Structure of MetalsDOE-HDBK-1017/1-93IMPERFECTIONS IN METALSInterfacial imperfections exist at an angle between any two faces of a crystal or crystalform. These imperfections are found at free surfaces, domain boundaries, grainboundaries, or interphase boundaries. Free surfaces are interfaces between gases andsolids. Domain boundaries refer to interfaces where electronic structures are differenton either side causing each side to act differently although the same atomic arrangementexists on both sides. Grain boundaries exist between crystals of similar lattice structurethat possess different spacial orientations. Polycrystalline materials are made up of manygrains which are separated by distances typically of several atomic diameters. Finally,interphase boundaries exist between the regions where materials exist in different phases(i.e., BCC next to FCC structures).Three-dimensional macroscopic defects are called bulk defects. They generally occur on a muchlarger scale than the microscopic defects. These macroscopic defects generally are introducedinto a material during refinement from its raw state or during fabrication processes.The most common bulk defect arises from foreign particles being included in the prime material.These second-phase particles, called inclusions, are seldom wanted because they significantlyalter the structural properties. An example of an inclusion may be oxide particles in a puremetal or a bit of clay in a glass structure.Other bulk defects include gas pockets or shrinking cavities found generally in castings. Thesespaces weaken the material and are therefore guarded against during fabrication. The workingand forging of metals can cause cracks that act as stress concentrators and weaken the material.Any welding or joining defects may also be classified as bulk defects.Rev. 0Page 21MS-01
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