In-situ investigation of dynamic deformation in NiTi shape memory alloys under laser induced shock

The free surface particle velocity of NiTi target shocked with a pulsed laser beam was measured with a photonic Doppler velocimetry (PDV) system to study the dynamic deformation behavior of NiTi alloys at ultra-high strain rate of 106∼107/s. Through the analysis of the particle velocity profiles, the shock wave intensity was found to have the influence on the process of austenite-martensite transformation. Theoretical analysis of shock wave propagation showed that the first plateau in the velocity profile was caused by martensitic transformation and the second plateau if existing is caused by the subsequent plastic yielding of shock induced martensite. Residual martensite of the NiTi, which exhibited as needlelike structures, was observed in the laser shocked region. Based on the present results, and the studies by Nemat-Nasser et al. (2005), Liao et al. (2012), and Wang et al. (2013), we concluded that laser induced shock can cause the martensitic transformation as long as the laser induced shock pressure reaches a critical value. The dynamic transition stress and the dynamic tensile strength of NiTi alloys were also determined from the experimentally measured surface velocity profile.