Slip planes and kink properties of screw dislocations in high-purity niobium
Résumé
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 at five or more resolved shear-strain rates between 6.5 × 10-5 and 3.5 × 10-3 s-1. The data are quantitatively interpreted in terms of the kink-pair generation and kink diffusion in a0 <111>/2 screw dislocations (a0 = cubic lattice parameter). From the stress dependence of the activation volume the spatial period of the Peierls barriers of the screw dislocations could be derived. In the temperature range investigated, the measured periodicity is in quantitative agreement with {112} as elementary slip planes but incompatible with predominant slip on {110} planes. The zero-stress enthalpy of formation of a pair of isolated kinks is 2Hk = (0.68 ± 0.02) eV. The Nb data are compared with those on Ta, Mo, W, and α-Fe. 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 a0 <111>/2 screw dislocations on {112} planes. For the more recently discovered 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.
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