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The influence of Ni4Ti3 precipitates orientation on two-way shape memory effect in a Ni-rich NiTi alloy
Journal of Alloys and Compounds 485 (2009) 320–323
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The influence of Ni4 Ti3 precipitates orientation on two-way shape memory effect in a Ni-rich NiTi alloy M.S. Shakeri a , J. Khalil-Allafi a,∗ , V. Abbasi-Chianeh a , Arash Ghabchi b,c a b c
Faculty of Materials Engineering, Research Center for Advance Materials, Sahand University of Technology, Tabriz, East Azarbaidjan, Iran Technical Research Center of Finland (VTT), Espoo, Finland Center for Thermal Spray Research, Material Science & Engineering Department, State University of New York, Stony Brook, United States
a r t i c l e
i n f o
Article history: Received 25 February 2009 Received in revised form 17 May 2009 Accepted 18 May 2009 Available online 23 May 2009 Keywords: Shape memory NiTi TWSME Thermomechanical cycles Ni4 Ti3 precipitates
a b s t r a c t In this present work two-way shape memory effect (TWSME) induced by thermomechanical cycling treatments in a Ni-rich NiTi alloy was investigated.Effect of orientation of metastable and lenticular Ni4 Ti3 precipitates as a governing parameter in different aging temperatures and time as well as applied strain on TWSME were studied. The results indicate that 450 ◦ C is the optimum aging temperature to achieve highest recovery rate. The increasing of applied strain up to 3.38% and aging time up to 2.5 h at 450 ◦ C improves the recovery rate. It has been concluded that formation of especial variants of Ni4 Ti3 precipitates during aging and increasing of applied strain on thermomechanical cycling treatments could enhance the TWSME. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The NiTi shape memory alloys exhibit two fascinating effects: thermal effect (one-way effect and two-way effect) and super elasticity. In order to obtain TWSME applying proper training process is decisive. The training processes result in formation of internal stresses, lattice defects and residual martensite in the parent phase. Formation of preferred martensite variants during direct martensitic transformation is due to the residual stresses acting on a certain direction. Among the effective parameters on TWSME, the generation of lattice defects during martensitic transformation, which are the source of oriented stress fields, is the most crucial factor [1–8]. Inducing TWSME can be realized by means of two treatments: mechanical and thermomechanical treatments. The maximum strain recovery ratio can be achieved by mechanical treatments and maximum stability of strain recovery can be obtained by thermomechanical treatments [8]. TWSME which is induced by thermomechanical methods can be affected by several different parameters such as aging time and temperature, magnitude and direction of applied stress and number of thermomechanical cycles [8–11]. Gall et al. showed that martensite variants are induced by coherent stresses around the Ni4 Ti3 precipitates [12,13]. It is confirmed
that the shape of Ni4 Ti3 precipitates is lenticular with (1 1 1)M habit plane [1]. In the early stages of aging, precipitates are coherent and the habit plane of the particles tends to be parallel to the direction of applied tensile-stress. Nevertheless, under compressive-stress habit plane tends to be perpendicular to the stress direction, as shown in Fig. 1A. Upon removal of external stress, particles create a tensile-stress perpendicular to their habit plane. When a sheet specimen is deformed into a round-shape as shown in Fig. 1A, the outer part and the inner part should be in a tensile-stress and compressive-stress state respectively. Fig. 1A illustrates a specimen which is aged in the bent state. Changes in the shape of the specimen due to martensitic transformation is called all-round shape memory. Occurance of this phenomenon is due to the different special martensite variants in outer and inner part of the specimen. However, coherency strains produced by the coherent Ni4 Ti3 precipitates is also important. To get the full advantage of coherency strains, precipitate size must be small (typically smaller than 150 nm). Therefore, aging temperature should not exceed 500 ◦ C, above which the precipitate size becomes too large and the coherency strains diminish. In this investigation, the relation between direction of oriented precipitates and original shape of specimens was examined as an effective parameter on TWSME. Then effect of aging time and applied strain was studied according to shape changes in a range of 0–100 ◦ C. 2. Materials and experimental procedure
∗ Corresponding author. Fax: +98 4123444334. E-mail address: allafi@sut.ac.ir (J. Khalil-Allafi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.084
A NiTi wire with nominal composition of 50.9 at.% Ni with 0.35 mm diameter purchased from Memry Co. (USA) was utilized in this study. To study the effect of
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
321
Fig. 1. (A) Schematic illustration of precipitates distribution in different states of all-round shape memory alloys [1], (B) schematic illustration of experimental procedure, and (C) different shapes and parameters for the computation of recovery ratio defined by Eq. (3) [15]. Table 1 Different aging conditions of specimens. Aging time (h)
Applied strain (%)
2.5 0.5, 1, 1.5, 2, 2.5
1.8, 2.14, 2.62, 3.38 2.14
Transformation temperatures were studied by means of DSC after the training procedure. In order to examine recovery ratio (R.r) of the specimens, shape changes were determined in the range of 0–100 ◦ C. By employing Eqs. (1)–(3), the R.r was calculated for all specimens [14,15]: εd =
orientation of Ni4 Ti3 precipitates on TWSME, the as received wire has been subjected to about 35% cold work (e = 0.35) with no heat treatment, were used. The stress–strain response of the as received wire is shown in Fig. 2. DSC (Differential Scanning Calorimetry) curve of as received wire did not show any peak during cooling and heating. The as received specimen was aged at 400, 450, 500 and 550 ◦ C for 1 h under 2.14%, 0% strain as shown in Fig. 1B.a(A and B) and 1B.a(C) respectively. Afterward in order to have the straight shape and the same aging time for all 3 specimens, the samples were shaped at the same temperature as aging for 15 min as shown in Fig. 1B.b. Then in training process, the cooling in liquid nitrogen (−196 for 10 s) and heating in vapor water (+100 ◦ C for 10 s), was done under 2.14% constant strain in three different states in comparison to constrained aging treatment as shown in Fig. 1B.c. Fig. 1B schematically illustrates the explained procedure. Because of the wires shape in aging treatment and thermomechanical cycling process, wires were named as following: specimen A (direct wire), specimen B (indirect wire) and specimen C (straight wire) (see the Fig. 1B). For evaluation of aging time and applied strain effect on TWSME, aging treatment was performed under different strains at 450 ◦ C in the bent form and then TWSME training process was performed in straight form using 50 thermomechanical cycles. Aging conditions are explained in detail in Table 1. It is worth noting that in this case we applied a different procedure from that used in evaluation of precipitates variant effect on TWSME.
Fig. 2. Loading–unloading response of as received wire.
d d + 2R
(1)
p − m × ε εtw = d 180
(2)
R.r =
(3)
εtw εd
× 100
where εd is deformation strain, R is the radius of bending, d is the diameter of wire, εtw is the TWSME strain, P and m are the arc angles of specimens at 100 and 0 ◦ C, respectively. total , m , P , R and d are defined in Fig. 1C [14,15].
3. Results and discussion Effect of the Ni4 Ti3 precipitates orientation on TWSME at different aging temperatures is considered with R.r value calculated by Eq. (3). Fig. 3 shows the R.r value of three different specimens (A, B, C) at aging temperatures of 400–550 ◦ C. These curves show a maximum for all 3 specimens at 450 ◦ C. The R.r value for specimen A (direct wire) is the highest for all aging temperatures. We hypothesize this is due to appropriate martensite variants limitation in specimen A. In the specimen that is aged without constraint (specimen C) which has all variants of precipitates, minimum R.r value was determined.
Fig. 3. Recovery ratio at different aging temperatures for three different orientations of precipitates.
322
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
Fig. 4. DSC curves at different aging temperatures for 1 h: aged at (a) 400 ◦ C, (b) 450 ◦ C, (c) 500 ◦ C, and (d) 550 ◦ C.
As a result of orientation of precipitates we can conclude that due to the formation of internal stresses in the special directions in the presence of oriented Ni4 Ti3 precipitates, martensitic variants may be limited. If these variants are suitable for TWSME, the R.r values will be increased. Applying of stress to specimens A and B results in formation of precipitates in a special state (as discussed previously) that could affect the TWSME. Considering all-round shape memory effect, it seems that indirect wire (specimen B) must have the highest R.r value, because the internal stresses may led to shape changes as same as trained shape. Nevertheless, the results indicate that the R.r value for specimen A is higher than specimen B. The all round shape memory effect was not observed in our specimens. It may be attributed to the different shape factor and heat treatment parameters from which reported in Ref. [16]. Even if the all round shape memory effect has been detected, it might lead to increase the R.r value in specimen B. So it seems that this cryptic phenomenon may be a result of different applied stresses in specimens A and B in order to reach the same strain. This different stresses are due to different internal stresses. On the other hand, R.r value may be induced by different density of lattice defects in specimens which could be the effect of different Ni4 Ti3 variants in specimens A and B. Fig. 4 indicates DSC curves of aged specimens at different temperatures for 1 h and after training process for 50 cycles. Transition temperatures for the same aged specimens with different thermomechanical cycles are approximately constant up to 550 ◦ C. It might be because of the constant value and kinds of the precipitates which are created during the same aging processes. Therefore orientation of precipitates has no effect on the transition temperatures while the amount of R.r can be induced. Fig. 3 illustrates, that changes in temperature leads to changes in R.r value but the effect of precipitates orientation in all temperatures is as the same as the 450 ◦ C which is discussed above. For all specimens, the highest R.r value was obtained at 450 ◦ C. In 400 ◦ C, precipitates are finely distributed. By increasing the temperature up to 450 ◦ C the Ni4 Ti3 precipitates grow. However, the precipitates are still coherent [13]. It seems that growth of these precipitates up to an appropriate level, until they are coherent, is the reason for higher R.r value. On the other hand polygonization process and dislocation density changes due to aging temperature and time variation can affect the R.r value [17,18]. By further increasing the aging temperature, the amount of coherency decreases gradually
and the precipitates become semi-coherent and then incoherent and the density of dislocations would decrease. Increasing aging temperature causes in changing the transition temperatures. In the specimens aged at 500 ◦ C, transition temperatures were changed in a way that on cooling to 0 ◦ C (minimum temperature of shape change examination) the specimens are in R phase while aged specimens at 450 ◦ C are in B19 phase at 0 ◦ C. For aged specimens at 550 ◦ C, B19 and R phase transformation temperatures were overlapped but also in this temperature, B19 transformation has not been completed. It can be inferred that in aging temperatures
Fig. 5. (a) recovery ratio with different deformation strains and (b) Recovery ratio at different aging times.
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
higher than 450 ◦ C, R.r for all specimens reach to a constant value which may be due to the coherent stress annihilation. Fig. 5a depicts the R.r versus deformation strain obtained for the sample aged at 450 ◦ C for 2.5 h with 50 thermomechanical cycles. As can be seen, with increasing of deformation strain, R.r was raised and in 3.38% deformation strain, reached to 4.4%. Perhaps this is because of the limitation of martensitic variants. Probably with increasing the deformation strain, a same variant of martensite remains and then it leads to highest R.r value. Fig. 5b illustrates the R.r values in different aging time, at T = 450 ◦ C, ε = 2.14% and 50 thermomechanical cycles. According to the results of the experiment, R.r was increased with increasing the aging time. In this case, increasing of R.r seems to be a result of increasing the size of Ni4 Ti3 precipitates and polygonization completeness precipitates. Therefore it seems that with further increasing of aging time, R.r reaches to a maximum value and then decreases because on coherency of precipitates annihilate and the best combination of fine precipitates with well developed polygonized structure will degrades. 4. Conclusions - Formation and orientation of Ni4 Ti3 precipitates is an important and effective factor on the recovery ratio (R.r). Orientation of precipitates leads to increasing of the R.r. - Aging temperature is a valuable parameter that can be effective in the R.r value. The optimum aging temperature to obtain the highest TWSME was realized at 450 ◦ C.
323
- Increasing applied strain up to 3.38% leads to improvement of the R.r value. - Increasing of aging time up to 2.5 h at 450 ◦ C causes enhancement of the R.r value. References [1] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1998. [2] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511. [3] H. Funakubo, Shape Memory Alloys, Sangyo Tosho K.K., Tokyo, 1984. [4] T.W. During, K.N. Melton, D. Stockel, Engineering Aspect of Shape Memory Alloys, Butterworth-Heinemann Ltd., 1990. [5] V. Brailovski, et al., Shape Memory Alloys: Fundamentals, Modeling and Applications, university du Quebec, 2003. [6] W.J. Cheng, Y.S. Wei, Scripta Metall. 2 (1989) 363. [7] J. Beyer, P.A. Besselink, A.J. Aartsen, Thermochim. Acta 85 (1985) 187. [8] J. Perkins, Scripta Metall. 8 (1974) 1469. [9] Z.G. Wang, X.T. Zu, X.D. Feng, et al., Mater. Sci. Eng. A 345 (2003) 249. [10] Z. Wang, X. Zu, et al., Mater. Lett. 54 (2002) 55. [11] E.P. Ryklina, S.D. Prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina, Mater. Sci. Eng. A (2008) 134. [12] K. Gall, H. Sehitoglu, Y.I. Chumlyakov, Y.L. Zuev, I. Karaman, Scripta Mater. 39 (1998) 699. [13] K. Gall, H. Sehltoglu, J. Eng. Mater. Technol. 121 (1999) 19. [14] X.L. Meng, W. Cai, Y.D. Fu, Q.F. Li, J.X. Zhang, L.C. Zhao, Intermetallics 16 (2008) 698. [15] C.Y. Chang, D. Vokoum, C.T. Hu, Metall. Mater. Trans. A 32 (2001) 1629. [16] M. Nishida, T. Honma, Scripta Metall. 18 (1984) 1293. [17] E.P. Ryklina, I.Yu. Khmelevskaya, S.D. prokoshkin, K.E. Inaekyan, R.V. Ipatkin, Mater. Sci. Eng. A 438–440 (2006) 1093. [18] E.P. Ryklina, S.D. prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina, Mater. Sci. Eng. A 481–482 (2008) 134.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The influence of Ni4 Ti3 precipitates orientation on two-way shape memory effect in a Ni-rich NiTi alloy M.S. Shakeri a , J. Khalil-Allafi a,∗ , V. Abbasi-Chianeh a , Arash Ghabchi b,c a b c
Faculty of Materials Engineering, Research Center for Advance Materials, Sahand University of Technology, Tabriz, East Azarbaidjan, Iran Technical Research Center of Finland (VTT), Espoo, Finland Center for Thermal Spray Research, Material Science & Engineering Department, State University of New York, Stony Brook, United States
a r t i c l e
i n f o
Article history: Received 25 February 2009 Received in revised form 17 May 2009 Accepted 18 May 2009 Available online 23 May 2009 Keywords: Shape memory NiTi TWSME Thermomechanical cycles Ni4 Ti3 precipitates
a b s t r a c t In this present work two-way shape memory effect (TWSME) induced by thermomechanical cycling treatments in a Ni-rich NiTi alloy was investigated.Effect of orientation of metastable and lenticular Ni4 Ti3 precipitates as a governing parameter in different aging temperatures and time as well as applied strain on TWSME were studied. The results indicate that 450 ◦ C is the optimum aging temperature to achieve highest recovery rate. The increasing of applied strain up to 3.38% and aging time up to 2.5 h at 450 ◦ C improves the recovery rate. It has been concluded that formation of especial variants of Ni4 Ti3 precipitates during aging and increasing of applied strain on thermomechanical cycling treatments could enhance the TWSME. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The NiTi shape memory alloys exhibit two fascinating effects: thermal effect (one-way effect and two-way effect) and super elasticity. In order to obtain TWSME applying proper training process is decisive. The training processes result in formation of internal stresses, lattice defects and residual martensite in the parent phase. Formation of preferred martensite variants during direct martensitic transformation is due to the residual stresses acting on a certain direction. Among the effective parameters on TWSME, the generation of lattice defects during martensitic transformation, which are the source of oriented stress fields, is the most crucial factor [1–8]. Inducing TWSME can be realized by means of two treatments: mechanical and thermomechanical treatments. The maximum strain recovery ratio can be achieved by mechanical treatments and maximum stability of strain recovery can be obtained by thermomechanical treatments [8]. TWSME which is induced by thermomechanical methods can be affected by several different parameters such as aging time and temperature, magnitude and direction of applied stress and number of thermomechanical cycles [8–11]. Gall et al. showed that martensite variants are induced by coherent stresses around the Ni4 Ti3 precipitates [12,13]. It is confirmed
that the shape of Ni4 Ti3 precipitates is lenticular with (1 1 1)M habit plane [1]. In the early stages of aging, precipitates are coherent and the habit plane of the particles tends to be parallel to the direction of applied tensile-stress. Nevertheless, under compressive-stress habit plane tends to be perpendicular to the stress direction, as shown in Fig. 1A. Upon removal of external stress, particles create a tensile-stress perpendicular to their habit plane. When a sheet specimen is deformed into a round-shape as shown in Fig. 1A, the outer part and the inner part should be in a tensile-stress and compressive-stress state respectively. Fig. 1A illustrates a specimen which is aged in the bent state. Changes in the shape of the specimen due to martensitic transformation is called all-round shape memory. Occurance of this phenomenon is due to the different special martensite variants in outer and inner part of the specimen. However, coherency strains produced by the coherent Ni4 Ti3 precipitates is also important. To get the full advantage of coherency strains, precipitate size must be small (typically smaller than 150 nm). Therefore, aging temperature should not exceed 500 ◦ C, above which the precipitate size becomes too large and the coherency strains diminish. In this investigation, the relation between direction of oriented precipitates and original shape of specimens was examined as an effective parameter on TWSME. Then effect of aging time and applied strain was studied according to shape changes in a range of 0–100 ◦ C. 2. Materials and experimental procedure
∗ Corresponding author. Fax: +98 4123444334. E-mail address: allafi@sut.ac.ir (J. Khalil-Allafi). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.084
A NiTi wire with nominal composition of 50.9 at.% Ni with 0.35 mm diameter purchased from Memry Co. (USA) was utilized in this study. To study the effect of
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
321
Fig. 1. (A) Schematic illustration of precipitates distribution in different states of all-round shape memory alloys [1], (B) schematic illustration of experimental procedure, and (C) different shapes and parameters for the computation of recovery ratio defined by Eq. (3) [15]. Table 1 Different aging conditions of specimens. Aging time (h)
Applied strain (%)
2.5 0.5, 1, 1.5, 2, 2.5
1.8, 2.14, 2.62, 3.38 2.14
Transformation temperatures were studied by means of DSC after the training procedure. In order to examine recovery ratio (R.r) of the specimens, shape changes were determined in the range of 0–100 ◦ C. By employing Eqs. (1)–(3), the R.r was calculated for all specimens [14,15]: εd =
orientation of Ni4 Ti3 precipitates on TWSME, the as received wire has been subjected to about 35% cold work (e = 0.35) with no heat treatment, were used. The stress–strain response of the as received wire is shown in Fig. 2. DSC (Differential Scanning Calorimetry) curve of as received wire did not show any peak during cooling and heating. The as received specimen was aged at 400, 450, 500 and 550 ◦ C for 1 h under 2.14%, 0% strain as shown in Fig. 1B.a(A and B) and 1B.a(C) respectively. Afterward in order to have the straight shape and the same aging time for all 3 specimens, the samples were shaped at the same temperature as aging for 15 min as shown in Fig. 1B.b. Then in training process, the cooling in liquid nitrogen (−196 for 10 s) and heating in vapor water (+100 ◦ C for 10 s), was done under 2.14% constant strain in three different states in comparison to constrained aging treatment as shown in Fig. 1B.c. Fig. 1B schematically illustrates the explained procedure. Because of the wires shape in aging treatment and thermomechanical cycling process, wires were named as following: specimen A (direct wire), specimen B (indirect wire) and specimen C (straight wire) (see the Fig. 1B). For evaluation of aging time and applied strain effect on TWSME, aging treatment was performed under different strains at 450 ◦ C in the bent form and then TWSME training process was performed in straight form using 50 thermomechanical cycles. Aging conditions are explained in detail in Table 1. It is worth noting that in this case we applied a different procedure from that used in evaluation of precipitates variant effect on TWSME.
Fig. 2. Loading–unloading response of as received wire.
d d + 2R
(1)
p − m × ε εtw = d 180
(2)
R.r =
(3)
εtw εd
× 100
where εd is deformation strain, R is the radius of bending, d is the diameter of wire, εtw is the TWSME strain, P and m are the arc angles of specimens at 100 and 0 ◦ C, respectively. total , m , P , R and d are defined in Fig. 1C [14,15].
3. Results and discussion Effect of the Ni4 Ti3 precipitates orientation on TWSME at different aging temperatures is considered with R.r value calculated by Eq. (3). Fig. 3 shows the R.r value of three different specimens (A, B, C) at aging temperatures of 400–550 ◦ C. These curves show a maximum for all 3 specimens at 450 ◦ C. The R.r value for specimen A (direct wire) is the highest for all aging temperatures. We hypothesize this is due to appropriate martensite variants limitation in specimen A. In the specimen that is aged without constraint (specimen C) which has all variants of precipitates, minimum R.r value was determined.
Fig. 3. Recovery ratio at different aging temperatures for three different orientations of precipitates.
322
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
Fig. 4. DSC curves at different aging temperatures for 1 h: aged at (a) 400 ◦ C, (b) 450 ◦ C, (c) 500 ◦ C, and (d) 550 ◦ C.
As a result of orientation of precipitates we can conclude that due to the formation of internal stresses in the special directions in the presence of oriented Ni4 Ti3 precipitates, martensitic variants may be limited. If these variants are suitable for TWSME, the R.r values will be increased. Applying of stress to specimens A and B results in formation of precipitates in a special state (as discussed previously) that could affect the TWSME. Considering all-round shape memory effect, it seems that indirect wire (specimen B) must have the highest R.r value, because the internal stresses may led to shape changes as same as trained shape. Nevertheless, the results indicate that the R.r value for specimen A is higher than specimen B. The all round shape memory effect was not observed in our specimens. It may be attributed to the different shape factor and heat treatment parameters from which reported in Ref. [16]. Even if the all round shape memory effect has been detected, it might lead to increase the R.r value in specimen B. So it seems that this cryptic phenomenon may be a result of different applied stresses in specimens A and B in order to reach the same strain. This different stresses are due to different internal stresses. On the other hand, R.r value may be induced by different density of lattice defects in specimens which could be the effect of different Ni4 Ti3 variants in specimens A and B. Fig. 4 indicates DSC curves of aged specimens at different temperatures for 1 h and after training process for 50 cycles. Transition temperatures for the same aged specimens with different thermomechanical cycles are approximately constant up to 550 ◦ C. It might be because of the constant value and kinds of the precipitates which are created during the same aging processes. Therefore orientation of precipitates has no effect on the transition temperatures while the amount of R.r can be induced. Fig. 3 illustrates, that changes in temperature leads to changes in R.r value but the effect of precipitates orientation in all temperatures is as the same as the 450 ◦ C which is discussed above. For all specimens, the highest R.r value was obtained at 450 ◦ C. In 400 ◦ C, precipitates are finely distributed. By increasing the temperature up to 450 ◦ C the Ni4 Ti3 precipitates grow. However, the precipitates are still coherent [13]. It seems that growth of these precipitates up to an appropriate level, until they are coherent, is the reason for higher R.r value. On the other hand polygonization process and dislocation density changes due to aging temperature and time variation can affect the R.r value [17,18]. By further increasing the aging temperature, the amount of coherency decreases gradually
and the precipitates become semi-coherent and then incoherent and the density of dislocations would decrease. Increasing aging temperature causes in changing the transition temperatures. In the specimens aged at 500 ◦ C, transition temperatures were changed in a way that on cooling to 0 ◦ C (minimum temperature of shape change examination) the specimens are in R phase while aged specimens at 450 ◦ C are in B19 phase at 0 ◦ C. For aged specimens at 550 ◦ C, B19 and R phase transformation temperatures were overlapped but also in this temperature, B19 transformation has not been completed. It can be inferred that in aging temperatures
Fig. 5. (a) recovery ratio with different deformation strains and (b) Recovery ratio at different aging times.
M.S. Shakeri et al. / Journal of Alloys and Compounds 485 (2009) 320–323
higher than 450 ◦ C, R.r for all specimens reach to a constant value which may be due to the coherent stress annihilation. Fig. 5a depicts the R.r versus deformation strain obtained for the sample aged at 450 ◦ C for 2.5 h with 50 thermomechanical cycles. As can be seen, with increasing of deformation strain, R.r was raised and in 3.38% deformation strain, reached to 4.4%. Perhaps this is because of the limitation of martensitic variants. Probably with increasing the deformation strain, a same variant of martensite remains and then it leads to highest R.r value. Fig. 5b illustrates the R.r values in different aging time, at T = 450 ◦ C, ε = 2.14% and 50 thermomechanical cycles. According to the results of the experiment, R.r was increased with increasing the aging time. In this case, increasing of R.r seems to be a result of increasing the size of Ni4 Ti3 precipitates and polygonization completeness precipitates. Therefore it seems that with further increasing of aging time, R.r reaches to a maximum value and then decreases because on coherency of precipitates annihilate and the best combination of fine precipitates with well developed polygonized structure will degrades. 4. Conclusions - Formation and orientation of Ni4 Ti3 precipitates is an important and effective factor on the recovery ratio (R.r). Orientation of precipitates leads to increasing of the R.r. - Aging temperature is a valuable parameter that can be effective in the R.r value. The optimum aging temperature to obtain the highest TWSME was realized at 450 ◦ C.
323
- Increasing applied strain up to 3.38% leads to improvement of the R.r value. - Increasing of aging time up to 2.5 h at 450 ◦ C causes enhancement of the R.r value. References [1] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1998. [2] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511. [3] H. Funakubo, Shape Memory Alloys, Sangyo Tosho K.K., Tokyo, 1984. [4] T.W. During, K.N. Melton, D. Stockel, Engineering Aspect of Shape Memory Alloys, Butterworth-Heinemann Ltd., 1990. [5] V. Brailovski, et al., Shape Memory Alloys: Fundamentals, Modeling and Applications, university du Quebec, 2003. [6] W.J. Cheng, Y.S. Wei, Scripta Metall. 2 (1989) 363. [7] J. Beyer, P.A. Besselink, A.J. Aartsen, Thermochim. Acta 85 (1985) 187. [8] J. Perkins, Scripta Metall. 8 (1974) 1469. [9] Z.G. Wang, X.T. Zu, X.D. Feng, et al., Mater. Sci. Eng. A 345 (2003) 249. [10] Z. Wang, X. Zu, et al., Mater. Lett. 54 (2002) 55. [11] E.P. Ryklina, S.D. Prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina, Mater. Sci. Eng. A (2008) 134. [12] K. Gall, H. Sehitoglu, Y.I. Chumlyakov, Y.L. Zuev, I. Karaman, Scripta Mater. 39 (1998) 699. [13] K. Gall, H. Sehltoglu, J. Eng. Mater. Technol. 121 (1999) 19. [14] X.L. Meng, W. Cai, Y.D. Fu, Q.F. Li, J.X. Zhang, L.C. Zhao, Intermetallics 16 (2008) 698. [15] C.Y. Chang, D. Vokoum, C.T. Hu, Metall. Mater. Trans. A 32 (2001) 1629. [16] M. Nishida, T. Honma, Scripta Metall. 18 (1984) 1293. [17] E.P. Ryklina, I.Yu. Khmelevskaya, S.D. prokoshkin, K.E. Inaekyan, R.V. Ipatkin, Mater. Sci. Eng. A 438–440 (2006) 1093. [18] E.P. Ryklina, S.D. prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina, Mater. Sci. Eng. A 481–482 (2008) 134.