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One-way and two-way shape memory effect in thermomechanically treated TiNi-based alloys
Materials Science and Engineering A 481–482 (2008) 134–137
One-way and two-way shape memory effect in thermomechanically treated TiNi-based alloys E.P. Ryklina ∗ , S.D. Prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina Moscow State Institute of Steel and Alloys, Technological University, 4 Leninsky Pr., 119049 Moscow, Russia Received 22 May 2006; received in revised form 1 March 2007; accepted 14 March 2007
Abstract One-way and two-way shape memory effects (SME and TWSME) and functional properties of Ti–50.7 at.% Ni alloy after low-temperature thermomechanical treatment (LTMT) and post-deformation annealing are studied. The combined influence of structure realized under isothermal annealing after LTMT and external training parameters on functional properties of this alloy is studied. Variation of strain aging time under isothermal annealing is effective for regulation of critical temperatures and other SME/TWSME parameters. LTMT with subsequent aging at 430 ◦ C for 10 h is favorable for achieving the best combination of TWSME parameters in this alloy. Variation of loading time is effective for regulation of TWSME magnitude under multi-cycle training procedure. The TWSME value achieved is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Shape memory alloys; Nitinol; Thermomechanical treatment; Two-way shape memory effect; Functional properties
1. Introduction Shape memory effect (SME) and two-way shape memory effect (TWSME) parameters are structure-sensitive, therefore, heat and thermomechanical treatment are effective for TWSME regulation [1,2]. A post-deformation annealing (PDA) after low-temperature thermomechanical treatment (LTMT) provokes softening processes of the deformed metal including stress relaxation, recovery, polygonization and recrystallization of austenite, and provides an additional possibility for the shape memory properties regulation. In Ti–Ni alloys with nickel concentration more than 50.0% a precipitating process during aging takes place in parallel [1–3]. The main factors affecting functional properties in this case are as follows: concentration changes in matrix; dislocation substructure changes; formation of oriented internal stress fields caused by coherent precipitates. Under SME and TWSME inducing in real devices, the external training parameters additionally affect functional properties: the load scheme (tension, twisting, bending), constrained total strain and loading time. Real technical and medical SME arti∗
Corresponding author. Fax: +7 495 2472301. E-mail address: [email protected] (E.P. Ryklina).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.03.118
cles in most cases are functioning in bending. Nevertheless, there is lack of information about the effect of external training parameters under bending on the functional properties. The study of the individual and combined (structural and training) influence on final functional properties is of great practical interest. In the present work the functional properties in Ti–50.7 at.% Ni alloy after LTMT and isothermal PDA as well as the influence of the TWSME training parameters under bending are examined. 2. Experimental The Ti–50.7 at.% Ni alloy was studied having martensitic transformation temperatures Ms = −8 ◦ C, Mf = −24 ◦ C, As = 0 ◦ C, Af = 9 ◦ C after quenching from 700 ◦ C. The lowtemperature thermomechanical treatment (LTMT) of the initial workpiece (a bar of 2 mm in diameter) was performed by wiredrawing (or rolling for the electron microscopy study) in several passes at the room temperature with strain ε = 0.25–0.3 in each pass and intermediate annealings at 700 ◦ C (15 min). The wire 0.3 mm in diameter for further study was obtained with the summary strain of 0.3 in the last pass. The strain aging of samples was performed at 430 ◦ C during 10 min, 30 min, 60 min, 3 h and 10 h for obtaining all stages of polygonized dislocation
E.P. Ryklina et al. / Materials Science and Engineering A 481–482 (2008) 134–137
substructure (from initial stage to well-developed) and various precipitation stages of the Ti3 Ni4 phase. Controlled heat treatment was carried out at 700 ◦ C for 20 min followed by water cooling. The oxidized surface layer was removed by chemical etching in 1HF + 3HNO3 + 6H2 O2 solution. Thin foils for transmission electron microscopy (TEM) were prepared using grinding and electropolishing by the “window” technique in an electrolyte containing 10 vol.% HClO4 + 90 vol.% CH3 COOH. The micro-structure after PDA and aging was studied using JEM-100C transmission electron microscope. The samples for the X-ray study have been prepared on special mandrels of the cut wire. The X-ray diffraction study was performed using DRON-3 diffractometer in filtered Co-radiation in the temperature range from 0 ◦ C to 100 ◦ C. The diffractograms were recorded in the 2θ = 52◦ to 56◦ range which comprised the 1 1 0B2, 3 3 0–3 3¯ 0R, 5 1 1Ti2 Ni and 2 1 1Ti3 Ni4 X-ray diffraction peaks. As and Af temperatures were determined visually as the temperatures of beginning and completion of the shape change on heating, respectively. The SME/TWSME inducing was performed using “positive” training method by bending the wire samples around mandrels of various diameters at 25 ◦ C with total strain εt from 7.5% to 15%, then cooling under stress down to −115 ◦ C. The induced strain εi was determined at a constraining temperature after unloading. The residual strain εf was determined when heating above Af temperature. The recovery strain εr was determined as the difference between εi and εf . The recoverable elastic strain εrel was determined as the difference between εt and εi . The exposure time in the constrained condition in the experiment with cyclic TWSME training varied as 10 s, 30 s or 60 s at 25 ◦ C with cooling under stress to 0 ◦ C. 3. Results and discussion The electron microscopy examination shows the formation of different structure types of the B2 austenite after various strain aging regimes [4]. The post-deformation annealing at 430 ◦ C (10 min) is accompanied by a partial polygonization of the austenite, but the dislocation density remains high, i.e., the thermomechanical strengthening remains high in this case. When annealed at 430 ◦ C for 60 min, polygonization develops, and very fine precipitated particles decorating subboundaries are observed. Some diffraction spots in the electron diffraction patterns can be attributed to Ti3 Ni4 phase [4]. Increasing of exposure time up to 3 h and then 10 h improves perfectness of the polygonized substructure [4]. Under the X-ray diffraction study after annealing for 10 h, the 2 1 1 X-ray line of Ti3 Ni4 phase is visible (Fig. 1). The temperature range of the shape recovery As –Af depends on isothermal aging time (Fig. 2). Increasing of strain aging time under isothermal annealing allows realizing As –Af temperatures regulation by means of changing parameters (size and distribution) of Ti3 Ni4 precipitates.
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Fig. 1. X-ray diffractograms of Ti–50.7 at.% Ni alloy after PDA at 530 ◦ C, 10 h. State: (a) quenching 700 ◦ C, 20 min (recording at 100 ◦ C); (b) LTMT (recording at 60 ◦ C).
Fig. 2. The As –Af range vs. post-deformation annealing time at 430 ◦ C.
SME/TWSME parameters after LTMT and post-deformation isothermal annealing at 430 ◦ C show the following regularities (Fig. 3): increasing of aging time from 10 min to 60 min does not bring pronounced changes in recoverable elastic strain εrel , induced strain εi and residual strain εf (εf is determined as εf = εi − εr and it is quite small in all cases studied). Further increasing of exposure time from 3 h to 10 h brings εrel reducing but slight εi , εr and εf growth. TWSME amplitude (εTW ) manifests a gradual growth under aging time changing from 10 min to 10 h. This allows supposing that quantity and size growth of Ti3 Ni4 particles
Fig. 3. SME and TWSME parameters after LTMT, post-deformation isothermal annealing at 430 ◦ C and SME inducing under εt = 10%, 1 cycle.
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is accompanied by generation of additional internal stress fields, responsible for recoverable strain and TWSME amplitude. SME/TWSME parameters under various total strains εt have been evaluated after aging at 430 ◦ C for 10 h (Figs. 4 and 5Figs. 4a-c and 5). The Af temperature, induced strain εi , residual strain εf , and TWSME amplitude εTW depend on the total strain εt . All mentioned parameters increase as the total strain εt increases (see Figs. 4a–c and 5). The total strain growth from 7.5% to 15% allows achieving the greater TWSME value εTW ,
Fig. 5. Changing of SME and TWSME parameters under total strain increasing εi , εTW – determined at a temperature T = −155 ◦ C; εr and εf – determined at Af ; – determined.
however, its growth is accompanied by slight increasing of the residual strain value εf which does not exceed 0.3% (Fig. 5). Fig. 6 shows the kinetics of εTW growth during TWSME inducing in dependence of the exposure time in a constrained condition. The exposure times 10 s and 30 s bring uniform effect growth up to four to five cycles. Subsequent training cycles do not affect TWSME amplitude. The exposure time 60 s brings a significant εTW growth in initial three training cycles up to highest value. In all cases, 4TWSME value remains stable after subsequent cycles. This fact allows supposing that stress fields formation with favorable orientation for TWSME is also determined by time factor, i.e., there takes place some lattice “adaptation” or “training”. The conclusion following from this result is that achievement of required TWSME value is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition.
Fig. 4. SME and TWSME manifestation after various total strains: (a) εt = 7.5%; (b) εt = 12%; (c) εt = 15%.
Fig. 6. Kinetics of TWSME amplitude under various constraining times at 0 ◦ C (LTMT + PDA at 430 ◦ C for 1 h).
E.P. Ryklina et al. / Materials Science and Engineering A 481–482 (2008) 134–137
137
4. Conclusions
Acknowledgement
(i) Variation of strain aging time under isothermal annealing is effective for regulation of critical temperatures and all SME/TWSME parameters. (ii) The low-temperature thermomechanical treatment with following aging at 430 ◦ C for 10 h is favorable for achieving the best combination of TWSME parameters εr , εTW and εf in Ti–50.7 at.% Ni alloy. (iii) The constraining strain value affects all studied SME parameters: temperature interval of shape recovery, recovery strain, TWSME amplitude, residual strain. (iv) Variation of loading time is effective for TWSME magnitude regulation under multi-cyclic training procedure. The achievement of required TWSME value is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition.
This work was carried out under financial support of the Ministry of Education and Science of the Russian Federation. References [1] V. Brailovski, S. Prokoshkin, P. Terriault, F. Troshu (Eds.), Shape Memory Alloys: Fundamentals, Modeling and Applications, Montreal, ETS Publ., 2003. [2] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511. [3] J. Khalil-Allafi, G. Eggeler, W. Schmahl, D. Sheptyakov, Mater. Sci. Eng. A 438–440 (2006) 593–596. [4] E.P. Ryklina, I.Yu. Khmelevskaya, S.D. Prokoshkin, K.E. Inaekyan, R.V. Ipatkin, Mater. Sci. Eng. A 438–440 (2006) 1093–1096.
One-way and two-way shape memory effect in thermomechanically treated TiNi-based alloys E.P. Ryklina ∗ , S.D. Prokoshkin, I.Yu. Khmelevskaya, A.A. Shakhmina Moscow State Institute of Steel and Alloys, Technological University, 4 Leninsky Pr., 119049 Moscow, Russia Received 22 May 2006; received in revised form 1 March 2007; accepted 14 March 2007
Abstract One-way and two-way shape memory effects (SME and TWSME) and functional properties of Ti–50.7 at.% Ni alloy after low-temperature thermomechanical treatment (LTMT) and post-deformation annealing are studied. The combined influence of structure realized under isothermal annealing after LTMT and external training parameters on functional properties of this alloy is studied. Variation of strain aging time under isothermal annealing is effective for regulation of critical temperatures and other SME/TWSME parameters. LTMT with subsequent aging at 430 ◦ C for 10 h is favorable for achieving the best combination of TWSME parameters in this alloy. Variation of loading time is effective for regulation of TWSME magnitude under multi-cycle training procedure. The TWSME value achieved is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Shape memory alloys; Nitinol; Thermomechanical treatment; Two-way shape memory effect; Functional properties
1. Introduction Shape memory effect (SME) and two-way shape memory effect (TWSME) parameters are structure-sensitive, therefore, heat and thermomechanical treatment are effective for TWSME regulation [1,2]. A post-deformation annealing (PDA) after low-temperature thermomechanical treatment (LTMT) provokes softening processes of the deformed metal including stress relaxation, recovery, polygonization and recrystallization of austenite, and provides an additional possibility for the shape memory properties regulation. In Ti–Ni alloys with nickel concentration more than 50.0% a precipitating process during aging takes place in parallel [1–3]. The main factors affecting functional properties in this case are as follows: concentration changes in matrix; dislocation substructure changes; formation of oriented internal stress fields caused by coherent precipitates. Under SME and TWSME inducing in real devices, the external training parameters additionally affect functional properties: the load scheme (tension, twisting, bending), constrained total strain and loading time. Real technical and medical SME arti∗
Corresponding author. Fax: +7 495 2472301. E-mail address: [email protected] (E.P. Ryklina).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.03.118
cles in most cases are functioning in bending. Nevertheless, there is lack of information about the effect of external training parameters under bending on the functional properties. The study of the individual and combined (structural and training) influence on final functional properties is of great practical interest. In the present work the functional properties in Ti–50.7 at.% Ni alloy after LTMT and isothermal PDA as well as the influence of the TWSME training parameters under bending are examined. 2. Experimental The Ti–50.7 at.% Ni alloy was studied having martensitic transformation temperatures Ms = −8 ◦ C, Mf = −24 ◦ C, As = 0 ◦ C, Af = 9 ◦ C after quenching from 700 ◦ C. The lowtemperature thermomechanical treatment (LTMT) of the initial workpiece (a bar of 2 mm in diameter) was performed by wiredrawing (or rolling for the electron microscopy study) in several passes at the room temperature with strain ε = 0.25–0.3 in each pass and intermediate annealings at 700 ◦ C (15 min). The wire 0.3 mm in diameter for further study was obtained with the summary strain of 0.3 in the last pass. The strain aging of samples was performed at 430 ◦ C during 10 min, 30 min, 60 min, 3 h and 10 h for obtaining all stages of polygonized dislocation
E.P. Ryklina et al. / Materials Science and Engineering A 481–482 (2008) 134–137
substructure (from initial stage to well-developed) and various precipitation stages of the Ti3 Ni4 phase. Controlled heat treatment was carried out at 700 ◦ C for 20 min followed by water cooling. The oxidized surface layer was removed by chemical etching in 1HF + 3HNO3 + 6H2 O2 solution. Thin foils for transmission electron microscopy (TEM) were prepared using grinding and electropolishing by the “window” technique in an electrolyte containing 10 vol.% HClO4 + 90 vol.% CH3 COOH. The micro-structure after PDA and aging was studied using JEM-100C transmission electron microscope. The samples for the X-ray study have been prepared on special mandrels of the cut wire. The X-ray diffraction study was performed using DRON-3 diffractometer in filtered Co-radiation in the temperature range from 0 ◦ C to 100 ◦ C. The diffractograms were recorded in the 2θ = 52◦ to 56◦ range which comprised the 1 1 0B2, 3 3 0–3 3¯ 0R, 5 1 1Ti2 Ni and 2 1 1Ti3 Ni4 X-ray diffraction peaks. As and Af temperatures were determined visually as the temperatures of beginning and completion of the shape change on heating, respectively. The SME/TWSME inducing was performed using “positive” training method by bending the wire samples around mandrels of various diameters at 25 ◦ C with total strain εt from 7.5% to 15%, then cooling under stress down to −115 ◦ C. The induced strain εi was determined at a constraining temperature after unloading. The residual strain εf was determined when heating above Af temperature. The recovery strain εr was determined as the difference between εi and εf . The recoverable elastic strain εrel was determined as the difference between εt and εi . The exposure time in the constrained condition in the experiment with cyclic TWSME training varied as 10 s, 30 s or 60 s at 25 ◦ C with cooling under stress to 0 ◦ C. 3. Results and discussion The electron microscopy examination shows the formation of different structure types of the B2 austenite after various strain aging regimes [4]. The post-deformation annealing at 430 ◦ C (10 min) is accompanied by a partial polygonization of the austenite, but the dislocation density remains high, i.e., the thermomechanical strengthening remains high in this case. When annealed at 430 ◦ C for 60 min, polygonization develops, and very fine precipitated particles decorating subboundaries are observed. Some diffraction spots in the electron diffraction patterns can be attributed to Ti3 Ni4 phase [4]. Increasing of exposure time up to 3 h and then 10 h improves perfectness of the polygonized substructure [4]. Under the X-ray diffraction study after annealing for 10 h, the 2 1 1 X-ray line of Ti3 Ni4 phase is visible (Fig. 1). The temperature range of the shape recovery As –Af depends on isothermal aging time (Fig. 2). Increasing of strain aging time under isothermal annealing allows realizing As –Af temperatures regulation by means of changing parameters (size and distribution) of Ti3 Ni4 precipitates.
135
Fig. 1. X-ray diffractograms of Ti–50.7 at.% Ni alloy after PDA at 530 ◦ C, 10 h. State: (a) quenching 700 ◦ C, 20 min (recording at 100 ◦ C); (b) LTMT (recording at 60 ◦ C).
Fig. 2. The As –Af range vs. post-deformation annealing time at 430 ◦ C.
SME/TWSME parameters after LTMT and post-deformation isothermal annealing at 430 ◦ C show the following regularities (Fig. 3): increasing of aging time from 10 min to 60 min does not bring pronounced changes in recoverable elastic strain εrel , induced strain εi and residual strain εf (εf is determined as εf = εi − εr and it is quite small in all cases studied). Further increasing of exposure time from 3 h to 10 h brings εrel reducing but slight εi , εr and εf growth. TWSME amplitude (εTW ) manifests a gradual growth under aging time changing from 10 min to 10 h. This allows supposing that quantity and size growth of Ti3 Ni4 particles
Fig. 3. SME and TWSME parameters after LTMT, post-deformation isothermal annealing at 430 ◦ C and SME inducing under εt = 10%, 1 cycle.
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E.P. Ryklina et al. / Materials Science and Engineering A 481–482 (2008) 134–137
is accompanied by generation of additional internal stress fields, responsible for recoverable strain and TWSME amplitude. SME/TWSME parameters under various total strains εt have been evaluated after aging at 430 ◦ C for 10 h (Figs. 4 and 5Figs. 4a-c and 5). The Af temperature, induced strain εi , residual strain εf , and TWSME amplitude εTW depend on the total strain εt . All mentioned parameters increase as the total strain εt increases (see Figs. 4a–c and 5). The total strain growth from 7.5% to 15% allows achieving the greater TWSME value εTW ,
Fig. 5. Changing of SME and TWSME parameters under total strain increasing εi , εTW – determined at a temperature T = −155 ◦ C; εr and εf – determined at Af ; – determined.
however, its growth is accompanied by slight increasing of the residual strain value εf which does not exceed 0.3% (Fig. 5). Fig. 6 shows the kinetics of εTW growth during TWSME inducing in dependence of the exposure time in a constrained condition. The exposure times 10 s and 30 s bring uniform effect growth up to four to five cycles. Subsequent training cycles do not affect TWSME amplitude. The exposure time 60 s brings a significant εTW growth in initial three training cycles up to highest value. In all cases, 4TWSME value remains stable after subsequent cycles. This fact allows supposing that stress fields formation with favorable orientation for TWSME is also determined by time factor, i.e., there takes place some lattice “adaptation” or “training”. The conclusion following from this result is that achievement of required TWSME value is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition.
Fig. 4. SME and TWSME manifestation after various total strains: (a) εt = 7.5%; (b) εt = 12%; (c) εt = 15%.
Fig. 6. Kinetics of TWSME amplitude under various constraining times at 0 ◦ C (LTMT + PDA at 430 ◦ C for 1 h).
E.P. Ryklina et al. / Materials Science and Engineering A 481–482 (2008) 134–137
137
4. Conclusions
Acknowledgement
(i) Variation of strain aging time under isothermal annealing is effective for regulation of critical temperatures and all SME/TWSME parameters. (ii) The low-temperature thermomechanical treatment with following aging at 430 ◦ C for 10 h is favorable for achieving the best combination of TWSME parameters εr , εTW and εf in Ti–50.7 at.% Ni alloy. (iii) The constraining strain value affects all studied SME parameters: temperature interval of shape recovery, recovery strain, TWSME amplitude, residual strain. (iv) Variation of loading time is effective for TWSME magnitude regulation under multi-cyclic training procedure. The achievement of required TWSME value is possible after minimum number of training cycles owing to exposure time increasing in a constrained condition.
This work was carried out under financial support of the Ministry of Education and Science of the Russian Federation. References [1] V. Brailovski, S. Prokoshkin, P. Terriault, F. Troshu (Eds.), Shape Memory Alloys: Fundamentals, Modeling and Applications, Montreal, ETS Publ., 2003. [2] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511. [3] J. Khalil-Allafi, G. Eggeler, W. Schmahl, D. Sheptyakov, Mater. Sci. Eng. A 438–440 (2006) 593–596. [4] E.P. Ryklina, I.Yu. Khmelevskaya, S.D. Prokoshkin, K.E. Inaekyan, R.V. Ipatkin, Mater. Sci. Eng. A 438–440 (2006) 1093–1096.