New Understanding of the Influence of the Pre-Training Phase Transformation Behaviour on the TWSME in NiTi SMA Wires

Changes in the phase transformation behaviour in two-way shape memory NiTi wires were quantified by X-ray diffraction as a function of temperature. The results were compared with those obtained from the electrical resistivity measurements and applied loading method. The weight fraction diagrams enable us to observe that the higher is the level of the R-phase in the pre-training path of the NiTi sample, the lower will be the two-way memory strain obtained. Substantial values of two-way memory strain were found to be associated with pre-training paths characterized by simultaneous transformation of austenite to R-phase plus martensite. Finally, a comparison of these three experimental techniques led to a new interpretation of the electrical resistivity curves for obtaining the transformation temperatures of two-way shape memory NiTi wires. This may help to clarify the relationship between the shape of the electrical resistivity curve and the two-way memory strain that occurs in the NiTi sample.