IMPERFECTIONS IN METALS

IMPERFECTIONS IN METALS DOE-HDBK-1017/1-93 Structure of Metals IMPERFECTIONS IN METALS The discussion of order in microstructures in the previous chapters assumed idealized microstructures. In reality, materials are not composed of perfect crystals, nor are they free of impurities that alter their properties. Even amorphous solids have imperfections and impurities that change their structure. EO 1.13 IDENTIFY the three types of microscopic imperfections found in crystalline structures. EO 1.14 STATE how slip occurs in crystals. EO 1.15 IDENTIFY the four types of bulk defects. Microscopic imperfections are generally classified as either point, line, or interfacial imperfections. 1. Point imperfections have atomic dimensions. 2. Line imperfections or dislocations are generally many atoms in length. 3. Interfacial imperfections are larger than line defects and occur over a two- dimensional area. Point imperfections in crystals can be divided into three main defect categories. They are illustrated in Figure 7. 1. Vacancy defects result from a missing atom in a lattice position. The vacancy type of defect can result from imperfect packing during the crystallization process, or it may be due to increased thermal vibrations of the atoms brought about by elevated temperature. 2. Substitutional defects result from an impurity present at a lattice position. 3. Interstitial defects result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of being at its lattice position. Interstitial refers to locations between atoms in a lattice structure. MS-01 Page 18 Rev. 0