An abnormal after-effect in metals

Abstract

A metal wire that has been subjected to plastic flow in torsion generally untwists at a rate such that equal strains occur in equal intervals of log time. It was found that this is true with single crystal and polycrystalline wires of iron and of zinc, both when coated with oxide films and when freshly cleaned of oxide. But the normal after-effect following plastic twisting of these wires is replaced by an abnormal after-effect (twisting instead of untwisting), which occurs as a transient, when an etchant is suddenly applied to an oxide-coated wire that has been twisted. The transient reversal of the after-effect was predicted on the basis that adherent oxide films on metals act as barriers to the movement of dislocations. When a wire is twisted, dislocations pile up beneath the oxide-metal interface; these escape through the surface when acid is applied. An after-effect in the normal direction but with a reduced rate follows the transient effect and is also accounted for by dislocations escaping that had previously been piled up beneath the surface.