Resumen de Improvement of the one-way and two-way shape memory effects in ti-ni shape memory alloys by thermomechanical treatments
Ti-Ni phase transformation behaviour is very sensitive to the thermal and mechanical history of the alloy. Thermomechanical cycling through the full transformation range may degrade the Ti-Ni functional properties (functional fatigue). These repeated transformation cycles cause changes in the SMA phase transformation behaviour due to the formation and accumulation of defects in the alloy microstructure. The main objective of this thesis is to establish the relationships between the changes in Ti-Ni phase transformation behaviour caused by thermomechanical processes, especially in the R-phase range, and the functional properties of the Ti-Ni shape memory alloys (SMAs). Establishing these relationships should allow us to find appropriate thermomechanical processes to substantially improve the Ti-Ni one-way and two-way shape memory effects. To achieve this objective, several experimental techniques are used including measuring variations of the electrical resistivity with temperature, X-ray diffraction, isothermal tension testing, thermal cycling under constant stress, and thermal cycling under zero stress. This study of the phase transformation changes caused by thermomechanical processes has led to a new way of interpreting resistivity curves for calculating the transformation temperatures. Moreover, we have determined how the R-phase influences the functional properties of SMA and, finally, we have substantially improved the properties of one-way and two-way shape memory effects by using thermal processes that avoid permanent deformation of the alloy.
