Study of Phas Transformation and Mechanical Response in NiTi Shape Memory Alloy under Different Thermomechanical Treatment

In this work, the martensitic transformation and mechanical response of a near equiatomic NiTi Shape Memory Alloy (SMA) subjected to different thermo-mechanical treatments has been studied. The NiTi wire in as drawn condition was subjected to seven heat treatment cycles in order to prepare the wires with different transformation characteristics. The transformation temperatures have been measured using DSC method. The wires were then subjected to tensile test at the temperature of 20, 80 and 90 C, in order to study the stress induced martensite.
According to the results, with increasing the heat treatment temperature or time, the Ms increases and the Rs decreases in such a way that the R-phase disappears when the temperature achieves to 600 C.
The stress at which, the martens tic reorientation takes place is independent of the heat treatment conditions. The stress induced martensite, however, forms under different stresses, depending on the heat treatment temperature and time. These samples show the superelastic effect at high temperature; but in the case of the samples with martensitic reorientation, the alloys are not able to present this effect. Therefore, it is possible to determine the mechanical response of the material with respect to the given applications.