In this thesis, deformation instability and domain morphology evolution during stress-induced martensitic phase transformation in superelastic NiTi polycrystalline shape memory alloy microtubes are investigated. Several mini testing systems for such microtubes are developed to facilitate the investigation of the dynamic and quasi-static events which take place in a displacement controlled quasi-static combined (proportional and non-proportional) tensile and torsional loading/unloading process. In situ profilometry is introduced to measure the non-uniform strain fields in the microtubes. In the experiments, both the stress-strain curves and surface morphology evolution of the microtubes during loading and unloading are recorded. From these, new types of deformation instability, new shape...[ Read more ]

In this thesis, deformation instability and domain morphology evolution during stress-induced martensitic phase transformation in superelastic NiTi polycrystalline shape memory alloy microtubes are investigated. Several mini testing systems for such microtubes are developed to facilitate the investigation of the dynamic and quasi-static events which take place in a displacement controlled quasi-static combined (proportional and non-proportional) tensile and torsional loading/unloading process. In situ profilometry is introduced to measure the non-uniform strain fields in the microtubes. In the experiments, both the stress-strain curves and surface morphology evolution of the microtubes during loading and unloading are recorded. From these, new types of deformation instability, new shapes of transformation domain, branched front structure, convoluted front motion, gradual deformation mode transition, and loading path dependence of the initial macroscopic transformation are discovered; critical values in dynamic band formation, self-merging, and annihilation are characterized; loading history dependence and deformation mode transition are clarified. The underlying physical mechanisms of deformation instability and morphology evolution are then discussed. The observed phenomena bring up several fundamental issues regarding the roles of front energy, front kinetics and their interplay with the strain energy in the instability and pattern evolution in tube configuration under mechanical force. These issues are believed to be essential elements in the theoretical modeling of shape memory alloys.