DISLOCATION AND KINK DYNAMICS IN FCC METALS STUDIED BY MECHANICAL SPECTROSCOPY

Mechanical spectroscopy has been developed essentially to study the dynamic properties of structural defects. Mechanical loss spectra can be described in terms of discrete contributions of relaxation processes, each being due to a mechanism which controls the mobility of a defect. In the case of dislocations, many mechanisms are able to control their motion and as a consequence many different relaxation mechanisms have been observed and attributed to different dislocation mechanisms. The Bordoni relaxation (BR) observed in the low temperature range of f.c.c. metals was attributed by Seeger to the thermally activated kink pair formation (KPF) on dislocations. From the relaxation energy of the BR the Peierls stress can be evaluated. The obtained values are considerably larger than those expected from the low temperature behaviour of the critical resolved shear stress. Ultrasonic experiments using the coupling method were performed in order to obtain qualitatively different results when different relaxation mechanisms are operating. The so-called ''signature'' of the BR relaxation was observed, analysed and attributed to the KPF mechanism, confirming the Seeger interpretation. Moreover, after low temperature plastic deformation and electron irradiation a softening mechanism was observed. This softening effect, linked to the presence of vacancies on dislocations, could explain the discrepancy between the low temperature behaviour of the resolved shear stress and mechanical spectroscopy measurements.