Influence of the recovery and recrystallization processes on the martensitic transformation of cold worked equiatomic Ti–Ni alloy

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Abstract

The martensitic transformation of Ti–Ni shape memory alloys is very sensitive to thermomechanical treatments (cold work and annealings). In the present paper, a Ti–Ni alloy close to the equiatomic composition has been 40% cold rolled and then submitted to various annealing treatments, each one characterized by temperature and time (Ta, ta). A large range of annealing temperatures (538–778 K) and times (10 s–123 d) has been covered. From differential scanning calorimetry measurements, three zones have been identified with reference to (Ta, ta) values. Zone I, at low temperatures and times, is characterized by badly defined transformations on cooling as well as on heating; it corresponds to a recovery–reverse martensitic transformation stage. Zone II exhibits large changes of the transformation features which occur in two steps on cooling (R phase then martensitic) and in one step on heating; this zone corresponds to the recrystallization and growth of very small stressed grains. Similar transformation behavior can be obtained with different (Ta, ta) annealings, which can be characterized by an activation energy of ≈3.4 eV in the range 698–778 K. Zone III relates to a well defined martensitic transformation taking place in one step with almost no further evolution; it corresponds to the growth of stress-free grains.

Introduction

For various commercial applications, the Ti–Ni shape memory alloys are often cold worked [1]. This process introduces defects, such as dislocations, that for large strain amplitudes, inhibits the martensitic transformation. During the subsequent thermal treatments, the martensitic transformation is able to regenerate. As already shown in the literature, this occurs with some changes, in particular the emergence of a two step transformation on cooling for low temperature annealings: austenite→R phase→martensite [2], [3], [4], [5]. Much research has been carried out to study the effect of heat treatments on the evolution of the martensitic transformation [5], [6], [7], [8]. However, no systematic study has been performed in order to explain the changes of the martensitic transformation during the recovery and recrystallization processes. According to the microstructure evolution followed by thermoelectric power (TEP), two heat treatment stages have been defined elsewhere by the present authors [9]. These domains have been identified as corresponding to recovery and recrystallization processes with special features. The first domain is observed for low annealing temperatures (T<650 K) and is characterized by an apparent activation energy of ≈2.7 eV. The second domain is observed for higher annealing temperatures (T>650 K) and is characterized by an apparent activation energy of ≈3.6 eV. The aim of the present paper is to observe the evolution of the martensitic transformation properties during these annealing treatments.

Section snippets

Experimental procedure

A Ti–49.60 at.% Ni alloy, provided by the CEA (Atomic Energy Center) of Grenoble, is used for this study. Plates 66×4×1 mm3 are first annealed for 2 h at 1143 K, quenched in water and then 40% cold rolled in the martensitic state at room temperature. Small samples of 15 mg weight, used for differential scanning calorimetry (DSC), are carefully cut from this plate with a low speed diamond saw. The remaining plate is used for thermoelectric power (TEP) measurements, as described in Ref. [9] and

DSC measurements and behavior of the martensite transformation

Samples as annealed (2 h at 1143 K) exhibit only one transformation, as shown in Fig. 1(a). On cooling, the austenite to martensite transformation peak is noted (AM) and on heating, the martensite to austenite transformation peak is noted (MA). Just after cold work, no clear transformation is observed in the temperature range explored (Fig. 1b).

Fig. 2 shows typical DSC curves observed following various annealing treatments (more results are presented in Ref. [10]). After only 1 min at 778 K,

Discussion

It is well known that severe cold work, such as 40%, inhibits the martensitic transformation by the introduction of defects, which are essentially dislocations. Following cold work, the annealing treatment, as defined by temperature and time (Ta, ta), induces different features of the martensitic transformation related to the recovery and recrystallization processes.

In zone I (Fig. 3), small recovery of the martensitic transformation takes place and little quantitative information can be

Conclusion

This work, in addition to the previous work [9], presents an exhaustive study of the effect of annealing treatments after plastic deformation of Ti–Ni alloys. Three annealing zones characterized by (Ta, ta) couples have been evidenced:

  • Zone I of badly defined displacive transformations that corresponds to a combined recovery–reverse martensitic transformation stage.

  • Zone II of large changes of the martensitic transformation features which corresponds to the crystallization and growth of very

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1

Currently at Max-Planck-Institut für Metallforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany.

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