3 2.3
1.5 1.5
0.14 0.14
– 0.23
0.04 0.04
0.02 0.02
Samples for optical, scanning and transmission electron microscopy were prepared using standard methods to examine the evolution of grain structure during different stages of thermomechanical processing and superplastic deformation of the base alloy containing Sc. In this communication, the microstructural data for the base alloy is not presented, whilst the plots of stress () vs. ˙ and strain rate sensitivity (m) vs. strain rate (ε) ˙ and strain rate (ε), the constant strain rate tensile test results for the base alloy are presented for comparison with those of the Sc-bearing alloy.
Fig. 2. Plots of stress vs. strain rate and ‘m’ vs. strain rate after incremental velocity tests at various temperatures for (a) Sc-bearing alloy and (b) base alloy.
Fig. 1. Optical micrographs obtained from the Sc-bearing alloy showing grain structures developed in (a) 15 mm thick hot rolled plates, and (b) 2.3 mm thick TMP sheets. (c) Transmission electron micrograph obtained from the Sc-bearing alloy showing subgrain structure in the TMP sheets.
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A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857
Fig. 3. Flat tensile samples of the Sc-bearing alloy (a) original and (b) tested at 475 ◦ C, 1.9 × 10−2 s−1 .
3. Results and discussion Fig. 1(a) and (b) represents optical micrographs obtained from the Sc-bearing alloy showing grain structures developed in 15 mm thick hot rolled plates and 2.3 mm thick thermo-mechanically pro-
Fig. 4. Scanning electron micrograph showing recrystallized grains and cavitations (marked by arrows) in the gage region of the Sc-bearing alloy sample tested at 475 ◦ C, 1.9 × 10−2 s−1 .
Fig. 5. Transmission electron micrographs obtained from the Sc-bearing alloy showing (a) formation of large misoriented subgrains (one labeled ‘B’) (ε = 0.34); the arrows show the directions of migration of subgrain boundaries, (b) formation of fewer recrystallized grains (one labeled ‘Rex grain’) and (c) the presence of subgrains after 100% elongation (ε = 0.69), and (d) fully recrystallized grain structure after 650% elongation. The insets in (b) and (d) are SAEDPs obtained from individual grains showing high angles of misorientations across the high angle boundaries of the new grains.
A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857 Table 2 Constant strain rate tensile test results showing percentage total elongation. Alloy
Temperature (◦ C)
Strain rate (s−1 ) −2
% Total Elongation
7010 + Sc
475 450 425
1.9 × 10 9 × 10−3 10−2
650 551 562
7010
475 450 425
3 × 10−4 3 × 10−4 10−3
210 250 273
cessed (TMP) sheets, respectively. The grain structures developed in the plates and sheets could be best described as unrecrystallized. The latter is evidenced by the presence of subgrains in the 2.3 mm thick TMP sheets, as shown in the transmission electron micrograph in Fig. 1(c). Fig. 2 represents incremental velocity tensile test results showing the plots of stress vs. strain rate, and ‘m’ vs. strain rate corresponding to both the base alloy and the Sc-bearing alloy. For the Sc-bearing alloy, the highest ‘m’ value of 0.70 was obtained at 475 ◦ C (748 K), 1.9 × 10−2 s−1 . Table 2 summarizes the constant strain rate tensile test results carried out at different temperature–strain rate combinations to determine the total percentage of elongation. It is noteworthy that the results given in Table 2 are in agreement with the ‘m’ values shown in Fig. 2. The maximum elongation of 650% was obtained at 475 ◦ C, 1.9 × 10−2 s−1 . The tested sample together with the original sample is shown in Fig. 3. Fig. 4 represents a scanning electron micrograph obtained from the gage region of the sample that showed the maximum elongation. The micrograph shows recrystallized grains having an average size of 10 m. On the other hand, the base alloy exhibited ‘m’ values in a close range (0.39–0.43) at different temperature (425–475 ◦ C)–strain rate (2 × 10−4 to 8 × 10−4 s−1 ) combinations, as shown in Fig. 2. The maximum percentage of elongation obtained for the base alloy at 425 ◦ C, 10−3 in 273, which is much lower than the maximum elongation (650%) for the Sc-bearing alloy. Further, the higher strain rate involved with the Sc-bearing alloy implies restrictions on recrystallization (as well as on grain growth) due to the known presence of a uniform and fine distribution of Al3 Scx Zr1−x dispersoids. Fig. 5(a) represents a transmission electron micrograph obtained from the gage portion of Sc-bearing alloy sample tested at 475 ◦ C (748 K), 1.9 × 10−2 s−1 after interrupting the tensile test at a strain value of 0.34. The most noticeable feature in Fig. 5, when compared with Fig. 1(c), is the growth of the subgrains under a combination of suitable strain rate and temperature. The rotation of subgrains during the tensile test is understood to be responsible for the growth of the suitably rotated/oriented subgrains [one labeled B in Fig. 5(a)] at the expense of the neighboring subgrains. The absence of any large particle within such growing subgrains further substantiated this fact. Such subgrains eventually merge with the transition bands (namely the deformation bands), thereby converting subgrain boundaries into high angle grain boundaries.
857
Fig. 5(b) represents a transmission electron micrograph obtained from the gage portion of the alloy after interrupting the tensile test at a strain value of 0.69 (corresponding to 100% elongation). The micrograph shows formation of fewer recrystallized grains [as evidenced by the selected area electron diffraction patterns (SAEDPs) (see insets) showing high angles of misorientation across the grain boundaries of the new grains]. Fig. 5(c) represents a transmission electron micrograph showing the presence of subgrains in a different region of the same thin foil. Fig. 5(d) represents a transmission electron micrograph obtained from the gage portion of the sample showing the maximum elongation. The grain structure was indeed fully recrystallized, once again as evidenced by the SAEDPs (see insets) obtained from individual grains. The present results demonstrate that the subgrain structure is thermally stable under static conditions. Whilst, under a combination of suitable temperature and strain rate, the deformation process begins in selected regions of the materials involving rotation of suitably oriented subgrains followed by their growth. This process gives rise to the evolution of high angle grain boundaries throughout the gage region of the alloy samples. These high angle boundaries slide and dominate the overall deformation process in these alloy samples. 4. Summary and conclusions An Al–6.3Zn–2.3Mg–1.5Cu–0.14Zr (in wt%) alloy 7010 containing 0.23 wt% Sc could be subjected to a suitable thermomechanical process prior to the superplastic deformation at the temperature–strain rate combination of 475 ◦ C, 1.9 × 10−2 s−1 . This enabled development of recrystallized grain structure, having an average grain size of 10 m, during the superplastic deformation process and yielded a total elongation of as high as 650%. Acknowledgement The authors wish to acknowledge the financial support from Defence Research and Development Organization (DRDO), Government of India. References [1] A. Kumar, A.K. Mukhopadhyay, K.S. Prasad, Metall. Mater. Trans. A 40A (2009) 278. [2] Y.V. Milman, D.V. Lotsko, O.I. Sirko, Mater. Sci. Forum 331–337 (2000) 1107. [3] A.K. Mukhopadhyay, Metals Mater. Process. 19 (2007) 1. [4] A.K. Mukhopadhyay, K.S. Prasad, A. Dutta, Mater. Sci. Forum 519–521 (2006) 871. [5] A.K. Mukhopadhyay, K.S. Prasad, V. Kumar, G.M. Reddy, S.V. Kamat, V.K. Varma, Mater. Sci. Forum 519–521 (2006) 315. [6] G.M. Reddy, A.K. Mukhopadhyay, A.S. Rao, Sci. and Technol. Welding Joining 10 (2005) 432. [7] L.S. Toropova, D.G. Eskin, M.L. Kharakterova, T.V. Dobatkina, Advanced Aluminium Containing Scandium: Structure and Properties, Gordon and Breach Science Publishers, The Netherlands, 1998. [8] J. Pilling, N. Ridley, Superplasticity in Crystalline Solids, The Institute of Metals, London, 1989. [9] O.A. Kaibysher, Superplasticity of Alloys, Intermetallics and Ceramics, SpringerVerlag, Berlin, 1992.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy AA7010 containing Sc A. Kumar, A.K. Mukhopadhyay ∗ , K.S. Prasad Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India
a r t i c l e
i n f o
Article history: Received 29 April 2009 Accepted 4 September 2009
Keywords: Al–Zn–Mg–Cu–Zr ally 7010 Sc addition Thermo-mechanical process Strain rate–temperature combination Recrystallization Superplasticity
a b s t r a c t The influence of small addition of Sc on the superplastic potential of Al–Zn–Mg–Cu–Zr alloy AA 7010 has been examined. The alloy was subjected to a three-step thermo-mechanical process (TMP) and the resultant material (having essentially an unrecrystallized grain structure) was subjected to a strain rate–temperature combination of 1.9 × 10−2 s−1 , 475 ◦ C to develop recrystallized grain structure amenable to superplastic deformation. This resulted in a total elongation of 650%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In a recent study involving a Zr-containing Al–Zn–Mg–Cu alloy AA 7449, it was demonstrated that the alloy could be suitably thermo-mechanically processed and subjected to superplastic tensile deformation using appropriate combinations of strain rates and temperatures [1]. The alloy exhibited high values of total elongation, and the corresponding final grain structures were fully recrystallized. The implication is that the presence of Zr in the alloy did not inhibit the process of recrystallization during the superplastic deformation. It is the purpose of this study to examine the superplastic behaviour of an Al–Zn–Mg–Cu alloy containing both Zr and Sc, wherein recrystallization is known to be extremely difficult to occur [2]. The incentive for the present investigation is two fold. Firstly, Al–Zn–Mg–Cu–Zr alloy containing Sc is worthy of a study, because Sc additions to such alloys bring about improvements in a variety of properties such as strength (through refinement of as-cast grain size and subgrain size, suppression of recrystallization and grain growth and refinement of dispersoids and strengthening precipitates) [3], elevated temperature compressive strength [4], stress corrosion cracking resistance [5] and weldability [6]. Secondly, in Al alloys containing both Zr and Sc, the Al3 Scx Zr1−x dispersoids are fine and more uniformly distributed compared to either Al3 Sc or Al3 Zr, i.e., when Sc or Zr is added singly [7]. The Al3 Scx Zr1−x dispersoids have proven most effective in pinning grain and subgrain bound-
∗ Corresponding author. E-mail address: ashim [email protected] (A.K. Mukhopadhyay). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.09.010
aries inhibiting recrystallization during mechanical and thermal processing of 7xxx series Al alloys post homogenization [2–5]. It would be, therefore, important to reveal how and at what stage recrystallization occurs during the superplastic tensile deformation of these alloys.
2. Experimental procedure The chemical compositions of the alloys, i.e., the base alloy AA 7010 (hereafter termed the base alloy) and the base alloy containing Sc (hereafter termed the Sc-bearing alloy) investigated are shown in Table 1. The alloys were prepared in an induction furnace under argon atmosphere, homogenized and scalped. The resultant slabs (395 mm × 300 mm × 85 mm) were then soaked at 435 ◦ C (708 K) for 4 h followed by rolling to produce plates of thickness 15 mm. While rolling down the slabs from 85 to 15 mm thickness, intermediate annealing was given each time at 435 ◦ C (708 K) for 25 min following 7% reductions in thickness. The resultant 15 mm thick plates were then subjected to a three-step thermo-mechanical process (TMP) in the following sequence: (1) solutionizing at 465 ◦ C (738 K) for 1 h followed by quenching in water, (2) over aging at 400 ◦ C (673 K) for 8 h, and (3) hot rolling at 400 ◦ C (673 K) to produce 2.3 mm thick sheets (without any intermediate annealing). Flat tensile samples were machined from 2.3 mm thick sheets to carry out incremental velocity tensile tests (jump tests) at temperatures and strain rates ranging from 425 to 475 ◦ C (698–748 K) and 3 × 10−5 to 1.9 × 10−2 s−1 , respectively to evaluate the strain rate sensitivity index ‘m’ [8,9]. Constant strain rate tensile tests were then performed at appropriate combinations of temperatures and strain rates to determine the total elongation.
A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857
855
Table 1 Chemical compositions (wt.%) of Al alloys (Fe and Si, associated with the primary aluminium, are present as impurities) examined in the present investigation. Alloy
Zn
Mg
Cu
Zr
Sc
Fe
Si
Base alloy 7010 Sc-bearing alloy
6.3 6.3
2.3 2.3
1.5 1.5
0.14 0.14
– 0.23
0.04 0.04
0.02 0.02
Samples for optical, scanning and transmission electron microscopy were prepared using standard methods to examine the evolution of grain structure during different stages of thermomechanical processing and superplastic deformation of the base alloy containing Sc. In this communication, the microstructural data for the base alloy is not presented, whilst the plots of stress () vs. ˙ and strain rate sensitivity (m) vs. strain rate (ε) ˙ and strain rate (ε), the constant strain rate tensile test results for the base alloy are presented for comparison with those of the Sc-bearing alloy.
Fig. 2. Plots of stress vs. strain rate and ‘m’ vs. strain rate after incremental velocity tests at various temperatures for (a) Sc-bearing alloy and (b) base alloy.
Fig. 1. Optical micrographs obtained from the Sc-bearing alloy showing grain structures developed in (a) 15 mm thick hot rolled plates, and (b) 2.3 mm thick TMP sheets. (c) Transmission electron micrograph obtained from the Sc-bearing alloy showing subgrain structure in the TMP sheets.
856
A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857
Fig. 3. Flat tensile samples of the Sc-bearing alloy (a) original and (b) tested at 475 ◦ C, 1.9 × 10−2 s−1 .
3. Results and discussion Fig. 1(a) and (b) represents optical micrographs obtained from the Sc-bearing alloy showing grain structures developed in 15 mm thick hot rolled plates and 2.3 mm thick thermo-mechanically pro-
Fig. 4. Scanning electron micrograph showing recrystallized grains and cavitations (marked by arrows) in the gage region of the Sc-bearing alloy sample tested at 475 ◦ C, 1.9 × 10−2 s−1 .
Fig. 5. Transmission electron micrographs obtained from the Sc-bearing alloy showing (a) formation of large misoriented subgrains (one labeled ‘B’) (ε = 0.34); the arrows show the directions of migration of subgrain boundaries, (b) formation of fewer recrystallized grains (one labeled ‘Rex grain’) and (c) the presence of subgrains after 100% elongation (ε = 0.69), and (d) fully recrystallized grain structure after 650% elongation. The insets in (b) and (d) are SAEDPs obtained from individual grains showing high angles of misorientations across the high angle boundaries of the new grains.
A. Kumar et al. / Materials Science and Engineering A 527 (2010) 854–857 Table 2 Constant strain rate tensile test results showing percentage total elongation. Alloy
Temperature (◦ C)
Strain rate (s−1 ) −2
% Total Elongation
7010 + Sc
475 450 425
1.9 × 10 9 × 10−3 10−2
650 551 562
7010
475 450 425
3 × 10−4 3 × 10−4 10−3
210 250 273
cessed (TMP) sheets, respectively. The grain structures developed in the plates and sheets could be best described as unrecrystallized. The latter is evidenced by the presence of subgrains in the 2.3 mm thick TMP sheets, as shown in the transmission electron micrograph in Fig. 1(c). Fig. 2 represents incremental velocity tensile test results showing the plots of stress vs. strain rate, and ‘m’ vs. strain rate corresponding to both the base alloy and the Sc-bearing alloy. For the Sc-bearing alloy, the highest ‘m’ value of 0.70 was obtained at 475 ◦ C (748 K), 1.9 × 10−2 s−1 . Table 2 summarizes the constant strain rate tensile test results carried out at different temperature–strain rate combinations to determine the total percentage of elongation. It is noteworthy that the results given in Table 2 are in agreement with the ‘m’ values shown in Fig. 2. The maximum elongation of 650% was obtained at 475 ◦ C, 1.9 × 10−2 s−1 . The tested sample together with the original sample is shown in Fig. 3. Fig. 4 represents a scanning electron micrograph obtained from the gage region of the sample that showed the maximum elongation. The micrograph shows recrystallized grains having an average size of 10 m. On the other hand, the base alloy exhibited ‘m’ values in a close range (0.39–0.43) at different temperature (425–475 ◦ C)–strain rate (2 × 10−4 to 8 × 10−4 s−1 ) combinations, as shown in Fig. 2. The maximum percentage of elongation obtained for the base alloy at 425 ◦ C, 10−3 in 273, which is much lower than the maximum elongation (650%) for the Sc-bearing alloy. Further, the higher strain rate involved with the Sc-bearing alloy implies restrictions on recrystallization (as well as on grain growth) due to the known presence of a uniform and fine distribution of Al3 Scx Zr1−x dispersoids. Fig. 5(a) represents a transmission electron micrograph obtained from the gage portion of Sc-bearing alloy sample tested at 475 ◦ C (748 K), 1.9 × 10−2 s−1 after interrupting the tensile test at a strain value of 0.34. The most noticeable feature in Fig. 5, when compared with Fig. 1(c), is the growth of the subgrains under a combination of suitable strain rate and temperature. The rotation of subgrains during the tensile test is understood to be responsible for the growth of the suitably rotated/oriented subgrains [one labeled B in Fig. 5(a)] at the expense of the neighboring subgrains. The absence of any large particle within such growing subgrains further substantiated this fact. Such subgrains eventually merge with the transition bands (namely the deformation bands), thereby converting subgrain boundaries into high angle grain boundaries.
857
Fig. 5(b) represents a transmission electron micrograph obtained from the gage portion of the alloy after interrupting the tensile test at a strain value of 0.69 (corresponding to 100% elongation). The micrograph shows formation of fewer recrystallized grains [as evidenced by the selected area electron diffraction patterns (SAEDPs) (see insets) showing high angles of misorientation across the grain boundaries of the new grains]. Fig. 5(c) represents a transmission electron micrograph showing the presence of subgrains in a different region of the same thin foil. Fig. 5(d) represents a transmission electron micrograph obtained from the gage portion of the sample showing the maximum elongation. The grain structure was indeed fully recrystallized, once again as evidenced by the SAEDPs (see insets) obtained from individual grains. The present results demonstrate that the subgrain structure is thermally stable under static conditions. Whilst, under a combination of suitable temperature and strain rate, the deformation process begins in selected regions of the materials involving rotation of suitably oriented subgrains followed by their growth. This process gives rise to the evolution of high angle grain boundaries throughout the gage region of the alloy samples. These high angle boundaries slide and dominate the overall deformation process in these alloy samples. 4. Summary and conclusions An Al–6.3Zn–2.3Mg–1.5Cu–0.14Zr (in wt%) alloy 7010 containing 0.23 wt% Sc could be subjected to a suitable thermomechanical process prior to the superplastic deformation at the temperature–strain rate combination of 475 ◦ C, 1.9 × 10−2 s−1 . This enabled development of recrystallized grain structure, having an average grain size of 10 m, during the superplastic deformation process and yielded a total elongation of as high as 650%. Acknowledgement The authors wish to acknowledge the financial support from Defence Research and Development Organization (DRDO), Government of India. References [1] A. Kumar, A.K. Mukhopadhyay, K.S. Prasad, Metall. Mater. Trans. A 40A (2009) 278. [2] Y.V. Milman, D.V. Lotsko, O.I. Sirko, Mater. Sci. Forum 331–337 (2000) 1107. [3] A.K. Mukhopadhyay, Metals Mater. Process. 19 (2007) 1. [4] A.K. Mukhopadhyay, K.S. Prasad, A. Dutta, Mater. Sci. Forum 519–521 (2006) 871. [5] A.K. Mukhopadhyay, K.S. Prasad, V. Kumar, G.M. Reddy, S.V. Kamat, V.K. Varma, Mater. Sci. Forum 519–521 (2006) 315. [6] G.M. Reddy, A.K. Mukhopadhyay, A.S. Rao, Sci. and Technol. Welding Joining 10 (2005) 432. [7] L.S. Toropova, D.G. Eskin, M.L. Kharakterova, T.V. Dobatkina, Advanced Aluminium Containing Scandium: Structure and Properties, Gordon and Breach Science Publishers, The Netherlands, 1998. [8] J. Pilling, N. Ridley, Superplasticity in Crystalline Solids, The Institute of Metals, London, 1989. [9] O.A. Kaibysher, Superplasticity of Alloys, Intermetallics and Ceramics, SpringerVerlag, Berlin, 1992.