Microstructure and mechanical behavior of small scale NiTi structures during tensile testing

Understanding microstructural evolution in NiTi is an important prerequisite for the design of small scale structures and applications. In many previous studies, large-scale bulk testing of NiTi materials and its effect on microstructural evolution has been analyzed. Experimental reports using small scale samples and small scale testing set-ups are still limited. In this contribution, we study the relation between the microstructure of thin NiTi layers and their mechanical properties during tensile testing. A physical vapor deposition (PVD) process was used to sputter thin NiTi dogbone samples with a thickness of 5 microns and a width of 500 microns (grain size: about 5 microns). In addition, small struts with a similar grain size and with dimensions of 15 µm x 15 µm were investigated. The material was annealed at 550 °C. The phase transition behavior and microstructural details were characterized by X-ray diffraction and scanning electron microscopy. Tensile tests were performed using a specially designed experimental set-up equipped with video microscopy for high-resolution optical strain measurements. During tensile testing, both dogbone and strut samples exhibit pseudoelastic material behavior, and both sample geometries exhibit localization phenomena, but the formation and propagation mechanism of the martensitic bands markedly differs. Moreover, the reverse transformation during unloading depends on the sample geometry. We discuss how, consequently, the transformation behavior during subsequent loading and unloading cycles also differs between the two sample geometries.