TWO TYPES OF THE FIRST CYCLE EFFECTS IN COPPERBASED SHAPE MEMORY ALLOYS

E. Hornbogena, V. Mertingerb and J. SpielfeldaaRuhr Universita¨t Bochum, Institut fu¨r Werkstoffe, Lehrstuhl Werkstoffwissenschaft, Bochum,

GermanybUniversity of Miskolc, Institute of Material Science, Department of Physical Metallurgy,Femtani Tanszk´, Hungary

(Received November 4, 1998)(Accepted in revised form February 11, 1999)

1. Introduction

The effects of thermo mechanical treatments on transformation temperatures and hysteresis areimportant for many shape memory applications. Depending on the field of application either a large orsmall hysteresis is required. A high hysteresis for the first cycle is especially important for one wayapplications, when the material has to be stored in the transformable martensitic condition. For thisreason the material is deformed slightly in the martensitic state (TMF , Mf). The As and Af temperaturesare increased by this treatment for the first transformation cycle. For the following cycles thesetemperatures nearly regain the value of the undeformed state. It is known from several investigationsthat the martensite transformation temperatures depend on the thermo-mechanical history of the alloy.Treppman et al.(1), Spielfeld et al.(2) and also Zhang et al. (3) found that the defects formed inmartensite domain boundaries are able to induce a shift of austenite temperatures to significantly highervalues and thus increase the transformation hysteresis of the first cycle.

In this work it will be shown that a different first cycle effect occurs after ausforming of the Copperbased shape memory alloy. For the first cycle the As temperature is lowered. For the following DSCcycles it is increased nearly to its original value after betatising (Fig. 1).

2. Alloy and Experimental Method

The investigated alloy was produced by Kobe Steel Company (Japan). The ausforming treatments havebeen described in detail in the earlier papers (1,4).The ausforming temperatures raised between 190°Cand 950°C. The amount of deformation waswAF 5 0.2–0.8. As MF.20°C marforming can beperformed by rolling at ambient temperature. Table 1 shows the chemical composition and transfor-mation temperatures of the as betatised alloy. The transformation temperatures were measured by DSC.

3. Experimental Results

Fig. 1a compares the subsequent cycles with the first cycle (dashed) after the martensitic state has beendeformed somewhat more than required for reorientation of the domains. This optimum amount of true

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plastic deformation in case of marforming iswMF'1%. At smaller amounts only reorientation ofdomains occurs. Consequently no effect can be found. At higher amounts the re-transformability isreduced or totally impeded (“dead martensite”wMF$4%). After marforming the transformation tem-perature is raised by aboutDTAp 5 20°C, while the subsequent cycles behave normally. The reversetransformation occurs repeatedly at the lower temperature as it is found for the undeformed (asbetatised) state. This effect can be explained by the introduction of lattice defects in the martensitewhich are annihilated by the first reverse transformation. Stabilisation of martensite due to ageingprocesses does not show this behaviour, because the shift of the transformation temperature ispermanent.

Fig. 2 provides an example for this effect showing the DSC peaks for the first and secondtransformation cycles of the marformed CuAlNiMnTi shape memory alloy for high temperature use.The third and all the following reverse transformation cycles coincided with the second one. The firstmartensitic transformation occurs during quenching, can not be analysed.

A completely different effect has been discovered recently after the ausforming of this alloy. For thefirst DSC cycle the transformation temperatures are lowered as compared to the undeformed state. Theaustenite start temperature is only slightly above the range of martensitic transformation (the hysteresisis very small) (Fig 1b). The reverse transformation in the subsequent cycles takes place at considerablyhigher temperatures. The Fig 3, 4, and 5 show results for this unusual first cycle effect.

Figure 1. Martensitic transformation cycles, schematic. Two types of first cycle effects (a) after marforming (MF type), (b) afterausforming at;850°C plus ausageing of the as quenched state at 200–300°C (AF type), first cycles: dashed.

TABLE 1Chemical composition and transformation temperatures of the

betatised starting condition: betatised at 850°C for 30 minsubsequenct water quenched. Ass-austenite start, Af-austenitefinish, Ms-martensite start, Mf-martensite finish temperature

determined by DSC (second cycle).

Element

Composition, wt %

Al Ni Mn Ti Cu

11,9 5 2 1 Rest.

Transformation temperatures, °C

Ms: 126 Mf: 105 As: 124 Af: 139

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In order to explain this phenomenon it has to be considered that the end of the reverse transformationAf interferes with the temperature range where thermally activated processes become considerable.Therefore ageing experiments were conducted from which it is quite evident that in a temperature rangebetween 100°C and 400°C an increase of the transformation temperature occurs as function of

Figure 2. Typical DSC curve of marformed state. Heating and cooling rate: 10°C/min, plastic deformation of martensitewMF'1%, (type MF first cycle).

Figure 3. Typical DSC curve of ausformed state. Heating and cooling rate: 10°C/min, ausformed at 700°C,wMA 5 40%, (typeMA first cycle).

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temperature and duration of ageing (5,6,7). Additional experiments showed that this effect was stillmore pronounced if the alloy was quenched from a highly deformed state i.e. after ausforming (Fig 4).From this follows an explanation for this different type of first cycle effect based on ageing. The lowdegree of perfection of order of theb phase in the as quenched condition is still more reduced if thealloy is quenched from the as ausformed condition. During heating for the DSC analysis (Fig 5) thealloy was unintentionally heated into the temperature range (;200°C) where the return to higherperfection of order can take place. Consequently the following reverse transformations took place at ahigher temperature. As expected this shift of the peak is a function of temperature and duration ofageing in austenite.

4. Discussion, Conclusion

Ageing of austenite provides the best interpretation of the results of the first cycle effect in theausformed alloy. It changes the perfection of order in theb phase. Simultaneously minor bainitictransformation is evidently not effecting the change in the matrix. Introduction of defects by ausforming

Figure 4. Austenite transformation temperatures of the ausformed (at 750°C) samples with different amounts of deformation(w 5 0. . . . . .0.8) after the first and the second DSC cycles, and the amount of the effect Ap(2)-Ap(1) 5 DT.

Figure 5. Transformation temperatures after several DSC cycles indicating confirmed ageing effects.

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increases imperfection in the matrix and therefore increases the effect. The fact that there is only aneffect on the reverse transformation into the austenite, but not on the transformation into the martensite,can be easily explained. Only the first reverse transformation takes place in the unaged condition. Thefirst observable transformation into martensite starts from a material which had already undergoneageing at above 200°C during the DSC experiments. The condition of the following martensiteformation corresponds to the second cycle. The first formation of martensite went on during quenchingto 20°C and it is not observable.

Transformation temperatures and hysteresis of the martensitic transformations can be varied in awide range by chemical composition and thermo mechanical treatment. Very often a high transforma-tion temperature and hysteresis is aspired only for the first cycle. This paper should contribute to a betterunderstanding of first cycle effects. It can be summarised that the two effects described in this paperhave completely different origin. The effect after marforming (MF type, Fig. 1a): The introduction ofa new type of “small burgers vector” defect into martensite. These defects are removed by the firsttransformation cycle (Fig 6). The effect after ausforming (AF type, Fig. 1b) requires: Reduction ofcrystallographic order and possibly introduction of point defects by thermo mechanical treatment. Theorder is regained by ageing during heating only slightly above Af temperature. This results in a shift tohigher transformation temperatures which depends on temperature and duration of ausageing (Fig. 5).

Acknowledgement

Thanks are due to the Volkswagen -Foundation for supporting a stay in Bochum for V. Mertinger. Theauthors wish to express their appreciation to K. Rittner, M. Hu¨hner and A. Yawny who have contributedto this paper with experimental results or valuable discussion.

References

1. D. Treppman, E. Hornbogen, and D. Wurzel, Z. Metallkd. 89, 126 (1998).2. J. Spielfeld, E. Hornbogen, and M. Franz, J. Phys. France. 7, C5–239 (1997).3. Y. Zhang and E. Horbogen, Z. Metallkd. 79, 13 (1988).4. E. Hornbogen, ICOMAT 1998, to be published.5. E. Hornbogen, J. Spielfeld, and V. Mertinger, Z. Metallkd. to be published.6. E. Hornbogen and E. Kobus, Z. Metallkd. 83, 105 (1992).7. K. Sugimoto, K. Kamei, and M. Nakaniwa, in Engineering Aspects of Shape Memory Alloys, ed. T. W. Duerig, p. 89,

Norwalk, (1990).

Figure 6. Defect structure after marforming and after reverse transformation: annihilation of defects during heating with theaustenite state, schematic.

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