Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry* 1

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1 Materials Transactions, Vol. 7, No. 3 () pp. 558 to 53 Special Issue on Shape Memory Alloys and Their Applications # The Japan Institute of Metals Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry* 1 Noriaki Sakurai* and Junjiro Takekawa Department of Information Technology and Electronics, Faculty of Science and Engineering, Ishinomaki Senshu University, Ishinomaki , Japan In order to fabricate the NiTi foams with greater than 8% porosity, a vacuum process applied to a slurry was developed. A mixture of elemental Ni and Ti powders was dipped into a solution of 7.5 mass% polyvinyl alcohol, and stirred to make the slurry. The lightly compacted lumps of slurry were then subjected to reduced atmospheric pressure to make foamed compacts. The green foams were debound and sintered under vacuum into NiTi sintered foams with 85% porosity. X-ray analysis showed alloying of NiTi was completed by the sintering at =11 C. X-ray diffraction analysis and DSC measurement also indicated that the NiTi foams consisted of B austenite and B19 martensite phases. Measurement of shape recovery strain showed the NiTi foams obtained by this process had the far excellent shape recovery characteristics compared with those of wrought NiTi alloys. Furthermore, repeated compressive deformation and heating greatly increased the shape recovery strains of these high-porosity NiTi foams. (Received June 1, 5; Accepted September, 5; Published March 15, ) Keywords: foams, shape memory alloy, NiTi, shape recovery strain, slurry, sintering, X-ray diffraction, differential scanning calorimeter, superelasticity, plateau 1. Introduction Metal foams, 1) having large interior porosity volumes, are expected to be new functional materials, because they are very light weight and have many unique properties, such as shock absorptivity, adiabaticty, and biocompatibility. Equiatomic NiTi alloy ) is a well known material having two remarkable characteristics: shape memory and pseudoelasticity. Over the last two decades, extensive studies have been carried out that advance the use of NiTi alloys in various fields. But the practical application of the NiTi alloys for use in devices has been limited because of their relatively low response rate and shape recovery strains. If NiTi alloys having a porosity of greater than 8%, so-called foams, can be realized, higher response rates and shape recovery strains are expected, because the foams can be more readily cooled due to their lower volume heat capacity and the shape recovery strains of NiTi alloys are always proportional to the porosity. Porous NiTi alloys have been fabricated by various methods such as conventional sintering, 3) combustion synthesis ) and the HIP process 5) etc., in addition to the ordinary powder metallurgical method using a polymeric foamprecursor. ) Generally in metal foams, the plateau regions in stress strain curves do not appear until the porosity is above 8%, but hitherto only a few NiTi foams have attained this level, so the shape memory characteristics of highly porous NiTi alloys have not yet been investigated. In this study, we attempt to fabricate NiTi foams with greater than 8% porosity using our own newly developed process 7) (a vacuum process applied to a slurry) and examine the shape memory characteristics of the resulting NiTi foams. * 1 This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Metall. 5 (5) 7. * Research student of Science and Engineering, Ishinomaki Senshu University, Ishinomaki , Japan Mixing powder Ti-5at.%Ni Ti<5 µ m, Ni 5 µ m g Fig. 1. Experimental Mixing Slurry Compaction Foaming Drying Debinding Sintering Valuation Binder 7.5mass%PVA solution 3.g Centrifugal mixer rpm, s 8hPa, 9.8L/min 9 C, 5h 5 C, 1h (1~ 1-3 Pa) 1 C~13 C, 1h (1~ 1-3 Pa) Microstructural examination, Compressive and heating test, XRD, DSC Flow chart of experimental procedure. The experiments were performed according to the procedure shown in Fig Fabrication of NiTi foams Carbonyl Ni powder of around 5 mm and Ti powder under 5 mm were used. These powders were uniformly mixed using a V-type mixer. Next, g of the mixed powder was dipped into a 7.5 mass% polyvinyl alcohol solution and thoroughly stirred to make the slurry. Lightly compacted

2 Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry Fig. Schematic illustration of the apparatus used to make NiTi foams from the slurry. Porosity(%) 7 lumps of the slurry were then subjected to reduced atmospheric pressures below 8 hpa to make foamed compacts. The apparatus used to make foams from the slurry is shown in Fig.. The foamed compacts, after they were debound at 5 C for 1 h under vacuum, were sintered at 1 13 C for 1 h in a vacuum (1{ 1 3 Pa). The density (porosity) was measured for the sintered foams and their microstructures were examined using both an optical microscope and a scanning electron microscope (SEM). X- ray diffraction was used for the characterization of the formed phases. A Seiko EXSTAR differential scanning calorimeter (DSC) was used to evaluate the austenitemartensite transformation reaction. The heating and cooling rate kept identical at.17 C/s Atmospheric pressure, P /hpa Fig. 3 Relationship between porosity of the green NiTi foam and atmospheric pressure.. Measurement of the shape recovery strain The NiTi foams were tested under compressive loads under quasi-static conditions using a Shimadzu AG-1A universal test machine. The sintered foams were cut into compressive test pieces of mm 3. Compressive tests were carried out at C at strain rates of about 5 1 s 1. After the compressive tests, the test pieces were heated to 15 C at a rate of.83 C/s, and the shape recovery strain was measured using a Shimadzu DT- thermomechanical analyzer (TMA). 3. Results and Discussion Fig. Visual comparison of sintered NiTi foams obtained at different atmospheric pressure (a) 113 hpa and (b) 7 hpa. (top view) 3.1 Structural examination Figure 3 shows the relationship between the porosity of the green NiTi foam and atmospheric pressure. Although the porosity of the green foam was little changed at pressures higher than 17 hpa, it abruptly increased at lower pressures, reaching 9% near hpa. Only the green foams with porosity over 8% were sintered in this study. Figure gives a visual comparison of sintered NiTi foams made under different atmospheric pressures. The initial volumes of the slurry lumps were almost the same. It can be seen that the slurry greatly expanded at the reduced pressures. Figure 5 shows a cross sectional view of the NiTi foam sintered at 1 C. In this image, black regions show pores while white areas reveal the NiTi matrix. The SEM micrograph of the same NiTi foam is given in Fig., which shows that all the pores are open and connected to each other. Figure 7 shows the XRD patterns for NiTi foams sintered at various temperatures. The XRD pattern of the starting powder mixture is also shown. The foam sintered at a relatively low temperature, 1 C, showed the formation of several intermetallic compounds. In the foams sintered at 11 and 1 C, both B and B19 phases 8,9) were formed.

3 5 N. Sakurai and J. Takekawa Heat flow, F /1 W C 1h Mf As Ms Cooling Af Heating Temperature, T/ C Fig. 5 Cross sectional view of NiTi foam sintered at 1 C for 1 h. Fig. 8 DSC (differential scanning calorimetry) curve for NiTi foam sintered at 1 C for 1 h. 18 Fig. X ray intensity Scanning electron micrograph of cross section of NiTi foam. Sintering time : 1h Powder : Ti 5at%Ni 13 C 1 C 11 C 1 C Powder mixture : B(NiTi) : B19'(NiTi) : Ni 3 Ti : Ti Ni : Ni Ti 3 : Ti : Ni Degrees, θ / Fig. 7 X-ray diffraction patterns for NiTi foams sintered at various temperatures. However, the B19 phase decreased with increasing sintering temperature and vanished at 13 C. Figure 8 shows the DSC curve of NiTi foam sintered at 1 C. The curve has two peaks, and the peaks are much Transformation temperature,t / C Fig M s Sintering temperature,t / C broader than those obtained for wrought NiTi alloys. The reason for the broad transformation temperature ranges is considered to be due to the formation of several compounds during sintering. The transformation temperatures (Mf, Ms, As, and Af) were determined according to Li. 1) Figure 9 shows the effect of the sintering temperature on the transformation temperature. Examination of the graph reveals the transformation temperatures were not significantly influenced by the sintering temperature. The transformation temperatures of these NiTi foams were approximately As: C, Af: 13 C, Ms: 8 C, Mf: 3 C. Figure 9 also shows the NiTi foams obtained in this work consisted of mixed phases of B austenite and B19 martensite at room temperature. A f A s M f Effect of sintering temperature on transformation temperature.

4 Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry Compressive test and measurement of shape recovery strain Compression tests of the NiTi foams were carried out under a uniaxial quasi-static loading condition at room temperature ( C). Figure 1 shows the compressive stress-strain curves of the NiTi foams. It can be seen that the compressive stress of the NiTi foams increased with decreasing porosity, particularly when the strain was smaller than 1%. The NiTi foam sintered at 1 C showed the clear plateau region (an increase in strain while keeping stress constant), which characterize foamed metals with high porosity. In this work, however, foams sintered at 1 C were mainly used for the / MPa 5 1 C 11 C 1 C 13 C Porosity 77.1% 81.5% subsequent compression and heating tests. This is because the foams sintered below 11 C were too fragile to withstand the rather complicated mechanical test and those sintered at 13 C were too dense to be regarded as foams. Figure 11 shows the strain-stress curves for four NiTi foams with nearly the same porosity (85%). The four specimens (a), (b), (c), (d) were subjected to compressive strains of 1,, 3, and 5%, respectively. Upon unloading, the elastic recovery strains in (a), (b), (c), (d) were., 8.8, 9.8, and 1.%, respectively. By heating at 15 C, the four specimens showed shape recovery strains (denoted with arrows) of., 7.3, 1., and 13.%, respectively. It can be seen from Fig. 11 that the shape recovery strains of the NiTi foams increased with increasing compressive strain as well as the residual strain. The microstructures indicating the shape memory characteristics of the NiTi foams are shown in Fig. 1. Figure 1(a) shows the microstructure of a NiTi foam before the compressive test. The same foam was subjected to a compressive strain of 3%, and, after unloading, foam (b) with % strain was obtained, after removal of an elastic σ Compressive Stress, % 8.1% Stress, σ / MPa Compressive strain, ε c (%) (a) (b) (c) (d) 1 C 1h Porosity 85% 8 1 Strain (%).% 7.3% 1.% ε p ( ε r ε p ) 13.% ε r Fig. 1 Compressive stress-strain curves for NiTi foams sintered at various temperatures. Fig. 11 Compressive stress-strain curves for four NiTi foams with nearly the same porosity of 85%. Fig. 1 Microstructural changes accompanying the deformation of a NiTi foam, (a) before compression, (b) after compression and (c) after heating at 15 C.

5 5 N. Sakurai and J. Takekawa 1 Shape recovery strain, ( ε r ε p ) (%) ( ε ε )/ ε =. r p r Maximum shape memory strain (8.5%) of bulk NiTi Bulk TiNi 1 3 Residual strain, ε (%) r Fig. 13 Relationship between the shape recovery strain and the residual strain of a NiTi foam having a porosity of 85%. recovery strain of 1%. Next, foam (b) was heated to 15 C, well above austenite finish temperature Af, and 1.% shape memory strain recovered as shown in (c). Figure 13 shows the relationship between the shape recovery strain (" r {" p ) and the residual strain of a NiTi foam having a porosity of 85%. The residual strain (" r ) is determined by subtracting the elastic recovery strain from the compressive strain, and " p is the permanent strain that remains after the compressive test shown in Fig. 11. The shape recovery strain increased with increasing residual strains. The maximum shape recovery strain of this foam was 13.%, which is about 1.5 times greater than that of wrought NiTi alloys (8.5%) 11) obtained at a compressive strain of 8%. The shape recovery strain was directly proportional to the residual strain at less than % and the slope, ð" r {" p Þ= " r, was.. This result indicates that more than half of the residual strain can recover by the shape memory effect. The triangle in Fig. 13 shows the shape recovery strain for a bulk NiTi alloy made in our laboratory. No treatments to improve the shape recovery strain were carried out on this bulk NiTi alloy. It was found that the shape recovery strain of the NiTi foam was more than two times larger than that of NiTi bulk alloy (untreated). If the shape recovery strain of this bulk NiTi alloy could be raised to near the maximum strains (8.5%) by some method, such as heat treatment and plastic working, and if the same method is applicable to NiTi foams, the shape recovery strains of our NiTi foams would be greatly improved. This suggests that NiTi foams may be more effective actuators with larger shape recovery functions. The Fig. 1 Microstructure of a NiTi foam after compression of 3% at room temperature. Compressive stress, σ /MPa st nd 3rd th 5th Shape recovery strain (%) 1st(7.%) nd(8.9%) 3rd(9.1%) th(9.%) 5th(9.5%) Strain(%) Fig. 15 Compressive stress-strain curves obtained from five cycles of compressive and heating tests for the NiTi foam with 85% porosity. reason for the large strain recovery of the NiTi foams in comparison with the wrought NiTi alloy is that these high porosity foams have cellular structures. Figure 1 shows the microstructure of NiTi foam subjected to 3% compressive strain. No structural damage, such as microcracks or collapsed pores, could be seen in the strained NiTi foam. It is considered that NiTi foams with more than 8% porosity are composed of cellular structures in which a major component of applied compressive strain buckles the cells and a minor part deforms the NiTi matrix. The NiTi foams obtained in this work were composed of two phases, austenite and martensite. Therefore, two kinds of shape recovery mechanisms, 1) namely martensite variant reorientation in a martensite phase, and stress-induced martensite phase transformation in an austenitic phase, were considered. Figure 15 shows the stress-strain curves obtained from five cycles of compressive and heating tests for the NiTi foam. In the compressive test, elastic recovery strains were also measured in each cycle, and the strain value in the stressstrain curve was reset to zero for each cycle. In the first cycle, the NiTi foam showed 8.% elastic recovery strain after the compressive test, and 7.% shape recovery strain by heating

6 Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry 53 Length of specimen, L /mm Compression(strain:%) Unloading (Superelasticity) Heating (Shape memory) Cooling between each compressive test, and consequently the shrinkage and the expansion of the specimen cyclically took place. After the third cycle, the compressive strains of the specimen were completely recovered by the effects of both the superelastic recovery and the shape recovery.. Conclusion The main results of this work are as follows. (1) NiTi foams with over 8% porosity were obtained by a vacuum process applied to a slurry. () The maximum shape recovery strain of a NiTi foam with 85% porosity was 13.% at a compressive strain of 8%, which is about 1.5 times larger than that of the wrought alloy. (3) In the NiTi foams, the repetition of compressive deformations and heat treatments greatly increased the shape recovery strains. REFERENCES.5 1 at 15 C. In the second cycle, 8.8% elastic strain after the compressive test and 8.9% shape recovery strain by heating were observed. Thus, the shape recovery strain increased when the test was repeated. After the second cycle, the shape recovery strains exceeded 8.5%, the maximum value for wrought NiTi alloy. In this way, the repetition of compressive deformations and heat treatments resulted in a considerable increase in the shape recovery strains of the NiTi foams. In addition, the elastic recovery strains of the NiTi foams were all over 8%, about three times larger than that of other metal foams, 13) implying that this NiTi foam may have superelasticity. Figure 1 shows the changes in the length of the specimen as a function of the number of test cycles. A specimen with 9.5 mm original length was subjected to cyclical tests of compression and heating. The specimen was heated at 15 C 3 Test cycle Fig. 1 Change in a length of the specimen as a function of the number of test cycles. 5 1) L. J. Gibson and M. F. Ashby: Cellular Solids: Structures and Properties, nd ed., (Cambridge University Press, 1987) pp ) K. Otsuka and C. M. Wayman ed.: Shape Memory Materials, (Cambridge University Press, 1998) pp ) K. Thangaraj, Y. C. Chen and K. Salama: Adaptive Structures and Material Systems () ) Y. H. Li, L. J. Rong and Y. Y. Li: J. Alloy. Compds 35 () ) D. C. Lagoudas and E. L. Vandygriff: J. Int. Mater. Systems Structures 13 () ) J. A. Shaw, A. Gremillet and D. S. Grummon: The Manufacture of NiTi Foams, Proceedings of ASME International Mechanical Engineering Congress and Exposition, New Orleans, Louisiana, () pp ) N. Sakurai and J. Takekawa: J. Jpn. Soc. Powder Powder Metall. 5 (3) ) B. Y. Li, L. J. Rong and Y. Y. Li: J. Mater. Res. 15 () ) T. H. Nam, T. Saburi, Y. Nakata and K. Shimizu: Mater. Trans., JIM 31 (199) ) B. Y. Li, L. J. Rong, X. H. Luo and Y. Y. Li: Metall. Master. Trans. A 3A (1999) ) D. E. Hodgson, M. H. Wu and R. J. Biermann: Metal Handbook, 1th ed., (ASM International, Vol., 199) pp ) S. Miyazaki, T. Sakuma and T. Shibuya ed.: Characteristics and Applications of Shape Memory Alloys, (CMC Inc., 1) pp ) Y. Sugimura, J. Meyer, M. Y. He, H. Bart-Smith, J. Grenstedt and A. G. Evans: Acta Mater. 5 (1997)