Elastic properties of shape memory TiNi alloys with different heat treatment

Comparative experimental studies of the elastic characteristics of commercial shape memory TiNi alloys (Nitinol) exposed to different heat treatment have been carried out within the (80 – 300) K temperature range, including the points of premartensitic and martensitic transitions. It was found that the distortion of the crystal structure at the premartensitic transition in the “as prepared” sample is accompanied by pronounced anomalies of the Young's modulus and internal friction. This transition occurs at $T_S\approx$ 270 K both upon heating and cooling. After an additional heat treatment, the premartensitic transition temperatures $T_{Su}$ and $T_{Sd}$ upon heating and cooling respectively, shift slightly and the hysteresis turns out to be more pronounced. However, the form of elastic anomalies remains nearly the same. In contrast, for the martensitic transition at $T\approx$ 220 K, the elastic anomalies are observed only upon cooling, and the temperature and shape of these anomalies undergo significant changes under effect of the heat treatment.