- Email: [email protected]
Recovery and recrystallization in the Ti50Pd50 martensite
February 1997
ELSEVIER
Materials Letters 30 (1997) 189-197
Recovery and recrystallization in the Ti 5O Pd 5Omartensite Ya Xu a, K. Otsuka a**, E. Furubayashi
b, T. Ueki ‘, K. Mitose
’
a Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan h Department of Materials Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan ’ Yokohama R & D Laboratory, The Furukawa Electric Co. Ltd., 2-4-3 Okano, Nishi-ku, Yokohama 220, Japan
Received 5 August 1996; accepted 8 August 1996
Abstract The recrystallization process in the martensitic state was clearly demonstrated for the cold-rolled Ti,,Pd,, high temperature shape memory alloy by electron microscopy and electron diffraction, along with the change in hardness with annealing. As a result, the martensite crystals were created by the diffusional process, which had the same orthorhombic structure as the transformed one, and which had no lattice invariant shear characteristic of the martensitic transformation. This is probably the first observation of the recrystallization process in martensite on electron microscopic scale. These results are briefly described. Keywords:
Recovery; Recrystallization; Martensite; Ti,,Pd,,
1. Introduction The Ti-Pd alloy system has attracted significant attention as a high temperature shape memory ahoy in recent years, and many researches have been done on this alloy system [l-7]. In the course of the investigations, a systematic research on recovery and recrystallization processes of several Ti-Pd-Ni alloys was recently done by the present authors [S]. As a result, a close relationship was found between the recovery-recrystallization and the reverse transformation of the alloys. That is, for isochronal annealing, the recovery-recrystallization was found to occur in the parent after the reverse transformation, since diffusion is much faster in the parent phase
^ Corresponding author. Tel: 81-298-53-5294 Fax: 81-298-557440. 00167-577X/97/$17.00 PI1 SOl67-577X(96)00203-0
compared to that in the martensite. Thus, it was shown in the present alloy system that the recovery-recrystallization is controlled by the reverse transformation. Then, we may argue whether recrystallization occurs in the martensitic state or not. Actually we found recrystallization in the martensitic state for a long-time isothermal annealing by choosing a proper annealing temperature and composition (Ti,,Pd,,). The purpose of the present paper is to report these results. Since diffusion is very slow in the martensite, the study of recrystallization in the martensite is very time consuming, but it is a good system to study the recrystallization process step by step, since recrystallization occurs very slowly. In the literature, there are two reports, which concern with recrystallization in ferrous martensites. Tokizane et al. [9] observed recrystallization in the lath martensite of Fe-0.16C-
Copyright 0 1997 Elsevier Science B.V. All rights reserved.
Y. Xu et al./Materials
t~~~~.~.~~~~~.~.~.~~1
0
10
20
30
40
50
60
70
80
Letters 30 (1997) 189-197
90 100
Amealiug time I hour Fig. 1. Micro-hardness changes after 5OO”C, 550°C and 650°C respectively.
isothermal
annealing
at
0.6Mn-Cr-0.2Mo and Fe-0.2C-0.45Mn low carbon steels, and reported that although the as-quenched lath martensite hardly recrystallized on tempering, the deformed lath martensite easily recrystallized on subsequent tempering. However, they observed the recrystallization only by optical microscopy, and they did not describe the recrystallization in detail. Enomoto and Furubayashi [lo] observed the recrystallization of the bee phase in the two-phase (a + ~1 region of a Fe-7Ni ahoy. However, it is not clear whether the bee phase is a martensite or not. Instead, it is more likely that the bee phase is an equilibrium (Y phase. Meanwhile, there is an interesting report in an opposite sense on the Ni-Al martensite with Ll,, structure [ 11,121. In this alloy system, the metastable martensite is obtained by rapid quenching from the P-phase (B2) region. Upon heating, however, the martensite does not revert to the l3 parent phase, but it transforms into a stable Ni, Al, phase. Thus, the shape memory effect is disturbed by the avoidance of the reverse transformation. By considering these situations, the present paper may be the first report on recrystallization in martensite on an electron microscopic scale.
2. Experimental
thick sheets with a 24.2-29.0% reduction in thickness at room temperature. Because of the ductility limit of the Ti50Pd50 alloy, a higher cold-rolling reduction rate was not available. The specimens for hardness tests, electrical resistivity measurements and TEM observations were spark cut, mechanically polished and then heat-treated at various temperatures. The specimens were isothermally annealed in fused quartz capsules, and the preparation was done in the same way as in Ref. [8]. The hardness measurements were carried out using a MVE-K micro-Viekcrs tester. The load was 1 kgf and the loading time was 20 s. Samples for electrical resistivity measurement were cut into rods about 1 X 2 X 30 mm. Electrical resistivity was measured during isothermal annealing and heating and cooling processes at a rate of about S”C/min in vacuum using the four-probe method with a constant current of 0.084 A. Microstructures of annealed bulk samples were observed at ambient
t
4 I
0
100
I
I,
I
200
300
I
I
400
500
600
700
Temperature/C
procedure
Ingots of Ti,,Pd,, alloy were plasma melted by using 99.99% Ti and 99.99% Pd in a water cooled copper mould. The ingots were remelted four times for homogenization. All the alloys were homogenized at 1000°C for 5 h, then were hot rolled at 800°C and subsequently cold rolled into 0.5-l mm
,I,,,
-1st cycle -----2nd cycle 0
100
200
300
400
500
600
_
700
Temperature /C Fig. 2. Electrical resistivity versus temperature plots: (a) after annealing at 550°C for 42 h; (b) after annealing at 550°C for 100 h.
Y. Xu et al. / Materials Letters 30 (1997) 189-197 Table 1 Martensitic
transformation
191
temperatures
of the Ti,,Pd,,
alloy
(“0
cold rolled state (CR) a CR + 550°C 42 h CR + 550°C 100 h 1st cycle CR + 550°C 100 h 2nd cycle
0
,
I
100
200
I,
300
I,
400
I,
500
I,
600
700
’ After Ref. [8] measured
540 549 547
526 537 540
517 590 592
589 596 597
546
535.5
581.5
589
by DSC.
Temperature /‘c Fig. 3. Electrical resistivity versus temperature plots: (1) heating at S”C/min; (2) cooling at 5°C /min after annealing at 500°C for 2 h.
temperature microscope.
by a JEOL 200CX transmission
electron
3. Results and discussion 3.1. Hardness ments
and
electrical
resistivity
measure-
Fig. 1 shows the results of micro-Vickers hardness test of cold-rolled Ti,,Pd,, alloy isothermally
annealed at 500°C 550°C and 650°C respectively. The A, (reverse transformation start temperature) and A, (reverse transformation finish temperature) of the cold-rolled Ti50Pd50 alloy were 577°C and 589°C respectively [8]. Thus, the annealings at 500°C and 550°C correspond to the annealing in the martensitic state, and 650°C corresponds to that in the parent state. We notice from the hardness curves of Fig. 1 that a pronounced hardness drop occurred in the early annealing period of 2 h for annealing at 500°C and 550°C which is in the martensitic state. This indicates that some restoration process took place. Further softening occurred after annealing at
192
Y. Xu et al./Materials
500°C and 550°C for about 50 h; especially for 550°C annealing the hardness decreases to almost the same value as that after annealing at 650°C for 20 h, which indicates that further restoration process took place. These will be discussed later in combination with the results of resistivity measurements and microstructure observations. We also see that a complete softening occurred after a short-time (about 1 min) annealing at 650°C which indicates that the reverse transformation affects the recrystallization process greatly, as we reported previously. The electrical resistivity was measured during annealing at 550°C. No obvious change, however, was observed even after annealing for 100 h. This is probably because the scattering of electrons by phonons is so strong at high temperatures that the
Fig. 5. Eletron micrograph
taken after annealing
Letters 30 (1997) 189-197
lattice restoration by recrystallization does not contribute much to the change in resistivity. Fig. 2a and 2b show the temperature dependence of the electrical resistivity for the specimens after annealing at 550°C for 42 h and 100 h, respectively. The specimens were heated up to 650°C at S”C/min, and then cooled to room temperature at the same rate. A square hysteresis loop which corresponds to the martensitic transformation was observed. The reverse transformation temperatures were determined as A, = 590°C A,. = 596°C after 42 h annealing, and A, = 592°C A, = 597°C after 100 h annealing, respectively. We see that the reverse transformation temperatures do not change during annealing at 550°C. Meanwhile, when the second heating cycle was made for the same specimen after annealing at 550°C for
at 500°C for 100 h. A, B, C, D, E and F indicate the recrystallized
grains.
Y. Xu et al. /Materials
Letters 30 (1997) 189-397
Fig. 6. (a) Bright-field image of the region containing grains A and B shown in Fig. 5, observed by tilting the specimen; (b) SAD patterns taken from the boundary region between A and B. Two sets of SAD pattern coming from A and B respectively, were confirmed; (c) DF image which shows grain B by using an 721, reflection shown in (b); (d) DF image which shows grain A by using a 212, reflection shown in 03).
Fig. 7. Electron micrograph
exhibiting
typical recrystallized
grains, after annealing
at 550°C for 100 h
194
Y. Xu et aI./Materials Letters 30 (19971 189-197
100 h, the reverse transformation temperatures were determined as A: = 5815”C, A; = 589°C as shown in Fig. 2b. The martensitic transformation temperatures measured after annealing at different temperatures and for different times are summarized in Table 1. We see that there is a decrease of the reverse transformation temperatures in the second cycle compared with the result for the first cycle. Since the increase of the reverse transformation temperature reflects the cold working effect [7,13], this decrease is considered to be related to some restoration process occurred during the first heating process up to 650°C. Thus, we concluded that even after annealing at 550°C for 100 h, the recrystallization process was still not finished, and further recrystallization occurred upon the following heating up to 650°C which is above A,. This also supports the above results of the hardness measurement that the recrystallization process in martensite is much slow, and the reverse transformation can promote the recrystallization process greatly. Some difference between the resistivities measured by the first and the second cycle in the low temperature range was also observed in Fig. 2b. This is attributed to an experimental error in controlling the cooling rate accurately in the low temperature range. On the other hand, the electrical resistivity changes during the heating and cooling processes after annealing at 550°C for 2 h were measured as shown in Fig. 3. Comparing curve (1) heating with curve (2) for cooling after annealing in Fig. 3, we see that an obvious decrease occurred after annealing for 2 h, which also indicates the occurrence of some restoration.
early annealing period in the martensitic state is ascribed to a recovery process due to climb motion of dislocations, etc. However, some subgrains with a diameter less than 0.5 pm were observed after annealing at 500°C for 20 h as shown in Fig. 4d. This indicates that a subgrain formation process occurred with the increase of the annealing time. Fig. 5 shows a typical microstructure after annealing at 500°C for 100 h. We find that some new recrystallized grains with a diameter greater than I km were formed as
3.2. TEM obseruations The microstructures in the cold-rolled state and those after isothermal annealing at 500°C and 550°C for various times were observed at room temperature. Fig. 4 shows such examples in cold-rolled state and after annealing at 500°C for I, 5 and 20 h, respectively. We find that the microstructure in cold-rolled state shows a typically deformed structure, which is inhomogeneous, as reported in our previous work [8]. No formation of subgrain structure was observed after annealing at 500°C for 5 h. Thus, compared with the results of the hardness and resistivity measurements, the hardness drop in the
Fig. 8. (a) Magnified image of the retangular region indicated in Fig. 7; (b) SAD pattern taken from the area indicated in (a), which shows a (I I I),,, twin pattern in [ilo],,, zone; (c) DF image taken from the spot 002, indicated in (b).
Y. Xu et al./Materials
marked by A, B, C, D, E and F in Fig. 5, which were confirmed by following observations. Fig. 6a shows the region containing grains A and B observed in Fig. 5, in different orientations after tilting the specimen. The selected area diffraction (SAD) patterns taken from the boundary region between A and B is shown in Fig. 6b. Fig. 6c and 6d show the dark-field (DF) images taken from the spots 721, and 212,, respectively. We see that these two sets of pattern represent gains A and B, respectively. Comparing Fig. 6a with Fig. 6d, some original subgrain boundaries in recrystallized grain A are still faintly visible after annealing at 500°C for 100 h, as indicated by an arrow in Fig. 6a. Thus, it can be considered that the recovery progressed by the annihilation and rearrangement of dislocations, and then subgrain growth until a critical size for recrystallized nucleus being reached. Further growth of the recrystallized nucleus will take place by the migration of the high-angle boundaries as a result of high mobility of high-angle boundaries (Fig. 5C and 5D). The mechanism of
Letters 30 (1997) 189-197
195
subgrain growth in the Ti,,Pd,, alloy, that is whether by subgrain boundary migration [ 14,151 or by subgrain rotation and coalescence [16,17] is not determined yet in the present work. Further researches on this are in progress. With the increase of annealing temperature, the recrystallization in the martensitic state in the Ti,OPd,, alloy progressed significantly. A partially recrystallized structure was observed after annealing at 550°C for 100 h, as typically shown in Fig. 7. This confirmed the result of electrical resistivity measurement described above that the recrystallization is still not finished after annealing at 550°C for 100 h. Some twins, dislocations and faults were observed within the recrystallized regions. Since the formation of twins was observed rarely during the recovery process (Fig. 4) and at the very onset of formation of recrystallized grains (Fig. 5), we see that these twins formed during the growth process of recrystallized grains. Two kinds of annealing twins were confirmed in the Ti,,Pd,, alloy. One is the (ill),,, twin
Fig. 9. (a) Electron micrograph of a recrystallized structure after annealing at 550°C for 100 h; (b) magnified image of the area indicated in (a); (c) SAD pattern taken from the area indicated in (b), which shows a (lOI),,, twin pattern in [OlO],,, zone; (d) DF image taken from the spot 702, indicated in cc).
196
Y. Xu et al./Materials Letters 30 (19971 189-197
shown in Fig. 8 which is a magnified image of the rectangular region indicated in Fig. 7. The SAD pattern is shown in Fig. 8b. Fig. SC shows the DF image taken from the 002, spot indicated in Fig. 8b, which was confirmed as the (11 I),,, twin in the zone. This twinning mode is the same as the m,,, lattice invariant shear upon martensitic transformation [3,18]. We see that the twins are very thin, and they join the other thin twins in the neighboring grains, as if the former induced the latter, as indicated by the arrows in Fig. 7. Thus, we suggest that this kind of twins is introduced by the deformation twinning upon quenching operation after the annealing process. Another one is the (lol),,, twin as shown in Fig. 9, in which the twin boundaries are not straight. We believe these are introduced by a diffusional process. Some dislocations were also observed within the recrystallized region. The dislocations in martensite will be discussed in detail in another paper. The martensites are usually considered as metastable phases rather than equilibrium phases. However, the fact that the martensite structure was obtained by a recrystallization process may mean that the martensite is an equilibrium phase in the present case. Since the phase diagram of the Ti-Pd system is not well established 119,201, the above experimental results need to be taken into account, when the phase diagram is reexamined. A study is being made in this direction.
teristic of the martensitic transformation. This is reasonable, since the martensite was formed by a diffusional process. (3) A pronounced hardness drop (which is smaller in magnitude compared to that in the parent phase) was observed in the early annealing period of 2 h, even in the martensitic state. This was ascribed to a recovery process due to climb motion of dislocations, etc. (4) Two types of twins were sometimes observed: { 11 l},,g twins and {IOl},,, twins. The former twinning mode is the same as the lattice invariant shear upon martensitic transformation, while the latter is not. The former twins were suggested to be introduced by the deformation twinning upon quenching operation after the annealing process.
Acknowledgements The authors wish to thank Dr. T. Kainuma of the National Research Institute for Metals for the use of a micro-Vickers hardness tester. The present work was supported by the Grant-in-Aid for General Scientific Research (Shiken B, 1995-7) from the Ministry of Education, Science and Culture of Japan.
References [I] H.C.
4. Conclusions The recovery and recrystallization processes in the deformed orthorhombic martensite in the Ti,,Pd,, alloy were studied by electron microscopy, hardness tests and electrical resistivity measurements, and the following conclusions were obtained. (1) The recrystallization in the martensitic state was clearly observed on electron microscopic level. Recovery and recrystallization occurred concurrently under isothermal annealing at 550°C (27°C below the A, temperature), and the recrystallized martensites appeared surrounded by small recovered martensite grains, after a long anneal of 100 h. (2) The recrystallized martensite was clearly strain-free, without the lattice invariant shear charac-
Donkersloot and J.H.N. Van Vucht, J. Less-Common Metals 20 (1970) 83. [2] N.M. Matveeva, Y.K. Kovneristyi, A.S. Savinov, V.P.Sivokha and V.N. Khachin, in: Proc. ICOMAT-82, eds. L. Delaey and M. Chandrasekaran (Leuven, Belgium, 1982) p. 249. 131K. Enami, Y. Kitano and K. Horii, Mater. Res. Sot. Proc. 9 (1989) 117. [41P.G. Lindquist and C.M. Wayman, in: Proc. Int. Conf. on Engineering Aspects of Shape Memory Alloys (ButterworthHeineman, London, 1990) p.58. [51K. Otsuka, K. Oda, Y. Ueno, Min Piao, T. Ueki and H. Horikawa, Ser. Metall. Mater. 29 (1993) 1355. [61 D. Golberg, Ya Xu, Y. Murakami, S. Morito, K. Otsuka, T. Ueki and H. Horikawa, Ser. Metall. Mater. 30 (1994) 1349. 171D. Golberg, Ya Xu, Y. Murakami, K. Otsuka, T. Ueki, H. Horikawa, Mater. L&t. 22 (1995) 241. [81 Ya Xu, S. Shimizu, Y. Suzuki, K. Otsuka, T. Ueki and K. Mitose, Acta Metall. Mater., in press (1996). 191M. Tokizane, N. Matsumura, K. Tsuzaki, T. Maki and 1. Tamura, Metall. Trans. 13 A (1982) 1379.
Y. Xu et al./Materials [IO] M. Enomoto and E. Furubayashi, Mater. Sci. Eng. 24 (1976) 123. [l I] J.H. Yang and C.M. Wayman, Intermetallics 2 (1994) 11 I, 121. [I21 D. Schryvers and Y. Ma, J. Alloys Compounds 221(1995) 227. [ 131 Min Piao, K. Otsuka, S. Miyazaki and H. Horikawa, Mater. Trans. JIM 34 (1993) 19. [ 141 F.J. Humphreys, Acta Metall. Mater. 25 (1977) 1323. [ 151 A. Betget, P.J. Wilbrandt, F. Ernst, U. Klement and P. Haasen. Prog. Mater. Sci. 32 (1988) 1.
Letters 30 (19971 189-197
197
[ 161 H. Hu, Recovery and recrystallization of metals, ed. L. Hirnmel (Interscience, New York, 1963) p. 311. [17] H.M. Chan and F.J. Humphreys, Proc. Electron Microsc. Sot. Am. (1984) 476. [ 181 D. Golberg, Ya Xu, Y. Murakami, K. Otsuka, T. Ueki and H. Horkawa, lntennetallics 3 (1995) 35. [ 191 V.N. Eremenko and T.D. Shtepa, Poroshkovaya Metallurgiya 111 (1972) 75. [20] Y. Shugo and T. Honma, J. Institute of Mineral-Dressing and Smelting in Tohoku Univ. 43 (1987) 128.
ELSEVIER
Materials Letters 30 (1997) 189-197
Recovery and recrystallization in the Ti 5O Pd 5Omartensite Ya Xu a, K. Otsuka a**, E. Furubayashi
b, T. Ueki ‘, K. Mitose
’
a Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan h Department of Materials Engineering, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan ’ Yokohama R & D Laboratory, The Furukawa Electric Co. Ltd., 2-4-3 Okano, Nishi-ku, Yokohama 220, Japan
Received 5 August 1996; accepted 8 August 1996
Abstract The recrystallization process in the martensitic state was clearly demonstrated for the cold-rolled Ti,,Pd,, high temperature shape memory alloy by electron microscopy and electron diffraction, along with the change in hardness with annealing. As a result, the martensite crystals were created by the diffusional process, which had the same orthorhombic structure as the transformed one, and which had no lattice invariant shear characteristic of the martensitic transformation. This is probably the first observation of the recrystallization process in martensite on electron microscopic scale. These results are briefly described. Keywords:
Recovery; Recrystallization; Martensite; Ti,,Pd,,
1. Introduction The Ti-Pd alloy system has attracted significant attention as a high temperature shape memory ahoy in recent years, and many researches have been done on this alloy system [l-7]. In the course of the investigations, a systematic research on recovery and recrystallization processes of several Ti-Pd-Ni alloys was recently done by the present authors [S]. As a result, a close relationship was found between the recovery-recrystallization and the reverse transformation of the alloys. That is, for isochronal annealing, the recovery-recrystallization was found to occur in the parent after the reverse transformation, since diffusion is much faster in the parent phase
^ Corresponding author. Tel: 81-298-53-5294 Fax: 81-298-557440. 00167-577X/97/$17.00 PI1 SOl67-577X(96)00203-0
compared to that in the martensite. Thus, it was shown in the present alloy system that the recovery-recrystallization is controlled by the reverse transformation. Then, we may argue whether recrystallization occurs in the martensitic state or not. Actually we found recrystallization in the martensitic state for a long-time isothermal annealing by choosing a proper annealing temperature and composition (Ti,,Pd,,). The purpose of the present paper is to report these results. Since diffusion is very slow in the martensite, the study of recrystallization in the martensite is very time consuming, but it is a good system to study the recrystallization process step by step, since recrystallization occurs very slowly. In the literature, there are two reports, which concern with recrystallization in ferrous martensites. Tokizane et al. [9] observed recrystallization in the lath martensite of Fe-0.16C-
Copyright 0 1997 Elsevier Science B.V. All rights reserved.
Y. Xu et al./Materials
t~~~~.~.~~~~~.~.~.~~1
0
10
20
30
40
50
60
70
80
Letters 30 (1997) 189-197
90 100
Amealiug time I hour Fig. 1. Micro-hardness changes after 5OO”C, 550°C and 650°C respectively.
isothermal
annealing
at
0.6Mn-Cr-0.2Mo and Fe-0.2C-0.45Mn low carbon steels, and reported that although the as-quenched lath martensite hardly recrystallized on tempering, the deformed lath martensite easily recrystallized on subsequent tempering. However, they observed the recrystallization only by optical microscopy, and they did not describe the recrystallization in detail. Enomoto and Furubayashi [lo] observed the recrystallization of the bee phase in the two-phase (a + ~1 region of a Fe-7Ni ahoy. However, it is not clear whether the bee phase is a martensite or not. Instead, it is more likely that the bee phase is an equilibrium (Y phase. Meanwhile, there is an interesting report in an opposite sense on the Ni-Al martensite with Ll,, structure [ 11,121. In this alloy system, the metastable martensite is obtained by rapid quenching from the P-phase (B2) region. Upon heating, however, the martensite does not revert to the l3 parent phase, but it transforms into a stable Ni, Al, phase. Thus, the shape memory effect is disturbed by the avoidance of the reverse transformation. By considering these situations, the present paper may be the first report on recrystallization in martensite on an electron microscopic scale.
2. Experimental
thick sheets with a 24.2-29.0% reduction in thickness at room temperature. Because of the ductility limit of the Ti50Pd50 alloy, a higher cold-rolling reduction rate was not available. The specimens for hardness tests, electrical resistivity measurements and TEM observations were spark cut, mechanically polished and then heat-treated at various temperatures. The specimens were isothermally annealed in fused quartz capsules, and the preparation was done in the same way as in Ref. [8]. The hardness measurements were carried out using a MVE-K micro-Viekcrs tester. The load was 1 kgf and the loading time was 20 s. Samples for electrical resistivity measurement were cut into rods about 1 X 2 X 30 mm. Electrical resistivity was measured during isothermal annealing and heating and cooling processes at a rate of about S”C/min in vacuum using the four-probe method with a constant current of 0.084 A. Microstructures of annealed bulk samples were observed at ambient
t
4 I
0
100
I
I,
I
200
300
I
I
400
500
600
700
Temperature/C
procedure
Ingots of Ti,,Pd,, alloy were plasma melted by using 99.99% Ti and 99.99% Pd in a water cooled copper mould. The ingots were remelted four times for homogenization. All the alloys were homogenized at 1000°C for 5 h, then were hot rolled at 800°C and subsequently cold rolled into 0.5-l mm
,I,,,
-1st cycle -----2nd cycle 0
100
200
300
400
500
600
_
700
Temperature /C Fig. 2. Electrical resistivity versus temperature plots: (a) after annealing at 550°C for 42 h; (b) after annealing at 550°C for 100 h.
Y. Xu et al. / Materials Letters 30 (1997) 189-197 Table 1 Martensitic
transformation
191
temperatures
of the Ti,,Pd,,
alloy
(“0
cold rolled state (CR) a CR + 550°C 42 h CR + 550°C 100 h 1st cycle CR + 550°C 100 h 2nd cycle
0
,
I
100
200
I,
300
I,
400
I,
500
I,
600
700
’ After Ref. [8] measured
540 549 547
526 537 540
517 590 592
589 596 597
546
535.5
581.5
589
by DSC.
Temperature /‘c Fig. 3. Electrical resistivity versus temperature plots: (1) heating at S”C/min; (2) cooling at 5°C /min after annealing at 500°C for 2 h.
temperature microscope.
by a JEOL 200CX transmission
electron
3. Results and discussion 3.1. Hardness ments
and
electrical
resistivity
measure-
Fig. 1 shows the results of micro-Vickers hardness test of cold-rolled Ti,,Pd,, alloy isothermally
annealed at 500°C 550°C and 650°C respectively. The A, (reverse transformation start temperature) and A, (reverse transformation finish temperature) of the cold-rolled Ti50Pd50 alloy were 577°C and 589°C respectively [8]. Thus, the annealings at 500°C and 550°C correspond to the annealing in the martensitic state, and 650°C corresponds to that in the parent state. We notice from the hardness curves of Fig. 1 that a pronounced hardness drop occurred in the early annealing period of 2 h for annealing at 500°C and 550°C which is in the martensitic state. This indicates that some restoration process took place. Further softening occurred after annealing at
192
Y. Xu et al./Materials
500°C and 550°C for about 50 h; especially for 550°C annealing the hardness decreases to almost the same value as that after annealing at 650°C for 20 h, which indicates that further restoration process took place. These will be discussed later in combination with the results of resistivity measurements and microstructure observations. We also see that a complete softening occurred after a short-time (about 1 min) annealing at 650°C which indicates that the reverse transformation affects the recrystallization process greatly, as we reported previously. The electrical resistivity was measured during annealing at 550°C. No obvious change, however, was observed even after annealing for 100 h. This is probably because the scattering of electrons by phonons is so strong at high temperatures that the
Fig. 5. Eletron micrograph
taken after annealing
Letters 30 (1997) 189-197
lattice restoration by recrystallization does not contribute much to the change in resistivity. Fig. 2a and 2b show the temperature dependence of the electrical resistivity for the specimens after annealing at 550°C for 42 h and 100 h, respectively. The specimens were heated up to 650°C at S”C/min, and then cooled to room temperature at the same rate. A square hysteresis loop which corresponds to the martensitic transformation was observed. The reverse transformation temperatures were determined as A, = 590°C A,. = 596°C after 42 h annealing, and A, = 592°C A, = 597°C after 100 h annealing, respectively. We see that the reverse transformation temperatures do not change during annealing at 550°C. Meanwhile, when the second heating cycle was made for the same specimen after annealing at 550°C for
at 500°C for 100 h. A, B, C, D, E and F indicate the recrystallized
grains.
Y. Xu et al. /Materials
Letters 30 (1997) 189-397
Fig. 6. (a) Bright-field image of the region containing grains A and B shown in Fig. 5, observed by tilting the specimen; (b) SAD patterns taken from the boundary region between A and B. Two sets of SAD pattern coming from A and B respectively, were confirmed; (c) DF image which shows grain B by using an 721, reflection shown in (b); (d) DF image which shows grain A by using a 212, reflection shown in 03).
Fig. 7. Electron micrograph
exhibiting
typical recrystallized
grains, after annealing
at 550°C for 100 h
194
Y. Xu et aI./Materials Letters 30 (19971 189-197
100 h, the reverse transformation temperatures were determined as A: = 5815”C, A; = 589°C as shown in Fig. 2b. The martensitic transformation temperatures measured after annealing at different temperatures and for different times are summarized in Table 1. We see that there is a decrease of the reverse transformation temperatures in the second cycle compared with the result for the first cycle. Since the increase of the reverse transformation temperature reflects the cold working effect [7,13], this decrease is considered to be related to some restoration process occurred during the first heating process up to 650°C. Thus, we concluded that even after annealing at 550°C for 100 h, the recrystallization process was still not finished, and further recrystallization occurred upon the following heating up to 650°C which is above A,. This also supports the above results of the hardness measurement that the recrystallization process in martensite is much slow, and the reverse transformation can promote the recrystallization process greatly. Some difference between the resistivities measured by the first and the second cycle in the low temperature range was also observed in Fig. 2b. This is attributed to an experimental error in controlling the cooling rate accurately in the low temperature range. On the other hand, the electrical resistivity changes during the heating and cooling processes after annealing at 550°C for 2 h were measured as shown in Fig. 3. Comparing curve (1) heating with curve (2) for cooling after annealing in Fig. 3, we see that an obvious decrease occurred after annealing for 2 h, which also indicates the occurrence of some restoration.
early annealing period in the martensitic state is ascribed to a recovery process due to climb motion of dislocations, etc. However, some subgrains with a diameter less than 0.5 pm were observed after annealing at 500°C for 20 h as shown in Fig. 4d. This indicates that a subgrain formation process occurred with the increase of the annealing time. Fig. 5 shows a typical microstructure after annealing at 500°C for 100 h. We find that some new recrystallized grains with a diameter greater than I km were formed as
3.2. TEM obseruations The microstructures in the cold-rolled state and those after isothermal annealing at 500°C and 550°C for various times were observed at room temperature. Fig. 4 shows such examples in cold-rolled state and after annealing at 500°C for I, 5 and 20 h, respectively. We find that the microstructure in cold-rolled state shows a typically deformed structure, which is inhomogeneous, as reported in our previous work [8]. No formation of subgrain structure was observed after annealing at 500°C for 5 h. Thus, compared with the results of the hardness and resistivity measurements, the hardness drop in the
Fig. 8. (a) Magnified image of the retangular region indicated in Fig. 7; (b) SAD pattern taken from the area indicated in (a), which shows a (I I I),,, twin pattern in [ilo],,, zone; (c) DF image taken from the spot 002, indicated in (b).
Y. Xu et al./Materials
marked by A, B, C, D, E and F in Fig. 5, which were confirmed by following observations. Fig. 6a shows the region containing grains A and B observed in Fig. 5, in different orientations after tilting the specimen. The selected area diffraction (SAD) patterns taken from the boundary region between A and B is shown in Fig. 6b. Fig. 6c and 6d show the dark-field (DF) images taken from the spots 721, and 212,, respectively. We see that these two sets of pattern represent gains A and B, respectively. Comparing Fig. 6a with Fig. 6d, some original subgrain boundaries in recrystallized grain A are still faintly visible after annealing at 500°C for 100 h, as indicated by an arrow in Fig. 6a. Thus, it can be considered that the recovery progressed by the annihilation and rearrangement of dislocations, and then subgrain growth until a critical size for recrystallized nucleus being reached. Further growth of the recrystallized nucleus will take place by the migration of the high-angle boundaries as a result of high mobility of high-angle boundaries (Fig. 5C and 5D). The mechanism of
Letters 30 (1997) 189-197
195
subgrain growth in the Ti,,Pd,, alloy, that is whether by subgrain boundary migration [ 14,151 or by subgrain rotation and coalescence [16,17] is not determined yet in the present work. Further researches on this are in progress. With the increase of annealing temperature, the recrystallization in the martensitic state in the Ti,OPd,, alloy progressed significantly. A partially recrystallized structure was observed after annealing at 550°C for 100 h, as typically shown in Fig. 7. This confirmed the result of electrical resistivity measurement described above that the recrystallization is still not finished after annealing at 550°C for 100 h. Some twins, dislocations and faults were observed within the recrystallized regions. Since the formation of twins was observed rarely during the recovery process (Fig. 4) and at the very onset of formation of recrystallized grains (Fig. 5), we see that these twins formed during the growth process of recrystallized grains. Two kinds of annealing twins were confirmed in the Ti,,Pd,, alloy. One is the (ill),,, twin
Fig. 9. (a) Electron micrograph of a recrystallized structure after annealing at 550°C for 100 h; (b) magnified image of the area indicated in (a); (c) SAD pattern taken from the area indicated in (b), which shows a (lOI),,, twin pattern in [OlO],,, zone; (d) DF image taken from the spot 702, indicated in cc).
196
Y. Xu et al./Materials Letters 30 (19971 189-197
shown in Fig. 8 which is a magnified image of the rectangular region indicated in Fig. 7. The SAD pattern is shown in Fig. 8b. Fig. SC shows the DF image taken from the 002, spot indicated in Fig. 8b, which was confirmed as the (11 I),,, twin in the zone. This twinning mode is the same as the m,,, lattice invariant shear upon martensitic transformation [3,18]. We see that the twins are very thin, and they join the other thin twins in the neighboring grains, as if the former induced the latter, as indicated by the arrows in Fig. 7. Thus, we suggest that this kind of twins is introduced by the deformation twinning upon quenching operation after the annealing process. Another one is the (lol),,, twin as shown in Fig. 9, in which the twin boundaries are not straight. We believe these are introduced by a diffusional process. Some dislocations were also observed within the recrystallized region. The dislocations in martensite will be discussed in detail in another paper. The martensites are usually considered as metastable phases rather than equilibrium phases. However, the fact that the martensite structure was obtained by a recrystallization process may mean that the martensite is an equilibrium phase in the present case. Since the phase diagram of the Ti-Pd system is not well established 119,201, the above experimental results need to be taken into account, when the phase diagram is reexamined. A study is being made in this direction.
teristic of the martensitic transformation. This is reasonable, since the martensite was formed by a diffusional process. (3) A pronounced hardness drop (which is smaller in magnitude compared to that in the parent phase) was observed in the early annealing period of 2 h, even in the martensitic state. This was ascribed to a recovery process due to climb motion of dislocations, etc. (4) Two types of twins were sometimes observed: { 11 l},,g twins and {IOl},,, twins. The former twinning mode is the same as the lattice invariant shear upon martensitic transformation, while the latter is not. The former twins were suggested to be introduced by the deformation twinning upon quenching operation after the annealing process.
Acknowledgements The authors wish to thank Dr. T. Kainuma of the National Research Institute for Metals for the use of a micro-Vickers hardness tester. The present work was supported by the Grant-in-Aid for General Scientific Research (Shiken B, 1995-7) from the Ministry of Education, Science and Culture of Japan.
References [I] H.C.
4. Conclusions The recovery and recrystallization processes in the deformed orthorhombic martensite in the Ti,,Pd,, alloy were studied by electron microscopy, hardness tests and electrical resistivity measurements, and the following conclusions were obtained. (1) The recrystallization in the martensitic state was clearly observed on electron microscopic level. Recovery and recrystallization occurred concurrently under isothermal annealing at 550°C (27°C below the A, temperature), and the recrystallized martensites appeared surrounded by small recovered martensite grains, after a long anneal of 100 h. (2) The recrystallized martensite was clearly strain-free, without the lattice invariant shear charac-
Donkersloot and J.H.N. Van Vucht, J. Less-Common Metals 20 (1970) 83. [2] N.M. Matveeva, Y.K. Kovneristyi, A.S. Savinov, V.P.Sivokha and V.N. Khachin, in: Proc. ICOMAT-82, eds. L. Delaey and M. Chandrasekaran (Leuven, Belgium, 1982) p. 249. 131K. Enami, Y. Kitano and K. Horii, Mater. Res. Sot. Proc. 9 (1989) 117. [41P.G. Lindquist and C.M. Wayman, in: Proc. Int. Conf. on Engineering Aspects of Shape Memory Alloys (ButterworthHeineman, London, 1990) p.58. [51K. Otsuka, K. Oda, Y. Ueno, Min Piao, T. Ueki and H. Horikawa, Ser. Metall. Mater. 29 (1993) 1355. [61 D. Golberg, Ya Xu, Y. Murakami, S. Morito, K. Otsuka, T. Ueki and H. Horikawa, Ser. Metall. Mater. 30 (1994) 1349. 171D. Golberg, Ya Xu, Y. Murakami, K. Otsuka, T. Ueki, H. Horikawa, Mater. L&t. 22 (1995) 241. [81 Ya Xu, S. Shimizu, Y. Suzuki, K. Otsuka, T. Ueki and K. Mitose, Acta Metall. Mater., in press (1996). 191M. Tokizane, N. Matsumura, K. Tsuzaki, T. Maki and 1. Tamura, Metall. Trans. 13 A (1982) 1379.
Y. Xu et al./Materials [IO] M. Enomoto and E. Furubayashi, Mater. Sci. Eng. 24 (1976) 123. [l I] J.H. Yang and C.M. Wayman, Intermetallics 2 (1994) 11 I, 121. [I21 D. Schryvers and Y. Ma, J. Alloys Compounds 221(1995) 227. [ 131 Min Piao, K. Otsuka, S. Miyazaki and H. Horikawa, Mater. Trans. JIM 34 (1993) 19. [ 141 F.J. Humphreys, Acta Metall. Mater. 25 (1977) 1323. [ 151 A. Betget, P.J. Wilbrandt, F. Ernst, U. Klement and P. Haasen. Prog. Mater. Sci. 32 (1988) 1.
Letters 30 (19971 189-197
197
[ 161 H. Hu, Recovery and recrystallization of metals, ed. L. Hirnmel (Interscience, New York, 1963) p. 311. [17] H.M. Chan and F.J. Humphreys, Proc. Electron Microsc. Sot. Am. (1984) 476. [ 181 D. Golberg, Ya Xu, Y. Murakami, K. Otsuka, T. Ueki and H. Horkawa, lntennetallics 3 (1995) 35. [ 191 V.N. Eremenko and T.D. Shtepa, Poroshkovaya Metallurgiya 111 (1972) 75. [20] Y. Shugo and T. Honma, J. Institute of Mineral-Dressing and Smelting in Tohoku Univ. 43 (1987) 128.