![]() ![]() If the rug we have just described has a second wrinkle at a different angle, however, it is very difficult to move the first one where the two wrinkles intersect (part (b) in Figure 12.4.3) this process is called pinning A process that increases the mechanical strength of a material by introducing multiple defects into a material so that the presence of one defect prevents the motion of another. (b) A second intersecting wrinkle prevents movement of the first by “pinning” it. Similarly, in a solid, the contacts between layers are broken in only one place at a time, which facilitates the deformation process.į igure 12.4.3 The Role of Dislocation in the Motion of One Planar Object across Another (a) Pushing a wrinkle across the rug results in a net movement of the rug with relatively little expenditure of energy because at any given time only a very small amount of the rug is not in contact with the floor. Moving the wrinkle requires only a small amount of energy because only a small part of the rug is actually moving at any one time. An easier solution is to create a small wrinkle at one end of the rug (an edge dislocation) and gradually push the wrinkle across, resulting in a net movement of the rug as a whole (part (a) in Figure 12.4.3). Alternatively, you could pick up the rug and try to set it back down exactly where you want it, but that requires a great deal of effort (and probably at least one extra person). Because of the large area of contact between the rug and the pad, however, they will probably move as a unit. ![]() To move the rug to its proper place, you could pick up one end and pull it. To illustrate the process, suppose you have a heavy rug that is lying a few inches off-center on a nonskid pad. occurs when a dislocation moves through a crystal. In both cases, the origin of the edge dislocation is indicated by the symbol ⊥ĭeformation A distortion that occurs when a dislocation moves through a crystal. For example, strontium and calcium are chemically similar and have similar radii, and as a result, strontium is a common impurity in crystalline calcium, with the Sr atoms randomly occupying sites normally occupied by Ca.įigure 12.4.2 Edge Dislocations Shown are two examples of edge dislocations: (a) an edge dislocation in an ear of corn and (b) a three-dimensional representation of an edge dislocation in a solid, illustrating how an edge dislocation can be viewed as a simple line defect arising from the insertion of an extra set of atoms into the lattice. Substitutional impurities are usually chemically similar to the substance that constitutes the bulk of the sample, and they generally have atomic radii that are within about 15% of the radius of the host. is a different atom of about the same size that simply replaces one of the atoms that compose the host lattice ( Figure 12.4.1 \). In contrast, a substitutional impurity A point defect that results when an impurity atom occupies a normal lattice site. *In addition to enough iron to bring the total percentage up to 100%, most steels contain small amounts of carbon (0.5%–1.5%) and manganese (<2%). Magnets in electric motors and transformers Knives, cutting tools, drill bits, and springsġ5%–20% Cr, 1%–5% Mn, 5%–10% Ni, 1%–3% Si, 1% C, 0.05% PĬutlery, instruments, and marine fittings Table 12.4.1Compositions, Properties, and Uses of Some Types of Steel Name of Steel ![]() We begin with solids that consist of neutral atoms, specifically metals, and then turn to ionic compounds. In this section, we discuss how defects determine some of the properties of solids. Applying an external stress to a crystal, such as a hammer blow, can cause microscopic regions of the lattice to move with respect to the rest, thus resulting in imperfect alignment.Cooling at more realistic rates usually results in one or more components being trapped in the “wrong” place in a lattice or in areas where two lattices that grew separately intersect. Even if a substance were 100% pure, forming a perfect crystal would require cooling the liquid phase infinitely slowly to allow all atoms, ions, or molecules to find their proper positions.It is impossible to obtain any substance in 100% pure form.These defects occur for three main reasons: (typically more than 10 4 per milligram), ranging from variable amounts of impurities to missing or misplaced atoms or ions. In contrast, real crystals contain large numbers of defects Errors in an idealized crystal lattice. ![]() The crystal lattices we have described represent an idealized, simplified system that can be used to understand many of the important principles governing the behavior of solids.
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