Crystal symmetry, plasticity affected

In a crystal lattice there is translation symmetry but in a polycrystalline solid it exists only approximately within one grain. Similar to the outer surfaces of the crystallites, the grain boundaries are two-dimensional defects the crystal lattice stops there. Dislocations are one-dimensional defects and pores are defects in solids having a dimension that is usually three but can be lower. Such higher-dimensional defects (Table 10.1) determine many properties the dislocations in metals affect plasticity and the porosity, if open, determines gas permeability. 

---