2013 Theses Doctoral

Modification and Integration of Shape Memory Alloys Through Thermal Treatments and Dissimilar Metal Joining

Satoh, Gen

While Shape Memory Alloys (SMAs) have been the topic of numerous studies throughout their history, over fifty years after the first observation of the shape memory effect, their widespread use is still limited by the complexity of tuning the shape memory response and furthermore the difficulty in incorporating the materials selectively into practical systems. Recent advancements, however, show the promise of SMAs for use in micro-electro-mechanical systems (MEMS) and medical devices where their unique properties can provide advanced functionalities. This dissertation investigates the use of laser-based treatments for the modification of shape memory properties as well as the joining of a shape memory alloy to a dissimilar metal through a novel process. The shape memory properties of SMAs are a strong function of composition, thermal treatments, microstructure, ambient temperature, and stress state. These effects are often intertwined, further disguising their true relationships. The use of thermal annealing for the formation of non-equilibrium precipitates in Ti-rich NiTi thin films is investigated for control over martensitic microstructure, transformation temperatures, and shape memory recovery. Modifications to shape memory properties are investigated through the use of temperature-dependent optical microscopy, temperature-dependent X-ray diffraction, and nano-indentation. As shape memory alloys are increasingly applied at smaller length scales due to advantages in achievable actuation frequency and the growth of micro-scale applications in medical devices, the anisotropy of the shape memory response at the grain level becomes an important consideration for optimizing device performance. The formation of crystallographic texture in NiTi thin films through controlled melting and abnormal grain growth during solidification is investigated through the use of x-ray diffraction and electron backscatter diffraction measurements. An experimentally validated Monte-Carlo grain growth model is developed to predict the texture formation based on the anisotropy in the surface energy between the growing grains and the adjacent liquid. Despite their unique properties, SMAs are not expected to entirely replace more commonly used alloys in most conceivable applications. Rather, these materials are envisioned to be used selectively, where their properties are most advantageous. Joining dissimilar metals, however, is oftentimes made difficult by the formation of brittle intermetallics when the two base materials are mixed. A novel joining process, Autogenous Laser Brazing, is described for the joining of a shape memory alloy to a dissimilar metal. The morphology and strength of the resultant joints is experimentally characterized. Fundamental understanding of the joint formation mechanism is developed through spatially-resolved composition and phase measurements and predictive numerical simulations. The ability to form joints between materials with different geometries is crucial for the wide applicability of a joining process. To this end, the Autogenous Laser Brazing process is further developed for application to tubular structures. The laser scanning scheme is revised to provide uniform heating both in the circumferential and radial directions. The resultant joints are characterized using spatially resolved phase and material property maps and are found to be formed under a different mechanism than the wire samples.