TEMPERATURE MEASUREMENT AS A NEW TECHNIQUE APPLIED TO THE PHASE TRANSFORMATION STUDY IN A TiNi SHAPE MEMORY ALLOY SUBJECTED TO TENSION

摘要

Temperature distributions obtained from studying the stress induced phase transformations in a TiNi shape memory alloy (SMA) were employed for the investigation into nucleation and further development of the bands of martensite and reverse transformations. A thermovision camera was used to register the distribution of infrared radiation emitted by the specimen surface and constructed thermograms with 50 Hz frequency. Basing on temperature changes and the relevant mechanical characteristics it was noticed that just after crossing a certain threshold stress, narrow lines of considerably higher temperature - up to 10 K, corresponding to the martensite phase, appeared starting from the central part of the specimen and developing towards the specimen grips. Their angle of inclination was about 42 deg. At higher stresses, a few such lines parallel to each other occurred and moved towards the specimen borders, as well as the next "family" of them, developing in the perpendicular direction. The heterogeneous field of the temperature distribution was observed also during the unloading process of TiNi SMA, while the reverse transformation austenite into martensite, took place. The reverse transformation was accompanied by a significant temperature decrease. The lines of reverse transformation were observed when the unloading process started and developed in the whole material volume, however the process remained inhomogeneous. The processes of relaxation do not bring about any changes in the nature of phase transitions, however, the relaxation involved temperature changes cause stress changes as the deformation (unloading) develops after relaxation.