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Intermetallics 19 (2011) 217e220

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Intermetallics

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Mechanical behavior in NiTiNb shape memory alloys with low Nb contentq

Chen Ying a, Jiang Hai-chang a,*, Rong Li-jian a, Xiao Li b, Zhao Xin-qing b

a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Chinab School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 12 June 2008Accepted 31 July 2008Available online 8 October 2010

Keywords:B. PrecipitatesB. Martensitic transformationsB. Mechanical propertiesB. Shape-memory effects

q This research was supported by the National Adment Project (863 Foundation) of China under grant* Corresponding author. Tel.: þ86 24 23971985.

E-mail address: hcjiang@imr.ac.cn (J. Hai-chang).

0966-9795/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.intermet.2010.08.011

a b s t r a c t

Effect of low Nb content (3.5 at.%, 4.5 at.%, 5.0 at.%) on the mechanical properties of NiTiNb shapememory alloy (SMA) has been investigated in detail by differential scanning calorimetry (DSC), tensiletest and scanning electron microscope (SEM). The Ms temperatures of three kinds of NiTiNb alloys withdifferent Nb content were fixed at about 188 K to eliminate the influence of testing temperature on themechanical behaviors. The experimental results show that with the increase of Nb content, the yieldstrength increased and the elongation decreased gradually. However, the NiTiNb alloys still keep highelongation property. Furthermore, it has been found that the shape recovery strain of NiTiNb alloy with5.0 at.% Nb can reach 7.4%. It is believed that NiTiNb alloys with low Nb content can obtain good shapememory characteristic.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

As an important functionalmaterial, NiTiNb shapememory alloyshave received considerable attention for their unique shape memoryeffect, excellent mechanical properties and wide hysteresis, whichhavebeenextensively used as coupling and fasteners [1e3]. However,the development of industry technique brought forward the requestfor further improvement ofmechanical properties. Taking pipe-jointsapplication for example, the yield stress of existing NiTi(Nb) alloysonly reaches 400 MPa at room temperature [4], which already can’tsatisfy the practical requirements for the high pressure tube.

So far, most of thework onmechanical behavior in NiTiNb alloyswas taken the alloy with 9 at.% Nb [4,5]. However, recent researchfocusmore eyes on theNiTiNb alloyswith lowNb. As amatter of fact,electing NiTiNb alloys with low Nb content for experiments caneffectively avoid the influence of eutectic structure and directlyobtain the effect of Nb elements. He et al. [6] found thatTi46.9Ni50.1Nb3 alloy has better shape memory effect thanTi44Ni47Nb9 and exhibits enough wide transform temperaturehysteresis after deformation. Zhao et al. [7] studied NiTiNb alloyswith 4.5 at.% Nb and indicated that Nb dissolved in NiTi matrix isresponsible for the hysteresis expansion. In these experiments,NiTiNb alloys are in single austenite state and the high strength canbe obtained. However, no systemic work about the effects of Nb onmechanical behavior in NiTiNb alloys has been conduced. The

vanced Technology Develop-No. 2006AA03Z102.

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purpose of thiswork is to investigate the effect of Nb addition on themechanical behavior in NiTiNb alloyswith different lowNb content.

2. Experimental

Ni50.2Ti46.3Nb3.5, Ni49.9Ti45.6Nb4.5 and Ni49.8Ti45.2Nb5.0 alloyswere prepared from 99.96 wt% Ni, 99.8 wt% Ti and 99.7 wt% Nb ina water-cooled copper crucible by WCE300 tungsten electrode arcfurnace. Buttons were melted four times and homogenized inquartz capsules at 1153 K for 2 h. The specimens for cast micro-structure observation were directly spark-cut from the buttonsafter homogenization. The buttons were hot-rolled to 1.2 mmthickness sheets at 1143 K, and then solution-treated at 1133 K for40 min followed by water quenching. The specimens for hot rollingmicrostructure were taken from the resulting buttons.

The phase transformation behaviors were measured by differen-tial scanning calorimetry (DSC) using PerkineElmer Diamond DSC inaheliumatmospherewith theheating/cooling rate of 10 K/min. In theexperiment, the specimens were heated firstly to complete austeni-tizing temperature and the heating/cooling/heating cycle wasperformed. For the mechanical behavior of the alloys, tensile testswere carried out at room temperature and 193 K, respectively. For theshapememorycharacteristics test, the specimenswerepre-deformedto different strains at 77 K, then unloaded and heated to 473 K.

3. Results and discussion

The SEM micrographs of the cast NiTiNb alloys were shown inFig. 1. The dark region is NiTi matrix phase dissolved in a small

Fig. 2. DSC cooling-heating curves of the alloys.

C. Ying et al. / Intermetallics 19 (2011) 217e220218

amount of Nb element. The light one around the grain boundarieswas the Nb-riched region. It is found that the b-Nb particles are fineand in small amount distributed at grain boundaries inNi50.2Ti46.3Nb3.5 and Ni49.9Ti45.6Nb4.5 alloys, as shown in Fig. 1(aeb). In Fig. 1(c), when the Nb content reached 5.0 at.%, thestripped b-Nb particles appear around the boundary. The resultsdenote that Nb cannot be dissolved completely into the NiTi matrixin Ni49.8Ti45.2Nb5.0 alloy. When the Nb content in NiTiNb alloy ishigher than 5.0 at.%, the eutectic phase will appear in the micro-structure. So, the highest Nb content in this work is fixed as 5.0 at.%Nb. Fig. 1(d) shows the hot rolling Ni49.8Ti45.2Nb5.0 microstructureafter solution-treated at 1133 K and water quenching. In the figure,b-Nb particles were prolonged and appear discontinuous bandsalong the rolling direction.

3.1. Phase transformation behavior

It has been found that testing temperature plays a significantrole amongmany factors impacting themechanical behavior in NiTi(Nb) alloys [8e10]. In order to avoid the influence of the testingtemperature on the mechanical properties, the transformationtemperatures of the samples in this investigation were designed tobe as closer as possible. Meanwhile, the Ms temperature of theNiTiNb alloy should be designed in the appropriate temperaturerange, about 173e193 K, as a result NiTiNb alloys will exhibita wider hysteresis after deformation and higher mechanical prop-erties at room temperature [10]. Through adjusting the ratio of Tiand Ni, the Ms temperatures of NiTiNb alloys with different Nbcontent can be fixed at the same temperature. Fig. 2 shows the DSCcurves of the alloys after solution treatment. The DSC curves stylesof the alloys in this investigation are almost same. All the alloysexhibit one step transformation whether in heating or coolingprocess. The exothermic peak on cooling represents the trans-formation from austenite phase to martensite phase. And the

Fig. 1. SEM micrographs of the NiTiNb alloys: (a) cast Ni50.2Ti46.3Nb3.5, (b) cast Ni49.9Ti45.6Nb1133 K for 40 min, followed by water quenching.

endothermic peak on heating is related to the reverse trans-formation from martensite to austenite phase. The transformationtemperatures were determined from the intersections of twotangential lines.

Transformation temperatures of Ms (the matrensite trans-formation start temperature), Mf (the matrensite transformationfinish temperature), As (the reverse transformation start tempera-ture) and Af (the reverse transformation finish temperature) of thealloys are listed in Table 1. The results confirmed that phasetransformation temperature Ms of these alloys are nearly same asexpected. Consequently, the impact of testing temperature onmechanical behavior can be neglected in subsequent experiments.

4.5, (c) cast Ni49.8Ti45.2Nb5.0 and (d) hot rolling Ni49.8Ti45.2Nb5.0 after solution-treated at

Table 1Results of DSC measurement of the alloys.

Temperature Ms (K) Mf (K) As (K) Af (K)

Ni50.2Ti46.3Nb3.5 187.1 175.2 121.3 263.7Ni49.9Ti45.6Nb4.5 181.3 175.6 122.7 262.8Ni49.8Ti45.2Nb5.0 191.7 168.5 123.2 270.3

Fig. 3. Stressestrain curves of the alloys at room temperature.

Fig. 5. Stressestrain curves of the alloys at 193 K.

C. Ying et al. / Intermetallics 19 (2011) 217e220 219

3.2. Mechanical behavior

The mechanical behaviors of NiTiNb alloys in austenite statewere tested at room temperature, as shown in Fig. 3. All the alloysexhibit one-stage yielding in the stressestrain curves, representing

Fig. 4. SEM images of fracture surfaces of tensile tested NiTiNb alloys at room te

the plastic deformation of parent phase itself. This is attributed todeformation temperature exceeding the maximal critical stress-induced martensitic transformation temperature [11]. It is foundthat yield strength and rupture strength risewith the increase of Nbcontent and Ni49.8Ti45.2Nb5.0 alloy have 640 MPa yield strength and930 MPa rupture strength, while the elongation decreased gradu-ally. Under the condition that the impact of test temperature canbeen ignored, the increment of yield strength is ascribed to thesolution strengthening of Nb dissolved in NiTi matrix. All the alloysin this study showexcellent ductility. For Ni49.8Ti45.2Nb5.0 alloywiththeworst ductility in the three alloys, the elongation can reach 24%.

The fracture surfaces of NiTiNb alloys tested at room tempera-ture were shown in Fig. 4. Apparent dimples can be observed. It is

mperature: (a)Ni50.2Ti46.3Nb3.5, (b) Ni49.9Ti45.6Nb4.5 and (c) Ni49.8Ti45.2Nb5.0.

Fig. 6. (a) The stressestrain curves of Ni49.8Ti45.2Nb5.0 alloy deformed at 77 K, (b) Recoverable strain in different pre-deformation.

C. Ying et al. / Intermetallics 19 (2011) 217e220220

clear that dimples appear smaller and shallower with the increaseof Nb content. This change is thought due to the increase of b-Nbparticles, which is also supported by the microstructure observa-tion. The surfaces topography exhibits evident ductile fracturefeature, which is the reasonwhy the alloys exhibit high elongation.

The mechanical behaviors at low temperature have also beeninvestigated. Fig. 5 shows the stressestrain curves of the alloys testedat193 K,whichwasclose to theirMs temperatures.The tensileprocesscan be divided into three stages, as denoted in Ni49.8Ti45.2Nb5.0 alloy.The yielding in the first stage was the result of the stress-inducedmartensitic transformation. The following stagewith sharply strengthincrease was related to the elastic deformation of NiTi matrix,accompanying the plastic deformation of b-Nb particles. Plasticdeformationof stress-inducedmartensiteandb-Nbparticlesoccurs inthe third stage at the higher strain. With increasing Nb content yieldstrength enhanced and the yield strength of Ni49.8Ti45.2Nb5.0 alloy canapproach 250 MPa, while the rupture strength decreased. The elon-gation also exhibits decline trend,whereas theworst one still reached22%. It is noted that the elongation of Ni49.8Ti45.2Nb5.0 alloy representsa sudden decline compared to others. This means that the criticalstress of stress-induced martensite was lower than that of b-Nbparticles. So the obstacle from the b-Nb particles becomespronounced in Ni49.8Ti45.2Nb5.0 alloy when the matrix transformedinto martensite state.

On account of the excellent properties, the recovery character-istics of Ni49.8Ti45.2Nb5.0 were further investigated and the results ofthe tensile tests were shown in Fig. 6. When pre-deformed strainwas less than 9.9%, the strain after unloaded can totally recover byheating. When the pre-deformed strain reached 14%, the recoverrate decreased to 75%.

Generally, the plastic deformation of b-Nb particles can inducethe incomplete recovery. Excessive precipitations of b-Nb particleswill damage the shape memory property. However, the maximalrecoverable strain of the Ni49.8Ti45.2Nb5.0 alloy can reach 7.4%,maintaining the level of NiTi alloys. Compared with that of classicalNi47Ti44Nb9 alloy, which maximal recoverable strain is less than 5%[12], Ni49.8Ti45.2Nb5.0 alloy apparently has better recovery charac-teristic. Ni47Ti44Nb9 alloy shows typical hypoeutectic microstruc-ture [6]. Contrast to discontinuous bands in Ni49.8Ti45.2Nb5.0, asshown in Fig. 1(d), Ni47Ti44Nb9 alloy has higher density of bands orsmaller particles after hot rolling. At the testing temperature, thematrix is soft phase. When the strain was small, the deformationcan be realized by the collaboration of stress-induced martensitearound the b-Nb particles in Ni49.8Ti45.2Nb5.0 alloy, whileNi47Ti44Nb9 alloy has taken plastic deformation. Therefore, it is

believed that NiTiNb alloys with low Nb content can obtain goodshape memory characteristic.

4. Conclusions

(1) It’s controllable of the Ms temperature through adjusting Nbcontent andNi/Ti ratio in NiTiNb alloys. Excluding the impact oftesting temperature, the effect of Nb on mechanical behaviorcan be obtained more clearly.

(2) The yield strength of NiTiNb alloys with low Nb content atroom temperature increases with increase of Nb content, whilethe elongation decreases, but still in a high level. The defor-mation behavior at 193 K appears the similar trend.

(3) Ni49.8Ti45.2Nb5.0 alloy have 640 MPa yield strength, 930 MParupture strength, 22% elongation at room temperature and 7.4%shape recovery strain. It is believed that NiTiNb alloys with lowNb content can obtain excellent shape memory characteristic.

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