Chapter 10 - Lattice Defects and Diffusion in Nanomaterials

Abstract

The diffusion rate in nanomaterials is influenced strongly by the type and densities of lattice defects. Generally, dislocations and grain boundaries are paths for fast diffusion; however, the actual atomic arrangement around these defects also influences the diffusion along them. In this chapter, the most important models, formulas, and methods for the evaluation of diffusion along grain boundaries and dislocations are overviewed. The effect of solute segregation and misorientation on grain boundary self-diffusion is revealed. It was found that the larger the free volume and the energy of grain boundaries, the faster the grain boundary diffusion. In ultrafine-grained metallic materials processed by severe plastic deformation (SPD), a hierarchical microstructure develops with nonequilibrium and relaxed grain boundaries, which are paths for fast and slow grain boundary diffusion, respectively. As the fraction of boundaries exhibiting fast diffusion is very small (under 0.5%), the diffusion rate in nanomaterials is determined by the relaxed boundaries. The diffusivity for these boundaries is very close to that observed for grain boundaries in coarse-grained materials. Therefore, the faster diffusion in nanomaterials compared to coarse-grained counterparts is caused basically by the larger amount of grain boundaries and not by their different quality. In nanomaterials processed by SPD, the fastest diffusion path is provided by the percolating pores at grain boundaries formed by agglomeration of excess vacancies. However, the contribution of these defects to the total diffusivity is very small due to their marginal volume fraction. In nanomaterials processed by powder metallurgy, the diffusivity for interagglomerate interfaces between particles and pores is several orders of magnitude larger than that for intraagglomerate boundaries. Similar bimodal diffusivity is observed for nanomaterials obtained by crystallization of amorphous materials. Here, the slow and fast diffusion pathways are the amorphous and conventional crystalline interfaces. The diffusion along amorphous boundaries is slower than that for crystalline interfaces.