Anelastic relaxations in aluminium with ultrafine grain structure

The elastic energy dissipation coefficient and dynamic elasticity modulus were measured in the 3-4000 Hz frequency range by an inverted torsion pendulum and a vibrating reed technique on nanocrystalline aluminium samples obtained by mechanical attrition. The anelasticity spectra in the 300-700 K range on as-prepared samples with average grain size in the 20-40 mn range strongly depends on low temperature annealing treatments. A relaxational damping peak p(1) at 460 K (3Hz) is observed in samples after thermal annealing without appreciable grain growth. A second peak P-2 located at higher temperatures occurs when the grain size exceeds 100 nm.