Materials Science and Engineering A325 (2002) 249–254

Tensile behavior of Ti50.6Pd30Ni19.4 alloy under different tensileconditions

Qingchao Tian *, Jiansheng WuKey Laboratory for High Temperature Materials and High Temperature Test, Shanghai Jiaotong Uni�ersity,

Shanghai 200030, People’s Republic of China

Received 22 November 2000; received in revised form 25 April 2001

Abstract

Ti50.6Pd30Ni19.4 high-temperature shape memory alloy (SMA) has been prepared to investigate the mechanical properties undervarious tensile conditions. It has been found that shape memory effect (SME) exists even at temperature higher than austenitefinish temperature (Af), the mechanism is the reverse transformation of the stress-induced martensite at a higher temperature. Thestress–strain curve exhibits a cycle when the alloy was subjected to loading–unloading cycling at different temperature. But theshape and characteristic of the hysteresis cycle are related to the loading history. Complete linear pseudoelasticity is obtainedwhen the specimens are tested at temperature near Af, while elastic hysteresis for the specimens of a stable single phase. © 2002Elsevier Science B.V. All rights reserved.

Keywords: TiPdNi; High temperature shape memory alloy; Elastic hysteresis; Pseudoelasticity

www.elsevier.com/locate/msea

1. Introduction

High-temperature shape memory alloys (SMAs) haveattracted significant attention because of their potentialas functional materials at high temperature, such asactuators for aircraft engines, automobiles and pipecouplings, etc. In Ti50PdxNi50−x alloys, it has beenfound that the temperature of martensitic transforma-tion increases with the increase of Pd content from 20to 50 in atom percent. While Ti50Pd30Ni20 alloy hasbeen received more attention because it’s martensitictransformation temperature is higher than 200°C, thatis enough for engineering application [1–8]. Alloying,aging and thermomechanical treatment have been triedin order to improve the properties of shape memoryand pseudoelasticity, by several authors [5,8]. It wasreported that much improvement has been attained bysuch ways.

When an alloy was used in structural system to besubjected to cycling loading, it is necessary to under-stand the mechanical behavior of the materials. There

generally exists elastic hysteresis in most metal materi-als, while pseudoelasticity in SMAs if stress-inducedmartensite can be produced. Due to the limitation oftesting, few papers concerned with the tensile propertiesof TiPdNi high temperature SMA under loading–un-loading cycling have been published up to now.

In the present work, the mechanical properties ofoff-stoichiometric Ti50.6Pd30Ni19.4 alloy have been ex-perimentally investigated. It is found that in this alloyboth elastic hysteresis and linear pseudoelasticity exist.

2. Experimental

2.1. Sample fabrication

Ti50.6Pd30Ni19.4 alloy was made by arc melting 99%Ti, 99% Ni and 99.9% Pd on a water-cooled coppermould under a controlled protective argon atmosphere.One ingot was remelted four times, and then, homoge-nized in vacuum at 1000 °C for 5 h. Finally, the ingotwas hot-rolled into plate 1.1 mm thick at 800 °C, andsubsequently cold-rolled into 1 mm thick. Tensile speci-mens with gauge portions 16×2×1 mm3 and shoul-

* Corresponding author.E-mail address: qctian911@mail1.sjtu.edu.cn (Q. Tian).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 21 -5093 (01 )01472 -1

Q. Tian, J. Wu / Materials Science and Engineering A325 (2002) 249–254250

Fig. 1. The dimensions of the tensile specimen.

3. Results

The phase transformation temperatures of the alloyare measured as follows, the austenite start temperature(As) is 218 °C, the austenite finish temperature (Af)248 °C, the martensite start temperature (Ms) 230 °C,and the martensite finish temperature (Mf) 189 °C.According to the characteristic temperatures, the phasetype of the alloy at different temperature can bedetermined.

3.1. Stress–strain cur�es of specimens at differentphase state

Two specimens were tested to failure at room tem-perature and 320 °C, respectively (Fig. 2a). It can beclearly seen that the specimen at austenitic state yieldsat a stress (660 MPa) which is much higher than that(260 MPa) at martensitic state. But the martensiticspecimen shows higher strain-hardening ability, result-ing in a similar failure stress for the two kinds ofspecimen (1075 MPa). It can be seen from Fig. 2a thatthe elastic modulus of specimen at the state of austeniteis higher than that of martensite.

3.2. Shape memory properties

Fig. 2b shows the stress–strain curves of specimenstested at room temperature. The arrows indicate therecovery strains, in this figure the recovery rate is alsogiven. Three virgin specimens were loaded to 450, 750and 940 MPa, respectively. The total strain of the firstspecimen is 7.2% with a recovery rate of 100%. Thetotal strain of the second specimen is much higher,which is 11%, and the SME is incomplete (95%). Thethird specimen was deformed to a total strain of 12.9%without fracture, however, the SME is very poor (55%).The residual strain in the three experiments is 3, 6, and6.8% as the increase of applied stress.

Fig. 2c shows the stress–strain curves of a specimentested at austenitic state near the temperature of Af (theaustenite is metastable at such temperatures, martensitecan not be induced by stress). The Arabic numbers inthe figure and in the following figures refer to theloading–unloading times. The recovery strain was de-termined as follows, the specimen was cooled to roomtemperature after loading at 270 °C, then heated up to400 °C and finally cooling to room temperature. Asindicated by curve 1 and 2 in Fig. 2c, the SME isincomplete at 270 °C. When the temperature was in-creased to 285 °C, a 100% recovery rate can be ob-tained (curve 3 in Fig. 2c). Obviously, SME depends onboth the load and temperature. If the specimen issubjected to loading–unloading cycling at 285 °C, thehysteresis of the loading curve with the unloading onebecomes a cycle (curve 6 in Fig. 2c) in the sixth cycling.

ders were spark cut along the rolling direction, asshown in Fig. 1. The specimens were mechanicallypolished and then annealed at 400 °C in a quartzcapsule filled with argon for 1 h. After annealing, thespecimens were quenched into ice water by crushing thecapsule.

2.2. The determination of transformation temperatures

The phase transformation temperatures were deter-mined using differential scanning calorimetry (DSC) ofthe type 2910 MDSC with the specimen of 20 mgweight, and the heating rate is 5 °C min−1.

2.3. Mechanical testing

Tensile tests were carried out by using ‘ShimadzuAutograph’ tensile machine with the strain rate is 1×10−4 s−1, and the detailed schedules are as follows.

Firstly, two specimens were tested to failure at roomtemperature martensitic state and high temperatureaustenitic state, respectively. Therefore, the tensileproperties of specimens of different phases can beknown.

Then, experiments were carried out to determine theshape memory effect (SME) at both room temperatureand high temperature. Virgin specimens were used inevery test. Before testing, two parallel scratches weremade near the edges of gauge section in tensile speci-mens. The distance between scratches was determinedbefore loading (L0), after loading (L1) and after heat-ing to the temperature (Af+100) °C (L2). The recov-ery rate can be calculated by the formulae(L1−L2)/(L1−L0)×100%.

Finally, a set of virgin specimens was tested undervarious loading–unloading cycling conditions, i.e. alter-natively loading at single martensitic phase, at twophases of martensite and austenite, and at singleaustenitic phase. The experiment was designed to findthe effect of cycling history on the mechanical proper-ties. In the experiments, the displacement of the pin,which was inserted into the hole of the specimen, wasrecorded, and the deformation outside the gauge sec-tion was ignored.

Q. Tian, J. Wu / Materials Science and Engineering A325 (2002) 249–254 251

Fig. 2. Stress–strain curves of Ti50.6Pd30Ni19.4 tested at varioustemperatures. (a) Specimen tested at austenitic and martensitic state,SME (b) at room temperature and (c) at high temperature.

increased to a higher level, the specimen yields at ahigher stress correspondingly. This is the well-knownBauschinger effect. As the cycling proceeds, the cyclebecomes stable. The recoverable strain is 6.6% at thisload level. This experiment indicates that different re-coverable strain can be obtained under different loadlevel of training.

Fig. 3b represents the experimental results of a speci-men tested at 270 and 320 °C in sequence, i.e. ataustenitic state. The evolution of cycling process isomitted in this and the following figures. Firstly, thespecimen was under loading–unloading cycling at270 °C for obtaining a hysteresis cycle. The loadingand unloading curves close after four times of cycling,and the cycle becomes stable afterwards. Then, thetemperature was increased to 320 °C, the cycling wascarried out under the same load level, but a permanentstrain exists in this time (curve 7). After cycling fourtimes (curve 11), a permanent strain still exists.

Fig. 3c represents the experimental results of a speci-men tested at room temperature and 320 °C subse-quently. It is found that the loading–unloading curvescan be a cycle even though the specimen is at marten-sitic state, but the hysteresis cycle is very narrow. Afterthe temperature was increased to 320 °C, a stablehysteresis cycle (curve 10 in Fig. 3c) is obtained in thetenth cycling. If the result was compared with curve 11in Fig. 2b that attained at 320 °C, that incompletehysteresis cycle can be understood then, i.e. the toohigh load applied to the specimen, as a result, irre-versible slip exists in every time.

Fig. 3d represents the experimental results of a speci-men tested at 230 and 270 °C and room temperature insequence. Thus, the specimen undergoes cycling at twophases of austenite and martensite, at austenitic phaseand at martensitic phase subsequently. A permanentstrain exists at the first cycling at each different temper-ature, and a cycle can be attained for different cyclingtimes.

4. Discussion

4.1. Shape memory effect

The SME has been evaluated by room temperaturetensile test. With the increase of the deformation strain,the recovery rate decreases accordingly, meaning theincrease of irreversible deformation occurred in thealloy.

As shown in Fig. 2c, SME exists even at temperaturehigher than Af. This means that martensite induced bystress still exists in the specimen and the reverse trans-formation can not take place after the load is removed,although the martensite is unstable in the meaning ofthermodynamics. Therefore, not only one phase exists

For the easy description, the cycle is called hysteresiscycle in this paper.

3.3. Elastic beha�ior under loading–unloading cycling

Fig. 3a represents the experimental results of a speci-men tested at 250 °C, which is near the austenite finishtemperature. A virgin specimen was repeatedly de-formed, as shown in this figure. The pseudoelastictendency of the specimen appears in the first loading–unloading cycling, with the increase of cycling times, ahysteresis cycle can be obtained in the third cycling,and the recoverable strain is 5%. Then, the load was

Q. Tian, J. Wu / Materials Science and Engineering A325 (2002) 249–254252

in the specimens, there is stress-induced martensite aswell. When heating the specimen to a high temperature,the reverse martensitic transformation occurs, and thespecimen recovers its extension. The irreversible strainis produced by the deformation of austenitic phase.When loading at a higher temperature (285 °C), auniverse stress-induced martensite phase exists in thespecimen, thereafter, a 100% recovery rate can be ob-tained after unloading and heating the specimen to hightemperature.

As discussed above, the stress-induced martensite cannot transform back into austenite, Why? This mayattribute to the specific process of alloy preparation, i.e.

rolling and subsequently annealing, which result intexture structure, large amount of dislocations and fineprecipitates [5]. It is known that martensite in TiPd–Nialloy like Ti50+xPd30Ni20−x is of B19 type structure,which is of higher symmetry and hence has less variantsthan that in TiNi (B19�). So it cannot accommodate theapplied stress field as effectively as the B19� kind ofmartensite does, especially in specimens of texture na-ture. On the other hand, owing to the stress fieldproduced by precipitation, the reverse transformationof stress-induced martensite is compressed. It is theorigin of shape memory at high temperature. This alsoindicates that the work temperature of SMA can be

Fig. 3. Stress–strain curves of Ti50.6Pd30Ni19.4 alloy, tested at austenitic state (a) at different stress, (b) at different temperature, (c) tested ataustenitic and martensitic state, (d) tested at various temperature, and (e) warping up characteristic of linear pseudoelastic cycle.

Q. Tian, J. Wu / Materials Science and Engineering A325 (2002) 249–254 253

increased to the temperatures higher than the transfor-mation temperature by proper heat treatment process.However, the mechanism is different from that usuallysaid one, it is due to the reverse martensitic transforma-tion here.

4.2. Elastic beha�ior under different loading conditions

As shown in Fig. 3, the hysteresis cycles are differentto a certain degree. Specimens at martensitic state showa very thin cycle (curve 6 in Fig. 3c, curve 12 in Fig.3d), while the hysteresis cycle of the austenite is muchwider (curve 11 in Fig. 3b, curve 10 in Fig. 3c). As forspecimens in the temperature region of austenite trans-formation (curve 5 in Fig. 3d), or near the austenitefinish temperature, (Fig. 3a, curve 6 in Fig. 3b, curve 10in Fig. 3d), the loading curves of the cycles showwarping up tendency, as shown in Fig. 3e.

4.2.1. Elastic hysteresisIt can be considered that the hysteresis cycle for

specimens of a stable single phase, i.e. martensite oraustenite, is the common phenomenon of elastic hys-teresis. The dislocations in the alloy repeatedly moveunder loading–unloading cycling, leading to the occur-rence of a hysteresis cycle. As for the hysteresis cycle ofa specimen of martensitic phase, other contributoryfactors possibly exist, the reorientation of martensiticvariant and the transformation of different types ofmartensite. Martensite in a specimen at low tempera-ture is of thermal-induced martensite, and it may trans-form into stress-induced martensite when load isapplied to the specimen. A reverse transformation oc-curs in the unloading process, resulting in the existenceof a hysteresis cycle.

4.2.2. Linear pseudoelasticityThe hysteresis cycle of curve 6 in Fig. 2c is the

complete linear pseudoelastic cycle [9], which is causedby the reorientation of stress-induced martensite vari-ants. Because the reverse martensitic transformationcan not proceed, the pseudoelastic cycle is greatly dif-ferent from that of TiNi or CuZnAl kind of SMAs[10–12], of which the cycles usually appear a platformon the stress–strain curves in the process of stress-in-duced martensite.

Fig. 2a indicates that the elastic modulus ofaustenitic phase is different from that of martensiticphase. When the specimens were loaded near the tem-perature of Af, austenite and stress-induced martensitedeforms harmoniously in the loading process, resultingin the warping up of the loading curves. If the load isincreased to a higher level at the unchanged tempera-ture (Fig. 3a), the residual austenite transforms intomartensite, complete linear pseudoelasticity can be at-tained as well.

As to the specimen tested at the temperature regionof phase transformation, there coexist martensite,austenite and stress-induced martensite in the alloy. Theobtained hysteresis cycle contains the component ofboth elastic hysteresis and pseudoelasticity.

5. Conclusions

The alloy exhibits SME at both room temperatureand high temperature, and the SME is weakened withthe increase of tensile strain at room temperature. Thehigh temperature SME depends on both the load andtemperature. The mechanism is the reverse transforma-tion of stress-induced martensite, which still exist afterunloading. The yield stress for specimen in austeniticstate is much higher than that in martensitic state. Themartensite specimen shows a higher strain-hardeningability, but failure stress is similar for the two kinds ofspecimens.

Training the specimen under loading–unloading cy-cling at various temperatures, the hysteresis cycles canbe attained. As in a single stable phase, the cyclesbelong to the elastic hysteresis existing in most metals.The hysteresis in the martensite specimens show a verythin cycle, while that in the austenite specimens muchwider. As for the specimens tested in the temperatureregion of austenite transformation, or near the austen-ite finish temperature, martensitic transformationwould be induced by stress. Complete linear pseudoe-lastic cycle is attained after cycling. Pseudoelastic recov-erable strain can be changed according to the load levelof training. Austenite and stress-induced martensitedeforms harmoniously, resulting in the warping-upcurves in the loading process.

Acknowledgements

This work is sponsored by the Science and Technol-ogy Commission of the Shanghai Municipal Govern-ment, No. 00JC14055. The authors wish to expressthanks to the anonymous reviewer for his/her valuablecomments on this paper.

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