Peierls barriers, kinks, and flow stress: Recent progress

Abstract

The intrinsic barriers against the glide of dislocations lying along low-index crystallographic directions, the so-called Peierls barriers, are overcome by the thermally activated and stress-assisted generation of pairs of kinks of opposite sign in the dislocations. In plastically deformed Al four low-temperature mechanical relaxation processes resulting from this are identified. In a sequence of falling temperatures, the corresponding internal-friction peaks are the original Bordoni peak (a o /2 screw dislocations on {111} slip planes; a 0 = edge length of the elementary cube), Ni-blett's 'subsidiary peak' (60°-dislocations on {111}), the Lax-Filson peak (edge dislocations on {111}), and the Kosugi-Kino peak (30°-dislocations on {111}). In other face-centred cubic (fcc) metals (e. g. Cu, Pd) the relaxation processes of the dislocations lying along as well as those lying along (112) overlap, giving rise to just two peaks, namely very broad Bordoni and Niblett-Wilks peaks. In the body-centred cubic (bcc) transition metals, the Peierls barriers of a 0 /2 screw dislocations are exceptionally high. Beyond the microstrain regime, overcoming them by the formation of kink pairs determines the flow stress up to the knee-temperature T K . The theory of the kink-pair formation enthalpy H kp (σ*) (effective stress σ* = difference between applied resolved shear stress σ and long-range internal stress σ M ) has been tested and verified with great accuracy on various high-purity bcc metals using the Ackermann-Mughrabi cyclic-deformation method as well as stress-relaxation measurements. The 'hump' in the flow-stress-temperature relationship of high-purity bcc metals follows from the theory without further assumptions. The comparison experiment-theory shows that the kink-pair generation takes place on {110} planes below the temperature T of the lower bend of the flow-stress-temperature curve and on {112} in the range T {112} slip systems with different slip directions. The {112} slin is also responsible for the tension-compression asymmetry of bcc metals, which, however, cannot be explained entirely in terms of the familiar twinning-antitwinning asymmetry of {112} slip but involves also stress components other than the effective resolved shear stress σ*. In 1971 it was proposed that the generation of kink-pairs in a 0 /2 screw dislocations gives rise to the so-called γ-relaxation in the internal friction of bcc metals. The agreement of the activation enthalpy of this process with twice the enthalpy of formation of isolated kinks in screw dislocations, H k , as deduced from flow-stress measurements not only verifies this proposal but also demonstrates that in the 'classical' γ-relaxation the kink-pair generation takes place on {112} planes. However, recent measurements on Nb and Ta have shown that annealing may lead to a modified γ-relaxation with a smaller activation enthalpy.