Processing and characterization of high-temperature nickel-titanium-hafnium shape memory thin films

Near-equatomic TiNi thin films are of interest as robust actuator materials in microelectromechanical systems (MEMS) due to the high mechanical energy density associated with reversible martensitic transformations. High-temperature (Ti+Hf)Ni shape memory thin films have been of particular interest because their high transformation temperatures allows use at higher temperatures at which binary TiNi films become inoperative, and also because they can improve the operation frequency for actuation. In this thesis, (Ti+Hf)Ni films having different hafnium contents were prepared by magnetron sputtering. Crystalline films exhibiting martensitic transformation were obtained either by depositing the films at elevated temperatures or by conducting a post-deposition anneal of as-sputtered amorphous films. Phase transformations, microstructure, mechanical properties and shape memory properties of films obtained by different procedures were investigated in order to find the best way to produce high-temperature shape memory films.; In films obtained by post-deposition annealing, the heating rates during annealing were found to significantly affect mechanical properties. Conventional vacuum annealing with heating rates ranging from 5°C/min to 40°C/min resulted in brittle films even though the as-deposited amorphous films were ductile, but rapid thermal annealing (RTA) with a heating rate of 6000°C/min resulted in ductile films having improved martensitic transformation characteristics. Embrittlement of the films annealed by conventional annealing occurred at temperatures well below the crystallization temperature. The RTA treated Ni _49.1Ti_36.2Hf_14.7 films had room-temperature ductility of 5.8%, fully recoverable strain of 1.7%, maximum recoverable strain of 1.9%, and maximum stress for full strain recovery 200 MPa.; In-situ deposited crystalline Ni_48.9Ti _36.6Hf_14.5 films demonstrated substantially better properties than the RTA treated samples. The transformation temperatures were higher, transformation hysteresis was lower, and most importantly, the mechanical properties and shape-memory properties were significantly improved. In-situ deposited crystalline Ni_48.9Ti_36.6Hf _14.5 films had room-temperature ductility of 8.3%, fully recoverable strain of 2.8%, maximum recoverable strain of 3.6%, and maximum stress for full strain recovery of 250 MPa. In addition, the transformation temperatures were effectively increased as more Hf was substituted for Ti. These properties of in-situ deposited crystalline Ni-Ti-Hf films put them in a good position for industrial applications where binary TiNi films are not applicable because of their lower transformation temperatures.