Anelastic Effects Arising from Precipitation in Aluminum-Zinc Alloys

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Abstract
Torsion experiments on an aluminum‐zinc alloy (21 atomic percent zinc) show that precipitation gives rise to large values of internal friction. After prolonged periods of aging, which are sufficient to produce growth of the precipitate particles, the internal friction begins to decrease. The activation energy for the elementary process producing the anelastic behavior is 23 kcal per mole. It is demonstrated, with the help of static measurements, that the curves of internal friction versus temperature do not show a peak but continue to rise indefinitely with increasing temperature. This behavior cannot readily be interpreted in terms of the usual concept of a relaxation spectrum but requires a new concept of coupled relaxations. Metallographic examination of the specimens reveals that the observations of large internal friction are correlated with the occurrence of discontinuous precipitation. The observed anelasticity is interpreted in terms of the fragmentation of the lattice; i.e., the formation of an irregular network of shear‐relaxing interfaces during discontinuous precipitation. It is demonstrated that in such a network the coupling, produced by an overlapping of regions within which shear stress is relaxed, makes possible a very large total relaxation. Additional measurements of elastic aftereffect in the supersaturated solid solution permit the determination of the mean time of stay of atoms in this solid solution, between 52°C and 72°C, by the anelasticity resulting from pair reorientation. The results obtained are in excellent agreement with the conventional high temperature diffusion data and correspond to a heat of activation for diffusion of 25.4 kcal per mole.