Slip planes and kink properties of screw dislocations in high-purity niobium‡

‡Dedicated to Professor Frank Reginald Nunes Nabarro on the occasion of his 90th birthday on March 7th, 2006.

Abstract

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 120,K ≤T≤350 ,K with high accuracy and reproducibility for five or more resolved shear-strain rates between 6.5×10^-5 ,s^-1 and 3.5×10^-3 ,s^-1. The data are quantitatively interpreted in terms of kink-pair generation and kink diffusion in a ₀〈111〉/ 2 screw dislocations (a ₀= 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 {112} as elementary slip planes (i.e. the slip planes of the screw dislocations between cross-slip events) but incompatible with predominant slip on {110} planes. Examples of further quantitative results are for the effective stress at the ‘upper bend’ of the flow-stress–temperature relationship, the enthalpy of formation of a pair of isolated kinks, 2H _k=(0.68 ±0.02),eV, and the activation energy of kink diffusion, . In agreement with the above-mentioned prediction, the same H _k values are obtained above and below . The Nb data are compared with those for Ta, Mo, W, and α-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 γ-relaxation of Nb – called irreversible by D'Anna and Benoit – is caused by the thermally activated generation of kink pairs in a ₀〈111〉/ 2 screw dislocations on {112} planes. For the reversible γ-relaxation two alternative mechanisms are discussed. The one based on kink-pair formation in screw dislocations on {110} planes appears to be the more likely one. This interpretation implies that the reversible γ-relaxation is identical with the β-relaxation and thus substantiates Chambers' claim of the intrinsic nature of the β-relaxation.

‡Dedicated to Professor Frank Reginald Nunes Nabarro on the occasion of his 90th birthday on March 7th, 2006.

Acknowledgements

The authors gratefully acknowledge the advice of Dr. U. Eßmann in the execution of the vacuum experiments, the skillful fabrication of the vacuum chamber by E. Günther, and the internal-friction measurements by Dipl. Phys. J. Berger. One of the authors (U.H.) would like to thank Dr. M. Werner for the introduction in the experimental technique and the theoretical background.

Notes

†Present address: European Commission, Joint Research Centre, Institute for Health and Consumer Protection, Cyclotron Laboratory, T.P.500, I--21020 Ispra (VA), Italy.

‡Dedicated to Professor Frank Reginald Nunes Nabarro on the occasion of his 90th birthday on March 7th, 2006.

Additional information

Notes on contributors

U. Holzwarth†

†Present address: European Commission, Joint Research Centre, Institute for Health and Consumer Protection, Cyclotron Laboratory, T.P.500, I--21020 Ispra (VA), Italy.