Internal heat sources and transformation enthalpy of a nanostructured superelastic NiTi wire
Résumé
Superelastic NiTi wires are manufactured by a sequence of hot and cold working
which induces a material constituted of a mixture of amorphous phase and highly
strained nanocrystals. In this work, we study the thermomechanical tensile behaviour
of a cold rolled Ti-50.8%at.Ni wire of diameter 0.5mm submitted to a heat treatment
at 325°C for 30min, which is bellow the recrystallization temperature (~350°C). With
this heat treatment the material when slowly loaded and unloaded in tension up to
6% strain exhibits fully recoverable deformations but no stress plateau and Lüderslike
behaviour as usually observed for wires treated at 500°C. For wires treated at
325°C, latent heat can not be measured using Differential Scanning Calorimetry
measurements which do not present peaks in the heat flow vs. temperature curve
In the present work a method was developed and used to calculate the internal heat
sources and transformation enthalpy of a uniform stress induced phase
transformation in a NiTi wire. using Digital Image Correlation (DIC) and Thermal
Field Measurement (TFM). Through this thermomechanical approach, it was possible
to perform a heat balance in order to calculate thermal power and thermal energy
during a stress induced phase transformation. From these results and a
thermodynamic approach, it was possible to calculate the forward and reverse phase
transformation enthalpies. It allows to estimate the martensite fraction in function of
strain or stress.