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Wear 263 (2007) 858–865

Effects of ZrW2O8 and tungsten additions on the temperature range inwhich a pseudoelastic TiNi alloy retains its maximum wear resistance

Iulian Radu ∗, D.Y. LiDepartment of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6

Received 10 August 2006; received in revised form 8 January 2007; accepted 9 January 2007Available online 26 March 2007

bstract

The near-equiatomic TiNi alloy has demonstrated its attractive properties for tribological applications. The high wear resistance of this alloyainly benefits from its pseudoelasticity (PE). However, PE occurs only within a small temperature range, which makes the wear behavior of

his alloy unstable as temperature changes due to frictional heating or environmental instability. Enlarging the temperature range in which PE isffective would make it possible to use TiNi alloys for various tribological applications in which temperature is not stable.

We have proposed a promising approach to widen the temperature range by introducing a temperature-dependant internal stress field so that thes is adjusted automatically to retain PE over a wider temperature range. Such an internal stress field could be achieved by taking the advantage

f the difference in thermal expansion between the pseudoelastic matrix and a reinforcing phase. Research was conducted to investigate effects ofrW2O8 and W additions on the temperature range within which the wear loss dropped. The former had a negative thermal expansion coefficient

NTE) while the latter had a positive but smaller one compared to that of the TiNi alloy. Changes in microstructure, mechanical properties, and

ear behavior were investigated.The research demonstrated that the temperature range in which wear loss reached a minimum was widened. Such an effect was enhanced as

he difference in thermal expansion between the reinforcing phase and the TiNi matrix was increased. In addition, the NTE reinforcing phaseonsiderably increased the overall wear resistance.

2007 Elsevier B.V. All rights reserved.

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eywords: TiNi; Pseudoelasticity; Wear; Negative thermal expansion

. Introduction

Wear is one of the most destructive processes in industry1]. Great efforts have been continuously made to develop orearch for new wear-resistant materials to battle with wear. Assmart material, the near-equiatomic TiNi shape memory alloyas been successful in many industrial and medical applications2–4]. In recent years, the TiNi alloy was demonstrated to pos-ess high resistance to wear and corrosive wear [5–9], comparedo many conventional wear-resistant materials. The high wearesistance of TiNi alloy largely benefits from its pseudoelasticityPE) [7,8]. The pseudoelasticity results from a reversible marten-

itic transformation. The near-equiatomic TiNi alloy may havehree phases: austenite (B2-cubic), martensite (B19′-monoclinichase), and R phase (rhombohedral). The B2 phase that is stable

∗ Corresponding author. Tel.: +1 780 492 8833; fax: +1 780 492 2881.E-mail address: radu@ualberta.ca (I. Radu).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.01.065

t high temperatures may transform to B19′ phase on cooling10]. The martensitic transformation could occur in one stepf B2 → B19′, in two steps of B2 → R → B19′ or even severalteps involving multiple-stage R-phase transformations [10,11],epending on the heat treatment and the chemical composition.he martensitic transformation could also be induced by stresshen temperature is slightly above the martensite start trans-

ormation temperature (Ms) but lower than Md (the temperaturebove where martensite cannot be induced by stress) [10]. Theeformation due to the stress-induced transformation is recover-ble after the stress is removed [12,13]. The resultant rubber-likeehavior is known as the pseudoelasticity. When stress is largerhan the yielding strength of martensite, irrecoverable plasticeformation occurs [12]. Above Md, the TiNi alloy behavesike an ordinary material. The alloy may also show PE when in

artensitic state due to the reorientation of martensitic variantsnder stress [10].

It was observed that the wear resistance markedly increasedn the temperature range in which the stress-induced martensitic

ear 263 (2007) 858–865 859

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Table 1Experimental conditions for re-melting of TiNi alloy

Parameters Melting conditions

Vacuum pressure before filling with Ar gas 27–28 psiRatio of vacuum/back-fill with Ar gas Three timesProtection gas ArgonGas flow during melting and cooling 2 l/minPressure during melting and cooling 10 psiElectrode type Thoriated tungstenElectrode diameter 4.8 mmAN

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I. Radu, D.Y. Li / W

ransformation occurred [8]. However, PE occurs only withinsmall temperature range, thus limiting the use of the alloy inear processes when temperature is stable. For example, duringear the surface temperature could rise due to frictional heatr environmental instability, so that the PE may not function,eading to a decrease in the wear resistance of the alloy. Enlarginghe temperature range in which PE is effective would therefore

ake it possible to use TiNi alloy as a tribo-material in a wideremperature range.

Recently, we proposed a promising approach to widen theemperature range in which PE was effective by introducing aemperature-dependent internal stress field in the alloy to auto-

atically adjust its martensitic transformation temperature (Ms)hen environmental temperature changes, so that PE could be

etained over a larger temperature range [14]. Such an internaltress could be achieved by adding a second phase having a neg-tive thermal expansion coefficient or one that is considerablymaller than that of the TiNi matrix. We demonstrated that theemperature range in which a pseudoelastic TiNi alloy retainedts maximum wear resistance could be considerably expandedhen ZrW2O8, an oxide compound having a negative thermal

xpansion coefficient (NTE), was added to the alloy. In addi-ion, the overall wear resistance of such modified material wasncreased by one order of magnitude [14].

Objectives of this work are: (1) to study the effect of themount of added NTE phase on widening of the temperatureange within which the wear loss drops and (2) to investigatehe effect of a third element addition having a positive but smalloefficient of thermal expansion (CTE) on the temperature rangef wear drop for the TiNi alloy.

In this study, ZrW2O8 and tungsten were added to a TiNilloy, respectively. Selection of these two materials was basedn the fact that ZrW2O8 has a negative thermal expansion coef-cient while W has its thermal expansion coefficient markedlymaller than that of TiNi alloy, both of which may lead to desirednternal stress fields for self-adjusting of the martensitic transfor-

ation point (Ms) with changes in environmental temperature.ungsten has a low solubility (0.3%) in TiNi, so that it shouldainly form a second phase rather than dissolves in the alloy.

. Experimental procedure

A near-equiatomic TiNi alloy was modified with differ-nt amounts of zirconium tungstate (ZrW2O8: 10, 20, and0 wt%) and tungsten (5, 10, and 20 wt%). The zirconiumungstate has a larger negative thermal expansion coefficient−8.7 × 10−6 ◦C−1) over a wide temperature range from 0.3o 1050 ◦K [15]; while W has a positive (4 × 10−6 ◦C−1) butmaller thermal expansion coefficient, compared to that of theiNi alloy (1.1 × 10−5 ◦C−1) [16]. The amounts of the addi-

ions were limited in the range from 5 to 30 wt% based onhe consideration that the presence of a large amount of sec-nd phases may suppress pseudoelasticity while a small amount

ay not introduce a desired self-adjusting internal stress toodify the martensitic transformation temperature. The TiNi

lloy (provided by the Special Metal Corporation, New York)as re-melted with added ZrW2O8 powder (−200 mesh) and

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rc current 160–200 Aumber of turning over and re-melting Three times

powder (12 �m size), respectively, in an arc melting furnaceMRF INC. SA338-V&G). The melting and solidification condi-ions are given in Table 1. The cast samples were turned over ande-melted three times in order to increase the degree of homog-nization. After arc-melting, the cast samples were annealed at00 ◦C for 1 h in argon atmosphere followed by furnace cooling.

Standard techniques were used to prepare specimens for opti-al metallography. Final polishing was performed using 0.05 �mlumina slurry. A solution of HF–HNO3–H2O (volume fraction:3:10) was used to etch the samples. Microstructures of theamples were observed using an optical microscope (OlympusME3-ADL). SEM-EDS technique was used to investigate theicrostructure of the cast samples. A Rikagu X-ray diffractome-

er with Cu Kl radiation (λ = 1.54056 A) and Co Kl radiationλ = 1.78899 A) was used to determine the crystal structures ofhases developed in the samples. Prior to X-ray investigation, theamples (10 mm × 15 mm × 1 mm) were polished with 0.05 umlumina slurry and then etched to remove the surface tension.

Macro-hardness was measured using a conventional Rock-ell hardness tester under a load of 1471 N. Each reported HRCalue is an average of at least six measurements. Micro-hardnessalues of different phases developed in the samples were deter-ined using a micro-mechanical probe (Fisher Technology Ltd.,insor, CT, USA) under 30 mN maximum load. Each reported

alue is an average of 20 measurements.Sliding wear tests were performed on a high-temperature

in-on-disc tribometer (CSEM Instruments, Neuchatel, Switzer-and). The disc was the sample under study (15 mm × 8 mm ×mm) and the pin was a silicon nitride ball with its diame-

er equal to 6 mm. All tests were performed at a sliding speedf 1 cm/s along a circle path of 0.8 mm in radius under a nor-al load of 5 N for 3000 rotations. The temperature range for

he wear tests was set from the room temperature to 300 ◦C.ome tests were performed at temperatures below the ambi-nt one (∼25 ◦C) to ensure that the entire temperature rangeith minimum wear was covered. This was achieved by plac-

ng the sample in a special metallic container that was thermallynsulated from the environment. The temperature inside the con-ainer was maintained below the room temperature using dryce. The temperature was monitored in situ during wear test

y a thermocouple. It should be indicated that because of theow sliding speed (1 cm/s) the temperature rise due to frictionould be minor and negligible. Surfaces of specimens were pol-shed using 1200# grit SiC sand-paper then etched. Each wear

8 ear 26

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est was repeated at least 3 times. The volume loss was deter-ined by measuring the profile of the wear track using a contact

rofilometer (Tencor Instruments, USA).

. Results and discussion

.1. Microstructure, phase analysis and mechanicalroperties

All samples were examined using the X-ray diffraction tech-ique to determine existing phases. Fig. 1 shows the X-rayiffraction pattern of an unmodified TiNi sample. Identifiedhases in this sample were: TiNi martensite (B19′), TiNi austen-te (B2) and Ni3Ti (hexagonal).

X-ray diffraction patterns of modified samples with 10 and0% ZrW2O8 (NTE), respectively, are presented in Fig. 2. The

ddition of 10% NTE resulted in the formation of: W/TiWcubic), Ti2Ni (cubic), ZrO2 (tetragonal), Ni4Ti3 (rhombohe-ral) and ZrW2O8. Below 770 ◦C the NTE phase is metastablehile above this temperature it may decompose and form

Fig. 1. X-ray diffraction pattern of an unmodified TiNi (HT) sample.

ig. 2. X-ray diffraction patterns of TiNi–10% ZrW2O8 and TiNi–30% ZrW2O8

amples.

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3 (2007) 858–865

ungsten oxide (WO3) and zirconium oxide (ZrO2) [17]. Theresence of ZrO2 and W/TiW phases indicated that the NTEhase was partially decomposed during the melting process.

The X-ray pattern of 30% NTE-containing TiNi sample wasimilar to that of the sample added with 10% NTE, but its ZrO2eaks were weaker which might imply less decomposition of theTE phase. Besides, the 30% NTE-containg sample showed Tieaks. It should be noted that W and TiW have the same spaceroup (Im3m, no. 229) and similar lattice constants [18], so thatheir X-ray patterns are similar or not distinguishable. Accord-ng to the Ti–W binary phase diagram [19], TiW solid solutions stable at high temperatures over a wide range of chemicalomposition but the maximum solubility of W in Ti at roomemperature is less than 2%. The cooling rate after melting waselatively high and some metastable TiW could thus be presentt room temperature. During the subsequent heat treatment, theetastable TiW solution could decompose to form a eutectoid ofi and W, which was however not identified by SEM and EDXt room temperature in the present study. This implies that theecomposition of TiW could be minor under the present exper-mental conditions. As a result, it is possible that both TiW and

inor W coexisted in the modified TiNi samples.Tungsten-containing TiNi samples were also examined using

he X-ray diffraction technique. Fig. 3 presents the X-ray patternf a 5% W-added sample. X-ray patterns of 10 and 20% W-addediNi samples were also determined, which are similar to thathown in Fig. 3. Identified phases from the X-ray pattern wereiNi martensite (B19′), TiNi austenite (B2), Ti2Ni and W/TiW.

There were some changes in microstructure when the NTEhase and W were added, respectively. Fig. 4 illustrates opti-al micrographs of TiNi samples with and without ZrW2O8ddition. The unmodified sample showed a typical dendriticicrostructure. When NTE was added to the TiNi alloy, theicrostructure became more complex.The change in microstructure after tungsten was added was

ot obvious. Fig. 5 shows an optical micrograph of a TiNi sampledded with 5% of tungsten. All the samples modified with 10 and0% tungsten showed similar dendritic microstructure but theicrostructure coarsened as the amount of tungsten increased.In order to obtain more information on changes in microstruc-

ure of the modified samples, SEM and micro-indentation testsere performed. Fig. 6 illustrates backscattered SEM imagesf TiNi samples with and without the NTE phase, respectively.he sample without the NTE addition showed a relatively

Fig. 3. X-ray diffraction pattern of a 5% tungsten-added TiNi sample.

I. Radu, D.Y. Li / Wear 263 (2007) 858–865 861

W2O

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(m1

Fig. 4. Optical micrographs of TiNi alloy modified with: (a) Zr

omogeneous TiNi matrix (Fig. 6a). According to the micro-ndentation test, the hardness of interdendritic regions (250 HV)as slightly higher than that of dendritic regions (215 HV).

hen NTE was added to TiNi, the microstructure became more

omplex, consisting of different domains in the TiNi matrix.he EDX analysis has shown that the black domain con-isted of titanium-rich phases having relatively high hardness

Fig. 5. An optical micrograph of 5% W-containing TiNi alloy.

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8 free; (b) 10% ZrW2O8; (c) 20% ZrW2O8; (d) 30% ZrW2O8.

750 HV) as measured using the micro-mechanical probe. Theicro-hardness of the black domain could be as high as about

000 HV when more NTE was added. EDX analysis indicatedhat some NTE phase was also present in the dark domain. Therey “eutectoid like” domain consisted of nickel-rich phaseshowing relatively lower hardness (∼570 HV). The whiteomain mainly consisted of hard NTE phase (∼1200 HV). Inhe 10% NTE-added sample, the NTE phase was found alson the dark domain and in the slightly white domain as wellFig. 6b). The remaining matrix consisted of TiNi and verymall precipitates having a micro-hardness of about 600 HV.

It is clear that the alloy was hardened by the NTE addi-

ion, demonstrated by the micro-indentation and macro-hardness

easurements (Table 2). Higher hardness could benefit the wearesistance.

able 2acro-hardness of investigated alloys

ample HRC Sample HRC

iNi 43 TiNi + 5% W 43.8iNi + 10% ZrW2O8 54.3 TiNi + 10% W 45.5iNi + 20% ZrW2O8 56.3 TiNi + 20% W 47.3iNi + 30% ZrW2O8 67.3

862 I. Radu, D.Y. Li / Wear 263 (2007) 858–865

) ZrW

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Fig. 6. SEM backscattered images of TiNi alloy modified with: (a

Fig. 7 illustrates a SEM image of a 20% W-added TiNi sam-le. As shown, the microstructure consisted of small (less than0 �m) W/TiW precipitates distributed in a TiNi matrix. SEM

mages of 5 and 10% W-added samples were also obtained,hich are similar to that of the 20% W-added one but has fewerrecipitates. Since W has a low solubility (0.3%) in TiNi [20],oth TiW and W should coexist in the TiNi matrix.

Fig. 7. An SEM image of 20% W-added TiNi sample.

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2O8 free; (b) 10% ZrW2O8; (c) 20% ZrW2O8; (d) 30% ZrW2O8.

The micro-indentation measurement of dentritic and inter-entritic regions of tungsten-added samples (see Fig. 5) showedhat micro-hardness increased with respect to the amount ofdded tungsten, reaching ∼300 HV for 20% W addition. Thereas almost no difference in micro-hardness between interden-

ric and dendritic regions. The addition of tungsten did not showtrong effect of precipitation-hardening or solution-hardening.

.2. Wear behavior

The wear performance of the samples as a function of tem-erature was evaluated using a high-temperature tribometerpin-on-disc). Fig. 8 illustrates wear volume losses of an unmod-fied TiNi sample from room temperature up to 300 ◦C.

As shown, the wear loss decreased in a temperature rangerom 80 to 140 ◦C (�T ∼ 60◦) for the TiNi sample. Previoustudies showed that the maximum drop of wear loss occurredbout 30 ◦C above Ms [8]. Since Ms point of the TiNi alloyas around 80 ◦C, the drop in wear loss should benefit from theseudoelasticity, from which the resultant rubber-like behaviorelped to diminish plastic deformation and retarded crack prop-

gation. The benefit of pseudoelasticity to the wear resistanceas been described in detail in previous papers [5,12].

When NTE phase and W were added to TiNi alloy, respec-ively, the temperature range within which the wear loss dropped

I. Radu, D.Y. Li / Wear 26

Fig. 8. Wear volume loss of an unmodified TiNi sample from room temperaturet

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increased with the amount of added NTE. It should be indicatedthat, however, adding too much NTE may not be beneficial, since

o 300 ◦C.

as much wider as shown in Figs. 9 and 10. As an example, 20%TE widened the temperature range, which was more than three

imes as wider as that of the unmodified sample. In addition tohe widening, the NTE phase led to a decrease in wear loss byne order of magnitude. Adding tungsten resulted in a similarffect, which was however much less effective, compared to theTE addition.It was noticed that the wear losses of the unmodified sample

nd those added with tungsten showed decreases in wear losst higher temperatures above the temperature ranges of wearrop. This could be attributed to the formation of oxide scalet elevated temperatures, since their surface color changed fromhite to brown. However, the NTE-added samples did not show

bvious changes in surface color as temperature increased withinhe temperature range under study.

tm

Fig. 9. Wear volume loss of NTE-added TiNi as a function of t

3 (2007) 858–865 863

The mechanism responsible for the widening of the temper-ture range corresponding to the wear drop can be explainedsing a well-known Clausius–Claperon equation [21]:

dσ

dMs= ρ�HB2→M

To�εB2→M (1)

here �εB2→M is the strain caused by the martensitic transfor-ation, �HB2→M the transformation enthalpy (negative), ρ the

ensity of the alloy and To is the thermodynamic equilibriumemperature. It has been estimated that dσ/dMs = 13.3 MPa/◦K22]. Therefore, a tensile stress can result in a positive �Ms,hile a compressive results in a negative one. The added NTEhase could thus introduce a T-dependent internal stress field,o that Ms varied with the environmental temperature, leadingo a wider temperature range in which the pseudoelasticity func-ions. This may happen because as temperature decreases, theTE phase expands while the TiNi matrix shrinks. As a result,

he austenite TiNi matrix is under compression, leading to aecrease in Ms.

In addition to the widening effect, the added NTE phasereatly increased the overall wear resistance as demonstrated.his improvement could arise from the increase in hardnessaused by the formation of new hard phases. Such improvementncreased with the amount of added NTE as Fig. 9 illustrates.

The widened temperature range of the wear loss drop also

he fraction of pseudoelastic matrix would be reduced and theaterial will lose its pseudoelasticity.

emperature: (a) 10% NTE; (b) 20% NTE, (c) 30% NTE.

864 I. Radu, D.Y. Li / Wear 263 (2007) 858–865

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Fig. 10. Wear volume loss of W-modified TiNi as a fu

Adding W resulted in a similar but much weaker effect onear resistance and widening of the temperature in which theear loss dropped. The weaker widening effect of W should be

ttributed to the smaller difference in thermal expansion coef-cients between W/TiW and TiNi austenite, which could notenerate a sufficiently large internal stress to effectively shifts. Besides, the W or/and TiW precipitates (Fig. 7) may not

arden the alloy as effectively as the complex microstructureaused by the addition of NTE phase (see Fig. 6).

It should be finally mentioned that unlike the unmodified TiNir W-modified samples, the wear loss of ZrW2O8-added samplesid not decrease as temperature was higher than 210 ◦C. Thisight be attributed to possible changes in structure and proper-

ies of their oxide scale, which did not show obvious color changet elevated temperatures, compared to those on the unmodifiednd W-added samples. Previous studies have demonstrated thatharder and more adherent oxide scale help to withstand theearing force [23]. In addition, the 30% NTE-containing sam-le showed a second drop in wear loss at negative temperatureshich is being clarified.

. Conclusions

The high wear resistance of pseudoelastic TiNi alloy largelyenefits from a thermoelastic martensitic phase transformation.owever, the resultant pseudoelasticity only functions within a

imited temperature range so that the wear performance of this

lloy is not stable with changes in temperature due to environ-ental instability and frictional heating. In this study, ZrW2O8

nd W were added to a TiNi alloy, respectively, with the aim ofidening the temperature range in which PE is effective. The

n of temperature: (a) 5% W; (b) 10% W; (c) 20% W.

econd phase additions could adjust the martensitic transforma-ion point (Ms) with changes in the environmental temperaturey introducing a temperature-dependent internal stress field aris-ng from the difference in thermal expansion coefficient betweenhe TiNi matrix and the second phase additions.

This study has demonstrated that adding second phaseshose thermal expansion coefficients are negative or positiveut markedly smaller than that of the TiNi austenite matrix isn effective way to enlarge the temperature range within whichhe wear loss is minimized. The larger the difference in ther-

al expansion coefficient between TiNi austenite and the addedecond phase, the larger is the temperature range of wear drop.esides, the added hard second phases reduced the overall wear

oss by one order of magnitude.

eferences

[1] E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1995.[2] J. van Humbeeck, R. Stalmans, P.A. Besselink, in: J.A. Helsen, H.J. Breme

(Eds.), Metals as Biomaterials, John Wiley & Sons, Chichester, 1998, pp.73–100.

[3] H. Fujita, H. Toshiyoshi, Micro actuators and their applications, Micro-electron. J. 29 (1998) 637–640.

[4] J. Ryhanen, M. Kallioinen, J. Tuukkanen, J. Junila, E. Niemela, P. Sand-vik, W. Serlo, In vivo biocompatibility evaluation of nickel-titanium shapememory metal alloy: muscle and perineural tissue responses and encapsulemembrane thickness, J. Biomed. Mater. Res. 41 (1998) 481–488.

[5] D.Y. Li, A new type of wear-resistant material: pseudo-elastic TiNi alloy,Wear 221 (1998) 116–123.

[6] Y.N. Liang, S.Z. Li, Y.B. Jin, W. Jin, S. Li, Wear behavior of a TiNi alloy,Wear 198 (1996) 236–341.

[7] D.Y. Li, Exploration of TiNi shape memory alloy for potential applica-tion in a new area: tribological engineering, Smart Mater. Struct. 9 (2000)717–726.

ear 26

[

[

[

[

[

[

[

[

[[

[

[

[22] K.N. Melton, O. Mercier, The mechanical-properties of NiTi-based shape

I. Radu, D.Y. Li / W

[8] T.C. Zhang, D.Y. Li, Variation in erosion resistance of pseudoelastic TiNialloy with respect to temperature, Mater. Sci. Eng. A Struct. Mater. Prop.Microstruct. Process. 329 (2002) 563–567.

[9] R.H. Richman, A.S. Rao, D. Kung, Cavitation erosion of NiTi explosivelywelded to steel, Wear 181–183 (1995) 80–85.

10] K. Otsuka, X. Ren, Physical metallurgy of Ti-Ni-based shape memoryalloys, Prog. Mater. Sci. 50 (2005) 511–678.

11] Y.N. Liu, J. Il Kim, S. Miyazaki, Thermodynamic analysis of ageing-induced multiple-stage transformation behaviour of NiTi, Philos. Mag. 84(2004) 2083–2102.

12] D.Y. Li, R. Liu, The mechanism responsible for high wear resistanceof Pseudo-elastic TiNi alloy—a novel tribo-material, Wear 229 (1999)777–783.

13] D.Y. Li, Wear behaviour of TiNi shape memory alloys, Scripta Mater. 34(1996) 195–200.

14] I. Radu, D.Y. Li, Enlarging the temperature range for maximum wear resis-tance of TiNi alloy using a NTE phase, Mater. Sci. Forum 539–543 (2007)3261–3266.

15] T.A. Mary, J.S.O. Evans, T. Vogt, A.W. Sleight, Negative thermal expansionfrom 0.3 to 1050 Kelvin in ZrW2O8, Science 272 (1996) 90–92.

[

3 (2007) 858–865 865

16] J. Uchil, K.P. Mohanchandra, K.G. Kumara, K.K. Mahesh, T.P. Murali,Thermal expansion in various phases of Nitinol using TMA, Phys. B-Condens. Matter 270 (1999) 289–297.

17] J.S.O. Evans, T.A. Mary, A.W. Sleight, Negative thermal expansion mate-rials, Physica B 241 (1998) 311–316.

18] JADE PDF cards #01-089-4900 and #01-049-1440.19] J.L. Murray (Ed.), Phase Diagrams of Binary Titanium Alloys, ASM Inter-

national, Metals Park, Ohio, 1987.20] K. Enami, M. Hara, H. Maeda, Effect of W addition on the martensitic

transformation and shape memory behaviour of the TiNi-base alloys, J.Phys. IV 5 (1995) 629–633.

21] P. Wollants, M. Debonte, J.R. Roos, Thermodynamic analysis of the stress-induced martensitic-transformation in a single-crystal, Z. Metall. 70 (1979)113–117.

memory alloys, Acta Metall. 29 (1981) 393–398.23] I. Radu, D.Y. Li, Investigation of the role of oxide scale on Stellite 21

modified with yttrium in resisting wear at elevated temperatures, Wear 259(2005) 453–458.