Thermomechanical analysis of the deformation in tension of a nanocrystalline superelastic NiTi thin wire

International audienceManufacturing processes of NiTi Shape Memory Alloys (SMA) wires induce a complexmicrostructure with a mix between amorphous phase and highly strained nanocrystals [1]. Such amaterial, a cold worked Ti 50.8 at.% Ni wire with a diameter 0.5mm, was studied during a tensileloading. Before loading, a heat treatment at 325°C for 30min, which is bellow the recrystallizationtemperature (~350°C), was applied. Thanks to these relative low heat treatment temperature,comparatively to the usual ones around 500°C, no strain localization was observed. Latent heatmeasurement during a thermal induced transformation was impossible through a DSC because anypeak can be observed. However, the knowledge of latent heat is important in some applications,i.e. elastocaloric effect [2], where temperature variations are expected.Our presentation will show a method to estimate this latent heat during a stress induced phasetransformation. From experimental full field measurements, thermal and kinematics, and using aprocessing presented in previous papers ([3-4]) we performed a heat balance in order to calculatethermal power and thermal energy induced by a tensile loading. From these results and athermodynamic approach, it was possible to estimate: (i) the forward and reverse phasetransformation enthalpies, (ii) the ratio between inelastic thermal power and mechanical power,(iii) the martensite fraction in function of strain or stress