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 (T-a, t(a)). A large range of annealing temperatures (538-778 K) and times (10s-123d) has been covered. From differential scanning calorimetry measurements, three zones have been identified with reference to (T-a, t(a)) values. Zone 1, 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 11 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 (T-a, t(a)) annealings, which can be characterized by an activation energy of approximate to 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. (C) 2003 Elsevier Science B.V. All rights reserved.