Influence of the recovery and recrystallization processes on the martensitic transformation of cold worked equiatomic Ti–Ni alloy

The martensitic transformation of Ti–Ni shape memory alloys is very sensitive to thermomechanical treatments (cold work and annealings). In the present paper, a Ti–Ni alloy close to the equiatomic composition has been 40% cold rolled and then submitted to various annealing treatments, each one characterized by temperature and time (Ta, ta). A large range of annealing temperatures (538–778 K) and times (10 s–123 d) has been covered. From differential scanning calorimetry measurements, three zones have been identified with reference to (Ta, ta) values. Zone I, at low temperatures and times, is characterized by badly defined transformations on cooling as well as on heating; it corresponds to a recovery–reverse martensitic transformation stage. Zone II exhibits large changes of the transformation features which occur in two steps on cooling (R phase then martensitic) and in one step on heating; this zone corresponds to the recrystallization and growth of very small stressed grains. Similar transformation behavior can be obtained with different (Ta, ta) annealings, which can be characterized by an activation energy of ≈3.4 eV in the range 698–778 K. Zone III relates to a well defined martensitic transformation taking place in one step with almost no further evolution; it corresponds to the growth of stress-free grains.