Stability of thermal-induced phase transformations in the severely deformed equiatomic Ni-Ti alloys

Nickel-Titanium (Ni-Ti) alloys of two different near-equiatomic chemical compositions (Ni-rich and Ti-rich) are subjected to severe plastic deformation by means of high pressure torsion (HPT) by higher rotation speed and larger total number of rotations. Further, the as-received and severely deformed specimens are subjected to heat treatments at 300 and 350 A degrees C. Phase transformations of the specimens under different conditions are analyzed by employing differential scanning calorimetry and by X-ray diffraction. The results obtained show that in Ti-rich Ni-Ti alloy the sequence of phase transformations is found to be stable against heat treatments and independent of previous HPT process. Also, in Ni-rich Ni-Ti alloy, when it is subjected to HPT, the sequence of phase transformations found to remain unaltered. However, with or without HPT, after the heat treatments at 300 and 350 A degrees C, the sequence of the phase transformation is found to be affected.