Pre-stressed NiTi: effects of the thermodynamic forces and time

The use of NiTi in damping devices for Civil Engineering requires stable behavior of the stress-strain curves with time and cycling. In particular, the use of pre-stressed samples improves the damping efficiency. Since earthquakes can appear after several years (or decades) of calmness, it is required that the alloy maintains their properties unchanged even after long time in the pre-stressed situation. The static experimental analysis in parent phase, in phase coexistence and in martensite shows minor changes after one month at the pre-stressed condition. These changes increase when the room temperature is increased from 293 to 303 K. The study of aging at 373 K by calorimetric measurements shows relevant changes of both the R-phase and the martensitic transformation temperatures, with time constants near 70 days, in agreement with X-ray diffraction analysis. Preliminary observations associated to dynamic actions (cycling and pauses) shows that the accumulated deformation partially recovers in the pauses.