Influence of ageing on the internal friction of magnesium

The internal friction of cp- (99.8 wt.%) and hp- (99.99 wt.%) magnesium was determined by measuring the logarithmic decrement of free vibrations of bending beams, which were clamped at one end. Independently of the purity and the thermomechanical pretreatment, for all samples the logarithmic decrement increases with increasing strain amplitude. The damping in hp-magnesium is generally higher than in cp-magnesium. This can be explained by the model of Granato and Lücke, assuming that the successive unpinning of dislocation segments between weak pinning points is the reason of the increment of damping with increasing strain amplitude. The weak pinning points are assumed to be solute atoms or vacancies. Based on this model, other effects, found also in time and stress dependent internal friction measurements, can be explained qualitatively. The damping decreases at room temperature with ageing time and annealing temperatures up to 150°C. For higher annealing temperatures the damping increases monotonously with the temperature of the heat treatment. The effect of ageing can also be reverted by dynamic strain in the range of about 10-4–10-3, which increases the internal friction of aged samples before they can age again.