Effect of Crystal Orientation on Amplitude-Dependent Damping in Magnesium

The amplitude-dependent internal friction and Young’s modulus have been measured as a function of the crystal orientation in magnesium single crystal specimens of 99.9% purity by using a flexural vibration method at room temperature. It has been found that, if only the basal slip system is operative, Schmid’s law holds for the orientation dependence of the breakaway stress σB given from the internal friction (or Young’s modulus) versus stress-amplitude curves. This suggests that the amplitude-dependent internal friction is caused by the movement of basal dislocations. In addition the analysis of the internal friction data based on the Granato-Lücke theory has also given evidence that the amplitude-dependence of internal friction is caused by the static hysteresis due to the interaction of basal dislocations with impurity atoms. Complementary experiments using polycrystalline specimens of magnesium and zinc showed that the amplitude-dependence of internal friction was extremely sensitive to the crystallographic orientation of individual grains. A specimen of magnesium with equiaxed grains had a very small value of σB and displayed a very large amplitude-dependence of internal friction. In contrast a specimen with columnar grains whose longitudinal direction was almost parallel to the specimen axis had a much larger value of σB and displayed very little amplitude-dependence of internal friction. Similar behaviour was observed in two zinc specimens having quasi-equiaxed grains and the (0001) rolling texture, respectively. It was concluded that the Schmid factors in individual grains played the most important role in the amplitude-dependent internal friction of hexagonal close-packed metals.