Elastic Bulk and Surface Properties of Thermally and Mechanically Cycled NiTi Shape-Memory Alloys

摘要

The elastic properties of nearly equiatomic NiTi alloy in thermally and mechanically cycled polycristalline samples have been studied by ultrasonic pulse echo techniques in the temperature range from 4.2 K to 700 K and as a function of axial pressure. Bulk and surface acoustic waves have been generated in order to be able to investigate modifications in the bulk and the near surface regions. In addition, the samples were characterized by DSC measurements and x-ray diffraction experiments. After passing one pressure cycle the as-prepared bulk samples show one-way effect which is characterised by an open hysteresis loop of the elastic constant. Distinct differences of the temperature variation of the elastic constants were also observed for the slowly and rapidly cooled samples after heating to 1000℃. Thermal and mechanical treatment widens the hysteresis and leads to the observation of a second transformation which is attributed to the transition to the R-phase.