2.1.4. From 10<sup>3</sup> s−<sup>1</sup> to 104 s−<sup>1</sup>

The critical transformation stress increases remarkably with the strain rate in this range. The rise of transformation stress can be explained by the dislocation drag mechanism in the plastic deformation around the martensite interface [73,74]. Dislocation slips at very high strain rates need a much higher driving stress compared to those at the low strain rate due to the phono drag effect [60]. The phase interface moves with the high-speed dislocations on it and is thus subjected to the dislocation drag effect as well. Since the velocity of dislocations is a key factor in shock dynamics, the velocity of martensite interfaces should be considered in the transformation mechanism at high strain rates [23,65,74].

Split-Hopkinson bar systems are commonly used to conduct experimental tests in this strain rate range. Hudspeth et al. [75] developed a new technique to investigate the dynamic behaviors of SMA materials through simultaneous X-ray imaging and diffraction and gained a strain rate of 5000 s−1. Nemat-Nasser et al. [23–25] improved the Split-Hopkinson bar systems at a high strain rate in 2005. They showed that strain rate sensitivity increased sharply and observed a curved interface of martensite in TEM micrographs. Therefore, the velocity of the martensite interface migration plays an important role at this stage. Yang et al. [28] used a dynamic ex-situ neutron diffraction technique to characterize the rate effect of NiTi SMAs. Based on Yang's results, as the strain rate increases, plastic deformation replaces the martensite reorientation and the volume fraction of detwinning martensite decreases.

Recently, several Molecular-Dynamics (MD) models have been built to analyze the deformation mechanism of NiTi SMAs at a high strain rate level of around 103 s−1. For instance, Wang et al. [76] and Yazdandoost et al. [77] studied SMA crystallographic structure change in the transformation progress with MD in the shock condition. Yin et al. [78] and Ko et al. [79] simulated a NiTi nanopillar with MD under various strain rates and temperatures. They showed that the phase stress of B19 → B19 increased at a high strain rate because there was not enough time for atoms to reach new positions. Yazdandoost et al. [80] focused on the dissipation energy in the shock condition and indicated that transformation provided the main dissipative function.
