THERMALLY ACTIVATED DISLOCATION MOTION IN FCC AND REFRACTORY BCC METALS

Amplitude-dependent internal friction evidence is presented which demonstrates the strong differences in dislocation mobility which exist between the two classes of cubic metals: the face-centered cubic, represented by Al and Cu, and the body-centered cubic, represented by Ta, Nb, Mo, and W. Amplitude- dependent internal friction due to dislocation motion results from the hysteretic unpinning and repinning of dislocation loops from impurity pinning points when the oscillating measuring stress exceeds the breakaway stress value. Results of experiments indicate that the modulus defect measured at 4.2 nif- K in the fcc metals is considerably larger than that associated with the Bordoni peak alone, while the modulus defect measured at 4.2 nif- K in the refractory bcc metals is quite small compared with that associated with the deformation peaks along. (H.G.G.)