A new low-temperature internal friction peak in NiTi based alloys

The elastic, anelastic and calorimetric behavior of Ni-rich binary Ni50.8Ti49.2 and quaternary Ni47Ti40Hf10Cu3 alloys have been investigated in vacuum annealed and plastically deformed materials. A thermally activated internal friction peak has been detected at around 150 K (1 kHz frequency), which is a distinct feature from the ones previously recorded in the aged and hydrogen-loaded alloys. It has also been found that the ultrasonic attenuation peak reported in the literature for an equi-atomic NiTi alloy is of the same nature as the present one. The common relaxation parameters of these two effects are W = 0.29 ± 0.02 eV and τ0 = (8 ± 2) × 10−14 s, where W is the activation enthalpy and τ0 is the limiting relaxation time. The suggested structural mechanisms for the relaxation are stress-assisted motions of vacancies interacting with twin boundaries (annealed state) and with bulk dislocations (plastically deformed state).