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Improvement of shape memory characteristics by precipitation-hardening of TiPdNi alloys
February 1998
Materials Letters 34 (1998) 23-29
ELSEVIER
Improvement of shape memory characteristics by precipitation-hardening of Ti-Pd-Ni alloys S. Shirnizu a,*, Ya Xu a, E. Okunishi a, S. Tanaka ‘, K. Otsuka ‘, K. Mitose ’ d Institute of Materials Science, UniL~ersityof h Yokohama R&D
Laborator\.
The Furukawa
Electric
Tsukuha, Tsukuba, Ibaraki
305, Japan
Co. Ltd., 2-4-3 Okano, Nishi-ku,
Yokohamcr 220. Jtrpan
Received 16 April 1997; revised 8 May 1997; accepted 9 May 1997
Abstract An attempt to improve the shape memory characteristics by precipitation-hardening was made for non-equiatomic Ti-Pd-Ni high temperature shape memory alloys (Ti:(Ni, Pd) # 5050) by differential scanning calorimetry, high temperature tensile tests and transmission electron microscopy. It was found that homogeneously distributed fine precipitates can be produced by ageing treatment at the proper temperature of 773 K for TiS0,,Pd3,Ni,,,, alloys. These precipitates increase the critical stress for slip and improve the shape memory characteristics substantially at high temperatures. $D1998 Elsevier Science B.V. Keyordst
Shape memory effect: High temperature shape memory alloy; Ageing; Precipitation; Ti-Pd-Ni
1. Introduction Ti-Pd-Ni alloys have attracted considerable attention as high temperature shape memory alloys (HTSMAS) in recent years, since their martensitic transformation start temperature (M,) can be varied from 8 13 K to the ambient temperature by substituting Pd by Ni in Ti,,,Pd,, [ 1,2]. Much research was done on the shape memory effect (SME) of this alloy system by torsion and compression [2,3], tensile tests at room temperature [4] and at high temperature [5-71. It was found that the shape memory characteristics are fairly good at room temperature, but becomes bad with increasing the tensile test temperature. The reason was found to be due to the low critical stress for slip at high temperature by Otsuka
‘ Corresponding author.
et al. [5,6]. Thus, it is necessary to increase the critical stress for slip for improving the shape memory characteristics at high temperatures, and several key factors were suggested as follows [5,6]: (I) increase the critical stress for slip by adding the ternary element; (2) strengthen the parent phase by age hardening; (3) strengthen the parent phase by a thermomechanical treatment. Thereafter, an improvement of the shape memory effect by thermomechanical treatments was made by Golberg et al. [8-IO]. It was found that the shape memory characteristics of Ni , (x = 10, 15, 20) can be remarkably Ti,,Pds,-, improved by proper thermomechanical treatment, consisting of cold rolling followed by annealing. But no precipitate was found in Ti,,Pd,,,_ ,Ni , alloys even after various annealing treatments. Since it is known that precipitates can be produced in Ti-Ni alloys with non-equiatomic composition by proper
00167-577X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOI 67-577X(97)00134-1
24
S. Shimizu et al. /Materials Letters 34 (1998) 23-29
ageing treatment, i.e., Ti,Ni or Ti,Ni,O for Ti-rich for Ni-rich compositions [ll-151, and Ti,Ni, [ 16,171, the attempt will be useful to produce some precipitates for Ti-Pd-Ni alloys by making the composition off-stoichiometric (Ti:(Pd, Ni) # 50:50) by proper ageing treatments. The purpose of the present research is to try to produce fine precipitates in Ti-Pd-Ni alloys by ageing treatments, so as to increase the critical stress for slip and to improve the shape memory characteristics of Ti-Pd-Ni alloys at high temperatures by precipitation-hardening.
2. Experimental
procedure
Ingots of Ti,,_,qPd,,Ni,,+, (x= -0.6-1.5) alloys were plasma melted and homogenized at 1273 K for 18 ks. The compositions are shown in Table 1. These ingots were hot rolled at 1023 K into about 1 mm thick sheets. The specimens for tensile tests, differential scanning calorimetry (DSC) measurements and transmission electron microscopy (TEM) observations were spark cut, mechanically polished and then annealed at various temperatures in argonfilled quartz capsules with subsequent quenching into ice water by crushing the capsules. The martensitic transformation temperatures were measured with a cooling and heating rate of 0.083 K/s by DSC, using a MAC DSC3 100 s calorimeter. The SME was measured by tensile tests, which were carried out at room temperature, 373 and 473 K using an Instrontype tensile machine ‘Shimadzu Autograph DSSIOT-S’ equipped with an AG-type high temperature tensile unit. The tensile specimens have a gauge
Table 1 Composition and transformation Ti-Pd-Ni alloys
temperatures
Composition
M, (K)
M, (K)
Ti VI ,PdGi,9
517.8
499.
524 528.8 495.5 455.5 384.4 313.6
502.2 504.2 413.5 435.8 366.4 271
4
Ti so.lPd,oNi19., Ti 5o.?Pd3oNi19s TisoPdjoNi,, Ti49.sPd,oNizo s Ti,,Pd,oNi,, Ti As5PdqoNi?I c
I
(before
ageing)
A,(K)
A, (K)
515.9
529.9
521.8 524.8 496.9 455.9 385.7 309.8
536.7 542.8 511.7 469.8 400.8 334.4
of
Et
:total strain
E,
:permanent strain
CsME:strain which recovers by shape memoryeffect &B :Bauschinger strain
(tsuperelastic
strain)
Recovery Rate: R=
&SUE+ &ES &SUEt Eo + CP Fig. 1. Definitions
(E, is eliminated)
of various strains.
section of 16 X 2 X 1 mm’. The strain rate was 5.2 X 10p4/s. Definitions of various strains are shown in Fig. 1. The tensile specimens were used for repeated tensile tests in the present study. The shape memory strain ( &sME) was assessed by measuring the permanent strain (s,) at about A, + 100 K by heating the specimens to the parent phase after deformation in the martensitic state. Thus, the &sME is assessed by subtracting the sp from the residual strain after unloading. The microstructure of specimens after various ageing treatments was examined by TEM using a JEM 200CX electron microscope. Specimens for TEM were prepared using a twin-jet electro-polishing apparatus in electrolytic solution (cH,coOH:HC~O, = 92.5:7.5).
3. Results and discussion 3.1. The results of DSC measurements The transformation temperatures prior to ageing treatments were measured for various compositions
S. Shimizu et al./Materials Letters 34 11998123-29
-Mf
280 4x
I 49
48.5
I 49.5
I 50
I
I 50.5
transformation temperatures alloys. rPd30Ni2,1+x (x = -0.6-1.5)
’
vs. com-
by DSC measurements. Only a single peak is observed upon cooling and heating for all the specimens used in the present study, which indicates a one-stage forward and reverse martensitic transformation. The martensite start and finish temperatures M,, Mr and the reverse transformation start and finish temperatures A,, A, are shown in Table 1. These results are plotted as a function of Ti content in Fig. 2. We can see that the transformation temperatures decrease with decreasing Ti content (Ti-poor side), but increase with increasing Ti content from 50 to 50.2 at%, then stay almost the same for 50.2 to 50.6 at% (Ti-rich side). For Ti48,5Pd3,,Ni21.5 alloy, the MS temperature is 150 K lower than that of alloys. Therefore, the Ti-rich alloys Tiso Pd,tPi,o are more suitable for HTSMAs than Ti-poor alloys. Then the changes of transformation temperatures
rI
I
I
400
500
Ageing
temperature
I
600
I
I
700
-o-As -MS tMf
’
400
’
500
/
600
’
700
’
800
’
’
900 100011
0
Ageing Temperature I K
Fig. 2. Plots of martensitic
400300
470 460 4501 300
51
Ti I at%
posmon for Tt,,_
1 +
25
800
I
I
Ii
90010001100
Temperature I K
of martensitic transformation for Ti,, (Pd,,,Ni,,, 5 alloys
temperatures
vs. ageing
Fig. 4. Plots of martensitic transformation temperature for TI so.sPd 10Ni , 94 alloys.
temperatures
vs. ageing
after ageing at various temperatures were investigated systematically. Figs. 3 and 4 show the transformation temperatures of Ti,,,,Pd,,Ni,, 5 and Ti 50.6Pd30Ni19.4 alloys which were aged at various temperatures for 3.6 ks, respectively. For A,, A, and M, have no obvious Ti49.+%&0.,~ changes, only M, increases obviously with the increase of ageing temperature. However, for MS and M, deTi 50.6WONi19.4 alloys, A,, A,, crease by about 10 K by ageing treatment at 673-773 K for 3.6 ks. These results indicate that some precipitation processes may occur during ageing at 673-773 K for Ti50,6Pd3,,Ni,9,4, since precipitation changes the composition of the matrix, and affects the martensitic transformation temperatures. 3.2. The results of tensile tests As indicated by DSC measurements, some precipitation processes may occur for Ti,,,,Pd,,Ni,,,, alloys during ageing at 673-773 K. Here we investigated the shape memory characteristics of Ti 50.6Pd30Ni19.4 alloys aged at 773 K for 3.6 ks by high temperature tensile tests, to see whether these precipitates can produce a precipitation-hardening effect and improve the shape memory characteristics of Ti-Pd-Ni alloys. In order to make a comparison, the shape memory characteristics of Ti,,Pd,,Ni,, specimens with the same ageing conditions were also investigated, since it was known that no precipitation process occurs during ageing treatments [9] in Ti,,Pd3,Ni 20 alloys. Fig. 5 shows the relation of
S. Shimizu et al. /Materids
26
Stress
/ MPa
Total
Fig. 5. Plots of permanent strain vs. applied stress for Ti so bPd3UNi19 4 and Ti,,Pd,,Niz, alloys. The test temperatures are room temperature, 373 and 473 K, respectively.
permanent strain and tensile stress of these two alloys by deformation at room temperature, 373 and 473 K, respectively. We see that the permanent alloys is smaller than that strain of Ti,,.,PdjONi,,,, at all these three test temperatures. of Ti,,Pd,,Ni,, The critical stress for slip at various test temperatures was also measured for these two alloys, which is shown in Fig. 6. We find that the critical stress for slip decreases from 260 to 180 MPa for but is almost the same for Ti,,Pd,,Ni,,, Ti 50.6Pd30Ni19.4 when the tensile test temperature increases from room temperature to 373 K. Thus, it is clear that the decrease of the critical stress for slip is slower than that for for Ti,,.,Pd,,Ni,,., Fig. 7 shows the recovery rate calcuTi,,Pd,,N&,. lated from the above tensile results as a function of
/
loot300’
, I ,
’ 320
’ 340
,
’ 360
, 1 ,
’ 380
I 400
I.
,
,
’ 420
’ 440
460
’
1
480
Temperature/K
Fig. 6. Comparison of the Ti so,sPd,,Ni,,, and Ti,,Pd,,Ni,,
34 (19981 23-29
6
so0
300
Letten
critical stress alloys.
for
slip
for
8
10
12
14
Strain : %
Fig. 7. Recovery rate plotted against the total strain for Ti so.6PdjoNi19.4 and TiSoPd,oNi,o alloys, which were aged at 773 K for 3.6 ks. The test temperature is room temperature, 373 and 473 K, respectively.
total strain for Ti,,Pd,,Ni,, and Ti,,,Pd,,Ni ,9.4 aged at 773 K for 3.6 ks. It is found that the recovery rate of Ti,,,,Pd,,Ni ,y.4 is better than that of at all these test temperatures. The Ti,,Pd,,Ni,, recovery rate decreases to about 78% for but is still about 90% for Ti,,Pd,,Ni,, Ti 50,6Pd30Ni,9,j up to a total strain of 6% at a test temperature of 473 K. Thus, we conclude that the shape memory characteristics of Ti-Pd-Ni alloys can be improved by adjusting Ti content (Ti-rich composition) and proper ageing treatment, i.e. ageing at 673-773 K for 3.6 ks for Ti,,.,Pd,,Ni,,,4 alloys. 3.3. The results of TEM observations In order to confirm the existence of precipitates in Ti 50.6Pd30Ni19.A alloy after ageing treatment, microstructure after ageing at various temperatures was observed by TEM. Fig. 8(a) shows the typical microstructure which was observed at room tempera,9,4 prior to ageing. No precipiture for Ti,,,Pd,,Ni tates were found. Fig. 8(b) shows the microstructure after ageing at 673 K for 3.6 ks, which was observed at room temperature. The confirmation of precipitates is difficult due to the existence of internal twins, variants and other defects introduced by the martensitic transformation. In order to make the analysis and interpretation of observation results simple, the microstructure after ageing were ob-
S. Shimizu
Fig. 8. TEM micrographs
of TI,,,
Pd&i,,,
et nl. /Materials
of TlsO 6 Pd,oNi,,
34 (19981
alloys observed at room temperature:
served in the parent state (B2) using a heating holder. The specimens were heated to about 573 K (A, I 540 K) in TEM, then the structure observation was carried out. Fig. 9 shows the typical structure after ageing at 673 (a) and 773 K (b), respectively. It was found that some contrast of precipitates is faintly visible after ageing at 673 K, as indicated by arrows in Fig. 9(a), and the precipitates become larger and higher in density after ageing at 773 K for 3.6 ks (Fig. 9(b)). These precipitates are about 40 nm in diameter, and homogeneously distributed within the observed area of specimens. In order to identify the crystal structure of the precipitates, the microstructure of the specimens aged at 873 K for 18 ks were observed. Fig. lo(a) and (c) show the typical structures observed at 573 K. Fig. 10(b) is the magnified image of Fig. IO(a). We see that the precipitates grow to some strips along some certain directions. Fig. IO(d) is the selected area diffraction (SAD) pattern taken from Fig. 10(c). It is found that in addition to the diffraction spots of { 1 IO},, reciprocal
Rg. 9. TEM mlcrographs ks.
Letters
23-29
27
(a) before ageing; (b) after ageing at 673 K for 3.6 ks.
lattice vectors coming from matrix, small diffraction spots located at about l/3 positions of { 1 lo},, reciprocal lattice vectors can be observed. These l/3 diffraction spots were proved coming from the precipitates. Since the following precipitate types can be considered to occur in Ti,,,,Pd,,Ni,,,,, according to the phase diagram researches on the Ti-Pd-Ni system: Ti,Pd (cubic (Fd3m)), Ti,Pd (tetragonal (14/mmm)), Ti,% (cubic (Fd3ml) and Ti,Ni,O (cubic (Fd3m)), we made reciprocal lattice vectors simulations for all above intermetallics, to index the SAD pattern of Fig. 10(d). The results show that the Ti,Ni structure is the most possible for the precipitate. Fig. IO(e) shows the simulation results of the diffraction pattern of the [l 1 l]n, zone axis and zone axis. The diffraction spots of Ti,Ni n321Ti2ti, in Fig. 10(d) can be considered coming from three different variants, as shown in Fig. 10(c). But it should be mentioned that Fig. IO(e) does not agree unanimously with Fig. 10(d). This indicates that the composition of precipitate may deviate from Ti,Ni.
4 alloy observed at 573 K: (a) after ageing at 673 K for 3.6 ks; (b) after ageing at 773 K for 3.6
Fig. 10. (a, c) Typical microstructures of Ti,,,,Pd,,Ni 1y4 alloys aged at 873 K for 3.6 ks, which were observed at 573 K; (b) magnified image of (a), which shows the strip shape of precipitates; (d) SAD pattern taken from (c); (e) simulated diffraction pattern of the 111 I],, zone axis and the [?32],,?,, zone axis.
The study of the precipitates in other orientations are necessary, and the study is continued in that direction.
4, Conclusions The martensitic transformation temperatures, shape memory characteristics and microstructure of Ti X-XPdXINi*O+J (x = -0.6-l .5) alloys aged at various temperatures were investigated. The following conclusions were obtained. temperatures (1) The ma r tensitic transformation decrease with decreasing Ti content from equiatomic composition (Ti-poor), but increase with increasing
Ti content from equi-atomic composition (Ti-rich) and keep almost the same for 50.2 to 50.6 at% Ti for alloys. Ti 50-.rPd30Ni20+., (x = -0.6-1.5) (2) The shape memory characteristics were improved substantially by ageing Ti 50.6Pd 3,,Ni ,9.4 alloys at 773 K for 3.6 ks. The recovery rate can be kept to be about 90% for a total strain of 6% at 473 K, which is about 10% higher than that of Ti,,Pd,,Ni,, alloys. (3) The reason for the improvement of shape memory characteristics was found to be due to the hardening effect by homogeneously distributed fine precipitates, which were produced by ageing at 773 K. These precipitates are considered to be of the Ti 2 Ni type structure.
S. Shimizu et al. /Materials
Acknowledgements The present work was supported by the Grant-in Aid for General Scientific Research (Kiban A, 1995 7) from the Ministry of Education, Science and Culture of Japan.
References [I] H.C. Donkersloot, J.H.N. Van Vucht, J. Less-Common Met. 20 (1970) 83. [2] V.N. Khachin, N.A. Matveeva, V.P. Sivokha, D.V. Chernov, Dokl. Acad. Nauk SSSR 257 (1981) 167. [3] W.S. Yang, D.E. Mikkola, Ser. Metall. 28 (1993) 161. [4] P.G.Lindquist, C.M. Wayman, Proc. of the Int. Conf. on Engineering Aspects of Shape Memory Alloys, ButterworthHeineman, London, 1990, p. 58. [5] K. Otsuka, K. Oda, Y. Ueno, M. Piao, T. Ueki, H. Horikawa, Ser. Metall. 29 (1993) 1355.
Letters 34 11998123-29
b1 Y.
29
Ueno, M. Piao, K. Oda, K. Otsuka, T. Ueki. H. Horikawa, Proc. 3rd International SAMPE Symposium, Chiba, Japan, 1993, p. 1274. [71 K. Enami, T. Hoshiya, in: C.T. Liu, K. Otsuka, K. Shimizu, Y. Suzuki, J. Van Humbeeck (Eds.), Proc. 3rd IUMRS Int. Conf. on Advanced Materials, Tokyo, Japan, 1993, p. 1013. t81D. Golberg, Y. Xu. Y. Murakami, S. Morito, K. Otsuka, T. Ueki, H. Horikawa, Ser. Metall. 30 (1994) 1349. [91 D. Golberg, Y. Xu, Y. Murakami, S. Morito. K. Otsuka, T. Ueki, H. Horikawa, Intermetallics 3 (1995) 35. [lOI D. Golberg, Y. Xu, Y. Murakami, K. Otsuka, T. Ueki, H. Horikawa, Mater. Lett. 22 (1995) 241. [I II M.H. Muller, H.W. Knott, Trans. Metall. Sot. AIME 227 (1963) 674. 1121 A. Ishida, M. Sato, A. Takei, S. Miyazaki. Mat. Trans. JIM 36 (1 I) (1995) 1349. [I31 H.C. Lin, S.-K. Wu, J.C. Lin, ICOMAT-92. 1992, p. 875. [I41 J. Li, J.S. Zhu, Y.N. Wang, Phys. Statis Solidi (a) 144 (1994) 105. J.L. Rodriguez-Galicia, [I51 H.F. Lopez, A. Salinas-Rodriguez, Scri. Metall. 34 (1996) 659. [I61M. Nishida, T. Honma, Scri. Metall. 18 (1984) 1293. [I71 M. Nishida, C.M. Wayman. T. Honma, Metall. Trans. A 17A (1986) 1505.
Materials Letters 34 (1998) 23-29
ELSEVIER
Improvement of shape memory characteristics by precipitation-hardening of Ti-Pd-Ni alloys S. Shirnizu a,*, Ya Xu a, E. Okunishi a, S. Tanaka ‘, K. Otsuka ‘, K. Mitose ’ d Institute of Materials Science, UniL~ersityof h Yokohama R&D
Laborator\.
The Furukawa
Electric
Tsukuha, Tsukuba, Ibaraki
305, Japan
Co. Ltd., 2-4-3 Okano, Nishi-ku,
Yokohamcr 220. Jtrpan
Received 16 April 1997; revised 8 May 1997; accepted 9 May 1997
Abstract An attempt to improve the shape memory characteristics by precipitation-hardening was made for non-equiatomic Ti-Pd-Ni high temperature shape memory alloys (Ti:(Ni, Pd) # 5050) by differential scanning calorimetry, high temperature tensile tests and transmission electron microscopy. It was found that homogeneously distributed fine precipitates can be produced by ageing treatment at the proper temperature of 773 K for TiS0,,Pd3,Ni,,,, alloys. These precipitates increase the critical stress for slip and improve the shape memory characteristics substantially at high temperatures. $D1998 Elsevier Science B.V. Keyordst
Shape memory effect: High temperature shape memory alloy; Ageing; Precipitation; Ti-Pd-Ni
1. Introduction Ti-Pd-Ni alloys have attracted considerable attention as high temperature shape memory alloys (HTSMAS) in recent years, since their martensitic transformation start temperature (M,) can be varied from 8 13 K to the ambient temperature by substituting Pd by Ni in Ti,,,Pd,, [ 1,2]. Much research was done on the shape memory effect (SME) of this alloy system by torsion and compression [2,3], tensile tests at room temperature [4] and at high temperature [5-71. It was found that the shape memory characteristics are fairly good at room temperature, but becomes bad with increasing the tensile test temperature. The reason was found to be due to the low critical stress for slip at high temperature by Otsuka
‘ Corresponding author.
et al. [5,6]. Thus, it is necessary to increase the critical stress for slip for improving the shape memory characteristics at high temperatures, and several key factors were suggested as follows [5,6]: (I) increase the critical stress for slip by adding the ternary element; (2) strengthen the parent phase by age hardening; (3) strengthen the parent phase by a thermomechanical treatment. Thereafter, an improvement of the shape memory effect by thermomechanical treatments was made by Golberg et al. [8-IO]. It was found that the shape memory characteristics of Ni , (x = 10, 15, 20) can be remarkably Ti,,Pds,-, improved by proper thermomechanical treatment, consisting of cold rolling followed by annealing. But no precipitate was found in Ti,,Pd,,,_ ,Ni , alloys even after various annealing treatments. Since it is known that precipitates can be produced in Ti-Ni alloys with non-equiatomic composition by proper
00167-577X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOI 67-577X(97)00134-1
24
S. Shimizu et al. /Materials Letters 34 (1998) 23-29
ageing treatment, i.e., Ti,Ni or Ti,Ni,O for Ti-rich for Ni-rich compositions [ll-151, and Ti,Ni, [ 16,171, the attempt will be useful to produce some precipitates for Ti-Pd-Ni alloys by making the composition off-stoichiometric (Ti:(Pd, Ni) # 50:50) by proper ageing treatments. The purpose of the present research is to try to produce fine precipitates in Ti-Pd-Ni alloys by ageing treatments, so as to increase the critical stress for slip and to improve the shape memory characteristics of Ti-Pd-Ni alloys at high temperatures by precipitation-hardening.
2. Experimental
procedure
Ingots of Ti,,_,qPd,,Ni,,+, (x= -0.6-1.5) alloys were plasma melted and homogenized at 1273 K for 18 ks. The compositions are shown in Table 1. These ingots were hot rolled at 1023 K into about 1 mm thick sheets. The specimens for tensile tests, differential scanning calorimetry (DSC) measurements and transmission electron microscopy (TEM) observations were spark cut, mechanically polished and then annealed at various temperatures in argonfilled quartz capsules with subsequent quenching into ice water by crushing the capsules. The martensitic transformation temperatures were measured with a cooling and heating rate of 0.083 K/s by DSC, using a MAC DSC3 100 s calorimeter. The SME was measured by tensile tests, which were carried out at room temperature, 373 and 473 K using an Instrontype tensile machine ‘Shimadzu Autograph DSSIOT-S’ equipped with an AG-type high temperature tensile unit. The tensile specimens have a gauge
Table 1 Composition and transformation Ti-Pd-Ni alloys
temperatures
Composition
M, (K)
M, (K)
Ti VI ,PdGi,9
517.8
499.
524 528.8 495.5 455.5 384.4 313.6
502.2 504.2 413.5 435.8 366.4 271
4
Ti so.lPd,oNi19., Ti 5o.?Pd3oNi19s TisoPdjoNi,, Ti49.sPd,oNizo s Ti,,Pd,oNi,, Ti As5PdqoNi?I c
I
(before
ageing)
A,(K)
A, (K)
515.9
529.9
521.8 524.8 496.9 455.9 385.7 309.8
536.7 542.8 511.7 469.8 400.8 334.4
of
Et
:total strain
E,
:permanent strain
CsME:strain which recovers by shape memoryeffect &B :Bauschinger strain
(tsuperelastic
strain)
Recovery Rate: R=
&SUE+ &ES &SUEt Eo + CP Fig. 1. Definitions
(E, is eliminated)
of various strains.
section of 16 X 2 X 1 mm’. The strain rate was 5.2 X 10p4/s. Definitions of various strains are shown in Fig. 1. The tensile specimens were used for repeated tensile tests in the present study. The shape memory strain ( &sME) was assessed by measuring the permanent strain (s,) at about A, + 100 K by heating the specimens to the parent phase after deformation in the martensitic state. Thus, the &sME is assessed by subtracting the sp from the residual strain after unloading. The microstructure of specimens after various ageing treatments was examined by TEM using a JEM 200CX electron microscope. Specimens for TEM were prepared using a twin-jet electro-polishing apparatus in electrolytic solution (cH,coOH:HC~O, = 92.5:7.5).
3. Results and discussion 3.1. The results of DSC measurements The transformation temperatures prior to ageing treatments were measured for various compositions
S. Shimizu et al./Materials Letters 34 11998123-29
-Mf
280 4x
I 49
48.5
I 49.5
I 50
I
I 50.5
transformation temperatures alloys. rPd30Ni2,1+x (x = -0.6-1.5)
’
vs. com-
by DSC measurements. Only a single peak is observed upon cooling and heating for all the specimens used in the present study, which indicates a one-stage forward and reverse martensitic transformation. The martensite start and finish temperatures M,, Mr and the reverse transformation start and finish temperatures A,, A, are shown in Table 1. These results are plotted as a function of Ti content in Fig. 2. We can see that the transformation temperatures decrease with decreasing Ti content (Ti-poor side), but increase with increasing Ti content from 50 to 50.2 at%, then stay almost the same for 50.2 to 50.6 at% (Ti-rich side). For Ti48,5Pd3,,Ni21.5 alloy, the MS temperature is 150 K lower than that of alloys. Therefore, the Ti-rich alloys Tiso Pd,tPi,o are more suitable for HTSMAs than Ti-poor alloys. Then the changes of transformation temperatures
rI
I
I
400
500
Ageing
temperature
I
600
I
I
700
-o-As -MS tMf
’
400
’
500
/
600
’
700
’
800
’
’
900 100011
0
Ageing Temperature I K
Fig. 2. Plots of martensitic
400300
470 460 4501 300
51
Ti I at%
posmon for Tt,,_
1 +
25
800
I
I
Ii
90010001100
Temperature I K
of martensitic transformation for Ti,, (Pd,,,Ni,,, 5 alloys
temperatures
vs. ageing
Fig. 4. Plots of martensitic transformation temperature for TI so.sPd 10Ni , 94 alloys.
temperatures
vs. ageing
after ageing at various temperatures were investigated systematically. Figs. 3 and 4 show the transformation temperatures of Ti,,,,Pd,,Ni,, 5 and Ti 50.6Pd30Ni19.4 alloys which were aged at various temperatures for 3.6 ks, respectively. For A,, A, and M, have no obvious Ti49.+%&0.,~ changes, only M, increases obviously with the increase of ageing temperature. However, for MS and M, deTi 50.6WONi19.4 alloys, A,, A,, crease by about 10 K by ageing treatment at 673-773 K for 3.6 ks. These results indicate that some precipitation processes may occur during ageing at 673-773 K for Ti50,6Pd3,,Ni,9,4, since precipitation changes the composition of the matrix, and affects the martensitic transformation temperatures. 3.2. The results of tensile tests As indicated by DSC measurements, some precipitation processes may occur for Ti,,,,Pd,,Ni,,,, alloys during ageing at 673-773 K. Here we investigated the shape memory characteristics of Ti 50.6Pd30Ni19.4 alloys aged at 773 K for 3.6 ks by high temperature tensile tests, to see whether these precipitates can produce a precipitation-hardening effect and improve the shape memory characteristics of Ti-Pd-Ni alloys. In order to make a comparison, the shape memory characteristics of Ti,,Pd,,Ni,, specimens with the same ageing conditions were also investigated, since it was known that no precipitation process occurs during ageing treatments [9] in Ti,,Pd3,Ni 20 alloys. Fig. 5 shows the relation of
S. Shimizu et al. /Materids
26
Stress
/ MPa
Total
Fig. 5. Plots of permanent strain vs. applied stress for Ti so bPd3UNi19 4 and Ti,,Pd,,Niz, alloys. The test temperatures are room temperature, 373 and 473 K, respectively.
permanent strain and tensile stress of these two alloys by deformation at room temperature, 373 and 473 K, respectively. We see that the permanent alloys is smaller than that strain of Ti,,.,PdjONi,,,, at all these three test temperatures. of Ti,,Pd,,Ni,, The critical stress for slip at various test temperatures was also measured for these two alloys, which is shown in Fig. 6. We find that the critical stress for slip decreases from 260 to 180 MPa for but is almost the same for Ti,,Pd,,Ni,,, Ti 50.6Pd30Ni19.4 when the tensile test temperature increases from room temperature to 373 K. Thus, it is clear that the decrease of the critical stress for slip is slower than that for for Ti,,.,Pd,,Ni,,., Fig. 7 shows the recovery rate calcuTi,,Pd,,N&,. lated from the above tensile results as a function of
/
loot300’
, I ,
’ 320
’ 340
,
’ 360
, 1 ,
’ 380
I 400
I.
,
,
’ 420
’ 440
460
’
1
480
Temperature/K
Fig. 6. Comparison of the Ti so,sPd,,Ni,,, and Ti,,Pd,,Ni,,
34 (19981 23-29
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critical stress alloys.
for
slip
for
8
10
12
14
Strain : %
Fig. 7. Recovery rate plotted against the total strain for Ti so.6PdjoNi19.4 and TiSoPd,oNi,o alloys, which were aged at 773 K for 3.6 ks. The test temperature is room temperature, 373 and 473 K, respectively.
total strain for Ti,,Pd,,Ni,, and Ti,,,Pd,,Ni ,9.4 aged at 773 K for 3.6 ks. It is found that the recovery rate of Ti,,,,Pd,,Ni ,y.4 is better than that of at all these test temperatures. The Ti,,Pd,,Ni,, recovery rate decreases to about 78% for but is still about 90% for Ti,,Pd,,Ni,, Ti 50,6Pd30Ni,9,j up to a total strain of 6% at a test temperature of 473 K. Thus, we conclude that the shape memory characteristics of Ti-Pd-Ni alloys can be improved by adjusting Ti content (Ti-rich composition) and proper ageing treatment, i.e. ageing at 673-773 K for 3.6 ks for Ti,,.,Pd,,Ni,,,4 alloys. 3.3. The results of TEM observations In order to confirm the existence of precipitates in Ti 50.6Pd30Ni19.A alloy after ageing treatment, microstructure after ageing at various temperatures was observed by TEM. Fig. 8(a) shows the typical microstructure which was observed at room tempera,9,4 prior to ageing. No precipiture for Ti,,,Pd,,Ni tates were found. Fig. 8(b) shows the microstructure after ageing at 673 K for 3.6 ks, which was observed at room temperature. The confirmation of precipitates is difficult due to the existence of internal twins, variants and other defects introduced by the martensitic transformation. In order to make the analysis and interpretation of observation results simple, the microstructure after ageing were ob-
S. Shimizu
Fig. 8. TEM micrographs
of TI,,,
Pd&i,,,
et nl. /Materials
of TlsO 6 Pd,oNi,,
34 (19981
alloys observed at room temperature:
served in the parent state (B2) using a heating holder. The specimens were heated to about 573 K (A, I 540 K) in TEM, then the structure observation was carried out. Fig. 9 shows the typical structure after ageing at 673 (a) and 773 K (b), respectively. It was found that some contrast of precipitates is faintly visible after ageing at 673 K, as indicated by arrows in Fig. 9(a), and the precipitates become larger and higher in density after ageing at 773 K for 3.6 ks (Fig. 9(b)). These precipitates are about 40 nm in diameter, and homogeneously distributed within the observed area of specimens. In order to identify the crystal structure of the precipitates, the microstructure of the specimens aged at 873 K for 18 ks were observed. Fig. lo(a) and (c) show the typical structures observed at 573 K. Fig. 10(b) is the magnified image of Fig. IO(a). We see that the precipitates grow to some strips along some certain directions. Fig. IO(d) is the selected area diffraction (SAD) pattern taken from Fig. 10(c). It is found that in addition to the diffraction spots of { 1 IO},, reciprocal
Rg. 9. TEM mlcrographs ks.
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(a) before ageing; (b) after ageing at 673 K for 3.6 ks.
lattice vectors coming from matrix, small diffraction spots located at about l/3 positions of { 1 lo},, reciprocal lattice vectors can be observed. These l/3 diffraction spots were proved coming from the precipitates. Since the following precipitate types can be considered to occur in Ti,,,,Pd,,Ni,,,,, according to the phase diagram researches on the Ti-Pd-Ni system: Ti,Pd (cubic (Fd3m)), Ti,Pd (tetragonal (14/mmm)), Ti,% (cubic (Fd3ml) and Ti,Ni,O (cubic (Fd3m)), we made reciprocal lattice vectors simulations for all above intermetallics, to index the SAD pattern of Fig. 10(d). The results show that the Ti,Ni structure is the most possible for the precipitate. Fig. IO(e) shows the simulation results of the diffraction pattern of the [l 1 l]n, zone axis and zone axis. The diffraction spots of Ti,Ni n321Ti2ti, in Fig. 10(d) can be considered coming from three different variants, as shown in Fig. 10(c). But it should be mentioned that Fig. IO(e) does not agree unanimously with Fig. 10(d). This indicates that the composition of precipitate may deviate from Ti,Ni.
4 alloy observed at 573 K: (a) after ageing at 673 K for 3.6 ks; (b) after ageing at 773 K for 3.6
Fig. 10. (a, c) Typical microstructures of Ti,,,,Pd,,Ni 1y4 alloys aged at 873 K for 3.6 ks, which were observed at 573 K; (b) magnified image of (a), which shows the strip shape of precipitates; (d) SAD pattern taken from (c); (e) simulated diffraction pattern of the 111 I],, zone axis and the [?32],,?,, zone axis.
The study of the precipitates in other orientations are necessary, and the study is continued in that direction.
4, Conclusions The martensitic transformation temperatures, shape memory characteristics and microstructure of Ti X-XPdXINi*O+J (x = -0.6-l .5) alloys aged at various temperatures were investigated. The following conclusions were obtained. temperatures (1) The ma r tensitic transformation decrease with decreasing Ti content from equiatomic composition (Ti-poor), but increase with increasing
Ti content from equi-atomic composition (Ti-rich) and keep almost the same for 50.2 to 50.6 at% Ti for alloys. Ti 50-.rPd30Ni20+., (x = -0.6-1.5) (2) The shape memory characteristics were improved substantially by ageing Ti 50.6Pd 3,,Ni ,9.4 alloys at 773 K for 3.6 ks. The recovery rate can be kept to be about 90% for a total strain of 6% at 473 K, which is about 10% higher than that of Ti,,Pd,,Ni,, alloys. (3) The reason for the improvement of shape memory characteristics was found to be due to the hardening effect by homogeneously distributed fine precipitates, which were produced by ageing at 773 K. These precipitates are considered to be of the Ti 2 Ni type structure.
S. Shimizu et al. /Materials
Acknowledgements The present work was supported by the Grant-in Aid for General Scientific Research (Kiban A, 1995 7) from the Ministry of Education, Science and Culture of Japan.
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