PhD Thesis Presentation
NiTi shape memory alloys are increasingly used in many fields due to its superelasticity, shape memory and large elastocoloric effects. However, the fatigue problem of NiTi material has been a major concern in the industrial applications. The required service life in industry is about 10 million cycles, while the real service life of bulk NiTi under tension and bending is only about several thousand cycles, which is much lower than the required service life, so there is a strong demand to enhance the tension and bending fatigue resistance of bulk NiTi.
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. Systematically study has shown 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, and displacement controlled bending fatigue tests show the fatigue life increases with the increase of pre-strain, the maximum fatigue life increases about 34 times after LSP treatment.
Finally, NiTi specimens with high bending fatigue life were fabricated with pre-strain laser shock peening 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 can achieve 10 GPa after laser treatment, which are caused by grain refinement and nano precipitates (Ti2N, TiN, Ni4Ti3, Ni3Ti, etc.) found at the LSP treated surfaces. And the maximum compressive residual elastic strain can achieve 4.3% at the LSP treated surfaces. The bending fatigue life increases about 461 times (0.47 million cycles) after LSP treatment comparing with the as received specimen (1026 cycles), such high bending fatigue life are mainly 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; Precipiates; Residual stress; Fatigue.
(Supervisor: Prof. Qingping SUN)