INTERNAL-FRICTION CAUSED BY MICROPLASTICITY IN ALUMINUM-MAGNESIUM ALLOYS

Mechanical hysteresis was observed at room temperature for polycrystalline Al-Mg dilute alloys by the three-point bending method. The form of hysteresis loops was not a breakaway type but a friction type, when the observation was done in the steady-state deformation of loading-unloading cycles. The results suggest that the amplitude dependence of internal friction is caused by dislocations moving across the field of dispersed solute atoms, and it should not be deduced from the Granato-Lucke theory based on a breakaway model of dislocations. Internal friction in the same alloys was measured at 190-370 K by the free decay method of flexual vibration. The amplitude-dependent internal friction was found to decrease with increasing Mg content and decreasing temperature. The data were analyzed on the basis of the microplastic theory, assuming a friction-type hysteresis, and converted into the microplastic strain expressed as a function of stress. The stress-strain responses showed that the flow stress increased more rapidly with alloying in the microplastic strain range than in the range of usual tensile tests.