Chapter 9 - Diffusion in Polycrystalline Materials

Abstract

Polycrystalline materials have a spectrum of defects of varying dimensionality, including zero-dimensional point defects (vacancies and interstitials), one-dimensional line defects (dislocations), two-dimensional surface defects (grain boundaries and free surfaces), and three-dimensional volume defects (pores). Higher-dimensional defects such as dislocations and surfaces lead to “short-circuit” pathways for dramatically accelerated diffusion. Hence, the diffusivity of a polycrystalline material depends strongly on the concentrations of such defects. The Harrison “ABC” model provides a description of three diffusion regimes in a polycrystalline material, depending on the relative diffusivity through the grains versus along the grain boundaries. Depending on the regime, the diffusion in polycrystalline materials can be treated using either a single diffusion equation with an effective diffusivity or a coupled set of diffusion equations on different time scales. While defects act to enhance diffusivity, there is no universal atomistic mechanism enabling this fast diffusion. The details of the enhancement in diffusivity depend on the structure and microstructure of the material in question.