Abstract
An investigation is made of porosity in polycrystals which have an idealised microstructure, comprising regular polyhedral grains of uniform size. The stability of networks of grain edge pores is considered in Chapter Two. In this chapter a new model of interlinked grain edge porosity is derived on physically realistic grounds and takes into account the variation in the dihedral angle of the porosity along its length. The model also improves extant work by paying careful attention to the surface morphology of the porosity in the region of the grain corners, where grain edge tunnels interlink. The relaxation of porous structures of general dihedral angle, to configurations of minimum energy, is the subject of Chapter Three. A mathematical model of the surface diffusion driven morphological changes in grain edge pores is developed. The model employs the sophisticated analysis of Chapter Two to describe the surface morphology of the porosity at any instant in time. In the later chapters the surface diffusion shape change model is extended to include the effects of diffusion in the grain boundaries. Thus the diminution of pore volume, by the condensation of vacancies on the grain boundaries, together with the enhancement of shape changes, by the mechanisms of grain boundary diffusion and surface diffusion acting in parallel, is introduced into the model. The results of a computer program capable of simulating the relaxation of both open, interlinked grain edge networks and closed, isolated edge pores is presented. The significance of these results to the phenomena of fission-gas release in nuclear fuels and sintering in ceramic polycrystals is discussed.