Internal friction and electron microscope studies of strain-ageing in zirconium

Abstract

Internal friction experiments and transmission electron microscopy have been used to throw further light on the mechanism of strain-ageing which occurs at about 300 °C in zirconium and zirconium-oxygen alloys. Internal friction specimens which have been heat-treated and deformed in a manner which leads to a discontinuous yield point in a tensile test show only logarithmic damping and the implication is that under these conditions the dislocations are strongly pinned. On the other hand in internal friction specimens treated in such a way that discontinuous yielding is not observed a transition from logarithmic to non-logarithmic damping takes place. Electron microscope studios of dislocation arrangements in sheet specimens given the same treatment as the internal friction wires have shown that the specimens which display a discontinuous yield point contain a well defined cellular structure of tangled dislocations. It is believed that with such a structure the dislocations are strongly pinned as a result of both solute atom locking and dislocation interactions. In specimens deformed under such conditions that no discontinuous yield point occurs a more uniform array of dislocations is formed. Here the pinning of the dislocations is much weaker because the effects of both solute locking and interaction locking are reduced and, under these conditions, the stresses imposed on the specimen during the internal friction experiments are sufficient to cause unpinning to take place. Some observations on the variation of logarithmic damping with temperature are also reported. These have been interpreted as arising from sub-grain boundary relaxation effects.