DISLOCATION AND KINK DYNAMICS IN FCC METALS STUDIED BY MECHANICAL SPECTROSCOPY

Mechanical spectroscopy has been developed essentially to study the dy namic 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 the ir motion and as a consequence many different relaxation mechanisms ha ve been observed and attributed to different dislocation mechanisms. T he Bordoni relaxation (BR) observed in the low temperature range of f. c.c. metals was attributed by Seeger to the thermally activated kink p air formation (KPF) on dislocations. From the relaxation energy of the BR the Peierls stress can be evaluated. The obtained values are consi derably larger than those expected from the low temperature behaviour of the critical resolved shear stress. Ultrasonic experiments using th e coupling method were performed in order to obtain qualitatively diff erent results when different relaxation mechanisms are operating. The so-called ''signature'' of the BR relaxation was observed, analysed an d attributed to the KPF mechanism, confirming the Seeger interpretatio n. Moreover, after low temperature plastic deformation and electron ir radiation a softening mechanism was observed. This softening effect, l inked to the presence of vacancies on dislocations, could explain the discrepancy between the low temperature behaviour of the resolved shea r stress and mechanical spectroscopy measurements.