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Factors influencing the development of two-way shape memory in NiTi
Acta metall, mater. Vol. 38, No. 7, pp. 1321-1326, 1990 Printed in Great Britain. All rights reserved
0956-7151/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc
FACTORS I N F L U E N C I N G THE DEVELOPMENT OF TWO-WAY SHAPE MEMORY IN NiTi Y I N O N G L1U and P. G. M c C O R M I C K Department of Mechanical Engineering, University of Western Australia, Nedlands, W.A. 6009, Australia (Received 27 September 1989)
Abstract--Factors influencing the development of two-way shape memory behaviour in NiTi have been investigated. The magnitude of the two-way memory is found to depend on whether the training procedure involves the formation of stress induced martensite or martensite reorientation, the number of training cycles, the training stress and prior heat treatment. The development of optimal two-way shape memory strains is associated with training conditions in which a full one-way transformation strain is achieved under conditions of minimum stress and permanent strain. R6sum6--On 6tudie les facteurs qui influencent le d6velopment de la m6moire de forme 5. double sens dans NiTi. L'importance de la m6moire d double sens d6pend de ce que le processus de formation implique la formation de martensite induite par contrainte ou bien une r6orientation de la martensite, du hombre de cycles au cours de la formation, de la contrainte de formation et du traitement thermique ant6rieur. Le d6veloppement des d6formations optimales de m6moire de forme d double sens est associ6 aux conditions de formation au cours desquelles une d6formation de transformation compl6te d sens unique est r6alis~e dans des conditions de contrainte minimale et de d6formation permanente. Zusammenfassung--Die Faktoren, die die Entwicklung des Zweiweg-Formged~ichtnisverhaltens in NiTI beeinflussen, werden untersucht. Die Gr6Be des Zweiwegged/ichtnisses h/ingt davon, ob die Vorbehandlung zur Bildung von spannungsinduziertem Martensit oder yon Martensitumlagerung ffihrt, yon der Zahl der Behandlungszyklen, vonder Behandlungsspannung und vonder vorausgehenden W~irmebehandlung ab. Die Entwicklung optimaler Verzerrungen f/ir das Zweiweg-Formged//chtnis h/ingt mit Behandlungsbedingungen zusammen, bei denen eine vollst~indige Einweg-Umwandlung unter Bedingungen minimaler Spannung und permanenter Verzerrung erreicht wird.
I. INTRODUCTION A two-way shape memory effect is well known to be developed in shape memory alloys by thermally cycling a shape memory element under an applied stress [1-3]. After a number of cycles, a reversible shape change associated with the parent ~-, martensite transformation will be established during subsequent thermal cycling under zero stress. This is known as thermomechanical two-way memory training [4-7]. In contrast to the extensive studies on CuZnAl alloys [2, 4, 7-9], less understanding of the factors influencing two-way memory behaviour in NiTi shape memory alloys has been achieved [1, 3, 5]. A significant difference between NiTi and CuZnAl alloys is that NiTi alloys may exhibit a separate transition from the parent phase to the rhombohedral R phase prior to the formation of martensite on cooling [10-12]. Previous investigations have clarified the important influence of transformation behaviour on the two-way memory in NiTi alloys [5, 13] and have concluded that a single stage transformation between the parent phase and martensite is essential for the development of a significant two-way memory. The transformation behaviour has been shown to be dependent on the heat treatment temperature AM387--J
following cold work. Heat treatment above a critical temperature has been shown to result in a single stage martensitic transformation [14-16] and a substantial two-way memory on training [5, 13]. However, optim u m training conditions are yet to be established. Heat treatment at lower temperatures results in a two-stage parent - , R phase --, martensite transformation and a negligible two-way memory. Recent studies have shown changes in mechanical properties and shape memory behaviour with increasing annealing temperature at temperatures above that required for a single stage transformation [17]. Since two-way memory behaviour is dependent on the internal stresses and dislocation structures resulting from training, which are dependent on the mechanical properties of the alloy, the heat treatment temperature should also influence two-way memory behaviour. This paper reports the results of an investigation of the effect of training and heat treatment on the two-way shape memory in NiTi. 2. EXPERIMENTAL
A commercial NiTi shape memory alloy of nominal composition T i - 5 0 . 2 % N i was used in this study. The as-received wire material was cold-rolled by 50%
1321
1322
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi A2
4 ¸
.~ 3' Temperature
Temperature
¢.0
2' BI
"~
B2
1'
I
-.m-
21MPa
"*"
3b'MPa
t-
68MPa
•" 0 -
103MPa
o
\X
20
40
60
\
80
100
120
Cycl~
Temperature
Temperature
load on, - . . . . . load off.
Fig. 1. Training procedures;
reduction to give a square cross-section of 0.87 x 0.87 mm. Following cold work the wires were annealed for 1.8 ks at various temperatures between 460 and 1200 K and air cooled to room temperature. The transformation behaviour of the specimens after heat treatment was determined using a Perkin-Elmer DSC-4 differential scanning calorimeter. Specimens having a gauge length of 20 mm were cut from the wires. The two-way memory training was carried out under various tensile stresses on a thermal cycling device which enabled thermal expansion compensated strain to be measured throughout the thermal cycle. The thermal cycling was conducted by using circulating liquid baths which allowed the temperature to be varied between 220 and 380 K. After a given number of thermal cycles under stress, the load was removed and the two-way strain was measured during the subsequent cycle. The main training procedure adopted was to heat the sample above A r prior to applying a load, which was then maintained for the entire thermal cycle. For comparison other training procedures were also performed by applying and maintaining loads at different stages of the training cycle. The various training modes used in this study are illustrated in Fig. 1. 3. RESULTS
3.1. Effect o f stress on two-way memory training
Fig. 3. Effect of number of training cycles and stress on transformation strain. induced martensitic transformation on reaching M s. During subsequent heating from below Mr the reverse transformation strain Ear, is recovered on heating to above A r. The difference of the forward and reverse transformation strains is equal to the permanent strain. As shown in Fig. 2, both Ear and the cumulative plastic strain are functions of the number of training cycles. The variation of Et~ with the number of training cycles and stress is shown in Fig. 3 for specimens annealed at 938 K. For applied stresses less than 60 MPa, ear increased with an increasing number of cycles, N, reaching a maximum value at large N. The number of cycles required to reach the maximum value of ear decreased with increasing stress. For stresses greater than ~ 60 MPa the maximum value of e~r occurred in the first one or two cycles and Etf decreased with further cycles. The maximum value of Earincreased with increasing stress as shown in Fig. 4. The cumulative permanent strain, %, also increased with increasing stress and number of cycles. As shown in Fig. 5, greater than 70 cycles were required for % to saturate. Measurements of the two-way strain, ~tw, as a function of the number of cycles and the training stress are shown in Fig. 6 for specimens annealed at 938 K. For small stresses, Etw increased with the number of cycles in a similar manner as Ear. At higher stress values the maximum value of Etw was reached after one or two cycles and decreased with further
Typical strain-temperature curves associated with the constant load training cycles (All are shown in Fig. 2. During cooling a strain accompanies the stress 6 5
70
4'
~o
2
1,1-3
2
r4 I o ,J 2"/3
....
3'
i ....
283
i ....
i ....
i ....
i ....
i ....
303 313 323 333 343 Temperature (gl 293
Fig. 2. Strain-temperature training curves at constant load; a = 21 MPa, annealing temperature--938 K.
1 0
•
.
20
.
,
40
.
,
60 a
.
,
•
.
-
,
80 100 120 (MPa)
.
.
140
Fig. 4. Effect of stress on maximum transformation strain.
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
5'
~
1323
4.5"
90
68MPa
4.0"
80
45NPa
3.5"
70
3.0"
60
103 MPa
4'
~ '~
2
35 MPa
1
~
0
-
j.,
tO
• 21MPa
, 20
•
, 40
•
, 60
"
, 80
"
, " , 100 120
2.5
. 10
,
.
,
30
.
50
,
. . . . . . .
70 90 ff (MPa)
Cycles
110
50
130
150
Fig. 5. Effect of number of training cycles and stress on the cumulative plastic strain.
Fig. 7. Effect of stress on maximum two-way strain and training efficiency.
training cycles. The effect of training stress on the maximum two-way strain, Et. . . . . . is shown in Fig. 7. The stress for which Et. . . . . is the greatest is defined as the optimum training stress, 6opt . Also shown in Fig. 7 are values of the training efficiency, qtr, defined as the ratio of E~..... and the corresponding value of Err. Maximum values of ~/,r of the order of 0.85 were achieved with low training stresses. For intermediate stresses, continuation of training to a greater number of cycles than that required for E~w(max)resulted in the E,w/Etr ratio reaching values as high as 0.97. This behaviour was associated with the decrease in E,~with increasing N while ~,~ remained relatively constant.
Mf under no load and the stress was applied, causing reorientation of the thermal martensite. The stress required for martensite reorientation was considerably higher than that required for the formation of stress induced martensite in procedures AI and A2. With B1 the load was then removed prior to heating, while with B2 the load was reduced to the same value as used with procedures AI and maintained at that value during heating to above A f. As shown in Fig. 8, lower values of •tw were obtained using procedures B1 and B2 as compared with AI and A2.
3.3. Effect of heat treatment 3.2. Effect of training method The variation of ~tw with the number of training cycles is shown in Fig. 8 for the four methods of training investigated. With procedures A1 and A2 the load was applied at T > A r and maintained during cooling to below Mf, thus causing stress induced martensite to form. With A1 the stress was maintained throughout the cycle, while with A2 the load was removed prior to heating and the reverse transformation occurred under zero stress. As shown in Fig. 8, the highest values of Etw were obtained in the specimens allowed to transform to the parent phase under no load. Procedures B1 and B2 correspond to the shape memory training procedure [4, 6]. The specimens were cooled to below
Typical curves showing the effect of heat treatment temperature on the development of two-way memory are given in Fig. 9. The maximum value of the two-way strain, Et. . . . . . the number of cycles and the optimum stress, aopt, required for Etw, max were all dependent on the heat treatment temperature. In Fig. 10 measurements of ¢~tw,max, ?]trand O'opt a r e plotted as a function of the annealing temperature. Maximum values ofE t. . . . . and r/tr of 4 and 90% respectively were obtained after annealing at temperatures between 870 and 950 K. As is evident in Fig. 10, the maximum values of Et. . . . . are associated with minimum values of aop,. The increase in ~ropt at higher and lower temperatures was accompanied by decreases in Etw,max and r/tr.
4¸ ~
0 20
40
60
80
100
~
~ *'
---
----... 1A2 AI
1'
.4t- 103 MPa 0
B
120
Cycles
Fig. 6. Effect of number of training cycles and stress on two-way strain.
0
0
20
40
60
80
100
120
Cycles
Fig. 8. Effect of training procedure on two-way strain.
1324
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi 5" 9381~ 35MPa 4" 855K, 45MPa
~"
3"
1143K, 150MPa
1"
738K, 186MPa 0 ~ 0
20
40 Cycles
80
60
Fig. 9. Effect of heat treatment temperature on ~t,, for optimum training stresses. 4. DISCUSSION The present results show that the development of significant two-way shape memory behaviour in NiTi is dependent on both the details of the training procedure and the prior heat treatment. Two-way memory is generally associated with the development of local internal stress fields and dislocation structures during the training procedure, which bias the orientation of martensite variants formed during subsequent thermal cycling [2, 7, 9]. While previous investigators [1, 18] have emphasised the role of permanent strain in providing the dislocation substructure necessary to favour the formation of a particular martensite variant, the present measurements show that optimum values of Et~ are obtained for training and heat treatment conditions which minimise training stresses and the buildup of permanent strain during training.
4.1. Training behaviour The increase in the transformation strain with increasing stress is in good agreement with previous investigations [19, 20]. The maximum transformation strain is limited by crystallographic considerations, and at low applied stresses it is evident that substantial self-accommodation of the martensite variants occurs. The increase in transformation strain with an increasing number of thermal cycles at low stresses indicates that the dislocation structure developed during cycling assists the formation and growth of
5"
100
4'
~
i
80
" 2
40
1
20
0 500
~ 700 900 1100 Annealing Temperature (IC)
stress directed martensite variants during subsequent cycling. The near proportionality between Etw and Etr at low stresses suggests that the dislocation structure responsible for the growth of the transformation strain with cycling is also responsible for the development of the two-way memory. The high values of P/tr are a clear indication of the effectiveness of the dislocation structures and local stress fields generated by cycling in biasing the orientation of the martensite variants which form during subsequent cycling under zero stress. As a consequence of the decrease in Etw with increasing stress and with increasing number of cycles at high stress, there is an optimum stress for a given number of training cycles (or optimum number of training cycles for a constant stress) required for the maximum value of Etw. The optimum stress/training cycles corresponds closely to that required to just produce a full one-way transformation strain. For lower stress/cycles, ~t+ is restricted by the value of the one-way strain developed. At higher values of stress/training cycles, Etwdecreases and the permanent strain increases, indicating that above a critical point dislocations and plastic strain introduced by cycling cause a degradation of the two-way memory. The optimum conditions to achieve maximum two-way strains are not critical, due apparently to the low value of the stress required to form oriented martensite relative to the yield stress of the martensitic and parent phases. Two-way shape memory training is basically a procedure to develop dislocation arrangements which guide the formation of martensite variants of a preferred orientation to give a net shape change. Although the development of a two-way memory during training is always associated with permanent deformation, there is not necessarily any unique relationship between the two-way memory and the plastic strain. This is illustrated in Figs l l and 12 where the data from the training tests under different stresses or procedures have been replotted to give ~tw as a function of Ep. It is seen that the relation between etw and ep depends on the stress and training method used. It is apparent that the training method and stress which result in the least permanent strain while
11
0 1300
Fig. 10. Effect of heat treatment on Etw(max),cro~t and qtr.
4" 3" 2"
II~ , q l l
"~ 21MPa -*- 3$MPa 68MPa
0
2
4
"*" 139MPa 6
p (~) Fig. 11. Plot of ~t+ as a function of Ep for various training stresses for procedure AI.
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
5" 4"~
B2 @4ir
2" 1' 0 2
3
4
e p (%) Fig. 12. Plot of ~,, as a function o f E o for different training procedures. producing a full one-way strain give the highest two-way memory. This conclusion is particularly evident from comparison of the curves for training procedures AI and A2. With procedure A2 the stress was removed on reaching the minimum cycle temperature and reversion to the parent phase occurred under zero stress. As reported previously [17], the permanent strain is not uniformly distributed over the cycle, but rather approximately 75% of the permanent strain occurs during reversion and only 25% during the forward transformation. It is apparent therefore that the higher values of Et~ exhibited by specimens trained using procedure A2 are associated with the lower values of Ep. Training procedures BI and B2 involved the stress directed reorientation of martensite variants rather than the formation of stress oriented martensite as occurred for A1 and A2. The higher values of etw exhibited by procedures AI and A2 relative to B1 and B2 agrees well with previous studies in copper based alloys [4]. Schroeder and Wayman [6] using CuZn single crystals showed that the formation of stress induced martensite associated with pseudoelastic training resulted in the formation of single martensite variants during subsequent thermal cycling, while multiple variants were found after shape memory training. With the present results, a direct comparison of the effectiveness of the two training methods is not possible because of the high stress (I10MPa) required for martensite reorientation. From Fig. 11 it is apparent that substantial plastic deformation and a reduced value of E~w will accompany thermal cycling under this stress. It was therefore decided to attempt to compare the two methods under conditions of optimum stress for procedure A, using the same stress during the reversion stage of procedure B after martensite reorientation at a stress of 110 MPa. Nonetheless it is clear that the two-way strains for procedure A1 and A2 are substantially higher than for either B1 or B2. Whether this is due to the lower stress used or to the higher inherent training efficiency associated with the formation of stress directed martensite, or both,
1325
cannot be unambiguously determined from the present results. Finally, it is of interest to note that the stresses used during training are substantially less than the yield strength of either the parent or martensitic phase [17, 21, 22]. The generation of significant permanent strain during cycling is indicative of high local stress concentrations at the martensite/parent interface boundaries during the transformation. It is evident that local yielding also occurs in unstressed specimens, but does not result in a net permanent strain due to the self accommodating nature of the transformation. 4.2. Heat treatment
The measurements in Fig. 10 show that maximum values of Etw and ~/tr are exhibited by specimens heat treated in the temperature range 850-950 K and correspond to minimum values of the optimum training stress. The decrease in Etw and increase in aopt at lower temperatures are directly associated with the transformation behaviour of the alloy as illustrated in Fig. 13. In region I the specimens showed no transformation, since the annealing temperature was too low to allow recovery or recrystallisation of the deformed structure. In region II only the R transformation occurred and in region III both the R and martensitic transformations occurred. Region IV corresponds to the conditions of stress and annealing temperature where only the martensitic transformation occurred (no R transformation). The intersection of the boundary between regions III and IV with the temperature axis gives the annealing temperature for which TR = Ms at zero stress. The value of 853 K, determined by extrapolating the boundary to zero stress, is in excellent agreement with that determined from DSC measurements. Curve A represents the minimum stress required to develop a two-way memory of greater than 1%, while curve B is the optimum stress to achieve maximum values of Etw. The boundary between regions III and IV decreases in temperature with increasing stress. This behaviour is due to the fact that the temperature difference (T RMs) at zero stress between the R and martensitic
300
0
UV
20O
100
0
I
~-
- - ,
- -
i - -
•
,
-
l P - -
400 600 800 1000 1200 Annealing Temperature (K)
Fig. 13. Effect of heat treatment temperature on transformation and shape memory behaviour; Curve A---stress required for etw= 1%, Curve B--#opr
1326
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
transformations increases as the annealing temperature decreases. In addition, the stress dependence of the martensitic transformation temperature is substantially greater than that of the R transformation. Therefore, at a sufficiently high stress, Ms > TR, and only a single stage transformation will occur. The lower the annealing temperature in this range, the larger the temperature difference between the zero stress values of TR and Ms and the higher the stress required to suppress the R transformation. The region where a significant two-way memory (i.e. Etw> 1%) was developed, i.e. above curve A, falls entirely within region IV, where only a single martensitic transformation occurred. The overlapping of curve A with the boundary between regions III and IV confirms the conclusion [13] that a single-stage martensitic transformation is essential for developing a substantial two-way memory. As discussed previously, curve B corresponds to the stress required to achieve a full transformation strain. The minima exhibited by curves A and B at temperatures between 870 and 1000 K correspond well with the results of a recent study of the effect of heat treatment on mechanical behaviour in this alloy [17]. Measurements of the stresses required for stress induced martensite, martensite reorientation and slip of the parent phase all exhibited minimum values after annealing in the temperature range of 870-1000 K. All three stresses increased with increasing annealing temperature above 1000 K in a similar manner as shown for curves A and B, due to the higher stress required for slip and twinning. 5. CONCLUSIONS I. Maximum two-way memory strains are associated with values of training stress and number of training cycles which are just sufficient to induce a full one-way transformation strain. The values of Etw decrease with increasing stress for stresses greater than that required to induce the full one-way strain in the first cycle. 2. Measurements of the effect of annealing temperature confirm that a single-stage martensitic transformation is a prerequisite for obtaining a significant two-way memory strain in NiTi. 3. The optimum annealing temperatures of 850-1000 K are associated with minimum values of applied stress required to obtain Et,,,,(~,~,o and correspond with recrystallisation of the prior cold worked structure.
4. The training procedure which yields the highest values of Etw (procedure A2) was that which resulted in the minimum cumulative plastic strain, due to the conditions of reverse transformation under zero load. 5. While dislocation structures and local strain fields are essential for the development of a two-way memory, it may be concluded from the present results that optimum two-way memory strains are associated with training conditions which minimise permanent strain. REFERENCES
1. A. Nagasawa, K. Enami, Y. Ishino, Y. Abe and S. Nenno, Scripta metall. 8, 1055 (1974). 2. J. Perkins, Proc. Int. Syrup. SMA-86, p. 201. China Academic, Beijing (1986). 3. T. Todoroki, J. Japan Inst. Metals 49, 439 (1985). 4. M. M. Reyhani and P. G. McCormick, Proc. ICOMAT-86, p. 896. Japan Inst. Metals (1986). 5. T. Todoroki, H. Tamura and Y. Suzuki, Proc. ICOMAT-86, p. 748, Japan Inst. Metals (1986). 6. T. A. Schroeder and C. M. Wayman, Scripta metall. 11, 225 (1977). 7. J. Perkins and R. O. Sponholz, Metall. Trans. A. 15, 313 (1984). 8. R. Rapacioli, V. Torra and E. Cesari, Scripta metall. 22, 261 (1988). 9. M. Zhu and D. Z. Yang, Scripta metall. 22, 5 (1988). 10. H. C. Ling and R. Kaplow, Metall. Trans. A 11, 77 (1980). 11. E. Goo and R. Sinclair, Acta metall. 33, 1717 (1985). 12. S. Miyazaki and K. Otsuka, Metall. Trans A 17, 53 (1986). 13. Y. Liu and P. G. McCormick, Scripta metall. 22, 1327 (1988). 14. T. Todoroki and H. Tamura, J. Japan Inst. Metals 50, 439 (1986). 15. H. Tamura and Y. Suzuki, Furukawa Electric Rev. 75, 101 (1985). 16. Y. Liu and P. G. McCormick, Proc. ICOMAT-89 (1989). 17. Y. Liu and P. G. McCormick, Iron Steellnst. Japan Int. 29, 417 (1989). 18. J. Perkins, Scripta metall. 8, 1469 (1974). 19. T. Todoroki and H. Tamura, J. Japan Inst, Metals 50, 546 (1986). 20. T. Todoroki and H. Tamura, J. Japan Inst. Metals 50, 538 (1986). 21. K. Otsuka and K. Shimizu, Proc. lnt Conf. Solid-Solid Phase Transf, p. 1267. T.M.S.-A.I.M.E., Warrendale, Pa (1982). 22. K. Otsuka, H. Sakamoto and K. Shimizu, Acta metall. 27, 585 (1979). 23. G. B. Stachowiak and P. G. McCormick, Acta metall. 36, 291 (1988).
0956-7151/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc
FACTORS I N F L U E N C I N G THE DEVELOPMENT OF TWO-WAY SHAPE MEMORY IN NiTi Y I N O N G L1U and P. G. M c C O R M I C K Department of Mechanical Engineering, University of Western Australia, Nedlands, W.A. 6009, Australia (Received 27 September 1989)
Abstract--Factors influencing the development of two-way shape memory behaviour in NiTi have been investigated. The magnitude of the two-way memory is found to depend on whether the training procedure involves the formation of stress induced martensite or martensite reorientation, the number of training cycles, the training stress and prior heat treatment. The development of optimal two-way shape memory strains is associated with training conditions in which a full one-way transformation strain is achieved under conditions of minimum stress and permanent strain. R6sum6--On 6tudie les facteurs qui influencent le d6velopment de la m6moire de forme 5. double sens dans NiTi. L'importance de la m6moire d double sens d6pend de ce que le processus de formation implique la formation de martensite induite par contrainte ou bien une r6orientation de la martensite, du hombre de cycles au cours de la formation, de la contrainte de formation et du traitement thermique ant6rieur. Le d6veloppement des d6formations optimales de m6moire de forme d double sens est associ6 aux conditions de formation au cours desquelles une d6formation de transformation compl6te d sens unique est r6alis~e dans des conditions de contrainte minimale et de d6formation permanente. Zusammenfassung--Die Faktoren, die die Entwicklung des Zweiweg-Formged~ichtnisverhaltens in NiTI beeinflussen, werden untersucht. Die Gr6Be des Zweiwegged/ichtnisses h/ingt davon, ob die Vorbehandlung zur Bildung von spannungsinduziertem Martensit oder yon Martensitumlagerung ffihrt, yon der Zahl der Behandlungszyklen, vonder Behandlungsspannung und vonder vorausgehenden W~irmebehandlung ab. Die Entwicklung optimaler Verzerrungen f/ir das Zweiweg-Formged//chtnis h/ingt mit Behandlungsbedingungen zusammen, bei denen eine vollst~indige Einweg-Umwandlung unter Bedingungen minimaler Spannung und permanenter Verzerrung erreicht wird.
I. INTRODUCTION A two-way shape memory effect is well known to be developed in shape memory alloys by thermally cycling a shape memory element under an applied stress [1-3]. After a number of cycles, a reversible shape change associated with the parent ~-, martensite transformation will be established during subsequent thermal cycling under zero stress. This is known as thermomechanical two-way memory training [4-7]. In contrast to the extensive studies on CuZnAl alloys [2, 4, 7-9], less understanding of the factors influencing two-way memory behaviour in NiTi shape memory alloys has been achieved [1, 3, 5]. A significant difference between NiTi and CuZnAl alloys is that NiTi alloys may exhibit a separate transition from the parent phase to the rhombohedral R phase prior to the formation of martensite on cooling [10-12]. Previous investigations have clarified the important influence of transformation behaviour on the two-way memory in NiTi alloys [5, 13] and have concluded that a single stage transformation between the parent phase and martensite is essential for the development of a significant two-way memory. The transformation behaviour has been shown to be dependent on the heat treatment temperature AM387--J
following cold work. Heat treatment above a critical temperature has been shown to result in a single stage martensitic transformation [14-16] and a substantial two-way memory on training [5, 13]. However, optim u m training conditions are yet to be established. Heat treatment at lower temperatures results in a two-stage parent - , R phase --, martensite transformation and a negligible two-way memory. Recent studies have shown changes in mechanical properties and shape memory behaviour with increasing annealing temperature at temperatures above that required for a single stage transformation [17]. Since two-way memory behaviour is dependent on the internal stresses and dislocation structures resulting from training, which are dependent on the mechanical properties of the alloy, the heat treatment temperature should also influence two-way memory behaviour. This paper reports the results of an investigation of the effect of training and heat treatment on the two-way shape memory in NiTi. 2. EXPERIMENTAL
A commercial NiTi shape memory alloy of nominal composition T i - 5 0 . 2 % N i was used in this study. The as-received wire material was cold-rolled by 50%
1321
1322
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi A2
4 ¸
.~ 3' Temperature
Temperature
¢.0
2' BI
"~
B2
1'
I
-.m-
21MPa
"*"
3b'MPa
t-
68MPa
•" 0 -
103MPa
o
\X
20
40
60
\
80
100
120
Cycl~
Temperature
Temperature
load on, - . . . . . load off.
Fig. 1. Training procedures;
reduction to give a square cross-section of 0.87 x 0.87 mm. Following cold work the wires were annealed for 1.8 ks at various temperatures between 460 and 1200 K and air cooled to room temperature. The transformation behaviour of the specimens after heat treatment was determined using a Perkin-Elmer DSC-4 differential scanning calorimeter. Specimens having a gauge length of 20 mm were cut from the wires. The two-way memory training was carried out under various tensile stresses on a thermal cycling device which enabled thermal expansion compensated strain to be measured throughout the thermal cycle. The thermal cycling was conducted by using circulating liquid baths which allowed the temperature to be varied between 220 and 380 K. After a given number of thermal cycles under stress, the load was removed and the two-way strain was measured during the subsequent cycle. The main training procedure adopted was to heat the sample above A r prior to applying a load, which was then maintained for the entire thermal cycle. For comparison other training procedures were also performed by applying and maintaining loads at different stages of the training cycle. The various training modes used in this study are illustrated in Fig. 1. 3. RESULTS
3.1. Effect o f stress on two-way memory training
Fig. 3. Effect of number of training cycles and stress on transformation strain. induced martensitic transformation on reaching M s. During subsequent heating from below Mr the reverse transformation strain Ear, is recovered on heating to above A r. The difference of the forward and reverse transformation strains is equal to the permanent strain. As shown in Fig. 2, both Ear and the cumulative plastic strain are functions of the number of training cycles. The variation of Et~ with the number of training cycles and stress is shown in Fig. 3 for specimens annealed at 938 K. For applied stresses less than 60 MPa, ear increased with an increasing number of cycles, N, reaching a maximum value at large N. The number of cycles required to reach the maximum value of ear decreased with increasing stress. For stresses greater than ~ 60 MPa the maximum value of e~r occurred in the first one or two cycles and Etf decreased with further cycles. The maximum value of Earincreased with increasing stress as shown in Fig. 4. The cumulative permanent strain, %, also increased with increasing stress and number of cycles. As shown in Fig. 5, greater than 70 cycles were required for % to saturate. Measurements of the two-way strain, ~tw, as a function of the number of cycles and the training stress are shown in Fig. 6 for specimens annealed at 938 K. For small stresses, Etw increased with the number of cycles in a similar manner as Ear. At higher stress values the maximum value of Etw was reached after one or two cycles and decreased with further
Typical strain-temperature curves associated with the constant load training cycles (All are shown in Fig. 2. During cooling a strain accompanies the stress 6 5
70
4'
~o
2
1,1-3
2
r4 I o ,J 2"/3
....
3'
i ....
283
i ....
i ....
i ....
i ....
i ....
303 313 323 333 343 Temperature (gl 293
Fig. 2. Strain-temperature training curves at constant load; a = 21 MPa, annealing temperature--938 K.
1 0
•
.
20
.
,
40
.
,
60 a
.
,
•
.
-
,
80 100 120 (MPa)
.
.
140
Fig. 4. Effect of stress on maximum transformation strain.
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
5'
~
1323
4.5"
90
68MPa
4.0"
80
45NPa
3.5"
70
3.0"
60
103 MPa
4'
~ '~
2
35 MPa
1
~
0
-
j.,
tO
• 21MPa
, 20
•
, 40
•
, 60
"
, 80
"
, " , 100 120
2.5
. 10
,
.
,
30
.
50
,
. . . . . . .
70 90 ff (MPa)
Cycles
110
50
130
150
Fig. 5. Effect of number of training cycles and stress on the cumulative plastic strain.
Fig. 7. Effect of stress on maximum two-way strain and training efficiency.
training cycles. The effect of training stress on the maximum two-way strain, Et. . . . . . is shown in Fig. 7. The stress for which Et. . . . . is the greatest is defined as the optimum training stress, 6opt . Also shown in Fig. 7 are values of the training efficiency, qtr, defined as the ratio of E~..... and the corresponding value of Err. Maximum values of ~/,r of the order of 0.85 were achieved with low training stresses. For intermediate stresses, continuation of training to a greater number of cycles than that required for E~w(max)resulted in the E,w/Etr ratio reaching values as high as 0.97. This behaviour was associated with the decrease in E,~with increasing N while ~,~ remained relatively constant.
Mf under no load and the stress was applied, causing reorientation of the thermal martensite. The stress required for martensite reorientation was considerably higher than that required for the formation of stress induced martensite in procedures AI and A2. With B1 the load was then removed prior to heating, while with B2 the load was reduced to the same value as used with procedures AI and maintained at that value during heating to above A f. As shown in Fig. 8, lower values of •tw were obtained using procedures B1 and B2 as compared with AI and A2.
3.3. Effect of heat treatment 3.2. Effect of training method The variation of ~tw with the number of training cycles is shown in Fig. 8 for the four methods of training investigated. With procedures A1 and A2 the load was applied at T > A r and maintained during cooling to below Mf, thus causing stress induced martensite to form. With A1 the stress was maintained throughout the cycle, while with A2 the load was removed prior to heating and the reverse transformation occurred under zero stress. As shown in Fig. 8, the highest values of Etw were obtained in the specimens allowed to transform to the parent phase under no load. Procedures B1 and B2 correspond to the shape memory training procedure [4, 6]. The specimens were cooled to below
Typical curves showing the effect of heat treatment temperature on the development of two-way memory are given in Fig. 9. The maximum value of the two-way strain, Et. . . . . . the number of cycles and the optimum stress, aopt, required for Etw, max were all dependent on the heat treatment temperature. In Fig. 10 measurements of ¢~tw,max, ?]trand O'opt a r e plotted as a function of the annealing temperature. Maximum values ofE t. . . . . and r/tr of 4 and 90% respectively were obtained after annealing at temperatures between 870 and 950 K. As is evident in Fig. 10, the maximum values of Et. . . . . are associated with minimum values of aop,. The increase in ~ropt at higher and lower temperatures was accompanied by decreases in Etw,max and r/tr.
4¸ ~
0 20
40
60
80
100
~
~ *'
---
----... 1A2 AI
1'
.4t- 103 MPa 0
B
120
Cycles
Fig. 6. Effect of number of training cycles and stress on two-way strain.
0
0
20
40
60
80
100
120
Cycles
Fig. 8. Effect of training procedure on two-way strain.
1324
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi 5" 9381~ 35MPa 4" 855K, 45MPa
~"
3"
1143K, 150MPa
1"
738K, 186MPa 0 ~ 0
20
40 Cycles
80
60
Fig. 9. Effect of heat treatment temperature on ~t,, for optimum training stresses. 4. DISCUSSION The present results show that the development of significant two-way shape memory behaviour in NiTi is dependent on both the details of the training procedure and the prior heat treatment. Two-way memory is generally associated with the development of local internal stress fields and dislocation structures during the training procedure, which bias the orientation of martensite variants formed during subsequent thermal cycling [2, 7, 9]. While previous investigators [1, 18] have emphasised the role of permanent strain in providing the dislocation substructure necessary to favour the formation of a particular martensite variant, the present measurements show that optimum values of Et~ are obtained for training and heat treatment conditions which minimise training stresses and the buildup of permanent strain during training.
4.1. Training behaviour The increase in the transformation strain with increasing stress is in good agreement with previous investigations [19, 20]. The maximum transformation strain is limited by crystallographic considerations, and at low applied stresses it is evident that substantial self-accommodation of the martensite variants occurs. The increase in transformation strain with an increasing number of thermal cycles at low stresses indicates that the dislocation structure developed during cycling assists the formation and growth of
5"
100
4'
~
i
80
" 2
40
1
20
0 500
~ 700 900 1100 Annealing Temperature (IC)
stress directed martensite variants during subsequent cycling. The near proportionality between Etw and Etr at low stresses suggests that the dislocation structure responsible for the growth of the transformation strain with cycling is also responsible for the development of the two-way memory. The high values of P/tr are a clear indication of the effectiveness of the dislocation structures and local stress fields generated by cycling in biasing the orientation of the martensite variants which form during subsequent cycling under zero stress. As a consequence of the decrease in Etw with increasing stress and with increasing number of cycles at high stress, there is an optimum stress for a given number of training cycles (or optimum number of training cycles for a constant stress) required for the maximum value of Etw. The optimum stress/training cycles corresponds closely to that required to just produce a full one-way transformation strain. For lower stress/cycles, ~t+ is restricted by the value of the one-way strain developed. At higher values of stress/training cycles, Etwdecreases and the permanent strain increases, indicating that above a critical point dislocations and plastic strain introduced by cycling cause a degradation of the two-way memory. The optimum conditions to achieve maximum two-way strains are not critical, due apparently to the low value of the stress required to form oriented martensite relative to the yield stress of the martensitic and parent phases. Two-way shape memory training is basically a procedure to develop dislocation arrangements which guide the formation of martensite variants of a preferred orientation to give a net shape change. Although the development of a two-way memory during training is always associated with permanent deformation, there is not necessarily any unique relationship between the two-way memory and the plastic strain. This is illustrated in Figs l l and 12 where the data from the training tests under different stresses or procedures have been replotted to give ~tw as a function of Ep. It is seen that the relation between etw and ep depends on the stress and training method used. It is apparent that the training method and stress which result in the least permanent strain while
11
0 1300
Fig. 10. Effect of heat treatment on Etw(max),cro~t and qtr.
4" 3" 2"
II~ , q l l
"~ 21MPa -*- 3$MPa 68MPa
0
2
4
"*" 139MPa 6
p (~) Fig. 11. Plot of ~t+ as a function of Ep for various training stresses for procedure AI.
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
5" 4"~
B2 @4ir
2" 1' 0 2
3
4
e p (%) Fig. 12. Plot of ~,, as a function o f E o for different training procedures. producing a full one-way strain give the highest two-way memory. This conclusion is particularly evident from comparison of the curves for training procedures AI and A2. With procedure A2 the stress was removed on reaching the minimum cycle temperature and reversion to the parent phase occurred under zero stress. As reported previously [17], the permanent strain is not uniformly distributed over the cycle, but rather approximately 75% of the permanent strain occurs during reversion and only 25% during the forward transformation. It is apparent therefore that the higher values of Et~ exhibited by specimens trained using procedure A2 are associated with the lower values of Ep. Training procedures BI and B2 involved the stress directed reorientation of martensite variants rather than the formation of stress oriented martensite as occurred for A1 and A2. The higher values of etw exhibited by procedures AI and A2 relative to B1 and B2 agrees well with previous studies in copper based alloys [4]. Schroeder and Wayman [6] using CuZn single crystals showed that the formation of stress induced martensite associated with pseudoelastic training resulted in the formation of single martensite variants during subsequent thermal cycling, while multiple variants were found after shape memory training. With the present results, a direct comparison of the effectiveness of the two training methods is not possible because of the high stress (I10MPa) required for martensite reorientation. From Fig. 11 it is apparent that substantial plastic deformation and a reduced value of E~w will accompany thermal cycling under this stress. It was therefore decided to attempt to compare the two methods under conditions of optimum stress for procedure A, using the same stress during the reversion stage of procedure B after martensite reorientation at a stress of 110 MPa. Nonetheless it is clear that the two-way strains for procedure A1 and A2 are substantially higher than for either B1 or B2. Whether this is due to the lower stress used or to the higher inherent training efficiency associated with the formation of stress directed martensite, or both,
1325
cannot be unambiguously determined from the present results. Finally, it is of interest to note that the stresses used during training are substantially less than the yield strength of either the parent or martensitic phase [17, 21, 22]. The generation of significant permanent strain during cycling is indicative of high local stress concentrations at the martensite/parent interface boundaries during the transformation. It is evident that local yielding also occurs in unstressed specimens, but does not result in a net permanent strain due to the self accommodating nature of the transformation. 4.2. Heat treatment
The measurements in Fig. 10 show that maximum values of Etw and ~/tr are exhibited by specimens heat treated in the temperature range 850-950 K and correspond to minimum values of the optimum training stress. The decrease in Etw and increase in aopt at lower temperatures are directly associated with the transformation behaviour of the alloy as illustrated in Fig. 13. In region I the specimens showed no transformation, since the annealing temperature was too low to allow recovery or recrystallisation of the deformed structure. In region II only the R transformation occurred and in region III both the R and martensitic transformations occurred. Region IV corresponds to the conditions of stress and annealing temperature where only the martensitic transformation occurred (no R transformation). The intersection of the boundary between regions III and IV with the temperature axis gives the annealing temperature for which TR = Ms at zero stress. The value of 853 K, determined by extrapolating the boundary to zero stress, is in excellent agreement with that determined from DSC measurements. Curve A represents the minimum stress required to develop a two-way memory of greater than 1%, while curve B is the optimum stress to achieve maximum values of Etw. The boundary between regions III and IV decreases in temperature with increasing stress. This behaviour is due to the fact that the temperature difference (T RMs) at zero stress between the R and martensitic
300
0
UV
20O
100
0
I
~-
- - ,
- -
i - -
•
,
-
l P - -
400 600 800 1000 1200 Annealing Temperature (K)
Fig. 13. Effect of heat treatment temperature on transformation and shape memory behaviour; Curve A---stress required for etw= 1%, Curve B--#opr
1326
YINONG LIU and McCORMICK: TWO-WAY SHAPE MEMORY IN NiTi
transformations increases as the annealing temperature decreases. In addition, the stress dependence of the martensitic transformation temperature is substantially greater than that of the R transformation. Therefore, at a sufficiently high stress, Ms > TR, and only a single stage transformation will occur. The lower the annealing temperature in this range, the larger the temperature difference between the zero stress values of TR and Ms and the higher the stress required to suppress the R transformation. The region where a significant two-way memory (i.e. Etw> 1%) was developed, i.e. above curve A, falls entirely within region IV, where only a single martensitic transformation occurred. The overlapping of curve A with the boundary between regions III and IV confirms the conclusion [13] that a single-stage martensitic transformation is essential for developing a substantial two-way memory. As discussed previously, curve B corresponds to the stress required to achieve a full transformation strain. The minima exhibited by curves A and B at temperatures between 870 and 1000 K correspond well with the results of a recent study of the effect of heat treatment on mechanical behaviour in this alloy [17]. Measurements of the stresses required for stress induced martensite, martensite reorientation and slip of the parent phase all exhibited minimum values after annealing in the temperature range of 870-1000 K. All three stresses increased with increasing annealing temperature above 1000 K in a similar manner as shown for curves A and B, due to the higher stress required for slip and twinning. 5. CONCLUSIONS I. Maximum two-way memory strains are associated with values of training stress and number of training cycles which are just sufficient to induce a full one-way transformation strain. The values of Etw decrease with increasing stress for stresses greater than that required to induce the full one-way strain in the first cycle. 2. Measurements of the effect of annealing temperature confirm that a single-stage martensitic transformation is a prerequisite for obtaining a significant two-way memory strain in NiTi. 3. The optimum annealing temperatures of 850-1000 K are associated with minimum values of applied stress required to obtain Et,,,,(~,~,o and correspond with recrystallisation of the prior cold worked structure.
4. The training procedure which yields the highest values of Etw (procedure A2) was that which resulted in the minimum cumulative plastic strain, due to the conditions of reverse transformation under zero load. 5. While dislocation structures and local strain fields are essential for the development of a two-way memory, it may be concluded from the present results that optimum two-way memory strains are associated with training conditions which minimise permanent strain. REFERENCES
1. A. Nagasawa, K. Enami, Y. Ishino, Y. Abe and S. Nenno, Scripta metall. 8, 1055 (1974). 2. J. Perkins, Proc. Int. Syrup. SMA-86, p. 201. China Academic, Beijing (1986). 3. T. Todoroki, J. Japan Inst. Metals 49, 439 (1985). 4. M. M. Reyhani and P. G. McCormick, Proc. ICOMAT-86, p. 896. Japan Inst. Metals (1986). 5. T. Todoroki, H. Tamura and Y. Suzuki, Proc. ICOMAT-86, p. 748, Japan Inst. Metals (1986). 6. T. A. Schroeder and C. M. Wayman, Scripta metall. 11, 225 (1977). 7. J. Perkins and R. O. Sponholz, Metall. Trans. A. 15, 313 (1984). 8. R. Rapacioli, V. Torra and E. Cesari, Scripta metall. 22, 261 (1988). 9. M. Zhu and D. Z. Yang, Scripta metall. 22, 5 (1988). 10. H. C. Ling and R. Kaplow, Metall. Trans. A 11, 77 (1980). 11. E. Goo and R. Sinclair, Acta metall. 33, 1717 (1985). 12. S. Miyazaki and K. Otsuka, Metall. Trans A 17, 53 (1986). 13. Y. Liu and P. G. McCormick, Scripta metall. 22, 1327 (1988). 14. T. Todoroki and H. Tamura, J. Japan Inst. Metals 50, 439 (1986). 15. H. Tamura and Y. Suzuki, Furukawa Electric Rev. 75, 101 (1985). 16. Y. Liu and P. G. McCormick, Proc. ICOMAT-89 (1989). 17. Y. Liu and P. G. McCormick, Iron Steellnst. Japan Int. 29, 417 (1989). 18. J. Perkins, Scripta metall. 8, 1469 (1974). 19. T. Todoroki and H. Tamura, J. Japan Inst, Metals 50, 546 (1986). 20. T. Todoroki and H. Tamura, J. Japan Inst. Metals 50, 538 (1986). 21. K. Otsuka and K. Shimizu, Proc. lnt Conf. Solid-Solid Phase Transf, p. 1267. T.M.S.-A.I.M.E., Warrendale, Pa (1982). 22. K. Otsuka, H. Sakamoto and K. Shimizu, Acta metall. 27, 585 (1979). 23. G. B. Stachowiak and P. G. McCormick, Acta metall. 36, 291 (1988).