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TRIP like deformation of NiTi shape memory alloys

Superelastic and shape memory functional properties of NiTi shape memory alloys /SMAs/ due to martensitic phase transformation in solid state are already utilized in medical devices, automotive and space industries, robotics and civil engineering applications. An obstacle for further spread of the technology is the applicability of binary NiTi alloy limited to temperatures below 100°C, which disqualifies wide range of very promising engineering applications, particularly in automotive and aerospace engines.

It is generally believed that, as the temperature increases above 100°C, dislocation slip substitutes the reversible martensitic transformation as a deformation mechanism of thermomechanically loaded NiTi. Apparently, this seems to be the case, since the functional stress-strain-temperature responses of NiTi become gradually irreversible as the temperature increases above 100°C. However, the temperature range in which both deformation mechanisms proceed in parallel is unrealistically wide (100-300°C) and the stress-strain-temperature responses are not very well known. Curiously, they are phase reversible but not strain reversible, which creates confusion in the SMA literature. This has recently motivated SMA researchers to investigate deformation of NiTi at elevated temperatures and to try to increase the NiTi functionality to higher temperatures. Development of ternary high temperature SMAs with elevated transformation temperatures and improved strength has been focused in the field in the last decade. For example, NASA currently explores aerospace engineering applications of NiTiHf actuators with transformation temperatures as high as 300°C. Nevertheless, the problems with the strain irreversibility due to transformation – plasticity coupling remains, only are shifted to higher temperatures. The motivation to investigate the deformation mechanism of NiTi at elevated temperatures thus persists.

Figure 1: TRIP like deformation mechanism in NiTi wire subjected to thermomechanical loading at elevated temperatures and stress [1,2].

Researchers from the Department of Functional Materials of Institute of Physics of the Czech Academy of Sciences (FZU AV ČR) and Laboratory of Ultrasonic Methods ÚT AV ČR carried out extensive thermomechanical loading experiments with in-situ neutron and synchrotron x-rays studies, transmission electron microscopy and mechanics modelling to analyze deformation behavior of NiTi at elevated temperatures. The results were presented in three extensive research articles, two of them published in Progress in Materials Science [1] and International Journal of Plasticity [2] and the third one submitted for publication in the latter journal.

Figure 2: A schematic figure explaining the nature of B2=>B19´=>B2T transformation into twinned austenite observed in NiTi wire upon tensile loading at high temperature and stress. Within the energy landscape of martensitically transforming lattice, the high symmetry austenite phase appears in original reference configuration I and rotated configuration R at the same energy level - in mirror symmetry with respect to the center. The low symmetry martensite phase forms from the austenite via martensitic transformation upon cooling or mechanical loading by overcoming the energy barrier. Taking into account the external applied stress, the mirror symmetry is broken (the rotated austenite configuration R has lower energy than I ) and the martensite variant J belonging to the Ericksen-Pitteri neighborhood for I (b) may form martensite variant N belonging to the E-P neighborhood for R (d) by deformation twinning involving combination of plastic shearing and shuffling (c). Parent phase with reference configuration R (e) is then obtained by reverse transformation on heating, unloading or further unloading from the martensitic variant N. The deformation twinning in martensite, by which the very high energy barrier between two E-P neighborhoods of I and R is overcome, thus plays key role in the B2=>B19´=>B2T transformation. Deformation twinning in martensite forms twin interfaces in martensite (f) which are after reverse transformation inherited into austenite as twin interfaces in austenite (g) observed in experiments (Fig.1). This way the B2=>B19´=>B2T martensitic transformation leads to the refinement of austenitic microstructure.

The results of systematic experiments as well as theoretical analysis consistently suggested that the deformation mechanism acting in NiTi at elevated temperatures is not just simultaneous martensitic transformation and dislocation slip, but that the wires deform by a kind of collaborative TRIP like deformation mechanism somehow similar to that known from modern TRIP steels (Transformation Induced Plasticity). To discover and consistently describe the activity of such deformation mechanism in SMAs (Fig. 1), it was necessary to start thinking beyond the concepts of reversible martensitic transformation deeply rooted in the SMA field (Fig. 2).

Results of systematic tensile experiments on thin NiTi wires [2] revealed that plastic deformation accompanies martensitic transformation only when it proceeds under external stress and that the magnitude of those plastic strains increases with increasing temperature and stress at which the transformation proceeds (Fig. 1 left). A theoretical model of the coupling between transformation and plasticity was developed within the framework of continuum mechanics based on the requirement for strain compatibility at moving phase interfaces and stationary grain boundaries [2]. If martensitic transformation proceeds at temperatures below 100°C, this deformation mechanism causes only incremental plastic strains anytime the transformation (forward or reverse) proceeds under external stress. Upon thermomechanical cycling of the wire, those plastic strains accumulate gradually introducing internal stress and damage into the microstructure and ultimately cause fatigue failure. That is why the mechanism is essential for the structural fatigue of NiTi.

Results of experiments in which martensitic transformation proceeds at temperatures exceeding 100°C were summarized in an extensive review article published in Progress in Materials Science [1]. The key lesson learned was that the stress induced martensitic phase, although it shall be thermodynamically stable at the high temperature – high stress conditions imposed in experiments, “feels uncomfortably” under such conditions and tends to escape back into the parent austenite. The way how the martensite does it is demonstrated in figure 1 (right) for the case of constrained heating of the stress induced martensite in NiTi wire. While the stress rises and falls upon the constrained heating, the reverse martensitic transformation B19´=>B2T proceeds into twinned austenite. The same apparently curious sequential transformation austenite=>martensite=>twinned austenite (B2=>B19´=>B2T), however, proceeds also upon tensile loading at elevated temperatures up to 300°C. This sequential transformation is the core of the newly proposed TRIP like deformation mechanism involving coupled martensitic transformation, deformation twinning in martensite and dislocation slip. It was introduced and implemented into an earlier developed SMA model to extend its applicability up to elevated temperatures. Although NiTi wires do not show functional stress-strain-temperature responses at elevated temperatures and stresses, they can be plastically formed via the TRIP like deformation mechanism to give it new shape, refined microstructure with grain sizes in nanometer range and functional properties as desired [1].

References

[1] P. Šittner, P. Sedlák, H. Seiner, P. Sedmák, J. Pilch, R. Delville, L. Heller, L. Kadeřávek, On the coupling between martensitic transformation and plasticity in NiTi: Experiments and continuum based modelling, Progress in Materials Science, 2018, 98; 249-298,
https://doi.org/10.1016/j.pmatsci.2018.07.003
[2] L. Heller, H. Seiner, P. Šittner, P. Sedlák, O. Tyc, L. Kadeřávek, On the plastic deformation accompanying cyclic martensitic transformation in thermomechanically loaded NiTi, International Journal of Plasticity, 2018, in press
https://doi.org/10.1016/j.ijplas.2018.07.007