Full length articleIn situ scanning electron microscopy study of the thermoelastic martensitic transformation in Ti–Ni shape memory alloy
Graphical abstract
Introduction
Near-equiatomic Ti–Ni alloys are widely used in engineering, aeronautics, dentistry, and medical fields because of their superior shape memory effect and superelasticity. These properties are associated with the thermoelastic martensitic transformation from the cubic (B2) to the monoclinic (B19′) structure [1]. The self-accommodation microstructure is characteristic of thermoelastic martensite because it reduces the elastic strain energy during transformations through combining multiple habit plane variants (HPVs) [1], [2], [3], [4], [5]. We have investigated the self-accommodation morphologies of B19′ martensite in Ti–Ni alloys as shown in the schematic illustration in Fig. 1 [6], [7], [8]. The HPV notation follows Miyazaki's notation [9]. We found that there are three pairs of minimum units consisting of two HPVs with V-shaped morphology connected to the type I twins that have the K1 plane normal perpendicular to [111]B2. The theoretical habit plane index {0.88888 0.40443 0.21523}B2 [9], [10] was approximated to {942}B2 to simplify the analysis. We defined this V-shaped habit plane variant cluster (HPVC) as ‘2HPVC’ [6], [7], [8]. There are twelve pairs of minimum units in the whole crystal. Three kinds of self-accommodation morphologies based on 2HPVC develop around the [111]B2 trace, which have triangular, rhombic, and hexangular shapes that consist of three HPVs (3HPVC), four HPVs (4HPVC), and six HPVs (6HPVC), respectively. There are six combinations of 3HPVC, three combinations of 4HPVC, and only one combination of 6HPVC; therefore, there are 24, 12, and 4 combinations in the whole crystal, respectively. 3HPVC is equivalent to the morphology reported by Madangopal [11]. 6HPVC has been observed in in-situ and ex-situ nanoindentation tests on <111> orientated surfaces of a Ti–Ni alloy [12]. Although the pentagonal morphology (5HPVC) can be imagined geometrically (gray lines in Fig. 1), we have not observed it experimentally [6], [7], [8]. Because these results were obtained from static observations, such as reverse transformation relief, there is no dynamical information, such as the nucleation and growth of the martensitic phase. We have investigated the multistage martensitic transformation in aged Ni-rich Ti–Ni alloys by in situ scanning electron microscopy (SEM), which enables extensive observation at a high resolution [13], [14], [15]. This technique has the advantages of optical microscopy and transmission electron microscopy (TEM).
In this study, we directly observed the microstructural changes during the thermoelastic martensitic transformation from B2 to B19′ structures by in situ SEM. We examined the nucleation and growth of the martensitic phase, the interaction between HPVCs, and the forward and reverse transformation sequences, focusing on the order of HPVC formation. We have proposed that 6HPVC is the most favorable morphology for relaxing the incompatibility at the junction planes of the HPVs, and that 3, 4, and 6HPVCs are derivatives of 2HPVC, based on the geometry of the transformation [6], [7], [8]. The in situ observation identified the HPVC with the lowest energy barrier for the formation and the microstructure properties.
Section snippets
Experimental procedures
A commercially available Ti-50.8 at% Ni alloy rod 3 mm in diameter was used. The rod was cut into disks 1 or 0.5 mm thick for the differential scanning calorimetry (DSC) measurements and SEM observations, respectively. The disks were solution treated at 1173 K for 3.6 ks and then quenched in ice water. The DSC measurements were performed on a calorimeter (DSC-60, Shimadzu) with a cooling and heating rate of 0.17 K/s. The martensite start temperature (Ms), martensite finish temperature (Mf),
Overview of forward and reverse transformations
Fig. 2 shows a series of in situ SEM images of the quenched Ti-50.8 at% Ni alloy after solution treatment at 1223 K (Supplementary Movie M1). The surface normal is [17 13 14]B2 in grain B. There are no characteristic microstructures in the parent phase at 290 K, except for grain boundaries and nonmetallic inclusions, visible as the darkest contrast in Fig. 2a. The martensitic phase with V-shaped (2HPVC) and hexangular morphologies (6HPVC) initially appear at 232 K in grains A and B (Fig. 2b).
Discussion
The Gibbs energy of the martensite phase is equal to that of the parent phase at the equilibrium temperature, To. In general, the martensitic transformation does not start at To because the driving force required to overcome the energy barrier that is the sum of elastic strain energy and dissipation energy [24]. Supercooling (or superheating in the reverse transformation) is necessary to supply the driving force. In Ti–Ni, the driving force is stored by supercooling or superheating at a slope
Conclusion
The following microstructural aspects were obtained by in situ SEM observations of thermoelastic martensitic transformation from the B2 to B19′ structures in the Ti–Ni shape memory alloy.
- 1.
The hexangular morphology consisting of 6HPVs (6HPVC) homogeneously appeared in the interior of grain during the initial stage of transformation. Nucleation of 2HPVC triggered the formation of 6HPVC.
- 2.
Other morphologies, such as V-shaped (2HPVC) and triangular (3HPVC) morphologies, appeared along the strain
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research (A: No. 26249090, B: No. 15H04143) from the Japanese Society for the Promotion of Science and a Grant-in-Aid for Scientific Research on Innovative Areas, “Bulk Nanostructured Metals”, No. 25102707, from MEXT, Japan. This work was also partially supported by the Advanced Low Carbon Technology Research and Development Program (JY220218) from the Japan Science and Technology Agency.
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