Imperfections and defects are unavoidable. These defects are driven by entropy since they increase the disorder of a system. In metals, defects can be vacancies or interstitials. Vacancies are empty sites in the lattice. Interstitials are when atoms occupy the sites between atoms (interstitials). The defects can be of the same type of atoms making up the lattice such as self-interstitials, or from different types of atoms as in substitutions or interstitial defects. These defects produce lattice strain in the vicinity of the defect.

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