Phase Transformation Behavior Of NiTi Shape Memory Alloy Under Impact Loading

Shape memory alloy（SMA）attracts many research attentions because of its smart behavior such as the shape memory effect and the superelasticity derived from the reversible transformation between the austenitic and martensitic phases.Therefore,NiTi SMA has important actual and potential uses in aeronautical and automobile industries as well as in the biomedical applications.The quasi-static mechanical behavior of NiTi SMA has been extensively investigated in the last decades,including the rate dependent behavior and the propagation of the phase transformation band.However,the behavior of NiTi alloys under impact loading is rarely reported.Besides,with the recent development of the manufacturing path of nanocrystalline NiTi SMA alloys,the influence of the grain size on its thermal-mechanical behavior becomes a new research interest.Such an influence of the grain size under impact loading has been never reported.This dissertation investigates,therefore,the NiTi SMA behavior and its grain size influence under impact loading by both experimental and numerical methods.It includes three following parts:The first part presents the investigation of common commercial NiTi plates in a wide range of strain rate from 10^-4/s to 10³/s.A modified Split Hopkinson tensile bar（SHTB）is used for impact tests with a maximum tensile speed up to 53m/s.At the same time,we use digital image correlation method（DIC）to measure the displacement field of the specimen（consequently the deformation and particle velocity fields）.Under quasi-static loading,the behavior of NiTi alloys are mainly governed by the thermal-mechanical effect.The latent heat released during the phase transformation will give a hardening of the stress strain curves and more phase transformation bands will nucleate.However,under impact loading,there will be a significant increase of the phase transformation stress compared with the quasi-static case.Another important feature under impact loading lies in the fact that the phase transformation band formation is due to the stress wave effect.The nucleation site and the number of phase band depend on the inhomogeneous stress field.For the 53m/s test,the transformation front velocity can reach up to 595m/s（faster than previously reported）.The front velocity depends on many factors,such as the loading velocity,the number of phase transformation fronts and the strain ahead/behind of the front.The maximum growing rate of the cumulated martensitic transformed length is found to be 1594m/s,exceeding the shear wave speed in the martensite phase.In the second part,we choose a NiTi alloy model and use VUMAT subroutine to implement it in ABAQUS.The field and history output of ABAQUS will help us to understand the phenomenon observed in our experiment.The simulation confirmed that the nucleation site of the phase transformation front usually appears at the two end of the specimen due to the reflection of the stress wave.The simulated strain ahead of phase transformation front can increase to a high level（exceeding the prescribed transformation strain）for the site located at the middle of the specimen,just as observed in the experiment.The analy of the martensitic phase volume fraction during numerical tests reveals that the transformation at these sites can be stopped many times by a localized unloading before the front arrives.Meanwhile,idealized high-speed impact tests（without rise time）can be simulated using the validated numerical model.It allows illustrating that there could be a limitation of the front speed which is higher than shear wave speed but lower than elastic wave speed.In the third part,we report the manufacture of the different grain size NiTi materials from nanocrystalline to micro grain sizes with the cold rolling and heat treatments with different temperatures and quenching times.The quasi-static and impact behavior of different grain size NiTi alloys are tested in order to investigate the grain size effect on the behavior of the phase transformation.The experimental results show that there will be a notable phase transformation stress increase under impact loading with respect to quasi-static one,except for the case of amorphous sheet.The growth will be 70%for 20μm and decrease to 26%for 40nm sheets.As to the transformation front propagation,the specimen with 40nm grain size exhibits a front speed of 725m/s at the early stage under 45m/s impact loading.A highest front speed of 811m/s is recorded for the 80nm grain size material at the middle stage of a 45m/s impact test.Besides,we find that the front speed increases with the grain size at the same impact velocity in general.