INTERNAL FRICTION STUDY OF ALUMINIUM ALLOY CONTAINING 7.5 WEIGHT PERCENT MAGNESIUM

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Abstract
Low-frequency internal-friction data were obtained using the torsion pendulum for the aluminium −7.5 wt% magnesium alloy in order to study the effects of quenching and aging on solute atom movements. A damping peak near 120 °C, attributed to the stress-induced reorientation of solute atoms (Zener relaxation), was found to shift to 80 °C on quenching. The activation energy for the peak at 80 °C was found to be 22.3 ± 1.5 kcal/mole, about 3 kcal/mole less than that for the Zener peak. The peak shift was found to be due to movement of solute atoms involving divacancies with reduced relaxation time. Aging of the quenched alloy at room temperature for one day was found sufficient to eliminate the effect of quenching in reducing relaxation time.Another weak damping peak near 40 °C in the as-quenched alloy specimen, attributed to the relaxation involving solute clusters, was found to be more pronounced on aging at room temperature for 24 hours, thereby indicating formation of zones in the quenched alloy during aging at room temperature. It was inferred from the results that about 100 atomic jumps were required to complete the relaxation responsible for this peak.Grain-boundary peaks were also observed in the furnace-cooled and the water-quenched specimens.
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