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Improving the high-temperature mechanical properties of a magnesium alloy by equal-channel angular pressing
Materials Science and Engineering A 410–411 (2005) 435–438
Improving the high-temperature mechanical properties of a magnesium alloy by equal-channel angular pressing Bing Q. Han a,∗ , Terence G. Langdon b a
b
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 9561, USA Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Received in revised form 16 March 2005
Abstract Equal-channel angular pressing (ECAP) was conducted on a Mg–0.55% Zr alloy at a temperature of 513 K. It is shown that processing by ECAP refines the grain size from ∼75 to ∼8.6 m. Tensile testing was performed at temperatures from 473 to 773 K and at initial strain rates from 10−5 to 10−2 s−1 using specimens in the as-received condition and after processing by ECAP through six passes. There is a higher strength in the as-received Mg–Zr alloy than in the Mg–Zr alloy after ECAP for six passes at a temperature of 513 K. The results show the tensile elongations are higher after ECAP and a maximum elongation of ∼380% was achieved in the as-pressed material at 773 K using an initial strain rate of 4.2 × 10−5 s−1 . The strain rate sensitivity for the as-pressed alloy was estimated as ∼0.17 and the activation energy were intermediate between the values for grain boundary and lattice diffusion in magnesium. © 2005 Elsevier B.V. All rights reserved. Keywords: Equal-channel angular pressing; Magnesium; Mechanical properties; Tensile testing
1. Introduction The application of severe plastic deformation (SPD) to bulk solids has become an established procedure for producing grain refinement to the submicrometer or nanometer level [1]. Although several different SPD techniques are now available, major interest has centered primarily on developing the process of equal-channel angular pressing (ECAP) [2]. This interest arises for two reasons. First, ECAP can be scaled up fairly easily to produce large bulk samples [3]. Second, it is possible to incorporate ECAP into continuous production operations, such as conshearing [4], continuous confined strip shearing [5] and the ECAP-conform process [6]. Low-density magnesium alloys are attractive as structural materials in a wide range of applications including in the automotive and aerospace industries [7–9]. They also have a major potential for hydrogen storage [10]. Nevertheless, the hexagonal close-packed (HCP) lattice structure generally leads to limited ductility so that an important current requirement is the development of processing procedures to improve the mechanical
∗
Corresponding author. Tel.: +1 530 752 9568; fax: +1 530 752 9554. E-mail address: [email protected] (B.Q. Han).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.084
properties of Mg-based alloys. The present investigation was initiated to evaluate the potential for improving the properties of a magnesium alloy through the use of ECAP. 2. Experimental material and procedures The experiments were conducted using a fully hydrided binary magnesium alloy, Mg–0.55 wt% Zr henceforth designated Mg–Zr55, which was received in the form of an extruded bar having a diameter of 14.3 mm. This material has been used extensively in creep testing, primarily because the precipitation of ZrH2 in the form of hydride stringers provides internal markers, which are convenient in assessing the importance of diffusional creep [11,12]. The extruded material was machined into billets for ECAP with diameters of 10 mm and lengths of 64 mm. The processing by ECAP was conducted at 513 K using a solid die having an internal channel bent through an angle of Φ = 90◦ and with an angle of Ψ ≈ 20◦ representing the outer arc of curvature where the two channels intersect. Pressing was conducted by heating the die to 513 K, placing the billet into the vertical channel, holding for ∼10 min to establish thermal equilibrium and then pressing the billet into the horizontal channel. The billet then remained in the horizontal channel until it was pushed out by the next sample. The billets were pressed repeti-
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B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
The microstructure of the Mg–Zr55 alloy is shown in Fig. 1 in (a) the as-received state and (b) after ECAP for six passes. Hydride stringers are clearly visible parallel to the extrusion direction in the as-received condition and the average linear intercept grain size was measured as ∼75 m in the longitudinal
direction and ∼55 m in the transverse direction. Observations showed these stringers, initially continuous, were sheared into discontinuous segments during the first pass of ECAP such that the numbers of stringers increased and the distances between the stringers decreased with subsequent pressings. The microstructure after six passes is shown in Fig. 1(b) where the average linear intercept grain size is ∼8.6 m along the longitudinal or pressing direction. This microstructure also shows the presence of an array of discontinuous Zr-rich stringers (dark areas). It is possible these stringers interfere with grain refinement because the measured grain size is somewhat larger than the ultrafine grains generally produced in ECAP processing. Fig. 2 shows the tensile properties achieved at 573 K when using an initial strain rate, ε˙ , of 1.0 × 10−4 s−1 for three different conditions: in the as-received condition, after one pass and after six passes of ECAP. It is apparent that the strength decreases with ECAP, as anticipated for the smaller grain size, but the elongation increases with increasing numbers of ECAP passes. A full summary of the mechanical properties is given in Fig. 3 for the as-received condition and after six passes of ECAP: (a) and (b) plot the elongations to failure against the absolute temperature, T, at an initial strain rate of 1.0 × 10−4 s−1 and against the strain rate at a temperature of 773 K, (c) plots the true stress, σ, at a true strain, ε, of 0.2 against the true strain rate and (d) plots the normalized stress against the reciprocal of the testing temperature. Fig. 3(a and b) confirm that higher elongations are achieved after ECAP and, in addition, the elongations after ECAP are generally >200%. It is apparent from Fig. 3(b) that the elongation increases with decreasing strain rate, with a maximum elongation of ∼380% recorded at 773 K at the lowest strain rate of 4.2 × 10−5 s−1 . It follows from Fig. 3(c) that the strain rate sensitivity, m, is ∼0.13 in the as-received condition and ∼0.17 after ECAP. The plot in Fig. 3(d) is derived from the nature of the conventional relationship describing hightemperature deformation where the strain rate may be expressed in the form: AGb σ n Q ε˙ = Do exp − (1) kT G RT
Fig. 1. Optical microstructures of Mg–Zr55 alloy in (a) the as-received condition and (b) after ECAP for six passes.
Fig. 2. True stress vs. true strain at 573 K with an initial strain rate of 1 × 10−4 s−1 for three different conditions.
tively using processing route BC in which the samples are rotated by 90◦ in the same sense between each separate pass [13]. Most of the pressings were continued to a total of six passes, equivalent to an imposed strain of ∼6 [14], when ECAP was terminated because of the appearance of heterogeneous shear bands on the surfaces of the samples. Tensile specimens were machined from the as-pressed billets with gauge lengths of 4 mm lying parallel to the pressing direction and with thicknesses and widths of 2 and 3 mm, respectively. Tensile testing was conducted at temperatures in the range from 298 to 773 K using an Instron testing machine operating at a constant rate of cross-head displacement with initial strain rates from 10−5 to 10−2 s−1 . All tensile specimens were heated to the required temperature and then held for ∼20 min prior to loading. 3. Experimental results
B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
437
Fig. 3. Mechanical properties of the Mg–Zr55 alloy in the as-received condition and after ECAP for six passes: (a) elongation vs. temperature, (b) elongation vs. initial strain rate, (c) true stress vs. true strain rate at a true strain of 0.2 when testing at 773 K and (d) temperature-compensated strain rate vs. the reciprocal of temperature at constant strain rate.
where A is the dimensionless constant, G the shear modulus, b the Burgers vector, k the Boltzmann’s constant, Do the preexponential frequency factor, Q the activation energy, R the gas constant and n (= 1/m) is the stress exponent. It follows from Eq. (1) that the value of Q can be obtained from a semilogarithmic plot of the normalized stress, σ n /G−1 T,against 1/T at a constant strain rate. This plot is shown in Fig. 3(d) where G was taken from published data [15] and the value of n was taken as 5 which is reasonably consistent with the strain rate sensitivity after ECAP. It follows from Fig. 3(d) that the activation energies are ∼100 and ∼80 kJ mol−1 for flow in the ECAP alloy and the as-received alloy, respectively, where the value for the ECAP alloy is intermediate between the anticipated activation energies of ∼135 and ∼92 kJ mol−1 for lattice and grain boundary diffusion in magnesium, respectively [15]. Finally, it is important to note that the refined grains are not stable at the elevated temperatures used in tensile testing. Careful inspection of samples after testing to failure at temperatures from 673 to 773 K showed the occurrence of extensive grain growth with average grain sizes typically >40 m.
4. Discussion These results show the potential for using ECAP to achieve significant grain refinement and an improvement in the hightemperature mechanical properties in the Mg–Zr55 alloy. In the present experiments, the longitudinal grain size was reduced from ∼75 to ∼8.6 m through ECAP at 513 K. An earlier report described the difficulty of achieving substantial grain refinement in pure Mg and Mg-based alloys through ECAP processing [16]. This difficulty was subsequently overcome by developing a two-step processing procedure, termed EX-ECAP, in which magnesium-based alloys are initially extruded and then subjected to grain refinement using ECAP [17,18]. The present investigation is consistent with this approach because the Mg–Zr55 alloy was supplied in an extruded condition. The advantage of using this two-step approach is generally attributed to the effect of extrusion, which produces an alignment of the basal planes in the extrusion direction [19–21]. The superior mechanical properties achieved after ECAP, including higher ductility by comparison with the as-received
438
B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
alloy, is due to the smaller grain size and the presence of easy non-basal slip at these high temperatures. The elongations achieved in the present investigation are consistent also with the elongations reported earlier for a Mg–0.6% Zr alloy where the grain size was refined to ∼1.4 m through pressing for a total of four passes at 573 K [22], in the Mg–0.6% Zr alloy, the maximum elongations were also in the range of 300–400% with the highest elongations occurring again at the slowest experimental strain rates. It is important to note that the pressing temperature of 513 K used in this investigation permits the occurrence of intensive shear deformation during ECAP so that the pressing can be conducted smoothly and reasonably homogeneously without the development of the macroscopic shear bands that lead to premature failure. Nevertheless, the fine grains produced in the Mg–Zr55 alloy are not stable at high temperatures because of the lack of suitable precipitates to restrict grain growth. Thus, the grains grow easily during high-temperature tensile testing. Despite these difficulties, it is apparent from Fig. 3 that the mechanical properties are enhanced by subjecting the Mg–Zr55 alloy to ECAP. 5. Summary and conclusions 1. Equal-channel angular pressing was performed on a Mg–Zr55 alloy at a temperature of 513 K. The grain size was refined from ∼75 to ∼8.6 m after six passes and the hydride stringers were broken and became discontinuous. 2. Excellent mechanical properties were recorded in the aspressed alloy at high temperatures including a maximum tensile elongation of ∼380% at 773 K when using an initial strain rate of 4.2 × 10−5 s−1 .
References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [2] M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, J. Mater. Sci. 36 (2001) 2835. [3] Z. Horita, T. Fujinami, T.G. Langdon, Mater. Sci. Eng. A318 (2001) 34. [4] Y. Saito, H. Utsunomiya, H. Suzuki, T. Sakai, Scripta Mater. 42 (2000) 1139. [5] J.-C. Lee, H.-K. Seok, J.-Y. Suh, Acta Mater. 50 (2002) 400. [6] G.J. Raab, R.Z. Valiev, T.C. Lowe, Y.T. Zhu, Mater. Sci. Eng. 382A (2004) 30. [7] F.H. Froes, D. Eliezer, E. Aghion, JOM 50 (9) (1998) 30. [8] H. Friedrich, S. Schumann, J. Mater. Proc. Technol. 117 (2001) 276. [9] K. Johnson, Adv. Mater. Proc. 160 (6) (2002) 62. [10] V.M. Skripnyuk, E. Rabkin, Y. Estrin, R. Lapovok, Acta Mater. 52 (2004) 405. [11] J.E. Harris, Met. Sci. J. 7 (1973) 1. [12] K.R. McNee, G.W. Greenwood, H. Jones, Scripta Mater. 46 (2002) 437. [13] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [14] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [15] H.J. Frost, M.F. Ashby, Deformation-Mechanism Maps: The Plasticity and Creep of Metals and Ceramics, Pergamon Press, Oxford, UK, 1982. [16] A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A300 (2001) 142. [17] Z. Horita, K. Matsubara, K. Makii, T.G. Langdon, Scripta Mater. 47 (2002) 255. [18] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langdon, Acta Mater. 51 (2003) 3073. [19] D.V. Wilson, J. Inst. Met. 98 (1970) 133. [20] M. Hilpert, A. Styczynski, J. Kiese, L. Wagner, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys Their Applications, Wiley-VCH, Weinheim, Germany, 1998, p. 319. [21] T. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Scripta Mater. 45 (2001) 89. [22] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langdon, Metall. Mater. Trans. 35A (2004) 1735.
Improving the high-temperature mechanical properties of a magnesium alloy by equal-channel angular pressing Bing Q. Han a,∗ , Terence G. Langdon b a
b
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 9561, USA Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Received in revised form 16 March 2005
Abstract Equal-channel angular pressing (ECAP) was conducted on a Mg–0.55% Zr alloy at a temperature of 513 K. It is shown that processing by ECAP refines the grain size from ∼75 to ∼8.6 m. Tensile testing was performed at temperatures from 473 to 773 K and at initial strain rates from 10−5 to 10−2 s−1 using specimens in the as-received condition and after processing by ECAP through six passes. There is a higher strength in the as-received Mg–Zr alloy than in the Mg–Zr alloy after ECAP for six passes at a temperature of 513 K. The results show the tensile elongations are higher after ECAP and a maximum elongation of ∼380% was achieved in the as-pressed material at 773 K using an initial strain rate of 4.2 × 10−5 s−1 . The strain rate sensitivity for the as-pressed alloy was estimated as ∼0.17 and the activation energy were intermediate between the values for grain boundary and lattice diffusion in magnesium. © 2005 Elsevier B.V. All rights reserved. Keywords: Equal-channel angular pressing; Magnesium; Mechanical properties; Tensile testing
1. Introduction The application of severe plastic deformation (SPD) to bulk solids has become an established procedure for producing grain refinement to the submicrometer or nanometer level [1]. Although several different SPD techniques are now available, major interest has centered primarily on developing the process of equal-channel angular pressing (ECAP) [2]. This interest arises for two reasons. First, ECAP can be scaled up fairly easily to produce large bulk samples [3]. Second, it is possible to incorporate ECAP into continuous production operations, such as conshearing [4], continuous confined strip shearing [5] and the ECAP-conform process [6]. Low-density magnesium alloys are attractive as structural materials in a wide range of applications including in the automotive and aerospace industries [7–9]. They also have a major potential for hydrogen storage [10]. Nevertheless, the hexagonal close-packed (HCP) lattice structure generally leads to limited ductility so that an important current requirement is the development of processing procedures to improve the mechanical
∗
Corresponding author. Tel.: +1 530 752 9568; fax: +1 530 752 9554. E-mail address: [email protected] (B.Q. Han).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.084
properties of Mg-based alloys. The present investigation was initiated to evaluate the potential for improving the properties of a magnesium alloy through the use of ECAP. 2. Experimental material and procedures The experiments were conducted using a fully hydrided binary magnesium alloy, Mg–0.55 wt% Zr henceforth designated Mg–Zr55, which was received in the form of an extruded bar having a diameter of 14.3 mm. This material has been used extensively in creep testing, primarily because the precipitation of ZrH2 in the form of hydride stringers provides internal markers, which are convenient in assessing the importance of diffusional creep [11,12]. The extruded material was machined into billets for ECAP with diameters of 10 mm and lengths of 64 mm. The processing by ECAP was conducted at 513 K using a solid die having an internal channel bent through an angle of Φ = 90◦ and with an angle of Ψ ≈ 20◦ representing the outer arc of curvature where the two channels intersect. Pressing was conducted by heating the die to 513 K, placing the billet into the vertical channel, holding for ∼10 min to establish thermal equilibrium and then pressing the billet into the horizontal channel. The billet then remained in the horizontal channel until it was pushed out by the next sample. The billets were pressed repeti-
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B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
The microstructure of the Mg–Zr55 alloy is shown in Fig. 1 in (a) the as-received state and (b) after ECAP for six passes. Hydride stringers are clearly visible parallel to the extrusion direction in the as-received condition and the average linear intercept grain size was measured as ∼75 m in the longitudinal
direction and ∼55 m in the transverse direction. Observations showed these stringers, initially continuous, were sheared into discontinuous segments during the first pass of ECAP such that the numbers of stringers increased and the distances between the stringers decreased with subsequent pressings. The microstructure after six passes is shown in Fig. 1(b) where the average linear intercept grain size is ∼8.6 m along the longitudinal or pressing direction. This microstructure also shows the presence of an array of discontinuous Zr-rich stringers (dark areas). It is possible these stringers interfere with grain refinement because the measured grain size is somewhat larger than the ultrafine grains generally produced in ECAP processing. Fig. 2 shows the tensile properties achieved at 573 K when using an initial strain rate, ε˙ , of 1.0 × 10−4 s−1 for three different conditions: in the as-received condition, after one pass and after six passes of ECAP. It is apparent that the strength decreases with ECAP, as anticipated for the smaller grain size, but the elongation increases with increasing numbers of ECAP passes. A full summary of the mechanical properties is given in Fig. 3 for the as-received condition and after six passes of ECAP: (a) and (b) plot the elongations to failure against the absolute temperature, T, at an initial strain rate of 1.0 × 10−4 s−1 and against the strain rate at a temperature of 773 K, (c) plots the true stress, σ, at a true strain, ε, of 0.2 against the true strain rate and (d) plots the normalized stress against the reciprocal of the testing temperature. Fig. 3(a and b) confirm that higher elongations are achieved after ECAP and, in addition, the elongations after ECAP are generally >200%. It is apparent from Fig. 3(b) that the elongation increases with decreasing strain rate, with a maximum elongation of ∼380% recorded at 773 K at the lowest strain rate of 4.2 × 10−5 s−1 . It follows from Fig. 3(c) that the strain rate sensitivity, m, is ∼0.13 in the as-received condition and ∼0.17 after ECAP. The plot in Fig. 3(d) is derived from the nature of the conventional relationship describing hightemperature deformation where the strain rate may be expressed in the form: AGb σ n Q ε˙ = Do exp − (1) kT G RT
Fig. 1. Optical microstructures of Mg–Zr55 alloy in (a) the as-received condition and (b) after ECAP for six passes.
Fig. 2. True stress vs. true strain at 573 K with an initial strain rate of 1 × 10−4 s−1 for three different conditions.
tively using processing route BC in which the samples are rotated by 90◦ in the same sense between each separate pass [13]. Most of the pressings were continued to a total of six passes, equivalent to an imposed strain of ∼6 [14], when ECAP was terminated because of the appearance of heterogeneous shear bands on the surfaces of the samples. Tensile specimens were machined from the as-pressed billets with gauge lengths of 4 mm lying parallel to the pressing direction and with thicknesses and widths of 2 and 3 mm, respectively. Tensile testing was conducted at temperatures in the range from 298 to 773 K using an Instron testing machine operating at a constant rate of cross-head displacement with initial strain rates from 10−5 to 10−2 s−1 . All tensile specimens were heated to the required temperature and then held for ∼20 min prior to loading. 3. Experimental results
B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
437
Fig. 3. Mechanical properties of the Mg–Zr55 alloy in the as-received condition and after ECAP for six passes: (a) elongation vs. temperature, (b) elongation vs. initial strain rate, (c) true stress vs. true strain rate at a true strain of 0.2 when testing at 773 K and (d) temperature-compensated strain rate vs. the reciprocal of temperature at constant strain rate.
where A is the dimensionless constant, G the shear modulus, b the Burgers vector, k the Boltzmann’s constant, Do the preexponential frequency factor, Q the activation energy, R the gas constant and n (= 1/m) is the stress exponent. It follows from Eq. (1) that the value of Q can be obtained from a semilogarithmic plot of the normalized stress, σ n /G−1 T,against 1/T at a constant strain rate. This plot is shown in Fig. 3(d) where G was taken from published data [15] and the value of n was taken as 5 which is reasonably consistent with the strain rate sensitivity after ECAP. It follows from Fig. 3(d) that the activation energies are ∼100 and ∼80 kJ mol−1 for flow in the ECAP alloy and the as-received alloy, respectively, where the value for the ECAP alloy is intermediate between the anticipated activation energies of ∼135 and ∼92 kJ mol−1 for lattice and grain boundary diffusion in magnesium, respectively [15]. Finally, it is important to note that the refined grains are not stable at the elevated temperatures used in tensile testing. Careful inspection of samples after testing to failure at temperatures from 673 to 773 K showed the occurrence of extensive grain growth with average grain sizes typically >40 m.
4. Discussion These results show the potential for using ECAP to achieve significant grain refinement and an improvement in the hightemperature mechanical properties in the Mg–Zr55 alloy. In the present experiments, the longitudinal grain size was reduced from ∼75 to ∼8.6 m through ECAP at 513 K. An earlier report described the difficulty of achieving substantial grain refinement in pure Mg and Mg-based alloys through ECAP processing [16]. This difficulty was subsequently overcome by developing a two-step processing procedure, termed EX-ECAP, in which magnesium-based alloys are initially extruded and then subjected to grain refinement using ECAP [17,18]. The present investigation is consistent with this approach because the Mg–Zr55 alloy was supplied in an extruded condition. The advantage of using this two-step approach is generally attributed to the effect of extrusion, which produces an alignment of the basal planes in the extrusion direction [19–21]. The superior mechanical properties achieved after ECAP, including higher ductility by comparison with the as-received
438
B.Q. Han, T.G. Langdon / Materials Science and Engineering A 410–411 (2005) 435–438
alloy, is due to the smaller grain size and the presence of easy non-basal slip at these high temperatures. The elongations achieved in the present investigation are consistent also with the elongations reported earlier for a Mg–0.6% Zr alloy where the grain size was refined to ∼1.4 m through pressing for a total of four passes at 573 K [22], in the Mg–0.6% Zr alloy, the maximum elongations were also in the range of 300–400% with the highest elongations occurring again at the slowest experimental strain rates. It is important to note that the pressing temperature of 513 K used in this investigation permits the occurrence of intensive shear deformation during ECAP so that the pressing can be conducted smoothly and reasonably homogeneously without the development of the macroscopic shear bands that lead to premature failure. Nevertheless, the fine grains produced in the Mg–Zr55 alloy are not stable at high temperatures because of the lack of suitable precipitates to restrict grain growth. Thus, the grains grow easily during high-temperature tensile testing. Despite these difficulties, it is apparent from Fig. 3 that the mechanical properties are enhanced by subjecting the Mg–Zr55 alloy to ECAP. 5. Summary and conclusions 1. Equal-channel angular pressing was performed on a Mg–Zr55 alloy at a temperature of 513 K. The grain size was refined from ∼75 to ∼8.6 m after six passes and the hydride stringers were broken and became discontinuous. 2. Excellent mechanical properties were recorded in the aspressed alloy at high temperatures including a maximum tensile elongation of ∼380% at 773 K when using an initial strain rate of 4.2 × 10−5 s−1 .
References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [2] M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, J. Mater. Sci. 36 (2001) 2835. [3] Z. Horita, T. Fujinami, T.G. Langdon, Mater. Sci. Eng. A318 (2001) 34. [4] Y. Saito, H. Utsunomiya, H. Suzuki, T. Sakai, Scripta Mater. 42 (2000) 1139. [5] J.-C. Lee, H.-K. Seok, J.-Y. Suh, Acta Mater. 50 (2002) 400. [6] G.J. Raab, R.Z. Valiev, T.C. Lowe, Y.T. Zhu, Mater. Sci. Eng. 382A (2004) 30. [7] F.H. Froes, D. Eliezer, E. Aghion, JOM 50 (9) (1998) 30. [8] H. Friedrich, S. Schumann, J. Mater. Proc. Technol. 117 (2001) 276. [9] K. Johnson, Adv. Mater. Proc. 160 (6) (2002) 62. [10] V.M. Skripnyuk, E. Rabkin, Y. Estrin, R. Lapovok, Acta Mater. 52 (2004) 405. [11] J.E. Harris, Met. Sci. J. 7 (1973) 1. [12] K.R. McNee, G.W. Greenwood, H. Jones, Scripta Mater. 46 (2002) 437. [13] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [14] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [15] H.J. Frost, M.F. Ashby, Deformation-Mechanism Maps: The Plasticity and Creep of Metals and Ceramics, Pergamon Press, Oxford, UK, 1982. [16] A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A300 (2001) 142. [17] Z. Horita, K. Matsubara, K. Makii, T.G. Langdon, Scripta Mater. 47 (2002) 255. [18] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langdon, Acta Mater. 51 (2003) 3073. [19] D.V. Wilson, J. Inst. Met. 98 (1970) 133. [20] M. Hilpert, A. Styczynski, J. Kiese, L. Wagner, in: B.L. Mordike, K.U. Kainer (Eds.), Magnesium Alloys Their Applications, Wiley-VCH, Weinheim, Germany, 1998, p. 319. [21] T. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Scripta Mater. 45 (2001) 89. [22] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langdon, Metall. Mater. Trans. 35A (2004) 1735.