Study On The Relaxation Process In Thermo-elastic Martensitic Phase Transformation

Thermoelastic martensitic phase transformation (MT) is one of the most important features of shape memory alloy (SMA) system, which plays a crucial role in shape memory effect, super-elasticity and high damping capacity of SMA. It is important to study the mechanical relaxation behaviors of SMA for revealing the thermoelastic MT and developing its applied fields.Cu-Al-Ni-Mn-Ti alloy, etc. were adopted as experimental materials to measure the relationship of the mechanical relaxation to temperature, strain amplitude, oscillation frequency and isothermal time. In the dynamic experiments, the forced vibration mode was taken to determine the mechanical relaxation characteristics under different measured conditions, to study the evolvement and decomposition of the relaxation peak, and still to reveal the relation between the heterogeneous relaxation processes and the MT, particularly the anelastic behaviors during the MT.It has been experimentally found that the internal friction (IF) peak measured in general MTs is actually composed of two independent peaks, i.e. low-temperature and high-temperature peaks, which correspond to different mechanisms, indicating that the thermoelastic MT is a process coupled by multiple mechanisms. The height of high-temperature peak is directly proportional to reciprocal frequencies and decreases as strain amplitudes increase, however its location at temperature spectra is unvaried with vibration frequencies. These facts show that the high-temperature peak is only associated with the volume change caused by the normal motion of phase interfaces. The maximum of low-temperature peak gives a symmetrical peak-like function against logarithmic frequencies and is almost independent of strain amplitudes, but the low-temperature peak shifts a few degrees in temperature spectra towards high temperatures with increasing the frequencies, exhibiting it is associated with the transversal viscous motion of phase interfaces. All the aspects prove that there exists the anelastic relaxation in the thermoelastic MT.