Martensitic-Austenitic phase transformation of Ni-Ti SMAs: Thermal properties

The main scope of this work concerns the definition of the thermal conductivity temperature dependence of fully dense NiTi SMAs in the temperature range where the Martensitic-Austenitic phase transformation occurs. The methodology used to evaluate this thermal property is based on an experimental-numerical approach that requires the definition of the heat capacity temperature dependence and the knowledge of the latent heat of transformation. The experimental work is based on the capability to heat the cylindrical NiTi samples uniformly on one side and to impose a variety of initial heating rates ranging from 0.1 to 5 K/s. Laser radiant energy was used as the heating source and the temperature history of the top and bottom NiTi sample surfaces were recorded using thermocouples. The numerical code considered the sample as a solid with a constant density and with thermal properties that were dependent on temperature. The heat capacity and latent heat of transformation were defined on the basis of the thermal analysis data, while the convection heat exchange coefficient was estimated from knowledge of the experimental configuration, lateral sample surface and the temperature field of the gas surrounding the sample. The results indicated that the thermal conductivity generally increased with temperature, but a minimum in the temperature range defining the Martensitic-Austenitic transformation has been pointed out. The higher thermal conductivity value of the Austenite phase is correlated with its electronic structure