Effect of hot-rolling on microstructures, phase transformation and mechanical properties of shape memory alloys Ti₅₀Ni_50-xAl_x (x=1,2,4)

The phase transformations and mechanical properties of shape memory alloys are remarkably affected by mechanical processing. In this study shape memory alloys Ti₅₀Ni_50-xAl_x (x=1,2,4) are rolled into a plate at 850 °C. The microstructures of the alloys are investigated by scanning electronic microscopy, transmission electron microscopy and X-ray diffraction. The results show that the alloys are composed of NiTi and Ti₂Ni phases and the solid solubility of Al in the latter is higher than that in the former. The differential scanning calorimetry and electric resistance vs temperature measurements indicate that with the increasing Al content both the R and martensitic transformation temperatures decrease, and the temperature difference between the two transformations is enlarged. After hot-rolling at 850 °C, the volume ratio of the Ti₂Ni phase in the alloy decreases and the solid solubility of Al in the NiTi phase increases. Furthermore, the hot-rolling induces the decrease of R and martensitic transformation temperatures and the increase of their transformation temperature difference as well as the improvement of plasticity of the alloys at room temperature.