Mechanical loss spectrum of aged Al-Mg-Si alloys

The low-frequency internal friction and elastic modulus have been measured as functions of temperature at different strain amplitudes in wrought Al-Mg -Si (6082) alloys after quenching and after ageing. The damping spectrum ex hibits a relaxation peak located at about 420 K in over-aged specimens. Thi s damping peak, with an activation enthalpy H = 1.57 +/- 0.04 eV and a limi t relaxation time tau = 2.6 x 10(19+/1) s, may be attributed to the draggin g of solute atoms by the dislocation loops. A strong damping-amplitude effe ct has been observed at lower temperature. This phenomenon, which leads to a high damping capacity at high strain amplitudes, may be interpreted by a breakaway mechanism. Damping would be due to the increase in the dislocatio n loop length by depinning and to the work done to overcome the binding ene rgy between the dislocations and the solute atoms.