Slip Planes and Kink Properties of Screw Dislocations in High-Purity Niobium

Temperature and strain-rate dependence of the flow stress of cyclically pre-deformed high-purity niobium single crystals have been measured in the temperature range 120K T 350K with high accuracy and reproducibility at or more resolved shear-strain rates between 6:5 10¡5s¡1 and 3:5 10¡3s¡1. The data are quantitatively interpreted in terms of the kink-pair generation and kink diffusion in a0h111i=2 screw dislocations (a0 = cubic lattice parameter). The prediction of a discontinuity in the stress dependence of the activation volume (occasionally dubbed `the hump') at a strain rate independent effective flow stress has been verified. From the stress dependence of the activation volume and from the magnitude of the discontinuity the spatial period of the Peierls barriers of the screw dislocations could be derived without having to assume a special shape of the Peierls potential. In the temperature range investigated, the measured periodicity is in quantitative agreement with f112g as elementary slip planes (i.e., the slip planes of the screw dislocations between cross-slip events) but incompatible with predominant slip on f110g planes. In agreement with the above mentioned prediction, the same Hk values are obtained. The Nb data are compared with those on Ta, Mo, W, and Fe, which all exhibit a similar pattern. The comparison with the internal-friction measurements of D'Anna and Benoit shows very clearly that the classical relaxation of Nb called irreversible by D'Anna and Benoit is caused by the thermally activated generation of kink pairs in a0h111i=2 screw dislocations on f112g planes. For the more recently discovered reversible relaxation two alternative mechanisms are discussed. The one based on kink-pair formation in screw dislocations on f110g planes appears to be the more likely one. This interpretation implies that the reversible relaxation is identical with the relaxation and thus substantiates Chambers' claim of the intrinsic nature of the relaxation.JRC.I.4-Nanotechnology and Molecular Imagin