TWSME improvement by thermal cycling at zero stress in NiTi shapememory alloys

摘要

This paper describes an experimental study of the influence of thermal cycling at zero stress and the associated R-phase on two different training procedures for developing the two-way shape memory effect (TWSME) in shape memory alloys (SMAs). Two different sets of NiTi wire are used in the study: one set has been heat treated to ensure a transformation path with no R-phase, whereas the other set has had the same heat treatment which is then followed by repeated thermal cycling at zero stress to develop the R-phase. The study analyzes how both thermal cycling and the R-phase affect the transformation temperatures and the SMA hardness. Subsequently, two different TWSME trainings, thermal cycling under constant load and isothermal tensile deformation, are performed on each sample set. The study then analyzes the training procedures, the training parameters, the recoverable two-way strain and the transformation temperatures. The results show that after 30 thermal cycles, the SMA develops the R-phase and becomes harder. To obtain a substantial ctw together with a minimum plastic strain, the results suggest that the NiTi wire should be thermally cycled at zero stress prior to training until the R-phase is developed and trained by thermal cycling under constant load.