T. Waitz et al. Acta Materialia 52(2004)137-147 the number of retained crystals in the amorphous 7 Xu X, Thadhani N later 2001 phase and therefore the grain size of the nanocrystal 8 Prader P, Kneissl AC. Z Metallkd 1997: 88: 410 line structure formed by annealing 9 Tatyanin EV, Borovnikov NF, Kurdyumov VG. Phys Solid State 7;39:1997 4. The martensitic transformation is suppressed in the [10 Koike J, Parkin DM, Nastasi M. Philos Mag Lett 1990: 62 ultrafine grains. It is concluded that forced by the constraints of the grain boundaries the R-phase pre- [l Kim Hs. J Mater Process Technol 2001; 113: 617. cedes the martensite and that atomic scale(00 1) [12] Tyumentsev AN, Pinzhin YP, Tretiak MV, et al. Theoret Appl compound twinning occurs in the martensite facilitat- Fract Mech 2001: 35: 15 [13] Ewert JC, Bohm I, Peter R, Haider F. Acta Mater 1997: 4: ing the accommodation of the transformation strains. 2197 5. In grains smaller than 60 nm no thermally induced [14] Crevoiserat S, Gotthard R. Mater Sci Eng A 1999: A273-275 martensitic transformation is observed. Therefore it is concluded that with decreasing grain size and de- [5] Lopez HF, Salinas A, Calderon H. Met Mater Trans A creasing twin separation both the strain energy and [16 Chen JZ, Wu SK. J Non-Cryst Sol 2001: 288: 15 the twin interfacial energy are leading to an ind [7 Kulik T J Non-Cryst Sol 2001: 287: 145 ing energy barrier. [18] Chrobak D, Morawiec H. Scr Mater 2001: 44:725. [19 Khahil AJ, Ren x, Eggeler G. Acta Mater 2000: 50: 793 O Olson GB. Metall Trans A 1976: 7A: 1969 Acknowledgements [21] Chen IW, Chiao YH. Acta Metall 1985: 33: 1827. 22 Saburi T, Nenno S. In: Tamura I, editor. Proceedings of the The authors thank professor A c. Kneissl for the kind International Conference on Martensitic Transformations. Japan: The Japan Inst Metals, Japan: 1986. p 671 provision of the NiTi alloy and Professor W. Pfeiler for [23] Gil FJ, Manero JM, Planell JA. J Mater Sci 1995: 30: 2526 is help with the DSC measurements. One of us(V [24 Chen Iw, Chiao YH, Tsuzaki K. Acta Metall 1985: 33: 1847 thanks Professor R.Z. valiev for the provision of labo- (251 Kajiwara S, Ohno S, Honma K Philos Mag A 1991:63:625 ratory facilities. Financial support from the Austrian (26 Ling HC,Owen wS.Acta Metall 1981: 29: I Science Fund(FWf)is acknowledged 27 Madangopal K. Acta Mater 1997: 45: 1997. [8 Khachaturyan AG, Shapiro SM, Semenovskaya S. Mater Tran JM199233:278 [9 Krishnan M, Singh JB. Acta Mater 2000: 48: 1325. References B0 Nishida M, Itai I, Kitamura K, Chiba A, Yamauchi K. J Phys Fujita H, Sinclair R. Scr Metall 1982: 16: 589. B31 Evans AG, Bur Bl] Evans AG, Burlingame N, Drory M, Kriven WM. Acta Metall [kOike J L. Nastasi ater Res1990;5:1414 l981:29:447 [] CC. Appl ettl986:49:1 [2 Zhang JX, Sato M, Ishida A. Acta Mater 2001: 49: 3001 C, Valiev RZ, Mukherjee AK. Mater Sci Eng [33] Yan WJ, Reisner G, Fischer FD. Acta Mater 1997; 45: 199 A2003;A339:159 34 Fischer FD, Appel F, Clemens H. Acta Mater 2003: 51: 1249 5 Nakayama H, Tsuchiya K, Umemoto M. Scr Mater 2001: 44: 1781 35] Ling HC, Kaplow R. Metall Trans A 1981; 12A: 2101 6 Nakayama H, Tsuchiya K, Liu ZG, Umemoto M, Morii 36] Murakami Y, Shindo D. Philos Mag Lett 2001: 81: 63 Shimizu t. Mater Trans 2001: 42: 198 37 Miyazaki S, Wayman CM. Acta Metall 1988: 36: 181the number of retained crystals in the amorphous phase and therefore the grain size of the nanocrystal￾line structure formed by annealing. 4. The martensitic transformation is suppressed in the ultrafine grains. It is concluded that forced by the constraints of the grain boundaries the R-phase pre￾cedes the martensite and that atomic scale (0 0 1) compound twinning occurs in the martensite facilitat￾ing the accommodation of the transformation strains. 5. In grains smaller than 60 nm no thermally induced martensitic transformation is observed. Therefore it is concluded that with decreasing grain size and de￾creasing twin separation both the strain energy and the twin interfacial energy are leading to an increas￾ing energy barrier. Acknowledgements The authors thank Professor A.C. Kneissl for the kind provision of the NiTi alloy and Professor W. Pfeiler for his help with the DSC measurements. One of us (V.K.) thanks Professor R.Z. Valiev for the provision of labo￾ratory facilities. Financial support from the Austrian Science Fund (FWF) is acknowledged. References [1] Thomas G, Mori H, Fujita H, Sinclair R. Scr Metall 1982;16:589. [2] Koike J, Parkin DM, Nastasi M. J Mater Res 1990;5:1414. [3] Schwarz RB, Koch CC. Appl Phys Lett 1986;49:146. [4] Sergueeva AV, Song C, Valiev RZ, Mukherjee AK. Mater Sci Eng A 2003;A339:159. [5] Nakayama H, Tsuchiya K, Umemoto M. Scr Mater 2001;44:1781. [6] Nakayama H, Tsuchiya K, Liu ZG, Umemoto M, Morii K, Shimizu T. Mater Trans 2001;42:1987. [7] Xu X, Thadhani N. Scr Mater 2001;44:2477. [8] Prader P, Kneissl AC. Z Metallkd 1997;88:410. [9] Tatyanin EV, Borovnikov NF, Kurdyumov VG. Phys Solid State 1997;39:1997. [10] Koike J, Parkin DM, Nastasi M. Philos Mag Lett 1990;62: 257. [11] Kim HS. J Mater Process Technol 2001;113:617. [12] Tyumentsev AN, Pinzhin YP, Tretjak MV, et al. Theoret Appl Fract Mech 2001;35:155. [13] Ewert JC, Bohm I, Peter R, Haider F. Acta Mater 1997;45: € 2197. [14] Crevoiserat S, Gotthard R. Mater Sci Eng A 1999;A273–275: 357. [15] Lopez HF, Salinas A, Calderon H. Met Mater Trans A 2001;32A:717. [16] Chen JZ, Wu SK. J Non-Cryst Sol 2001;288:159. [17] Kulik T. J Non-Cryst Sol 2001;287:145. [18] Chrobak D, Morawiec H. Scr Mater 2001;44:725. [19] Khahil AJ, Ren X, Eggeler G. Acta Mater 2000;50:793. [20] Olson GB. Metall Trans A 1976;7A:1969. [21] Chen IW, Chiao YH. Acta Metall 1985;33:1827. 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