The inverse Swift effect: Experiments and theory

Experiments are described in which previously twisted wires are loaded in "free end" extension. The prior shears are applied at either low or elevated homologous temperatures. When prior twisting takes place at room temperature, extensions beyond 1-2% produce continued twisting. Conversely, when the prior shear is applied at elevated temperatures, free extension is accompanied by untwisting. In both casts, untwisting occurs during the first 1-2% of plastic extension. By means of stress relief annealing experiments and a simple analysis, the untwisting that takes place during the first stages of stretching is shown to be largely due to a residual stress effect. The shear parallel and anti-shear twists that are observed at larger extensions are attributed to asymmetries in the sample yield surfaces; these are tilted in the negative and positive shear directions according to whether prior shearing was carried out at low or at elevated homologous temperatures, respectively. The room temperature yield surface tilts are demonstrated to be associated with the lattice rotations produced by dislocation glide; by contrast, the high temperature tills are attributed to the occurrence of dynamic recrystallization. The latter phenomenon is shown to be responsible for significant grain rotations, which are additional to those predicted by the conventional methods of crystal plasticity. (C) 1997 Acta Metallurgica Inc.