R-phase influence on different two-way shape memory training methods in NiTi shape memory alloys

Abstract

This paper studies how the R-phase developed by thermal cycling at zero stress influences two different training procedures for developing the two-way shape memory effect (TWSME) in NiTi shape memory alloys (SMAs). The two training procedures are thermal cycling under constant load and isothermal tensile deformation under martensite state. This paper studies how R-phase developed by thermal cycling at zero stress influences these training procedures and their training parameters, the two-way memory strain obtained and the transformation temperatures. The results show first, that the presence of the R-phase did not prevent substantial two-way memory strain from being obtained in any instance of training and thermal treatment; and second, that thermal cycling at zero stress prior to training has a considerable impact depending on the training method used. Finally, to obtain the best results (that is, a substantial two-way memory strain together with a minimum of final plastic strain and easy TWSME activation temperatures), the NiTi wire should be thermally cycled at zero stress prior to training until the R-phase is totally developed and trained by thermal cycling under constant load.