COURSE: The Structure of Metals mperfections in the Crystal Structure of Metals
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Imperfections in the Crystal Structure of Metals The actual strength of metals is found to be approximately one to two orders of magnitude lower than the strength levels obtained from theoretical calculations based on molecular dynamics. This discrepancy is explained in terms of defects and imperfections in the crystal structure. Unlike in idealized models, actual metal crystals contain a large number of defects and imperfections, which generally are categorized as follows: 1. Point defects 2. Linear or one dimensional defects 3. Planar, or two-dimensional, imperfections, such as grain boundaries and phase boundaries 4. Volume, or bulk, imperfections, such as voids, inclusions (non metallic elements such as oxides, sulfides, and silicates), other phases, or cracks.
Point defects Point defects, such as a vacancy (missing atom), an interstitial atom (extra atom in the lattice), or an impurity (foreign atom that has replaced the atom of the pure metal). Self-interstitial atom Vacancy Interstitial impurity atom Substitutional impurity atom
Linear or one dimensional defects Linear or one-dimensional, defects, called dislocations Screw dislocation (a) edge dis ocation; and (b) screw dislocation
Structure sensitive and Structure insensitive Mechanical and electrical properties of metals, such as yield stress, fracture strength, and electrical conductivity, are adversely affected by defects; these properties are known as structure sensitive. By contrast, physical and chemical properties, such as melting point, specific heat, coefficient of thermal expansion, and elastic constants (e.g., modulus of elasticity and modulus of rigidity), are not sensitive to these defects; these properties are known as structure insensitive
Dislocations Dislocations are defects in the orderly arrangement of a metal's atomic structure. Because a slip plane containing a dislocation requires less shear stress to allow slip than does a plane in a perfect lattice, dislocations are the most significant defects that explain the discrepancy between the actual and theoretical strengths of metals There are two types of dislocations: edge and screw. An analogy to the movement of an edge dislocation is the progress of an earthworm, which moves forward by means of a hump that starts at the tail and moves toward the head. Slip plane
Work Hardening (Strain Hardening) Although the presence of a dislocation lowers the shear stress required to cause slip, dislocations can: 1. Become entangled and interfere with each other, and 2. Be impeded by barriers, such as grain boundaries, impurities, and inclusions in the material The increased shear stress required to overcome entanglements and impediments results in an increase in the overall strength and the hardness of the metal and is known as work hardening or strain hardening. The greater the deformation, the greater is the number of entanglements and hence the higher the increase in the metal's strength. Work hardening is used extensively for strengthening in metalworking processes at ambient temperatures. Typical examples are producing sheet metal for automobile bodies and aircraft fuselages by cold rolling, producing the head of a bolt by forging, and strengthening wire by reducing its cross section by drawing it through a die.
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