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Effects of ECAE process on microstructure and transformation behavior of TiNi shape memory alloy
Materials & Design Materials and Design 27 (2006) 324–328 www.elsevier.com/locate/matdes
Effects of ECAE process on microstructure and transformation behavior of TiNi shape memory alloy Zhenhua Li, Guoquan Xiang, Xianhua Cheng
*
School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 3 June 2004; accepted 18 October 2004 Available online 8 January 2005
Abstract Equal channel angular extrusion (ECAE) process was a deep pure shear deformation method for structural materials, developed in early 90s. ECAE technique has been successfully applied to several pure metals and alloys to refine the microstructure and improve the mechanical properties of materials. In the present paper, TiNi shape memory alloys have been treated by ECAE process. The effects of ECAE process on microstructures and transformation behaviors have been investigated. The initial coarse grains of TiNi alloy were refined after two passes of ECAE processes and short annealing at 1023 K. Transformation temperatures of TiNi alloy sharply decreased after two ECAE processes, then rapidly rose back when annealed at 773 K for 2 h. 2004 Published by Elsevier Ltd. Keywords: TiNi shape memory alloy; Equal channel angular extrusion; Phase transformations
1. Introduction Ultra-fine grained materials exhibit superior mechanical properties. The equal channel angular extrusion (ECAE) technique, introduced by Segal et al. [1], has been successfully applied to produce various ultra-fine grained materials, such as low carbon steel [2,3], copper [4–6], Al alloys [7–10], pure Ti [11,12] and Ti–6Al–4V [13] et al. However, the properties and microstructures of TiNi shape memory alloys treated by ECAE process have been reported few. Recently, Nakayama et al. [14] analyzed crystal refinement of TiNi shape memory alloy by cold rolling. Khmelevskaya et al. [15] investigated microstructure of TiNi-based shape memory alloys after severe plastic deformation. Transformation behavior of TiNi alloy after ECAE process has not been reported yet. In the present paper, the microstructure evolutions
*
Corresponding author. E-mail address: [email protected] (X. Cheng).
0261-3069/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.matdes.2004.10.025
and transformation behavior of TiNi shape memory alloy after high temperature ECAE processes have been analyzed. 2. Experimental method Experimental materials were Ti-50.6at% Ni alloy rods, with a diameter of 25 mm, after 1123 K hot forging and 773 K annealing for 2 h. Billets for ECAE process with dimensions of 10 · 10 · 55 mm3 were cut from the TiNi rod. The ECAE die was designed to yield an effective strain of 1 by a single pass. The inner contact angle (U) and the arc of curvature (w) at the outer point of contact between channels of the die were 90 and 90, respectively, as shown in Fig. 1. Two ECAE processes were conducted at high temperature. Keeping the die at 823 K, billets were preheated at 1123 K for 20 min before the first ECAE extrusion, and 1023 K for 20 min before the second ECAE extrusion. During ECAE process, the billet was not rotated between passages.
Z. Li et al. / Materials and Design 27 (2006) 324–328
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Fig. 1. Schematic illustration of the ECAE facility in the experiment. Fig. 2. Optical micrograph of TiNi alloy before the first ECAE process.
Samples for DSC and microstructure observation were cut from billets after ECAE process along extruding direction. Some of them were subsequently annealed at 1023 K for 5–20 min to observe their microstructural changes during static re-crystallization. After mechanically polishing and eroding in a mixture solution of HF:HNO3:H2O with a ratio of 1:3:10, samples were observed on a NEOPHOT-1 optical microscope. Differential scanning calorimeter (DSC) measurements were carried out at heating and cooling rates of 10 K/min to analyze the phase transformation behaviors.
3. Results and discussion 3.1. Evolution of the microstructures of TiNi alloys The initial microstructure of as-received hot forged and 773 K annealed TiNi alloy rod was shown in Fig. 2. The grains were coarse and unequal, a little extending along the axial direction of alloy rod. During the first ECAE process, billet was preheated at 1123 K for 20 min. It was found that the microstructure was coarse after the preheating. After the first ECAE extrusion, grains were elongated in a direction parallel to the shear direction and forming an angle of 30 respect to the extruding direction, as shown in Fig. 3(a) After the first ECAE extrusion, the billet was heated to 1023 K and held for 20 min before the second ECAE extrusion. It was observed that elongated grains after the first ECAE process became equaixed grains about 10 lm in size after 1023 K annealing for 20 min, as shown in Fig. 3(b), which revealed that there was static re-crystallization occurred during 1023 K annealing. Comparing Fig. 2 with Fig. 3(b), it was seen that TiNi grains remarkably refined after the first
high temperature ECAE process and static recrystallization at 1023 K for 20 min. Compared with the first ECAE treatment, the second ECAE extrusion resulted in denser shear bands, as seen in Fig. 3(c). Grains were elongated dramatically and parallel to the shear direction, forming an angle of 45 respect to the extrusion direction. From Figs. 2 and 3(a)–(c), it was reasonably suggested that there were not dynamical re-crystallization occurred during high temperature ECAE processes. However, there was probably dynamical recovery occurred during high ECAE processes, especially for the first ECAE process, because these ECAE processes were performed at so high temperature and under changing temperature condition, in which the die was kept at 823 K and billets were preheated at 1123 K, 20 min and 1023 K, 20 min for the first ECAE process and the second ECAE process, respectively. Recrystallized fine grains appeared in the specimen annealed at 1023 K for 5 min after above two passages of ECAE. Elongated grains became nearly full equaixed grains when specimens were heated to 1023 K for 10–20 min, as shown in Fig. 3(d). Compared to Fig. 3(b), grains of Fig. 3(d) were finer. Fig. 4 showed the microstructure of specimen annealed at 773 K for 2 h after above two passages of ECAE. Grains were still elongated as before, but a little bit widened. The microstructure of specimen annealed at 873 K for 2 h were similar to that in Fig. 4, which revealed that there was no re-crystallization occurred during annealing at 773 or 873 K for 2 h. It was suggested that the static re-crystallization temperature for TiNi specimen after above two high temperature ECAE processes was higher than 873 K. However, there possibly were static recoveries occurred since the elongated
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Fig. 3. Optical microstructure of TiNi specimens after ECAE processes: (a) after the first ECAE process; (b) annealed at 1023 K for 20 min; (c) after the second ECAE process and (d) annealed at 1023 K for 10 min.
grains were widened after annealing at 773 or 873 K for 2 h, comparing Fig. 4 with Fig. 3(c). 3.2. Transformation behaviors The transformation behavior of TiNi specimen, 1123 K hot forged and aged at 773 K for 2 h, was shown in Fig. 5(a). Two exothermic peaks were seen when the specimen was cooled down from 373 K, which were corresponding to the B2 parent phase to the R-phase and then to martensite phase transformations [16]. Transformation temperatures were higher than that of quenched binary TiNi alloy with a same composition [17]. The mainly reason was that hot forged Ti-50.6at% Ni had
been annealed at 773 K for 2 h, which resulted in the precipitation of Ti3Ni4 and the decrease of Ni content in the matrix, therefore, increased transformation temperatures [18]. During heating, there was only one endothermic peak observed, which revealed that reverse R-phase transformation and reverse martensitic transformation were overlapped together [19]. Table 1 showed the transformation temperature values of Ms, Mf, Rs, Rf, As and Af at different procedures. The transformation behaviors of TiNi alloy after above two ECAE processes were shown in Fig. 5(b). It was found that there was only one broadened exothermic peak on the cooling curve and the B2 to Rphase transformation peak was unclear. Meanwhile,
Z. Li et al. / Materials and Design 27 (2006) 324–328
Fig. 4. Microstructure of the sample annealed at 773 K for 2 h after two passages of ECAE.
the transformation temperatures decreased sharply. There were two factors which possibly caused the decrease of transformation temperatures. One was the dissolving of Ti3Ni4 precipitates into the matrix when TiNi billet was preheated at 1123 and 1023 K for 20 min before the first and the second ECAE process,
327
respectively. It was reasonably suggested that Ti3Ni4 phase was unable to precipitate again during the ECAE process due to the high temperature of ECAE process and the short extrusion time. As a result, Ni content in the matrix increased and then transformation temperatures decreased correspondingly. The other factor was the severe shear deformation of TiNi matrix during the ECAE process, which was under further discussion. Compared with those of the specimen in Fig. 5(b), transformation temperatures of specimen annealed at 1023 K for 5–20 min after above two ECAE passages were increased obviously, as shown in Fig. 6, while there was few change in transformation temperatures as the annealing time extended from 5 to 20 min, which revealed that there was no Ni content change in the matrix with increasing the annealing time. It was known that annealing at high temperature like 1023 K, Ti3Ni4 phase would not precipitate. Other precipitates, such as TiNi2 or TiNi3, usually appeared after long time aging at 873 K or above [20]. Considering the microstructure change after 1023 K annealing as shown in Fig. 3(d), it was reasonably suggested that the re-crystallization may give rise to the increase of transformation temperatures in Fig. 6. The mechanism of the effect of re-crystallization on phase transformation was still under investigation.
Fig. 5. Transformation behavior of TiNi alloy before (a) and after (b) two ECAE processes. Table 1 Transformation temperatures of TiNi specimens Temperatures (K)
As
Af
Rs
Rf
Ms
Mf
Procedures Received TiNi alloy specimen After ECAE process Annealed at 673K for 5 min Annealed at 773 K for 5 min Annealed at 773 K for 2 h Annealed at 873 K for 5 min Annealed at 1023 K for 5 min Annealed at 1023 K for 10 min Annealed at 1023 K for 20 min
327.9 260.08 272.27 277.49 326.95 263.13 261.51 262.09 266.5
353.15 283.15 291.9 300.65 335.65 289.4 293.15 288.15 288.15
320.38 – – 275.65 320.65 – – – –
303.15 – – 243.15 302.99 – – – –
294.49 227.38 267.4 226.9 289.87 261.99 266.17 263.65 264.06
276.9 211.9 230.65 225.65 281.9 224.4 231.9 226.9 239.4
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Z. Li et al. / Materials and Design 27 (2006) 324–328
Fig. 6. DSC curves of TiNi specimens annealed at 1023 K for 5 min (a), 20 min (b) after two ECAE processes.
Acknowledgement This project was supported by National Natural Science Foundation of China, Grant No. A50071034.
References
Fig. 7. Transformation behavior of TiNi alloy annealed 2 h at 773 K, furnace cooling after two ECAE processes.
Fig. 7 showed DSC curves of TiNi specimen annealed at 773 K for 2 h after above two ECAE processes. Compared with Figs. 5 and 6, it was obviously seen that transformation temperatures remarkably increased and similar to the ones in Fig. 5(a). It was suggested that Ti3Ni4 phase had precipitated again in matrix after annealed at 773 K for 2 h, which decreased the Ni content in the matrix and resulted in the increase of transformation temperatures. 4. Conclusions During high temperature ECAE process, there were not dynamical re-crystallization occurred, while there was probably dynamical recovery occured. Annealed for 5 min at 1023 K after two passes of ECAE, grains were refined and became even. After two passes of ECAE, transformation temperatures of the billet of TiNi alloy sharply decreased. Transformation temperature of the sample remarkably increased annealed for 2 h at 773 K after two ECAE processes, similar to the one of TiNi alloy before ECAE process, which was related to Ni content in the matrix.
[1] Segal VM, Reznikov VI, Drobyshevskiy AD, et al.. Russ Metall 1981;1:99. [2] Shin Dong Hyuk, Kim Byung Cheol, Park Kyung-Tae, et al.. Acta Mater 2000;48:3245–3252. [3] Aoki Koshiro, Kimura Youichiro, Asada Yousuke, et al.. Mater Sci Forum 2003;426–432:2705–2710. [4] Valiev RZ, Alexandrov IV. Nanostruct Mater 1999;12:35. [5] Guo Z, Solas D, Etter AL, et al.. Mater Sci Forum 2003;426– 432:2723–2728. [6] Short K, Wuhrer R, Collins G, et al.. Mater Sci Forum 2003;426– 432:2369–2374. [7] Bowen JR, Mishin OV, Prangnell PB, Juul Jensen D. Scripta Mater 2002;47:289–294. [8] Vogel SC, Alexander DJ, Beyerlein IJ, et al.. Mater Sci Forum 2003;426–432:2661–2666. [9] Furukawa Minoru, Fukuda Yukihide, Oh-ishi Keiichiro, et al.. Mater Sci Forum 2003;426–432:2711–2716. [10] Pithan C, Hashimoto T, Kawazoe M, et al.. Mater Sci Eng A 2000;280:62–68. [11] Stolyarov Vladimir V, Zhu Yuntian T, Lowe Terry C, et al.. Mater Sci Eng A 2001;303:82–89. [12] Sergueeva AV, Stolyarov VV, Valiev RZ, Mukherjee AK. Scripta Mater 2001;45:747–752. [13] Semiatin SL, Delo DP. Mater Des 2000;21:311–322. [14] Nakayama H, Tsuchiya K, Umemoto M. Scripta Mater 2001;44:1781–1785. [15] Khmelevskaya IYu, Trubitsyna IB, Prokoshkin SD, et al.. Mater Sci Forum 2003;426–432:2765–2770. [16] Matsumoto O, Honma T. Trans JIM 1976;17:199. [17] Filip Peter, Mazanec KareL. Scripta Metal Mater 1995;32:1375–1380. [18] Nishida M, Wayman CM, Honma T. Metall Trans 1986;17A:1505. [19] Nam TH, Saburi T, Kawamura Y, Shimizu K. Mater Trans JIM 1990;31:262. [20] Nishida M, Wayman CM. Scripta Metal 1985;19:983–987.
Effects of ECAE process on microstructure and transformation behavior of TiNi shape memory alloy Zhenhua Li, Guoquan Xiang, Xianhua Cheng
*
School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 3 June 2004; accepted 18 October 2004 Available online 8 January 2005
Abstract Equal channel angular extrusion (ECAE) process was a deep pure shear deformation method for structural materials, developed in early 90s. ECAE technique has been successfully applied to several pure metals and alloys to refine the microstructure and improve the mechanical properties of materials. In the present paper, TiNi shape memory alloys have been treated by ECAE process. The effects of ECAE process on microstructures and transformation behaviors have been investigated. The initial coarse grains of TiNi alloy were refined after two passes of ECAE processes and short annealing at 1023 K. Transformation temperatures of TiNi alloy sharply decreased after two ECAE processes, then rapidly rose back when annealed at 773 K for 2 h. 2004 Published by Elsevier Ltd. Keywords: TiNi shape memory alloy; Equal channel angular extrusion; Phase transformations
1. Introduction Ultra-fine grained materials exhibit superior mechanical properties. The equal channel angular extrusion (ECAE) technique, introduced by Segal et al. [1], has been successfully applied to produce various ultra-fine grained materials, such as low carbon steel [2,3], copper [4–6], Al alloys [7–10], pure Ti [11,12] and Ti–6Al–4V [13] et al. However, the properties and microstructures of TiNi shape memory alloys treated by ECAE process have been reported few. Recently, Nakayama et al. [14] analyzed crystal refinement of TiNi shape memory alloy by cold rolling. Khmelevskaya et al. [15] investigated microstructure of TiNi-based shape memory alloys after severe plastic deformation. Transformation behavior of TiNi alloy after ECAE process has not been reported yet. In the present paper, the microstructure evolutions
*
Corresponding author. E-mail address: [email protected] (X. Cheng).
0261-3069/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.matdes.2004.10.025
and transformation behavior of TiNi shape memory alloy after high temperature ECAE processes have been analyzed. 2. Experimental method Experimental materials were Ti-50.6at% Ni alloy rods, with a diameter of 25 mm, after 1123 K hot forging and 773 K annealing for 2 h. Billets for ECAE process with dimensions of 10 · 10 · 55 mm3 were cut from the TiNi rod. The ECAE die was designed to yield an effective strain of 1 by a single pass. The inner contact angle (U) and the arc of curvature (w) at the outer point of contact between channels of the die were 90 and 90, respectively, as shown in Fig. 1. Two ECAE processes were conducted at high temperature. Keeping the die at 823 K, billets were preheated at 1123 K for 20 min before the first ECAE extrusion, and 1023 K for 20 min before the second ECAE extrusion. During ECAE process, the billet was not rotated between passages.
Z. Li et al. / Materials and Design 27 (2006) 324–328
325
Fig. 1. Schematic illustration of the ECAE facility in the experiment. Fig. 2. Optical micrograph of TiNi alloy before the first ECAE process.
Samples for DSC and microstructure observation were cut from billets after ECAE process along extruding direction. Some of them were subsequently annealed at 1023 K for 5–20 min to observe their microstructural changes during static re-crystallization. After mechanically polishing and eroding in a mixture solution of HF:HNO3:H2O with a ratio of 1:3:10, samples were observed on a NEOPHOT-1 optical microscope. Differential scanning calorimeter (DSC) measurements were carried out at heating and cooling rates of 10 K/min to analyze the phase transformation behaviors.
3. Results and discussion 3.1. Evolution of the microstructures of TiNi alloys The initial microstructure of as-received hot forged and 773 K annealed TiNi alloy rod was shown in Fig. 2. The grains were coarse and unequal, a little extending along the axial direction of alloy rod. During the first ECAE process, billet was preheated at 1123 K for 20 min. It was found that the microstructure was coarse after the preheating. After the first ECAE extrusion, grains were elongated in a direction parallel to the shear direction and forming an angle of 30 respect to the extruding direction, as shown in Fig. 3(a) After the first ECAE extrusion, the billet was heated to 1023 K and held for 20 min before the second ECAE extrusion. It was observed that elongated grains after the first ECAE process became equaixed grains about 10 lm in size after 1023 K annealing for 20 min, as shown in Fig. 3(b), which revealed that there was static re-crystallization occurred during 1023 K annealing. Comparing Fig. 2 with Fig. 3(b), it was seen that TiNi grains remarkably refined after the first
high temperature ECAE process and static recrystallization at 1023 K for 20 min. Compared with the first ECAE treatment, the second ECAE extrusion resulted in denser shear bands, as seen in Fig. 3(c). Grains were elongated dramatically and parallel to the shear direction, forming an angle of 45 respect to the extrusion direction. From Figs. 2 and 3(a)–(c), it was reasonably suggested that there were not dynamical re-crystallization occurred during high temperature ECAE processes. However, there was probably dynamical recovery occurred during high ECAE processes, especially for the first ECAE process, because these ECAE processes were performed at so high temperature and under changing temperature condition, in which the die was kept at 823 K and billets were preheated at 1123 K, 20 min and 1023 K, 20 min for the first ECAE process and the second ECAE process, respectively. Recrystallized fine grains appeared in the specimen annealed at 1023 K for 5 min after above two passages of ECAE. Elongated grains became nearly full equaixed grains when specimens were heated to 1023 K for 10–20 min, as shown in Fig. 3(d). Compared to Fig. 3(b), grains of Fig. 3(d) were finer. Fig. 4 showed the microstructure of specimen annealed at 773 K for 2 h after above two passages of ECAE. Grains were still elongated as before, but a little bit widened. The microstructure of specimen annealed at 873 K for 2 h were similar to that in Fig. 4, which revealed that there was no re-crystallization occurred during annealing at 773 or 873 K for 2 h. It was suggested that the static re-crystallization temperature for TiNi specimen after above two high temperature ECAE processes was higher than 873 K. However, there possibly were static recoveries occurred since the elongated
326
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Fig. 3. Optical microstructure of TiNi specimens after ECAE processes: (a) after the first ECAE process; (b) annealed at 1023 K for 20 min; (c) after the second ECAE process and (d) annealed at 1023 K for 10 min.
grains were widened after annealing at 773 or 873 K for 2 h, comparing Fig. 4 with Fig. 3(c). 3.2. Transformation behaviors The transformation behavior of TiNi specimen, 1123 K hot forged and aged at 773 K for 2 h, was shown in Fig. 5(a). Two exothermic peaks were seen when the specimen was cooled down from 373 K, which were corresponding to the B2 parent phase to the R-phase and then to martensite phase transformations [16]. Transformation temperatures were higher than that of quenched binary TiNi alloy with a same composition [17]. The mainly reason was that hot forged Ti-50.6at% Ni had
been annealed at 773 K for 2 h, which resulted in the precipitation of Ti3Ni4 and the decrease of Ni content in the matrix, therefore, increased transformation temperatures [18]. During heating, there was only one endothermic peak observed, which revealed that reverse R-phase transformation and reverse martensitic transformation were overlapped together [19]. Table 1 showed the transformation temperature values of Ms, Mf, Rs, Rf, As and Af at different procedures. The transformation behaviors of TiNi alloy after above two ECAE processes were shown in Fig. 5(b). It was found that there was only one broadened exothermic peak on the cooling curve and the B2 to Rphase transformation peak was unclear. Meanwhile,
Z. Li et al. / Materials and Design 27 (2006) 324–328
Fig. 4. Microstructure of the sample annealed at 773 K for 2 h after two passages of ECAE.
the transformation temperatures decreased sharply. There were two factors which possibly caused the decrease of transformation temperatures. One was the dissolving of Ti3Ni4 precipitates into the matrix when TiNi billet was preheated at 1123 and 1023 K for 20 min before the first and the second ECAE process,
327
respectively. It was reasonably suggested that Ti3Ni4 phase was unable to precipitate again during the ECAE process due to the high temperature of ECAE process and the short extrusion time. As a result, Ni content in the matrix increased and then transformation temperatures decreased correspondingly. The other factor was the severe shear deformation of TiNi matrix during the ECAE process, which was under further discussion. Compared with those of the specimen in Fig. 5(b), transformation temperatures of specimen annealed at 1023 K for 5–20 min after above two ECAE passages were increased obviously, as shown in Fig. 6, while there was few change in transformation temperatures as the annealing time extended from 5 to 20 min, which revealed that there was no Ni content change in the matrix with increasing the annealing time. It was known that annealing at high temperature like 1023 K, Ti3Ni4 phase would not precipitate. Other precipitates, such as TiNi2 or TiNi3, usually appeared after long time aging at 873 K or above [20]. Considering the microstructure change after 1023 K annealing as shown in Fig. 3(d), it was reasonably suggested that the re-crystallization may give rise to the increase of transformation temperatures in Fig. 6. The mechanism of the effect of re-crystallization on phase transformation was still under investigation.
Fig. 5. Transformation behavior of TiNi alloy before (a) and after (b) two ECAE processes. Table 1 Transformation temperatures of TiNi specimens Temperatures (K)
As
Af
Rs
Rf
Ms
Mf
Procedures Received TiNi alloy specimen After ECAE process Annealed at 673K for 5 min Annealed at 773 K for 5 min Annealed at 773 K for 2 h Annealed at 873 K for 5 min Annealed at 1023 K for 5 min Annealed at 1023 K for 10 min Annealed at 1023 K for 20 min
327.9 260.08 272.27 277.49 326.95 263.13 261.51 262.09 266.5
353.15 283.15 291.9 300.65 335.65 289.4 293.15 288.15 288.15
320.38 – – 275.65 320.65 – – – –
303.15 – – 243.15 302.99 – – – –
294.49 227.38 267.4 226.9 289.87 261.99 266.17 263.65 264.06
276.9 211.9 230.65 225.65 281.9 224.4 231.9 226.9 239.4
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Fig. 6. DSC curves of TiNi specimens annealed at 1023 K for 5 min (a), 20 min (b) after two ECAE processes.
Acknowledgement This project was supported by National Natural Science Foundation of China, Grant No. A50071034.
References
Fig. 7. Transformation behavior of TiNi alloy annealed 2 h at 773 K, furnace cooling after two ECAE processes.
Fig. 7 showed DSC curves of TiNi specimen annealed at 773 K for 2 h after above two ECAE processes. Compared with Figs. 5 and 6, it was obviously seen that transformation temperatures remarkably increased and similar to the ones in Fig. 5(a). It was suggested that Ti3Ni4 phase had precipitated again in matrix after annealed at 773 K for 2 h, which decreased the Ni content in the matrix and resulted in the increase of transformation temperatures. 4. Conclusions During high temperature ECAE process, there were not dynamical re-crystallization occurred, while there was probably dynamical recovery occured. Annealed for 5 min at 1023 K after two passes of ECAE, grains were refined and became even. After two passes of ECAE, transformation temperatures of the billet of TiNi alloy sharply decreased. Transformation temperature of the sample remarkably increased annealed for 2 h at 773 K after two ECAE processes, similar to the one of TiNi alloy before ECAE process, which was related to Ni content in the matrix.
[1] Segal VM, Reznikov VI, Drobyshevskiy AD, et al.. Russ Metall 1981;1:99. [2] Shin Dong Hyuk, Kim Byung Cheol, Park Kyung-Tae, et al.. Acta Mater 2000;48:3245–3252. [3] Aoki Koshiro, Kimura Youichiro, Asada Yousuke, et al.. Mater Sci Forum 2003;426–432:2705–2710. [4] Valiev RZ, Alexandrov IV. Nanostruct Mater 1999;12:35. [5] Guo Z, Solas D, Etter AL, et al.. Mater Sci Forum 2003;426– 432:2723–2728. [6] Short K, Wuhrer R, Collins G, et al.. Mater Sci Forum 2003;426– 432:2369–2374. [7] Bowen JR, Mishin OV, Prangnell PB, Juul Jensen D. Scripta Mater 2002;47:289–294. [8] Vogel SC, Alexander DJ, Beyerlein IJ, et al.. Mater Sci Forum 2003;426–432:2661–2666. [9] Furukawa Minoru, Fukuda Yukihide, Oh-ishi Keiichiro, et al.. Mater Sci Forum 2003;426–432:2711–2716. [10] Pithan C, Hashimoto T, Kawazoe M, et al.. Mater Sci Eng A 2000;280:62–68. [11] Stolyarov Vladimir V, Zhu Yuntian T, Lowe Terry C, et al.. Mater Sci Eng A 2001;303:82–89. [12] Sergueeva AV, Stolyarov VV, Valiev RZ, Mukherjee AK. Scripta Mater 2001;45:747–752. [13] Semiatin SL, Delo DP. Mater Des 2000;21:311–322. [14] Nakayama H, Tsuchiya K, Umemoto M. Scripta Mater 2001;44:1781–1785. [15] Khmelevskaya IYu, Trubitsyna IB, Prokoshkin SD, et al.. Mater Sci Forum 2003;426–432:2765–2770. [16] Matsumoto O, Honma T. Trans JIM 1976;17:199. [17] Filip Peter, Mazanec KareL. Scripta Metal Mater 1995;32:1375–1380. [18] Nishida M, Wayman CM, Honma T. Metall Trans 1986;17A:1505. [19] Nam TH, Saburi T, Kawamura Y, Shimizu K. Mater Trans JIM 1990;31:262. [20] Nishida M, Wayman CM. Scripta Metal 1985;19:983–987.