Improved fatigue resistance of pseudo-elasticNiTi alloys by thermo-mechanical treatmentVerbesserung der Ermudungsfestigkeit von pseudoelastischen NiTi-Legierungendurch thermomechanische Behandlung

E. Hornbogen, A. Heckmann*

Herrn Prof. Dr.-Ing. M. Pohl zum 60. Geburtstag gewidmet

Shape memory alloys are susceptible to two types of fatigue inaddition to classical fatigue: 1. Pseudo-elastic fatigue leads to anincrease in the slope of the pseudo-elastic plateau and final lossof pseudo-elasticity 2. A change in transformation temperature.Usually the martensite temperature is lowered with the numberof cycles until final loss of transformability. This paper describesmeasures to improve stability against both types of fatigue. Suchmethods are simple ageing in order to achieve precipitation in aus-tenite, and thermo-mechanical treatments such as ausforming thatintroduce lattice defects into austenite, which transforms subse-quently into martensite. Another method consists in the introduc-tion of defects into martensite by marforming plus subsequent age-ing. This ageing treatment has two purposes. It increases the clas-sical strength and restores the b-phase from residual martensite andconsequently it recreates transformability. It is shown that the lastmentioned method leads to the greatest effect in respect to stabili-sation against both types of fatigue. An additional effect of thesetreatments is a transition of localised to more homogeneous strain.Its relevance for fatigue resistance is still under investigation.

Key words: Shape memory; pseudo-elasticity; fatigue; thermo-mechanical treatment; transformation temperatures

Formgedachtnislegierungen sind zusatzlich zur klassischen Er-mudung anfallig fur zwei Arten von Ermudung. 1. Pseudo-elasti-sche Ermudung fuhrt zu einemAnstieg der Steigung im pseudo-ela-stischen Plateau. Dies kann schließlich zu Verlust der Pseudo-ela-stizitat fuhren. 2. Eine Veranderung der Phasenumwandlungstem-peraturen, haufig Absinken der Martensittemperaturen mit der Zy-klenzahl, bis zum Verlust der Umwandlungsfahigkeit. Es werdenMaßnahmen beschrieben, die der Erhohung der Stabilitat gegenbeide Arten der Ermudung dienen. Bei diesen Methoden handeltes sich sowohl um einfache Alterung des Austenits, die zur Bildungvon Ausscheidungen fuhrt, als auch um thermo-mechanische Be-handlungen wie, Ausformen, welches Gitterfehler im Austenit er-zeugt, die anschließend martensitisch umwandeln. BeimMartensit-Verformen plus nachfolgender Alterung, werden Gitterdefekte imMartensit erzeugt. Die beim Marformen angeschlossene Alterungverfolgt zwei Ziele. Sie erhoht die klassische Festigkeit und stelltdie Umwandlungsfahigkeit der b-Phase wieder her. Es hat sich ge-zeigt, daß die zuletzt genannte Methode den großten Effekt in Be-zug auf Erhohung der Resistenz gegen beide Arten von Ermudungerzielt. Ein weiterer Effekt dieser Behandlung ist ein Ubergang vonstark lokalisierter zu homogenerer Verteilung der Dehnung. Die Re-levanz dieses Effekts fur Ermudung wird noch untersucht.

Schlagworte: Formgedachtnis, Pseudoelastizitat, thermo-mecha-nische Behandlung, Umwandlungstemperaturen

1 Introduction

There have been many attempts to find practical uses forshape memory alloys. These have been especially successfulin medical engineering. However for many applications thelimited fatigue resistance of these new materials has been aproblem. Stents can serve as an example for fatigue in pseu-do-elastic applications. These are implanted to human arteriesand exposed to 60 loading cycles in a minute by the beat of thehuman heart (Fig 1) [1]. An example for applications wherethermal cycles are used are gripper systems which could beapplied for positioning (Fig. 2) [2]. Again in this case fre-quently repeated shape changes are required. A completelydifferent field comes from applications where the extremelyhigh damping capacity is used. In such cases the number ofcycles is by many orders of magnitude higher than for the ap-plications which have been mentioned earlier. In any case de-velopments are important by which the fatigue resistance ofthis new material is improved.

* Institut fur Werkstoffe, Ruhr-Universitat Bochum

Fig. 1. Stent implanted in human artery loaded with ~ 60 min-1

Abb. 1. Ein in eine menschliche Arterie implantierter Stent zy-klisch beansprucht mit ~ 60 min-1

464 0933-5137/03/0505-0464$17.50 þ .50/0 Mat.-wiss. u. Werkstofftech. 34, 464–468 (2003)F 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Temperature ranges

Primarily thermal cycles may range between the stable aus-tenite T1>Af and the stable Martensite T6<Mf. For all pseu-do-elastic applications the temperature at which the mechan-ical load is applied is important. We distinguish mainly threetemperature ranges (Table 1). The temperature range wherethe austenite phase is thermo-dynamically stable. This meansthe austenite is not transforming by any applied load. In thelow temperature range 100% Martensite has formed which isonly deforming, T6. In an intermediate temperature range thealloy is susceptible to transformation. In this temperaturerange we distinguish: 1. Strain or plasticity induced: Plasticstrain in austenite is preceding the transformation into Mar-tensite, T2. 2. Stress induced: The martensitic transformationoccurs above the martensite start temperature Ms, but withoutpreceding plastic deformation of the austenite, T3, T4. 3. Stressinduced and detwining: The state where the alloy is partiallytransformed below Ms but additional stress induced marten-

sitic transformation and reorientation of the previouslyformed martensite crystals is observed, T5 (Table 1).

The temperature range of stable Austenite can be subdi-vided further in the lower range T1 where no diffusion willoccur and the still higher temperature range T0 above 1/3of the melting temperature T>200 �C. The use of the alloyshould always be safely below the level where diffusioncan occur, in which however primary shaping can be done.

For applications of the alloy and for fatigue investigations ithas always to be verified first in which of these temperatureranges the testing procedure or the use is desired. All experi-ments were conducted with a binary NiTi alloy consisting of50.7 at % Ni. Initial transformation temperatures in the solu-tion heat treated state were Mr ¼ �20 8C and Af ¼ 15 8C.

3 Thermo-mechanical treatments

If methods are considered to improve the fatigue perfor-mance of shape memory alloys two requirements have tobe taken into account. 1. The functional properties whichare associated with the martensitic phase transformationshould not be deteriorated. 2. The classical, i.e. strength re-lated properties should be improved. In this context earlierwork in the field of ultra high strength steels is useful. Twomethods are known which all have to do with martensiticallytransforming iron base alloys. 1. Ausforming: this procedureimplies hot deformation of austenite before martensitic trans-formation can occur. In this way lattice defects especially dis-locations are introduced into austenite, that are in turn incor-porated into the martensite phase, which forms immediatelyafterwards by cooling [3]. This method leads to the highestyield strength which can be achieved up to the present datein steels. The second method is a two step process. Plastic de-formation of martensite plus re-heating to produce precipita-tion. This method cannot be applied for classical carbon steel.It is used for increasing the already high strength of mar-age-ing steels. These steels are carbon-free. They produce a mar-tensite phase which is only moderatly hardened. The majorstrength is achieved by a subsequent tempering treatment[4]. Again this method leads to steels with extremely highstrength. In the present paper results are reported in which aus-forming and marforming with subsequent ageing treatments,are applied for shape memory alloys [5]. Different from steelsnot only strengthening but also preservation of the transform-ability are additional requirements.

Table 1. Temperature ranges for shape memory alloys

Tabelle 1. Temperaturbereiche fur Formgedachtnis-Legierungen

Temperature range Condition / Material behaviour Phase-Transformation

1/3 Tm < T0 < Tm normal classic Diffusion processes

Md < T1 < 1/3 Tm normal classic

Mr < T2 < Md Plastic + transforming Strain induced: b!a

Ar < T3 < Mr Pseudo-elastic)

Stress induced: b!a)Detwinning: a + a* !a

Ms < T4 < Af Pseudo-plastic, one-way effect

Mf < T5 < Ms Partially transformed

T6 < Mf Detwinning of stable thermal martensite

Fig. 2. NiTi Gripper [2]

Abb. 2. NiTi-Greifer [2]

Mat.-wiss. u. Werkstofftech. 34, 464–468 (2003) NiTi 465

4 Pseudo-elastic stress strain curves

Pseudo-elastic stress strain curves are obtained in the tem-perature range considerably above Af but also clearly belowthe classical yield stress of the alloy. Fig. 3a shows the effectof repeated cycling of an as betatized (Solution heat treated =SHT) and untreated alloy. The pseudo-elastic behaviour is di-minished after a few cycles. These results are compared to thesame alloy after two special treatments. The first consists inageing of the austenite of an alloy composition which is sus-ceptible to fine scale precipitation (Ageing heat treated =AHT) . The second shows the effect of a thermo-mechanicaltreatment consisting in marforming plus ageing (thermo-me-chanical treated = TMT). The simple ageing treatment showsalready a shift towards stabilisation of the pseudo-elastic be-haviour. This effect is much more pronounced for the thermo-mechanical treatments. An important feature of fatigue is thechange in shape, especially the slope of the plateau stress ofthe pseudo-elastic stress strain curve Figure 3c indicates thatthe slope is increasing. It can be expected that after a still high-er number of cycles, pseudo-elasticity diminishes further anda normal, classical linear elastic behaviour is reached.

Single stress strain curves (first cycle) characterise the dis-tinct behaviour of the differently treated alloys (Fig. 4). Thepreservation of the plateau indicates that transformability ispreserved in all cases. The mayor difference consists in an in-crease in the yield strength and plastic elongation of the trans-formed condition.

5 Transformation temperatures

The preservation of transformability after the three differ-ent treatments is also confirmed by DSC measurements (Fig.5). The mayor feature of cycling is a shift in transformationtemperature. This is undesired for most applications. Fig. 5ashows the transformation behaviour after the different treat-ments. Beside the change in martensitic transformation tem-

Fig. 3. Pseudo-elastic cycling: a) SHT b) AHT c) TMT; d) Change in plateau slope

Abb. 3. Pseudo-elastisches Zyklieren: a) SHT b) AHT c) TMT; d) Anderung der Plateau-Steigung

Fig. 4. Tensile test until fracture for all conditions

Abb. 4. Zugversuch bis zum Bruch fur alle Zustande

466 E. Hornbogen and A. Heckmann Mat.-wiss. u. Werkstofftech. 34, 464–468 (2003)

perature the occurrence of an preceding R-phase can be no-ticed. This phase however leads to an order of magnitudesmaller strains as compared to the martensitic transformation(eR � 1%). Consequently it is not of great relevance for thehigh strain amplitude in pseudo-elastic applications (eba �8%). Fig. 5b shows the change in transformation temperaturefor the aged state. It indicates relative stability of transforma-tion temperatures but also change in transformation character-istic. The results of cycling are summarised in Fig. 6. It isquite evident that the untreated condition shows least stability.The transformation temperature is lowered while the thermo-mechanically treated condition provides an almost constanttransformation temperature.

6 Homogeneity of strain and effect offrequency

As pseudo-elastic behaviour implies a high damping capa-city, effects from the enthalpy of phase transformation cannotbe neglected.. As a consequence the frequency of mechanicalloading should be of importance. Because heating effects ofthe materials are unavoidable due to the thermal irreversibilityin the cycles. This in turn leads locally to an changing positionof the material in the temperature range (Table 1). An in-creased local temperature implies that higher stress is requiredfor further stress induced transformation. Consequently for-mation of new transformation nuclei is favored. In this contexta related phenomenon has to be mentioned. Both thermo-me-chanical treatment as well as an increase in loading frequencywill lead to a transition from localised transformation to amore homogeneous transformation. This means that in theas-betatized condition and at low frequency the transforma-tion occurs in a single band that moves across the specimenlength. The effects of thermo-mechanical treatment and an in-crease in frequency lead to much more homogeneous strain[6]. The relevance of this strain homogeneity on the bulk fa-tigue strength is still under investigation.

7 Conclusions

Thermo-mechanical treatments like ausforming and mar-forming as well as ageing have the effect to raise the classicalyield strength of shape memory alloys, as it is well knownfrom steels. In addition treatments can be found which pre-serve the reversible transformability and consequently thefunctional properties of shape memory alloys. A generalrule can be established for the improvement of the shapememory alloys in respect to mechanical as well as thermalfatigue. The transformation stress rba (internal or external)should be much less than the classical yield stress of the ma-terial. As the transformation stress is fixed for example at am-bient temperature, the improvement requires a raise of theclassical yield strength of the material without loss in trans-formability.

Fig. 5. DSC measurements for a) the initial state of all studied conditions b) AHT after 1 and after 12 cycles at various frequencies

Abb. 5. DSC-Untersuchungen fur a) alle Ausgangszustande und b) TMT-Zustand nach pseudo-elastischer Ermudung bei 1 und bei 12Zyklen

Fig. 6. Change in martensite peak temperature and transformationcharacteristic due to mechanical cycling (R-phase transformation isnot shown).

Abb. 6. Veranderung der Martensit-Peak Temperatur und Um-wandlungscharakteristik durch pseudo-elastische Zyklen. (R-Pha-senumwandlung ist nicht gezeigt).

Mat.-wiss. u. Werkstofftech. 34, 464–468 (2003) NiTi 467

8 References

1. T.W. Duerig, A.R. Pelton, D. Stockel, presented at Int. Conf. onDisplacive Phase Transformations, Urbana-Champaign, USA,1996, pp. 141–147.

2. J. Hesselbach, R. Pittschellis, R. Thoben, H.S. Oh, Zeitschrift furWirtschaftlichen Fabrikbetrieb 1996, 91, 437.

3. V.F. Zackey, E.R. Parker et al., UCRL Report 18466, 1968.

4. J.M. Chilton, P.M. Kelly, Acta metallurgica 1968, 16, 637.5. E. Hornbogen, as [1], pp. 27–35.6. A. Heckmann, E. Hornbogen, Mater. Sci. Forum, 2002,

394–395, 325.

Prof. Dr. E. Hornbogen, Institut fur Werkstoffe, Ruhr-UniversitatBochum, Universitatsstr. 150, D-44780 Bochum, e-mail: andreas.heckmann@ruhr-uni-bochum.de

Received in final form: 2/6/03 [T 640]

Fachschrift

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468 E. Hornbogen and A. Heckmann Mat.-wiss. u. Werkstofftech. 34, 464–468 (2003)