Internal oxidized copper was tested by isothermal mechanical spectroscopy in
a medium temperature range (300–600 K). Experimental results show the
existence of a non-thermally activated effect at low temperature and of a
relaxation peak at higher temperatures. The material microstructure was
studied by combination of Transmission Electron Microscopy (TEM) and
Electron Energy Loss Spectrometry (EELS). The TEM study allowed us to
investigate the distribution of fine spherical particles and the presence of
particular network dislocations inside the grains. The EELS method was used
to identify the nature of these fine particles as Cu2O. The internal
friction has revealed a non thermally activated maximum occurring at 0.1 Hz
for temperatures ranging from 290 K to 394 K, and a relaxation peak obtained
after annealing at 573 K. This peak is stable after successive annealings at
723 K and 873 K. Comparison of the microstructure observations, their
evolution with annealing and the evolution of the relaxation effect with
annealing temperature enables us to interpret the phenomena described in
this work: on the one hand, the microstructural characterisation using TEM
and EELS allows us to assign the first effect to the result of a
transformation of metastable Cu2O particles to CuO under the cyclic
stress; on the other hand, the relaxation peak that does not change after
high temperature annealing is linked with a particular stable dislocation
network observed in many grains.