Time-Dependent Internal Friction in Aluminum and Magnesium Single Crystals

Magnesium and aluminum single crystals subjected to varying amounts of oscillatory strain of audio frequency have been investigated by measuring the strain amplitude-dependent complex dynamical-mechanical modulus at audio frequencies as a function of temperature and time. If the amplitude of the applied excitation exceeds a critical breakaway strain, an excited mechanical state is manifested in a modification of the amplitude dependence of the complex modulus. Upon cessation of the excitation, the excited state decays. The decay following short excitation times obeys a t1/3 law, becoming a t2/3 law as excitation approaches saturation. The short excitation decay rate is found to be governed by an activation energy in the range of 7-10 kcal per mole.The time dependence is considered to be caused by the dispersal and condensation of a pinning atmosphere which has high diffusivity and is easily dispersed by a dislocation segment undergoing pinning interactions with the atmosphere.