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Straining of Metastable Austenite as a Way to Improve NiTi Alloy Functional Properties
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2S (2015) S961 – S964
International Conference on Martensitic Transformations, ICOMAT-2014
Straining of metastable austenite as a way to improve NiTi alloy functional properties A. Bragovb, A. Danilova, A. Konstantinova, A. Lomunovb, A. Motorina, A. Razova,* a
Saint-Petersburg State University, Universitetskii pr. 28, Saint-Petersburg, 198504, Russia Lobachevsky State University of Nizhni Novgorod, Gagarin pr., 23, korp.6, GSP-1000 Nizhny Novgorod, 603163, Russia
b
Abstract The effect of metastable austenite straining on the basic functional properties of an NiTi alloy in quasi-equilibrium state was studied. Straining of austenite in a pre-martensitic state was shown to result in improvement of one-way and two-way shape memory effects (OWSME and TWSME). This improvement took place after straining in a narrow temperature range, and its manifestation at quasi-static and high-rate tensile tests was different: the increment of both characteristics was sufficiently large after quasi-static straining, but smaller after high-rate straining. Straining at temperatures that deviated significantly from the Ms temperature led to deterioration of OWSME and TWSME. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility of CC the chairs of the International Conference on Martensitic Transformations 2014. Keywords: NiTi, metastable austenite, tension, functional properties, shape memory, two-way shape memory, high strain rate.
1. Introduction One of the general requirements of NiTi devices, elaborated during the last two decades for the solution of numerous practical problems in different fields of techniques, is the long-term stability of their mechanical and functional properties. This requirement is conventionally satisfied by aging at moderate temperatures in the range of 300–500 oC. However, such aging leads to a decrease in the basic functional properties of these materials in
* Corresponding author. Tel.: +7-812-428-4210; fax: +7-812-428-6944. E-mail address: [email protected]
2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.441
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comparison with the same properties achieved after high-temperature solution treatment followed by quenching [1]. Thus, the search for ways to enhance the functional properties of NiTi alloys in quasi-equilibrium state appears to be a pressing task for further progress in the application of NiTi alloys, in particular, in the fields of civil and aerospace engineering, where the high and stable functional properties of NiTi devices are in great demand [2, 3]. Since the origin of NiTi shape memory effects lies in the reversible structural mechanisms of straining, the latter appears to be one of the basic ways of shaping NiTi functional properties. The magnitudes of the reversible strains accumulated due to the de-twinning and reorientation of crystals in martensite, or due to stress-induced martensitic transformations in austenite are, by and large, determined by the crystallographic routes along which straining processes predominantly run. The distinctive features of the NiTi quasi-equilibrium structures - the presence of Ni4Ti3 precipitates and heterogeneity of their distribution within the grain - result in heterogeneous distribution of Ni content and of the oriented structural stresses in matrix. The heterogeneity of these stresses and nickel distribution in matrix are the basic reasons for the existence of distinct paths of martensitic transformations [4]. As has been shown [5], different paths of martensitic transformations are preceded by different behavior of the B2 matrix anisotropy factor A=c44/c (which defines the strength of non-basal and basal shear / shuffle modes coupling) in the vicinity of the Ms temperature. According to [5], prior to B2 B19 transformation, the anisotropy factor sharply decreases with the approaching martensitic transformation temperature, while prior to B2 R transformation, as the initial stage of the B2 R B19 reaction, the anisotropy factor becomes unchanged near the transformation temperature. Application of external stress to such metastable austenite can assist the predominant realization of one of the feasible transformation paths, thus affecting the basic functional properties of NiTi. The consequences of applying external stress to metastable austenite in devices designed for the mitigation of extreme loads in civil or aerospace engineering [2, 3] should be taken into consideration when choosing the material state for functioning under corresponding conditions. However, information about the basic shape memory characteristics acquired by quasiequilibrium NiTi alloys after straining in the vicinity of the Ms temperature, as well as about the effect of the strain rate on them is absent. The aim of the present study was to explore the effect of quasi-static and high-rate tension in the vicinity of the Ms temperature on functional properties of a quasi-equilibrium NiTi alloy. 2. Material and experimental methods The cylindrical specimens (with operative parts 5 mm in diameter and 10 mm in length) for high-rate and quasistatic tensile tests were made of binary NiTi alloy hot-rolled rods. To remove the residual stress and to create a quasi-equilibrium structure, the specimens were annealed at 500 oC for 1 hour followed by cooling in a furnace. The characteristic temperatures of the direct and reverse transformations were determined in a differential scanning calorimeter (DSC) Mettler Toledo 822e. The heating and cooling rates during DSC measurements were 10 degree/min. Transformation temperatures, determined from DSC peaks by a slope line extension method, were as follows: Ms = 74 oC, Mf = 32 oC, As = 74 oC, Af = 98 oC. The high-rate tension at a strain rate of about 103 sec-1 in the temperature range of 20–300 oC was performed using the modifiedSplit Hopkinson Pressure Bar (SHPB) technique [6]. The quasi-static tension at a strain rate of 10-3 sec-1 in the same temperature range was executed in the universal machine for mechanical tests (Lloyd 30K Plus) equipped with a thermal chamber. To reveal the effect of metastable austenite straining on the basic functional properties of NiTi, test temperatures in the range of 60–100 oC were reached by two paths. In the first path – basically used for the whole set of experiments, specimens were heated up to the test temperature from room temperature; in the second – the test temperature was attained by specimens cooling down after preliminary exposure at 180 oC. The analysis of the alloy’s functional properties, acquired after both types of tensile tests, was performed in a special device without external load by heating/cooling cycling of tested specimens within the temperature range of martensitic transformations. On first heating, a magnitude of restored strain ( sm), which characterized the shape memory effect of the alloy after high-rate and quasi-static tension, was determined. On cooling, the magnitude of an accumulated strain ( twsm), which characterized the induced effect of two-way shape memory, was controlled. To smooth out the influence of poorly controlled variations in a total strain value achieved in different tensile experiments, the sm/ res and twsm/ p ratios ( res represented a residual stress value, and p= res - sm represented the
A. Bragov et al. / Materials Today: Proceedings 2S (2015) S961 – S964
irreversible plastic strain) were used in analyzing one-way and two-way shape memory effects, acquired after both types of straining. 3. Results and discussion Figures 1 and 2 represent the indices of one-way and two-way shape memory effects (OWSME and TWSME), initiated by quasi-static and high-rate tension at different temperatures reached by the heating of specimens from room temperature.
Fig. 1. Dependences of the sm/ res ratio that characterized one-way shape memory (shape memory effect) on tensile test temperature. (○ – after quasi-static tension, ● – after high-rate tension).
Fig. 2. Dependences of the twsm/ p ratio that characterized two-way shape memory on tensile test temperature. Open signs define the results obtained after quasi-static tension, while solid signs define the results obtained after high-rate tension. , ▲ – martensitic twoway shape memory, , ▼ – austenitic two-way shape memory.
The maxima on the plots of OWSME and TWSME as functions of test temperatures corresponded with test temperatures slightly higher than the As temperature. (The shift to lower temperatures of these maxima on the plots corresponding to the high-rate tension is most probably associated with the temperature increase inherent in highrate straining). The maximal magnitudes of OWSME and TWSME might be explained by the lowest threshold stresses, which launch de-twinning in martensite and stress-induced austenite-to-martensite transformation close to the As temperature. However, the closeness of the As and Ms temperatures for B2 R transformation leaves room for consideration of the OWSME and TWSME maximal magnitudes as the result of the metastable (pre-martensitic) austenite presence. Following this consideration, the straining of such metastable austenite that could be obtained by cooling from temperatures higher than Af should provide higher functional properties.
Fig. 3. Dependences of a sm/ res ratio that characterized one-way shape memory (shape memory effect) on tensile test temperature. ( , ▲ – test temperature was reached by heating from room temperature, , ▼ - test temperature was reached by cooling from 180оC. Solid triangles – high-rate tension, open triangles – quasistatic tension).
Fig. 4. Plots of a twsm/ p ratio (that characterized two-way shape memory effect) as functions of tensile test temperature, reached by two different paths. Signs define the magnitudes of TWSME after quasi-static tension at temperatures reached by heating from - the same characteristics after quasi-static room temperature, tension at temperatures reached by cooling from 180oC. Solid signs correspond to the results obtained after high-rate tension.
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The comparison of OWSME and TWSME acquired after quasi-static and high-rate tension at 60, 77, 87 and 100оC attained by heating from room temperature and by cooling from 180 оC is shown in Figs. 3 and 4. Positive increments of both sm/ res and twsm/ p relations after quasi-static tension at 60, 77 and 87 oC, attained by cooling from 180 oC, confirm the effectiveness of metastable austenite straining for the improvement of NiTi functional properties. The analysis of the sm and twsm absolute values obtained in this set of experiments showed that their increments sm and twsm were the biggest after straining at 77 oC and comprised 4.83% and 1.77% respectively. The deviations from this temperature led to a decrease in absolute values of both sm and twsm, thus pointing to a narrow temperature interval, within which the enhancement of functional properties was maximal. The negligible increments of sm/ res and twsm/ p relations after quasi-static tension at 77 oC attained by cooling from 180oC were conditioned by an extreme plasticity inherent in austenite in the vicinity of the Ms temperature. In this connection, it is worth noting that the absolute values of sm and twsm, in fact, determine the displacements, which can be executed by NiTi actuators and, from this point of view, are more representative indices of functional properties than the relations sm/ res and twsm/ p. Negative increments of sm/ res and twsm/ p relations after quasi-static tension at 100 oC and after high-rate tension at 77, 87 and 100 oC attained by cooling from 180 oC (Figs. 3 and 4) were the result of transition to the austenitic type of TWSME. As shown earlier, the straining of austenite at temperatures higher than A f leads to the formation of austenitic TWSME, in which the reversible strains are several times lower than the reversible strains accumulated at the expense of martensitic TWSME. Taking into consideration the temperature increase inherent in high-rate straining, one may affirm that the above negative increments were characteristic for the tests conducted at temperatures higher than the Af temperature (Af = 98 oC) and, thus, were related to the formation of austenitic TWSME. On the heterogeneity of precipitates distribution within a grain, the existence of austenite areas with different anisotropy factors becomes evident. One of the reasons for the observed improvement of NiTi functional properties after straining of metastable austenite could be the predominant shear along 1-10 {001} assisted by external stress in the areas of austenite with intermediate values of c44, which promoted B2 B19 transformation instead of B2 R transformation. Although the participation of plastic deformation at the earliest stages of high-rate tension and specimens’ selfheating contribute to the peculiarities of the functional properties formation by metastable austenite straining, they do not change the positive effect of the latter. 4. Conclusion The conducted study has demonstrated that quasi-static and high-rate tension of metastable austenite in the vicinity of the Ms temperature promote the increase of the basic functional properties of NiTi shape memory alloy. Acknowledgements The authors are grateful to Russian Foundation for Basic Research (grant #13-01-00050) and Saint-Petersburg State University (grant # 6.38.74.2012) for supporting this research. References [1] K. Otsuka, X. Ren, Prog. Mat. Sci. 50 (2005) 511–678. [2] V. Torra, G. Carreras, S. Casciati, P. Terriault, Smart. Struct. Syst. 13 (3) (2014) 353–374. [3] A. Razov, Phys. Met. Metallogr. 97 Suppl.1 (2004) 97–126. [4] N. Zhou, C. Shen, M.F.-X. Wagner, G. Eggeler, M.J. Mills, Y. Wang, Acta Mater. 58 (2010) 6685–6694. [5] X. Ren, N. Miura, J. Zhang, K. Otsuka, K. Tanaka, M. Koiwa, T. Suzuki, Y.I. Chumlyakov, M.A. Asai, Mater. Sci. Eng. A 312 (2001) 196– 206. [6] A.M. Bragov, A.K. Lomunov, I.V. Sergeichev, AIP Conference Proceedings 845 (2006) 705–708.
ScienceDirect Materials Today: Proceedings 2S (2015) S961 – S964
International Conference on Martensitic Transformations, ICOMAT-2014
Straining of metastable austenite as a way to improve NiTi alloy functional properties A. Bragovb, A. Danilova, A. Konstantinova, A. Lomunovb, A. Motorina, A. Razova,* a
Saint-Petersburg State University, Universitetskii pr. 28, Saint-Petersburg, 198504, Russia Lobachevsky State University of Nizhni Novgorod, Gagarin pr., 23, korp.6, GSP-1000 Nizhny Novgorod, 603163, Russia
b
Abstract The effect of metastable austenite straining on the basic functional properties of an NiTi alloy in quasi-equilibrium state was studied. Straining of austenite in a pre-martensitic state was shown to result in improvement of one-way and two-way shape memory effects (OWSME and TWSME). This improvement took place after straining in a narrow temperature range, and its manifestation at quasi-static and high-rate tensile tests was different: the increment of both characteristics was sufficiently large after quasi-static straining, but smaller after high-rate straining. Straining at temperatures that deviated significantly from the Ms temperature led to deterioration of OWSME and TWSME. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility of CC the chairs of the International Conference on Martensitic Transformations 2014. Keywords: NiTi, metastable austenite, tension, functional properties, shape memory, two-way shape memory, high strain rate.
1. Introduction One of the general requirements of NiTi devices, elaborated during the last two decades for the solution of numerous practical problems in different fields of techniques, is the long-term stability of their mechanical and functional properties. This requirement is conventionally satisfied by aging at moderate temperatures in the range of 300–500 oC. However, such aging leads to a decrease in the basic functional properties of these materials in
* Corresponding author. Tel.: +7-812-428-4210; fax: +7-812-428-6944. E-mail address: [email protected]
2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.441
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A. Bragov et al. / Materials Today: Proceedings 2S (2015) S961 – S964
comparison with the same properties achieved after high-temperature solution treatment followed by quenching [1]. Thus, the search for ways to enhance the functional properties of NiTi alloys in quasi-equilibrium state appears to be a pressing task for further progress in the application of NiTi alloys, in particular, in the fields of civil and aerospace engineering, where the high and stable functional properties of NiTi devices are in great demand [2, 3]. Since the origin of NiTi shape memory effects lies in the reversible structural mechanisms of straining, the latter appears to be one of the basic ways of shaping NiTi functional properties. The magnitudes of the reversible strains accumulated due to the de-twinning and reorientation of crystals in martensite, or due to stress-induced martensitic transformations in austenite are, by and large, determined by the crystallographic routes along which straining processes predominantly run. The distinctive features of the NiTi quasi-equilibrium structures - the presence of Ni4Ti3 precipitates and heterogeneity of their distribution within the grain - result in heterogeneous distribution of Ni content and of the oriented structural stresses in matrix. The heterogeneity of these stresses and nickel distribution in matrix are the basic reasons for the existence of distinct paths of martensitic transformations [4]. As has been shown [5], different paths of martensitic transformations are preceded by different behavior of the B2 matrix anisotropy factor A=c44/c (which defines the strength of non-basal and basal shear / shuffle modes coupling) in the vicinity of the Ms temperature. According to [5], prior to B2 B19 transformation, the anisotropy factor sharply decreases with the approaching martensitic transformation temperature, while prior to B2 R transformation, as the initial stage of the B2 R B19 reaction, the anisotropy factor becomes unchanged near the transformation temperature. Application of external stress to such metastable austenite can assist the predominant realization of one of the feasible transformation paths, thus affecting the basic functional properties of NiTi. The consequences of applying external stress to metastable austenite in devices designed for the mitigation of extreme loads in civil or aerospace engineering [2, 3] should be taken into consideration when choosing the material state for functioning under corresponding conditions. However, information about the basic shape memory characteristics acquired by quasiequilibrium NiTi alloys after straining in the vicinity of the Ms temperature, as well as about the effect of the strain rate on them is absent. The aim of the present study was to explore the effect of quasi-static and high-rate tension in the vicinity of the Ms temperature on functional properties of a quasi-equilibrium NiTi alloy. 2. Material and experimental methods The cylindrical specimens (with operative parts 5 mm in diameter and 10 mm in length) for high-rate and quasistatic tensile tests were made of binary NiTi alloy hot-rolled rods. To remove the residual stress and to create a quasi-equilibrium structure, the specimens were annealed at 500 oC for 1 hour followed by cooling in a furnace. The characteristic temperatures of the direct and reverse transformations were determined in a differential scanning calorimeter (DSC) Mettler Toledo 822e. The heating and cooling rates during DSC measurements were 10 degree/min. Transformation temperatures, determined from DSC peaks by a slope line extension method, were as follows: Ms = 74 oC, Mf = 32 oC, As = 74 oC, Af = 98 oC. The high-rate tension at a strain rate of about 103 sec-1 in the temperature range of 20–300 oC was performed using the modifiedSplit Hopkinson Pressure Bar (SHPB) technique [6]. The quasi-static tension at a strain rate of 10-3 sec-1 in the same temperature range was executed in the universal machine for mechanical tests (Lloyd 30K Plus) equipped with a thermal chamber. To reveal the effect of metastable austenite straining on the basic functional properties of NiTi, test temperatures in the range of 60–100 oC were reached by two paths. In the first path – basically used for the whole set of experiments, specimens were heated up to the test temperature from room temperature; in the second – the test temperature was attained by specimens cooling down after preliminary exposure at 180 oC. The analysis of the alloy’s functional properties, acquired after both types of tensile tests, was performed in a special device without external load by heating/cooling cycling of tested specimens within the temperature range of martensitic transformations. On first heating, a magnitude of restored strain ( sm), which characterized the shape memory effect of the alloy after high-rate and quasi-static tension, was determined. On cooling, the magnitude of an accumulated strain ( twsm), which characterized the induced effect of two-way shape memory, was controlled. To smooth out the influence of poorly controlled variations in a total strain value achieved in different tensile experiments, the sm/ res and twsm/ p ratios ( res represented a residual stress value, and p= res - sm represented the
A. Bragov et al. / Materials Today: Proceedings 2S (2015) S961 – S964
irreversible plastic strain) were used in analyzing one-way and two-way shape memory effects, acquired after both types of straining. 3. Results and discussion Figures 1 and 2 represent the indices of one-way and two-way shape memory effects (OWSME and TWSME), initiated by quasi-static and high-rate tension at different temperatures reached by the heating of specimens from room temperature.
Fig. 1. Dependences of the sm/ res ratio that characterized one-way shape memory (shape memory effect) on tensile test temperature. (○ – after quasi-static tension, ● – after high-rate tension).
Fig. 2. Dependences of the twsm/ p ratio that characterized two-way shape memory on tensile test temperature. Open signs define the results obtained after quasi-static tension, while solid signs define the results obtained after high-rate tension. , ▲ – martensitic twoway shape memory, , ▼ – austenitic two-way shape memory.
The maxima on the plots of OWSME and TWSME as functions of test temperatures corresponded with test temperatures slightly higher than the As temperature. (The shift to lower temperatures of these maxima on the plots corresponding to the high-rate tension is most probably associated with the temperature increase inherent in highrate straining). The maximal magnitudes of OWSME and TWSME might be explained by the lowest threshold stresses, which launch de-twinning in martensite and stress-induced austenite-to-martensite transformation close to the As temperature. However, the closeness of the As and Ms temperatures for B2 R transformation leaves room for consideration of the OWSME and TWSME maximal magnitudes as the result of the metastable (pre-martensitic) austenite presence. Following this consideration, the straining of such metastable austenite that could be obtained by cooling from temperatures higher than Af should provide higher functional properties.
Fig. 3. Dependences of a sm/ res ratio that characterized one-way shape memory (shape memory effect) on tensile test temperature. ( , ▲ – test temperature was reached by heating from room temperature, , ▼ - test temperature was reached by cooling from 180оC. Solid triangles – high-rate tension, open triangles – quasistatic tension).
Fig. 4. Plots of a twsm/ p ratio (that characterized two-way shape memory effect) as functions of tensile test temperature, reached by two different paths. Signs define the magnitudes of TWSME after quasi-static tension at temperatures reached by heating from - the same characteristics after quasi-static room temperature, tension at temperatures reached by cooling from 180oC. Solid signs correspond to the results obtained after high-rate tension.
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A. Bragov et al. / Materials Today: Proceedings 2S (2015) S961 – S964
The comparison of OWSME and TWSME acquired after quasi-static and high-rate tension at 60, 77, 87 and 100оC attained by heating from room temperature and by cooling from 180 оC is shown in Figs. 3 and 4. Positive increments of both sm/ res and twsm/ p relations after quasi-static tension at 60, 77 and 87 oC, attained by cooling from 180 oC, confirm the effectiveness of metastable austenite straining for the improvement of NiTi functional properties. The analysis of the sm and twsm absolute values obtained in this set of experiments showed that their increments sm and twsm were the biggest after straining at 77 oC and comprised 4.83% and 1.77% respectively. The deviations from this temperature led to a decrease in absolute values of both sm and twsm, thus pointing to a narrow temperature interval, within which the enhancement of functional properties was maximal. The negligible increments of sm/ res and twsm/ p relations after quasi-static tension at 77 oC attained by cooling from 180oC were conditioned by an extreme plasticity inherent in austenite in the vicinity of the Ms temperature. In this connection, it is worth noting that the absolute values of sm and twsm, in fact, determine the displacements, which can be executed by NiTi actuators and, from this point of view, are more representative indices of functional properties than the relations sm/ res and twsm/ p. Negative increments of sm/ res and twsm/ p relations after quasi-static tension at 100 oC and after high-rate tension at 77, 87 and 100 oC attained by cooling from 180 oC (Figs. 3 and 4) were the result of transition to the austenitic type of TWSME. As shown earlier, the straining of austenite at temperatures higher than A f leads to the formation of austenitic TWSME, in which the reversible strains are several times lower than the reversible strains accumulated at the expense of martensitic TWSME. Taking into consideration the temperature increase inherent in high-rate straining, one may affirm that the above negative increments were characteristic for the tests conducted at temperatures higher than the Af temperature (Af = 98 oC) and, thus, were related to the formation of austenitic TWSME. On the heterogeneity of precipitates distribution within a grain, the existence of austenite areas with different anisotropy factors becomes evident. One of the reasons for the observed improvement of NiTi functional properties after straining of metastable austenite could be the predominant shear along 1-10 {001} assisted by external stress in the areas of austenite with intermediate values of c44, which promoted B2 B19 transformation instead of B2 R transformation. Although the participation of plastic deformation at the earliest stages of high-rate tension and specimens’ selfheating contribute to the peculiarities of the functional properties formation by metastable austenite straining, they do not change the positive effect of the latter. 4. Conclusion The conducted study has demonstrated that quasi-static and high-rate tension of metastable austenite in the vicinity of the Ms temperature promote the increase of the basic functional properties of NiTi shape memory alloy. Acknowledgements The authors are grateful to Russian Foundation for Basic Research (grant #13-01-00050) and Saint-Petersburg State University (grant # 6.38.74.2012) for supporting this research. References [1] K. Otsuka, X. Ren, Prog. Mat. Sci. 50 (2005) 511–678. [2] V. Torra, G. Carreras, S. Casciati, P. Terriault, Smart. Struct. Syst. 13 (3) (2014) 353–374. [3] A. Razov, Phys. Met. Metallogr. 97 Suppl.1 (2004) 97–126. [4] N. Zhou, C. Shen, M.F.-X. Wagner, G. Eggeler, M.J. Mills, Y. Wang, Acta Mater. 58 (2010) 6685–6694. [5] X. Ren, N. Miura, J. Zhang, K. Otsuka, K. Tanaka, M. Koiwa, T. Suzuki, Y.I. Chumlyakov, M.A. Asai, Mater. Sci. Eng. A 312 (2001) 196– 206. [6] A.M. Bragov, A.K. Lomunov, I.V. Sergeichev, AIP Conference Proceedings 845 (2006) 705–708.