Cyclic deformation behavior of NiTi shape memory alloys at microscale

Abstract

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 NiTi with different microstructures produced via severe cold rolling and annealing. The nanocrystalline NiTi micropillars with large grain sizes 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. SEM and 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. It is found that the micropillars with smaller grain sizes are more resistant to functional degradation than the large-grain-size counterparts. Reducing the grain size significantly increases the resistance of NiTi to functional degradation. Optimal cyclic deformation behavior is achieved by a composite structure of 10 nm nanocrystals embedded in amorphous phase 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 10⁸ cycles of compression under a maximum stress of 1.8 GPa. The high cyclic stability of phase transformation in the micropillars with a composite structure stems from the high yield strength of 2.34 GPa and the low initial hysteresis loop area (<2 MPa).