THESIS
2019
xiv, 94 pages : color illustrations ; 30 cm
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 form...[
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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 10
8 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).
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