Low-temperature relaxation in faulted Cu-based martensites

Abstract

An extensive investigation of elastic and anelastic properties, at an ultrasonic frequency of around 100 kHz, of a variety of binary and ternary Cu-based alloys forming faulted martensites evidences the existence of a strong anelastic relaxation over a wide temperature range propagating down to lowest temperatures achieved in experiments of around 10 K. The anelastic phenomenon observed is the most pronounced for low oscillatory strain amplitudes (approximately from 10 −7 to 10 −6) and results in a rapid decrease of Young's modulus (measured at low strain amplitudes) with increasing temperatures from the lowest values of around 10 K. The relaxation of Young's modulus is accompanied by a linear increase of the low-amplitude internal friction with increasing temperature. The low-temperature relaxational behaviour is strongly dependent on concentration of quenched-in vacancies: the relaxation is enhanced by β-phase ageing and is strongly suppressed by direct quenching of ternary Cu–Al–Ni, Cu–Zn–Al and Cu–Al–Be alloys. The anelastic effect observed exhibits an unusual property: the amplitude-independent internal friction background emerging as a result of this relaxation is non-additive with the amplitude-dependent internal friction. We suggest that this low-temperature relaxation is associated with a reversible transition from a cooperative to an individual mode of motion of the system of partial dislocations.