Laser Induced Two Way Shape Memory Effect in NiTi Alloy*

Shape memory alloys (SMAs) are a unique class of smart materials widely used in biomedical and aerospace technologies. Here, we report an efficient and low-cost direct imprinting method to create thermally controllable surface patterns. Patterned micro-indents were generated on Ni₅₀Ti₅₀ (at. %) using an Nd:YAG laser with 1064 nm wavelength at 10 Hz. The effects of laser energy, shape recovery of NiTi, and simulation of shock wave was studied. Laser pulses were focused on the NiTi surface and generated pressure pulses of up to a few GPa. Optical microscope images showed that surface patterns with tailorable sizes can be obtained. The depth of the patterns increases with laser power and irradiation time. Upon heating, the depth profile of SMA surfaces changed where the maximum depth recovery ratio of 30 % was observed. The recovery ratio decreased and leveled off at about 15 % when the number of pulses and thus the well depth was increased. A numerical simulation of pressure evolution in SMAs showed a good agreement with the experimental results. The stress wave closely followed the rise time of the laser pulse to its peak value and initial decay. Rapid attenuation and dispersion of the stress wave were observed.