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Most metals and alloys are melted and cast into semifinished or finished shapes. During the solidification of a metal into a casting, nuclei are formed that grow into grains, creating solidified cast metal with a polycrystalline grain structure. For most industrial applications a very small grain size is desirable. The grain size may be indirectly determined by ASTM grain size number n or directly determined by finding the average grain diameter. Large single crystals are rarely made in industry. However, an exception is the large single crystals of silicon produced for the semiconductor industry. For this material special solidification conditions must be used and the silicon must be of very high purity.

Crystal imperfections are present in all real crystalline materials, even at the atomic or ionic size level. Vacancies or empty atomic sites in metals can be explained in terms of the thermal agitation of atoms and are considered equilibrium lattice defects. Dislocations (line defects) occur in metal crystals and are created in large numbers by the solidification process. Dislocations are not equilibrium defects and increase the internal energy of the metal. Images of dislocations can be observed in the transmission electron microscope. Grain boundaries are surface imperfections in metals created by crystals of different orientations meeting each other during solidification. Other important types of defects that affect the properties of materials are twins, low angle boundaries, high angle boundaries, twists, stacking faults, and precipitates.

Materials scientists and engineers use high-tech instruments to learn about the internal structure (including defect structure), behavior, and failure of materials. Instruments such as metallographs, SEM, TEM (HRTEM), and SPM allow analysis of materials from macro to nano range. Without such instruments understanding the behavior of materials would be impossible.







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