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Achieving exceptional superplasticity in a bulk aluminum alloy processed by high-pressure torsion
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Scripta Materialia 58 (2008) 1029–1032 www.elsevier.com/locate/scriptamat
Achieving exceptional superplasticity in a bulk aluminum alloy processed by high-pressure torsion Zenji Horitaa,* and Terence G. Langdonb,c,d a
b
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, USA c Department of Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA d Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK Received 9 January 2008; revised 18 January 2008; accepted 25 January 2008 Available online 2 February 2008
Bulk specimens of an aluminum–magnesium–scandium alloy, in the form of small cylinders, were processed by high-pressure torsion at room temperature. Following processing, the microstructure was inhomogeneous with larger grains in the center and ultrafine grains of 130 nm at the periphery. Tensile testing after processing revealed the potential for achieving exceptional superplastic elongations but the measured elongations depended upon the positions of the specimens within the cylinder. The highest tensile elongation recorded in these experiments was 1600%. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum alloy; High-pressure torsion; Severe plastic deformation; Superplasticity
Processing via the application of severe plastic deformation (SPD) is now an established procedure for refining the grain size of bulk samples to the submicrometer or nanometer level [1]. Several SPD procedures are available and the basic principles of these processes were described in a recent report [2]. Two of these procedures, high-pressure torsion (HPT) [3] and equalchannel angular pressing (ECAP) [4], are receiving considerable attention, but HPT is especially attractive because experiments show that this procedure leads to greater grain refinement than can be achieved by ECAP [5]. Nevertheless there is a significant disadvantage because the samples used in HPT are generally in the form of very thin disks having thicknesses typically of the order of only 0.5–0.8 mm. When materials are processed by SPD they exhibit several attractive properties including high strength at ambient temperatures and a potential for use in superplastic forming operations at high-temperatures. Superplasticity is defined as the ability to achieve elongations in tension of at least 500%, and there are numerous reports to date of the occurrence of superplastic flow in a range of metals processed by ECAP including a
* Corresponding author. Tel.: +81 92 802 2958; fax: +81 92 802 2992; e-mail: [email protected]
number of aluminum, copper and magnesium alloys: a detailed tabulation of these various results is given elsewhere [6]. By contrast, there are only very limited reports of the occurrence of superplastic flow in alloys processed by HPT. Specifically, there are reported elongations of 500% in an Al–3% Mg–0.2% Sc alloy [7], 570% in an Al-2024 alloy [8], 620% in an AZ61 alloy [9], 750% in an Al-1420 alloy [10], 800% in an Al–4% Cu–0.5% Zr alloy [11] and 810% in a Mg–9% Al alloy [12] where all alloys were processed by HPT. By contrast, and despite the slightly larger grain sizes introduced by ECAP, the elongations in ECAP are often much higher and occasionally exceed 1000%. A probable explanation for these lower elongations in HPT lies in the very small thicknesses of the gauge sections within the tensile specimens. It is well-established in superplastic research of conventional materials that the measured elongations to failure are dependent upon the geometry of the test specimens, and an empirical equation was developed in early work relating the total elongation to the specimen diameter and the gauge length [13]. Subsequently, it was demonstrated experimentally that the superplastic elongations are higher when the gauge widths of the tensile specimens are increased [14]. Accordingly, it is reasonable to anticipate that higher superplastic elongations may be achieved in HPT samples if the thicknesses in the gauge sections are also increased.
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.043
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The present investigation was initiated to evaluate the factors influencing the superplastic properties of an Al– 3% Mg–0.2% Sc alloy, where the composition of the alloy is given in weight per cent. This material was selected because earlier research demonstrated excellent superplastic capabilities after processing by ECAP [15,16]. To overcome the problems inherent in the very thin disks generally used in HPT, the experiments were conducted using a facility described earlier and developed specifically to extend the capability of HPT processing to larger bulk samples [17]. The initial samples were in the form of small cylinders 8.57 mm high and 10.0 mm in diameter. The straining facility and the significance of the specimen shape were described in detail in an earlier report [17] but it is important to note that the samples were compressed between upper and lower anvils under an imposed pressure P, the dies were shaped such that the upper and lower receptacles for the sample had vertical surfaces inclined outwards at an angle of 5° to the perpendicular, and on application of the pressure the sample was compressed and forced outwards into a barrel-shaped configuration to fill the die. In this form, the sample height was reduced to 8.0 mm and the outer appearance of the sample was as shown in Figure 1 after a single turn of torsional straining where the upper and lower side faces are inclined outwards with respect to the top and bottom surfaces by 5°: the small ridge visible in Figure 1 around the mid-section of the sample is due to a minor outflow of material between the upper and lower anvils during the straining operation. The experiments were conducted using the same Al– 3% Mg–0.2% Sc alloy and the same material preparation procedures described in earlier reports of testing by ECAP [15,16,18] and HPT [1,17,19]. Briefly, the alloy was prepared from high-purity materials, homogenized in air at 753 K for 24 h, cut and swaged into a 12 mm diameter rod, lathed to a diameter of 10 mm, cut into cylinders 8.6 mm high, solution treated in air at 873 K for 5 h, and then lightly ground to the required height of 8.57 mm. In this initial condition, the measured grain size was 0.5 mm. Processing by HPT was conducted in air under an applied load of 9.3 tonnes, equivalent to an
imposed pressure of P = 1 GPa, using a rotation speed of 1 rpm. The number of rotations, N, imposed on the samples was 2. To evaluate the microstructural characteristics after HPT through two turns, a sample was cut along a vertical section into two equal parts; one exposed face was mechanically polished to a mirror-like finish and the face was then electropolished in an aqueous solution of 5% HBF4 for inspection by optical microscopy. Figure 2 shows a montage of the microstructure where the lower inset indicates sectioning on the central vertical plane. It is apparent that this section exhibits the characteristic flow pattern anticipated for a cylindrical sample subjected to torsional straining when the straining plane lies horizontally across the mid-point of the section. The microstructure is clearly inhomogeneous with very heavily deformed zones around the edges of the cylinder wherein the grains are extremely small and with larger grains in the center of the sample. There is also evidence for a directionality in the shapes of the larger grains which delineates the local flow direction. This result is generally consistent with microstructural features reported earlier after the processing of a bulk sample through one-quarter and one turn apart from the fact that there is now a larger peripheral region of ultrafine grains. Using transmission electron microscopy, the measured grain size on the central plane near the outer edge of the cylinder was 130 nm. It is interesting to note that this is smaller than the grain sizes reported previously for the same alloy processed by different SPD procedures including 200 nm after one turn of HPT for a bulk sample with P = 1 GPa [17], 150 nm after five turns of HPT for a thin disk with P = 6 GPa [7] and 200 nm after ECAP through eight passes [15]. Following HPT, small tensile samples, each 1 mm thick, were cut from the cylinders using two different procedures to give samples oriented either in the straining plane or perpendicular to the straining plane: for convenience, the samples in these two configurations are termed the horizontal and vertical samples, respectively. The procedure for preparing the horizontal samples is shown in Figure 3. A disk with a thickness of 1 mm was sliced from the mid-section of a cylinder after torsional straining as illustrated on the left in Figure 3
Figure 1. Appearance of a bulk sample after HPT through a single turn.
Figure 2. Montage of microstructures after two turns: the plane of sectioning is shown at the lower left.
Z. Horita, T. G. Langdon / Scripta Materialia 58 (2008) 1029–1032 φ 10
1031
2000
S plane
2
1
0.
R
1
3
2
2
Elongation (%)
T = 573 K
Slicing 1
Al-3%Mg-0.2%Sc
L plane 1500
ε = 3.3x10-3 s-1 Specimen horizontal
1000
vertical
500
(unit:mm)
0 0
Figure 3. Procedure for preparing horizontal specimens in the plane of torsional straining.
and tensile specimens were then cut from the disk using electrodischarge machining (EDM) as indicated on the right in Figure 3. In order to avoid the inhomogeneous microstructures associated with the centers of each cylinder, these tensile specimens were cut from off-center positions whereby their longitudinal axes were aligned at a distance of 2.5 mm from the central diameter through the disk: as shown in Figure 3 and following the notation introduced earlier [17], the central vertical or longitudinal plane is termed the L plane and a sectional plane parallel to the L plane but displaced by a distance of 3 mm is termed the S plane. As indicated in Figure 3, the tensile specimens had a gauge length of 1 mm and a gauge section of 1 1 mm2. The procedure for preparing the vertical specimens is shown in Figure 4, where the front view on the left delineates the position of each specimen and the side view on the right shows the shape of the specimens. These specimens were prepared by cutting a block into the overall shape of the tensile specimens using EDM and then slicing this block into the individual specimens. The shape and thickness of these vertical specimens were identical to those of the horizontal specimens. After preparing these miniature tensile specimens, each specimen was mounted horizontally, heated to a temperature of 573 K over a period of 40 min and then pulled to failure using a testing machine operating at a constant rate of cross-head displacement with an initial strain rate, e_ , of 3.3 10 3 s 1. The measured elongations to failure are shown in Figure 5, where the upper point is for a horizontal specimen and the lower points are for the vertical specimens. Three important conclusions may be reached from inspection of Figure 5.
1.5
1
2
3 4.5 (unit:mm)
Figure 4. Procedure for preparing vertical specimens perpendicular to the plane of torsional straining.
1
2
3
4
5
Distance from center (mm)
Figure 5. Variation of elongation with distance from the center of the cylinders for horizontal and vertical specimens.
First, the elongations recorded in the outer peripheral region of the bulk samples are exceptionally high. These elongations of more than 1000% are the highest recorded to date for specimens subjected to HPT processing and the maximum elongation of 1600% in the horizontal specimen contrasts with a reported elongation of 1270% for the same alloy tested under the same conditions of temperature and strain rate after processing through ECAP for eight passes [15]. This result demonstrates conclusively that processing by HPT leads to elongations which are comparable to those attained after ECAP, and indeed the larger elongation now achieved by HPT is attributed to the smaller grain size of 130 nm in HPT compared to 200 nm in ECAP. Conversely, it is concluded that the relatively lower elongations reported in earlier studies for specimens processed by HPT are not an accurate reflection of the superplastic capabilities of the processed materials but rather are a consequence of using specimens having very small thicknesses within the gauge sections. Second, the elongation recorded in the horizontal specimen is unusually high and equal to 1600%, whereas the elongations are slightly lower in the vertical specimens. This difference is reasonable because the horizontal specimen lies in the plane of torsional straining and therefore any elongations of the grains lie parallel to, rather than perpendicular to, the tensile axis. Third, the measured elongations in the vertical specimens are very significantly lower near the center of the cylinder by comparison with the elongations recorded in the peripheral region. This difference is a direct consequence of the larger grains clearly visible in the central region in Figure 2 and also to the wellestablished variations in microhardness and microstructural characteristics across the diameters of conventional disks processed by HPT [20–25]. This result contrasts with the situation in processing by ECAP where experiments have shown there is no significant anisotropy in the superplastic properties when tensile specimens are machined from the pressed billets in three mutually perpendicular directions [26]. Accordingly, the present results provide a clear demonstration of the necessity for cutting tensile specimens in HPT both in a horizontal configuration with respect to the straining plane and from an off-center position to avoid the region of minimum strain in the center of the sample.
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Z. Horita, T. G. Langdon / Scripta Materialia 58 (2008) 1029–1032
Figure 6. Appearance of the horizontal specimen giving an elongation of 1600%: the upper specimen is untested.
Figure 6 shows the appearance of the horizontal specimen that pulled out to 1600% at failure together with an undeformed specimen. It is apparent from Figure 6 that the deformed specimen exhibits all of the characteristics of true superplasticity or quasi-stable plastic flow, including uniform deformation with no evidence for the occurrence of any incipient necking within the gauge length, a pulling down almost to a point at failure and a measured macroscopic elongation greater than 1000% [27]. In summary, experiments were undertaken to determine whether high superplastic elongations may be achieved after processing by HPT when using tensile specimens 1 mm thick. The experiments were conducted using an Al–Mg–Sc alloy in the form of bulk cylinders. The results demonstrate excellent superplastic characteristics after HPT including a maximum tensile elongation of 1600% at a testing temperature of 573 K. It is concluded that the relatively lower elongations to failure generally observed after superplastic testing in HPT specimens are a direct consequence of using specimens with very thin gauge sections. These results show also that the measured elongations depend critically upon the location of the tensile specimens within the bulk cylinder and on whether the specimens are cut parallel or perpendicular to the straining plane. We are grateful to Mr. Genki Sakai and Mr. Masaaki Kai for experimental assistance. This work was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, in the Priority Area ‘‘Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects” and in part by the National Science Foundation of the United States under Grant No. DMR0243331.
[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [2] R.Z. Valiev, Y. Estrin, Z. Horita, T.G. Langdon, M.J. Zehetbauer, Y.T. Zhu, JOM 58 (4) (2006) 33. [3] A.P. Zhilyaev, G.V. Nurislamova, B.K. Kim, M.D. Baro´, J.A. Szpunar, T.G. Langdon, Acta Mater. 51 (2003) 753. [4] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881. [5] J. Dutkiewicz, J. Kus´nierz, W. Maziarz, M. Lejkowska, H. Garbacz, M. Lewandowska, A.V. Dobromyslov, K.J. Kurzydlowski, Phys. Status Solidi (a) 202 (2005) 2309. [6] M. Kawasaki, T.G. Langdon, J. Mater. Sci. 42 (2007) 1782. [7] G. Sakai, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 393 (2005) 344. [8] S.V. Dobatkin, E.N. Bastarache, G. Sakai, T. Fujita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 408 (2005) 141. [9] Y. Harai, M. Kai, K. Kaneko, Z. Horita, T.G. Langdon, Mater. Trans. 49 (2008) 76. [10] R.S. Mishra, R.Z. Valiev, S.X. McFadden, R.K. Islamgaliev, A.K. Mukherjee, Philos. Mag. A 81 (2001) 37. [11] R.Z. Valiev, O.A. Kaibyshev, R.I. Kuznetsov, R.Sh. Musalimov, N.K. Tsenev, Dokl. Akad. Nauk SSSR 301 (1988) 864. [12] M. Kai, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A, in press. [13] W.B. Morrison, Trans. Met. Soc. AIME 242 (1968) 2221. [14] Y. Ma, T.G. Langdon, Metall. Mater. Trans. A 25 (1994) 2309. [15] S. Komura, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 32 (2001) 707. [16] S. Komura, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 297 (2001) 111. [17] G. Sakai, K. Nakamura, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 406 (2005) 268. [18] S. Komura, P.B. Berbon, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 38 (1998) 1851. [19] Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 410–411 (2005) 422. ˇ´ızˇek, I. Procha´zka, O. Melikhova, G. Brauer, W. [20] J. C Anwand, R. Kuzˇel, M. Cieslar, R.K. Islamgaliev, Appl. Surf. Sci. 194 (2002) 140. ˇ´ızˇek, I. Procha´zka, G. Brauer, W. Anwand, R. Kuzˇel, [21] J. C M. Cieslar, R.K. Islamgaliev, Phys. Status Solidi (a) 195 (2003) 335. [22] A. Vorhauer, R. Pippan, Scripta Mater. 51 (2004) 921. [23] Z. Yang, U. Welzel, Mater. Lett. 59 (2005) 3406. [24] C. Xu, Z. Horita, T.G. Langdon, Acta Mater. 55 (2007) 203. [25] Zs. Kova´cs, S. Ho´bor, P.J. Szabo, J. Lendvai, A.P. ´ . Re´ve´sz, Mater. Sci. Eng. A 449–451 (2007) Zhilyaev, A 1139. [26] G. Sakai, Z. Horita, T.G. Langdon, Mater. Trans. 45 (2004) 3079. [27] T.G. Langdon, Metal Sci. 16 (1982) 175.
Scripta Materialia 58 (2008) 1029–1032 www.elsevier.com/locate/scriptamat
Achieving exceptional superplasticity in a bulk aluminum alloy processed by high-pressure torsion Zenji Horitaa,* and Terence G. Langdonb,c,d a
b
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, USA c Department of Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA d Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK Received 9 January 2008; revised 18 January 2008; accepted 25 January 2008 Available online 2 February 2008
Bulk specimens of an aluminum–magnesium–scandium alloy, in the form of small cylinders, were processed by high-pressure torsion at room temperature. Following processing, the microstructure was inhomogeneous with larger grains in the center and ultrafine grains of 130 nm at the periphery. Tensile testing after processing revealed the potential for achieving exceptional superplastic elongations but the measured elongations depended upon the positions of the specimens within the cylinder. The highest tensile elongation recorded in these experiments was 1600%. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum alloy; High-pressure torsion; Severe plastic deformation; Superplasticity
Processing via the application of severe plastic deformation (SPD) is now an established procedure for refining the grain size of bulk samples to the submicrometer or nanometer level [1]. Several SPD procedures are available and the basic principles of these processes were described in a recent report [2]. Two of these procedures, high-pressure torsion (HPT) [3] and equalchannel angular pressing (ECAP) [4], are receiving considerable attention, but HPT is especially attractive because experiments show that this procedure leads to greater grain refinement than can be achieved by ECAP [5]. Nevertheless there is a significant disadvantage because the samples used in HPT are generally in the form of very thin disks having thicknesses typically of the order of only 0.5–0.8 mm. When materials are processed by SPD they exhibit several attractive properties including high strength at ambient temperatures and a potential for use in superplastic forming operations at high-temperatures. Superplasticity is defined as the ability to achieve elongations in tension of at least 500%, and there are numerous reports to date of the occurrence of superplastic flow in a range of metals processed by ECAP including a
* Corresponding author. Tel.: +81 92 802 2958; fax: +81 92 802 2992; e-mail: [email protected]
number of aluminum, copper and magnesium alloys: a detailed tabulation of these various results is given elsewhere [6]. By contrast, there are only very limited reports of the occurrence of superplastic flow in alloys processed by HPT. Specifically, there are reported elongations of 500% in an Al–3% Mg–0.2% Sc alloy [7], 570% in an Al-2024 alloy [8], 620% in an AZ61 alloy [9], 750% in an Al-1420 alloy [10], 800% in an Al–4% Cu–0.5% Zr alloy [11] and 810% in a Mg–9% Al alloy [12] where all alloys were processed by HPT. By contrast, and despite the slightly larger grain sizes introduced by ECAP, the elongations in ECAP are often much higher and occasionally exceed 1000%. A probable explanation for these lower elongations in HPT lies in the very small thicknesses of the gauge sections within the tensile specimens. It is well-established in superplastic research of conventional materials that the measured elongations to failure are dependent upon the geometry of the test specimens, and an empirical equation was developed in early work relating the total elongation to the specimen diameter and the gauge length [13]. Subsequently, it was demonstrated experimentally that the superplastic elongations are higher when the gauge widths of the tensile specimens are increased [14]. Accordingly, it is reasonable to anticipate that higher superplastic elongations may be achieved in HPT samples if the thicknesses in the gauge sections are also increased.
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.043
1030
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The present investigation was initiated to evaluate the factors influencing the superplastic properties of an Al– 3% Mg–0.2% Sc alloy, where the composition of the alloy is given in weight per cent. This material was selected because earlier research demonstrated excellent superplastic capabilities after processing by ECAP [15,16]. To overcome the problems inherent in the very thin disks generally used in HPT, the experiments were conducted using a facility described earlier and developed specifically to extend the capability of HPT processing to larger bulk samples [17]. The initial samples were in the form of small cylinders 8.57 mm high and 10.0 mm in diameter. The straining facility and the significance of the specimen shape were described in detail in an earlier report [17] but it is important to note that the samples were compressed between upper and lower anvils under an imposed pressure P, the dies were shaped such that the upper and lower receptacles for the sample had vertical surfaces inclined outwards at an angle of 5° to the perpendicular, and on application of the pressure the sample was compressed and forced outwards into a barrel-shaped configuration to fill the die. In this form, the sample height was reduced to 8.0 mm and the outer appearance of the sample was as shown in Figure 1 after a single turn of torsional straining where the upper and lower side faces are inclined outwards with respect to the top and bottom surfaces by 5°: the small ridge visible in Figure 1 around the mid-section of the sample is due to a minor outflow of material between the upper and lower anvils during the straining operation. The experiments were conducted using the same Al– 3% Mg–0.2% Sc alloy and the same material preparation procedures described in earlier reports of testing by ECAP [15,16,18] and HPT [1,17,19]. Briefly, the alloy was prepared from high-purity materials, homogenized in air at 753 K for 24 h, cut and swaged into a 12 mm diameter rod, lathed to a diameter of 10 mm, cut into cylinders 8.6 mm high, solution treated in air at 873 K for 5 h, and then lightly ground to the required height of 8.57 mm. In this initial condition, the measured grain size was 0.5 mm. Processing by HPT was conducted in air under an applied load of 9.3 tonnes, equivalent to an
imposed pressure of P = 1 GPa, using a rotation speed of 1 rpm. The number of rotations, N, imposed on the samples was 2. To evaluate the microstructural characteristics after HPT through two turns, a sample was cut along a vertical section into two equal parts; one exposed face was mechanically polished to a mirror-like finish and the face was then electropolished in an aqueous solution of 5% HBF4 for inspection by optical microscopy. Figure 2 shows a montage of the microstructure where the lower inset indicates sectioning on the central vertical plane. It is apparent that this section exhibits the characteristic flow pattern anticipated for a cylindrical sample subjected to torsional straining when the straining plane lies horizontally across the mid-point of the section. The microstructure is clearly inhomogeneous with very heavily deformed zones around the edges of the cylinder wherein the grains are extremely small and with larger grains in the center of the sample. There is also evidence for a directionality in the shapes of the larger grains which delineates the local flow direction. This result is generally consistent with microstructural features reported earlier after the processing of a bulk sample through one-quarter and one turn apart from the fact that there is now a larger peripheral region of ultrafine grains. Using transmission electron microscopy, the measured grain size on the central plane near the outer edge of the cylinder was 130 nm. It is interesting to note that this is smaller than the grain sizes reported previously for the same alloy processed by different SPD procedures including 200 nm after one turn of HPT for a bulk sample with P = 1 GPa [17], 150 nm after five turns of HPT for a thin disk with P = 6 GPa [7] and 200 nm after ECAP through eight passes [15]. Following HPT, small tensile samples, each 1 mm thick, were cut from the cylinders using two different procedures to give samples oriented either in the straining plane or perpendicular to the straining plane: for convenience, the samples in these two configurations are termed the horizontal and vertical samples, respectively. The procedure for preparing the horizontal samples is shown in Figure 3. A disk with a thickness of 1 mm was sliced from the mid-section of a cylinder after torsional straining as illustrated on the left in Figure 3
Figure 1. Appearance of a bulk sample after HPT through a single turn.
Figure 2. Montage of microstructures after two turns: the plane of sectioning is shown at the lower left.
Z. Horita, T. G. Langdon / Scripta Materialia 58 (2008) 1029–1032 φ 10
1031
2000
S plane
2
1
0.
R
1
3
2
2
Elongation (%)
T = 573 K
Slicing 1
Al-3%Mg-0.2%Sc
L plane 1500
ε = 3.3x10-3 s-1 Specimen horizontal
1000
vertical
500
(unit:mm)
0 0
Figure 3. Procedure for preparing horizontal specimens in the plane of torsional straining.
and tensile specimens were then cut from the disk using electrodischarge machining (EDM) as indicated on the right in Figure 3. In order to avoid the inhomogeneous microstructures associated with the centers of each cylinder, these tensile specimens were cut from off-center positions whereby their longitudinal axes were aligned at a distance of 2.5 mm from the central diameter through the disk: as shown in Figure 3 and following the notation introduced earlier [17], the central vertical or longitudinal plane is termed the L plane and a sectional plane parallel to the L plane but displaced by a distance of 3 mm is termed the S plane. As indicated in Figure 3, the tensile specimens had a gauge length of 1 mm and a gauge section of 1 1 mm2. The procedure for preparing the vertical specimens is shown in Figure 4, where the front view on the left delineates the position of each specimen and the side view on the right shows the shape of the specimens. These specimens were prepared by cutting a block into the overall shape of the tensile specimens using EDM and then slicing this block into the individual specimens. The shape and thickness of these vertical specimens were identical to those of the horizontal specimens. After preparing these miniature tensile specimens, each specimen was mounted horizontally, heated to a temperature of 573 K over a period of 40 min and then pulled to failure using a testing machine operating at a constant rate of cross-head displacement with an initial strain rate, e_ , of 3.3 10 3 s 1. The measured elongations to failure are shown in Figure 5, where the upper point is for a horizontal specimen and the lower points are for the vertical specimens. Three important conclusions may be reached from inspection of Figure 5.
1.5
1
2
3 4.5 (unit:mm)
Figure 4. Procedure for preparing vertical specimens perpendicular to the plane of torsional straining.
1
2
3
4
5
Distance from center (mm)
Figure 5. Variation of elongation with distance from the center of the cylinders for horizontal and vertical specimens.
First, the elongations recorded in the outer peripheral region of the bulk samples are exceptionally high. These elongations of more than 1000% are the highest recorded to date for specimens subjected to HPT processing and the maximum elongation of 1600% in the horizontal specimen contrasts with a reported elongation of 1270% for the same alloy tested under the same conditions of temperature and strain rate after processing through ECAP for eight passes [15]. This result demonstrates conclusively that processing by HPT leads to elongations which are comparable to those attained after ECAP, and indeed the larger elongation now achieved by HPT is attributed to the smaller grain size of 130 nm in HPT compared to 200 nm in ECAP. Conversely, it is concluded that the relatively lower elongations reported in earlier studies for specimens processed by HPT are not an accurate reflection of the superplastic capabilities of the processed materials but rather are a consequence of using specimens having very small thicknesses within the gauge sections. Second, the elongation recorded in the horizontal specimen is unusually high and equal to 1600%, whereas the elongations are slightly lower in the vertical specimens. This difference is reasonable because the horizontal specimen lies in the plane of torsional straining and therefore any elongations of the grains lie parallel to, rather than perpendicular to, the tensile axis. Third, the measured elongations in the vertical specimens are very significantly lower near the center of the cylinder by comparison with the elongations recorded in the peripheral region. This difference is a direct consequence of the larger grains clearly visible in the central region in Figure 2 and also to the wellestablished variations in microhardness and microstructural characteristics across the diameters of conventional disks processed by HPT [20–25]. This result contrasts with the situation in processing by ECAP where experiments have shown there is no significant anisotropy in the superplastic properties when tensile specimens are machined from the pressed billets in three mutually perpendicular directions [26]. Accordingly, the present results provide a clear demonstration of the necessity for cutting tensile specimens in HPT both in a horizontal configuration with respect to the straining plane and from an off-center position to avoid the region of minimum strain in the center of the sample.
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Z. Horita, T. G. Langdon / Scripta Materialia 58 (2008) 1029–1032
Figure 6. Appearance of the horizontal specimen giving an elongation of 1600%: the upper specimen is untested.
Figure 6 shows the appearance of the horizontal specimen that pulled out to 1600% at failure together with an undeformed specimen. It is apparent from Figure 6 that the deformed specimen exhibits all of the characteristics of true superplasticity or quasi-stable plastic flow, including uniform deformation with no evidence for the occurrence of any incipient necking within the gauge length, a pulling down almost to a point at failure and a measured macroscopic elongation greater than 1000% [27]. In summary, experiments were undertaken to determine whether high superplastic elongations may be achieved after processing by HPT when using tensile specimens 1 mm thick. The experiments were conducted using an Al–Mg–Sc alloy in the form of bulk cylinders. The results demonstrate excellent superplastic characteristics after HPT including a maximum tensile elongation of 1600% at a testing temperature of 573 K. It is concluded that the relatively lower elongations to failure generally observed after superplastic testing in HPT specimens are a direct consequence of using specimens with very thin gauge sections. These results show also that the measured elongations depend critically upon the location of the tensile specimens within the bulk cylinder and on whether the specimens are cut parallel or perpendicular to the straining plane. We are grateful to Mr. Genki Sakai and Mr. Masaaki Kai for experimental assistance. This work was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, in the Priority Area ‘‘Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects” and in part by the National Science Foundation of the United States under Grant No. DMR0243331.
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