With very few exceptions metals in the solid state are crystalline, the individual crystals are normally small (typically in the range 5–500 nm). Metallic crystals contain a number of defects that have a profound influence on their properties. Point defects (missing atoms, vacancies and additional atoms, interstitials) influence diffusion, and in turn properties such as creep at elevated temperature. Line defects (dislocation) are responsible for the ductility of metals (the ability to deform permanently when stressed). Planar defects (grain boundaries and twin boundaries) provide some strengthening and also act as sites of enhanced diffusion and of segregation of impurities with important consequences for corrosion. While pure metals will have all grain with the same crystal structure, alloys often have two or more phases (regions of different crystal structure and composition), and many microstructures can be produced depending on the thermal and mechanical history of the material. Again the exact microstructure can have very strong influence on the mechanical and corrosion properties of the alloy.