The superelastic cyclic deformation behavior of nanocrystalline NiTi at microscale is investigated. Cuboidal micropillars with sizes of 2 µm to 500 nm are fabricated by FIB from bulk nanocrystalline NiTi with average grain sizes ranging from 10 nm to around 100 nm produced via severe cold rolling and annealing. It is found that the micropillars with smaller grain sizes are more resistant to functional degradation than the large-grain-size counterparts. The NiTi micropillars with average grain sizes above 10 nm show significant functional degradation where the hysteresis loop area and the transformation stress demonstrate power-law decreasing trends, and the residual strain shows a power-law increasing trend as the cycle number increases. TEM observations show that the functional degradation is microscopically attributed to the motion and accumulation of transformation-induced dislocations and the resulting residual martensite. The former leads to the formation of multiple localized shear bands which result in steps and shear cracks on the micropillar surface and the latter is pinned by the created internal stress-fields of the former. Reducing the grain size significantly increases the resistance of NiTi to functional degradation. Optimal cyclic deformation behavior is achieved at a grain size of around 10 nm where the micropillars demonstrate exceptional resistance to functional fatigue and shows highly stable superelastic stress-strain curve with less than 0.2% decrease in the total elastic strain (including elastic strains of the two phases and the transformation strain) and less than 1% residual strain even after cycles of compression under a maximum stress of 1.8 GPa. The high cyclic stability of phase transformation in the 10-nm-GS micropillars stems from the high yield strength of 2.34 GPa and the low initial hysteresis loop area (<2 MPa) due to continuous and compatible martensitic transformation in the small grains.
(Supervisor: Prof. Qingping SUN)