Internal friction peaks observed in single or polycrystals are clearly due to a dislocation
relaxation mechanism. Because a sample observed by transmission electron microscopy (TEM)
often exhibits in the same time various dislocation microstructures (isolated dislocations,
dislocation walls, etc.) it is very difficult to connect the observed relaxation peak with a particular
dislocation microstructure. Using isothermal mechanical spectroscopy (IMS), it is easier to
compare, for instance, the evolution of a relaxation peak with measurement temperature to the
microstructural evolution observed by in-situ TEM at the same temperatures. IMS was used to
study a relaxation peak in a 5N aluminium single crystal firstly 1% cold worked and then annealed
at various temperatures. TEM experiments performed in the same material at various temperatures
equal to the temperatures used for the damping experiments made possible to link this internal
friction peak with a relaxation effect occurring inside dislocation walls. In two other experiments in
a 4N aluminium polycrystal and in a metal matrix composite with SiC whiskers, it is shown that the
observed relaxation peaks are connected to the motion of dislocations inside polygonization
boundaries in the first case and in dislocation pile-ups around each whisker in the second one.
Theoretical models proposed to explain such relaxation peaks due to a dislocation motion inside a
dislocation wall or network are discussed.
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