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Structure, phase transformations and properties of the TiNi-TiCu alloys subjected to high pressure torsion
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 4846–4850
www.materialstoday.com/proceedings
SMA 2016
Structure, phase transformations and properties of the TiNi-TiCu alloys subjected to high pressure torsion Alexander Lukyanova,b*, Natalia Kuranovaa, Artem Pushina, Vladimir Pushina, Dmitry Gunderovb,c a Institute of Physics of Metals UB RAS, S. Kovalevskoy st. 18, Ekaterinburg 620990, Russia b Ufa State Aviation Technical University, K. Marks st. 12, Ufa 450000, Russia c Institute of Molecule and Crystal Physics RAS, Prospekt Oktyabrya st. 151, Ufa 450075, Russia
Abstract It was performed a complex study of stability, crystallization kinetics and martensitic transformations of the melt-spun (MS) Ti50Ni25Cu25 alloy subjected to high pressure torsion (HPT). The amorphous MS Ti50Ni25Cu25 alloy was subjected to highpressure torsion (HPT) at temperature of 20°C 10 turnes. HPT processing of the melt-spun amorphous Ti-Ni-Cu alloy does not effect on intensive nanocrystallization. HPT processing leads to a decrease of the energy of structural relaxation during heating up to 4000C, in comparison with the initial MS ribbons. The temperature of crystallization of the MS Ti50Ni25Cu25 alloy decreases after HPT processing. In the MS Ti50Ni25Cu25 alloy HPT processing and subsequent annealing produces noticeably finer grains than annealing at 4500C of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the MS Ti50Ni25Cu25 alloy. The size effect on the stabilization of martensitic transformation in nanocrystalline TiNiCu alloy has been established.
© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of The second conference “Shape memory alloys”. Keywords: TiNi - based alloys, microstructure, severe plastic deformation, thermoelastic martensitic transformations, shape memory effects, size effect on thermoelastic martensitic transformations, structural relaxation
* Corresponding author. Tel.: +7 917 048 14 78; fax: +7 -347-2733422.. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of The second conference “Shape memory alloys”.
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
4847
1. Introduction Alloys with thermoelastic martensite transformations (TMT) are a special class of structural and functional materials with the shape memory effect (SME), especially the alloys based on titanium nickelide [1–5]. Application of thermomechanical treatment (TMT) methods, including severe plastic deformation (SPD) and post-deformation heat treatment (PHT), provides the formation of high strength nanocrystalline (NC) and submicrocrystalline (SMC) structural states in TiNi alloys with a priority of the grain–subgrain nanofragments smaller than 100 nm in size [6– 9]. It is known that SPD (with ultrahigh degrees of true strain e, e.g., more than 3–4) achieved by special techniques, such as high pressure torsion (HPT) makes it possible to obtain an amorphized state (with a mixed amorphous– nanocrystalline structure) in these alloys [10–14]. In this case, upon the devitrification of amorphized alloys an appropriate choice of the HPT regime (temperature and time) makes it possible to control the process of the formation of an nanocrystalline (NC) structure and thus to regulate required physical and mechanical properties. Some of these studies were provided on a binary Ti-Ni alloys and published in [6-14]. Back in the 1990s, we performed a series of works on the synthesis and research of rapidly quenched alloys of the ternary quasibinary TiNi–TiCu system and showed that amorphous and nanostructured states can be obtained in these alloys with increased copper content, with an optimum combination of strength and plastic properties at 25 % Cu [15–19]. Recent studies have shown that these alloys have also a high propensity to amorphization upon SPD, which was revealed in [20–21]. This article is aimed at studying the effect of severe plastic deformation by HPT and postdeformation heat treatment on the structure and martensitic transformation in the initially amorphous Ti50Ni25Cu25 alloy. 2. Experimental procedure The Ti50Ni25Cu25 ingots were re-melted in quartz crucibles under a purified atmosphere of inert gas and ejected onto the surface of a fast rotating copper wheel at cooling rates around 106 K/s. The melt-spun ribbons had a thickness of 0.04 mm and a width of about 2 mm. A detailed description of the single-roller melt-spinning technique is given elsewhere [16,17]. Thin flakes of rapidly quenched TiNiCu alloy were cut from ribbons of the amorphous alloy and subjected to SPD. HPT processing was carried out by torsion under a pressure of 6 GPa at room temperature (RT). The HPT processing was performed for up to 10 revolutions at a rotation speed of 1 turn/min. As a result, solid samples with a thickness of 0.2–0.3 mm and a diameter of 10 mm were produced from the initial asspun ribbons. The structure of the samples was investigated by a JEM-2100 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The foils for the electron microscopy were prepared on the twin jet polishing machine «Tenupol-5» by the standard method at 50V using a mix of chloric acid (10%) and butanol (90 %) as the electrolyte. Foil etching by argon ions was additionally carried out using JEOL ION SLICER EM-09100, at a voltage of 3 kV and an inclination angle of 20, with the polishing duration about 45 min. The DSC tests were performed on a Netzsch DSC 204 F1 Phoenix calorimeter; the heating temperature was 520 °C (higher than the crystallization temperature in this alloy) and the typical heating rate was 20°C/min. The measurement accuracy counts 4.5%. The disc-shaped samples for TEM and DSC studies were cut out in such a manner that the foil center and the center of the disc corresponded to the point «½ radius of the HPT – sample». 3. Results and Discussion The TEM study of the initial melt-spun Ti50Ni25Cu25 alloy reveals the amorphous state (fig. 1,a). In the selected area electron diffraction (SAED) pattern from an area of 100 nm2, only the halo of the amorphous phase is observed. Thus, nanocrystals in the observed region are not present, or their volume fraction is not significant. Figure 1b shows the TEM image of the melt-spun Ti50Ni25Cu25 alloy subjected to HPT for up to 10 revolutions at a temperature of T=20ºC. The SAED pattern also contains the amorphous halo. But in the bright-field image there can
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Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
be seen a rather complex picture. The bright field TEM images exhibit a weakly pronounced «salt–pepper» contrast typical of the amorphous state with separate nanocrystallites up to 5 nm in a size [18]. According to the TEM bright-field structure of the MS Ti50Ni25Cu25 changes in a complex manner after HPT processing. TEM analysis demonstrates that in HPT processed samples and subsequent annealing were produced markedly finer grains in comparison with the initial MS alloy. Thus, subsequent annealing at 300-350°С for 10 min of the samples after HPT n=10 turns leads to nanocrystallization of the alloy with a grain size of 15–35 nm of the В2 phase. No haloes were observed in the SAED patterns and the innermost ring of the superlattice reflections of the (100)B2 type became clear [19]. The grain size in the Ti50Ni25Cu25 alloy subjected to HPT and subsequent annealing at 450°C the grain size is below 100 nm (fig. 1d), whereas in the MS Ti50Ni25Cu25 alloy after annealing at 450°C for 10 min reaches 1.5 μm (with martensite plate observed within the grains) (fig. 1c). In this case, along with the В2austenite reflections, there are observed reflections of the В19 martensite, located separately in the SAED patterns. Thus, HPT processing also markedly alters the crystallization kinetics of the amorphous phase in the MS alloy, which can be accounted for by structural transformations in the material during HPT.
Fig. 1. TEM images of the melt-spun (MS) Ti50Ni25Cu25 alloy: a) the initial MS alloy, bright-field image and SAED; b) bright-field TEM image and the corresponding SAED pattern for the MS Ti50Ni25Cu25 alloy alloy after HPT n=10 at T = 20ºC; c) Bright-field TEM image and the corresponding SAED pattern for the MS Ti50Ni25Cu25 alloy alloy after annealing at 450°C for 10 min; d) bright-field image and the related SAED pattern for the MS Ti50Ni25Cu25 alloy subjected to HPT n=10 (T=200C) and subsequent annealing at 450°C for 10 min.
The temperatures and energies, obtained by DSC, for the processes of structural relaxation in a temperature range of 20-400ºC and crystallization are shown in Table 1. As a result of HPT 10 turns, the temperature of the crystallization start drops down by about 40°C in comparison with the initial as-spun amorphous state. The evaluation of the energy of structural relaxation during heating up to 400°C was made in the following way: three successive heating were conducted in the temperature range of 120° - 400°C, and the relaxation energy was determined from the difference in the areas under the curves for the 1st and 3rd heating (Table 1). HPT processing leads to a decrease in the energy of structural relaxation in the amorphous phases (in the region of 20-400ºC) in comparison with the initial as-spun ribbons (Table 1). The reduction of the energy of structural relaxation may be associated with a reduced free volume in the amorphous phase.
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
4849
Thus, according to the DSC data, the TEM bright-field structure of the MS Ti50Ni25Cu25 changes in a complex manner after HPT processing. HPT processing also markedly alters the crystallization kinetics of the amorphous phase in the MS alloy, which can be accounted for by structural transformations in the material during HPT. Table 1. Temperatures and energies of structural transformations occurring during heating for as-spun and HPT-processed samples Sample
Crystallization peak, °C
Crystallization start, °C
Crystallization finish, °C
Relaxation energy, J/g
As-spun
460
444
463
41
HPT n=10 turnes
432
401
450
37
The microstructural analysis and temperature dependence of the resistivity of the Ti50Ni25Cu25 alloy a size effect of the stabilization of the В2 austenite has been revealed. Аt a size of nano grains less than 20 nm the martensitic B2–B19 transformations either do not take place at all or there is possible only the В2↔R transition [14-19]. In grains with a size greater than 20-50 nm, during cooling, the В2↔В19 martensitic transition is feasible; however, at this, the completely different structural mechanism of martensitic transformation is realized, namely of the «single crystal – single crystal» type, with no twins to occur. In the alloys with the grain size ~100–300 nm the transformation proceeds with the formation of one-packet twinned martensite. At this, quasi-isotropic spatial compensation for the elastic stresses initiated by the martensitic transformation in the nano-structured Ti–Ni alloys is attained precisely in a large group of adjacent nano grains, whose ensembles form a self-accommodation elastic system, rather than in separate nano grains. And only at large grain sizes, the accommodation of stresses takes place – due to the formation of a many-packet martensite in grain boundaries. 4. Conclusions In summary, it has been shown that the structure of the MS Ti50Ni25Cu25 alloy remains mainly amorphous after HPT processing. HPT leads to a decrease of the energy of structural relaxation during heating up to 400ºC, in comparison with the initial as-spun ribbons and its. The temperature of crystallization start decreases from 444ºC in the initial as-spun ribbons to 401°C after HPT processing at 20°C. In the MS Ti50Ni25Cu25 alloy, HPT processing and subsequent annealing produce much finer grains than the annealing of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the phase in the MS Ti50Ni25Cu25 alloy. In the nanostructured Ti50Ni25Cu25 alloy a size effect of the stabilization of the В2 austenite has been revealed. Acknowledgement The authors gratefully acknowledge the financial support of the structural studies provided by the Russian Science Foundation through project No. 15-12-10014. References [1] K. Otsuka, C.M. Wayman, Shape memory materials. Cambridge University Press. (1998) 284. [2] J. Koike, D.M. Parkin, M. Nastasi J Mater Res. 5 (1990) 1414-1418. [3] E.V. Tatianin, V.G.Kurdumov, Phys. Stat. sol. (A) 121 (1990) 455-459. [4] H. Nakayama, K. Tsuchiya, M. Umemoto, Scr Mater. 44 (2001) 1781-1785. [5] S. D. Prokoshkin, I. Yu. Khmelevskaya, S. V. Dobatkin, E. V. Tatyanin, I. B. Trubitsyna, Mater. Sci. Forum 503–504 (2006) 481–486. [6] V. G. Pushin. Ed. by M. Zehetbauer and R. Valiev. Wiley, Weinheim (2004) 822–828. [7] A.V. Sergueeva, C. Song, R.Z. Valiev, A.K. Mukherjee, Mater. Sci. Eng. A339 (2003) 159-165. [8] S.D. Prokoshkin, V. Brailovskii, A. V. Korotitskii, K. E. Inaekyan, A. M. Glezer, Phys. Met. Metallogr. 110 (2010) 289–304. [9] J.Y. Huang, Y.T. Zhu, X.Z. Liao, R.Z. Valiev, Phyl. Mag. Lett. 84 (2004) 183–190 [10] T. Waitz, V. Kazykhanov, H. P. Karnthaler, Acta Mater. 52 (2004) 137–147. [11] R.Z. Valiev, D. V. Gunderov, A.V. Lukyanov, V. G. Pushin, Journal of Materials Science. 47 (2012) 7848-7853.
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Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
[12] D.V. Gunderov, N.N. Kuranova, A.V. Luk’yanov, Phys. Metals Metallogr. 108 (2009) 131-138. [13] S. D. Prokoshkin, I. Yu. Khmelevskaya, S. V. Dobatkin, I. B. Trubitsyna, E. V. Tatyanin, V. V. Stolyarov, E. A. Prokofiev, Acta Mater. 53 (2005) 2703–2714. [14] V. G. Pushin, S. B. Volkova, N. M. Matveeva, Phys. Met. Metallogr. 83 (1997) 283–288. [15] V. G. Pushin, S. B. Volkova, N. M. Matveeva, Phys. Met. Metallogr. 83 (1997) 435–443. [16] V. G. Pushin, S. B. Volkova, N. M. Matveeva, L. I. Yurchenko, A. S. Chistyakov, Phys. Met. Metallogr. 83 (1997) 673–678. [17] V. G. Pushin, S. B. Volkova, N. M. Matveeva, L. I. Yurchenko, A. S. Chistyakov, Phys. Met. Metallogr. 83 (1997) 679–683. [18] V.G. Pushin, Pushin, V.G., Kuranova, N.N., Pushin, A.V. et al., Phys. Metals Metallogr. 113 (2012) 271-282. [19] D. V. Gunderov, V. G. Pushin, R. Z. Valiev, E. Z. Valiev, Deform. Razrush. Mater. 4 (2006) 22–25 (in Russian). [20] D.V. Gunderov, V.Yu. Slesarenko, A.V. Lukyanov, A.A. Churakova, E.V. Boltynjuk, V.G. Pushin, E.V. Ubyivovk, A.V. Shelyakov, R.Z. Valiev, Adv. Eng. Mat. 17 (2016) 1728–1732. [21] V.G. Pushin, N.N. Kuranova, A.V. Pushin, E.Z. Valiev, N.I. Kourov, A.E. Teplykh, A.N. Uksusnikov, Phys. Met. Metallogr. 113 (2012) 271–282.
ScienceDirect Materials Today: Proceedings 4 (2017) 4846–4850
www.materialstoday.com/proceedings
SMA 2016
Structure, phase transformations and properties of the TiNi-TiCu alloys subjected to high pressure torsion Alexander Lukyanova,b*, Natalia Kuranovaa, Artem Pushina, Vladimir Pushina, Dmitry Gunderovb,c a Institute of Physics of Metals UB RAS, S. Kovalevskoy st. 18, Ekaterinburg 620990, Russia b Ufa State Aviation Technical University, K. Marks st. 12, Ufa 450000, Russia c Institute of Molecule and Crystal Physics RAS, Prospekt Oktyabrya st. 151, Ufa 450075, Russia
Abstract It was performed a complex study of stability, crystallization kinetics and martensitic transformations of the melt-spun (MS) Ti50Ni25Cu25 alloy subjected to high pressure torsion (HPT). The amorphous MS Ti50Ni25Cu25 alloy was subjected to highpressure torsion (HPT) at temperature of 20°C 10 turnes. HPT processing of the melt-spun amorphous Ti-Ni-Cu alloy does not effect on intensive nanocrystallization. HPT processing leads to a decrease of the energy of structural relaxation during heating up to 4000C, in comparison with the initial MS ribbons. The temperature of crystallization of the MS Ti50Ni25Cu25 alloy decreases after HPT processing. In the MS Ti50Ni25Cu25 alloy HPT processing and subsequent annealing produces noticeably finer grains than annealing at 4500C of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the MS Ti50Ni25Cu25 alloy. The size effect on the stabilization of martensitic transformation in nanocrystalline TiNiCu alloy has been established.
© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of The second conference “Shape memory alloys”. Keywords: TiNi - based alloys, microstructure, severe plastic deformation, thermoelastic martensitic transformations, shape memory effects, size effect on thermoelastic martensitic transformations, structural relaxation
* Corresponding author. Tel.: +7 917 048 14 78; fax: +7 -347-2733422.. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of The second conference “Shape memory alloys”.
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
4847
1. Introduction Alloys with thermoelastic martensite transformations (TMT) are a special class of structural and functional materials with the shape memory effect (SME), especially the alloys based on titanium nickelide [1–5]. Application of thermomechanical treatment (TMT) methods, including severe plastic deformation (SPD) and post-deformation heat treatment (PHT), provides the formation of high strength nanocrystalline (NC) and submicrocrystalline (SMC) structural states in TiNi alloys with a priority of the grain–subgrain nanofragments smaller than 100 nm in size [6– 9]. It is known that SPD (with ultrahigh degrees of true strain e, e.g., more than 3–4) achieved by special techniques, such as high pressure torsion (HPT) makes it possible to obtain an amorphized state (with a mixed amorphous– nanocrystalline structure) in these alloys [10–14]. In this case, upon the devitrification of amorphized alloys an appropriate choice of the HPT regime (temperature and time) makes it possible to control the process of the formation of an nanocrystalline (NC) structure and thus to regulate required physical and mechanical properties. Some of these studies were provided on a binary Ti-Ni alloys and published in [6-14]. Back in the 1990s, we performed a series of works on the synthesis and research of rapidly quenched alloys of the ternary quasibinary TiNi–TiCu system and showed that amorphous and nanostructured states can be obtained in these alloys with increased copper content, with an optimum combination of strength and plastic properties at 25 % Cu [15–19]. Recent studies have shown that these alloys have also a high propensity to amorphization upon SPD, which was revealed in [20–21]. This article is aimed at studying the effect of severe plastic deformation by HPT and postdeformation heat treatment on the structure and martensitic transformation in the initially amorphous Ti50Ni25Cu25 alloy. 2. Experimental procedure The Ti50Ni25Cu25 ingots were re-melted in quartz crucibles under a purified atmosphere of inert gas and ejected onto the surface of a fast rotating copper wheel at cooling rates around 106 K/s. The melt-spun ribbons had a thickness of 0.04 mm and a width of about 2 mm. A detailed description of the single-roller melt-spinning technique is given elsewhere [16,17]. Thin flakes of rapidly quenched TiNiCu alloy were cut from ribbons of the amorphous alloy and subjected to SPD. HPT processing was carried out by torsion under a pressure of 6 GPa at room temperature (RT). The HPT processing was performed for up to 10 revolutions at a rotation speed of 1 turn/min. As a result, solid samples with a thickness of 0.2–0.3 mm and a diameter of 10 mm were produced from the initial asspun ribbons. The structure of the samples was investigated by a JEM-2100 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The foils for the electron microscopy were prepared on the twin jet polishing machine «Tenupol-5» by the standard method at 50V using a mix of chloric acid (10%) and butanol (90 %) as the electrolyte. Foil etching by argon ions was additionally carried out using JEOL ION SLICER EM-09100, at a voltage of 3 kV and an inclination angle of 20, with the polishing duration about 45 min. The DSC tests were performed on a Netzsch DSC 204 F1 Phoenix calorimeter; the heating temperature was 520 °C (higher than the crystallization temperature in this alloy) and the typical heating rate was 20°C/min. The measurement accuracy counts 4.5%. The disc-shaped samples for TEM and DSC studies were cut out in such a manner that the foil center and the center of the disc corresponded to the point «½ radius of the HPT – sample». 3. Results and Discussion The TEM study of the initial melt-spun Ti50Ni25Cu25 alloy reveals the amorphous state (fig. 1,a). In the selected area electron diffraction (SAED) pattern from an area of 100 nm2, only the halo of the amorphous phase is observed. Thus, nanocrystals in the observed region are not present, or their volume fraction is not significant. Figure 1b shows the TEM image of the melt-spun Ti50Ni25Cu25 alloy subjected to HPT for up to 10 revolutions at a temperature of T=20ºC. The SAED pattern also contains the amorphous halo. But in the bright-field image there can
4848
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
be seen a rather complex picture. The bright field TEM images exhibit a weakly pronounced «salt–pepper» contrast typical of the amorphous state with separate nanocrystallites up to 5 nm in a size [18]. According to the TEM bright-field structure of the MS Ti50Ni25Cu25 changes in a complex manner after HPT processing. TEM analysis demonstrates that in HPT processed samples and subsequent annealing were produced markedly finer grains in comparison with the initial MS alloy. Thus, subsequent annealing at 300-350°С for 10 min of the samples after HPT n=10 turns leads to nanocrystallization of the alloy with a grain size of 15–35 nm of the В2 phase. No haloes were observed in the SAED patterns and the innermost ring of the superlattice reflections of the (100)B2 type became clear [19]. The grain size in the Ti50Ni25Cu25 alloy subjected to HPT and subsequent annealing at 450°C the grain size is below 100 nm (fig. 1d), whereas in the MS Ti50Ni25Cu25 alloy after annealing at 450°C for 10 min reaches 1.5 μm (with martensite plate observed within the grains) (fig. 1c). In this case, along with the В2austenite reflections, there are observed reflections of the В19 martensite, located separately in the SAED patterns. Thus, HPT processing also markedly alters the crystallization kinetics of the amorphous phase in the MS alloy, which can be accounted for by structural transformations in the material during HPT.
Fig. 1. TEM images of the melt-spun (MS) Ti50Ni25Cu25 alloy: a) the initial MS alloy, bright-field image and SAED; b) bright-field TEM image and the corresponding SAED pattern for the MS Ti50Ni25Cu25 alloy alloy after HPT n=10 at T = 20ºC; c) Bright-field TEM image and the corresponding SAED pattern for the MS Ti50Ni25Cu25 alloy alloy after annealing at 450°C for 10 min; d) bright-field image and the related SAED pattern for the MS Ti50Ni25Cu25 alloy subjected to HPT n=10 (T=200C) and subsequent annealing at 450°C for 10 min.
The temperatures and energies, obtained by DSC, for the processes of structural relaxation in a temperature range of 20-400ºC and crystallization are shown in Table 1. As a result of HPT 10 turns, the temperature of the crystallization start drops down by about 40°C in comparison with the initial as-spun amorphous state. The evaluation of the energy of structural relaxation during heating up to 400°C was made in the following way: three successive heating were conducted in the temperature range of 120° - 400°C, and the relaxation energy was determined from the difference in the areas under the curves for the 1st and 3rd heating (Table 1). HPT processing leads to a decrease in the energy of structural relaxation in the amorphous phases (in the region of 20-400ºC) in comparison with the initial as-spun ribbons (Table 1). The reduction of the energy of structural relaxation may be associated with a reduced free volume in the amorphous phase.
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
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Thus, according to the DSC data, the TEM bright-field structure of the MS Ti50Ni25Cu25 changes in a complex manner after HPT processing. HPT processing also markedly alters the crystallization kinetics of the amorphous phase in the MS alloy, which can be accounted for by structural transformations in the material during HPT. Table 1. Temperatures and energies of structural transformations occurring during heating for as-spun and HPT-processed samples Sample
Crystallization peak, °C
Crystallization start, °C
Crystallization finish, °C
Relaxation energy, J/g
As-spun
460
444
463
41
HPT n=10 turnes
432
401
450
37
The microstructural analysis and temperature dependence of the resistivity of the Ti50Ni25Cu25 alloy a size effect of the stabilization of the В2 austenite has been revealed. Аt a size of nano grains less than 20 nm the martensitic B2–B19 transformations either do not take place at all or there is possible only the В2↔R transition [14-19]. In grains with a size greater than 20-50 nm, during cooling, the В2↔В19 martensitic transition is feasible; however, at this, the completely different structural mechanism of martensitic transformation is realized, namely of the «single crystal – single crystal» type, with no twins to occur. In the alloys with the grain size ~100–300 nm the transformation proceeds with the formation of one-packet twinned martensite. At this, quasi-isotropic spatial compensation for the elastic stresses initiated by the martensitic transformation in the nano-structured Ti–Ni alloys is attained precisely in a large group of adjacent nano grains, whose ensembles form a self-accommodation elastic system, rather than in separate nano grains. And only at large grain sizes, the accommodation of stresses takes place – due to the formation of a many-packet martensite in grain boundaries. 4. Conclusions In summary, it has been shown that the structure of the MS Ti50Ni25Cu25 alloy remains mainly amorphous after HPT processing. HPT leads to a decrease of the energy of structural relaxation during heating up to 400ºC, in comparison with the initial as-spun ribbons and its. The temperature of crystallization start decreases from 444ºC in the initial as-spun ribbons to 401°C after HPT processing at 20°C. In the MS Ti50Ni25Cu25 alloy, HPT processing and subsequent annealing produce much finer grains than the annealing of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the phase in the MS Ti50Ni25Cu25 alloy. In the nanostructured Ti50Ni25Cu25 alloy a size effect of the stabilization of the В2 austenite has been revealed. Acknowledgement The authors gratefully acknowledge the financial support of the structural studies provided by the Russian Science Foundation through project No. 15-12-10014. References [1] K. Otsuka, C.M. Wayman, Shape memory materials. Cambridge University Press. (1998) 284. [2] J. Koike, D.M. Parkin, M. Nastasi J Mater Res. 5 (1990) 1414-1418. [3] E.V. Tatianin, V.G.Kurdumov, Phys. Stat. sol. (A) 121 (1990) 455-459. [4] H. Nakayama, K. Tsuchiya, M. Umemoto, Scr Mater. 44 (2001) 1781-1785. [5] S. D. Prokoshkin, I. Yu. Khmelevskaya, S. V. Dobatkin, E. V. Tatyanin, I. B. Trubitsyna, Mater. Sci. Forum 503–504 (2006) 481–486. [6] V. G. Pushin. Ed. by M. Zehetbauer and R. Valiev. Wiley, Weinheim (2004) 822–828. [7] A.V. Sergueeva, C. Song, R.Z. Valiev, A.K. Mukherjee, Mater. Sci. Eng. A339 (2003) 159-165. [8] S.D. Prokoshkin, V. Brailovskii, A. V. Korotitskii, K. E. Inaekyan, A. M. Glezer, Phys. Met. Metallogr. 110 (2010) 289–304. [9] J.Y. Huang, Y.T. Zhu, X.Z. Liao, R.Z. Valiev, Phyl. Mag. Lett. 84 (2004) 183–190 [10] T. Waitz, V. Kazykhanov, H. P. Karnthaler, Acta Mater. 52 (2004) 137–147. [11] R.Z. Valiev, D. V. Gunderov, A.V. Lukyanov, V. G. Pushin, Journal of Materials Science. 47 (2012) 7848-7853.
4850
Alexander Lukyanov et al./ Materials Today: Proceedings 4 (2017) 4846–4850
[12] D.V. Gunderov, N.N. Kuranova, A.V. Luk’yanov, Phys. Metals Metallogr. 108 (2009) 131-138. [13] S. D. Prokoshkin, I. Yu. Khmelevskaya, S. V. Dobatkin, I. B. Trubitsyna, E. V. Tatyanin, V. V. Stolyarov, E. A. Prokofiev, Acta Mater. 53 (2005) 2703–2714. [14] V. G. Pushin, S. B. Volkova, N. M. Matveeva, Phys. Met. Metallogr. 83 (1997) 283–288. [15] V. G. Pushin, S. B. Volkova, N. M. Matveeva, Phys. Met. Metallogr. 83 (1997) 435–443. [16] V. G. Pushin, S. B. Volkova, N. M. Matveeva, L. I. Yurchenko, A. S. Chistyakov, Phys. Met. Metallogr. 83 (1997) 673–678. [17] V. G. Pushin, S. B. Volkova, N. M. Matveeva, L. I. Yurchenko, A. S. Chistyakov, Phys. Met. Metallogr. 83 (1997) 679–683. [18] V.G. Pushin, Pushin, V.G., Kuranova, N.N., Pushin, A.V. et al., Phys. Metals Metallogr. 113 (2012) 271-282. [19] D. V. Gunderov, V. G. Pushin, R. Z. Valiev, E. Z. Valiev, Deform. Razrush. Mater. 4 (2006) 22–25 (in Russian). [20] D.V. Gunderov, V.Yu. Slesarenko, A.V. Lukyanov, A.A. Churakova, E.V. Boltynjuk, V.G. Pushin, E.V. Ubyivovk, A.V. Shelyakov, R.Z. Valiev, Adv. Eng. Mat. 17 (2016) 1728–1732. [21] V.G. Pushin, N.N. Kuranova, A.V. Pushin, E.Z. Valiev, N.I. Kourov, A.E. Teplykh, A.N. Uksusnikov, Phys. Met. Metallogr. 113 (2012) 271–282.