Strength and Deformation of Ionic Materials

Abstract

Summary The necessary conditions for plastic deformation in ionic solids are readily defined. There must be mobile dislocations in sufficient numbers, on a sufficient number of slip systems for a general strain to take place at the applied strain rate. Provided these conditions are fulfilled an ionic solid may be ductile. In fully ionic solids dislocations are highly mobile on some slip systems at least, down to very low temperatures, whereas in the more covalent solids this is not so. In most ionic solids the number of available slip systems does decrease with fall of temperature, and the multiplication of dislocations by cross-slip also becomes increasingly difficult at low temperatures. We conclude that the necessary conditions for deformation are unlikely to be fulfilled at low temperatures, and this is in agreement with the experimental facts. While these are the necessary conditions for ductility they may not be sufficient ones. As a result of the occurrence of plastic deformation cracks may form in ionic solids. For the material to flow rather than fracture, these cracks, or pre-existing cracks for that matter, must be non-propagating at the stress levels and strain-rates concerned. Here it is more difficult to define the conditions for flow precisely. The relevant factors include the stress state at the head of the crack, and the temperature dependence and the strain-rate dependence of the yield stress of these solids as a function of purity. For polycrystalline materials, a knowledge of these factors, together with the grain-size dependence of the yield stress, should permit some prediction of the transition between ductile and brittle behaviour. At high temperatures, deformation occurs by creep at very low stresses. New mechanisms of deformation appear, in which the movement of dislocations by glide is less important, and at the highest temperatures diffu-sional processes play an important role. Steady-state creep represents a balance between the hardening of the material by deformation and thermal recovery. The onset of recrystallization can lead to a sudden increase in the rate of creep.