Progress and problems in the understanding of the dislocation relaxation processes in metals

Abstract

The decomposition of the motion of flexible dislocation lines in the Peierls potential of crystals into four classes of almost independent degrees of freedom is discussed and applied to the calculation of the equilibrium density of kinks in the presence of so-called geometrical kinks. The theory of the mechanical relaxation by kink-pair generation is generalised to include the effects of geometrical kinks and internal stresses. The prediction that the non-Debye features of the relaxation, e.g. the ‘extra width’ of the peaks in the temperature dependence of the internal friction, should increase with decreasing measuring frequency agrees with the experiments on the Bordoni relaxation in copper. Whereas in most face-centred cubic (fcc) metals the kink-pair generation in dislocations with Burgers vectors a0〈110〉/2 running along 〈110〉 or 〈112〉 gives rise to two rather broad relaxation peaks (the Bordoni and the Niblett–Wilks peak, respectively), in aluminium the relaxation processes of 0°-, 60°-, 90°-. and 30°-dislocations show up in four well separated internal-friction maxima. In this sequence, they are attributed to the Bordoni peak, the ‘subsidiary’ or Niblett–Wilks peak, the Lax–Filson peak, and the Kosugi–Kino peak. Comparison of flow-stress and internal-friction measurements on high-purity refractory body-centred cubic (bcc) metals confirms the interpretation of the γ-relaxation in terms of the kink-pair generation in a0〈111〉/2 screw dislocations. The cores of these dislocations may exist in two different configurations with different slip planes, the {112} configuration being responsible for the γ-relaxation, the {110} configuration for the β-relaxation. The existence of the two distinct core configurations may also account for the difference between the ‘reversible’ and the ‘irreversible’ γ-relaxation in niobium and tantalum and appears to be the reason for the striking dependence of the α-relaxation on the temperature of deformation. It is argued that the kink-pair formation in non-screw dislocation on {110} planes gives rise to the high-temperature side of the α-relaxation and that on {112} planes to the low-temperature side.