From functional properties to micro/nano-structures: A TEM study of titanium nickel (X) shape memory alloys

Shape memory alloys (SMA), among which Ni-Ti is the most popular for applications, have the fascinating property to remember their shape when heated up above a certain temperature. It is possible to train them to give them any desired shape which can be repeatedly obtained. At different composition and temperature, the same alloys can also show 'superelastic' property. In this case the alloys have a rubber-like behavior and can withstand deformation much larger than any other metals or alloys without permanent deformation. Both properties stem from a phase transformation between a high temperature phase (austenite) and low temperature phase (martensite).;These unusual properties have led to numerous applications, especially in the medical field. However further development of SMAs is hindered by the degradation of functional properties (degradation of shape memory or superelastic effects) as they go through repeated phase transformations. To tackle this problem a better understanding of the fundamental mechanisms occurring at the micro/nano-scale during the martensitic phase transformation is necessary. The deterioration of functional properties is intimately linked to plastic relaxation mechanisms taking place at the nano/micro-scale and coupled with the phase transformation. The present work brings new evidences of these degradation mechanisms and proposes new ways to improve the functional properties.;One approach taken in this thesis was to add a ternary element to Ni-Ti to improve its functional properties. Such alloying can give specific lattice parameters for which perfect crystallographic compatibility between austenite and martensite is achieved. Because of the good fit between the two phases less energy is dissipated by the propagation of their mutual interface which facilitates the phase transformation. A full characterization of the microstructures of these alloys was obtained with transmission electron microscopy (TEM).;Another approach undertaken for this work was to optimize superelastic Ni-Ti microstructures using thermo-mechanical treatment. Using a newly developed electropulse annealing technique under stress is was possible to obtain a range of different microstructures among which some were highly resistant against the degradation of functional properties (dislocation slip) due to their nano-sized structures as characterized by TEM. Microstructures annealed for longer times and showing a rapid degradation of their superelastic properties were studied with TEM to understand some of the fundamental mechanisms leading to this degradation.