| Shape memory alloys (SMAs) are well known for their unique shape memory effect (SME) and pseudoelasticity. Their ability to recover large strains and dissipate mechanical work without macroscopic permanent deformation has generated significant interest in various fields of science and technology. Among all SMA, titanium nickel (TiNi) is the most important alloy. Currently, the most successful application of SMA is the use of TiNi in the medical field. This is attributed to its adjustable SME and pseudoelasticity, high corrosion resistance, and excellent biocompatibility. Despite significant insight into macroscopic behaviors of TiNi, basic knowledge of the underlying physical phenomena that control deformation at the nanometer scale, especially under thermal-mechanical contact loading, is relatively sparse.; In this thesis, TiNi films fabricated by the sputtering deposition technique were studied systematically to understand their thermal-mechanical behaviors, especially SME and pseudoelasticity of TiNi films under the nanocontact conditions. Surface force microscopy techniques were used to perform nanoindentations with diamond tips of different sizes in a temperature-controlled manner. Results reveal the occurrence of pseudoelasticity in indented austenitic and martensitic TiNi films at the nanometer scale, and the significance of normal load, nanoindenter tip radius, indentation cycles, time at maximum contact load, and loading/unloading rate on the transition from pseudoelastic to elastic-plastic deformation of the TiNi films. Transmission electron microscopy (TEM) with in-situ heating/cooling capability and electrical resistivity measurement were used to obtain insight into the effects of phase transformation on nanocontact deformation of TiNi films at different temperatures. An acoustic emission sensor was attached to the indenter holder to continuously monitor the contact deformation process of the TiNi films. Fingerprints of the damage processes of TiNi films in frequency domain were extracted by using fast Fourier transformation and short-time Fourier transformation analysis.; The findings of this thesis demonstrate the high potential of TiNi films in high-density storage, microactuator, dynamic micro/nano-device, and biomedical applications. |
