Structure of MetalsDOE-HDBK-1017/1-93 GRAIN STRUCTURE AND BOUNDARYGRAIN STRUCTURE AND BOUNDARYMetals contain grains and crystal structures. The individual needs a microscopeto see the grains and crystal structures. Grains and grain boundaries helpdetermine the properties of a material. EO 1.6DEFINE the following terms:a.Grain b.Grain structurec.Grain boundaryd.CreepIf you were to take a small section of a common metal and examine it under a microscope, youwould see a structure similar to that shown in Figure 3(a). Each of the light areas is called agrain, or crystal, which is the region of space occupied by a continuous crystal lattice. The darklines surrounding the grains are grain boundaries. The grain structure refers to the arrangementof the grains in a metal, with a grain having a particular crystal structure. The grain boundaryrefers to the outside area of a grain that separates it from the other grains.The grain boundary is a region of misfit between the grains and is usually one to three atomdiameters wide. The grain boundaries separate variously-oriented crystal regions(polycrystalline) in which the crystal structures are identical. Figure 3(b) represents four grainsof different orientation and the grain boundaries that arise at the interfaces between the grains.A very important feature of a metal is the average size of the grain. The size of the graindetermines the properties of the metal. For example, smaller grain size increases tensile strengthand tends to increase ductility. A larger grain size is preferred for improved high-temperaturecreep properties. Creep is the permanent deformation that increases with time under constantload or stress. Creep becomes progressively easier with increasing temperature. Stress andstrain are covered in Module 2, Properties of Metals, and creep is covered in Module 5, PlantMaterials.Rev. 0Page 9MS-01
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