Mechanical behavior of nanocrystalline and ultrafine-grained NiTi thin films

Nanoscale NiTi thin films are highly suitable for high frequency thermal actuation in microelectromechanical devices because of their small thermal mass and large surface-to-volume ratio. However, there have been few studies on the mechanical behavior of NiTi films with thicknesses below 200 nm. Here, we synthesized nanocrystalline (NC) and ultrafine-grained (UFG) NiTi films, 150 and 120 nm thick, respectively, by crystallizing amorphous NiTi films embedded with Cr seed crystals, and investigated their mechanical behavior via uniaxial tensile experiments. The experiments reveal that the stress–strain response of NC and UFG NiTi film samples is considerably different from their bulk counterparts. NC NiTi film samples show a combination of pseudoelasticity and shape memory effect even when the deformation temperature is above the austenite finish temperature, while UFG samples show significant increase in stress during phase transformation from austenite to martensite. Furthermore, small changes in the microstructure cause marked differences in the stress–strain response of the NiTi thin films, emphasizing the need to carefully control their chemical composition and thermal history to achieve reproducible properties.