The evolution of the characteristic transformation temperatures of the martensitic phase transformation in Ni-rich binary NiTi wires is studied as a function of the heat treatment and the size of the wires. Wires of diameter between 1.52 mm and 0.05 mm, were obtained by cold drawing the original 1.52 mm wire. It is shown that in this range of diameter, no size effect can be observed. This study shows that the transformation sequence of the martensitic transformation is strongly affected by the heat treatment temperature. A special case of martensitic transformation, called multiple step transformation, is observed when the wire is first solution treated at 900°C and further aged between 300°C and 550°C. The present work shows that the multiple step transformation is due to the appearance of a two step transformation of the R-phase into the martensite.
Conclusion:
1.- The heat treatments have the same effect on wires which diameters are between 1.52 and 0.05 mm. No size effect has been detected.
2.- The multiple step transformation was observed after a wide variety of thermomechanical treatments, but was studied in the case of a sample which was solution treated at 900°C and further aged at 520°C.
3.- In this type of sample, the transformation sequence was found to be the following:
A->R->M1+R->M1+M2 on cooling and M2->A and M1->R->A on heating.
4.- The multiple step transformation is due to local stress fields around precipitates of the Ni4Ti3 type, which appear during the ageing treatment. M1 is a locally stress induced martensite, and M2 is the stress-free martensite.
5.- The multiple step transformation was found to be perfectly stable as a function of thermal cycling, probably because the precipitates provide easy nucleation sites and are not affected by thermal cycling.