hot-rolling at 1200 C followed by cold-rolling. Co-(0â14) at%Al and ... cold-rolling and subsequent annealing at 900 C for 3 minutes ... (γ Co) + CoAl. Magnetic ...
Materials Transactions, Vol. 44, No. 12 (2003) pp. 2732 to 2735 #2003 The Japan Institute of Metals RAPID PUBLICATION
Shape Memory Effect Associated with FCC–HCP Martensitic Transformation in Co-Al Alloys Toshihiro Omori* 1 , Yuji Sutou1 , Katsunari Oikawa2 , Ryosuke Kainuma1 and Kiyohito Ishida1 1 2
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan National Institute of Advanced Industrial Science and Technology, Tohoku Center, Sendai 983-8551, Japan
The martensitic transformation and shape memory (SM) effect in Co-Al alloys containing 0–16 at%Al were investigated by differential scanning calorimetry (DSC) and bending tests. It was found that the martensitic transformation temperatures decrease and the thermal hystereses increase with increasing Al content. It was also found that an incomplete SM effect occurring in pure Co can be enhanced by the addition of Al over 4 at% and that Co-Al alloys containing Al over 10 at% show an excellent SM effect. Co-Al SM alloys possessing high reverse transformation temperatures over 200 C and martensitic transformation in the ferromagnetic state show promise as a new type of SM alloys. (Received September 17, 2003; Accepted October 15, 2003) Keywords: shape memory effect, " martensite, martensitic transformation temperature, cobalt-aluminum, high-temperature shape memory alloy
1.
Introduction
Since shape memory (SM) effect was first reported in a Au-47.5 at%Cd alloy,1) many sorts of SM alloy systems have been studied. Among them, extensive work has been devoted to ordered bcc Hume-Rothery alloys and Ni-Ti based alloys accompanied by thermoelastic martensite transformation such as Cu-Al-Ni, Cu-Zn-Al, Cu-Al-Mn, Ni-Al-based and Ni-Ti-Cu alloys.2–5) In particular, Ni-Ti based alloys have been put to various practical uses such as pipe couplings, medical materials, etc.,2,6) due to their excellent SM properties. The maximum Af temperature of conventional Ni-Ti alloys is limited to about 120 C, although high-temperature SM alloys are required in a lot of fields. Some attempts to develop high-temperature SM alloys have been performed for Ti-NiPd, Ti-Ni-(Hf or Zr), Cu-Al-Ni-Ti-Mn, Ni-Al, Cu-Zr, Zr-CuCo and Ni-Mn-Ga alloys, with limited success due to poor SM and mechanical properties, poor thermal stability or high cost.7) It is known that martensitic transformation takes place in pure Co from the phase (fcc) to the " phase (hcp) and that it proceeds by the movement of a/6h112ifcc Shockley partial dislocations. The martensitic transformation starting temperature (Ms ¼ 400{419 C) and the reverse transformation starting temperature (As ¼ 429{445 C) of pure Co8) depend on the purity, the cooling/heating rates and the grain size. It has been reported that polycrystalline Co and Co-Ni alloys show no or only slight SM effects,9–11) while the SM effect can be obtained in Co-32 mass%Ni single crystal only in the [001]fcc compressive direction with As ¼ 156 C.9) Although Koval has reported the SM effect of Co-Al, Ge, Mo, Zr alloys where the shape recovery is less than 40%,12) there have been no systematic studies on the SM effect in Co-based alloys except Co-Ni. It has been widely accepted that an ordered structure of the parent phase is one of the essential conditions for a perfect SM effect, and the addition of Al is thus supposed to be favorable for the improvement of the SM *Graduate
Student, Tohoku University
effect of Co due to the promotion of L12 ordering. Moreover, the solubility (about 16 at%) of Al in Co is relatively high and the addition of Al does not reduce the ductility. Recently, the present authors have found that polycrystalline Co-Al alloys exhibit an excellent SM effect due to martensitic transformation from fcc to hcp. The Co-Al alloys, therefore, show promise as high-temperature SM alloys due to their high transformation temperatures and high thermal stability. The purpose of this paper is to report the martensitic transformation and SM effect in a polycrystalline Co-Al alloy system in the range of Co-(0–16) at%Al. 2.
Experimental
Pure Co and Co-X at%Al (X ¼ 4, 8, 10, 14 and 16) binary alloys ingots were prepared in an induction furnace under an argon atmosphere from pure cobalt (99.9%) and aluminum (99.7%). Sheet specimens were obtained from the ingots by hot-rolling at 1200 C followed by cold-rolling. Co-(0–14) at%Al and Co-16 at%Al specimens were solution treated at 1200 C for 1 hour and 1300 C for 15 minutes in the singlephase region, respectively, and then quenched in water. The microstructures were observed using an optical microscope (OM), a solution of 75% hydrochloric acid and 25% nitric acid being used as an etchant. The martensitic transformation temperatures (Ms , Mf , As and Af ) were measured by differential scanning calorimetry (DSC) at heating and cooling rates of 10 C/min. These temperatures were defined as points of intersection between the base line and the tangent of maximum or minimum inclination in the DSC curves. The SM effect was evaluated by bending a rectangular specimen of 0:24 4 50 mm3 into a round shape at room temperature, unloading it (surface strain "c ) and then heating it up to 800 C (surface strain "h ). The surface strain is defined as " ¼ t=2r 100, where t is the thickness of the specimen and r is the radius of curvature. The shape recovery RSME and recoverable strain "SME were evaluated by RSME ¼ ð"c "h Þ="c 100 and "SME ¼ "c "h , respectively. SM testing was also carried out for a Co-10 at%Al specimen subjected to
Shape Memory Effect Associated with FCC–HCP Martensitic Transformation in Co-Al Alloys
1600
2733
(a)
Temperature, T / °C
1400 1495
1000
(γ Co) + CoAl
800
Magnetic Transition
600 400
422 (ε Co)
200 600
100µm
(γ Co)
1200 1121
(b) 500 Temperature, T / °C
Fig. 1 OM micrograph of Co-14 at%Al at room temperature. The typical " martensite structure is observed.
a thermomechanical treatment (TMT) which consists of 10% cold-rolling and subsequent annealing at 900 C for 3 minutes after solution treatment at 1200 C. 3.
Results and Discussion
400 300
Af As
200
Ms Mf
100
3.1 Martensitic transformation and microstructure Figure 1 shows the microstructure of Co-14 at%Al alloy at room temperature. " martensites are observed, which are considered to be formed by the motion of ða=6Þh112ifcc -type Shockley partial dislocations. It seems that the morphologies of the " martensites are similar to those observed in Co and Co-Ni alloys.13,14) A small amount of phase was also detected by X-ray diffraction and TEM observation. All the specimens in this study included some residual phase at room temperature, although the Mf temperatures of all the specimens were located above room temperature. It is well known that the residual austenite phase exists in hcp martensite phase of Fe-Mn-based alloys, Co-based alloys and so on.11,15–17) Figure 2 shows (a) the phase diagram in Co-Al system18) and (b) the martensitic transformation temperatures of supersaturated solid solutions of Co-Al alloys. The addition of Al gradually lowers the magnetic transition temperature according to the phase diagram shown in Fig. 2(a), and that of Co-14 at%Al alloy is 689 C, which was measured in the present study. The martensitic transformation temperatures decrease with increasing Al content. Both of these trans-
0
0
5
10 15 Al Content (at%)
20
Fig. 2 (a) Phase diagram in Co-rich portion of Co-Al system15) and (b) martensitic transformation temperatures.
formation temperatures are high compared with those of conventional SM alloys. The martensitic transformation temperatures, the transformation temperature hystereses TTH (Af Ms and As Mf ) and the transformation temperature intervals TTI (Ms Mf and As Af ) are listed in Table 1. The TTH of pure Co is quite small (Af Ms ¼ 57 C and As Mf ¼ 67 C) compared with that of other nonthermoelastic martensite transformations. While the TTI does not strongly depend on Al content, the TTH drastically increases with increasing Al content. The DSC heating and cooling curves of Co-8 at%Al water-quenched (WQ) and aircooled (AC) from 1200 C are shown in Fig. 3. The AC specimen shows a single peak of the reverse transformation in heating, while the WQ specimen shows a reverse transformation peak at around 300 C and an additional small peak
Table 1 Martensitic transformation temperatures (Ms , Mf , As and Af ), transformation temperature hystereses (Af Ms and As Mf ) and transformation temperature intervals (Ms Mf and Af As ) in pure Co and Co-Al alloys. Martensitic transformation temperature/ C
Al (at%) 0 4
Hysteresis/ C
Interval/ C
Ms
Mf
As
Af
Af Ms
As Mf
Ms Mf
Af As
404 283
372 246
439 355
461 386
57 103
67 109
32 37
22 31
8
182
151
287
311
129
136
31
24
10
162
134
264
283
121
130
28
19
14
112
75
236
260
148
161
37
24
16
100
55
228
253
153
173
45
25
T. Omori, Y. Sutou, K. Oikawa, R. Kainuma and K. Ishida
Shape Recovery, RSME (%)
Heat Flow (arb. unit) exothermic
2.0
100
(a) WQ
(b) AC
TMT
80
1.5
60
1.0 40
TMT RSME
20
εSME
0
100
200 300 400 500 Temperature, T / °C
600
Fig. 3 Heating and cooling DSC curves of (a) water-quenched and (b) aircooled Co-8 at%Al specimens.
at around 430 C. In cooling, only one peak of forward transformation appears in both specimens. The sum of the latent heat of the two peaks during heating in the WQ specimen is almost equal to that in AC specimen, and the smaller peak was not seen in subsequent heating cycles. From the X-ray diffraction analysis at high temperatures, it is considered that a part of martensite is stabilized by quenching and that the small peak corresponds to a reverse transformation of the stabilized martensite. A small peak at higher temperature was also detected in Co-10 at%Al. Although affecting the transformation behavior, the cooling process hardly influences the SM properties of the specimens. 3.2 Shape memory effect Figure 4 shows the shape recovery RSME of a Co-14 at%Al sheet as a function of heating temperature. It is seen that the deformed shape drastically recovers in the temperature range of 200 C to 300 C, which corresponds to the reverse transformation temperatures, As ¼ 236 C and Af ¼ 260 C,
100 Shape Recovery, RSME (%)
90 80 70 60 50 40 30 20 10 0
0
200
400 600 800 Temperature, T / °C
1000
Fig. 4 Shape recovery of Co-14 at%Al gradually heated from 100 C up to 800 C. Applied surface strain was 1.2%.
0
0
5
ASS = 1.2 % ASS = 2.0 % ASS = 1.2 % ASS = 2.0 %
10 15 Al Content (at%)
0.5
20
Recovered Strain, εSME (%)
2734
0
Fig. 5 Shape recovery ratio "SEM and recovered strain RSME of pure Co and Co-Al alloys for the applied surface strains of 1.2% and 2.0% obtained by bending shape memory tests. Shape recovery of TMT specimen is also shown.
as listed in Table 1. This result means that the shape recovery is due to the martensitic reverse transformation and that CoAl is a high-temperature SM alloy. The shape recovery and the recovered strain "SME evaluated by the bending test are plotted against the Al content in Fig. 5. For an applied surface strain (ASS) of 1.2%, pure Co exhibits a poor SM effect of RSME ¼ 40%, while the addition of Al drastically improves the SM effect. The SM effect is almost independent of the Al content up to 4 at%Al, but is considerably improved by the addition of Al over 4 to 10 at%. Over 10 at%Al, the SM effect reaches about 80% and becomes steady again. The tendency of the RSME for ASS ¼ 2:0% is the same as that for ASS ¼ 1:2%, while the RSME is always lower than that of the ASS ¼ 1:2% specimens. In the case of SM strain "SME , the "SME for ASS ¼ 1:2% and 2.0%, which are almost the same in pure Co, increase with increasing Al content and the "SME of ASS ¼ 2:0% becomes larger than that of ASS ¼ 1:2%. These results suggest that the maximal recoverable strain, namely, the maximum of the SM ability in the materials, is limited to about 0.3% in pure Co, while it increases with increasing Al content. An "SME of 1.1% was obtained in Co-16 at%Al for ASS ¼ 2:0%. It is known that TMT is effective to improve the SM effect in Fe-Mn-Si alloys.19) As shown in Fig. 5, the TMT also improves the SM effect in the Co-Al alloys. It has been proposed that alloys satisfying the following requirements exhibit shape memory effects with a perfect reversibility:20,21) i) The martensitic transformation is thermoelastic. ii) The structure of the parent phase is ordered or the lattice deformation associated with the transformation is small. iii) The lattice invariant strains are caused not by dislocations but by twins. It is known, however, that the perfect shape memory effect can be obtained in Fe-Mn-Si alloys, which have a disordered structure with non-thermoelastic martensite transformation under certain conditions.19,22) When discussing the SM effect in the Co-Al alloys, it is helpful to compare these alloys with
Shape Memory Effect Associated with FCC–HCP Martensitic Transformation in Co-Al Alloys
Fe-Mn-Si alloys since these SM alloys are similar in the nonthermoelastic martensite transformation from disordered fcc to hcp with low stacking fault energy. Several explanations have been proposed for the origin of the SM effect in the FeMn-Si alloys. One of them is that the existence of internal stress causes backward stress on the reversible movement of Shockley partial dislocations upon reverse transformation,23) and another is that the SM effect is due to the short range ordered structure or small coherent ordered particles.24) Although both explanations might be applied to Co-Al alloys, the report by Bradley and Seager25) and Edwards,26) in which there is a metastable phase thought to be a Co3 Al L21 ordered structure, supports the latter. It should be noted that even though they are deformed at a temperature below Mf with only a slight amount of retained fcc phase, the Co-Al alloys show a good SM effect, while Fe-Mn-Si alloys exhibit an excellent SM effect associated only with stress-induced martensite. This fact suggests that there is an essential difference in the mechanism of the SM effect between the Co-Al and the Fe-Mn-Si alloys. The deformation in the martensite state may be important to obtain the excellent SM effect and further investigations such as the observation of the microstructural change by deformation are necessary to understand the mechanism. From the results obtained in the present study, the Co-Al SM alloys show promise as high-temperature SM alloys. Other characteristic features of Co-Al SM alloys are that the martensitic transformation occurs in a ferromagnetic state and the Co-Al can be easily cold-worked in contrast to other ordered ferromagnetic SM alloys such as Ni2 MnGa. Thus, the Co-Al SM alloys also have potential as ferromagnetic SM alloys with high Curie temperatures above 600 C and high As temperatures above 200 C. 4.
Conclusion
(fcc)/"(hcp) transformations of Co-Al alloys in the range of 0-16 at%Al were investigated. The martensitic transformation temperatures of these alloys were found to decrease with increasing Al content, whereas the transformation temperature hysteresis was observed to increases. With regard to stabilized martensite, an additional peak was observed in the DSC heating curves of as-quenched Co8 at%Al and Co-10 at%Al specimens, although it hardly affects the recoverable strain due to the SM effect. The SM effect is enhanced by the addition of Al over 4 at% and Co-Al alloys over 10 at% exhibit an excellent SM effect. The shape recovery of "SME ¼ 1:1% and RSME ¼ 80% was obtained in
2735
Co-16 at%Al. The TMT is also effective for improving the SM effect. Since the SM effect can be obtained at over 200 C, Co-Al SM alloys show promise as high-temperature SM alloys. Acknowledgements This work was supported by the Grant-in-aids for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. REFERENCES 1) L. C. Chang and T. A. Read: Trans. AIME 191 (1951) 47-52. 2) L. Delaey, R. V. Krishnan, H. Tas and H. Warlimont: J. Mater. Sci. 9 (1974) 1521-1535. 3) S. Miyazaki and K. Otsuka: ISIJ Int. 29 (1989) 353-377. 4) Y. Sutou, T. Omori, R. Kainuma, N. Ono and K. Ishida: Metall. Mater. Trans. A 33A (2002) 2817-2824. 5) K. Enami and S. Nenno: Metall. Trans. 2 (1971) 1487-1490. 6) J. Van Humbeeck: Mater. Sci. Eng. A A273-275 (1999) 134-148. 7) J. Van Humbeeck: J. Eng. Mater. Tech. 121 (1999) 98-101. 8) T. Nishizawa and K. Ishida: Bull. Alloy Phase Diagram 4 (1983) 387390. 9) W. M. Zhou, Y. Liu, B. H. Jiang, X. Qi and Y. N. Liu: Appl. Phys. Lett. 82 (2003) 760-762. 10) A. Nagasawa: Phys. Status Solidi A 8 (1971) 531-538. 11) H. C. Shin, S. H. Lee, J. H. Jun and C. S. Choi: Mater. Sci. Technolo. 18 (2002) 429-432. 12) Yu. N. Koval: Mater. Sci. Forum 327-328 (2000) 271-278. 13) H. Bibring, F. Sebilleau and C. Bu¨ckle: J. Inst. Metals 87 (1957) 71-76. 14) S. Takauchi and T. Homma: Sci. Rep. RITU A 9 (1957) 492-507. 15) Z. Nishiyama: Martensitic Transformation, (Academic Press, New York, 1978) 16) H. Otsuka, H. Yamada, T. Maruyama, H. Tanahashi, S. Matsuda and M. Murakami: ISIJ Int. 30 (1990) 674-679. 17) J. W. Christian: Proc. R. Soc. London Sect. A 206 (1951) 51-64. 18) T. B. Massalski: Binary Alloy Phase Diagrams, (ASM International, Ohio, 1990) pp. 136-138. 19) H. Otsuka, M. Murakami and S. Matsuda: Proc. MRS Int. Meeting on Advanced Materials, Shape Memory Materials, Tokyo, 1988, Materials Research Society (Materials Research Society, Pittsburgh, 1988) 9 (1989) pp 451-456. 20) K. Otsuka and K. Shimizu: Scr. Metall. 4 (1970) 469-472. 21) C. M. Wayman and K. Shimizu: Met. Sci. J 6 (1972) 175-183. 22) A. Sato, E. Chishima, K. Soma and T. Mori: Acta Metall. 30 (1982) 1177-1183. 23) T. Maki and K. Tsuzaki: Proc. International Conference on Martensitic Transformations (ICOMAT-92), (Monterey Inst. for Adv. Studies, Monterey, 1993) pp. 1151-1162. 24) M. Sade, K. Halter and E. Hornbogen: Z. Metallkd. 79 (1988) 487-491. 25) A. J. Bradley and G. C. Seager: J. Inst. Met. 64 (1939) 81-88. 26) O. S. Edwards: J. Inst. Met. 67 (1941) 67-77.
