Temperature and strain-rate dependence of the flow stress of cyclically pre-deformed high-purity Nb single crystals were measured at 120 to 350K with high accuracy and reproducibility for 5 or more resolved shear-strain rates between 6.5 x 10-5/s and 3.5 x 10-3/s. The data were quantitatively interpreted in terms of kink-pair generation and kink diffusion in a0<111>/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, σ*, was 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 was 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 were σ* = 93MPa for the effective stress at the 'upper bend' of the flow-stress-temperature relationship, the enthalpy of formation of a pair of isolated kinks, 2Hk = 0.68eV, and the activation energy of kink diffusion, HkM = 15meV. In agreement with the above-mentioned prediction, the same Hk values were obtained above and below σ*. The Nb data were compared with those for Ta, Mo, W, and α-Fe, which all exhibited a similar pattern. The comparison with the internal-friction measurements of D’Anna and Benoit showed very clearly that the classical γ-relaxation of Nb - called irreversible by D’Anna and Benoit - was caused by the thermally activated generation of kink pairs in a0<111>/2 screw dislocations on {112} planes. For the reversible γ-relaxation two alternative mechanisms were considered. One based upon kink-pair formation in screw dislocations on {110} planes appeared to be the more likely. This interpretation implied that the reversible γ-relaxation was identical with the β-relaxation and thus substantiated Chambers’ claim of the intrinsic nature of the β-relaxation.

Slip Planes and Kink Properties of Screw Dislocations in High-Purity Niobium. A.Seeger, U.Holzwarth: Philosophical Magazine, 2006, 86[25-26], 3861-92