NiTi shape memory alloys are increasingly used in many fields due to their superelasticity, shape memory and large elastocoloric effects. However, the fatigue problem of NiTi material has been a major concern in industrial applications. In this thesis, new characterization methods to measure the residual stress and fabrication methods to enhance the fatigue resistance of nanocrystalline NiTi by Laser Shock Peening (LSP) were developed. Focused ion beam (FIB) was combined with digital image correlation (DIC) to measure the residual elastic strain distributions of nanocrystalline NiTi after pre-strain LSP treatment. A dual-pillar method was developed to get the true stress-strain curves of micron scale rectangular pillars; the accuracy of this method was verified by finite element...[ Read more ]

NiTi shape memory alloys are increasingly used in many fields due to their superelasticity, shape memory and large elastocoloric effects. However, the fatigue problem of NiTi material has been a major concern in industrial applications. In this thesis, new characterization methods to measure the residual stress and fabrication methods to enhance the fatigue resistance of nanocrystalline NiTi by Laser Shock Peening (LSP) were developed. Focused ion beam (FIB) was combined with digital image correlation (DIC) to measure the residual elastic strain distributions of nanocrystalline NiTi after pre-strain LSP treatment. A dual-pillar method was developed to get the true stress-strain curves of micron scale rectangular pillars; the accuracy of this method was verified by finite element method (FEM). Then, a novel pre-strain LSP without surface coating method was proposed to enhance the fatigue resistance of nanocrystalline NiTi material with gradient grain size and residual stress layers. The systematic study shows that the nanostructure and thermo-mechanical behavior of NiTi can be dramatically changed by increasing the pre-strain (from 0%-9%) during LSP treatment in water. The surface hardness can achieve 6 - 11.5GPa, compressive residual elastic strains at the surfaces increase with the pre-strain, and nano precipitates (Ni₄Ti₃, Ni₃Ti and Ti₂Ni) were found at the LSP treated surfaces. The non-isothermal cyclic tension and bending stability is significantly improved with the increase of pre-strain after LSP treatment. Displacement controlled bending fatigue tests show the fatigue life increases with the pre-strain, the maximum fatigue life increases 34 times after LSP treatment. Finally, NiTi specimens with high bending fatigue life were fabricated with pre-strain LSP in air, and the effects of pre-strain, temperature and coating on nanostructure evolution and mechanical properties of nanocrystalline NiTi were investigated. The maximum surface hardness achieves 10 GPa after laser treatment, caused by grain refinement and nano precipitates (Ti₂N, TiN, Ni₄Ti₃, Ni₃Ti, etc.) found at the treated surfaces. The maximum compressive residual elastic strain achieves 4.3% at the LSP treated surfaces. The bending fatigue life increases about 461 times (0.47 million cycles) after LSP treatment compared with the as received specimen (1026 cycles). Such a high bending fatigue life is due to the high surface compressive residual stress, heterogeneous gradient nanostructure as well as nano precipitates after nanosecond laser treatment. This work opens up a new pathway for developing next generation fatigue resistance NiTi shape memory alloys.

Keywords: Laser shock peening; NiTi; Gradient; Precipitates; Residual stress; Fatigue.