Chinese Journal of Aeronautics

Chinese Journal of Aeronautics 22(2009) 540-543 www.elsevier.com/locate/cja

Mechanical Properties and Transformation Behavior of NiTiNb Shape Memory Alloys

Liu Wei, Zhao Xinqing*

School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China

Received 15 May 2009; accepted 10 September 2009

Abstract

NiTiNb shape memory alloys have attracted much attention in pipe coupling or sealing system because of their large transfor-mation hysteresis upon a proper pre-deformation. In order to clarify the effects of adding Nb on the mechanical properties as well as the transformation behavior of NiTiNb shape memory alloys, Ni47Ti44Nb9 and Ni49.8Ti45.2Nb5 alloys with different microstruc-tures but with similar martensitic transformation start temperature, are prepared. Comparative studies on the microstructures, mechanical properties and transformation characteristics are conducted by means of scanning electron microscopy (SEM), phase transformation measurements and mechanical property tests. It is found that Ni47Ti44Nb9 and Ni49.8Ti45.2Nb5 alloys possess similar transformation hysteresis in the as-annealed state. However, the presence of Nb and its status exerts important effects on the me-chanical properties, especially the yield strength and the yield behavior of the alloys. Ni49.8Ti45.2Nb5 alloy exhibits remarkable increase in the yield strength than the Ni47Ti44Nb9 alloy. The transformation hysteresis of both alloys under pre-deformation is characterized and the relative mechanism is discussed.

Keywords: NiTiNb; martensitic transformation; mechanical properties; transformation hysteresis; microstructure

1. Introduction1

In recent years, NiTiNb shape memory alloys, espe-cially the one with nominal composition of Ni47Ti44Nb9, have attracted a lot of attention for their remarkable transformation hysteresis expansion upon a proper pre- deformation in martensitic state or by stress-induced martensitic transformation[1-7]. Transformation hystere-sis expansion proves to be very useful in engineering applications, such as pipe coupling or sealing, in which the operation could be carried out at the room tem-perature. In addition to the shape memory effect in-cluding pseudo-elasticity, the mechanical properties are very important parameters for the application of shape memory alloys. As is known, the microstructure of Ni47Ti44Nb9 is composed of NiTi matrix and a large amount of �-Nb phase. The existence of latter is des-tined to have significant influences on the shape mem-

*Corresponding author. Tel.: +86-10-82338559. E-mail address: xinqing@buaa.edu.cn Foundation items: National Natural Science Foundation of China (50971009); National High-Tech Research and Development Program of China (2006 AA03Z102); Aeronautical Science Foundation of China (2009ZF51059) 1000-9361/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi: 10.1016/S1000-9361(08)60138-7

ory effects and the mechanical properties of Ni47Ti44Nb9 alloy. This is because the �-Nb exhibits different features from the NiTi matrix which contrib-utes to the shape memory effect. Previous experimental researches were concentrated on the microstructures dependence of Nb addition in NiTiNb alloys as well as the transformation behaviors before and after pre- deformation of Ni47Ti44Nb9 alloy. Actually, the me-chanical mechanism and shape recovery behavior of Ni47Ti44Nb9 alloy after the concurrent deformation of both NiTi matrix and �-Nb phases are not clear.

In the present work, a comparative study is carried out on the transformation behavior and mechanical properties of Ni47Ti44Nb9 and Ni49.8Ti45.2Nb5 alloys which have different microstructures yet similar mart-ensitic transformation start temperatures in an attempt to elucidate the effects of adding Nb on the mechanical properties and transformation behavior of NiTiNb shape memory alloys.

2. Experimental Procedures

The alloys with nominal compositions of Ni49.8Ti45.2- Nb5 (for short Nb5 in the following) and Ni47Ti44 Nb9 (for short Nb9 in the following) were prepared by arc melting Ti (99.8 wt%), Ni (99.96 wt%) and Nb (99.9

No.5 Liu Wei et al. / Chinese Journal of Aeronautics 22(2009) 540-543 · 541 ·

wt%) in a vacuum induction melting furnace. The in-gots weighing 10 kg were hot-forged and hot-rolled into rods 18 mm diameter. All specimens were cut from the rods after annealing at 1 123 K for 7.2 h and then furnace cooled for Nb9 alloy and air cooled for Nb5 alloy. An MTS-800 mechanical tester was used to re-cord the stress-strain curves. The phase transformation temperatures were taken from the curves of electrical resistance versus temperature. Microstructure observa-tions and the quantitative analysis were conducted us-ing the JXA-8100 electron probe micro analyzer (EPMA). X-ray diffraction (XRD) was carried out on a Regaku D/max 2200pc diffractometer. The reverse martensitic transformation temperature after pre-defor- mation was determined by heating the samples in an alcohol bath and a water bath from 223 K to 373 K.

3. Results and Discussion

3.1. Characterization of microstructure and transfor-mation

Fig.1 shows the scanning electron microscope (SEM) images of Nb5 and Nb9 alloys as-rolled. Previous inves-tigations indicated that the microstructure and the transformation temperature of NiTiNb alloys are closely related to the Nb-addition and the Ni/Ti ratio. From the pseudo-binary of NiTiNb compiled by M. Piao, et al.[3], the composition of Nb9 alloy belongs to the category of alloys that form the typical hypoeutec-tic microstructure containing primary NiTi matrix and a lot of eutectic NiTi and �-Nb mixture. As shown in Fig.1(a), apart from a little of Nb-rich phase (marked as B) appearing in the microstructure of Nb5 alloy, the dominant phase is NiTi matrix (marked as A) with some Nb atoms dissolved inside. This is supported by XRD. In addition, in the microstructures of both Nb5 and Nb9 alloys, can be found a few dark particles (marked as C), which are identified as oxides of Ni and Ti, (Ti, Nb)4Ni2O[8-9].

The �-Nb phase can be inhibited from growing if in-creasing Ni/Ti ratio, however, it should be noted that increasing Ni/Ti ratio could result in lowering the martensitic transformation temperature. In order to compare the transformation hysteresis and the me-chanical behavior of Nb5 and Nb9 alloys, it is nece-

(a) Nb5 alloy

(b) Nb9 alloy

Fig.1 SEM micrographs.

ssary to control the martensitic transformation of two alloys which are in close temperature range. In the dual-phase NiTiNb alloys, only the NiTi matrix exhib-its martensitic transformation. NiTiNb alloys with similar Ni/Ti ratio and similar Nb content dissolved in the matrix should have similar transformation tem-peratures. Actually, the chemical composition analysis by EPMA suggested that the Nb content dissolved in the NiTi matrixes of Nb5 and Nb9 alloys are almost the same. Moreover, they have very close averages of Ni/Ti ratio in NiTi matrix: 1.060 for Nb5 alloy and 1.067 for Nb9 alloy.

From the curves of electrical resistance vs tem- perature (see Fig.2), quite similar martensitic trans- formation start temperatures are found: 172 K for Nb5 alloy and 171 K for Nb9 alloy. Further, both alloys have very similar martensitic transformation finish tem- peratures (Mf): 133 K for Nb5 alloy and 146 K for Nb9 alloy, and reverse martensitic transformation finish temperatures (Af): 255 K for Nb5 alloy and 247 K for Nb9 alloy.

It should be noted that before pre-deformation, Nb9 exhibits wider transformation hysteresis (As-Ms) than Nb5. Considering the difference between their micro- structures and the similarity of their matrix composition, the wider of hysteresis of Nb9 can be attributed to the existence of a large amount of �-Nb phase. With fewer �-Nb phase in Nb5 alloy, the hysteresis before pre- deformation is 37 K similar to that of binary NiTi alloys, 20-30 K for example.

Fig.2 Electrical resistance vs temperature of NiTiNb alloys.

· 542 · Liu Wei et al. / Chinese Journal of Aeronautics 22(2009) 540-543 No.5

3.2. Mechanical properties of Nb5 and Nb9 alloys

Fig.3 shows the stress-strain curves of the Nb5 and Nb9 alloys at the room temperature. From Fig.3, it is clear that the yield stress of Nb5 alloy is higher than that of the Nb9 alloy. This can be explained in two ways. On the one hand, the presence of larger amount of soft �-Nb phase in Nb9 alloy would naturally decrease its yield and the fracture strength in comparison with Nb5 alloy. On the other hand, the enhancement of the yield strength is closely related to the strengthening effects by the solution of Nb. This can explain why both Nb5 and Nb9 alloys possess higher yield strength than bi-nary NiTi alloys in austenitic state. It is interesting that obvious yielding of Lüders band type is observed on the stress-strain curve of Nb9 alloy, although it cannot be disclosed during deformation of Nb5 alloy. Un-doubtedly, this must be associated with the presence of a large amount of �-Nb phase in the Nb9 alloy.

Fig.3 Stress-strain curves of Nb5 and Nb9 alloys at room temperature.

As is already known, shape memory alloys exhibit different yielding behavior when the alloys are in dif-ferent states, martensitic or austenitic, because alloys could yield by self-orientation of martensite or stress induced martensite[8]. In order to examine the me-chanical behavior of Nb5 and Nb9 alloys in martensitic state, mechanical characteristics were determined at their Ms temperatures, as shown in Fig.4, from which it

Fig.4 Stress-strain curves of Nb5 and Nb9 alloys at their Ms temperatures.

is observed that at their Ms temperatures the martensite self-orientation induced yield stress of Nb5 alloy is much higher than that of Nb9 alloy. From Fig.3 and Fig.4, it follows that Nb9 alloy performs better ductility than Nb5 alloy at the room temperature while at low temperatures, the exact reverse is the case. This sug-gests that the presence of a large amount of �-Nb phase can improve the ductility of NiTiNb alloys only at the room temperature.

3.3. Effects of deformation on recovery and hysteresis

Concerning the mechanism of enlargement of hys-teresis of NiTiNb shape memory alloys by pre-defor- mation in martensitic state, early investigation attrib-utes the wide hysteresis to the recovery hindered by deformed �-Nb[3-4,10]. Some researchers considered that the release of elastic strain energy during pre-defor-mation postpones the reverse martensitic transfor- mation to occur at higher temperatures, thus leading to expansion of transformation hysteresis[11-15]. Fig.5 shows the variation of strain recovery ratio and trans-formation hysteresis as function of the pre-deformation at Ms. It is found that both alloys have as good shape memory effect as binary NiTi shape memory alloys. For Nb5 alloy, if the total strain does not exceed 8%, the strain can be recovered completely after heating to 423 K. In the case of Nb9 alloy with 8% pre-defor- mation, very few of residual strain can be observed after heating to 423 K. With the pre-deformation fur- ther increased above 8%, the recovery ratio decreases gradually. For example, the recovery ratio of 65% for Nb5 alloy and 69% for Nb9 alloy can be achieved with 18% of total pre-deformation. This suggests that in the range of pre-deformation under study, �-Nb phase does not exert substantial influences on the shape recovery of Nb5 and Nb9 alloys. Soft �-Nb of NiTiNb alloys in martensitic state can plastically deformed during pre- deformation. Comparison of the hysteresis curves in Fig.4 indicates that deformation of �-Nb in Nb9 alloy contributes to the expansion of transformation hystere-sis, resulting in a wider hysteresis than that in Nb5 alloy with the same pre-deformation. However, one can see

Fig.5 Transformation hysteresis and recovery ratio of Nb5

and Nb9 alloys vs pre-deformation at Ms temperature.

No.5 Liu Wei et al. / Chinese Journal of Aeronautics 22(2009) 540-543 · 543 ·

from Fig.4 that with little �-Nb phase present, Nb5 alloy can still achieve quite wide hysteresis after pre-defor- mation. This means that deformation of �-Nb might not be the primary mechanism to boost the hysteresis ex-pansion. Though binary NiTi shape memory alloys show hysteresis expansion after pre-deformation in martensitic state, this hysteresis expansion is much weaker than that in NiTiNb alloys with or without large amount of �-Nb in microstructures. Therefore, it can be concluded that pre-deformation induced hysteresis ex-pansion is closely associated with solution of Nb in the matrix. It is reasonable to consider that the solution of Nb in the NiTi matrix can change the kinetics of mart-ensitic or reverse martensitic transformation. More elastic strain energy might be stored due to the solution of Nb. When the alloys are subject to deforming, the elastic strain energy which is the driven force of the reverse martensitic transformation releases during the orientation of martensite variants, resulting in the stabi-lization of martensite. Therefore, reverse martensitic transformation occurs at higher temperature after the elastic strain energy is released upon deformation.

4. Conclusions

In the present work, Ni47Ti44Nb9 and Ni49.8Ti45.2Nb5 shape memory alloys with different microstructures yet similar Ms temperature are prepared and studied to elu-cidate effects of Nb content on their mechanical prop-erties and transformation behavior.

Adding Nb can effectively improve the yield stress of NiTiNb shape memory alloys. Solution of Nb in NiTi matrix is found to play a key role in strengthening NiTiNb alloys.

The presence of Nb either as a �-Nb phase or as a solution can enhance the transformation hysteresis, and the solution of Nb in the NiTi matrix is the primary reason for the hysteresis expansion upon pre-deforma-tion in martensitic state.

References

[1] Otsuka K, Kakeshita T. Shape memory materials. Lon-don: Cambridge University Press, 1998.

[2] Melton K N, Mercier O. The mechanical properties of NiTi-based shape memoryalloys. Acta Metallurgica 1981; 29(2): 393-398.

[3] Piao M, Miyazaki S, Otsuka K, et al. Effect of Nb addi-tion on the microstructure of Ti-Ni alloys. Materials Transactions JIM 1992; 33(4): 337-345.

[4] Piao M, Miyazaki S, Otsuka K. Characteristics of de-

formation and transformation in Ti44Ni47Nb9 shape memory alloys. Materials Transactions JIM 1992; 33(4): 346-357.

[5] Duerig T W, Melton K, Proft J L. Wide hysteresis shape memoey alloys. London: Btterworth-Heinemann Press, 1990; 130-136.

[6] Zheng Y F, Zhao L C, Ye H Q. HREM studies on the interface structure of deformed stress induced marten-site variants in a Ti-Ni-Nb shape memory alloy. Materi-als Science and Engineering A 1999; 273-275: 271-274.

[7] Miyazaki S, Otsuka K. Deformation and transition be-havior associated with the R-phase in Ti-Ni alloys. Metallurgical and Materials Transastions A 1986; 17(1): 53-63.

[8] Yang Y Z, Zhao X Q, Meng L J. Microstructure and transformation behavior of Ni-Ti-Nb shape memory al-loys with low Nb content. Acta Metallurgica Sinica 2005; 41(6): 627-632.

[9] Zhao X Q, Yan X M, Yang Y Z, et al. Wide hysteresis NiTi(Nb) shape memory alloys with low Nb content (4.5at.%). Materials Science and Engineering A 2006; 438-440: 575-578.

[10] Zhao L C, Duerig T W, Justi S, et al. The study of nio-bium-rich precipitates in a Ni-Ti-Nb shape memory al-loy. Scripta Metallurgica 1990; 24(2): 221-225.

[11] Lin H C, Wu S K, Chou T S, et al. The effects of cold rolling on the martensitic transformation of an equia-tomic TiNi alloy. Acta Metallurgica et Materialia 1991; 39(9): 2069-2080.

[12] Abramov V Y, Aleksandrova N M, Borovkov D V, et al. Martensitic transformations and functional properties of thermally and thermomechanically treated Ti-Ni-Nb- based alloys. Materials Science and Engineering A 2006; 438-440: 553-557.

[13] He M, Rong L J, Yan D S, et al. TiNiNb wide hysteresis shape memory alloy with low niobium content. Materi-als Science and Engineering A 2004; 371(1-2): 193- 197.

[14] He X M, Rong L J, Yan D S, et al. Effect of deformation on martensitic transformation behavior in Ni50.1Ti46.9Nb3 shape memory alloy. Journal of Materials Science 2005; 40(19): 5311-5313.

[15] Meng X L, Chen F, Cai W, et al. Two-way shape mem-ory effect and its stability in a Ti-Ni-Nb wide hysteresis shape memory alloy. Materials Transactions 2006; 47(3): 724-727.

Biography:

Zhao Xinqing Born in 1962, he received Ph.D. degree from Institute of Metal Research, Chinese Academy of Sci-ences in 1995. Now, he is a professor in Beijing University of Aeronautics and Astronautics. E-mail: xinqing@buaa.edu.cn