Interface morphology and its stability in martensitic phase transformation of nickel-titanium shape memory alloy

Interface morphology and its stability during stress-induced martensitic phase transformation in the superelastic NiTi polycrystalline shape memory alloys are investigated in this thesis. A theoretical model is proposed to conduct the stability analysis and the instability criterion is established. Based on the calculation of the thermodynamic driving force along the interfaces and the incorporation of different kinetics laws, the interface instability and its corresponding modes are well addressed which can be compared with the experimental phenomena of the microtube. Starting from an unstable equilibrium state, the FEM simulation can verify the theoretical predictions and track the succeeding evolution of interface morphology to reach another stable equilibrium. The effect of various parameters such as surface energy density, transformation strain and critical driving force are discussed. Moreover, the structure effects are incorporated to consider the uniaxial tension of thin strips. The experimental nominal stress-strain curve and corresponding domain morphology can be roughly captured with the FEM simulation. The variation of domain patterns and especially the phenomena of interface switch (rotation) are studied subject to different parameters such as critical driving force, surface energy density, nucleation position and strip geometry.