Low and High Temperature Limits of Plastic Instabilities of Cu-Al Polycrystals

Abstract

Polycrystalline samples of f.c.c. Cu 10 at%Al solid solutions are deformed in a wide temperature range (270K < Τ < 1000K) in uniaxial tension. Within this range of temperatures the occurrence and behaviour of boundaries of load instabilities (Portevin -Le Chatelier (PLC) effect) are investigated. At low temperatures the dynamic strain ageing processes which induce the PLC instabilities are investigated by strain rate change experiments within the region of stable deformation preceding the instabilities. An analysis of the resulting strain rate sensitivity of the flow stress and a comparison of the temperature and strain rate dependence of the PLC boundaries shows that jerky flow in Cu-Al is induced by pipe diffusion processes at low temperatures. At high temperatures the PLC instabilities vanish as a consequence of the transition from the discontinuous, thermally activated overcoming of obstacles to the continuous, viscous motion of dislocations by dragging a Cottrell cloud. This transition is accompanied by one from pipe to bulk diffusion and is shown to depend not only on temperature and strain rate but also on stress. This is investigated by changes of the deformation conditions from constant strain rate to constant stress (creep test).