Quantitative analysis of the microstructure in NiTi wires by Synchrotron X-ray diffraction: An approach to the cause of the Two Way Shape Memory Effect

Abstract To understand the complex behaviour of the NiTi shape memory alloys, the use of advanced characterization techniques as Synchrotron X-ray diffraction, becomes more and more necessary. In this paper, this technique was performed in heat-treated, thermally cycled at zero stress and two-way memory trained NiTi wires to evaluate through microstrain and retained martensite which is the predominant micro-mechanism that cause the Two Way Shape Memory Effect. The microstructural effects in the structure of the Austenite by thermal and training methods were quantified separately for each NiTi sample, allowing us to calculate how much of the accumulated microstrain in the Austenite phase is due to the increment in density of dislocations and how much is consequence of the decrease in crystallite size. To achieve this, two different methodologies were used. The mean microstrain and shift index were calculated by analysing the whole diffractogram with the Rietveld method and the Double-Voigt Approach. The individual peak shift and the peak broadening of the Austenite phase were evaluated by the modified Williamson-Hall plot. The results show that as higher is the increase in microstrain by thermal and training procedures, higher is the two-way memory strain of the NiTi sample. The micromechanical results pointed out that there is a certain formation of retained martensite during two-way memory training, but is not the main cause of the two-way shape memory effect. What is more, it seems that an early formation of retained martensite during training may decrease the generation of two-way shape memory effect. Pre-training thermal procedures that minimize the formation of retained martensite and maximize the increase in microstrain are fundamental to improve the generation of the NiTi two-way memory strain by subsequent training.