Résumé
Superelastic NiTi wires are manufactured by a sequence of hot and cold workingwhich induces a material constituted of a mixture of amorphous phase and highlystrained nanocrystals. In this work, we study the thermomechanical tensile behaviourof a cold rolled Ti-50.8%at.Ni wire of diameter 0.5mm submitted to a heat treatmentat 325°C for 30min, which is bellow the recrystallization temperature (~350°C). Withthis heat treatment the material when slowly loaded and unloaded in tension up to6% strain exhibits fully recoverable deformations but no stress plateau and Lüderslikebehaviour as usually observed for wires treated at 500°C. For wires treated at325°C, latent heat can not be measured using Differential Scanning Calorimetrymeasurements which do not present peaks in the heat flow vs. temperature curveIn the present work a method was developed and used to calculate the internal heatsources and transformation enthalpy of a uniform stress induced phasetransformation in a NiTi wire. using Digital Image Correlation (DIC) and ThermalField Measurement (TFM). Through this thermomechanical approach, it was possibleto perform a heat balance in order to calculate thermal power and thermal energyduring a stress induced phase transformation. From these results and athermodynamic approach, it was possible to calculate the forward and reverse phasetransformation enthalpies. It allows to estimate the martensite fraction in function ofstrain or stress.