Thermal Diffusivity Measurement of a NiTi Shape Memory Alloy Using a Periodic Temperature Field

The aim of this study is to estimate the thermal diffusivity of martensitic and austenitic microstructures of a NiTi SMA. To this end, an experimental apparatus supported on the Angstrom’s method was designed, where one face of a cylindrical sample is subjected to a periodic heat flux and the other face is kept at a constant temperature. This apparatus was developed to provide the three fundamental requirements of the applied method: (1) uniformity of the thermal field in any cross section of the medium; (2) complete damping of the heat wave before it reaches the face at a prescribed temperature; (3) thermophysical properties independent of temperature within the adopted measurement range. Fourteen thermocouples are fixed at specific locations on the side of the sample, for measuring the amplitude and phase-lag of the thermal wave supplied from periodic heating at a certain frequency. The measurements of these parameters, carried out on a permanent periodic regime, are replaced into a mathematical model, in order that the thermal diffusivity can be determined by linear regression. The results obtained for the two microstructures showed good agreement with the literature, with absolute deviations below 8.5 %, proving the effectiveness of the experimental arrangement. The estimated thermal diffusivity of austenite is 48.21 % higher than that of martensite. The uncertainty quantification, assessed by the Monte Carlo Method, indicates a low dispersion, below 1.5 %, which delivers high precision and reliability to the estimated thermophysical property.