Dynamic Mechanical Spectroscopy of Nanograined Thin NiTi Wires
In this work, thermomechanical tensile behaviour of thin NiTi wires was investigated by means of a device which combines a conventional tensile device with a Dynamic Mechanical Analyser (DMA). In this device, the displacements of the specimen upper and lower extremities are controlled by a servo-motor with static drive and by a shaker with dynamic drive, respectively. Wires containing nanograined microstructures were used in order to minimize plastic deformation.
This non conventional tensile device permits to study storage modulus (E’) and damping properties (tand) as functions of tensile stress, strain and temperature history, both in the A-R-M transformations window in the stress-temperature space and outside this window (specimen fully austenitic or martensitic).
First, E’ and tand were measured during usual isothermal tensile tests over a wide testing temperature range between -100°C and 150°C, as functions of stress and strain history, on one hand during cyclic tests and on the other hand during monotonic tests conducted till the rupture of the wire.
Second, E’ and tand were measured during tensile tests performed on identical wires and consisting in temperature sweeps under constant tensile stress, over wide testing stress and temperature ranges (100MPa; 1000MPa and -150°C; 250°C).
These tests are analysed to study dependences of phase transformations (A-R, R-M and A-M) on stress and temperature and to bring new information concerning the value of the elastic moduli in the different stress-temperature regions. These values are analysed with regard to the deformation mechanisms which are involved, depending on the material state.
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