Internal friction of diluteAl/Cualloys in homogenized and precipitated states between 100 and 350 °C

The activation energy of the Zener relaxation due to Cu pairs was found to be 31.4 kcal/mol, with τ0= 10−15.6s. The results show that the relaxation strength of this effect obeys the quadratic concentration dependence. The small peak around 180 °C, at about 1 Hz, appearing in the presence of the θ″-phase, has «Zener» character. In the case of the θ′-phase a broad peak near 160°C is attributed to the misfit dislocations around the semi-coherent precipitates. The activation energy of this relaxation is 23 kcal/mol, with τ0= 10−12.4s. From the spectra of specimens pre-annealed at 300°C, or above, we infer that in this temperature range mostly θ-phase was formed. The results are complemented by measurements on plastically deformed specimens. In order to investigate the stability and transformations of different precipitated phases, internal-friction measurements were also made after various, subsequent heat treatments.