Mechanical behaviors of polycrystalline NiTi SMAs of various grain sizes under impact loading

This work presents mechanical properties of the NiTi polycrystalline superelastic shape memory alloys (SMA) of 5 different grain sizes under high-speed impacts. The amorphous, nanocrystalline (40, 80, 120 nm) and coarse grain (20 μm) sheets are manufactured with cold rolling and suitable heat treatments. A Hopkinson tensile bar is used to perform tests up to 45 m/s. High-speed camera system and digital image correlation method are used to get the strain field and particle velocity field at a sampling frequency of 2×10⁶ frames/s with a resolution of 924×768 pixels. Nominal stress-strain curves are obtained for all the sheets with a strain rate of about 1000 s⁻¹ and they have a similar evolution to the quasi-static case but with much higher stress levels. The rate sensitivity is increased with the grain size and the stress level can reach up to a 70% growth for a coarse grain sheet but be totally insensitive for the amorphous sheet in the strain rate from 10⁻⁴ to 10³ s⁻¹. A single transformation front can be found under high-speed impact (45 m/s) at the early loading stage. The speed of the transformation front is calculated from strain time histories and the highest front speed of 811 m/s is observed which is never observed before. It also reveals that the front speed depends also on the grain size. With the same loading speed, the bigger the grain size is, the slower the transformation front speed is.